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A simple PAN-based fabrication method formicrostructured carbon electrodes for organicfield-effect transistors

http://dx.doi.org/10.1016/j.carbon.2015.02.0400008-6223/� 2015 Elsevier Ltd. All rights reserved.

* Corresponding authors: Fax: +82 42 821 8910.E-mail addresses: jslee@gist.ac.kr (J.-S. Lee), jaehakchoi@cnu.ac.kr (J.-H. Choi).

Chan-Hee Jung a,b, Wan-Joong Kim a,d, Chang-Hee Jung a, In-Tae Hwang a,Dongyoon Khim b, Dong-Yu Kim b, Jae-Suk Lee b,*, Bon-Cheol Ku d, Jae-Hak Choi c,*

a Research Division for Industry and Environment, Korea Atomic Energy Research Institute, Jeollabuk-do 580-185, Republic of Koreab Department of Advanced Materials Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Koreac Department of Polymer Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Koread Carbon Convergence Materials Research Center, Institute of Advanced Composites Materials, Korea Institute of Science and

Technology, Jeollabuk-do 565-902, Republic of Korea

A R T I C L E I N F O

Article history:

Received 25 September 2014

Accepted 9 February 2015

Available online 14 February 2015

A B S T R A C T

Facile and efficient fabrication of polyacrylonitrile (PAN)-based conductive graphitic carbon

microstructures (GCMs) and their application to the electrodes of organic field-effect tran-

sistors (OFETs) is described. The PAN thin films spin-coated on a SiO2-deposited Si wafer

was irradiated through a pattern mask with 150 keV H+ ions at various fluences, and sub-

sequently developed to form PAN microstructures. The resulting PAN microstructures were

carbonized at various temperatures to create the GCMs. The analytical results revealed that

the optimized fluence and carbonization temperature for well-defined GCMs was 3 · 1015 -

ions cm�2 and 100 �C, respectively, and that the resulting GCMs created at the optimized

condition exhibited a greatly low surface roughness of 0.36 nm, a good electrical conduc-

tivity of about 600 S cm�1, and a high work function of 5.11 eV. Noticeably, the GCM elec-

trodes-based p-type OFET showed a comparable performance to that of the gold

electrode-based one, demonstrating that the practical use of GCMs as cheap electrodes

to replace expensive metallic ones for organic electronic devices.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Organic material-based graphitic carbon microstructures

(GCMs) have elicited enormous attention in various fields of

organic electronics owing to their low cost, eco-friendly nat-

ure, good electrical conductivity, good chemical resistance,

and compatibility with integrated organic electronic devices

[1–4]. Electrically-conductive GCMs have been fabricated by

high-temperature carbonization of organic material

microstructures formed through various patterning methods,

including photolithography, nano-imprint lithography, elec-

tron beam lithography, chemical vapor deposition, soft lithog-

raphy, and inkjet printing [4–10]. However, most of these

methodologies have several weaknesses, such as complex

process; low conductivity; necessity of special chemicals such

as photoresists and additives; and limited high-volume pro-

duction [11,12]. Thus, a more facile and efficient method for

the fabrication of GCMs with a high resolution and electrical

258 C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8

conductivity is still a prerequisite for their versatile use in

electronic applications.

The fabrication of the electrically-conductive microstruc-

tures using carbon nanomaterials (carbon nanotube (CNT)

and graphene) and conducting polymers have been numer-

ously reported for the application of organic electronic

devices, such as solar cells, light-emitting diodes, and organic

field effect transistors (OFETs) [13–15]. However, their disad-

vantages including high surface roughness (CNT), low ther-

mal and chemical stability (conducting polymers), and

complicated fabrication processes (graphene) are great bottle-

necks for their practical applications.

