732

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Sung Ho Song , Kwang Hyun Park , Bo Hyun Kim , Yong Won Choi , Gwang Hoon Jun , Dong Ju Lee , Byung-Seon Kong , Kyung-Wook Paik , and Seokwoo Jeon *

Enhanced Thermal Conductivity of Epoxy–Graphene Composites by Using Non-Oxidized Graphene Flakes with Non-Covalent Functionalization

The thermal properties of materials are becoming increasingly important in tandem with the need for more effi cient heat removal in numerous systems such as LEDs, [ 1 ] CMOS, [ 2 ] and automotive [ 3 ] and aerospace [ 4 ] products. Light-weight, polymeric nanocomposites are promising candidates to address heat dis-sipation problems. For example, excellent thermal conductivity ( ∼ 3,000 W/mK) of carbon nanotubes improves heat transports when they are mixed in composites. [ 5 ] However, graphitic nano-material fi llers, which have extremely high thermal conductivity with low specifi c weight, do not show expected improvement of thermal conductivities, mostly due to poor dispersion and dis-continuous polymer/nanomaterial interfaces. [ 6 , 7 ] Consequently, the search for advanced polymeric nanocomposites has been intensifi ed and fundamental studies of thermal conduction in nanoscale materials have been prompted.

Novel two dimensional (2D) graphene fl akes (GFs) have drawn interest from researchers due to their extreme thermal conductivity ( ∼ 5,300 W/mK) through phononic transport. [ 8–10 ] Their potential use in polymer composites for thermal man-agement and thermal conductivity enhancement have been demonstrated. [ 11–15 ] Haddon and coworkers reported a thermal conductivity of 6.44 W/mK from a polymer composite with ∼ 25 vol% of graphite nanoplatelets (GNPs). GNPs, few graphene layers prepared from acid treatment for intercala-tion and then exfoliated via thermal shock, provide excellent thermal enhancements when embedded in an epoxy matrix. [ 16 ] Ganguli and coworkers dispersed a functionalized graphene oxide (GO) by silane into epoxy resin, and showed thermal con-ductivity reaching 5.8 W/mK in resin containing 20 wt% GO. [ 17 ] At comparable loading levels, the non-oxidized GFs will provide more effective fi llers than GO for producing polymeric nano-composites with high thermal conductivity.

However, the highly cohesive van der Waals energy (5.9 kJ/mol) of non-oxidized GFs makes it impossible to form well dispersed graphene polymer composites. [ 18 ] Although

wileyonlinelibrary.com © 2013 WILEY-VCH Verlag G

S. H. Song, K. H. Park, B. H. Kim, Y. W. Choi, G. H. Jun, D. J. Lee, Prof. S. Jeon, K.-W. PaikDepartment of Material Science and EngineeringKorea Advanced Institute of Science and TechnologyDaejeon 305-701, Republic of Korea E-mail: jeon39@kaist.ac.kr Dr. B.-S. KongKCC Central Research InstituteGyeonggi-do, 446–912, Republic of Korea

DOI: 10.1002/adma.201202736

graphene oxide fl akes exfoliated from graphite can address the dispersion issues, [ 19 , 20 ] they undergo signifi cant losses in thermal conductivity even after reduction due to defects and disruption of the π -orbital structure during oxidation. To resolve these problems, several methods such as solvent- [ 21 ] or surfactant-assisted exfoliation [ 22 , 23 ] and use of a graphite inter-calation compound (GIC) have been suggested for the fabrica-tion of non-oxidized GFs. [ 24–27 ] Still, the poor dispersion of the resultant GFs in polymer matrix should be improved to achieve better composite properties.

