Embed Size (px)
Citation preview
Exciton dynamics in luminescent carbon nanodots: Electron-hole exchange interaction
Bo Peng1,† (*), Xin Lu1, Evren Mutlugun3, Shi Chen1, Cheng Hon Alfred Huan1, Siu Fung Yu4, Hilmi Volkan Demir3, and Qihua Xiong1,2 (*) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0936-2
http://www.thenanoresearch.com on November. 2, 2015
© Tsinghua University Press 2015
Just Accepted
This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” 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 “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or 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 “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.
TABLE OF CONTENTS (TOC)
Nano Research DOI 10.1007/s12274-015-0936-2
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
64 Nano Res.
Exciton dynamics in luminescent carbon nanodots:
electron-hole exchange interaction
Bo Peng*, Xin Lu, Evren Mutlugun, Shi Chen, Cheng
Hon Alfred Huan, Siu Fung Yu, Hilmi Volkan Demir,
and Qihua Xiong*
1 Division of Physics and Applied Physics, School of
Physical and Mathematical Sciences, Nanyang
Technological University, Singapore 2 NOVITAS, Nanoelectronics Centre of Excellence,
School of Electrical and Electronic Engineering,
Nanyang Technological University, Singapore 3 LUMINOUS, Centre of Excellence for Semiconductor
Lighting and Displays, School of Electrical and
Electronic Engineering, Nanyang Technological
University, Singapore 4 Department of Applied Physics, the Hong Kong
Polytechnic University, Hong Hum, Hong Kong SAR,
China
The electron-hole exchange interaction is found in luminescent carbon
nanodots, which results in that the exciton states of split to the
singlet-bright and triplet-dark state with an energy splitting ΔE.
Qihua Xiong, http://www1.spms.ntu.edu.sg/~qihuagroup/
Bo Peng, http://www.me.uestc.edu.cn/teacher/teacherMain?id=247
Exciton dynamics in luminescent carbon nanodots: electron-hole exchange interaction
Bo Peng1, † (*), Xin Lu1, Evren Mutlugun3, Shi Chen1, Cheng Hon Alfred Huan1, Siu Fung Yu4, Hilmi Volkan Demir3, and Qihua Xiong1,2 (*) 1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 2 NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 3 LUMINOUS, Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 4 Department of Applied Physics, the Hong Kong Polytechnic University, Hong Hum, Hong Kong SAR, China
† Present address: National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, No.4, Block 2, North Jianshe Road, Chengdu, 610054, China
Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014
KEYWORDS Carbon dots, Photoluminescence, Pyrolysis, Electron-hole exchange interaction, Energy splitting.
ABSTRACT Electron-hole exchange interaction significantly influences the optical properties of excitons and radiative decay. However, exciton dynamics in luminescent carbon dots (Cdots) are still not clear. Herein, we have developed a simple and efficient one-step strategy for luminescent Cdots by the pyrolysis of oleylamine. The sp2 clusters of a few aromatic rings are responsible for the observed blue photoluminescence. The sp2 clusters size can be tuned by controlling the reaction time and the energy gap between π-π* states of sp2
domains decreases as the sp2 cluster size increases. More importantly, the strong electron-hole exchange interaction results in that the exciton states of sp2
clusters split to the singlet-bright and triplet-dark state with an energy splitting ΔE, which decreases as the increasing sp2 cluster size due to the decrease of confinement energy and electron-hole exchange interaction.
Nano Research DOI (automatically inserted by the publisher) Research Article
Address correspondence to Qihua Xiong, [email protected]; Bo Peng, [email protected]
1 Introduction As a new fascinating class of carbon nanomaterials, luminescent carbon dots have attracted wide attention owing to their favorable properties of good biocompatibility, low toxicity and low cost, which offer the opportunity to substitute high toxic semiconductor quantum dots (CdSe, CdTe) in the application of bio-imaging and light-emitting diodes (LEDs) [1-5]. Since the fluorescent carbon materials were purified from arc-discharge soot by an electrophoretic method [6], the past few years have witnessed considerable progress in developing physical and chemical preparation methods for carbon dots (Cdots). Graphite was judiciously cut into ultra-small luminescent carbon dots using a laser ablation approach [7], electrochemical synthesis [8] and hydrothermal cutting method [9]. The soot from burning candles or natural gas was oxidized to synthesize carbon dots [10, 11]. The pyrolysis approach was carried out using carbohydrates, critic acid and ethylenediaminetetraacetic acid as the carbon precursors [12-15]. Previous reports have demonstrated that carbon architectures show similar photoluminescence (PL) behavior, whether they are carbon nanotubes [16]. or amorphous [17] and crystalline [18] carbon particles. Until now, based on the luminescent carbon dots by cutting graphite/graphene sheets, three main explanations have been proposed to argue the origin of photoluminescence: emissive surface defects [12, 17], localized electron-hole pairs from π-π* states in sp2 clusters [19] and zigzag sites with a carbene-like triplet ground state [9, 13]. However, there is still no work on the mechanism of exciton dynamics in carbon dots. Herein we demonstrate a simple one-step synthesis of luminescent carbon dots by the pyrolysis of oleylamine. This approach can be extended to other alkylamine and silanes. In the synthetic system, oleylamine is the only chemical,
acting as a solvent, as well as a precursor and a passivation agent at the same time. We have synthesized different luminescent carbon dots by controlling the reaction time and further investigated the exciton dynamics. The FTIR, Raman HNMR and XPS data confirm that the alkylamines are oxidized into sp2 domains. Our experimental data demonstrate that the absorption and PL position of carbon dots are red-shifted due to the decreasing of energy gap between π-π* states of sp2 domains as the reaction time increases, which results in the increasing sp2 cluster size. What’s more, the exciton dynamics in carbon dots show that the electron-hole exchange interaction in sp2 domains gives rise to a large splitting between optically active (singlet-bright) and optically passive (triplet-dark) electrons (electron-hole excitations). The electron-hole exchange interaction and confinement energy decreased as the increasing sp2 cluster size.
2 Experimental sections 2.1 Synthesis of carbon dots Photoluminescent carbon dots were synthesized by the carbonization of one organic solvent. Typically, 8 mL glass vials were heated at 380 oC for 10 min, and then 50 µL oleylamine was quickly added into the vial. The reaction time was controlled at 1, 5, 10, 20, and 30 min. After cooling to room temperature, the obtained carbon dots were directly dispersed in 4 mL hexane. 0.2 ml Cdots dispersion solution were withdrawn and diluted to 2 ml by hexane for absorption and PL measurements. Oleylamine can be substituted by other organic source, such as Octadecanethiol, Dodecanethiol, Hexadecyltrimethoxysilane (HDTMSi), Hexadecylamine, Dodecylamine, 3-aminopropyltrimethoxysilane (APTMS), N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AEPTMS),
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
2.2 Characterization TEM investigations were performed on a JEOL1400 transmission electron microscope with
an accelerating voltage of 100 kV. UV/Vis absorption
c
500 600 700 800
HDTMSi
DodecanethiolOctadecanethiol
Nor
mal
ized
PL
Wavelength (nm)
Ex: 457 nm
AEPTMSAPTMS
Oleylamine
HexadecylamineDodecylamine
d
SiO
O
O
SH
SH
a
200 300 400 500 600 700 800
0.0
4.0x105
8.0x105
1.2x106
1.6x106
2.0x106
300 nm275 nm
Abs
PL
10 min
x15 x20
Oleyamine
Wavelength (nm)
457 nm 488 nm 532 nm 633 nm
230 nm
200 300 400 500 600 700 800
b
Figure 1 (a) PL spectra of carbon dots (Cdots) in hexane excited with different lasers and the corresponding absorption spectrum.
The insets are the Cdots dispersion with blue fluorescence under 254 nm UV light. (b) Typical TEM image of Cdots. (c)
Normalized PL spectra of Cdots from other Mercaptans, Alkylamines and silanes: Octadecanethiol, Dodecanethiol,
Hexadecyltrimethoxysilane (HDTMSi), Hexadecylamine, Dodecylamine, 3-Aminopropyltrimethoxysilane (APTMS),
N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AEPTMS). (d) The chemical structure of corresponding mercaptans,
alkylamines and silanes.
spectra were recorded at room temperature using a Lambda 950 spectrophotometer. Room-temperature steady-state PL was recorded by a home-built optical setup, excited with 457, 488, 532, and 633 nm lasers. FT-IR spectra were recorded by Bruker Vertex 80v at room temperature in a continuous vacuum environment with typically 256 scans per loop at a 4 cm-1 resolution in the transmission and reflection mode. Raman scattering spectroscopy was conducted using a micro-Raman spectrometer (Horiba-JY T64, 000) excited with a solid-state laser (λ = 514 nm) in the backscattering configuration. The backscattered signal was collected through a 100× objective and dispersed
by a 1,800 g/mm grating, and the laser power on the sample surface was measured to about 1.0 mW. The typical time was 30 s. The XPS was measured by using an Omicron EA125 analyzer with monochromatic Al K alpha (hυ=1486.7 eV) radiation. The room-temperature time resolved PL spectra were taken by Becker-Hickl DCS-120 confocal scanning fluorescence lifetime imaging microscopy (FLIM) system, using a laser diode operating at 375 nm for excitation. 1HNMR spectra of oleylamine and carbon dots were recorded by Bruker BioSpin GmbH NMR300. To detect the feature peak of sp2 at 7.2 ppm, the carbon dots were dispersed in CD2Cl2.
