Organic Electronics 12 (2011) 2215–2224

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Organic Electronics

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Poly(ionic liquid)-stabilized graphene sheets and their hybridwith poly(3,4-ethylenedioxythiophene)

Tran Thanh Tung a, Tae Young Kim a,b,⇑, Jong Pil Shim a,c, Woo Seok Yang b, Hyeongkeun Kim b,Kwang S. Suh a,⇑a Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 137-713, Republic of Koreab Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Koreac Purchasing Center, LG Display Co. Ltd., 1007 Deogeun-ri, Wollong-myeon, Paju-si, Gyunggi-do 413-811, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 May 2011Received in revised form 12 September2011Accepted 17 September 2011Available online 5 October 2011

Keywords:GraphenePoly(ionic liquid)Conducting polymerTransparent conductive filmHybrid composites

1566-1199/$ - see front matter � 2011 Elsevier B.Vdoi:10.1016/j.orgel.2011.09.012

⇑ Corresponding authors. Address: Departmentand Engineering, Korea University, 5-1 Anam-dong137-713, Republic of Korea. Tel.: +82 29274546; faxKim).

E-mail addresses: thomas75@korea.ac.kr (T.Ykorea.ac.kr (K.S. Suh).

Hybrid materials of reduced graphene oxide (RG-O) and poly(3,4-ethylenedioxythiophene)(PEDOT) were prepared by poly(ionic liquid)-mediated hybridization. In this hybrid mate-rial, poly(ionic liquid)s (PILs) are found to be preferentially physisorbed onto the RG-Oplatelets, and allow them to be dispersed as a homogeneous colloidal system. In additionto the function as an effective stabilizer, the PIL also promotes PEDOT growth on RG-Oplatelets through favorable molecular interaction of PIL with PEDOT chains. The resultingmaterial, a hybrid of RG-O and PEDOT showed an electrical conductivity of 18.8 S/cm at aRG-O loading of 0.3 wt.%, and its thin film on glass substrate showed a surface resistivity aslow as 1.8 � 104 O/sq at an optical transmittance of 91.18%.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Graphene has emerged as a promising carbon-basednanomaterials with outstanding thermal, mechanical, andelectronic properties because of their two-dimensionalsp2-hybridized bonded structure [1,2]. While single- tofew-layered graphenes can be prepared by a variety ofmethods such as micromechanical exfoliation [3], chemicalvapor deposition [4], and epitaxial growth [5], a chemicalexfoliation route (i.e. sequential oxidation–exfoliation–reduction route) shows distinct advantages in terms of yieldand cost [6–8]. This chemical method typically producesbulk quantities of reduced graphene oxide (RG-O), whichresembles graphene but with some oxygen groups and

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of Materials Science, Seongbuk-gu, Seoul: +82 29294408 (T.Y.

. Kim), suhkwang@

structural defects. RG-O yields an electrical conductivitythat is comparable to that of doped conductive polymers,and it has been proposed as an attractive alternative tographene for a range of applications such as highly sensitivegas sensors [9], mechanical resonator [10], supercapacitors[11], and field-effect transistor (FET) devices [12].

However, RG-O sheets prepared via the chemical exfoli-ation route are intrinsically defective and their lateral sizesusually ranges up to a few micrometers, which may limitintra- and inter-plate charge transport to achieve a macro-scopic conductivity of RG-O films. One way to achievemore effective charge transport within and/or betweenthe RG-O platelets is to hybridize with conducting poly-mer. This hybridized RG-O material with intrinsically con-ducting polymer may offer advantages in applications suchas supercapacitors and optoelectronic devices which havebeen previously reported with improved device perfor-mance [13–16].

Herein, we report on a new route to the hybridization ofRG-O platelets with conducting polymer of poly(3,4-ethyl-enedioxythiophene) (PEDOT) using poly(ionic liquid) (PIL).

