Fabrication of highly conducting and transparent graphene films

Transcription

1 CARBON 48 (21) available at journal homepage: Fabrication of highly conducting and transparent graphene films Shu Jun Wang, Yan Geng, Qingbin Zheng, Jang-Kyo Kim * Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ARTICLE INFO ABSTRACT Article history: Received 12 October 29 Accepted 16 January 21 Available online 22 January 21 Graphene-based transparent conducting films were prepared using the following method. A chemically-reduced graphene dispersion was synthesized and graphene films were prepared from it by transfer printing, followed by thermal treatment. The resulting graphene films possessed an excellent electrical conductivity with a high transparency. A sheet resistance lower than 2 KX/sq and a transparency well over 8% were achieved at a typical wavelength of 55 nm. These properties are considered quite sufficient for many applications, such as transparent conductor films for touch panels. Ó 21 Elsevier Ltd. All rights reserved. 1. Introduction A transparent conductor (TC) is a critical component in many photoelectronic devices such as liquid crystal displays (LCD), organic solar cells, organic light emitting diodes (OLED), smart windows, etc. [1 3]. Indium tin oxide (ITO) has been the predominant material for the fabrication of TC because of its high electrical conductivity and optical transparency. However, there are several disadvantages of using ITO: (1) the cost of indium has gone up continuous, making ITO an increasingly expensive material, (2) ITO itself falls short in meeting property requirements for new applications such as flexible devices (i.e. flexible LCD, organic solar cell) due to its brittle nature and (3) ITO is inherently incapable of undergoing etching and high temperature processing, which makes patterning of ITO films difficult to realize. Considerable research has been conducted on the possibility of applying carbon nanotubes (CNTs) as a material for TC [1 9]. In one approach, TC was prepared in the form of composite thin film where CNTs are incorporated into a polymer. The transparent thin films prepared with multi-walled carbon nanotube (MWCNT) reinforced poly(2-vinylpyridine) composite showed optimal transmittance 7% and sheet resistance 9 KX/sq [1]. Another approach was through direct fabrication of neat CNT films on a substrate, which yielded TCs of more applicable quality. The thin neat single-walled carbon nanotube (SWCNT) films [11] had visible light transmittance 9 97% and sheet resistance 2 5 X/sq, which approached those of practical ITO films. Although the above finding opened the potential of replacing ITO with neat SWCNTs for TC applications, SWCNTs are prohibitively too expensive to make profits for low-cost consumable electronic products. Graphene is a new material, which has attracted significant attention only recently [12]. Due to its unusual physical and mechanical properties such as high carrier concentration and mobility [13] along with a room temperature quantum Hall effect [14], high thermal conductivity [15,16], and highest mechanical strength measured to date [17], it holds a number of promising applications [18]. One encouraging aspect of graphene is to replace CNTs as a cheaper but better solution in various applications including TCs because the naturally abundant and cheap graphite flakes are used to produce graphene. Since chemical oxidation method has been regarded as the most promising way to produce graphene sheets from both scale and cost perspectives [19,2], one typical approach to fabricate graphene TC is to coat graphite oxide (GO) films on a substrate, followed by converting GO * Corresponding author: Fax: address: mejkkim@ust.hk (J.-K. Kim) /$ - see front matter Ó 21 Elsevier Ltd. All rights reserved. doi:1.116/j.carbon

2 1816 CARBON 48 (21) films into graphene films through thermal or vapor chemical reduction [21,22]. Several techniques have been devised to fabricate the GO films, such as spin- or spray-coating, transfer printing and electrophoretic deposition. Spin-coating is a procedure used to deposit uniform thin films on flat substrates. Individual graphene oxide sheets were incorporated into silica sols followed by spin-coating, chemical reduction, and thermal curing [23]. However, the electrical conductivity of such TC films, compared to the requirement of optoelectronic devices, was relatively low, typically ranging from 1 3 to 1 S/ cm. Blake et al. [24] fabricated graphene TC film based on spray-coating, showing that its high transparency and low resistivity were ideally suitable for electrodes in LCDs. In transfer printing method, the GO aqueous dispersion was