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Nano Res
1
Low haze transparent electrodes and highly conducting
air dried films with ultra-long silver nanowires
synthesized by one-step polyol method Teppei Araki () Jinting Jiu Masaya Nogi Hirotaka Koga Shijo Nagao Tohru Sugahara Katsuaki Suganuma Nano Res Just Accepted Manuscript bull DOI 101007s12274-013-0391-x
httpwwwthenanoresearchcom on November 13 2013
copy Tsinghua University Press 2013
Just Accepted
This is a ldquoJust Acceptedrdquo manuscript which has been examined by the peer‐review process and has been
accepted for publication A ldquoJust Acceptedrdquo manuscript is published online shortly after its acceptance
which is prior to technical editing and formatting and author proofing Tsinghua University Press (TUP)
provides ldquoJust Acceptedrdquo as an optional and free service which allows authors to make their results available
to the research community as soon as possible after acceptance After a manuscript has been technically
edited and formatted it will be removed from the ldquoJust Acceptedrdquo Web site and published as an ASAP
article Please note that technical editing may introduce minor changes to the manuscript text andor
graphics which may affect the content and all legal disclaimers that apply to the journal pertain In no event
shall TUP be held responsible for errors or consequences arising from the use of any information contained
in these ldquoJust Acceptedrdquo manuscripts To cite this manuscript please use its Digital Object Identifier (DOIreg)
which is identical for all formats of publication
Nano Research DOI 101007s12274‐013‐0391‐x
1
Table of Contents
Ultra-long silver nanowires (u-long AgNWs) with lengths up to 230 μm have been
successfully prepared by a one-step polyol process at a low reaction temperature and a
low stirring speed of 110 degC and 60 rpm respectively Compared to conventional
AgNWs with lengths of 10 μm and indium tin oxide u-long AgNW electrodes achieved
a low haze of 22 with low sheet resistance of 49 Ω at an optical transmittance of
96 and the electrodes also showed a low sheet resistance without post-treatment at
room temperature and pressure
2
Low haze transparent electrodes and highly conducting air dried films with ultra-long
silver nanowires synthesized by one-step polyol method
Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru
Sugahara2 Katsuaki Suganuma2
1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka
567-0047 Japan
Corresponding author
Teppei Araki
teppeiecosankenosaka-uacjp
Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
3
Abstract
Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility
when compared to those made of tin doped indium oxide (ITO) and are expected to be
applied in plastic electronics However these transparent electrodes composed of
AgNWs show high haze because the wires cause strong light scattering in the visible
range Reduction of the wire diameter has been proposed to weaken light scattering
although there have seldom been any studies focusing on the haze because of the
difficulty involved in controlling the wire diameter In this report we show that the haze
can be easily reduced by increasing the length of AgNWs with a large diameter
Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum
length of 230 μm have been successfully synthesized by adjusting the reaction
temperature and the stirring speed of a one-step polyol process Compared to typical
AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a
transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a
low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of
94ndash97 Even when fabricated at room temperature without any post-treatment the
electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a
transmittance of 80 which was six orders of magnitude lower than that of typical
4
AgNWs
Keywords
Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze
Introduction
Fabrication of flexible and stretchable transparent electrodes has attracted much
attention in the view of the increasing demand for plastic electronics in displays touch
screens and solar cells Attempts have been made to replace the widely used albeit stiff
and rarely available tin doped indium oxide (ITO) with new materials such as carbon
nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among
the many candidates silver nanowire (AgNW) films show excellent electrical
conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals
and exhibits high stability and flexibility AgNW electrodes with a transparency of 80
and sheet resistances of around 10 Ω have been fabricated and applied to the above
mentioned devices at the laboratory scale [11ndash18]
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
1
Table of Contents
Ultra-long silver nanowires (u-long AgNWs) with lengths up to 230 μm have been
successfully prepared by a one-step polyol process at a low reaction temperature and a
low stirring speed of 110 degC and 60 rpm respectively Compared to conventional
AgNWs with lengths of 10 μm and indium tin oxide u-long AgNW electrodes achieved
a low haze of 22 with low sheet resistance of 49 Ω at an optical transmittance of
96 and the electrodes also showed a low sheet resistance without post-treatment at
room temperature and pressure
2
Low haze transparent electrodes and highly conducting air dried films with ultra-long
silver nanowires synthesized by one-step polyol method
Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru
Sugahara2 Katsuaki Suganuma2
1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka
567-0047 Japan
Corresponding author
Teppei Araki
teppeiecosankenosaka-uacjp
Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
3
Abstract
Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility
when compared to those made of tin doped indium oxide (ITO) and are expected to be
applied in plastic electronics However these transparent electrodes composed of
AgNWs show high haze because the wires cause strong light scattering in the visible
range Reduction of the wire diameter has been proposed to weaken light scattering
although there have seldom been any studies focusing on the haze because of the