Polyacrylonitrile (PAN) is a semi-crystalline homopolymer

with unique properties, such as good hardness and rigidity,

good chemical resistance, capability of heat-induced cycliza-

tion and radiation-induced crosslinking, and good formability

of fibers and thin films [16–18]. Thus, it has been commonly

used not only in the fabrication of clothing and soft furnish-

ing fibers but also as an important precursor for the fabrica-

tion of carbon fibers having high mechanical strength and

high conductivity [19–21]. In this respect, PAN can be a

promising polymer precursor for the formation of electrical-

ly-conductive GCMs.

Ion beam contact lithography is an attractive method for

the formation of well-defined polymer microstructures due

to the high linear energy transfer and straight penetration tra-

jectory of the ion beams [22,23]. This technique offers several

advantages, including simplicity; controllability; good resolu-

tion; reliability; and no need for special chemicals, such as

crosslinking agents [24,25]. Due to these benefits, the forma-

tion of polymer microstructures on various substrates by

ion beam contact lithography has been widely studied [26–

28]. However, the fabrication of electrically-conductive GCMs

by ion beam contact lithography and carbonization and their

application as source/drain (S/D) electrodes for organic field-

effect transistors (OFETs) has not been reported yet.

In this study, a simple and effective method for the forma-

tion of PAN-based GCMs by the combination of ion beam con-

tact lithography and carbonization was developed without

time-consuming thermal oxidative stabilization, pattern

transfer, and etching processes, which offers several benefits,

such as easiness, low cost of raw material, no necessity of

additives without crosslinking agents and photo-acid gen-

erators, and the capability of forming well-defined microstruc-

tures with a good electrical conductivity and a high work

function. PAN thin films on silicon dioxide (SiO2)-deposited

silicon (SiO2/Si) wafers were patterned by ion beam contact

lithography under various conditions for optimization. The

formation of GCMs by the carbonization of the formed PAN

microstructures at various temperatures was investigated in

terms of the remaining thickness, morphology, chemical

structure and composition, crystalline structure, work func-

tion, and electrical conductivity, and thus, a plausible mechan-

ism for the formation of PAN-based GCMs can be suggested. To

the best of our knowledge, this is the first report on the prepa-

ration of GCMs by using ion beam contact lithography and car-

bonization techniques without any thermal oxidative

stabilization and additional lithographic or etching processes.

Moreover, the direct application of the GCMs formed on SiO2/Si

substrates as S/D electrodes for OFETs was demonstrated.

2. Experimental

2.1. Materials

PAN (Mw: 150,000, Sigma–Aldrich) and DMF (grade: anhy-

drous, purity: 99.8%, Sigma–Aldrich) were used as a precursor

and solvent for ion beam contact lithography, respectively.

For the fabrication of OFETs, p-type semiconducting P3HT

(region regularity: 98.5%, Rieke Metal), n-type semiconducting

P(NDI2OD-T2) (grade: ActivInk N2200, Polyera), PMMA (Mw:

120,000, Sigma–Aldrich), chlorobenzene (CB, grade: anhy-

drous, purity: >99%, Sigma–Aldrich), and n-butyl acetate

(BA, grade: anhydrous, purity: 99.8%, Sigma–Aldrich) were

used. All the chemicals in this work were used without fur-

ther purification. Pattern masks (50 lm thick stainless steel)

with 100 lm line spaces and 300 lm pitches for the contrast

curve and with 50 lm channel length (L) and 1000 lm width

(W) for the formation of S/D electrodes were obtained from

Youngjin Astech Co., Ltd.

2.2. Formation of PAN microstructures by ion beamcontact lithography

For the formation of the PAN thin films, a PAN solution was

prepared by dissolving 2.4 g of PAN in 37.6 g of DMF. The

resulting PAN solution was spin-coated on cleaned 300 nm-

thick SiO2/Si wafers at 7000 rpm for 40 s. The thickness of

the resulting PAN thin films was about 400 nm. Ion beam con-

tact lithography was performed on a 300-keV ion implanter at

the Advanced Radiation Technology Institute (ARTI) of the

Korea Atomic Energy Research Institute (KAERI). To estimate

the desirable patterning condition, a contrast curve for the

PAN thin films was drawn by proton irradiation in the pres-

ence of a pattern mask with a linear increase in the fluence.