first vacuum filtrated through a membrane and then transferred onto a substrate by placing the membrane with the film side down to the substrate and applying force. The GO film can be finally obtained after dissolving the membrane in a solvent [25]. Recently, researchers reported a new technique, namely electrophoretic deposition, to fabricate graphene TC films [26,27]. The graphene oxide was reduced to graphene nanosheets in a strongly alkaline solution and the reduced graphene sheets were electrophoretically deposited onto a substrate. The film thickness can be controlled by changing the deposition time. Different starting materials, such as giant polycyclic aromatic hydrocarbons (PAHs) that contained nanographene molecules bonded to alkyl side chains, have also been used to produce graphene films. It was claimed that the nanographene molecules would form a large-size graphene film on a substrate through cross-linking upon the decomposition of alkyl side chains during the thermal treatment. TCs obtained thereby had a transmittance of 85% and a sheet resistance of 18 KX/sq [2], which was still quite high. This paper is part of a larger project on the development and applications of graphene and graphite nanoplatelets (GNPs) [19,28 31]. In this paper, graphene films are initially prepared on a cellulous ester (CE) filter membrane through vacuum filtrating graphene aqueous colloid which was made by chemical reduction of graphene oxide. Not only being able to produce films with well-packet graphene sheets, the vacuum filtration technique employed here has a flexibility to control the thickness by varying either the concentration or volume of the graphene aqueous colloid. In contrast, the conventional spin-coating method can produce only very thin films. First of all, natural graphite flakes (Asbury Graphite Mills) were exfoliated in a formic acid (HCOOH, 98%, supplied by BDH) through ultrasonication (Bransonic 151-DTH) to obtain GNPs, according to the procedure described previously [19]. The GNPs were processed to produce GO using the modified Hummer s method [32]: GNPs were put into a flask, and nitric acid (69 72%, supplied by Fisher), sulfuric acid ( %, supplied by General Chemical) as well as potassium permanganate (P99%, supplied by RDH) were added into the flask sequentially while magnetically stirring the mixture in an ice bath. After diluting the mixture with distilled water, the resulting slurry was subjected to dialysis for purification over a week. Upon the completion of dialysis, GO was stored either in a colloidal state or in the form of solid after drying in a vacuum oven. Prior to reduction, the as-prepared GO was first exfoliated in water by ultrasonication for 2 h and followed by centrifugation at 4 rpm for 1 h to remove the precipitates, yielding GO colloid [33]. The GO colloid was subsequently reduced to graphene colloid in the presence of hydrazine solution (N 2 H 4 in water, Aldrich) and ammonium hydroxide solution (NH 3 ÆH 2 O, 28 3 wt.%, Wako), similar to a recent study [19,2]. Fig. 1 shows the images of the as-prepared GO colloid and the corresponding graphene colloid Preparation of graphene films on quartz substrates Substrates for graphene films were prepared using 2 mm thick 25 mm square quartz slides, which were washed in an acetone bath by ultrasonication to remove any organic contamination. The quartz pieces were pretreated with a trithoxysilane solution (3% in toluene). The silane treatment allowed the substrate surface to become more adhesive to the transferring graphene film. The clean substrates were stored in a vacuum oven at 8 C before being used in the next step. Chemically converted graphene films were prepared on CE filter membranes (Roshi Kaisha) through vacuum filtration of the chemically converted graphene colloid. The thickness of film was carefully controlled by varying the volume of colloids. The CE membranes with graphene films on were cut into appropriate sizes when they were still wet and subsequently pasted onto the quarts substrates with the film surface against the substrate surface. A dead weight of 1 kg was applied overnight in vacuum to assure that the graphene films firmly stuck to the substrate surface and any entrapped air bubbles could be eliminated. The CE membrane was dissolved in an acetone bath, leaving the graphene films on the quartz substrates. To further remove the residual oxygen functional groups and thus improve the electrical conductivity, the as-produced graphene films were heat treated as follows. A tube furnace 2. Experimental 2.1. Preparation of graphene aqueous colloid Fig. 1 (a) Graphene oxide colloid (.25 mg/ml) and (b) the corresponding graphene colloid.