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Since a successful hybridization of RG-O/conducting poly-mer requires RG-O platelets to be fully exfoliated in a sus-pension and provide a modified and active sites for thehybridization, we used PIL to modify the surface of RG-Oplatelets. The specific PIL of poly(1-vinyl-3-ethyimidazoli-um) salts was explored to stabilize RG-O platelets to besuspended in organic solvent, and provide an effectivelinkage during the polymerization of PEDOT molecules onthe RG-O surface. The resulting hybrid material of RG-Oand PEDOT are found to be readily dispersed as a colloidalsystem without any agglomeration and its film showedsynergistic effects in terms of electrical conductivity andthermal stability.

2. Experimental

2.1. Synthesis of graphene oxide and poly(ionic liquid)

Graphite oxide (GO) powder was prepared from graph-ite flakes using modified Hummers method [17,18]. Briefly,5 g of graphite (Bay carbon SP-1) and 3.75 g of NaNO3 wereplaced in a flask. Then, 375 mL of H2SO4 was added while

Fig. 1. The process for preparation of PIL:RG-O hybrids with

stirring in an ice-water bath, and 25 g of KMnO4 wereslowly added for 1 h. Stirring was continued for 2 h inthe ice-water bath. The reaction mixture was washed withdeionized water, and reacted with a 30 wt.% aqueous solu-tion of H2O2 to complete oxidation. The ions of oxidant andother inorganic impurity were removed by repeating cycleof centrifugation, removal of the supernatant liquid, andredispersing the solid using 3 wt.% HCl aqueous solution.Finally, the solid was dispersed again in water using ultra-sonication for 2 h and centrifuged at 6000 rpm for 30 minto remove the multilayered species.

PIL of poly(1-vinyl-3-ethylimidazolium) salts bearingthe bis(trifluoromethylsulfonyl)amide anion (NTf�2 orCF3SO2–N–SO2CF3) was synthesized according to a previ-ously reported procedure [19,20].

2.2. Poly(ionic liquid)-mediated thermal reduction ofgraphene oxide in propylene carbonate

Graphene oxide (G-O) suspension with a concentrationof 1 mg/mL was obtained by dispersing GO powder inpropylene carbonate (PC) with the aid of mild sonication

conducting polymer PEDOT using poly(ionic liquids).

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for 30 min. Then, PIL was added to the G-O suspension in PCand stirred for 30 min to produce PIL-modified G-O. PIL-modified G-O was prepared with different ratios of PIL/G-O (1/1, 3/1, 5/1, 10/1) by weight. In a typical process,300 mg of PIL(NTf�2 ) were dissolved in 30 mL of anhydrousPC and added to 60 mL of G-O suspension. Under continuesstirring, heating these mixture at 150 �C for 3 h yielded athermally reduced RG-O modified with PIL (PIL:RG-O), aspreviously reported [21]. After cooling, a part (e.g., 10 mL)of heated suspension was filtered through a Omniporemembrane (JGWP, 0.2 lm pore size, Millipore, Ireland) toobtain the resultant PIL:RG-O, followed by drying at 80 �Cin vacuum for 2 days.

2.3. Hybridization of PIL-modified reduced graphene oxideand conducting polymer

The as-prepared PIL:RG-O suspension was subjected tohybridization with PEDOT. In situ polymerization of EDOTwas carried out by chemical oxidative method in PIL:RG-Osuspension. Typical preparation procedure for the PIL-mediated hybridization of RG-O and PEDOT (PEDOT-PIL:RG-O) composites is as follows: a mixture of EDOT

Fig. 2. (a) FTIR, (b) TGA, (c) XRD, and (d) Raman spectra of G-O (black curve)references to color in this figure legend, the reader is referred to the web versio

(0.268 g), 1.25 mL of PIL:RG-O suspension and 60 mL ofPC were introduced into a 250 mL one-necked round bot-tom flask and stirred for 1 h. The chemical polymerizationwas effected by the addition of a solution of Iron(III) chlo-ride (FeCl3, 0.30 g in 20 mL PC) into the mixture and thereaction was continued under vigorous stirring for 48 hat room temperature, yielding organic solvent dispersiblePEDOT-PIL:RG-O composites. For comparison, the PIL-modified PEDOT (without any RG-O) (PEDOT:PIL) samplewas also prepared separately under same conditions.