A proton beam acceleration voltage of 150 keV and current

density of 1.0 lA cm�2 were used for the contrast curve. The

fluence of the proton beam ranged from 1 · 1014 to 2 · 1016 -

ions cm�2. After the irradiation, the selectively-irradiated

PAN thin films were developed by DMF, blown with an N2

stream, and finally dried in a vacuum oven at 40 �C at a resi-

dual pressure of 1 · 10�3 mbar for 24 h. The contrast curve

was normalized to the initial thickness.

2.3. Formation of GCMs by carbonization

The PAN microstructures formed on SiO2/Si wafers were

placed in a horizontal tube furnace (Lindberg/Blue M, USA).

The samples were heated to 800, 900, and 1000 �C at a heating

rate of 5 �C min�1 under a nitrogen atmosphere, and then

naturally cooled to room temperature.

2.4. Characterization

The thickness and surface profile of the microstructures

before and after carbonization were measured by using an

Alpha Step IQ surface profiler (KLA Tencor, USA) and a 3D

surface profiler (NanoSystem, Korea), respectively. The

morphological change in the microstructures before and after

the carbonization was observed by using and an FE-SEM

(JSM-7500F, JEOL, Japan) equipped with an EDX and an AFM

Fig. 2 – Contrast curve of PAN thin films. (A color version of

this figure can be viewed online.)

C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8 259

(XE-100, Park System, Korea). The chemical structure and

compositions were analyzed by using a FT-IR spectrometer

(Varian 640, Australia) and an XPS (MultiLab 2000, Ther-

moElectron Corporation, England) employing MgKa radiation.

XRD analysis was carried out on an X-ray diffractometer

(X’Pert Pro Multi-Purpose, PANlytical, Netherland) with CuKa

radiation in a range of 2h from 5 to 60�. Raman spectroscopy

was performed on a Raman spectrometer (LABRAM-HR Jobi-

nYvon LabRAM system, Horiba Scientific, UK) with an Ar-

ion laser at an excitation wavelength of 514.5 nm. High

resolution-transmission electron microscopy (HR-TEM) ana-

lysis was performed on a cross-section of the GCM generated

by carbonization of the PAN microstructures on the 300 nm-

thick SiO2/Si wafers at temperatures of 1000 �C using a JEM-

2100F HR-TEM equipment (JEOL, Japan), and the diffraction

pattern was established by fast Fourier transform. The TEM

sample was prepared by the deposition of a protective plat-

inum (Pt) layer followed by focused ion milling (Quanta 3D

FEG, FEI, USA). The work function was measured using an

ultraviolet photoelectron spectrometer (UPS, AXIS-NOVA,

Kratos Inc., UK) with a He I discharge lamp (hm = 21.2 eV).

The work function was determined from the secondary elec-

tron cutoff of the UPS spectra using metallic gold as a refer-

ence. To measure the current–voltage (I–V) characteristic of

the formed graphic microstructures, silver electrodes with

dimensions of 100 lm (length) · 200 lm (width) · 0.1 lm

(height) were thermally deposited at both edges of the formed

GCMs and the average distance between the electrodes was

about 8000 lm. The I–V curves were recorded on a probe sta-

tion (MST-4000A) equipped with a Keithley 2400 source meter.

The electrical conductivity was measured by using a four-

point probe measurement system (Mitsubishi LORESTA-GP).

