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Nanocomposites of reduced graphene oxide nanosheets and conductingpolymer for stretchable transparent conducting electrodes†

Young Gug Seol,‡a Tran Quang Trung,‡a Ok-Ja Yoon,a Il-Yung Sohna and Nae-Eung Lee*ab

Received 19th June 2012, Accepted 19th September 2012

DOI: 10.1039/c2jm33949h

Nanocomposites of functionalized reduced graphene oxide (FR-GO) nanosheets and a conducting

polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), were investigated

for their applicability as stretchable transparent conducting electrode (TCE) materials. Adding FR-GO

nanosheets functionalized by a surfactant, phenyl isocyanate, through the reduction of functionalized

GO nanosheets into the PEDOT:PSS solution was an effective way of increasing the dispersibility of R-

GO and, in turn, the electrical conductivity of the nanocomposite thin layers. A sheet resistance as low

as �68 U ,�1 and optical transmittance of �86% at the wavelength of 550 nm was obtained for the

nanocomposite layer with FR-GO well-dispersed in the PEDOT:PSS matrix. A patterned

nanocomposite electrode formed on a pre-stretched polydimethylsiloxane substrate showed potential

applicability as an excellent TCE for transparent and stretchable electronics.

Introduction

With the advent of transparent, flexible and stretchable elec-

tronics, transparent conductive electrodes (TCEs) with

mechanical flexibility and stretchability, light weight, scalability,

low cost, high optical transparency and electrical conductivity

are one of the holy grails in new-concept smart digital elec-

tronics.1–4 Indium tin oxide (ITO) thin films have widely been

used as TCEs in conventional flat panel displays, solar cells, and

touch panels.5 ITO coated on a glass substrate has excellent

electrical and optical properties, with a sheet resistance less than

30 U,�1 at an optical transmittance of�90%. ITO films coated

on flexible plastic substrates typically show sheet resistances of

30–50 U ,�1 at an optical transmittance of �85%. With

increasing demand for ITO in modern digital electronic devices,

the cost of ITO is expected to increase continuously due to

limited resources. Furthermore, for new applications in flexible/

stretchable electronics mechanically flexible or stretchable TCEs

on flexible or stretchable substrates are essential because ITO

thin films under mechanical bending or deformation are easily

susceptible to cracking.6 As such, the tremendous demand for

alternative TCEs to replace ITO for newly developing flexible

aSchool of Advanced Materials Science and Engineering, SungkyunkwanUniversity (SKKU), Suwon, Kyunggi-do 440-746, Korea. E-mail:nelee@skku.edubSKKU Advanced Institute of Nanotechnology (SAINT) and SamsungAdvanced Institute of Health Sciences and Technology (SAIHST),Sungkyunkwan University (SKKU), Suwon, Kyunggi-do 440-746, Korea

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2jm33949h

‡ These authors contributed equally to this work.

This journal is ª The Royal Society of Chemistry 2012

and stretchable electronics has sparked a furious world-wide race

in the last decade.3,7–15

The search for alternative flexible/stretchable TCEs to replace

the rigid ITO is still under progress due to limited optoelectronic

properties of various candidates, including conductive polymer

coatings,16 carbon nanotube films,17,18 films of graphene or net-

worked reduced graphene oxide,19–22 metal gratings,23 and

metallic nanowire networks.24 Among the many candidates for

flexible/stretchable TCEs, films of carbon nanomaterials,

including carbon nanotubes (CNTs),7 graphene,10,11 networked

reduced graphene oxide (R-GO) nanosheets,25 and nano-

composites of CNTs and R-GOs incorporated into conducting

polymers,3,12–15 are very promising due to their mechanical flex-

ibility and lower cost of manufacture. However, achieving elec-

trical conductivity, mechanical flexibility/stretchability, and

optical transparency simultaneously applicable to transparent

and stretchable electronics is still challenging.