2.4. Characterization

Fourier transform infrared (FT-IR) spectra were re-corded on a Nicolet Avatar 320 using KBr pellets. X-rayphotoelectron spectroscopy (XPS) measurements wereperformed with ESCA2000 (VG Microtech) system using amonochromatized aluminum Ka anode. Raman spectrawere recorded over 500–4000 cm�1 with an excitationwavelength of 642.8 nm using RFS-100/S Raman spectrom-eter equipment. Thermogravimetric analysis (TGA) mea-surement was performed at a heating rate of 10 �C min�1

under nitrogen flow using a NETZSCH STA 409 PC/PG

, RG-O (red curve) and PIL:RG-O (blue curve). (For interpretation of then of this article.)

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instrument. X-ray diffraction (XRD) data were collected ona Rigaku D/Max Ultima II Powder X-ray diffractometer.Scanning electron microscopy (SEM) images were recordedusing a JEOL JSM-700F microscope operated at 15.0 kV.High resolution transmission electron microscopy (HR-TEM) images were obtained using a TECNAI 20 operatingat 200 kV and equipped with energy dispersive X-ray fluo-rescence spectrometer for EDX analysis. Atomic forcemicroscopy (AFM) measurements were performed in tap-ping mode with an AFM XE-100 (Park System). The surfaceresistance was obtained using a four-point probe method(CMT-SR2000N) at room temperature.

3. Results and discussion

A hybrid of RG-O and PEDOT was obtained through atwo-step procedure illustrated in Fig. 1. The first step in-volved thermal reduction of G-O in the presence of PILand this process typically resulted in the RG-O modifiedwith PIL. In the second step, we carried out in situ chemicalpolymerization of EDOT in PIL:RG-O suspension usingFeCl3 as an oxidizing agent. This process leads to the for-mation of a hybridized material PEDOT-PIL:RG-O that arehomogeneously dispersed as a colloidal system in PC.

3.1. Thermal reduction of the PIL-functionalized grapheneoxide in propylene carbonate

The addition of PIL into G-O suspension in propylenecarbonate (PC) typically yielded the PIL-modified G-O bythe electrostatic interaction between the cation in PILand negatively charged G-O platelets. Then, heating thissuspension at 150 �C led to the reduced graphene oxidemodified with PIL. Fig. 2a shows the FT-IR spectra of G-O,RG-O and PIL:RG-O. The spectra of G-O sheets (black curve)showed C=O (1725 cm�1), aromatic C=C (1621 cm�1), car-boxy C�O (1382 cm�1), epoxide/ether C�O (1231 cm�1),and alkoxy/alkoxide C�O stretches (1027 cm�1). The

Fig. 3. Photograph of (a) G-O, (b) RG-O, and (c) PIL:RG-O dispersed inpropylene carbonate after 3 days of free-standing.

RG-O (red curve) showed a decrease in C=O and C�Ostretch peak intensities, indicating de-oxygenation of theRG-O sheets after thermal reduction. In the FT-IR spectraof PIL:RG-O (blue curve), the peaks at 1792, 1569, 1352,1189, 744, and 647 cm�1 correspond to the PIL(NTf�2 ), indi-cating an adsorption of PIL(NTf�2 ) onto the RG-O surface.

Fig. 2b shows the TGA data of RG-O (red curve) andPIL:RG-O (blue curve) in comparison with pristine G-O(black curve). G-O shows a continuing weight loss of

Fig. 4. C 1s XPS spectra of (a) G-O, (b) RG-O, and (c) PIL:RG-O.

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7–8% in the temperature range up to 80 �C, and this is re-lated to the removal of physisorbed water. A weight lossaround 168 �C can be ascribed to the decomposition of dif-ferent oxygen containing groups in the G-O layer [22]. Theweight loss of RG-O (red curve) is much smaller due to adecreased amount of oxygen functional groups in the RG-O. For the case of PIL:RG-O (blue curve), the weight lossof 25% occurs from 316 to 428 �C, possibly due to thedecomposition of the surface-bound PIL molecules.