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
3 Results and discussion
3.1 UV-Vis absorption and PL properties of Cdots
Figure 1(a) shows the ultraviolet-visible (UV-vis) absorption spectrum of carbon dots synthesized by pyrolysis of oleylamine with the reaction time of 10 min. The typical feature at ~230 nm is attributed to the π-π* transition of aromatic sp2 domains and the two shoulder absorption peaks at ~275 and ~300 nm are assigned to n-π* transition [20]. The insets in Fig. 1(a) show the Cdots dispersion with blue fluorescence under 254 nm UV light illumination. Figure 1(b) represents the TEM image of Cdots. The results reveal that the Cdots are mostly irregularly spherical dots with diameters in the range of 4-13 nm. The PL properties show excitation-dependent behavior. The PL emission shift to longer wavelengths and its intensity decreases as the increase of the excitation wavelength, which agrees with previous report [1, 9, 17, 21]. The emission band maximum is 554, 560, 580, and 656 nm in the case that carbon dots are excited under 457, 488, 532, and 633 nm lasers, respectively. The carbon dots were dispersed in hexane and the laser power on the sample surface was measured to be about 80 mW. However, the PL intensity decreases drastically when the sample was excited with 532 and 633 nm lasers. Until now, several mechanisms have been proposed to explain the excitation-dependent behavior, such as several different fluorophores within the carbogenic network [17] and a distribution of different emissive sites and size [10, 22]. However, it is debatable on the essence of excitation-dependent behaviour. Using our approach, other Mercaptans, Alkylamines and silanes, such as: Octadecanethiol, Dodecanethiol, Hexadecyltrimethoxysilane (HDTMSi), hexadecylamine, dodecylamine, 3-aminopropyltrimethoxysilane (APTMS), N-(2-Aminoethyl)-3-aminopropyltrimethoxysilan
e (AEPTMS), can also be used as source to synthesize carbon dots. Figure 1(c) shows the corresponding PL spectra of carbon dots using those different sources under excitation with a 457 nm laser. Figure 1(d) shows the chemical structure of those
b
0 1 2 3 4 5 6 7 8 9
0
5000
10000
15000
20000
25000
30000
35000
OA
ppm
CD2Cl
2
CH3 -(CH2)6 -CH2 -C=C-CH2 -(CH2)5 -CH2 -CH2 -NH2| |H H
G E C A A C E D B F
FG
E
D
C
B
A
3500 3000 2500 2000 1500 1000 500
dNH
uC-CH
3u
uC=O
1600
dNH
uNH u
C-N
w=CH
uC=C
u=CH
rCH
ds
CH3das
CH
us
CH
1710
1621
3375
3005
29242853
1571
1467
1378
967
1073
1306
796
Oleylamine
Wavenumber (cm-1)
Carbon dots
722
uas
CH
a
c
0 1 2 3 4 5 6 7 8 9
0
20000
40000
60000
80000
100000
120000
Cdots
ppm
CD2Cl
2
6 7 8 9
0
200
400
OA
Cdots
sp2
Figure 2 (a-d) FTIR spectra of oleylamine and Cdots, (b-c)
The 1HNMR spectra of oleylamine and Cdots. The insets in
(c) show that sp2 clusters were formed in Cdots.
corresponding alkylamines and silanes.
3.2 Mechanism of the pyrolysis of oleylamine to Cdots
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
The basic procedures to synthesize carbon dots include the decomposition and pyrolysis of oleylamine in air. Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance spectroscopy (1H NMR) have been used to confirm the reaction process. To eliminate the FTIR signal interference due to the H2O in air, the carbon dots were drop-casted on 50 nm Au films to form thin films, and then measured in a continuous vacuum environment through the reflection mode. Oleylamine liquid films were checked in vacuum environment by the transmission mode. Figure 2(a) shows the FTIR spectra of oleylamine and carbon dots. The bands at 2924 and 2853 cm-1 can be ascribed to the symmetric and asymmetric stretching vibrations of the C–H bonds from CH2 and CH3 [23-26]. The peaks at 1467 and 1378 cm-1 can be assigned to the asymmetric in-plane and symmetric rocking mode of C-H [25]. The peaks at 1306, 1073 cm-1 for oleylamine are assigned to the stretching vibrations of CH2-CH3 and C-N. However, direct FTIR analysis of Cdots does not offer detectable FTIR signals, indicating that the CH2-CH3 and C-NH2 were carbonized. The peak of oleylamine at 722 cm-1 corresponds to the in-plane rocking vibration of the CH2 bond from (CH2)n chains, where n>4 [24]. However, the fractions of peak at 722 cm-1 drastically decrease in carbon dots, which indicate that the long chains of oleylamine are broken during carbonization. After carbonization, the stretching vibrations and in-plane rocking mode of N-H at 3375, 1571 and 796 cm-1, and the C-N stretch peak of oleylamine at 1073 cm-1 disappear [27, 28], which indicate that the -C-NH2 groups of oleylamine are oxidized. It is important to note that a new band appears at 1710 and 1600 cm-1, which are assigned to the stretching vibrations of C=O and ring stretching vibrations of phenyl group, respectively. The peaks of the oleylamine at 3005, 1621 and 967 cm-1 can be assigned to the
stretching vibrations of =C-H and -C=C-, and out-plane rocking mode of =C-H [25]. However, the peaks at 3005 and 1621 cm-1 disappear, which indicates that the CH=CH groups were broken, and the left small peak at ~967 cm-1 is attributed to the C-O groups in carbon dots. To
288 287 286 285 284 283 282
sp3
C-O/C-HInte
nsity
(a.
u.)
sp2 C=CC (1s)
Bonding Energy (eV)288 287 286 285 284 283 282
b
1000 1200 1400 1600 1800 2000
514 nm excitation
Graphite
Cdots
1586
1347
D
Raman shifts (cm-1)
G
a
536 534 532 530 528
Inte
nsity
(a.
u.)
O-H
O=C
O (1s)
Bonding Energy (eV)536 534 532 530 528
c
Figure 3 (a) Raman spectra of Cdots and graphite excited
with a 514 nm laser, (b, c) XPS spectra of Cdots. The solid
lines are fit to the data.
further confirm the formation of sp2 clusters,
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
1HNMR spectra of oleylamine and carbon dots were checked in CD2Cl2 solvent, shown in Fig. 2(b) and 2(c). It is important to note that the 1HNMR feature peak of phenyl group at ~7.2 ppm was detected in Cdots. However, that feature was not found in oleylamine. Therefore, O2 from air participated in the pyrolysis of oleylamine and sp2 clusters or fragments were formed in the carbon dots. Oleylamines are oxidized into carbon compounds consisting of sp2 clusters (phenyl group), C=O, C-O, -CH2- and -CH3 species.
The Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) are further used to investigate the composition of carbon dots. Figure 3(a) shows the Raman spectra of carbon dots excited with a 514 nm laser. Two fingerprint features of carbon, the D band and G band, are clearly shown at 1347 and 1586 cm-1, respectively [29]. The D band is related to the presence of sp3 defects and the G band corresponds to the in-plane stretching vibration of sp2 carbon atoms, which is a doubly degenerate (TO and LO) phonon mode (E2g symmetry) at the Brillouin zone center [30, 31]. The increasing phenyl group units in sp2 domain result in a blue-shift of G band [32]. The XPS data of C1s from carbon dots are shown in Fig. 3(b). The peaks of at 284.5 and 285.4 eV are assigned to sp2 carbon (C=C) and sp3 carbon (C-O/C-H), respectively, in agreement with FTIR, 1HNMR, and Raman results [30, 33, 34]. As shown in the O1s XPS data, the peak at 530.5 eV contributions from C=O groups, while the peak at 531.4 eV is assigned to C-O and O-C-O groups [35-37], which further verify the FTIR results. It is important to note that the possible contribution from water molecules can be excluded based upon four arguments: (1) the carbon dots are hydrophobic passivated with long change alkyl groups, (2) the sample was extracted by centrifugation and washing in THF, (3) the sample was heated at 100°C and pumped before XPS was taken, (4) FTIR data measured in
vacuum don’t show any feature peak of water molecules. Therefore, the Raman and XPS data have proven that the carbon dots contain a mixture of sp2 and a few conjugated repeating units dispersed in an insulating sp3 matrix, where a fraction of carbons are bonded with oxygen (C=O, O=C–OH groups), which is consistent with the UV-Vis, 1HNMR and FTIR results. 3.3 Aromatic sp2 domains for the observed blue PL To understand the PL mechanism, we have synthesized the carbon dots at different reaction times and further investigated the time dependence of absorption, PL and lifetime. As shown in Fig. 4(a) , the UV absorption peak of π-π* transition of aromatic sp2 domains is red-shifted from 217 to 230 nm as the reaction time increases from 1 to 10 min, and further slightly shifted to 232 nm at 30 min. Li et al. calculated the dependence of HOMO–LUMO gap on the size of the graphene fragments and suggested that the gap decreases gradually as the size of the fragment increases [8]. Eda et al. have calculated the energy gap between the π-π* states, which decreases as the increasing size of sp2 clusters or conjugation length [19]. Therefore, we propose that small sp2 clusters or fragments of few aromatic rings are responsible for the observed blue PL of our carbon dots synthesized by the pyrolysis process. The red-shift of π-π* transition absorption peak is attributed to the increased size of sp2 clusters. The broad peak of π-π* transition absorption indicates that the size distribution of the sp2 cluster is inhomogeneous. The most obvious absorption peak line-shape at 5 mins is probably due to the conjunction of sp2 domains and C=O and -CH3 groups, consistent with previous works [38]. The absorption peaks of n-π* transition at ~275 and 300 nm are attributed to the electronic transition of C-OH and C=O groups, respectively. The FTIR and XPS data have confirmed that the two groups of
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
C-OH and C=O are formed in the carbon dots. The electronic orbital energy of n state of the C-OH group is lower than that of the C=O group [39]. Therefore, the energy gap between n-π*
transition of the C-OH group is larger than the C=O group, resulting in a blue shift of the absorption peak.