Single-walled CNT films with acid treatment showed an

excellent sheet resistance of �70 U ,�1 at the optical trans-

mittance of �80%.4 These values are quite close to those of

commercial ITO on a flexible substrate. Another approach for

obtaining low sheet resistance at high optical transmittance is to

increase the electrical connectivity of CNTs by incorporating

them into conductive polymers such as poly(3,4-ethyl-

enedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).26

Nanocomposite films of CNT and PEDOT:PSS films typically

showed sheet resistance of well over 500 U ,�1 at an optical

transmittance of �80%,26 while well-prepared PEDOT:PSS films

show sheet resistance of over 500 U ,�1 at an optical trans-

mittance of �80%.27,28 Recently, a significant enhancement was

reported in the electrical performance of SWCNT-PEDOT:PSS

nanocomposite films formed by dip coating on a plastic

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substrate, with a sheet resistance of 66 U ,�1 at the optical

transmittance of �80%.29 It was indicated that increasing the

electrical junction between high conductivity CNTs by con-

ducting polymers can be an excellent approach for producing

highly conductive and flexible TCEs with optoelectronic

performance comparable to ITO films. However, the cost of

producing CNTs is still high, even though solution processing

can be utilized.

Recently, graphene, a two-dimensional carbon nano-scale

material, has shown excellent optoelectronic properties such as

extremely high mobility, high elasticity, and high optical trans-

mittance,30,31 which enables graphene films to be used in TCEs.

Optoelectronic properties superior to those of ITO were achieved

using graphene on a large-area flexible substrate formed by roll-

to-roll transfer of a high-temperature-grown graphene layer on a

Cu substrate.22,32,33 It was reported that with chemical doping of

a four-layer graphene film, a sheet resistance of 30 U ,�1 at the

optical transmittance of 90% was obtained.33 This is a major

breakthrough in the search for flexible TCEs. However, the

complexity of the fabrication processes and the additionally

required patterning might limit wide-spread use of the dry-

transferred graphene film due to the potentially high cost. In this

respect, solution processing of reduced graphene oxide (R-GO)

nanosheets chemically derived from graphite in the form of thin

films is very interesting for generation of high-performance

flexible/stretchable TCEs. However, the properties of solution-

processed R-GO films showed limited optoelectronic properties,

with 102� 2� 103U,�1 sheet resistance at the transmittance of

�80% due to high junction resistance between R-GO

sheets.19,20,34 Hybrid films of graphene and CNTs also showed a

sheet resistance of 240 U ,�1 at the transmittance of �80%.35

Nanocomposite films of R-GO/conducting polymer with a sheet

resistance of over 106 U ,�1 at the transmittance of �80% were

still of poor quality due to insufficient dispersion of R-GO sheets

in the conducting polymer matrix.12 Additional advantages of

Fig. 1 Graphical illustration showing the process of n

23760 | J. Mater. Chem., 2012, 22, 23759–23766

nanocomposites of R-GO and polymers include the improve-

ment of mechanical and thermomechanical properties compared

to the conventional conducting polymers,15,36,37 which can

provide enhanced performance in the flexible/stretchable appli-

cations of solution-processable nanocomposite-based TCEs.

In this work we report that improved dispersion of R-GO

nanosheets functionalized by a surfactant into PEDOT:PSS, a

conducting polymer, enhanced the optoelectronic properties of

the nanocomposites significantly. Achieving high optoelectronic

performance of nanocomposites and enhancing the dispersion of

R-GO nanosheets in the conducting polymer matrix critically

requires enhanced inter-connectivity of R-GO nanosheets in the

conducting polymer matrix, as mentioned. In our approach, GO

nanosheets were first functionalized by a surfactant, phenyl

isocyanate, and then the reduction of functionalized GO (FGO)

was followed to obtain functionalized R-GO (FR-GO) nano-

sheets. Phenyl isocyanate has been utilized for the functionali-

zation of GO because it provides good dispersibility of FGO

nanosheets in polar aprotic solvents.38 As a result, a sheet resis-

tance of �68 U ,�1 at the optical transmittance of 86% and the

wavelength of 550 nm was obtained for the nanocomposite films

with the well-dispersed FR-GO in the PEDOT:PSS matrix,

which makes the solution-processed nanocomposite suitable for

various TCE applications. Patterned nanocomposite electrodes

formed on pre-stretched PDMS (polydimethylsiloxane) substrate

showed potential applicability as a stretchable TCE on a

stretchable substrate.

Experimental details

Preparation of graphene oxide (GO) nanosheets

Fig. 1 illustrates the fabrication process of nanocomposite

materials (FR-GO/PEDOT:PSS) of functionalized reduced gra-

phene oxide (FR-GO) and PEDOT:PSS conducting polymer.

anocomposite (FR-GO/PEDOT:PSS) formation.