2.5. Fabrication of top-gate bottom-contact OFETs

The 86 nm-thick GCMs as S/D electrodes with the defined

channel length of 50 lm (L) and width of 1000 lm (W) were

directly formed on 100 nm-thick SiO2/Si wafers through ion

beam contact lithography with an S/D pattern mask at a flu-

ence of 3 · 1015 ions cm�2 and carbonization at a temperature

of 1000 �C. For the comparison, 20 nm-thick gold patterns

with a 3 nm-thick nickel adhesion layer for the S/D electrodes

were fabricated on SiO2/Si wafers by thermal evaporation in

the presence of the same mask as mentioned above. For the

Fig. 1 – Schematic illustration of the fabrication of electrically-c

carbonization. (A color version of this figure can be viewed onli

formation of a semiconductor layer, 1 wt% of p-type P3HT or

n-type P(NDI2OD-T2) solutions in CB were spin-coated on

the S/D electrodes-formed wafers at 2000 rpm for 60 s, and

subsequently annealed at 150 �C for 20 min to remove the

residual solvents and improve the crystallinity of the semi-

conductor layer. Afterwards, to form the 500 nm-thick poly-

mer gate dielectric layer, 8 wt% PMMA solutions in BA were

spin-coated on the semiconductor layer-deposited wafers at

2000 rpm for 60 s and then baked at 80 �C for 30 min. Finally,

50 nm-thick aluminum (Al) gate electrodes were formed by

thermal evaporation in the presence of a metal mask. The

electrical characteristics of the fabricated OFETs were mea-

sured using a Keithley 4200 semiconductor characterization

system in a nitrogen-filled glove box.

3. Results and discussion

The simple fabrication of electrically-conductive GCMs by ion

beam contact lithography and carbonization is schematically

illustrated in Fig. 1 PAN thin films on a SiO2/Si substrate were

formed by spin-coating of a PAN solution in DMF, and drying

to completely remove the residual solvent. For ion beam con-

tact lithography, light protons were selected to modify only

onductive GCMs by ion beam contact lithography and

ne.)

260 C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8

the PAN films without a detrimental effect on the SiO2/Si sub-

strate. To uniformly penetrate the 400 nm-thick PAN films,

the accelerating energy of 150 keV was determined as shown

in the TRIM depth profile of 150 keV protons into the PAN

(Fig. S1). The resulting PAN thin films were selectively irradi-

ated by 150 keV proton ions in the presence of a pattern mask,

and they were subsequently developed with DMF to generate

negative-type PAN microstructures. Afterwards, the formed

PAN microstructures were carbonized to create electrically-

conductive GCMs. Finally, the GCMs formed on the SiO2/Si

wafers were directly utilized as S/D electrodes for OFETs.

3.1. Fabrication of GCMs by ion beam contact lithographyand carbonization

To estimate the performance of PAN thin films for ion beam

contact lithography, a contrast curve was plotted as shown

Fig. 3 – FE-SEM and EDX spectra of PAN microstructures prepar

carbonization at temperatures of 800 (d–f), 900 (g–i), and 1000 �C

in Fig. 2. The normalized thickness of the PAN microstruc-

tures after development was increased with an increasing flu-

ence up to 3 · 1015 ions cm�2, after which it levelled off. The

contrast (c) is determined by c = [log10(D100/D0)]�1, where D100

and D0 are the fluences at the normalized thickness of 100%

and the onset of the complete development, respectively

[29,30]. The calculated contrast and sensitivity of the PAN thin

films were 3.3 and 1.9 · 1015 ions cm�2, respectively. Moreover,

the FE-SEM images shown in (Fig. S2) indicate that well-de-

fined 100 lm negative-type PAN microstructures were formed

at fluences of 3 · 1015 and 5 · 1015 ions cm�2; however, at a flu-

ence of 1 · 1016 ions cm�2, the line widths of the formed

microstructures seemed to be widened because of the inten-

sified ion scattering effect at a higher fluence [31]. Thus, the

optimum fluence for the formation of well-defined PAN

microstructures was 3 · 1015 ions cm�2 in this system, which

was further applied to the formation of electrically-

ed at a fluence of 3 · 1015 ions cm�2 before (a–c) and after

(j–l). (A color version of this figure can be viewed online.)

Fig. 4 – 3D surface profiles of PAN microstructures prepared at a fluence of 3 · 1015 ions cm�2 before (a) and after carbonization

at temperatures of 800 (b), 900 (c), and 1000 �C (d). (A color version of this figure can be viewed online.)