b
500 600 700 800
0.00.20.4
0.60.81.01.2
1.41.61.8
30 min20 min10 min5 min
1 min0.0
0.20.4
0.60.81.01.2
1.41.61.8
Nor
mal
ized
PL
Wavelength (nm)
1 min 5 min 10 min 20 min 30 min
200 300 400 5000.0
0.2
0.4
0.6
0.8
1.0
Abs
Wavelength (nm)
Oleylamine 1 min 5 min 10 min 20 min 30 min
x 2.2
a
Figure 4 (a) UV-Vis absorption spectra of oleylamine and Cdots at different reaction times varied from 1 to 30 min. To clearly
show the red-shift of absorption peak, the absorption intensity of carbon dots at 30 min is multiplied by 2.2. The inset shows the
original absorption intensity data. (b) Normalized PL spectra of Cdots excited with a 457 nm laser. The solid lines are the fitting
curves to the data using Gaussian peak function. The inset is the non-normalized PL spectra of Cdots. Theoretically, the n-π* energy gap is independent of the size of sp2 clusters. Therefore, the absorption peaks of n-π* transition for our carbon dots are not shifted as the reaction time increases from 1 to 30 min.
Figure 4(b) shows the normalized PL spectra of carbon dots synthesized at different reaction times varied from 1 to 30 min excited with a 457 nm laser. It is clearly observed that the PL position is red-shifted from 544 to 553 nm in the case that the reaction time increases from 1 to 30 min, which indicates that the energy gap between the π-π* states decreases, consistent with the absorption data. Therefore, the size of sp2 clusters increases as the reaction time increases. All luminescent carbon dots have a quantum efficiency of ~3%. It is important to note that the PL and abs measurement conditions for all Cdots are nearly the same, which ensures that the comparisons of PL and abs absolute intensity are convincing. As shown from the PL spectra in absolute scale (the inset in Fig. 4(b)), it is observed that the PL intensity monotonously increases to a maximum at 10 min and then
decreases with the increasing reaction time from 10 to 30 min. From the absorption data in the inset in Fig. 4(a), the absorption intensity of the π-π* transition first increases and is maximized at 20 min, and then drastically decreases at 30 min, which indicates that the concentration of luminescent sp2 clusters in the carbon dots achieves a maximum at ~20 min. Therefore, the observed increase in the PL intensity is attributed to the increased concentration of the strongly localized sp2 sites. The concentration of sp2 sites at 20 min is larger than that at 10 min, however, the PL intensity at 20 min is smaller than 10 min. Therefore, the subsequent PL intensity decrease at 20 min is due to the percolation among the sp2 configurations, which facilitates the transport of excitons to nonradiative recombination sites, and thus leading to a PL quenching [19]. With a longer reaction time, excessive carbonization occurs, which results in the drastically decreased concentration of luminescent carbon dots. Therefore, the absorption and PL intensity drastically decrease. However, it is important to note that both of the absorption and PL data
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
Nano Res.
indeed show that the size of sp2 clusters increases as the increasing reaction time. The absorption and PL intensity depend on the concentration of
the formed sp2 clusters. However, high concentration facilitates the percolation
b
Ground state
Singlet-bright state
Ene
rgy
Electronic Transitions
Triplet-dark state
|G>
|J=2>
|J=1>ΔE
Γb Γd
0 20 40 60 80 100 120 140 160 180 2000
1
2
3
4
5
6
7
30 min
20 min
10 min
5 min
1 min
Life
time
(ns)
Energy splitting (meV)
Theoretical Lifetime
0 20 40 60 80 100 120 140 160 180 2000
1
2
3
4
5
6
7
Experimental Data
c
0 20 40 60 80 100 120 140 160 180 200
0.6
0.8
1.0
1.2
1.4S
ize
of s
p2 clu
ster
(nm
)
Energy splitting (meV)
Theoretical Size
0 20 40 60 80 100 120 140 160 180 200
0.6
0.8
1.0
1.2
1.4 Experimental Data
30 min
20 min
10 min
5 min
1 min
d
a
0 2 4 6 8 10
10
100
1000
Log 10
(inte
nsity
)
Decay Lifetime (ns)
1 min 5 min 10 min 20 min 30 min
Fitting curves
Figure 5 (a) Time-resolved PL spectra of Cdots synthesized at different reaction times. The lifetime decreases as the reaction time
increases. The solid lines are the fitting curves to the data using a biexponential function. (b) Schematic diagram of a three-level
model with the singlet-bright, triplet-dark, and ground state. (c) Lifetime as a function of the energy splitting between the
singlet-bright and triplet-dark state (ΔE). The red solid circles are the experimental long-living lifetime extracted from (a) and the
black solid line is fit to the data using Eq. 2 as described in the text. (d) Size of sp2 clusters (d) vs energy splitting (ΔE). The red
solid circles are the energy splitting extracted from (c) and the black solid curve is fit to the data using Eq. (3).