This journal is ª The Royal Society of Chemistry 2012

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Firstly, graphene oxide (GO) nanosheets were synthesized from

purified natural graphite sheets (purchased from Sigma Aldrich)

by the modified Hummers method.39 H2SO4 (50 ml) was added to

a beaker filled with graphite sheets (2 g) at room temperature.

Then, KMnO4 powder was added slowly in an ice bath at 0 �C.The mixture was stirred by a magnetic stirrer bar for 2 h on a

heating plate at 35 �C. Distilled water (700 ml) was added to the

mixture slowly at 0 �C (ice bath). After adding DI water (920 ml),

the mixture was stirred for 2 h and then diluted with DI water

three times. 30 wt% H2O2 (in water from Sigma Aldrich) (50 ml)

was added to the mixture. At the end of this process, the mixture

had a bright yellow color with bubbling. The mixture was kept

for a day after the removal of the supernatant. The remaining

mixture was centrifuged and washed with 10 wt% HCl solution

(1 l) to remove the acid solution. The mixture was passed through

a filter with a 0.2 mm pore diameter, and dried on a Teflon film

heated at 80 �C. In general, the size of GO was randomly

distributed between 200 and 500 nm. The thickness of the GO

sheet was approximately 1 nm.

Functionalization of R-GO nanosheets

Reduction of GO nanosheets causes severe agglomeration in

organic solvents due to p–p interaction between R-GO nano-

sheets, which makes dispersion of R-GO nanosheets in a solution

of PEDOT:PSS very difficult. Furthermore, improving disper-

sion of R-GO nanosheets in the matrix of conductive polymer is

limited because of the difficulty of direct functionalization of

R-GO nanosheets due to the lack of surface functional groups

after the reduction of GO. As shown in Fig. 1, therefore, the

functionalization of GO nanosheets followed by the reduction of

functionalized GO nanosheets was carried out to minimize the

agglomeration of functionalized GO (FGO) nanosheets in the

polymer matrix. Since GO nanosheets were difficult to disperse in

organic solvents such as dimethylsulfoxide (DMSO), GO nano-

sheets were first chemically functionalized for good dispersion.

To prepare the FGO nanosheets, GO (50 mg) was dispersed in

anhydrous N,N-dimethylformamide (DMF) (5 ml) by sonication

for 24 hours to create an inhomogeneous suspension of GO

nanosheets. For production of FGO, phenyl isocyanate

(C6H5NCO, 0.5 ml) was added and stirred for 24 hours with a

magnetic stir bar. After 24 h of reaction, the reactant was poured

into dichloromethane (50 ml) to coagulate the product. After

that, the product was washed several times with additional DCM

(dichloromethane) (50 ml), and dried under vacuum. The

thicknesses of the FGO nanosheets produced were about 1 nm

on average, with a lateral dimension of several hundred nano-

meters.40 Chemical reduction of the FGO was carried out in

anhydrous DMF with hydrazine hydrate (24–26%) for 24 h at

100 �C with stirring by a magnetic stir bar. Finally, the product

of functionalized reduced graphene oxide (FR-GO) nanosheets

was washed with DCM (50 ml) and dried under vacuum.

Fabrication of FR-GO/PEDOT:PSS nanocomposite films

Nanocomposite solutions of FR-GO nanosheets and

PEDOT:PSS with different concentrations of FR-GO (0–18 mg/

10 ml) were fabricated. The PEDOT:PSS solution was purchased

from H.C. Starck (CLEVIOS� PH1000) with a suggested

This journal is ª The Royal Society of Chemistry 2012

maximum conductivity of �900 S cm�1 for spin-coated films

(after addition of 5% of DMSO). The weight ratio of PEDOT to

PSS was 1 : 2.5. The dried FR-GO nanosheets of various

concentrations (0–18 mg/10 ml) were dispersed into DMSO

(5 ml) by sonication for 1 h to create a homogeneous suspension

of FR-GO nanosheets in DMSO. The suspension of FR-GO

nanosheets in the DMSO solvent was quite stable. Then

PEDOT:PSS (10 ml) solution was added to 5 ml of FR-GO

dispersion in the DMSO solvent, and the mixed solution was

stirred until dissolution was complete. Finally, isopropanol (2

ml) was added to the composite solution of PEDOT:PSS and

FR-GO for improving the wetting properties for spin coating.