difficulty involved in controlling the wire diameter In this report we show that the haze
can be easily reduced by increasing the length of AgNWs with a large diameter
Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum
length of 230 μm have been successfully synthesized by adjusting the reaction
temperature and the stirring speed of a one-step polyol process Compared to typical
AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a
transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a
low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of
94ndash97 Even when fabricated at room temperature without any post-treatment the
electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a
transmittance of 80 which was six orders of magnitude lower than that of typical
4
AgNWs
Keywords
Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze
Introduction
Fabrication of flexible and stretchable transparent electrodes has attracted much
attention in the view of the increasing demand for plastic electronics in displays touch
screens and solar cells Attempts have been made to replace the widely used albeit stiff
and rarely available tin doped indium oxide (ITO) with new materials such as carbon
nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among
the many candidates silver nanowire (AgNW) films show excellent electrical
conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals
and exhibits high stability and flexibility AgNW electrodes with a transparency of 80
and sheet resistances of around 10 Ω have been fabricated and applied to the above
mentioned devices at the laboratory scale [11ndash18]
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
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Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
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[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
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[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
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[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
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[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
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[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
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[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
2
Low haze transparent electrodes and highly conducting air dried films with ultra-long
silver nanowires synthesized by one-step polyol method
Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru
Sugahara2 Katsuaki Suganuma2
1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka
567-0047 Japan
Corresponding author
Teppei Araki
teppeiecosankenosaka-uacjp
Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan
3
Abstract
Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility
when compared to those made of tin doped indium oxide (ITO) and are expected to be
applied in plastic electronics However these transparent electrodes composed of
AgNWs show high haze because the wires cause strong light scattering in the visible
range Reduction of the wire diameter has been proposed to weaken light scattering
although there have seldom been any studies focusing on the haze because of the
difficulty involved in controlling the wire diameter In this report we show that the haze
can be easily reduced by increasing the length of AgNWs with a large diameter
Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum
length of 230 μm have been successfully synthesized by adjusting the reaction
temperature and the stirring speed of a one-step polyol process Compared to typical
AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a
transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a
low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of
94ndash97 Even when fabricated at room temperature without any post-treatment the
electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a
transmittance of 80 which was six orders of magnitude lower than that of typical
4
AgNWs
Keywords
Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze
Introduction
Fabrication of flexible and stretchable transparent electrodes has attracted much
attention in the view of the increasing demand for plastic electronics in displays touch
screens and solar cells Attempts have been made to replace the widely used albeit stiff
and rarely available tin doped indium oxide (ITO) with new materials such as carbon
nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among
the many candidates silver nanowire (AgNW) films show excellent electrical
conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals
and exhibits high stability and flexibility AgNW electrodes with a transparency of 80
and sheet resistances of around 10 Ω have been fabricated and applied to the above
mentioned devices at the laboratory scale [11ndash18]
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
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Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
3
Abstract
Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility
when compared to those made of tin doped indium oxide (ITO) and are expected to be
applied in plastic electronics However these transparent electrodes composed of
AgNWs show high haze because the wires cause strong light scattering in the visible
range Reduction of the wire diameter has been proposed to weaken light scattering
although there have seldom been any studies focusing on the haze because of the
difficulty involved in controlling the wire diameter In this report we show that the haze
can be easily reduced by increasing the length of AgNWs with a large diameter
Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum
length of 230 μm have been successfully synthesized by adjusting the reaction
temperature and the stirring speed of a one-step polyol process Compared to typical
AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a
transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a
low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of