Nano Res
1
between sp2 clusters, leading to nonradiative energy transfer and thus PL quenching, which is similar to occurrence in CdSe/ZnS quantum dot films [40]. 3.4 Electron-hole exchange interaction The time-resolved PL spectra of luminescent carbon dot drop-casted films are recorded by a confocal scanning fluorescence lifetime imaging microscopy system and shown in Fig. 5(a) [41, 42]. The time-resolved PL decay curve can be well fitted as a bi-exponential function, I(t) = A1exp(-t/τ1) + A2exp(-t/τ2), with two time constants: a fast decay (τ1) accompanied by a long-living emission (τ2), where I(t) is the PL intensity [43, 44]. Because of the inhomogeneous distribution of sp2 cluster lifetimes, the emission kinetics shows a multiexponential behavior distributed on a large time scale [45]. The lifetime of long-lived decay component is assumed to correspond to lifetime of film samples [46]. The lifetime is found to be dependent on the reaction time. As the reaction time increases, the lifetime of carbon dots decreases. The fluorescence lifetimes τ1 are 1.2±0.06, 0.7±0.01, 0.5±0.01, 0.4±0.01, and 0.3±0.01 ns and τ2 are 6.5±0.01, 3.9±0.02, 2.5±0.02, 1.9±0.02, and 1.2±0.02 ns in the case that the reaction time is 1, 5, 10, 20, and 30 min, respectively. The absorption and PL data of carbon dots show that the size of sp2 clusters increase as the increasing reaction time. Therefore, we propose that the electron-hole exchange interaction occurs in the luminescent carbon dots, leading to the splitting of excitonic states and thus the size dependence of lifetime, consistent with previous reports [46, 47]. Donegá et al. [48] and Brongersma et al. [49] have investigated the size-dependence of the band-edge PL decay of CdSe quantum dots with diameters ranging from 1.7 to 6.3 nm and Si nanocrystals with diameters in the region of 2 to 5.5 nm. Both of Donegá and Brongersma’s works are on the basis of a three-level system consisting of a zero exciton ground level, excitonic singlet and triplet states and they have demonstrated that lifetime decreases with increasing quantum dot size due to the decreased electron-hole exchange interaction, which results in energy gap between the excitonic singlet and triplet states decreases. For the electronic states of graphene structure, 16 exciton states are proposed due to the spin and degeneracy of band structures at the K and K′ valleys in the momentum space. However, the optical transition from only one exciton state to the ground state is dipole-allowed, defined as the singlet-bright exciton state. The other 15 exciton states are optically forbidden, defined as the triplet-dark exciton state [47]. Therefore, the exciton states of sp2 clusters can be classified by singlet and triplet state of the electron-hole spin configuration. Figure 5(b) shows a schematic of a three-level model with singlet-bright (J=1) and triplet-dark (J=2) excitonic states and a zero exciton ground state (G). The dark state lies below the bright state with an energy splitting ΔE. Γb and Γd are the radiative rate from the singlet-bright (J=1) and the triplet-dark (J=2) state to the ground state (G), respectively. The |J=1> →|J=2> and |J=2> →|J=1> rate are, respectively, γb = γ0(NB +1) and γd = γ0NB, where NB = [exp(ΔE/kBT)-1]-1 is the Bose-Einstein phonon number at the temperature T. γ0 is the relaxation rate of |J=1> →|J=2> transition at zero temperature.[45] And γb nb = γd nd, where nb and nd are the number of excitons at the singlet-bright and triplet-dark states respectively. Therefore, we can obtain nb exp(ΔE/kBT) = nd. Defining N = nb + nd as the total number of excitons, the decay rate (Γ) of excitons is given by [46, 50]
- = G + Gb b d d
dNn n
dt (1)
Nano Res
2
/
/
11t
D
D
G + GG = =
+
B
B
E k Tb d
E k T
ee
(2)
where τ is the lifetime and kB is Boltzmann constant, 8.617×10−5 eV/K. The temperature T is 298 K in our experiments. Using equation (2), the experimental long-living lifetime data are fitted, shown in Fig. 5(c). As the increases of lifetime, ΔE increases. The calculated energy splitting ΔE are 179, 86.6, 63.3, 51.7, and 33.4 meV respectively in the case that the reaction times are 1, 5, 10, 20, and 30 min. The previous reports have shown that the energy splitting ΔE and the exciton lifetime increase with the decreasing size of luminescent nanoparticles [45, 47, 49]. Brovelli et al. have investigated the strong electron-hole exchange interaction in CdSe/CdS core-shell semiconductor nanocrystals and demonstrated the relationship between energy bright-dark splitting and the nanocrystal size can be fitted by the equation of 70/d12 meV, where d1 is the diameter of nanocrystals (nm) [46]. Matsunaga et al. have demonstrated the tube-diameter dependence of the energy splitting for carbon nanotubes, ΔE = 70/d22 (meV), where d2 is the diameter of carbon nanotubes (nm) [51]. Therefore, the size of sp2 clusters, d, is defined as
70 /d E= D (3)
where the units of d and ΔE are nm and meV, respectively. Figure 5(d) shows the fitting curve. And the size of sp2 clusters increases with the decrease of energy splitting. The sp2 cluster size is 0.6, 0.9, 1.1, 1.2, and 1.4 nm in the case that the reaction time is 1, 5, 10, 20, and 30 min, respectively, which is in good agreement with the absorption and PL data. The decreasing of sp2 cluster size gives rise to an increasing confinement energy and stronger electron-hole exchange interaction [48]. Therefore, the energy splitting ΔE increases, resulting in the increasing lifetime. 4 Conclusions In summary, luminescent carbon dots have been synthesized by a simple one-step pyrolysis of oleylamine, which is the only chemical, acting simultaneously as a solvent, as well as a precursor and passivation agent in the synthetic system. FTIR data show that oleylamines are oxidized into carbon compounds including (C=C) n, C-O, O-H, and C-H species. The 1HNMR, Raman and XPS data have proven that the sp2 clusters are formed and dispersed in an insulating sp3 matrix. The absorption peaks of π-π* transition and PL peaks of carbon dots are redshifted as the reaction time increases. Therefore, sp2 clusters are responsible for the observed blue PL, and the sp2 cluster size increases with the increasing reaction time. The exciton states of sp2 clusters are classified by the singlet-bright and triplet-dark states. The bright-dark energy splitting decreases as the sp2 cluster size increases, as a result of the decreasing confinement energy and weaker electron-hole exchange interaction, which results in that the decay lifetime of carbon dots decreases. Acknowledgements Q.X. gratefully thanks the strong support from Singapore National Research Foundation through a Competitive Research Program (NRF-CRP-6-2010-2), and Singapore Ministry of Education via two Tier2 grants (MOE2011-T2-2-051 and MOE2011-T2-2-085) and a Tier1 grant (2013-T1-002-232). References:
Nano Res
3
[1] Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem. Int. Edit.
2010, 49, 6726-6744.
[2] Li, H. T.;Kang, Z. H.;Liu, Y.; Lee, S. T. Carbon nanodots: synthesis, properties and applications. J. Mater.
Chem. 2012, 22, 24230-24253.
[3] Pan, J.;Utama, M. I. B.;Zhang, Q.;Liu, X. F.;Peng, B.;Wong, L. M.;Sum, T. C.;Wang, S. J.; Xiong, Q. H.
Composition-Tunable Vertically Aligned CdSxSe1-x Nanowire Arrays via van der Waals Epitaxy:
Investigation of Optical Properties and Photocatalytic Behavior. Adv. Mater. 2012, 24, 4151-4156.
[4] Loh, K. P.;Bao, Q. L.;Eda, G.; Chhowalla, M. Graphene oxide as a chemically tunable platform for optical
applications. Nat. Chem. 2010, 2, 1015-1024.
[5] Wang, J.;Wang, C. F.; Chen, S. Amphiphilic Egg-Derived Carbon Dots: Rapid Plasma Fabrication,
Pyrolysis Process, and Multicolor Printing Patterns. Angew. Chem. Int. Edit. 2012, 51, 9297-9301.
[6] Xu, X. Y.;Ray, R.;Gu, Y. L.;Ploehn, H. J.;Gearheart, L.;Raker, K.; Scrivens, W. A. Electrophoretic analysis
and purification of fluorescent single-walled carbon nanotube fragments. J. Am. Chem. Soc. 2004, 126,
12736-12737.
[7] Zhang, W. F.;Zhu, H.;Yu, S. F.; Yang, H. Y. Observation of Lasing Emission from Carbon Nanodots in
Organic Solvents. Adv. Mater. 2012, 24, 2263-2267.
[8] Li, H.;He, X.;Kang, Z.;Huang, H.;Liu, Y.;Liu, J.;Lian, S.;Tsang, C. H. A.;Yang, X.; Lee, S.-T.
Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem. Int. Edit. 2010,
49, 4430-4434.
[9] Pan, D. Y.;Zhang, J. C.;Li, Z.; Wu, M. H. Hydrothermal Route for Cutting Graphene Sheets into
Blue-Luminescent Graphene Quantum Dots. Adv. Mater. 2010, 22, 734-738.
[10] Liu, H. P.;Ye, T.; Mao, C. D. Fluorescent carbon nanoparticles derived from candle soot. Angew. Chem. Int.
Edit. 2007, 46, 6473-6475.
[11] Tian, L.;Ghosh, D.;Chen, W.;Pradhan, S.;Chang, X.; Chen, S. Nanosized Carbon Particles From Natural Gas
Soot. Chem. Mat. 2009, 21, 2803-2809.
[12] Wang, X.;Qu, K.;Xu, B.;Ren, J.; Qu, X. Microwave assisted one-step green synthesis of cell-permeable
multicolor photoluminescent carbon dots without surface passivation reagents. J. Mater. Chem. 2011, 21,
2445-2450.
[13] Pan, D.;Zhang, J.;Li, Z.;Wu, C.;Yan, X.; Wu, M. Observation of pH-, solvent-, spin-, and
excitation-dependent blue photoluminescence from carbon nanoparticles. Chem. Commun. 2010, 46,
3681-3683.
[14] Xie, Z.;Wang, F.; Liu, C. Y. Organic-Inorganic Hybrid Functional Carbon Dot Gel Glasses. Adv. Mater.
2012, 24, 1716-1721.
[15] Wang, F.;Pang, S. P.;Wang, L.;Li, Q.;Kreiter, M.; Liu, C. Y. One-Step Synthesis of Highly Luminescent
Carbon Dots in Noncoordinating Solvents. Chem. Mat. 2010, 22, 4528-4530.
[16] Ju, S.-Y.;Kopcha, W. P.; Papadimitrakopoulos, F. Brightly Fluorescent Single-Walled Carbon Nanotubes
via an Oxygen-Excluding Surfactant Organization. Science 2009, 323, 1319-1323.
[17] Bourlinos, A. B.;Stassinopoulos, A.;Anglos, D.;Zboril, R.;Karakassides, M.; Giannelis, E. P. Surface
functionalized carbogenic quantum dots. Small 2008, 4, 455-458.
[18] Zhou, J.;Booker, C.;Li, R.;Zhou, X.;Sham, T.-K.;Sun, X.; Ding, Z. An Electrochemical Avenue to Blue
Luminescent Nanocrystals from Multiwalled Carbon Nanotubes (MWCNTs). J. Am. Chem. Soc. 2007, 129,
744-745.