3 CARBON 48 (21) (Thermcraft, Eurotherm) equipped with high vacuum was used for annealing the films at 4 C. For further graphitization, the films were heated to 11 C for 3 min. To avoid burning of graphene films at such a high temperature due to the self-released oxygen from the graphene films, the furnace was vacuumed and then purged using an argon flow three times, followed by a low rate of argon flow for the rest of heat treatment. The oven was cooled to room temperature to obtain the final graphene films on quarts substrates, which are shown in Fig Characterization of graphene films The thickness and the surface roughness of film were measured using an atomic force microscopy (AFM, Digital Instruments). To measure the thickness, the films were scratched to expose the substrate and the AFM tapping mode was used to scan from the film top to the exposed substrate surface. The elemental compositions and the assignments of the carbon peaks were characterized using the X-ray photoelectron spectroscopy (XPS, PHI56 Physical Electronics). The transparency of the graphene films was characterized using the UV/ VIS spectroscopy (Perkin Elmer Lambda 2) and the Fourier transform infrared spectroscopy (FTIR, Bio-Rad FTS6 System). Raman spectroscopy (Renishaw MicroRaman/Photoluminescence System) was used to analyze the effects of heat treatments on the crystal quality of graphene films. The electrical conductivity of the graphene films was measured using the four-point probe method (Scientific Equipment & Services). To reduce the contact resistance between the probes and the film surface, the samples were coated with silver at the positions of probe contact. 3. Results and discussion 3.1. Surface morphology of graphene films Fig. 2 Finished graphene films with different thicknesses (a) 86 nm, (b) 22 nm and (c) 14 nm. The thermal treatment was aimed at eliminating the oxygen functionality by decomposition. Through this process, it is expected that the graphene sheets would reorient and thus the film becomes denser and smoother. Fig. 3(a) shows the AFM image of the chemically-reduced graphene sheets. The images of Fig. 3(b) (e) are the 3D surface morphologies of the graphene films. The surface roughness of the 22 nm thick film were R a 9 and 3 nm before and after the thermal treatment ( Fig. 3(b) and (c)), respectively; whereas for the 78 nm thick film the corresponding surface roughness values became much higher, i.e. R a 15 and 7 nm, respectively (Fig. 3(d) and (e)). Fig. 4 summaries the roughness of graphene films plotted as function of thickness after different treatments. As chemically-reduced graphene films were prepared through vacuum filtration, the surface roughness measured before thermal treatment may have been influenced by the roughness of filter paper, which in turn affected the final roughness after thermal treatment. The pore size of the filter paper employed here was.2 lm. It is envisaged that the final surface roughness could have been considerably reduced if a filter paper with a much smaller pore size had been chosen Crystallization of graphene films after heat treatments Fig. 5 shows the Raman spectra obtained for four different graphite materials, and the corresponding intensity ratios of the prominent bands, namely D and G bands at wave numbers of 1356 and 1586 cm 1, are given in Table 1. It is wellknown that the G band indicates sound graphite carbon structure (sp 2 ), whereas the D band is typical of defects in carbon materials [34]. The intensity ratio of D (I D ) to G band (I G ) can be used as an indication of defects quantity: a low I D /I G corresponds to a small defect quantity. The intensity ratio, I D /I G, decreased consistently when graphene oxide was reduced by hydrazine and subsequently heat treated at 4 and 11 C to produce the final product. Along with the XPS findings in Section 3.3, it can be said the thermal treatments restored the carbon carbon bonds thereby reducing the defect quantity when the oxygen functional groups were decomposed, which in turn promoted the crystallization of graphene films Surface chemistry The high oxygen content in the form of functional groups contained by graphene oxide inevitably makes it an insulator. Reduction is necessary to enable electrical conduction. Chemical reduction (in the presence of hydrazine) could effectively reduce the oxygen contents of graphene oxide. However, due