Fig. 5 – C1s core-level spectra of pure PAN (a) and PAN irradiated at a fluence of 3 · 1015 ions cm�2 (b), and carbon films

prepared by carbonization at temperatures of 800 (c), 900 (d), and 1000 �C (e). (A color version of this figure can be viewed

online.)

C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8 261

conductive GCMs by carbonization at various temperatures.

Taking account of the penetration depth of 150 keV protons

presented in the TRIM depth profiles (Fig. S1), microstructures

with a higher aspect ratio can be reliably generated even at a

fluence of 3 · 1015 ions cm�2 within the film thickness of

1.5 lm [32].

The change in the morphology and elemental composi-

tions of PAN microstructures formed at the optimum fluence

of 3 · 1015 ions cm�2 after carbonization at various tem-

peratures was investigated by using an FE-SEM equipped with

an EDX. As shown in Fig. 3a, d, g, and j, the width of the

PAN microstructures was almost maintained during

carbonization, even at 1000 �C, while their thickness was

reduced remarkably. Moreover, as shown in Fig. 3c, f, i, and

l, only two elemental peaks corresponding to the silicon (Si)

and oxygen (O) of the SiO2-deposited Si wafer appeared in

the EDX spectra for the space regions between the PAN

microstructures, indicating that the non-irradiated regions

were completely eliminated during development. As shown

in Fig. 3b, two elemental peaks assigned to the carbon (C)

and nitrogen (N) elements besides the Si and O elements

appeared in the EDX spectra for the PAN microstructures

without carbonization, indicating the manifest presence of

crosslinked PAN. On the other hand, as shown in Fig. 3e, h,

Fig. 6 – N1s core-level spectra of pure PAN (a) and PAN irradiated at a fluence of 3 · 1015 ions cm�2 (b), and carbon films

prepared by carbonization at temperatures of 800 (c), 900 (d), and 1000 �C (e). (A color version of this figure can be viewed

online.)

Fig. 7 – [O]/[C] (a) and [N]/[C] ratio (b) of pure PAN and PAN irradiated at a fluence of 3 · 1015 ions cm�2, and carbon films

prepared by carbonization at various temperatures.

262 C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8

and k, only the C, O, and Si elements were observed in the

EDX spectra for the PAN microstructures treated at 800, 900,

and 1000 �C. This result implies that the formed PAN

microstructures were converted to the carbonized ones dur-

ing carbonization within the detectable limit of the EDX spec-

tra [33].

To further investigate the change in the thickness of the

PAN microstructures formed at the optimized fluence of

3 · 1015 ions cm�2 after carbonization at various tem-

peratures, a 3D surface profile analysis was performed, and

the results are shown in Fig. 4. As shown in Fig. 4a, the thick-

ness of the PAN microstructures without carbonization was

381 nm. On the other hand, as presented in Fig. 4c and d,

the thicknesses of the microstructures carbonized at tem-

peratures of 800, 900, and 1000 �C were 108, 92, and 86 nm,

respectively. Furthermore, as shown in the AFM images relat-

ed to the carbonization-induced surface morphological

change of PAN microstructures (Fig. S3), all the carbonized

microstructures at the given temperatures exhibited similar

smooth morphologies and low root-mean-square (RMS)

roughnesses below 0.36 nm greatly lower than those prepared

with carbon nanomaterials [14,15], indicating that there was

no significant change in the PAN microstructures during

carbonization.

The structural changes in the PAN thin films after the

sequential process of proton irradiation at a fluence of

3 · 1015 ions cm�2 and carbonization at a temperature of

1000 �C were analyzed by FT-IR, and the results are shown

in Fig. S4. Three characteristic vibrations of pure PAN were

identified at 2925 cm�1 (mCAH in CH2), 2242 cm�1 (mC„N in

CN), and 1450 cm�1 (dCAH in CH2), respectively [34]. In the case

of the irradiated PAN, the respective peaks assigned to the

C@O and C@N newly appeared at 1710 and 1590 cm�1 with a

simultaneous reduction in the peak intensities of C„N at

2242 cm�1 and CH2 at 1450 cm�1 [35], indicating the proton

irradiation-induced intra-and inter-molecular cyclization,

Fig. 8 – Raman spectra of GCMs generated by carbonization

of PAN microstructures at temperatures of 800 (a), 900 (b),

and 1000 �C (c). (A color version of this figure can be viewed

online.)