Nano Res
4
[19] Eda, G.;Lin, Y. Y.;Mattevi, C.;Yamaguchi, H.;Chen, H. A.;Chen, I. S.;Chen, C. W.; Chhowalla, M. Blue
Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505-509.
[20] Luo, Z.;Lu, Y.;Somers, L. A.; Johnson, A. T. C. High Yield Preparation of Macroscopic Graphene Oxide
Membranes. J. Am. Chem. Soc. 2009, 131, 898-899.
[21] Wang, F.;Xie, Z.;Zhang, H.;Liu, C. Y.; Zhang, Y. G. Highly Luminescent Organosilane-Functionalized
Carbon Dots. Adv. Funct. Mater. 2011, 21, 1027-1031.
[22] Sun, Y. P.;Zhou, B.;Lin, Y.;Wang, W.;Fernando, K. A. S.;Pathak, P.;Meziani, M. J.;Harruff, B. A.;Wang,
X.;Wang, H. F.;Luo, P. J. G.;Yang, H.;Kose, M. E.;Chen, B. L.;Veca, L. M.; Xie, S. Y. Quantum-sized
carbon dots for bright and colorful photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756-7757.
[23] Wu, H.; Chen, W. Synthesis and reaction temperature-tailored self-assembly of copper sulfide nanoplates.
Nanoscale 2011, 3, 5096-5102.
[24] Cattley, C. A.;Stavrinadis, A.;Beal, R.;Moghal, J.;Cook, A. G.;Grant, P. S.;Smith, J. M.;Assender, H.; Watt,
A. A. R. Colloidal synthesis of lead oxide nanocrystals for photovoltaics. Chem. Comm. 2010, 46,
2802-2804.
[25] Hou, X.;Zhang, X.;Yang, W.;Liu, Y.; Zhai, X. Synthesis of SERS active Ag2S nanocrystals using
oleylamine as solvent, reducing agent and stabilizer. Mater. Res. Bull. 2012, 47, 2579-2583.
[26] Hu, S.-L.;Niu, K.-Y.;Sun, J.;Yang, J.;Zhao, N.-Q.; Du, X.-W. One-step synthesis of fluorescent carbon
nanoparticles by laser irradiation. J. Mater. Chem. 2009, 19, 484-488.
[27] Kisner, A. Ultrathin Gold Nanowires: Chemistry, Electrical Characterization and Application to Sense
Cellular Biology; Forschungszentrum Jülich GmbH Zentralbibliothek, ISBN: 978-3-89336-824-2, 2012.
[28] Gericke, A.; Huhnerfuss, H. Infrared Spectroscopic Comparison of Enantiomeric and Racemic
N-Octadecanoylserine Methyl Ester Monolayers at the Air/Water Interface. Langmuir 1994, 10, 3782-3786.
[29] Wang, Y.;Alsmeyer, D. C.; Mccreery, R. L. Raman-Spectroscopy of Carbon Materials - Structural Basis of
Observed Spectra. Chem. Mater. 1990, 2, 557-563.
[30] Lu, J.;Yang, J.;Wang, J.;Lim, A.;Wang, S.; Loh, K. P. One-Pot Synthesis of Fluorescent Carbon
Nanoribbons, Nanoparticles, and Graphene by the Exfoliation of Graphite in Ionic Liquids. ACS Nano 2009,
3, 2367-2375.
[31] Ray, S. C.;Saha, A.;Jana, N. R.; Sarkar, R. Fluorescent Carbon Nanoparticles: Synthesis, Characterization,
and Bioimaging Application. J. Phys. Chem. C. 2009, 113, 18546-18551.
[32] Tommasini, M.;Castiglioni, C.; Zerbi, G. Raman scattering of molecular graphenes. Phys Chem Chem Phys
2009, 11, 10185-10194.
[33] Haerle, R.;Riedo, E.;Pasquarello, A.; Baldereschi, A. sp2/sp3 hybridization ratio in amorphous carbon from
C 1s core-level shifts: X-ray photoelectron spectroscopy and first-principles calculation. Phys. Rev. B 2001,
65, 045101.
[34] Yumitori, S. Correlation of C1s chemical state intensities with the O1s intensity in the XPS analysis of
anodically oxidized glass-like carbon samples. J. Mater. Sci. 2000, 35, 139-146.
[35] Yu, X.;Tong, S.;Ge, M.;Wu, L.;Zuo, J.;Cao, C.; Song, W. Adsorption of heavy metal ions from aqueous
solution by carboxylated cellulose nanocrystals. Journal of Environmental Sciences 2013, 25, 933-943.
[36] Yang, D.;Velamakanni, A.;Bozoklu, G.;Park, S.;Stoller, M.;Piner, R. D.;Stankovich, S.;Jung, I.;Field, D.
A.;Ventrice, C. A.; Ruoff, R. S. Chemical analysis of graphene oxide films after heat and chemical
treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009, 47, 145-152.
[37] Chen, J. P.; Yang, L. Study of a Heavy Metal Biosorption onto Raw and Chemically Modified Sargassum sp.
via Spectroscopic and Modeling Analysis. Langmuir 2006, 22, 8906-8914.
Nano Res
5
[38] Sk, M. P.;Jaiswal, A.;Paul, A.;Ghosh, S. S.; Chattopadhyay, A. Presence of Amorphous Carbon
Nanoparticles in Food Caramels. Sci. Rep. 2012, 2, 383.
[39] Taylor, P. R. On the origins of the blue shift of the carbonyl n-p* transition in hydrogen-bonding solvents. J.
Am. Chem. Soc. 1982, 104, 5248-5249.
[40] Nizamoglu, S.; Demir, H. V. Excitation resolved color conversion of CdSe/ZnS core/shell quantum dot
solids for hybrid white light emitting diodes. J. Appl. Phys. 2009, 105, 083112.
[41] Peng, B.;Li, Z.;Mutlugun, E.;Hernandez Martinez, P. L.;Li, D.;Zhang, Q.;Gao, Y.;Demir, H. V.; Xiong, Q.
H. Quantum Dots on Vertically Aligned Gold Nanorod Monolayer: Plasmon Enhanced Fluorescence.
Nanoscale 2014, 6, 5592-5598.
[42] Peng, B.;Li, G.;Li, D.;Dodson, S.;Zhang, Q.;Zhang, J.;Lee, Y. H.;Demir, H. V.;Yi Ling, X.; Xiong, Q. H.
Vertically Aligned Gold Nanorod Monolayer on Arbitrary Substrates: Self-Assembly and Femtomolar
Detection of Food Contaminants. ACS Nano 2013, 7, 5993-6000.
[43] Peng, B.;Zhang, Q.;Liu, X. F.;Ji, Y.;Demir, H. V.;Huan, C. H. A.;Sum, T. C.; Xiong, Q. H.
Fluorophore-doped core-multishell spherical plasmonic nanocavities: resonant energytransfer toward a loss
compensation. ACS Nano 2012, 6, 6250-6259.
[44] Zhang, Q.;Liu, X.;Utama, M. I. B.;Zhang, J.;de la Mata, M.;Arbiol, J.;Lu, Y.;Sum, T. C.; Xiong, Q. Highly
Enhanced Exciton Recombination Rate by Strong Electron–Phonon Coupling in Single ZnTe Nanobelt.
Nano Lett 2012, 12, 6420-6427.
[45] Labeau, O.;Tamarat, P.; Lounis, B. Temperature Dependence of the Luminescence Lifetime of Single
CdSe/ZnS Quantum Dots. Phys. Rev. Lett. 2003, 90, 257404.
[46] Brovelli, S.;Schaller, R. D.;Crooker, S. A.;García-Santamaría, F.;Chen, Y.;Viswanatha, R.;Hollingsworth, J.
A.;Htoon, H.; Klimov, V. I. Nano-engineered electron–hole exchange interaction controls exciton dynamics
in core–shell semiconductor nanocrystals. Nat. Commun. 2011, 2, 280.
[47] Matsuda, K. Novel Excitonic Properties of Carbon Nanotube Studied by Advanced Optical Spectroscopy. In
Progress in Nanophotonics 2. M. Ohtsu, Ed.; Springer Berlin Heidelberg, 2013; pp 33-70.
[48] de Mello Donegá, C.;Bode, M.; Meijerink, A. Size- and temperature-dependence of exciton lifetimes in
CdSe quantum dots. Phys. Rev. B 2006, 74, 085320.
[49] Brongersma, M. L.;Kik, P. G.;Polman, A.;Min, K. S.; Atwater, H. A. Size-dependent electron-hole exchange
interaction in Si nanocrystals. Appl. Phys. Lett. 2000, 76, 351-353.
[50] Crooker, S. A.;Barrick, T.;Hollingsworth, J. A.; Klimov, V. I. Multiple temperature regimes of radiative
decay in CdSe nanocrystal quantum dots: Intrinsic limits to the dark-exciton lifetime. Appl. Phys. Lett. 2003,
82, 2793-2795.
[51] Matsunaga, R.;Matsuda, K.; Kanemitsu, Y. Observation of Charged Excitons in Hole-Doped Carbon
Nanotubes Using Photoluminescence and Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 037404.