The XRD patterns of G-O, RG-O and PIL:RG-O are shownin Fig. 2c. Similar to the previous report, G-O and RG-Oshowed a XRD peak at 2h = 11.2� and 23.8�, indicating aninterlayer spacing of �0.78 nm and �0.36 nm. The de-creased interlayer spacing for RG-O is due to the removalof oxygen-based functional groups on the basal plane bythermal reduction. Compared with RG-O, the PIL:RG-Oshowed a broad peak at 2h = 13.9�, indicating an increasedinterlayer spacing of �0.62 nm. This suggests that PIL arecoated on the surface of RG-O surface and act as a spacerfor RG-O platelets with an increased interlayer spacing.

Raman spectroscopy was employed to investigate achange in the graphitic structure during the reduction pro-cess of RG-O and PIL:RG-O as shown in Fig. 2d. The Ramanspectrum shows a typical D band (1332 cm�1) and G band(1560 cm�1) for G-O, which corresponds to the presence ofsp3 defects and tangential vibration of sp2 carbon atoms inhexagonal plane, respectively. While the intensity ratio ofD and G band (ID/IG) of G-O is about 0.82, the ID/IG of

Fig. 5. (a and b) SEM image with different magnification of PIL

RG-O increased to 1.12 due to a decrease in the averagesize of the sp2 carbon network upon thermal reduction ofthe exfoliated G-O [22,23]. In case of PIL:RG-O, the ID/IG

was measured as 1.03 which is lower than that of RG-O.This suggests that PIL:RG-O has decreased density of de-fects compared with RG-O without PIL.

The thermal reduction of G-O in the presence of PIL pro-duces a suspension of PIL:RG-O in PC. While the reductionof G-O without PIL resulted in an agglomerated RG-Osheets which can be viewed as black particles in the vial(Fig. 3b), the PIL:RG-O was found to be dispersed as ahomogeneous colloidal system with no visible agglomer-ates (Fig. 3c). This dispersion can be filtered to obtainPIL:RG-O platelets that can be re-dispersed in a wide rangeof organic solvents such as PC, acetone, acetonitrile (AN),dimethylformamide (DMF), N-methyl-pyrollidone (NMP)and nitromethane (NM). It should be noted that an in-crease in the amount of PIL added to G-O suspension en-hances the stability of suspension (Fig. S-1, supportinginformation). When the PIL/G-O weight ratio is equal toor lower than 3/1, PIL:RG-O in PC precipitated out afterseveral days, suggesting that PIL concentration was notsufficient to fully stabilize RG-O platelets in PC. On theother hand, when the ratio is higher than 5/1, the disper-sion remained stable for more than 6 months.

The chemical composition of G-O, RG-O, and PIL:RG-Owas investigated by XPS. The C 1s XPS spectrum of G-O(Fig. 4a) can be deconvoluted into four main components

:RG-O and (c) the EDX spectra corresponding to panel b.

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corresponding to carbon atoms in different functionalgroups: the non-oxygenated ring C (284.8 eV), the C inC–O bonds (286.8 eV), the carbonyl C (C=O, 288.0 eV),and the carboxylate carbon (O–C=O, 289.1 eV). As shownin Fig. 4b, RG-O sample showed a decreased intensity ofthese oxygen groups confirming the effective removal ofoxygen groups by thermal reduction [21]. The C 1s XPS

Fig. 6. (a) TEM and (b) AFM histogram shows the statistic analysis ofPIL:RG-O sample. The inset in (a) is the SAED patterns from PIL:RG-Osheet.

spectrum of PIL:RG-O (Fig. 4c) also shows the removal ofoxygen groups on the G-O basal plane and the subsequentrestoration of sp2-hybridized carbon network. Moreover,the C/O atomic ratio calculated from the XPS spectra is2.0, 6.2 and 8.6 for G-O, RG-O and PIL:RG-O, respectively.The higher C/O atomic ratio observed for PIL:RG-O isattributed to the fact that the thermal reduction of G-Oplatelets occurred more rapidly in the presence of PIL, thanin a control experiment with no PIL present [21]. The rapidreduction of suspended G-O described here may comefrom the reducing capability of imidazolium-based PIL,which has been previously reported for ionic liquid reduc-tion of Au precursors [24]. Additionally, there is a newcomponent at 288.4 eV corresponding to the C in the C–Sbonds of NTf�2 (CF3SO2–N–SO2CF3) anion, suggesting theexistence of PIL on the RG-O surfaces.