The transparent conducting films of FR-GO/PEDOT:PSS

composite were prepared by spin-coating onto transparent glass

and polyethylene terephthalate (PET) or PDMS substrate. After

the spin-coating of the FR-GO/PEDOT:PSS nanocomposite, the

samples were cured at 150 �C for 1 h to remove the residual

solvent. For stretchability of the nanocomposite films, the

nanocomposite solution was spin-coated on pre-strained PDMS

substrate, and the spin-coated nanocomposite films were cured at

150 �C for 1 h.

For characterization of the GO, FGO, and FR-GO nano-

sheets, Fourier-transformed infrared (FT-IR) spectroscopy

(Bruker, IFS-66/S) was carried out. The sheet resistance was

measured for electrical characterization of the nanocomposite

films using a four-point probe. The optical transmittance of

the spin-coated films on the glass substrate was measured by a

UV-vis spectrometer (Scinco, S-3100). Changes in the electrical

resistance of the PEDOT:PSS and nanocomposite films on the

pre-strained PDMS with the ‘‘effective’’ strains of 10 and 15%

were measured in stretching mode by varying the distance

between the electrical contact pads in a custom-built stretching

tester. Electrical contact pads were formed by evaporating Au

(50 nm)/Cr (5 nm) on pre-strained PDMS substrates before spin-

coating of the PEDOT:PSS and nanocomposite films. For eval-

uation of the durability of the stretchable nanocomposite films,

cyclic stretching of the PEDOT:PSS and nanocomposite film on

the pre-strained PDMS with a maximum strain of 15% was also

carried out, and electrical resistances of the films were measured

by varying the bending cycles.

Results and discussion

Fig. 2 shows the FT-IR spectra of GO, FGO, and FR-GO

nanosheets in the powder form. FGO nanosheets obtained by

treating GO nanosheets with phenyl isocyanate were chemically

reduced to obtain FR-GO nanosheets [see the Experimental

section]. After the treatment of GO nanosheets by phenyl

isocyanate, the characteristic features in the FT-IR spectra of

FGO nanosheets were significantly different from those of the

GO nanosheets. The FT-IR spectra of GO apparently showed

C]O carbonyl stretching (1733 cm�1), aromatic C]C (1631

cm�1), O–H deformation vibration (1396 cm�1), and C–O

stretching (1053 cm�1).40 When GO nanosheets were treated with

phenyl isocyanate, the adsorption peak of the C]O stretching

vibration related to the carbonyl stretching vibration of the

carbamate esters of the surface hydroxyls in FGO nanosheets

was shifted to 1703 cm�1 in FGO, from 1733 cm�1 in GO.38 In

addition, the newly appeared stretch peak at 1646 cm�1 can be

J. Mater. Chem., 2012, 22, 23759–23766 | 23761

Fig. 2 FT-IR spectra of graphene oxide (GO), functionalized GO

(FGO), and functionalized reduced graphene oxide (FR-GO) nanosheets

in powder form.

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assigned to an amide carbonyl-stretching mode corresponding to

NH2 deformation. The new band at 1554 cm�1 is related to either

amides or carbamate esters, and corresponds to the coupling of

the C–N stretching vibration with the CHN deformation vibra-

tion.38 After reduction of the FGO nanosheets, the peak intensity

of amide and carbamate ester functional groups in the FR-GO

were decreased. However, the peak of NH3+ deformation and

C–N stretch corresponding to 1486 cm�1 and 1170 cm�1 still

remained, as shown in Fig. 2. The amide and carbamate ester

functional groups are believed to improve the dispersibility of

FR-GO in polar aprotic solvents such as DMSO (dimethyl

sulfoxide) and DMF.38 The reduction of hydrogen bonds caused

by the functionalization of GO with isocyanate treatment would

render the F-GO surfaces less hydrophilic, and the strength of

hydrogen bonding between the FGO nanosheets would be

attenuated, possibly allowing for improved dispersion in organic

solvents.38 Direct evidence of improved dispersibility of F-GO

nanosheets in DMSO solvent, compared to GO nanosheets, is

provided in ESI (Fig. S1†). The FGO nanosheets in the DMSO

solvent were not agglomerated while the GO nanosheets in the

DMSO solvent were agglomerated within a day of preparation.