to the electrostatic repulsion introduced by residual charged functional groups [2], the chemically-reduced graphene sheets remain in an aqueous colloidal state in water. As noted in Section 1, thermal annealing is a very effective method capable of removing oxygen from GO, where reduction is accomplished by decomposition of oxygen containing groups and simultaneous restoration of carbon carbon bonding. The XPS analysis can trace the chemical elements of the films throughout the whole process of different treatments. Fig. 6 presents the oxygen to carbon ratios measured after heat treatments at varying temperatures. The O/C ratio decreased from.4 in the original GO to less than.2 in the finished graphene films, confirming the effectiveness of heat treatments. Fig. 7 illustrates curve fitting of the C1s peak of XPS spectra and Table 2 summarizes the relative percentages of these carbon structures and those containing functional groups. They signify the gradual reduction of oxygen con-

4 1818 CARBON 48 (21) Fig. 3 AFM images of (a) chemically-reduced graphene single sheets on the quartz substrate (the height of the sheet being around 1.2 nm), (b) a 22 nm thick graphene film before thermal treatment, (c) after thermal treatment, (d) a 78 nm graphene film before thermal treatment and (e) after thermal treatment. tents. The intensities of the peaks 1 and 2 (corresponding to sp 2 - and sp 3 - C C, respectively) increased, whereas the intensities of the peaks 3 and 4 (corresponding to C O and O, respectively) were reduced significantly when the graphene oxide was reduced to graphene and was further treated thermally. Chemical reduction in the presence of hydrazine was proven to be effective in reducing the oxygen content of graphene oxide. Annealing at elevated temperatures further removed the residual oxygen from GO. The reduction process was accomplished by decomposition of oxygen containing

5 CARBON 48 (21) Ra (nm) hydrazine reduced hydrazine+4 C hydrazine+4 C+11 C O/C Ratio Film thickness (nm) Fig. 4 Surface roughness plotted as a function of film thickness at different stages of treatment.. a b c d Treatments Fig. 6 Ratio between oxygen and carbon contents at different stages of treatment: (a) GO, (b) hydrazine-reduced, (c) hydrazine + 4 C and (d) hydrazine + 4 C + 11 C. Intensity (a.u.) Hydrazine+4 o C+11 o C Hydrazine+4 o C Hydrazine reduced GO Wavenumber (cm -1 ) Fig. 5 Raman spectra of samples at different stages of treatment. groups and the simultaneous restoration of sp 2 C C bonds as the change in C C concentration indicates (Table 2) Enhanced electrical conductivity The chemical reduction and the following heat treatments resulted in significant improvements in electrical conductivity of graphene films. Fig. 8 shows the sheet resistance and the corresponding electrical conductivity of graphene films plotted as a function of film thickness after different treatments. The electrical conductivity increased by approximately three Table 1 Intensity ratio of G and D bands. D G I D /I G GO.921 Hydrazine-reduced.916 Hydrazine + 4 C.98 Hydrazine + 4 C + 11 C.89 orders of magnitude for a given thickness after thermal annealing of hydrazine-reduced graphene films at 4 C. The final thermal annealing at 11 C further enhanced the conductivity by approximately one order of magnitude. It is shown that the 14 nm thick graphene film possessed an acceptable transparency of 8% and an electrical conductivity of 29 S/cm (or sheet resistance R s =2KX/sq). The highest electrical conductivity obtained was 649 S/cm (R s = X/ sq) with a transparency of 35% at a wavelength of 55 nm. In general, the electrical conductivity increased with increasing the thickness of films, which was attributed to improved orientation and structure of the film as for the spin-coated GO films [2,22]. There were two possible mechanisms responsible for the remarkable, four orders of magnitude improvement in electrical conductivity due to the heat treatments. They are: (1) restoration of sp 2 C C bonds and (2) cross-linking between chemically-reduced graphene sheets. The heat treatments accelerated the reduction process by decomposing the oxygen functionalities and simultaneous restoration of sp 2 C C bonds that ubiquitously exist in graphitic structures. The sp 2 bond structure can provide p electrons that are loosely constrained by the nucleus than common electrons and