A top-view SEM image of PIL:RG-O deposited on Si/SiO2

substrates (Fig 5a) reveals that the RG-O platelets are indi-vidually isolated. In Fig. 5b, it can be seen that the surfaceof RG-O was covered with a thin layer of PIL molecules,which is also evidenced by energy dispersive X-ray (EDX)analysis in Fig. 5c.

TEM was used to investigate the level of exfoliation andthe dispersion. As can be seen in Fig. 6a, PIL:RG-O materialwas observed as thin platelets with some folded area andsmall fragments. In the selected area electron diffraction(SAED) patterns (inset of Fig. 6a), a ring-like pattern consist-ing of many diffraction spots was observed indicating a ran-dom overlay of individual RG-O platelets coated with PIL, orfrom folds that cause overlays [21]. Fig. 6b shows an AFMimage and height profile from the sample, confirming thatPIL:RG-O are comprised of large-scale (several lm) and iso-lated graphitic sheets with some winkles. From the AFMheight profile, the thickness of PIL:RG-O was measured as1.64 nm that is higher than that of theoretical thickness ofa single layer graphene (0.34 nm) [25], G-O platelets exfoli-ated in water (�1 nm) [26,27], and RG-O in PC (�0.6 nm)[21]. The higher thickness value for PIL:RG-O is likely dueto the PIL coating of RG-O platelet [28].

Fig. 7 shows the surface resistivity vs. weigh ratio of PIL/G-O. The surface resistivity was measured for PIL:RG-O

Fig. 7. The surface resistivity of PIL:RG-O films with different weightratios of PIL vs. G-O.

Fig. 8. (a) FTIR curves of PEDOT-PIL:GR-O (blue line) and pristine PEDOT-PIL (red line). (b) C 1s and (c) S 2p spectra of PEDOT-PIL:RG-O composites.(For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

T.T. Tung et al. / Organic Electronics 12 (2011) 2215–2224 2221

using a standard four-probe method. For the measurement,the PIL:RG-O paper-like films with the film thickness of�534 nm (Fig. S-2, SI) were prepared by casting PIL:RG-Osuspension on the Si/SiO2 wafer. The randomly stackedPIL:RG-O film was peeled off the substrate and subject tothe resistivity measurement. The surface resistivity ofPIL:RG-O decreases from 22.6 to 3.3 kO/sq with increasingPIL/G-O ratio of up to 5. This result suggest that thesp2-carbon network is more effectively restored in thePIL:RG-O than RG-O. However, when PIL/G-O ratio is above5, the surface resistivity increases to 23 kO/sq, indicatingthat an excess of PIL negatively affects the electrical con-nectivity of PIL:RG-O platelets. Therefore, the optimizedPIL/G-O ratio was set as 5 and further experiments werecarried out.

3.2. Hybridization of PIL-modified reduced graphene oxideand conductive polymer

In this section, we describe the preparation and charac-terization of PEDOT-PIL:RG-O composites by in situ poly-merization of EDOT (Fig. 1, step 2), in which EDOT ispolymerized in the presence of PIL:RG-O. The driving forcefor the formation of hybridized PEDOT and PIL:RG-O islikely due to electrostatic attraction between oppositecharges. Since the growing PEDOT chains have positivecharge and PIL carries a negative charge by NTf�2 anions,it is conceivable that PIL on the RG-O surface function asa charge balancing stabilizer to the positively doped PEDOTchains during the polymerization process.