Our group recently introduced a novel GIC approach to gen-erate GFs by using a ternary eutectic system of alkali salts and fabricated high quality GFs with extremely low oxygen content of 2.9% (from a raw graphite with oxygen content of 2.1%). Even though the highly transparent conducting ( ∼ 930 Ω / � at ∼ 75% transmission) fi lms were produced through non-oxidized GFs, non-oxidized GFs remained dispersed only in a pyridine-salts solution. [ 27 ] Eliminating the alkali metal salts from the solution while preserving the highly dispersive and excellent material properties of the non-oxidized GFs is a key challenge for broad applications of nanocomposites and fi lm formation. One pos-sible route is non-covalent functionalization. Very recently, non-covalent functionalization of CNTs [ 28 , 29 ] and reduced graphene oxide (RGO) [ 30 ] with π – π interaction have been suggested, and it was shown that the GFs dispersed in diverse solvents without degrading the physical and chemical properties. Here, we sug-gest a method for obtaining noncovalently functionalized GFs ( f -GFs) with 1-pyrenebutyric acid (PBA) that are highly soluble in various solvents, and these GFs were used to synthesize an epoxy–graphene composite showing enhanced thermal conduc-tivity. We have developed a novel approach to prepare the f -GFs with the most effective and nondestructive method using the non-oxidized, low-defect GFs. The f -GFs were used as fi llers for the fi rst time in polymeric nanocomposites that exhibited improved thermal conductivities. When embedded in a polymer matrix, the f -GFs demonstrated remarkable enhancement of the thermal conductivity at low loadings. The incorporation of f -GFs into epoxy resin yielded 1.53 W/mK thermal conductivity at 10 wt% and a modulus of 1.03 GPa at 1 wt%. The matrix properties including the thermal conductivity and mechanical properties provided by the f -GFs are much higher than those of previous nanomaterials.

Figure 1 a shows the GFs dispersed in pyridine after exfolia-tion from graphite through a previously reported method. [ 27 ] Sequentially, the f -GFs are fabricated via PBA treatment and dispersed in various solvents (Figure S1). Figure 1 b shows the

mbH & Co. KGaA, Weinheim Adv. Mater. 2013, 25, 732–737

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 1 . Schematic diagram showing the overall processing required for the f -GFs and f -GFs-nanocomposites: a) GFs using ternary eutectic system of the alkali salts and digital photography image of dispersed f -GFs in pyridine. b) Non-covalent functionlized GFs by PBA and digital photograph image of dispersed f -GFs in acetone. c) Mixing epoxy resin, Curing Agent, and f -GFs through sonication. d) Curing process for the fabrication f -GFs-nanocomposites for 1h at 175 ° C and Digital photograph image of f -GFs–Nanocomposites.

dispersed f -GFs in acetone and a schematic illustration of the functionalized state of GFs in the solution. The prepared f -GFs are around 200nm in size and less than 5 nm in thickness, as deter-mined from AFM images and a statistical analysis (Figure S2). Finally, a f -GF nanocomposite with an epoxy polymer is synthesized through solution mixing and a curing process (Figure 1 c). The mixture of homogeneously dispersed f -GFs with epoxy is casted into a stainless steel mold and then cured at 70 ° C in a vacuum for 12 hours. The composite is subjected to an additional curing process at 175 ° C for 1 hour. Figure 1 d shows a disk shaped nanocomposite used for measuring thermal proper-ties and a schematic illustration of the nanocomposite structure. The disc-shaped nanocomposites have surface area of ∼ 5.07 cm 2 and thickness of 2 mm (See details in Experimental Section).

The surface functionalization of GFs is verifi ed by spectro-scopic methods and TGA. Figure 2 a presents a comparison of UV–vis absorption spectra of PBA, GFs, and f -GFs dispersed in solution. Three characteristic absorption peaks of PBA in acetone, marked by arrows, are observed in the spectrum of f -GFs with a slight shift, and these peaks do not appear in the spectrum of the GFs. Also, in the FT–IR spectra of both PBA and f -GFs three characteristic peaks related to C–O stretching, C–OH stretching, and C = O stretching in the carboxylic acid and carbonyl moie-ties are seen, although these peaks in f -GFs are relatively shifted (Figure 2 b). There is another peak appearing at 2930 cm − 1 which originates from C–H bonds in alkyl chains. The peaks observed in the FT-IR spectra of PBA and f -GFs do not appear in the