The remaining functional groups in the FR-GO, after reduction

of GO nanosheets, presumably improve the dispersibility of FR-

GO nanosheets in organic solvent and in PEDOT:PSS solution

by reducing the p–p interaction between the FR-GO nanosheets.

To improve the electrical properties of PEDOT:PSS and

nanocomposite films coated onto glass substrates, the effects of

different organic solvents on the electrical properties of as-

prepared PEDOT:PSS and nanocomposite films were charac-

terized and compared using four different methods. It is well

known that the use of DMSO, a polar aprotic organic solvent,

improves the electrical conductivity of coated PEDOT:PSS films

significantly, because the DMSO solvent causes doping.41 To

improve the wetting property of the FR-GO/PEDOT:PSS

nanocomposites, 2 ml of IPA (isopropanol) was added. Table 1

shows the electrical conductivity and sheet resistance of the

four nanocomposite films fabricated. The total thickness of

all samples was kept at 110 nm. The electrical conductivity and

sheet resistance of the coated films made from commercially

23762 | J. Mater. Chem., 2012, 22, 23759–23766

available PEDOT:PSS used in this experiment were �50 S cm�1

and 457 U ,�1, respectively. As expected, the conductivity of

spin-coated PEDOT:PSS film is limited, and is lower than the

maximum value suggested by the manufacturer (�800 U ,�1),

because of the very thin nature of the coated film in our experi-

ments. After the addition of DMSO and IPA into PEDOT:PSS,

the electrical conductivity and sheet resistance were significantly

improved to 518 S cm�1 and 209 U ,�1, respectively.

To investigate and compare the effects of R-GO and FR-GO

incorporation in PEDOT:PSS on the electrical, optical, and

mechanical properties of the composites, two different nano-

composite solutions were prepared on the glass substrate, with

R-GO (5 mg) and FR-GO (5 mg) incorporated into 10 ml of

PEDOT:PSS solutions mixed with DMSO and IPA. For elec-

trical and optical characterizations of the nanocomposite films

with varying amounts of FR-GO, the nanocomposite film

thickness was kept at 110 nm. When the R-GO or FR-GO

nanosheets were incorporated into the PEDOT:PSS solutions,

the electrical conductivity and sheet resistance of spin-coated

nanocomposite films incorporated with R-GO and FR-GO

nanosheets were improved to 860 and 1010 S cm�1, and 129 and

95 U ,�1, respectively, compared to 50 S cm�1 and 457 U ,�1

of PEDOT:PSS films. In the case of R-GO/PEDOT:PSS nano-

composite films, partially agglomerated R-GO nanosheets were

clearly observed at the surface of the films. The agglomeration

phenomenon at the surface of the R-GO/PEDOT:PSS nano-

composite films caused larger surface roughness and non-

uniform thickness. In contrast, the greater enhancement in

electrical conductivity and sheet resistance values of the FR-GO/

PEDOT:PSS nanocomposite film compared to those of the

R-GO/PEDOT:PSS nanocomposite film was attributed to better

dispersion of FGO than that of GO nanosheets in the

PEDOT:PSS solution and, in turn, in the spin-coated nano-

composite film (see Fig. S2 and S3 in the ESI†).

For further investigation of electrical and optical properties

of FR-GO/PEDOT:PSS nanocomposite films, the amount of

FR-GO in the nanocomposite films was systematically varied,

and the effects of the FR-GO content on the properties of the

nanocomposite films were measured. Fig. 3 shows the effects of

the amount of FR-GO in PEDOT:PSS solution (10 ml) on the

sheet resistance and optical transmittance of nanocomposite

films spin-coated on the glass substrate (for the whole optical

spectrum of the nanocomposites, see the Fig. S4(a) in the ESI†).

As seen in Fig. 3, the optical transmittance of the films was

initially decreased with increase of the FR-GO amount.

However, it was not appreciably changed within the instrumental

resolution at the concentration range of 7–15 mg FR-GO in

10 ml PEDOT:PSS solution (see Fig. S4†) even though the

reason for this was not clearly understood. The sheet resistance

of nanocomposite films depends strongly on the amount of

FR-GO. As seen in Fig. 3, the sheet resistance of the nano-

composite films decreased gradually as the amount of FR-GO

was increased up to 10 mg. A sheet resistance of 68 U ,�1 and

optical transmittance (at 550 nm) of 86% were obtained for the

FR-GO (10 mg)/PEDOT:PSS (10 ml) nanocomposite film.