94ndash97 Even when fabricated at room temperature without any post-treatment the
electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a
transmittance of 80 which was six orders of magnitude lower than that of typical
4
AgNWs
Keywords
Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze
Introduction
Fabrication of flexible and stretchable transparent electrodes has attracted much
attention in the view of the increasing demand for plastic electronics in displays touch
screens and solar cells Attempts have been made to replace the widely used albeit stiff
and rarely available tin doped indium oxide (ITO) with new materials such as carbon
nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among
the many candidates silver nanowire (AgNW) films show excellent electrical
conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals
and exhibits high stability and flexibility AgNW electrodes with a transparency of 80
and sheet resistances of around 10 Ω have been fabricated and applied to the above
mentioned devices at the laboratory scale [11ndash18]
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
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Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
4
AgNWs
Keywords
Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze
Introduction
Fabrication of flexible and stretchable transparent electrodes has attracted much
attention in the view of the increasing demand for plastic electronics in displays touch
screens and solar cells Attempts have been made to replace the widely used albeit stiff
and rarely available tin doped indium oxide (ITO) with new materials such as carbon
nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among
the many candidates silver nanowire (AgNW) films show excellent electrical
conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals
and exhibits high stability and flexibility AgNW electrodes with a transparency of 80
and sheet resistances of around 10 Ω have been fabricated and applied to the above
mentioned devices at the laboratory scale [11ndash18]
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
5
However one of the disadvantages of using AgNW films is the high haze derived from
light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low
haze leads to less obscuration of transparent materials which is immensely required in
display-based applications Conventional ITO films exhibit a haze of about 1ndash3
with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used
in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum
(which is disadvantageous for high speed production) or high temperature (over 300 degC
which limits the use of low-heat-resistant substrates) On the other hand CNT graphene
and AgNW electrodes can be fabricated by solution processing for high speed
production by roll-to-roll processing with treatments that preclude damage to the
polymer substrate With new nanomaterials like CNTs or graphene haze values below
1 have been achieved [22 23] while the sheet resistances are generally far above 100
Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can
achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag
[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the
application of AgNW electrodes in display devices [10 20 25 26] If the haze can be
decreased to the level of ITO films in combination with the high conductivity AgNW
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
6
electrodes will open new frontiers in terms of applications in devices
Kim et al reported an inverse relationship between haze and transmittance in AgNW
electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance
When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3
[20] which is similar to the values shown by ITO films Unfortunately the AgNW
electrodes of such low haze showed poor conductivity or were even non-conductive
because the junctions between the wires collapsed in the film comprising wires with
typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the
haze while maintaining the high conductivity it is necessary to extend the length of the
wires to make network junctions between the AgNWs Moreover smaller diameter
AgNWs directly decrease the haze in the visible wavelength range because of the
decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining
the conductivity as well as transparency has been a great challenge In general
decreasing the diameter and extending the length of wires have been expected to be a
promising strategy to achieve low haze with high conductivity and high transparency
Synthesis of AgNWs by the polyol process is now widely known as a scalable and a
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
7
simpler method when compared to other techniques [27 28] Recently AgNWs with
lengths typically below 20 μm were elongated to several hundred micrometers by a
multistep repetitive process involving the polyol method [14 29] Transparent
electrodes consisting of long AgNWs has been shown to possess superior electrical
properties at high transmittances ranging to up to 95 when compared to carbon
nanomaterials and other nanometals However AgNWs with diameters above 100 nm
are considered to lead to an increase in haze [25 26] In this report low-haze AgNW
electrodes with high conductivity were obtained when the length of the AgNWs was
increased while maintaining their large diameter A modified one-step polyol method
was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to
100 μm (the maximum length achieved was 230 μm) by adjusting the reaction
temperature and stirring speed AgNW electrodes based on these u-long NWs showed
lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high
optical transmittance of 94ndash97 The sheet resistance was comparable to or even
better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]
Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a
transmittance of 80 just after drying at room temperature without any post-treatments
which was six orders of magnitude lower than the values shown by typical AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
8
1 Experiment