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, 25, 732–737

spectrum of the GFs. These results show that the surface of the GFs is functionalized by PBA. Also, the shift of characteristic peaks of f -GFs in both the UV–vis and FT-IR spectra might be attributed to π – π stacking between six-membered carbon rings of GFs and pyrene molecules of PBA. [ 30–33 ] The π -interaction results in a signifi cant charge transfer effect, which is confi rmed by Raman spectroscopy (Figure S3). [ 34–36 ] In addition, the fl uo-rescence of PBA at 530 nm is completely quenched after func-tionalization on the surface of the GFs (Figure S4). [ 30 , 31 , 37 ] These results also support the noncovalent PBA functionalization of GFs through π – π stacking on the surface.

The chemical bonding of carbon of f -GFs is further exam-ined by the XPS results (Figure 2 c). The C1s signal consisted of four different peaks: the C–C bond (284.5 eV) of sp 2 carbon in the basal plane of f -GFs, a C–O bond (286.6 eV), C = O groups (288.2 eV) of the carbonyl group, and the OH-C = O bond (290.1 eV) of carboxylic carbon. Compared with the Cls peak from GFs, carbon-oxygen bonding peaks mostly originated from the carboxylic moieties of PBA. The wide XPS scanning of a thin fi lm shows no other element content, suggesting that most of the eutectic salts were washed out during the fi ltration step using water (Figure S5). From the TGA results for the f -GFs up to 900 ° C, the quantity of functionalized PBA (Figure 2 d) is estimated to be ∼ 13.7%.

The dispersion properties of f -GFs were evaluated with 12 solvents for a period of more than two weeks (Figure S1). Figure 3 a presents the Lambert-Beer behavior, [ 22 , 38–40 ] showing

733wileyonlinelibrary.commbH & Co. KGaA, Weinheim

734

www.advmat.dewww.MaterialsViews.com

wileyonlinelibrary.com

CO

MM

UN

ICATI

ON

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2 . a) UV–vis spectra results for PBA, GFs and f -GFs suspension. The spectrum shows π – π stacking between PBA and GFs. b) FT-IR spectra of PBA, GFs and f -GFs powders. The PBA, GFs and f -GFs powders were dispersed in KBr discs. c) C1s XPS spectra of the f -GFs on silicon oxide. The f -GFs spectra were deconvoluted into four peaks of sp 2 , COOH, C–O, and C = O. d) Thermal gravimetric analysis (TGA) of PBA, GFs and f -GFs in the temperature range from 0 ° C to 900 ° C at heating rate of 10 ° C/min under N 2 fl ow.

Figure 3 . a) Optical absorbance slopes at excitation wavelength 660 nm as a function of the f -GFs concentration in the each solvent showing Lambert-Beer behavior. f -GFs have a uniform dispersion and different dispersion properties in diverse solvents. Through the straight line fi t, acetone displays the best dispersing ability. b) Dispersibility of f -GFs in the tested solvents and their Hansen solubility parameter. c) Concentration remaining as a function of sedimentation tie for f -GFs dispersion in acetone over the course of 17 days. The fi t constants indicate that 49 wt% of the sample is stable over the 17 days. d) Zeta potential of f -GFs dispersions in acetone as a function of pH (adjusted by addition of 1 M HCl and NaOH). The formation of stable f -GFs colloids should be attributed to electrostatic repulsion through the ionization of carboxylic acid.