C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8 263

dehydrogenation, and oxidation of the PAN. On the other

hand, the FT-IR spectrum of the carbonized PAN exhibited

only the peak at 1580 cm�1 corresponding to aromatic C@C.

This result suggests that GCMs were successfully formed by

the carbonization of the cyclized and oxidized PAN

microstructures.

To further elucidate the chemical change in the PAN thin

films after proton irradiation at a fluence of 3 · 1015 ions cm�2

and carbonization at various temperatures, the XPS analysis

Fig. 9 – Cross-sectional TEM (a) and HR-TEM images of the 85 nm

PAN microstructures at the temperature of 1000 �C (b). The SAED

can be viewed online.)

was carried out, and the results are shown in Fig. 5. As shown

in Fig. 5a, the CAN and CAC peaks of pure PAN thin films was

observed at 286.2 and 285.6 eV, respectively [36]. As shown in

Fig. 5b, new peaks such as C@O, CAO, and C@C were generat-

ed at 287.5, 286.2, and 284.5 eV, respectively, in the C1s spectra

owing to cyclization and oxidation induced by proton irra-

diation [37]. After carbonization, a strong C@C peak and

relatively weak C@O and CAN/O peaks were identified in

the C1s spectra and their intensities were dependent on the

carbonization temperature, as shown in Fig. 5c–e. Moreover,

the N1s spectra of the irradiated PAN thin films exhibited a

new pyridinic N@C peak at 398.0 eV, which was absent from

the N1s spectra of the pure PAN films, indicating the occur-

rence of cyclization brought about by proton irradiation, as

shown in Fig. 6a and b. After carbonization, the graphitic

C@C peak in addition to the pyridinic N@C peak was newly

identified at 401.2 eV in all the N1s spectra of the carbonized

films as shown in Fig. 6c–e, [38]. As shown in Fig. 7, in compar-

ison to those of the pure PAN thin film, the [O]/[C] atomic ratio

of the irradiated PAN thin films increased, whereas the [N]/[C]

atomic ratio decreased. On the other hand, the [O]/[C] and [N]/

[C] atomic ratios of the carbonized films were further

decreased with an increase in the carbonization temperature.

These results indicate that GCMs were effectively generated

by the carbonization of the oxidized and cyclized PAN

microstructures formed by proton irradiation, and the extent

of their graphitization was dependent on the carbonization

temperature [39].

-thick GCMs on a SiO2 substrate prepared by carbonization of

pattern of the same sample (c). (A color version of this figure

Fig. 10 – Plausible mechanism for the formation of graphitic carbon structures by proton irradiation and carbonization.

264 C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8

To investigate the formation of graphitic carbon structures

by the carbonization of PAN films irradiated at a fluence of

3 · 1015 ions cm�2 at various temperatures, Raman and TEM

analyses were carried out, and the results are shown in Figs. 8

and 9. The G, D, and 2D bands corresponding to the sp2-hy-

bridized and disordered carbons clearly appeared at 1350,

1604, and 2700 cm�1 in all the Raman spectra, respectively,

and their intensities were increased with an increase in the

carbonization temperature [40]. The D-band to G-band inten-

sity ratios (ID/IG) of the formed carbon films were decreased by

as much as 0.81 with an increase in the carbonization tem-

perature due to the better formation of ordered graphitic

structures at a higher temperature. Moreover, as shown in

the cross-sectional TEM images and the selected-area elec-

tron diffraction (SAED) pattern for the further investigation

of the graphitic structure of the GCMs (Fig. 9), curvy and par-

allel fringes corresponding to the graphic structure were

observed in the highly magnified HR-TEM image of the GCMs

prepared by carbonization of PAN microstructures at the tem-

peratures of 1000 �C (Fig. 9b), and its SAED pattern exhibited

an weak and diffuse circles (Fig. 9c), indicating the presence

of a turbostratic graphite structure in the GCMs [39]. These

results indicate that the graphitic and amorphous carbon

structures simultaneously existed in the formed GCMs, which

were dependent on the carbonization temperature.