11 Synthesis of AgNWs and fabrication of the electrodes
The AgNWs were synthesized by the reduction of silver nitrate in the presence of
polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from
Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)
was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600
μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene
glycol) were added rapidly into the PVP solution within a minute The mixture was
subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm
for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm
After the reaction the solutions were washed in ethanol thrice centrifuged and the
product was dispersed in ethanol for further use To break the ultralong AgNWs their
suspension was ultrasonicated at 195 KHz and 300 W for 10 min
A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the
suspension was streamed along the longitudinal and transverse directions on the
substrate surface followed by air drying Then the AgNW electrode was prepared with
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
9
and without heating at 200 degC for 10 min The sheet resistance of the transparent
electrode fabricated by this method showed no large deviation at different locations of
the film
12 Characterization of the AgNW electrodes
The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was
measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co
Ltd) with the substrate as the reference The haze and Tp were measured under D65
illumination haze meter with a strong visible light source (HZ-V3 Suga Test
Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter
light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference
substrate was 92 and 01 respectively under D65 illumination Many researchers
consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis
as the generally accepted data Here we compared the Tp measured with the UV-Vis
spectra at 550 nm and the haze meter The results were identical Hence the measuring
the Tp with the haze meter is also an acceptable method to determine the transmittance
of the AgNW electrode (Figure S-1)
The sheet resistance of the electrode of 25 mm times 30 mm was measured using the
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
10
four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The
AgNWs on the substrate were observed using an optical microscope (VH-Z500
VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)
(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working
distance of 8 mm
2 Results and Discussion
Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)
exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)
The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It
has been known that lower reaction temperatures used in the polyol method results in
longer wires and the length was about 20 μm at 130 degC [30] In order to extend the
length we attempted to further decrease the reaction temperature to 110 degC When the
temperature was decreased to 110 degC at the same stirring speed the AgNWs
(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative
frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of
74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of
typical AgNWs However the low-temperature sample included some nanoparticles too
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
11
The two reaction temperatures affording different AgNW lengths can be reasoned as
follows High concentrations of silver nuclei upon being subjected to high temperatures
during the nucleation stage led to a large number of seeds (namely a limited amount of
the Ag+ source) that tended to develop into short AgNWs In contrast low reaction
temperatures produced limited silver nuclei which were stable in solution When new
silver nuclei were reduced in the same solution slowly due to the low temperature these
fresh nuclei tended to accumulate on the previously present silver nuclei which led to
long wires However with the severe disturbance caused by high stirring speeds some
silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)
In order to observe the effect of the stirring speed the stirring speed was maintained at
60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm
(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1
(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10
for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1
d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our
knowledge these lengths have never been realized with a one-step polyol process until
now The average length of the u-long wires was 44 μm which is over four times longer
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
12
than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh
nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging
to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the
contrary low stirring speeds and low temperatures ie low flow velocities and low
growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to
form stable and long wires under a relatively quiet growth environment (Figure 1f)
Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes
fabricated at room temperature with only a coat-drying process The sheet resistances of
the AgNW electrodes increased with transparency The electrodes with the u-long
AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The
typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were
over the measurement range With only coating at room temperature without any
post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was
six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical
SEM images of the two kinds of AgNW electrodes at the same transmittance The
number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2
times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
13
times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin
et al [30] it can be assumed that the difference remains at the same level as observed in
the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network
structure because of the contact points between the wires the number of contact points
was completely different in the two kinds of wires In addition a contact resistance