Adv. Mater. 2013, 25, 732–737

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 4 . a) SEM images of fracture surfaces of the f -GFs-nanocomposites. Isolated f -GFs were homogeneously dispersed in the polymer matrix. b) DMA storage modulus curves and tan δ curves of the epoxy nanocomposites with GO and f -GFs at 1 wt%. The mechanical properties of f -GFs com-posites are enhanced by improving the interfacial interaction between f -GFs and polymer matrix and better dispersion of f -GFs within a polymer matrix. c) Thermal conductivity of epoxy nanocomposites with C/B, graphite, MWNT, GO and f -GFs. The highest value of thermal conductivity is 1.53 W/mK at a 10 wt% loading of f -GFs in epoxy. d) Digital photograph image of water evaporation above pure-epoxy and f -GFs–nanocomposites.

different slopes of each suspension of dispersed f -GFs in diverse solvents. The correlation between absorbance ( A ) and concen-tration ( c ) measured by UV–vis shows that the suspension in acetone provides the highest absorption intensity, suggesting that acetone is the best dispersing solvent of f -GFs. Figure 3 b shows the concentration of f -GFs as a function of δ p + δ h , where δ p and δ h are the polarity cohesion and the hydrogen bonding cohesion, used as Hansen solubility parameters. [ 41 ] The Hansen parameters have been used to predict the solubility of fuller-enes, [ 42 ] single-walled carbon nanotubes, [ 43 ] and graphene [ 41 , 44 ] . For graphene, they are 18.0, 9.3, and 7.7, respectively. [ 41 ] The value of δ p + δ h is 17.0, which is consistent with our results in acetone. However, the f -GFs in this experiment were not only dispersed in solutions with δ p + δ h of 13 ∼ 29, in accordance with the fi ndings of Ruoff et al, [ 41 ] but also well dispersed in methanol and water with δ p + δ h > 29 (Table S1). This disper-sion property of our f -GFs could be ascribed to the exposed–COOH groups of PBA, which noncovalently functionalized the surface of the GFs, allowing the dispersibility of f -GFs in relatively highly polar solvents including even water. The sedi-mentation of f -GFs in acetone was monitored by optical absorb-ance at 660nm (Figure 3 c, Figure S6). From the line fi tting with sedimentation theory it was found that 49 wt% of the f -GFs is stable for more than 12 days. [ 44 , 45 ] Among the remaining f -GFs, 32 wt% falls out rapidly within 3 days and 19 wt% falls out between 3 days and 12 days. Figure 3 d shows the zeta poten-tial of the f -GFs measured at different pH in acetone. [ 22 , 46 ] The variation of the zeta potential shows that the surface of f -GFs is

© 2013 WILEY-VCH Verlag GAdv. Mater. 2013, 25, 732–737

negatively charged when dispersed in acetone even in low pH. This may be caused by the high ionization property of the carboxylic acid groups, suggesting that the formation of stable f -GFs colloids in various solvents is attributed to the electrostatic repulsion. This is the fi rst report of f -GFs manufactured from GIC processing to date. The most notable point is that non-covalent functionalization of GFs greatly enhances the disper-sion properties without deteriorating the physical and chemical properties of the specimens, which facilitates incorporation of f -GFs into homogeneous nanocomposites with various organic polymers.

Nanocomposites of f -GFs with epoxy were prepared with different wt% of f -GFs compared to that of epoxy. The SEM images in Figure 4 a and Figure S7 show that isolated f -GFs are homogeneously dispersed in the polymer matrix and no large bundles are observed. The white arrows in the inset fi gure point out graphene fl akes well dispersed in epoxy without agglomeration. The fracture surface exhibits that f -GFs have good adhesion and compatibility with the epoxy matrix due to surface functionalization. The mechanical properties, such as storage modulus and tan δ (tan δ , defi ned as the loss modulus to storage modulus ratio), were also measured. Figure 4 b shows a comparison of the modulus between pure epoxy, epoxy with GO (1 wt% loading ratio, Figure S8,9), and epoxy with f -GFs (1 wt% loading ratio). The storage modulus of the f -GF com-posites increased by as much as 100% relative to the pure epoxy modulus, and by 57% compared with that of GO composites. Moreover, the addition of f -GFs to the epoxy matrix increases T g