thus can easily escape from the nucleus to become free electrons if there is external electric field. Hence, the more are the restored sp 2 C C bonds, the higher is the carrier concentration that graphene films will possess, and consequently the higher is the electrical conductivity. Another potential mechanism behind the enhanced conductivity is cross-linking between chemically-reduced graphene sheets during the thermal annealing process. It is assumed that during the vacuum filtration of graphene colloid the graphene films were formed through continuous stacking of the individual graphene sheets on top of each other, similar to producing graphene oxide papers [35]. Alkyl chains are decomposed upon heat treatment at elevated temperatures, and cross-linking takes place between the PAHs cores [36]. 11 C is known as the graphitization temperature of carbon materials [21] and therefore similar cross-linking

6 182 CARBON 48 (21) Fig. 7 Curve fitting of C1s peaks in XPS spectra for materials at different stages of treatment: (a) GO, (b) hydrazine-reduced, (c) hydrazine + 4 C and (d) hydrazine + 4 C + 11 C. (Peaks 1 4 are assigned to different carbon chemical states specified in Table 2). Table 2 Summary on relative percentages of the functional groups (%). Binding energy (ev) and assignation 1: : : : C C C O C@O COO Graphene oxide Hydrazine-reduced Hydrazine + 4 C Hydrazine + 4 C + 11 C mechanisms would occur among the adjacent graphene sheets, which may contribute to the improvement of electrical performance Transparency of graphene films and correlation with electrical conductivity Figs. 9 and 1 present the UV vis spectra and IR spectra, respectively, of graphene films with different thicknesses prepared at three different stages, showing the changes in transmittance in the wavelength range from 2 to 3 nm. There were two observations regarding transparency of graphene films. As expected, the transparency of the film decreased with increasing film thickness for a given treatment. The increase in thickness increased both the back reflection and absorption of light, resulting in the reduction in transparency. The other phenomenon was that the heat treatment had a detrimental effect on transparency of the graphene film. All transmittance curves in both the UV vis (Fig. 9) and FTIR spectra (Fig. 1) exhibited an obvious tendency to shift downward along the transmittance axis upon heat treatments, suggesting that the transparency decreased after annealing at 4 C and further decreased after subsequent graphitization at 11 C. The degradation of transparency in graphene films during thermal treatment was attributed to the improvement of reduction level [21]. Special note should be made on the FTIR spectra for the graphene film of thickness 14 nm obtained after the heat treatment at 11 C (Fig. 1(c)). The transmittance was recorded well over 85% throughout the whole wavelength

7 CARBON 48 (21) Sheet resistance (ohm/sq) hydrazine reduced hydrazine+4 o C hydrazine+4 o C+11 o C Electrical conductivity (S/cm) Film thickness (nm) Film thickness (nm) (a) (b) hydrazine reduced hydrazine+4 o C hydrazine+4 o C+11 o C Fig. 8 (a) Sheet resistance and (b) electrical conductivity of graphene sheets as a function of film thickness at different stages of treatment. studied between 11 and 3 nm. This is a remarkable observation because a recent work on conductive graphene electrodes for dye-sensitized solar cells showed only about 7% transmittance for a 1 nm thick graphene film for the same range of wavelength [22] ITO and fluorine tin oxide (FTO) that have been widely used as window electrodes in optoelectronic devices showed much lower transmittance especially at high wavelengths. This indicates that the graphene films developed in this study are of particular importance for applications in solar cells and other optoelectronic devices. Further analysis was made to establish the correlation between the electrical conductivity and transmittance measured at a wavelength of 55 nm, as well as the thickness of grapheme film, as shown in Fig. 11. The wavelength 55 nm is known as typical of indicating the transparency of TCs. It nm 22 nm 59 nm 78 nm 86 nm Wavelength (nm) Wavelength (nm) (a) (b) Wavelength (nm) (c) Fig. 9 UV vis spectra of graphene films obtained at different stages of treatment: (a) hydrazine-reduced, (b) hydrazine + 4 C and (c) hydrazine + 4 C + 11 C.