always existed between the wires The higher the number of contact points the higher is
the resistance With the typical AgNWs the electrodes did not show high conductivity
because the network included a large number of contact points with excessively high
contact resistance The small diameter also caused high contact resistance because of
the small contact area In contrast in the case of the u-long wires the contact resistance
was low because of the limited number of contact points and the large contact area
derived from the longer lengths and larger diameters which resulted in high
conductivities even without heat treatment (Figure 2a) The results indicated that the
contact resistance was crucial for achieving high-performance transparent AgNW
electrodes without post-treatments like heating pressing or illumination by a flash
lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics
which require low-temperature production with just air drying
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
14
The relationship between the sheet resistance and transparency after heating are shown
in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of
u-long AgNWs was always lower than the electrodes consisting of typical wires
especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω
for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94
the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while
in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112
Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased
to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with
typical AgNWs were not conductive at over 96 transparency because of the lack of
junctions only the u-long AgNW electrodes showed a conductive network and
exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω
at a high optical transmittance of 97 was comparable or even better than monolayer
graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with
the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of
94ndash98 which was identical haze value that shown by ITO films The haze of the
AgNW films will be discussed further subsequently
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
15
It is known that the haze of the AgNW films closely depends on the light scattering in
wires caused by the large diameters as mentioned before [25 26] The differences in
the diameters between the u-long AgNWs and typical AgNWs were clearly observed in
the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass
substrates at different transparencies All the spectra showed a wide flat region from 450
nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition
to a weak dip at 700 nm The wide flat region is advantageous for display applications
to see clearly under visible light and in solar cells as well However the strong dips at
around 355 nm and 400 nm were detected with the excitation of the localized surface
plasmon resonance due to oscillations of free electrons in a direction transverse to the
individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was
red-shifted especially around 400 nm and was broad when compared to the spectrum
of typical AgNWs The red-shift and peak broadening supported the increase in the
diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was
most likely caused by the excitation of the propagating surface plasmon polaritons
along the longitudinal direction of the AgNWs [31] According to Groep et al the dip
should have been blue-shifted and intensified with increase in the diameter which is in
agreement with our results A difference in the diameter (about 20 nm from SEM)
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
16
between the typical and u-long AgNW was detected from the UV-Vis spectra
According to the simulation results if the diameter of the wires is small enough low
haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)
shows the relationship between the haze and transparency of the AgNW electrodes The
haze of the electrodes was identical before and after heating The two kinds of AgNW
electrodes showed the same trend in terms of the haze which always linearly decreased
with increase in transparency The haze shown by the typical wires was always lower
than that shown by the u-long wires when the transparency was below 95 because the
diameter of the typical wires was only 68 nm while the diameter of the u-long wires
was larger (91 nm) The increase in the probability of light scattering owing to an
increase in diameter can be expected to be more exaggerated resulting in an increase in
haze [26] which corresponds to the slightly high haze shown by the u-long wires below
95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs
electrodes gradually evened up to the level (below 3) shown by the typical AgNWs
and was close to the values shown by traditional ITO films The result seems to imply
that the light scattering drastically decreased because of the excessively small number of
wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
17
AgNWs electrodes still maintained a network structure to achieve high conductivities
with the high Tp (Figure 3)
It is worth noting that AgNW electrodes that exhibit high performance at low haze
values have been achieved with the u-long wires In order to examine the relationship
between the haze and the length the haze shown by broken u-long AgNWs is shown in
Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs
but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and
broken showed almost the same trend however the slope observed for the typical
AgNWs was smaller than the other two The nearly identical slope shown by the u-long
and broken u-long wires suggests that the haze is independent of the length Meanwhile
these coefficients in the large diameter wires are always larger than that of the typical
wires with smaller diameter which indicated that the dependence of the scattering effect
on the diameter is a key aspect Importantly comparing the haze of AgNWs with same
diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it
was found that the haze was very slightly lower in the case of the longer wires because