735wileyonlinelibrary.commbH & Co. KGaA, Weinheim

736

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

to 97.8 ° C from 91.1 ° C of pure epoxy, which can be compared

with a T g ∼ 93.9 ° C of the GO/epoxy nanocomposite. The tan δ of the f -GFs/epoxy nanocomposite is also much lower than that of pure epoxy. This is attributed to the interaction between the car-boxylic group of PBA functionalized on the surface of the GFs and the epoxy matrix, which enhances interfacial adhesion and restricts the motion of the epoxy segmental chains. [ 47 ]

Figure 4 c shows a comparison of the thermal conductivity of the f -GF nanocomposite with that of other carbon material composites. The thermal conductivity of the f -GF composite at 10 wt% loading in epoxy is 1.53 W/mK, which is much higher than those of other composites. This value is comparable to that of previously nanomaterials. Interestingly, the f -GF nano-composites display better fi ller performance than composites of MWNTs or GO; this is attributed to the high quality of nono-xidized graphene fl akes and their outstanding dispersion prop-erty through noncovalent functionalization by PBA. Also, it might be associated with the reduced effective aspect ratio due to nanotube bending [ 14 , 47 ] and with increased phonon scattering due to a high concentration defects of GO. [ 48 , 49 ] The interfacial bonding between f -GFs and the polymer matrix is responsible for the superior thermal conductivity enhancement. Figure 4 d presents photograph images showing that water above the f -GFs composites more rapidly evaporated than in pure epoxy com-posites. This demonstrates that the thermal conductivities of f -GFs nanocomposites are superior to those of pure epoxy com-posites. Consequently, the homogeneous dispersion and inter-facial bonding of f -GFs with the polymer matrix are responsible for the remarkable enhancement of the thermal conductivity and mechanical properties.

In conclusion, we have developed a novel approach to pre-pare non-covalently functionalized GFs ( f -GFs), which are used to produce nanocomposites with epoxy showing improved mechanical properties and thermal conductivities. The non-covalently functionalized GFs with PBA facilitate dispersion of GFs in organic or aqueous solvents with high solubility and stability, thus facilitating their manipulation via various proc-esses that involve mixing, blending or dispersion in a polymer matrix. The f -GFs reported herein provide the highest effi ciency in terms of enhancing thermal conductivity and mechanical properties of composites amongst all reported materials. At low loading levels, the f -GFs were apparently more effective fi llers than GO and MWNTs for producing polymeric nanocomposites with high thermal conductivity. Furthermore, for maximizing the effect of f -GFs its size needs to be increased as microscale.

With our approach it is possible to fabricate graphene-based materials at low cost and with environmentally friendly processing techniques. This work opens up new opportunities for potential applications, ranging from sensors to transparent electrodes and conductive composites.

Experimental Section Preparation of GFs and f-GFs and Dispersion : GIC were manufactured

by following the method previously reported in ref 27. Briefl y, a ternary eutectic system (KCl (99%, Aldrich), NaCl (99%, Aldrich), and ZnCl 2 (99%, Aldrich)) was used for GIC at 350 ° C and the prepared GIC was exfoliated in a pyridine solution. For the f -GFs, PBA (20 mg, Aldrich) was added to GFs (20 mg) dispersed in pyridine (40 ml, Aldrich) and then

wileyonlinelibrary.com © 2013 WILEY-VCH Verlag

these mixtures were mildly sonicated for 12 hours. The mixtures were subsequently stored at 70 ° C for 24 hours. The mixture was fi ltered and washed with de-ionized (DI) water using a 0.1 micron Anodisc fi lter to remove the salts and residual pyridine. The product was dried under 100 ° C in a furnace.

The f -GFs (10 mg) were dispersed in 30ml of various solvents (acetone, DMF, THF, ethanol, pyridine, methanol, water, IPA, dichlorobenzene (DCB), chloroform, dichloromethane (DCM) and chlorobenzene) with sonication for 3 hours. All solvents were purchased from Sigma - Aldrich. After 2 weeks, absorbance (A) was measured for each suspension. After making a baseline with each pure solvent, a quartz cell was fi lled with the f -GF suspension and pure solvent with different concentrations, such as 1:2, 2:1, and 3:0.