Reduction in the sheet resistance is caused by increased

networking of FR-GO nanosheets in the conducting polymer

matrix. However, the sheet resistance of the nanocomposite films

with amounts of FR-GO larger than 10 mg started to increase.

This journal is ª The Royal Society of Chemistry 2012

Table 1 The electrical conductivity and sheet resistance of fabricated four nanocomposite films. The layer thickness in all samples was kept at 110 nm

Sample no.

1 2 3 4

PEDOT:PSSPEDOT:PSS +DMSO + IPA

PEDOT:PSS + DMSO +IPA/RG-O composite

PEDOT:PSS + DMSO +IPA/FR-GO composite

Sheet resistance (U ,�1) 457 209 128.8 95.4Conductivity (S cm�1) 50.45 518 860 1010

Fig. 3 Sheet resistance and optical transmittance (at 550 nm) of the

PEDOT:PSS and the nanocomposite films prepared on the glass

substrate by varying the amount of FR-GO nanosheets in the

PEDOT:PSS conducting electrodes. The sheet resistance of the nano-

composite films is reduced significantly by incorporating FR-GO nano-

sheets, but increases again due to agglomeration at higher FR-GO

nanosheet contents. Increased incorporation of FR-GO gradually

reduces the optical transmittance.

Fig. 4 (a) Optical transmittance at 550 nm of the PEDOT:PSS (10 ml)/

FR-GO (10 mg) nanocomposite film on the glass substrate as a function

of film thickness. (b) Relationship between the optical transmittance and

electrical properties (sheet resistance and electrical conductivity)

obtained by varying the film thickness in (a).

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The reason for the increase in sheet resistance for additions of

FR-GO nanosheets above 10 mg is related to the limited dis-

persibility of FR-GO nanosheets caused by the incorporation of

a large amount of FR-GO in the PEDOT:PSS solution. In the

case of the nanocomposite solutions with 15 and 18 mg of

FR-GO, the FR-GO nanosheets could not be dispersed well

despite long sonication times. In the case of optical trans-

mittance, the increase in the FR-GO amount reduced the

transmittance value at the wavelength of 550 nm from 89% for

the PEDOT:PSS film to 81.5% for the FR-GO (18 mg)/

PEDOT:PSS (10 ml) nanocomposite film. Reduction of the

optical transmittance with the FR-GO contents is consistent with

the expectation that the increased amount of FR-GO nanosheets

in the PEDOT:PSS absorbs more light. From these results, the

content ratio between the PEDOT:PSS solution and the FR-GO

nanosheets is an important parameter in controlling the electrical

conductivity and optical transmittance. A compromise in the

amount of FR-GO nanosheets in the PEDOT:PSS film might be

necessary to obtain optimal performance in terms of electrical

and optical properties for applications of the nanocomposite

films to TCE materials.

The thickness of the TCE film is an important parameter

because the film thickness greatly affects the optical trans-

mittance and sheet resistance. To investigate the effect of the film

thickness on the optical transmittance, sheet resistance, and

This journal is ª The Royal Society of Chemistry 2012

electrical conductivity of the nanocomposite films, the FR-GO/

PEDOT:PSS nanocomposite films (10 mg/10 ml) were fabricated

by varying the film thickness on the glass substrate. The film

thickness was varied by changing the number of spin coating

processes. Fig. 4(a) shows the optical transmittance at 550 nm for

the UV-visible range of FR-GO/PEDOT:PSS nanocomposite

films with various thicknesses (for the whole optical spectrum of

the nanocomposites, see the Fig. S4(b) in the ESI†). When the

nanocomposite film thickness was increased, the optical trans-

mittance of the nanocomposite films decreased gradually, as

expected.

Based on the results of the transmittance of nanocomposite

films with various thicknesses in Fig. 4(a), the electrical

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conductivity and the sheet resistance were measured and plotted

as a function of the optical transmittance, as shown in Fig. 4(b).