6
Silver Nanowires with Semiconducting Ligands for Low Temperature Transparent Conductors
Brion Bob,1 Ariella Machness,1 Tze-Bin Song,1 Huanping Zhou,1 Choong-Heui Chung,2 and Yang Yang1,*
1 Department of Materials Science and Engineering and California NanoSystems Institute,
University of California Los Angeles, Los Angeles, CA 90025 (USA)
2 Department of Materials Science and Engineering, Hanbat National University, Daejeon
305-719, Korea
Abstract
Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet resistance values at very low deposition temperatures. A commonly encountered obstacle in the formation of conductive nanowire electrodes is establishing high quality electronic contact between nanowires in order to facilitate long range current transport through the network. A new system of nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high performance transparent electrodes at dramatically reduced temperatures with minimal need for post-deposition treatments of any kind.
Keywords: Silver Nanowires, Sol-Gel, Transparent Electrodes, Nanocomposites
7
1. Introduction. Silver nanowires (AgNWs) are long, thin, and possess conductivity values on the same order of magnitude as bulk silver
(Ag) [1]. Networks of overlapping nanowires allow light to easily pass through the many gaps and spaces between nanowires, while transporting current through the metallic conduction pathways offered by the wires themselves. The high aspect ratios achievable for solution-grown AgNWs has allowed for the fabrication of transparent conductors with very promising sheet resistance and transmission values, often approaching or even surpassing the performance of vacuum-processed materials such as indium tin oxide (ITO) [2-6].
Significant electrical resistance within the metallic nanowire network is encountered only when current is required to pass between nanowires, often forcing it to pass through layers of stabilizing ligands and insulating materials that are typically used to assist with the synthesis and suspension of the nanowires [7, 8]. The resistance introduced by the insulating junctions between nanowires can be reduced through various physical and chemical means, including burning off ligands and partially melting the wires via thermal annealing [9, 10], depositing additional materials on top of the nanowire network [11-14], applying mechanical forces to enhance network morphology [15-17], or using various other post-treatments to improve the contact between adjacent wires [18-21]. Any attempt to remove insulating materials the network must be weighed against the risk of damaging the wires or blocking transmitted light, and so many such treatments must be reined in from their full effectiveness to avoid endangering the performance of the completed electrode.
We report here a process for forming inks with dramatically enhanced electrical contact between AgNWs through the use of a semiconducting ligand system consisting of tin oxide (SnO2) nanoparticles. The polyvinylpyrrolidone (PVP) ligands introduced during AgNW synthesis in order to encourage one-dimensional growth are stripped from the wire surface using ammonium ions, and are replaced with substantially more conductive SnO2, which then fills the space between wires and enhances the contact geometry in the vicinity of wire/wire junctions. The resulting transparent electrodes are highly conductive immediately upon drying, and can be effectively processed in air at virtually any temperature below 300 °C. The capacity for producing high performance transparent electrodes at room temperature may be useful in the fabrication of devices that are damaged upon significant heating or upon the application of harsh chemical or mechanical post-treatments.
2. Results and Discussion
2.1. Ink Formulation and Characterization
Dispersed AgNWs synthesized using copper chloride seeds represent a particularly challenging material system for promoting wire/wire junction formation, and often require thermal annealing at temperatures near or above 200 °C to induce long range electrical conductivity within the deposited network [22, 23]. The difficulties that these wires present regarding junction formation is potentially due to their relatively large diameters compared to nanowires synthesized using other seeding materials, which has the capacity to enhance the thermal stability of individual wires according to the Gibbs-Thomson effect. We have chosen these wires as a demonstration of pre-deposition semiconducting ligand substitution in order to best illustrate the contrast between treated and untreated wires.
Completed nanocomposite inks are formed by mixing AgNWs with SnO2 nanoparticles in the presence of a compound capable of stripping the ligands from the AgNW surface. In this work, we have found that ammonia or ammonium salts act as effective stripping agents that are able to remove the PVP layer from the AgNW surface and allow for a new stabilizing matrix to take its place. Figure 1 shows a schematic of the process, starting from the precursors used in nanowire and nanoparticle synthesis and ending with the deposition of a completed film. The SnO2 nanoparticle solution naturally contains enough ammonium ions from its own synthesis to effectively peel the insulating ligands from the AgNWs and allow the nanoparticles to replace them as a stabilizing agent. If not enough SnO2 nanoparticles are used in the mixture, then the wires will rapidly agglomerate and settle to the bottom as large clusters. Large amounts of SnO2 in the mixture gradually begin to increase the sheet resistance of the nanowire network upon deposition, but greatly enhance the uniformity, durability, and wetting properties of the resulting films. We have found that AgNW:SnO2 weight ratios ranging between 2:1 and 1:1 produce well dispersed inks that are still highly conductive when deposited as films.
The nanowires were synthesized using a polyol method that has been adapted from the recipe described by Lee et al. [22, 23] Silver nitrate dissolved in ethylene glycol via ultrasonication was used as a precursor in the presence of copper chloride and PVP to provide seeds and produce anisotropic morphologies in the reaction products. Synthetic details can be found in the experimental section. Distinct from previous recipes, we have found that repeating the synthesis two times without cooling down the reaction mixture generally produces significantly longer nanowires than a single reaction step. The lengths of nanowires produced using this method fall over a wide range from 15 to 65 microns, with diameters between 125 and 250 nm. This range of diameters is common for wires grown using copper chloride seeds, although the double reaction produces a number of wires with roughly twice their usual diameter. The morphology of the as-deposited AgNWs as determined via SEM is shown in Figure 2(a), higher magnification images are also provided in Figures 2(c) and 2(d).
8
The SnO2 nanoparticles were synthesized using a sol-gel method typical for multivalent metal oxide gelation reactions. A large excess of deionized water was added to SnCl4·5H2O dissolved in ethylene glycol along with tetramethylammonium chloride and ammonium acetate to act as surfactants. The reaction was then allowed to progress for at least one hour at near reflux conditions, after which the resulting nanoparticle dispersion can be collected, washed, and dispersed in a polar solvent of choice. The material properties of SnO2 nanoparticles formed using a similar synthesis method have been reported previously [24], although the present recipe uses excess water to ensure that the hydrolysis reaction proceeds nearly to completion.
After mixing with SnO2 nanoparticles, films deposited from AgNW/SnO2 composite inks show a largely continuous nanoparticle layer on the substrate surface with some nanowires partially buried and some sitting more or less on top of the film. Representative scanning electron microscopy (SEM) images of nanocomposite films are shown in Figure 2(b). Regardless of their position relative to the SnO2 film, all nanowires show a distinct shell on their outer surface that gives them a soft and slightly rough appearance, as is visible in the higher magnification images shown in Figure 2(e) and 2(f). The SnO2 nanoparticles do a particularly good job coating the regions near and around junctions between wires, and frequently appear in the SEM images as bulges wrapped around the wire/wire contact points.
The precise morphology of the SnO2 shell that effectively surrounded each AgNW was analyzed in more detail using transmission electron microscopy (TEM) imaging. Figures 3(a) to 3(c) show individual nanowires in the presence of different ligand systems: as-synthesized PVP in Figure 3(a), inactive SnO2 in Figure 3(b), and SnO2 activated with trace amounts of ammonium ions in Figure 3(c). The as-synthesized nanowires show sharp edges, and few surface features. In the presence of inactive SnO2, which is formed by repeatedly washing the SnO2 nanoparticles in ethanol until all traces of ammonium ions are removed, the nanowires coexist with somewhat randomly distributed nanoparticles that deposit all over the surface of the TEM grid. When AgNWs are mixed with activated SnO2, a thick and continuous SnO2 shell is formed along the nanowire surface. In when sufficiently dilute SnO2 solutions are used to form the nanocomposite ink, nearly all of the nanoparticles are consumed during shell formation and effectively no nanoparticles are left to randomly populate the rest of the image.
As the AgNWs acquire their metal oxide coatings in solution, the properties of the mixture change dramatically. Freshly synthesized AgNWs coated with residual PVP ligands slowly settle to the bottom of their vial or flask over a time period of several hours to one day, forming a dense layer at the bottom. The AgNWs with SnO2 shells do not settle to the bottom, but remain partially suspended even after many weeks at concentrations that are dependent on the amount of SnO2 present in the solution.
A comparison of the settling behavior of various AgNW and SnO2 mixtures after 24 hours is shown in Figures 3(d) and 3(e). The ratios 8:4, 8:16, and 8:8 indicate the concentrations of AgNWs and SnO2 (in mg/mL) present in each solution. The 8:8 uncoupled solution, in which the PVP is not removed from the AgNW surface with ammonia, produces a situation in which the nanowires and nanoparticles do not interact with one another, and instead the nanowires settle as in the isolated nanowire solution while the nanoparticles remain well-dispersed as in the solution of pure SnO2. The mixtures of nanowires and nanoparticles in which trace amounts of ammonia are present do not settle to the bottom, but instead concentrate themselves until repulsion between the semiconducting SnO2 clusters is able to prevent further settling.