Preparation of composites : Epoxy resin (diglycidyl ether of bispenol A, EPON 862) was added to the f -GF suspension, and the curing agent (diethytolenediamine, SEIKA-S) was added under sonication in a ratio of epoxy to curing agent of 100: 26 by weight. The mixture of epoxy with the homogeneously dispersed f -GFs was loaded into a stainless steel mold, and cured at 70 ° C for 12 hours under a dynamic vacuum. An additional curing step was applied at 175 ° C for 1 hour to complete the curing cycle. A series of composites was prepared with f -GF loading between 1 and 10 wt%. For comparison studies, epoxy composites fi lled with graphite (Timcal. CO. Ltd), carbon black (Cabot. CO. Ltd.), MWNTs (Hanwha Nanotech. Co. Ltd.), and GO were prepared via the same procedure.

Characterization : Morphology of f -GFs was analysed using an atomic force microscope (AFM, SPA400, SII, Japan) in tapping mode under ambient conditions. UV–vis spectra (UV-3101PC spectrometer), fl uorescence spectra (Perkin-Elmer LS 55 luminescence spectrometer), X-ray photoelectron spectroscopy (XPS, Sigma Probe, AlK α ), transmission electron microscopy (TEM, Tecnai G2 F30) analyses were condcuted. TEM samples were prepared by drying a droplet of the f -GFs suspensions on a carbon grid. A thermogravimetric analysis (TGA) was performed using a G 209 F3 at a heating rate of 10 ° C/min under a N 2 atmosphere. Raman spectra were obtained from 1200 to 3000 cm − 1 using a Raman spectrometer (LabRAM HR UV/Vis/NIR, excitation at 514nm). The FT-IR spectrum was measured using a FT-IR-4100 type-A FT-IR spectrometer with pure KBr as the background from 1000 and 3000 cm − 1 . The thermal conductivity of disc-shaped composite samples with 1 inch diameter was measured through the laser fl ash technique (LFA 447). The laser fl ash technique has been generally known as the standard and the popular method for measuring thermal diffusivities of solid materials above room temperature. For a given geometric sample, the applying heat propagates from the top to the bottom surface of the material.

And the mechanical properties of nancomposites were conducted with DMA (Dynamic mechanical analyzer, Seiko EXSTAR 6000).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011K000623) and through a grant (2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. It was also partially supported by the project with the Ministry of Knowledge Economy of KOREA (N02110414).

Received: July 6, 2012 Revised: September 24, 2012

Published online: November 14, 2012

GmbH & Co. KGaA, Weinheim Adv. Mater. 2013, 25, 732–737

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

[ 1 ] M. Arik , S. Weaver , Proc. SPIE 2004 , 5530 , 214. [ 2 ] S. Lin , K. Banerjee , IEEE. 2008 , 5 , 245 . [ 3 ] J. Yang , T. Caillat , MRS Bull. 2006 , 31 , 224 . [ 4 ] F. Monteverdew , L. Scatteia , J. Am. Chem. Soc. 2007 , 90 , 1130 . [ 5 ] M. J. Biercuk , M. C. Llaguno , M. Radosavljevic , J. K. Hyun ,

A. T. Johnson , Appl. Phys. Lett. 2002 , 80 , 2766 . [ 6 ] S. T. Huxtable , D. G. Cahill , S. Shenogin , L. Xue , R. Ozisik , P. Barone ,

M. Userey , M. L. Strano , G. Siddons , M. Shim , P.Keblinski , Nat. Mater. 2003 , 2 , 731 .

[ 7 ] S. Shenogin , L. P. Xue , R. Ozisik , P. Keblinski , D. G. Cahill , Appl. Phys. Lett. 2004 , 95 , 8136 .