Fig. 8a shows the FT-IR spectra of the PEDOT-PIL:RG-Ocomposite and pristine PEDOT-PIL. The characteristicbands for the PEDOT-PIL:RG-O composites appear in therange of 500–4000 cm�1, showing almost the same num-bers and positions of the main peaks as in the pristine PED-OT:PIL; C–C stretching in the thiophene ring (1350 cm�1),vibration modes of the ethylenedioxy group (1207 and1093 cm�1), S–O stretched vibration (988 cm�1) and vibra-tion modes of the PIL(NTf�2 ) (1790, 1400 and 780 cm�1).However, there were two additional peaks at 1725 and1634 cm�1, corresponding to the stretching modes of thearomatic C=C in RG-O sheets.

The chemical composition of the PEDOT-PIL:RG-O com-posites were also investigated by XPS spectra. The C 1s XPSspectrum (Fig. 8b) of the composite film can be deconvo-luted into four main components corresponding to carbonatoms in different functional groups: the non-oxygenatedring C (284.9 eV), the C–O (286.4 eV), C–S (287.7 eV) andthe C=O (289.5 eV). Moreover, the S 2p core-line spectra(Fig. 8c) showed the peak at 168.2 eV (NTf�2 anion in thePIL dopant), and the lower binding energy peaks at 165.3and 164.1 eV (spin-split doublets of sulfur atoms in PEDOTbackbone), which confirms the existence of PEDOT.

SEM was used to observe the fine morphological struc-ture of the composites. Fig. 9 shows representative SEMimages of PEDOT-PIL:RG-O. In the top-view image(Fig. 9a), the conducting polymer formed a continuousphase with the RG-O submerged in the polymer phase,while cross-section image (Fig. 9b) shows stacked RG-Osheets of which their surfaces are likely to be covered withPEDOT. A TEM image of the composite sample (Fig. 9c)

displays that each RG-O sheet is encapsulated by PEDOT.The PEDOT attachment on the RG-O sheet was also con-firmed by the EDX analysis (Fig. S-3), in which the sulfurfrom the PEDOT was found. AFM image (Fig. 9d) alsoreveals that PEDOT-PIL:RG-O is composed of thin platelets

Fig. 9. (a) Top-view, (b) cross-section SEM image, (c) TEM image of PEDOT-PIL:RG-O composites, and (d) AFM image of PEDOT-PIL:RG-O composites.

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with some wrinkles. The large surface area of RG-O wascovered by a thin layer of PEDOT with the total thickness10–15 nm.

In order to investigate the role of PIL:RG-O on theelectrical properties of conducting polymer composites,slab samples were prepared by solvent casting PEDOT–PIL:RG-O suspension onto the filter, followed by drying at80 �C for 2 h and further annealing at 200 �C for 3 h underargon atmosphere. The conductivity of sample was calcu-lated from the value of sheets resistivity and thickness(measured by SEM). As shown in Fig. 10a, while thePIL-modified PEDOT sample (PEDOT:PIL without RG-O)showed a conductivity of 0.92 S/cm, PEDOT-PIL:RG-O hy-brid composite showed enhanced conductivity values ofup to 20 S/cm with increasing PIL:RG-O contents in thecomposites. The conductivity value of the composite filmincreases sharply with increasing PIL:RG-O loading levelup to 0.3 wt.%, and then saturates over PIL:RG-O concen-tration, which gives an estimation that a percolationthreshold is in the proximity of 0.1 wt.%. This percolationthreshold value is comparable to the values previously re-ported in graphene/polystyrene composites [29,30]. More-over, the enhancement of conductivity observed for thiscomposite is consistent with other graphene/conductingpolymer composites including PEDOT/G-O composites

(5 S/cm) [31], polypyrrole/GO composites (7 S/cm) [32],graphene/polyaniline composites mediated by PIL (7.5 S/cm) [33], and graphene/polyaniline nanofiber compositesamples (16.8 S/cm) [34]. Such enhancement could be ex-plained by the better electrical connectivity between theRG-O platelets afforded by interconnecting PEDOT chains.