8 1822 CARBON 48 (21) nm 22 nm 59 nm 78 nm 86 nm Wavelength(nm) (a) Wavelength(nm) (b) Wavelength (nm) (c) Fig. 1 IR spectra of graphene films obtained at different stages of treatment: (a) hydrazine-reduced, (b) hydrazine + 4 C and (c) hydrazine + 4 C + 11 C. is clearly seen that the electrical conductivity gradually decreased as transmittance increased, suggesting an inverse relationship between the conductivity and optical transparency. This observation has practical importance in that applicable TCs can be obtained by controlling the film thickness to balance these two properties. As exhibited in this study, transparent conducting films with transparency well over 8% and electrical conductivity over 2 S/cm (or a sheet Electrical Conductivity (S/cm) hydrazine reduced hydrazine+4 o C hydrazine+4 o C+11 o C Transmittance at 55 (%) Fig. 11 Electrical conductivity as a function of transmittance at a wavelength of 55 nm at different stages of treatment. resistance 1 2 KX/sq) were successfully produced. These properties are sufficient for many important applications, including the TCs for touch panels [5]. The lowest sheet resistance achieved in this study was X/sq (or electrical conductivity = 649 S/cm) with a transmittance around 35% at a wavelength of 55 nm. 4. Conclusions Vacuum filtration method was used to fabricate transparent conducting graphene films with improved electrical conductivity and transparency from the chemically-reduced graphene colloids. The chemically-reduced graphene films were transferred onto the quartz substrates, which were subjected to annealing and graphitization at elevated temperatures. The transparent conducting films obtained possessed remarkable properties: the transparency was well over 8% and the electrical conductivity was over 2 S/cm (or a sheet resistance 1 2 KX/sq) at a typical wavelength of 55 nm. These properties are sufficient for many important applications, including the TCs for touch panels [5]. These observations confirmed that the synthesis technique devised in this study could yield transparent conducting films with acceptable quality and functional performances. Acknowledgements This project was financially supported by Henkel International (formerly Imperial Chemical Industries Limited, with

9 CARBON 48 (21) Project number: ICIPLC1.7/8). Technical assistance from the Materials Characterization and Preparation Facilities (MCPF) of HKUST is appreciated. REFERENCES [1] Rowell MW, Topinka MA, McGehee MD, Prall HJ, Dennler G, Sariciftci NS, et al. Organic solar cells with carbon nanotube network electrodes. Appl Phys Lett 26;88(23): [2] Wang X, Zhi LJ, Tsao N, Tomovic Z, Li JL, Mullen K. Transparent carbon films as electrodes in organic solar cells. Angew Chem Int 28;47(16): [3] Wu ZC, Chen ZH, Du X, Logan JM, Sippel J, Nikolou M, et al. Transparent, conductive carbon nanotube films. Science 24;35(5688): [4] Zhang M, Fang SL, Zakhidov AA, Lee SB, Aliev AE, Williams CD, et al. Strong, transparent, multifunctional, carbon nanotube sheets. Science 25;39(5738): [5] Gruner G. Carbon nanotube films for transparent and plastic electronics. J Mater Chem 26;16(35): [6] Schindler A, Brill J, Fruehauf N, Novak JP, Yaniv Z. Solutiondeposited carbon nanotube layers for flexible display applications. Physica E 27;37(1 2): [7] Atkinson K, Hawkins SC, Huynh C, Skourtis C, Dai J, Zhang M, et al. Multifunctional carbon nanotube yarns and transparent sheets: fabrication, properties, and applications. Physica B 27;394(2): [8] Ulbricht R, Lee SB, Jiang XM, Inoue K, Zhang M, Fang SL, et al. Transparent carbon nanotube sheets as 3-D charge collectors in organic solar cells. Sol Energy Mater Sol Cells 27;91(5): [9] Kim DO, Lee MH, Lee JH, Lee TW, Kim KJ, Lee YK, et al. Transparent flexible conductor of poly(methylmethacrylate) containing