[ 8 ] A. A. Balandin , S. Ghosh , W. Bao , I. Calizo , D. Teweldbrhan , F. Miao , C. N. Lau , Nano Lett. 2008 , 8 , 902 .

[ 9 ] A. A. Balandin , Nat. Mater. 2011 , 10 , 569 . [ 10 ] S. Ghosh , W. Bao , D. L. Nika , S. Subrina , E. P. Pokatilov , C. N. Lau ,

A. A. Balandin , Nat. Mater. 2010 , 9 , 555 . [ 11 ] A. Yu , P. Ramesh , X. Sun , E. Bekyarova , M. E. Itkis , R. C. Haddon ,

Adv. Mater. 2008 , 20 , 4740 . [ 12 ] L. M. Veca , M. J. Meziani , W. Wang , X. Wang , F. Lu , P. Zhang , Y. Lin ,

R. Fee , J. W. Connell , Y. P. Sun , Adv. Mater. 2009 , 21 , 2088 . [ 13 ] K. M. F. Shahil , A. A. Balandin , Nano Lett. 2012 , 12 , 861 . [ 14 ] V. Goyal , A. A. Balandin , Appl. Phys. Lett. 2012 , 100 , 073113 . [ 15 ] K. M. F. Shahil , A. A. Balandin , Solid State Commun. , 2012 , 152 ,

1331 . [ 16 ] A. Yu , P. Ramesh , M. E. Itkis , E. Bekyarova , R. C. Haddon , J. Phys.

Chem. C 2007 , 111 , 7565 . [ 17 ] S. Ganguli , A. K. Roy , D. P. Anderson , Carbon 2008 , 46 , 806 . [ 18 ] R. Zacharia , H. Ulbricht , T. Hertel , Phys Rev. B 2004 , 306 , 666 . [ 19 ] W. S. Hummers , R. E. Offeman , J. Am. Chem. Soc. 1958 , 80 , 1339 . [ 20 ] G. Eda , G. Fanchini , M. Chhowalla , Nat. Nanotech. 2008 , 3 , 270 . [ 21 ] Y. Hernandeze , V. Nicolosi , M. Lotya , F. M. Blighe , Z. SunUN , S. De ,

I. T. Mcgovern , B. Holland , M. Byrne , Y. K. Gun’ko , J. J. Boland , P. Niraj , G. Duesber , S. Krishnamurthy , R. Goodhue , J. Hutchison , V. Scardaci , A. C. Ferrari , J. N. Coleman , Nat. Nanotech. 2008 , 3 , 563 .

[ 22 ] M. Lotya , Y. Hernandez , P. J. King , R. J. Smith , V. Nicolosi , L. S. Karlsson , F. M. Blighe , S. De , Z. Wang , I. T. McGovern , G. S. Duesberg , J. N. Coleman , J. Am. Chem. Soc. 2009 , 131 , 3611 .

[ 23 ] M. Lotya , P. J. King , U. Khan , S. De , J. N. Coleman , ACS Nano 2010 , 4 , 3155 .

[ 24 ] L. M. Visulis , J. J. Mack , R. B. Kaner , Science 2003 , 299 , 1361 . [ 25 ] J. Wang , K. K. Manga , Q. Bao , K. P. Loh , J. Am. Chem. Soc . 2011 ,

133 , 8888 . [ 26 ] J. Kwon , S. H. Lee , K. H. Park , D. H. Seo , J. Lee , B. S. Kong , K. Kang ,

S. Jeon , Small 2011 , 7 , 864 .

© 2013 WILEY-VCH Verlag GmAdv. Mater. 2013, 25, 732–737

[ 27 ] K. H. Park , B. H. Kim , S. H. Song , J. Kwon , K. Kang , S. Jeon , Nano Lett. 2012 , 12 , 2871 .

[ 28 ] R. J. Chen , Y. Zhang , D. Wang , H. Dai , J. Am. Chem. Soc. 2001 , 123 , 3838 .