The plausible mechanism for the formation of graphitic

carbon structures by proton irradiation and carbonization

is illustrated in Fig. 10. Free radicals in the PAN main chain

are formed by hydrogen abstraction induced by proton irra-

diation under a vacuum. The resulting polymer radicals

cause intramolecular cyclization and intermolecular cou-

pling reactions, thus generating the formation of cyclic net-

work structures in the PAN films. After the atmospheric

exposure, the remaining free radicals in the cyclic network

structure readily react with oxygen in the air; thus, the for-

mation of oxidized cyclic network structures is similar to

that of the PAN treated by conventional thermal oxidative

stabilization. The resulting oxidized cyclic network struc-

tures in the PAN were converted to graphitic carbon ones

during carbonization [41,42].

Fig. 11 – Current–voltage (I–V) curves (a), electrical conductivity (b), and work function (c) of GCMs prepared by carbonization of

PAN microstructures at various temperatures. (A color version of this figure can be viewed online.)

Fig. 12 – Transfer curves of p-type P3HT (a) and n-type P(NDI2OD-T2) OFETs with the GCM and gold (Au) S/D electrodes (b) and

output curves of p-type P3HT (c) and n-type P(NDI2OD-T2) OFETs with the GCM S/D electrodes (d). (A color version of this

figure can be viewed online.)

C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8 265

The current–voltage (I–V) curves, electrical conductivity,

and work function of the GCMs prepared by the carbonization

of PAN microstructures at various temperatures are shown in

Fig. 11. As shown in Fig. 11a, all the GCMs exhibited the linear

I–V behaviors, implying the metallic property of the patterns

and the formation of Ohmic contact between the GCMs and

silver (Ag) electrodes [7]. As shown in Fig. 11b, the electrical

conductivities of the GCMs measured using a 4-probe conduc-

tivity meter was increased up to 600 S cm�1 with an increase

in the carbonization temperature. Moreover, the work func-

tion of the GCMs was higher than that of the gold (Au) refer-

ences, which was dependent on the carbonization

Table 1 – Performances of p-type P3HT (a) and n-type P(NDI2OD-T2) OFETs with the GCM and gold (Au) S/D electrodes.

Semiconductors S/D electrodes FET mobility (lFET)[cm2 V�1 s�1]

Threshold voltage (VTh) [V] On/off ratio [IOn/IOff]

P3HT GCM 0.060 �15.1 5 · 103

Au 0.051 �18.3 104

P(NDI2OD-T2) GCM 0.073 13.1 5 · 103

Au 0.130 13.4 104

266 C A R B O N 8 7 ( 2 0 1 5 ) 2 5 7 – 2 6 8

temperature. These temperature-dependent properties of the

GCMs can be ascribed to the fact that the carbonization at a

higher temperature created highly ordered graphitic struc-

tures in the GCMs, thereby improving the electrical

conduction.