highly-dispersed multiwalled carbon nanotube. Org Electron 28;9(1):1 13. [1] Bocharova V, Kiriy A, Oertel U, Stamm M, Stoffelbach F, Jerome R, et al. Ultrathin transparent conductive films of polymer-modified multiwalled carbon nanotubes. J Phys Chem B 26;11(3): [11] Trottier CM, Glatkowski P, Wallis P, Luo J. Properties and characterization of carbon-nanotube-based transparent conductive coating. J Soc Inf Display 25;13(9): [12] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 24;36(5696): [13] Morozov SV, Novoselov KS, Shedin F, Jiang D, Firsov AA, Geim AK. Two-dimensional electron and hole gases at the surface of graphite. Phys Rev B 25;72(2):2141. [14] Novoselov KS, Jiang Z, Zhang Y, Morozov SV, Stormer HL, Zeitler U, et al. Room-temperature quantum hall effect in graphene. Science 27;315(5817):1379. [15] Balandin AA, Ghosh S, Bao WZ, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 28;8(3):92 7. [16] Ghosh S, Calizo I, Teweldebrhan D, Pokatilov EP, Nika DL, Balandin AA, et al. Extremely high thermal conductivity of graphene: prospects for thermal management applications in nanoelectronic circuits. Appl Phys Lett 28;92(15): [17] Lee CG, Wei XD, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 28;321(5887): [18] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 27;6(3): [19] Geng Y, Li J, Wang SJ, Kim JK. Amino functionalization of graphite nanoplatelet. J Nanosci Nanotechnol 28;8(12): [2] Li D, Müller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat Nanotechnol 28;3(2):11 5. [21] Becerril HA, Mao J, Liu ZF, Stoltenberg RM, Bao ZN, Chen YS. Evaluation of solution-processed reduced graphene oxide films as transparent conductors. ACS Nano 28;2(3): [22] Wang X, Zhi LJ, Mullen K. Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Lett 28;8(1): [23] Watcharotone S, Dikin DA, Stankovich S, Piner R, Jung I, Dommett GHD, et al. Graphene-silica composite thin films as transparent conductors. Nano Lett 27;7(7): [24] Blake P, Brimicombe PD, Nair RR, Booth TJ, Jiang D, Schedin F, et al. Graphene-based liquid crystal device. Nano Lett 28;8(6): [25] Eda G, Fanchini G, Chhowalla M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nat Nanotechnol 28;3(5):27 4. [26] Chen Y, Zhang X, Yu P, Ma Y. Stable dispersions of graphene and highly conducting graphene films: a new approach to creating colloids of graphene monolayers. Chem Commun 29;27: [27] Lee V, Whittaker L, Jaye C, Baroudi KM, Fischer DA, Banerjee S. Large-area chemically modified graphene films: electrophoretic deposition and characterization by soft X-ray absorption spectroscopy. Chem Mater 29;21(16): [28] Li J, Vaisman L, Marom G, Kim JK. Br treated graphite nanoplatelets for improved electrical properties of conducting polymer composites. Carbon 27;45(4): [29] Li J, Kim JK. Percolation threshold of conducting polymer composites containing 3D randomly distributed graphite nanoplatelets. Compos Sci Technol 27;67(1): [3] Green M, Marom G, Li J, Kim JK. The electrical conductivity of graphite nanoplatelet filled conjugated polyacrylonitrile. Macromol Rapid Commun 28;29(14): [31] Geng Y, Wang SJ, Kim JK. Preparation of graphite nanoplatelets and graphene sheets. J Colloid Interface Sci 29;336(2): [32] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;8(6):1339. [33] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 26;442(71): [34] Ouyang Y, Cong LM, Chen L, Liu QX, Fang Y. Raman study on single-walled carbon nanotubes and multi-walled carbon nanotubes with different laser excitation energies. Physica E 28;4(7): [35] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature 27;448(7152): [36] Zhi LJ, Wu JS, Li J, Kolb U, Müllen K. Carbonization of disclike molecules in porous alumina membranes: toward carbon nanotubes with controlled graphene-layer orientation. Angew Chem Int 25;44(14):212 3.