Our current explanation for the settling behavior of the wire/particle mixtures is that the PVP coating on the surface of the as-synthesized wires is sufficient to prevent interaction with the nanoparticle solution. The addition of ammonia into the solution quickly strips off the PVP surface coating and allowing the nanoparticles to coordinate directly with the nanowire surface. This explanation is in agreement with the effects of ammonia has on a solution of pure AgNWs, which rapidly begin to agglomerate into clusters and sink to the bottom as soon as any significant quantity of ammonia is added to the ink.
We attribute the stripping ability of ammonia in these mixtures to the strong dative interactions that
occur via the lone pair on the nitrogen atom interacting with the partially filled d-orbitals of the Ag atoms
on the nanowire surface. These interactions are evidently strong enough to displace the existing
coordination of the five-membered rings and carbonyl groups contained in the original PVP ligands and
allow the ammonia to attach directly to the nanowire surface. Since ammonia is one of the original
surfactants used to stabilize the surface of the SnO2 nanoparticles, we consider it reasonable that ammonia
coordination on the nanowire surface would provide an appropriate environment for the nanoparticles to
adhere to the AgNWs.
9
Scanning Energy Dispersive X-ray (EDX) Spectroscopy was also conducted on nanoparticle-coated AgNWs in order to image the presence of Sn and Ag in the nanowire and shell layer. The line scan results are shown in Figure 3(f), having been normalized to better compare the widths of the two signals. The visible broadening of the Sn lineshape compared to that of Ag is indicative of a Sn layer along the outside of the wire. The increasing strength of the Sn signal toward the center of the AgNW is likely due to the enhanced interaction between the TEM’s electron beam and the dense AgNW, which then improves the signal originating from the SnO2 shell as well. It is also possible that there is some intermixing between the Ag and Sn x-ray signals, but we consider this to be less likely as the distance between their characteristic peaks should be larger than the detection system’s energy resolution.
2.2. Network Deposition and Device Applications
For the deposition of transparent conducting films, a weight ratio of 2:1 of AgNWs to SnO2 nanoparticles was chosen in order to obtain a balance between the dispersibility of the nanowires, the uniformity of coated films, and the sheet resistance of the resulting conductive networks. Nanocomposite films were deposited on glass by blade coating from an ethanolic solution using a scotch tape spacer, with deposited networks then being allowed to dry naturally in air over several minutes.
The as-dried nanocomposite films are highly conductive, and require only minimal thermal treatment to dry and harden the film. Without the use of activated SnO2 ligands, deposited nanowire networks are highly insulating, and become conductive only after annealing at above 200 °C. The sheet resistance values of representative films are shown in Figure 4(a). The capability to form transparent conductive networks in a single deposition step that remain useful over a wide range of processing temperatures provides a high degree of versatility for designing thin film device fabrication procedures.
Figure 5(a) shows the sheet resistance and transmission of a number of nanocomposite films deposited from inks containing different nanowire concentrations. The deposited films show excellent conductivity at transmission values up to 85%, and then rapidly increase in sheet resistance as the network begins to reach its connectivity limit. The optimum performance of these networks at low to moderate transmission values is a consequence of the relatively large nanowire diameters, which scatter a noticeable amount of light even when the conditions required for current percolation are just barely met. Nonetheless, the sheet resistance and transmission of the completed nanocomposite networks place them within an acceptable range for applications in a variety of optoelectronic devices. Figure 5(b) shows the wavelength dependent transmission spectra of several nanowire networks, which transmit light well out into the infrared region. The presence of high transmission values out to wavelengths well above 1300 nm, where ITO or other conductive oxide layers would typically begin to show parasitic absorption, is due to the use of semiconducting SnO2 ligands, which is complimentary to the broad spectrum transmission of the silver nanowire network itself.
Avoiding the use of highly doped nanoparticles has the potential to provide optical advantages, but can create difficulties when attempting to make electrical contact to neighboring device layers. In order to investigate their functionality in thin film devices, we have incorporated AgNW/SnO2 nanocomposite films as electrodes in amorphous silicon (a-Si) solar cells. Two contact structures were used during fabrication: one with the nanocomposite film directly in contact with the p-i-n absorber structure and one with a 10 nm Al:ZnO (AZO) layer present to assist in forming Ohmic contact with the device. The I-V characteristics of the resulting devices are shown in Figure 6(a).
The thin AZO contact layers typically show sheet resistance values greater than 2.5 kΩ/⧠, and so cannot be responsible for long range lateral current transport within the electrode structure. However, their presence is clearly beneficial in improving contact between the nanocomposite electrode and the absorber material, as the SnO2 matrix material is evidently not conductive enough to form a high quality contact with the p-type side of the a-Si stack. We hope that future modifications to the AgNW/SnO2 composite, or perhaps the use of islands of high conductivity material such as a discontinuous layer of doped nanoparticles will allow for the deposition of completed electrode stacks that provide both rapid fabrication and good performance.
Figure 6(b) contains the top view image of a completed device. The enhanced viscosity of the nanowire/sol-gel composite inks allows for films to be blade coated onto substrates with a variety of surface properties without reductions in network uniformity. In contrast with traditional back electrodes deposited in vacuum environments, the nanocomposite can be blade coated into place in a single pass under atmospheric conditions and dried within moments. We anticipate that the use of sol-gel mixtures to enhance wetting and dispersibility may prove useful in the formulation of other varieties of semiconducting and metallic inks for deposition onto a variety of substrate structures.
3. Conclusions
In summary, we have successfully exchanged the insulating ligands that normally surround as-synthesized AgNWs with shells of substantially more conductive SnO2 nanoparticles. The exchange of one set of ligands for the other is mediated by
10
the presence of ammonia during the mixing process, which appears to be necessary for the effective removal of the PVP ligands that initially cover the nanowire surface. The resulting nanowire/nanoparticle mixtures allow for the deposition of nanocomposite films that require no annealing or other post-treatments to function as high quality transparent conductors with transmission and sheet resistance values of 85% and 10 Ω/⧠, respectively. Networks formed in this manner can be deposited quickly and easily in open air, and have been demonstrated as an effective n-type electrode in a-Si solar cells when a thin interfacial layer is deposited first to ensure good electronic contact with the rest of the device. The ligand management strategy described here could potentially be useful in any number of material systems that presently suffer from highly insulating materials that reside on the surface of otherwise high performance nano and microstructures.
4. Experimental Details
Tin oxide nanoparticle synthesis. Tin chloride pentahydrate was dissolved in ethylene glycol by
stirring for several hours at a concentration of 10 grams per 80 mL to serve as a stock solution. In a typical
synthesis reaction, 10 mL of the SnCl4·5H2O stock solution is added to a 100 mL flask and stirred at room
temperature. Still at room temperature, 250 mg ammonium acetate and 500 mg ammonium acetate were
added in powder form to regulate the solution pH and to serve as coordinating agents for the growing
oxide nanoparticles. 30 ml of water was then added, and the flask was heated to 90 °C for 1 to 2 hours in
an oil bath, during which the solution took on a cloudy white color. The gelled nanoparticles were then
washed twice in ethanol in order to keep trace amounts of ammonia present in the solution. Additional
washing cycles would deactivate the SnO2, and then require the addition of ammonia to coordinate with
as-synthesized AgNWs.
Silver nanowire synthesis. Copper(ii) chloride dihydrate was first dissolved in ethylene glycol at
1 mg/ml to serve as a stock solution for nanowire seed formation. 20 ml of ethylene glycol was then added
into a 100 ml flask, along with 200 µL of copper chloride solution. the mixture was then heated to 150 °C
while stirring at 325 rpm, and .35g of PVP (MW 55,000) was added. In a small separate flask, .25 grams of
silver nitrate was dissolved in 10 ml ethylene glycol by sonicating for approximately 2 minutes, similar to
the method described here.22 The silver nitrate solution was then injected into the larger flask over
approximately 15 minutes, and the reaction was allowed to progress for 2 hours. After the reaction had
reached completion, the various steps were repeated without cooling down. 200 µL of copper chloride
solution and .35g PVP were added in a similar manner to the first reaction cycle, and another .25g silver
nitrate were dissolved via ultrasonics and injected over 15 minutes. The second reaction cycle was allowed
to progress for another 2 hours, before the flask was cooled and the reaction products were collected and
washed three times in ethanol.
Nanocomposite ink formation. After the synthesis of the two types of nanostructures is complete,
11
the double washed SnO2 nanoparticles and triple-washed nanowires can be combined at a variety of weight
ratios to form the completed nanocomposite ink. The dispersibility of the mixture is improved when more
SnO2 is used, although the sheet resistance of the final networks will begin to increase if they contain
excessive SnO2. AgNW agglomeration during mixing is most easily avoided if the SnO2 and AgNW
solutions are first diluted to the range of 10 to 20 mg/ml in ethanol, with the SnO2 solution being added
first to an empty vial and the AgNW solution added afterwards. The dilute mixture was then be allowed to
settle overnight, and the excess solvent removed to concentrate the wires to a concentration that is
appropriate for blade coating.