A 110 nm thick nanocomposite film with 81% transmittance at

550 nm and sheet resistance of 71U,�1 was obtained. The sheet

resistance of our FR-GO/PEDOT:PSS nanocomposite film was

similar to that of the ITO film coated onto the PET substrate by

sputtering.6 The optical transmittance and sheet resistance of the

nanocomposite films obtained in this work are superior to other

reported values of 102� 2� 103U,�1 for sheet resistance at the

optical transmittance of �80%, which were obtained from

solution-processed R-GO films.4,5,8 The nanocomposite film

with the optical transmittance of 81% and the sheet resistance of

71 U ,�1 obtained in this work is expected to be applicable to

typical transparent electronics applications.

Solution-processable, printable, and stretchable TCE films are

very interesting. In order to examine the applicability of the

nanocomposite films to stretchable electronics, their electrical

properties under mechanical stretching were further measured,

and the effect of cyclic stretching on the stability of the

Fig. 5 (a) Optical image of the FR-GO (10 mg)/PEDOT:PSS (10 ml) nanocom

after release. (b) Electrical resistance of the nanocomposite film on the 10% ‘‘e

stretching. (c) Electrical resistance of the nanocomposite film on the 15% ‘‘ef

stretching. (d) Electrical and mechanical stabilities of nanocomposite film on t

cyclic stretching tests.

23764 | J. Mater. Chem., 2012, 22, 23759–23766

nanocomposite films was investigated. The nanocomposite film

of FR-GO (10 mg)/PEDOT:PSS (10 ml) was spin-coated onto

plasma-treated, pre-strained PDMS substrates to examine the

stretchability.42 For uniform coating of the film, the surface

hydrophilicity of the PDMS substrate was found to be critical.

The 20–30% pre-strained PDMS substrates were treated by O2

microwave plasma for 1 min. The PDMS substrates were uni-

axially pre-strained by 20 and 30% from the original length, but

they did not return to their original length after fabricating the

FR-GO/PEDOT:PSS nanocomposite thin film. Therefore, the

nanocomposite films produced smaller ‘‘effective’’ pre-strains of

10 and 15%, respectively, corresponding to the 20 and 30% pre-

strained PDMS substrates. These smaller strains generated in the

pre-strained PDMS substrate presumably come from increased

cross-linking or plastic deformation of the PDMS when exposed

to the heat (150 �C) required to anneal the substrate for curing

the FR-GO/PEDOT:PSS nanocomposite.42

Fig. 5(a) shows an optical image of the nanocomposite film

patterned on the 10% ‘‘effective’’ pre-strained PDMS substrate.

posite films formed on the 10% ‘‘effective’’ pre-strained PDMS substrate

ffective’’ pre-strained PDMS substrates was measured during mechanical

fective’’ pre-strained PDMS substrates was measured during mechanical

he ‘‘effective’’ pre-strained (15%) PDMS substrate was evaluated by using

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The FR-GO/PEDOT:PSS film pattern on the PDMS substrate

after the release of the pre-strained PDMS substrate exhibited

buckles in the nanocomposite films perpendicular to the axis of

pre-strain for accommodation of the pre-strain formed during

the preparation of the stretchable film. The formation of buckles

in the nanocomposite films occurs because of the compressive

force generated in the nanocomposite films after PDMS

substrate release.43–46 The mismatch in the thermal expansion

between the PDMS substrate and the FR-GO/PEDOT:PSS layer

may also affect the generation of the buckles.47 The morphology

properties of buckles in the nanocomposite films such as

uniformity, amplitude, and wavelength are expected to be

affected by film thickness, width of film pattern, adhesion

between film and substrate, and the elastic modulus and the

Poisson ratio of the film and the PDMS.43–46

The electrical resistance of the nanocomposite films on the 10

and 15% ‘‘effective’’ pre-strained PDMS substrates was

measured during mechanical stretching, and the results are

shown in Fig. 5(b) and (c), respectively. The data for the 10%

‘‘effective’’ pre-strained PDMS substrate in Fig. 5(b) indicate

that the electrical resistance of the film initially decreases with the

strain increased up to 10%, but starts to increase above the strain

of 10%. The electrical resistance of the nanocomposite film on the

15% ‘‘effective’’ pre-strained PDMS substrate in Fig. 5(c) initially

decreases with the strain increased up to the strain of 15%, but

starts to increase above the strain of 15%. The observed decrease

and increase in the normalized resistance are related to the pie-

zoresistivity of the FR-GO/PEDOT:PSS nanocomposite film.