3.2. Fabrication of top-gate bottom-contact OFET usingGCM S/D electrodes

To evaluate the potential use of the GCMs as S/D electrodes for

OFETs, top-gate/bottom-contact OFET devices based on the

GCM S/D electrodes were fabricated. GCM S/D electrodes with

the defined length of 50 lm and width of 1000 lm were directly

formed on SiO2/Si wafers by ion beam contact lithography and

carbonization under the optimized conditions (Fig. S5),

where p-type poly(3-hexylthiophene) (P3HT) and n-type poly

{[N,N-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicrboximide)-

2,6-diyl]-alt-5,5-(2,2-bithiophene)} (P(NDI2OD-T2)) OFETs with

poly(methyl methacrylate) (PMMA) as a gate dielectric were

fabricated. The transfer and output characteristics of the

P3HT and P(NDI2OD-T2) OFETs are displayed in Fig. 12. The

measured fundamental parameters of the OFETs, namely

FET mobility (lFET), threshold voltage (VTh), and on/off-current

ratio (Ion/Ioff), are summarized in Table 1. Based on the gradual

channel approximation, lFET was calculated in the saturation

regime at a drain voltage (Vd) of �60 V (p-channel) or 60 V (n-

channel). The P3HT OFETs with GCM S/D electrodes exhibited

an ideal p-channel transfer characteristic with a lFET of

0.060 cm2 V�1 s�1, which is slightly higher than that with Au

S/D electrodes. The output curves exhibited desirably linear

and saturation regimes with a gate-voltage dependence.

However, the P(NDI2OD-T2) OFETs with the GCM S/D as

an n-channel device showed a slightly lower lFET of 0.073

cm2 V�1 s�1 in comparison to that with Au S/D electrodes.

These results are attributed to an energy level mismatch in

the charge injection between the semiconductor molecular

orbital (electron: lowest unoccupied molecular orbitals

(LUMO), hole: highest occupied molecular orbitals (HOMO))

and the electrode. The work function of the optimized GCM

electrode is 5.1 eV, which is similar to 5.0 eV of the Au elec-

trode (see Fig. 11c). At the interface between Au and organic

semiconductor, the work function of the Au electrode could

be reduced to �4.4 eV compared to that of an atomically clean

Au surface (5.0 eV), due to interface dipole formation resulting

from the ‘‘push-back’’ effect [43]. The interface dipole typically

favors electron injection by lowering the metal work function,

which consequently reduce the electron injection barrier in

the device. Therefore, P(NDI2OD-T2) OFETs with Au electrodes

exhibited a high electron lFET of 0.180 cm2 V�1 s�1 although

the LUMO level of P(NDI2OD-T2) was �4.0 eV. However, the

graphene-organic semiconductor interface generally forms a

relatively-weak interface dipole layer, unlike the Au-organic

semiconductor interface [44]. Therefore, the CGM work func-

tion at the interface could be much higher than that of the

Au electrode, which is favorable for hole injection to the

HOMO level (�5.2 eV) of P3HT and slightly unfavorable for

electron injection to the LUMO level (�4.0 eV) of P(NDI2OD-

T2), compared to those of the Au electrode device.

4. Conclusions

PAN-based GCMs were fabricated by ion beam contact lithog-

raphy and carbonization and directly used as efficient S/D

electrodes for OFETs without thermal oxidative stabilization

or additional transfer and etching processes. It is clearly con-

firmed from the contrast curve, 3D surface profile, FE-SEM,

and EDX analyses that well-defined negative-type PAN

microstructures with the a greatly low surface roughness of

0.36 nm were formed at a fluence of 3 · 1015 ions cm�2 and

carbonized even at 1000 �C almost entirely without shrinkage

in the horizontal direction. Based on the results of the FT-IR,

XPS, and Raman spectroscopy, the cyclized and oxidized

PAN was simply generated by proton irradiation and then

transformed to a graphitic carbon material by high-tem-

perature carbonization. The analytical results of the electrical

properties showed that the formed GCMs exhibited a good

electrical conductivity of 600 S cm�1 and a high work function

of 5.11 eV. Noticeably, the GCM electrodes-based p-type OFET

showed a comparable performance to that of the gold elec-

trode-based one due to its higher work function, which is

responsible for the reduced charge carrier injection barrier.

Thus, the GCMs fabricated by this simple and profitable tech-

nique can be a promising carbon material to replace expen-

sive metallic electrodes for organic electronic devices.

Acknowledgements

This work was supported by Radiation Technology R&D pro-

gram through the National Research Foundation of Korea

(NRF) grant funded by the Ministry of Science, ICT & Future

Planning.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2015.02.040.

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