the less number of contact points between wires may be less exaggerate scattering
Similarly when the Tp was over 95 the relationship between haze and Tp in the
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
18
broken u-long wires followed a nearly similar trend like other two type wires However
a conductive electrode consisted by the broken u-long AgNWs was not achieved with
Tp over 95 due to less contacts between short wires Hence enough length in these
AgNWs is also an important factor as well as the diameter for low haze Therefore the
low haze of AgNWs electrodes can be achieved with small diameter or with sufficient
length ie u-long wires in the present work
Figure 5 (c) shows the model of the random network in a grid square in which five
wires were placed in a square to make eight contact points The area coverage of these
wires Ac was analyzed with a model which involved wires of the same diameter and
length The model was determined by observation of a network of real AgNWs network
in which the ratio of the number of contact points to the number of wires (designated as
M) is assumed to be 16 as nearly limit of making conductive networks The value of M
was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was
17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes
showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased
with decrease in the M value in the two even transparent electrodes because of the small
number of wires After determining the model the value of Ac can be analyzed by
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
19
counting the black colored parts which is fitted by an expanded scale over a square grid
of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac
was used to recalculate Tp using the following equation [30]
AcC 100Tp (1)
In the above equation C is a coefficient to determine the slop for liner relationship
between Tp and Ac Ac is also related to the wire length L wire diameter D and number
of wires per unit area N by the following equation [30]
DLNAc (2)
The value of C in eq (1) was determined to be 103 and 125 for typical and u-long
AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)
and the data of N L and D which have already been shown in Fig 1 and 2 Combining
eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and
length was obtained and is shown in Fig 5 (d) Several kinds of structures were made
with the minimum number of wires to obtain the value of Ac A small difference below
1 for Ac was observed in each various structure In fact the difference was not larger
than the wobbly borders at the edge of wires in the bitmap image which have been
shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm
and 91 nm were assumed for the typical and u-long AgNWs respectively In order to
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
20
decrease the haze below 4 the length of the grid ie the wire length was dramatically
increased The calculation implied that the long wires were required to improve the haze
while maintaining a network structure For the typical AgNWs with a Tp of 96 to
achieve a haze value below 15 a wire length over 11 microm was required This is the
reason why the typical AgNWs could not achieve a network with only 11 microm in real
average length On the contrary the u-long AgNWs could accomplish the lowest haze of
09 with a Tp of 98 because the real average length of 44 microm was sufficient to make
a network when compared to the required length of 32 microm Even with the large diameter
a haze below 1 which is superior to that of ITO can be realized with sufficiently long
wires Hence transparent electrodes with a low haze can be obtained even with large
diameter AgNWs by ensuring that the wires are long enough to create a network
structure
3 Conclusions
U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of
20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the
one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
21
rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous
high haze confronted with the widely reported typical AgNWs In the Tp range
94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24
Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the
performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of
97 was comparable or even better than that of monolayer graphene sheet at 125 Ω
In general decreasing the diameter and extending the length of the wires is expected to
be the only effective strategy to fabricate highly conductive AgNW electrodes with low
haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at
a Tp of 98 and a further lowered haze of 09 that can potentially be achieved
according the calculations by using large diameter highly conductive AgNWs is
extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs
at room temperature without post-treatment by a simple drying process showed a sheet
resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude
lower than shown by electrodes of typical AgNWs The advantages of using u-long
AgNWs of high electricaloptical performance and the room temperature processing
involved for the fabrication can be used to realize soft and flexible electrodes for plastic
electronics
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
22
Acknowledgments
T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows
The authors would like to thank the members of Showa Denko K K for constructive
discussions and encouragement
Electronic Supplementary Material Supplementary material (Comparison of Tp
measured by UV-Vis spectroscopy and by haze meter) is available in the online version
of this article at http
dxdoiorg101007
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
23
References
[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K
Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent
Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276
[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A
Transparent Flexible Organic Thin-Film Transistors That Use Printed
Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511
[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of
Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843
[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced
graphene oxide as a transparent and flexible electronic material Nat Nanotechnol
2008 3 270ndash274
[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327
[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T
Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S
Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat
Nanotechnol 2010 5 574ndash578
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
24
[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic
devices Adv Mater 2007 19 2893ndash2897
[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and
conductive electrodes based on unpatterned thin metal films Appl Phys Lett
2008 93 223304
[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using
nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413
[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of
transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963
[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J
B Boland J J Coleman J N Silver Nanowire Networks as Flexible
Transparent Conducting Films Extremely High DC to Optical Conductivity
Ratios ACS Nano 2009 3 1767ndash1774
[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for
transparent electrodes MRS Bulletin 2011 36 760ndash765
[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application
in touch screens Nanotechnology 2011 22 245201
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
25
[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire
synthesis and its application in a highly transparent conductive and flexible metal
electrode touch panel Nanoscale 2012 4 6408ndash6414
[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K
Fabrication of silver nanowire transparent electrodes at room temperature Nano
Res 2011 4 1215ndash1222
[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J
Improvements in purification of silver nanowires by decantation and fabrication
of flexible transparent electrodes Application to capacitive touch sensors
Nanotechnology 2013 24 215501
[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks
for polymer light emitting diode electrodes Nanotechnology 2013 24 125202
[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar
cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34
[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric
pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
26
[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray
Deposition of Highly Transparent Silver Nanowire Electrode on Flexible
Substrate ACS Appl Mater Interfaces 2013 5 788minus794
[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent
conductive coatings on glass US6268059 B1 2001
[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic
P Carbon-nanotube film on plastic as transparent electrode for resistive touch
screens Journal of the SID 2009 17 941ndash946
[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low
temperature using a roll-to-roll microwave plasma chemical vapor deposition
Thin Solid Films 2013 532 89ndash93
[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P
Organic light-emitting diodes on solution-processed graphene transparent
electrodes ACS Nano 2010 4 43ndash48
[25] Katagiri K Hunakubo T Metal Nanowires method for producing same
transparent conductor and touch panel US 20120255762 A1 2012
[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent
and conductive silver nanowire films Nano Res 2013 in press
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
27
[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft
solution processing Nano Lett 2002 2 165ndash168
[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods
with high yield by polyol process Materials Chemistry and Physics 2009 114
333ndash338
[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization
of very long silver nanowires via successive multistep growth Cryst Growth Des
2012 12 5598minus5605
[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J
The effect of nanowire length and diameter on the properties of transparent
conducting nanowire films Nanoscale 2012 4 1996minus2004
[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire
Networks Nano Lett 2012 12 3138minus3144
[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of
silver nanowires with a nonregular cross section Phys rev B 2001 64 235402
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
28
Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm
(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs
synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to
300 nm (d) Relative frequencies of the length of typical low temperature and u-long
AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively
Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)
at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The
maximum length of u-long AgNWs observed after the reaction was 230 μm
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
29
Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images
at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs
revealed lesser number of wires leading to lesser number of contact points The scale
bars of the SEM images correspond to 5 μm
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
30
Figure 3 The relationship between the sheet resistance haze and Tp for typical and
u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance
below 5 haze and over 90 Tp
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
31
Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the
wavelength of 400 nm in u-long AgNWs because of their larger diameters
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs
32
Figure 5 (a) The relationship between haze and Tp with an approximate formula
expressed using percentages (b) Optical micrographs of short wires fabricated by
breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds
to 10 μm (c) The model for the random network of AgNWs (d) The relationship
between haze and length of the wires obtained by using the model and eq (1) The plot
demonstrates that low haze can be realized by increasing the length of AgNWs