[ 29 ] S. H. Park , S. H. Jin , G. H. Jun , S. Jeon , S. H. Hong , Nano Res. 2011 , 4 , 1129 .

[ 30 ] Y. Xu , H. Bai , G. Lu , C. Li , G. Shi , J. Am. Chem. Soc. 2008 , 130 , 5856 .

[ 31 ] Y. Liang , D. Wu , X. Feng , K. Mullen , Adv. Mater. 2009 , 21 , 1679 . [ 32 ] F. Li , Y. Bao , J. Chai , Q. Zhang , D. Han , L. Niu , Langmuir 2010 , 26 ,

12 314 . [ 33 ] Y. Chen , X. Zhang , P. Yu , Y. Ma , Chem. Commun. 2009 , 4527 . [ 34 ] Q. Su , S. Pang , V. Alijani , C. Li , X. Feng , K. Mullen , Adv. Mater. 2009 ,

21 , 3191 . [ 35 ] V. Z. Poenitzsch , D. C. Winters , H. Xie , G. R. Dieckmann ,

A. B. Dalton , I. H. Musselman , J. Am. Chem. Soc. 2007 , 127 , 14724 . [ 36 ] H. J. Shin , S. M. Kim , S. M. Yoon , A. Benayad , K. K. Kim , S. J. Kim ,

H. K. Musselman , J. Am. Chem. Soc. 2008 , 130 , 2062 . [ 37 ] X. Qi , K. Y. Pu , H. Li , X. Z. Zhou , S. Wu , Q. L. Fan , B. Liu , F. Boey ,

W. Huang , H. Zhang , Angew. Chem. Int. Ed. 2010 , 49 , 9426 . [ 38 ] J. I. Paredes , S. V. Rodil , A. Martinez-Alonso , J. M. D. Tascon ,

Langmuir 2008 , 24 , 10560 . [ 39 ] M. Lotya , P. J. King , U. Khan , S. De , J. N. Coleman , ACS Nano 2010 ,

4 , 3155 . [ 40 ] J. H. Lee , D. W. Shin , V. G. Makotchenko , A. S. Nazarov , V. E. Fedorov ,

J. H. Yoo , S. M. Yu , J. M. Kim , J.-B. Yoo , Small 2010 , 6 , 58 . [ 41 ] S. Park , J. An , I. Jung , R. D. Piner , S. J. An , X. Li , A. Velamakanni ,

R. S. Ruff , Nano Lett. 2009 , 9 , 153 . [ 42 ] R. S. Ruff , D. S. Tse , R. Malhotra , D. C. Lorents , J. Phys. Chem. 1993 ,

97 , 3379 . [ 43 ] K. D. Ausman , R. Piner , O. Lourie , R. S. Ruff , J. Phys. Chem. B 2000 ,

104 , 8911 . [ 44 ] Y. Hernandez , M. Lotya , D. Rickard , S. D. Bergin , J. N. Coleman ,

Langmuir 2010 , 26 , 3208 . [ 45 ] V. Nicolosi , D. Vrbanic , A. Mrzel , J. McCauley , S. O’Flaherty ,

C. McGuinness , G. Compagnini , D. Mihailovic , W. J. Blau , J. N. Coleman , J. Phys. Chem. B 2005 , 109 , 7124 .

[ 46 ] D. Li , M. B. Muller , S. G. Je , R. B. Kaner , G. G. Wallace , Nat Nano-technol. 2008 , 3 , 101 .

[ 47 ] W. Cui , F. Du , J. Zhao , W. Zhang , Y. Yang , X. Xie , Y. Mai , Carbon 2011 , 49 , 495 .

[ 48 ] S. Shenogin , A. Bodapati , L. Xue , R. Ozisik , P. Keblinski , J. Appl. Phys. 2004 , 85 , 2229 .

[ 49 ] M. T. Hung , O. Choi , Y. S. Ju , H. T. Hahn , Appl. Phys. 2006 , 89 , 023117 .

737wileyonlinelibrary.combH & Co. KGaA, Weinheim