The nanocomposite thin films on the released PDMS substrate

are under compressive strain exerted due to the difference

between the Young’s moduli of the nanocomposite film and the

PDMS substrate. Under elongation, the compressive strain in the

nanocomposite film is reduced, and therefore, the normalized

electrical resistance decreases. The decrease in the resistance is

mainly contributed by modulation of mobility in the FR-GO/

PEDOT:PSS nanocomposite.48 The FR-GO/PEDOT:PSS film

pattern on the PDMS substrate after release of the pre-strain

exhibited buckles in the nanocomposite which can be ascribed to

the non-flatness for buckled geometries. And the non-flatness of

the composite film can act as an additional source of scattering

for charge carriers (decrease in carrier mobility).49 Therefore,

under elongation, the non-flatness of the nanocomposite film is

reduced and reduction in scattering of charge carriers increases

carrier mobility, thus making the normalized electrical resistance

decrease. As seen in Fig. S5 in the ESI,† the resistance values of

the nanocomposite film before release of the pre-strained PDMS

and after elongation of the released PDMS to the nanocomposite

strain of 0% are nearly the same. However, after release of the

pre-strained PDMS giving a compressive strain of 5% in the

nanocomposite film, the resistance was increased. The reason for

the increase in the resistance of the nanocomposite film under

compressive strain is presumably attributed to the non-flatness of

the nanocomposite films causing increased surface scattering of

charges in micro-scale buckles. However, when the elongation is

increased over the ‘‘effective’’ pre-strains of 10 and 15%, the

tensile strain generated in the nanocomposite film causes an

increase in the electrical resistance. The observed increase in the

resistance of the tensile-strained nanocomposite film is ascribed

to less percolation of the FR-GO nanosheets inside the

This journal is ª The Royal Society of Chemistry 2012

nanocomposite films under tensile strain.50–52 Abrupt increases in

the resistance above the strains of 15 and 20% in Fig. 5(b) and (c),

respectively, are caused by the mechanical failure of the FR-GO/

PEDOT:PSS nanocomposite film.

The electrical and mechanical stabilities of the stretchable FR-

GO (10 mg)/PEDOT:PSS (10 ml) nanocomposite film spin-

coated on the ‘‘effective’’ pre-strained (15%) PDMS substrate

was evaluated using cyclic stretching tests. The results shown in

Fig. 5(d) indicate the changes in the electrical resistance of the

FR-GO (10 mg)/PEDOT:PSS (10 ml) nanocomposite film on the

15% ‘‘effective’’ pre-strained PDMS substrate cyclically stretched

up to the strain of 15%. The normalized electrical resistance was

not changed as the number of elongation cycles was increased to

60. The results indicate reasonable electrical and mechanical

stabilities after cyclic elongations.

Conclusions

In this work, we have demonstrated that the incorporation of

functionalized reduced graphene oxide (FR-GO) nanosheets into

a PEDOT:PSS conducting polymer matrix though the reduction

of functionalized GO (FGO) nanosheets in the PEDOT:PSS

solution is an effective way of improving the electrical properties

of the conducting polymer for stretchable TCE applications.

Improved dispersion of FR-GO nanosheets in the FR-GO/

PEDOT:PSS nanocomposite resulted in excellent electrical

conductivity and optical transparency that are close to those of

conventional ITO on flexible substrates. Solution-processed

nanocomposite films exhibited excellent sheet resistance as low as

68 U ,�1 and optical transmittance as high as 86% at the

wavelength of 550 nm. The stretchability tests of FR-GO/

PEDOT:PSS nanocomposite films on the pre-strained PDMS

substrate indicated a good stability under static stretching

conditions, as well as after cyclic stretching up to the strain of

15%. Solution processing of the nanocomposite TCE such as

printing can provide interesting applications in flexible and

stretchable electronics that are comparable to those in a

PEDOT:PSS film or networked films of R-GO nanosheets.

Acknowledgements

This research was supported by the Basic Science Research

Program (Grant no. 2010-0015035) and the WCU Program

(Grant no. R32-2008-000-10124-0) through the National

Research Foundation of Korea (NRF) funded by theMinistry of

Education, Science and Technology.

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