Film and electrode deposition. The completed nanocomposite ink was deposited onto any desired
substrates using a razor blade and scotch tape spacer. The majority of the substrates used in this study were
Corning soda lime glass, but the combined inks also deposited well on silicon, SiO2, and any other
substrates tested. Electrode deposition onto a-Si substrates was accomplished by masking off the desired
cell area with tape, and then depositing over the entire region. The p-i-n a-Si stacks and 10 nm AZO
contact layers were deposited using PECVD and sputtering, respectively.
ACKNOWLEDGMENTS The authors would like to acknowledge the use of the Electron Imaging Center for Nanomachines
(EICN) located in the California NanoSystems Institute at UCLA.
REFERENCES [1] Sun, Y.; Gates, B.; Mayers, B.; Xia, Y., Crystalline silver nanowires by soft solution
processing. Nano Lett. 2002, 2, 165-168.
[2] Kim, T.; Kim, Y. W.; Lee, H. S.; Kim, H.; Yang, W. S.; Suh, K. S., Uniformly
interconnected silver-nanowire networks for transparent film heaters. Adv. Funct.
Mater. 2013, 23, 1250-1255.
[3] Hu, L.; Wu, H.; Cui, Y., Metal nanogrids, nanowires, and nanofibers for transparent
electrodes. MRS Bull. 2011, 36, 760-765.
12
[4] van de Groep, J.; Spinelli, P.; Polman, A., Transparent conducting silver nanowire
networks. Nano Lett. 2012, 12, 3138-3144.
[5] Yang, L.; Zhang, T.; Zhou, H.; Price, S. C.; Wiley, B. J.; You, W., Solution-processed
flexible polymer solar cells with silver nanowire electrodes. ACS Appl. Mater.
Interfaces 2011, 3, 4075-4084.
[6] Scardaci, V.; Coull, R.; Lyons, P. E.; Rickard, D.; Coleman, J. N., Spray deposition of
highly transparent, low-resistance networks of silver nanowires over large areas. Small
2011, 7, 2621-2628.
[7] Wiley, B.; Sun, Y.; Xia, Y., Synthesis of silver nanostructures with controlled shapes
and properties. Acc. Chem. Res. 2007, 40, 1067-1076.
[8] Korte, K. E.; Skrabalak, S. E.; Xia, Y., Rapid synthesis of silver nanowires through a
cucl- or cucl2-mediated polyol process. J. Mater. Chem. 2008, 18, 437-441.
[9] Anuj, R. M.; Akshay, K.; Chongwu, Z., Large scale, highly conductive and patterned
transparent films of silver nanowires on arbitrary substrates and their application in
touch screens. Nanotechnology 2011, 22, 245201.
[10] Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P., Solution-processed metal nanowire
mesh transparent electrodes. Nano Lett. 2008, 8, 689-692.
[11] Zhu, R.; Chung, C.-H.; Cha, K. C.; Yang, W.; Zheng, Y. B.; Zhou, H.; Song, T.-B.;
Chen, C.-C.; Weiss, P. S.; Li, G.; Yang, Y., Fused silver nanowires with metal oxide
nanoparticles and organic polymers for highly transparent conductors. ACS Nano 2011,
5, 9877-9882.
[12] Chung, C.-H.; Song, T.-B.; Bob, B.; Zhu, R.; Duan, H.-S.; Yang, Y., Silver nanowire
composite window layers for fully solution-deposited thin-film photovoltaic devices.
Adv. Mater. 2012, 24, 5499-5504.
13
[13] Kim, A.; Won, Y.; Woo, K.; Kim, C.-H.; Moon, J., Highly transparent low resistance
zno/ag nanowire/zno composite electrode for thin film solar cells. ACS Nano 2013, 7,
1081-1091.
[14] Ajuria, J.; Ugarte, I.; Cambarau, W.; Etxebarria, I.; Tena-Zaera, R. n.; Pacios, R.,
Insights on the working principles of flexible and efficient ito-free organic solar cells
based on solution processed ag nanowire electrodes. Sol. Energy Mater. Sol. Cells
2012, 102, 148-152.
[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, 1215-1222.
[16] Lim, J.-W.; Cho, D.-Y.; Jihoon, K.; Na, S.-I.; Kim, H.-K., Simple brush-painting of
flexible and transparent ag nanowire network electrodes as an alternative ito anode for
cost-efficient flexible organic solar cells. Sol. Energy Mater. Sol. Cells 2012, 107,
348-354.
[17] De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.;
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,
1767-1774.
[18] Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y., Scalable coating and properties of
transparent, flexible, silver nanowire electrodes. ACS Nano 2010, 4, 2955-2963.
[19] Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Greyson Christoforo,
M.; Cui, Y.; McGehee, M. D.; Brongersma, M. L., Self-limited plasmonic welding of
silver nanowire junctions. Nat. Mater. 2012, 11, 241-249.
14
[20] Yu, Z.; Zhang, Q.; Li, L.; Chen, Q.; Niu, X.; Liu, J.; Pei, Q., Highly flexible silver
nanowire electrodes for shape-memory polymer light-emitting diodes. Adv. Mater.
2011, 23, 664-668.
[21] Song, T.-B.; Chen, Y.; Chung, C.-H.; Yang, Y.; Bob, B.; Duan, H.-S.; Li, G.; Tu,
K.-N.; Huang, Y., Nanoscale joule heating and electromigration enhanced ripening of
silver nanowire contacts. ACS Nano 2014, 8, 2804-2811.
[22] Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H.,
Highly stretchable and highly conductive metal electrode by very long metal nanowire
percolation network. Adv. Mater. 2012, 24, 3326-3332.
[23] Lee, J. H.; Lee, P.; Lee, D.; Lee, S. S.; Ko, S. H., Large-scale synthesis and
characterization of very long silver nanowires via successive multistep growth. Cryst.
Growth Des. 2012, 12, 5598-5605.
[24] Bob, B.; Song, T.-B.; Chen, C.-C.; Xu, Z.; Yang, Y., Nanoscale dispersions of gelled
Sno2: Material properties and device applications. Chem. Mater. 2013, 25, 4725-4730.
15
Figure 1. Process flow diagram showing the synthesis of AgNWs and SnO2 nanoparticles followed
by stirring in the presence of ammonium salts to create the final nanocomposite ink. Transparent
conducting films were produced by blade coating the completed inks onto the desired substrate.
16
Figure 2. (a,c,d) SEM images of as-synthesized AgNWs at various magnifications. (b,e,f) SEM
images of nanocomposite films, showing the tendency of the SnO2 nanoparticles to coat the entire
outer surface of the AgNWs, increasing their apparent diameter and giving them a soft appearance.
17
Figure 3. Schematic diagrams and TEM images of (a) a single untreated AgNW, (b) an AgNW in the
presence of uncoupled SnO2 (all ammonium ions removed), and (c) an AgNW with a coordinating
SnO2 shell. Scale bars in images (a), (b), and (c) are 300 nm, 400 nm, and 600 nm, respectively. (d,e)
Optical images of AgNW and SnO2 nanoparticle dispersions mixed in varying amounts (d) before and
(e) after settling for 24 hours. The numbers associated with each solution represent the AgNW:SnO2
concentrations in mg/ml. The uncoupled solution contains AgNWs and non-coordinating SnO2
nanoparticles, and shows settling behavior similar to the pure AgNW and pure SnO2 solutions. (f)
Normalized Ag and Sn EDX signal mapped across the diameter of a single nanowire, with the inset
showing the scanning path across an isolated wire.
18
Figure 4. Sheet resistance versus temperature for films deposited using (red) AgNWs that have been
washed three times in ethanol and (blue) mixtures of AgNW and SnO2 with weight ratio of 2:1. The
annealing time at each temperature value was approximately 10 minutes. The large sheet resistance
values of the bare AgNWs when annealed below 200 °C is typical for nanowires fabricated using
copper chloride seeds, which clearly illustrate the impact of SnO2 coordination at low treatment
temperatures.
19
Figure 5. (a) Sheet resistance and transmission data for samples deposited from solutions of varying
nanostructure concentration. Each of these samples were fabricated starting from the same
nanocomposite ink, which was then diluted to a range of concentrations while maintaining the same
AgNW to SnO2 weight ratio. (b) Transmission spectra of several transparent conducting networks
chosen from the plot in plot (a).
20
Figure 6. (a) I-V characteristics of devices made with AgNW/SnO2 rear electrodes with (blue) and
without (red) a 10 nm AZO contact layer. The dramatic double diode effect is likely a result of a
significant barrier to charge injection at the electrode/a-Si interface. (b) Top view SEM image of the
AgNW/SnO2 composite films on top of the textured a-Si absorber. (c) Schematic cross section of the
a-Si device architecture used in solar cell fabrication. The thickness of the thin AZO contact layer is
exaggerated for clarity.