To evaluate the possible application of hybrid compos-ite as a transparent electrode, 1.3 wt.% of PEDOT-PIL:RG-O composites (with 0.3 wt.% of RG-O in the composite)was dispersed in AN and coated on PET film to form 500-nm-thick conductive layer. This film showed a surfaceresistivity of 1.5 � 104 O/sq with light transmittance of86.0% at 550 nm. Considering the optical transmittance ofPET film itself 87.6%, PEDOT-PIL:RG-O caused a transmit-tance loss less than 2%. Moreover, upon treatment of SOCl2

solution for 1 h the PEDDOT-PIL:RG-O composite filmshows further decrease in the surface resistivity to5.3 � 103 O/sq. On the other hand, the hybrid compositescan be spin-coated onto glass substrate to form a homoge-neous conductive composite film with a thickness of 50 nmlayer as shown in Fig. 10b. The PEDOT-PIL:RG-O film wasshowed a surface resistivity of 9.6 � 104 O/sq with a trans-parence of 91.18%. Subsequently, the sample was annealedat 200 �C for 3 h to remove the residual solvent andoxygen-containing functional groups from RG-O. After

Fig. 10. (a) The electrical conductivity of PEDOT-PIL:RG-O compositefilms as a function of PIL:RG-O content in the composites. (b) Aphotograph of PEDOT-PIL:RG-O on glass substrate using a spin-coatingmethod with KU logo underneath. And (c) sheet surface resistance andtransmittance varies with different thicknesses of films.

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annealing we found that the surface resistivity of film de-creased from 9.6 � 104 to 1.8 � 104 O/sq without any sig-nificant change in light transmittance. In Fig. 10c, thesurface resistivity and transmittance varies with thick-nesses of films. As the film thickness increases, the surfaceresistivity decreases at the expense of light transmittance.For example, a surface resistivity of 800 O/sq can be ob-tained with 150 nm film thickness, but transmittance

was decreased to 86.8%. The transparent and conductiveproperties of the present film are consistent with other re-ports involving graphene/poly(3,4-ethylenedioxy thio-phene) composite films [35,37], RG-O thin films [38–42],and lower than that of the aqueous dispersible graphene/PEDOT:PSS composite electrode in term of conductivity[43,44]. However, at the same time, the hybrid materialsdescribed here have additional advantages in that it is dis-persible in a range of organic solvents and free of waterwith no acidic moieties like PSS, which may be valuablefor its long-term utilization in many practical applicationssuch as organic light emitting diodes and organic photo-voltaic cells [45,46].

The thermal stability of the PEDOT-PIL:RG-O compos-ites was also investigated and compared with that of pris-tine PEDOT:PIL as shown in Fig S-4. The PEDOT:PIL showedan onset degradation temperature at 298 �C and a weightloss was�78%. On the other hand, the PEDOT-PIL:RG-O hy-brid composite showed the decomposition temperature of312 �C, with the weight loss of �54%. These results suggestthat the coupling of PIL:RG-O and PEDOT could delay thethermal degradation of PEDOT chains due to a strong inter-action between PEDOT matrix and PIL:RG-O. Therefore, theadvantage of this method for fabricating RG-O/conductingpolymer is that PIL-modified RG-O not only enhances thesolubility of reduced graphene oxide, but also functionsas ionic stabilizers and/or counterions of conducting poly-mer to facilitate the formation of homogeneous compos-ites on the molecular scale.

4. Conclusions

In this work, we have demonstrated hybridization ofgraphene/PEDOT composite with an aid of PIL. Here, PILcoating on the surface of the RG-O platelets plays dualroles by increasing the solubility of RG-O in organic sol-vents and providing effective anchoring sites to bond withPEDOT chains. It was found that this results in good electri-cal conductivity due to inter-sheets enhanced carrier hop-ping. This non-acidic, organic solvent dispersible PEDOT-PIL:RG-O hybrid composites holds a great potential forapplication in optoelectronic devices especially wheretransparency, electrical conductivity, and stability proper-ties are required.

Acknowledgements

This work was supported by InsCon Tech. Co. Ltd.,Korea, and Korea Ministry of Environment as the Eco-Innovation Project (Global-Top Project).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.orgel.2011.09.012.

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