Polyaniline Nanocomposite for Hydrogen Sensing - The

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Graphene/Polyaniline Nanocomposite for Hydrogen Sensing Laith Al-Mashat,*,† Koo Shin,*,‡ Kourosh Kalantar-zadeh,† Johan D. Plessis,§ Seung H. Han,‡ Robert W. Kojima,| Richard B. Kaner,| Dan Li,⊥ Xinglong Gou,⊥ Samuel J. Ippolito,§ and Wojtek Wlodarski† School of Electrical and Computer Engineering, School of Applied Sciences, RMIT UniVersity, Melbourne VIC 3001, Australia, Department of Applied Chemistry, Sejong UniVersity, Seoul 143-747, Korea, Department of Chemistry and Biochemistry and California NanoSystems Institute, UniVersity of California, Los Angeles, California 90095-1569, and Department of Materials Engineering, Monash UniVersity, Clayton VIC 3800, Australia ReceiVed: April 7, 2010; ReVised Manuscript ReceiVed: August 1, 2010

Here we report on the synthesis of a graphene/polyaniline (PANI) nanocomposite and its application in the development of a hydrogen (H2) gas sensor. Using a chemical synthetic route, graphene was prepared and ultrasonicated with a mixture of aniline monomer and ammonium persulfate to form PANI on its surface. The developed material was characterized by scanning electron microscopy (SEM), transmission electron microscopy, Raman spectroscopy, and X-ray photoemission spectroscopy. The SEM study revealed that the PANI in the composite has a nanofibrillar morphology. We investigated the H2 gas sensing performance of this material and compare it with that of the sensors based on only graphene sheets and PANI nanofibers. We found that the graphene/PANI nanocomposite-based device sensitivity is 16.57% toward 1% of H2 gas, which is much larger than the sensitivities of sensors based on only graphene sheets and PANI nanofibers. Introduction Graphene is a two-dimensional sheet of carbon atoms bonded through sp2 hybridization.1 Several studies have reported on the excellent properties of graphene including Young’s modulus ∼1100 GPa,2 thermal conductivity ∼5000 Wm-1 K-1,3 mobility of charge carriers of up to 200 000 cm2/(V s),4 specific surface area (a calculated value of 2630 m2 g-1),5 and interesting transport phenomena such as the quantum Hall effect.6 The first successful experimental preparation of graphene was reported in 2004 by Novoselov et al. through the mechanical exfoliation of small mesas of highly oriented pyrolytic graphite.7 However, this method has a very low yield, which is not suitable for large scale production as well as it is difficult to control the number of exfoliated layers. Other methods have been reported in the literature such as chemical vapor deposition (CVD) and epitaxial growth which often require very high temperatures ∼1000 °C, ultrahigh vacuum equipment, and/or lengthy purification procedures.8–10 An alternative promising approach is the chemical modification of graphene/graphite oxide or graphite fluoride to generate homogeneous colloidal suspensions known as chemically modified graphene (CMG).11 This approach has the advantages of being an economical solution suitable for largescale production and simultaneously viable for chemical functionalization. Because of the disruption of the “graphitic” networks, CMGs are electrically insulating and hence reduction of the graphene oxide by chemical means such as hydrazine is needed to restore electrical conductivity.12 Through the CMG approach, aqueous dispersions of graphene nanosheets were * To whom correspondence should be addressed. E-mail: (L.A.M.) [email protected]; (K.S.) [email protected]. † School of Electrical and Computer Engineering, RMIT University. ‡ Sejong University. § School of Applied Sciences, RMIT University. | University of California. ⊥ Monash University.

recently prepared without the inclusion of any stabilizers or surfactants.13 Instead, electrostatic repulsion between negatively charged graphene oxide sheets was found to be responsible for generating a stable aqueous suspension. In this study, we have fabricated and tested H2 gas sensors based on these graphene nanosheets for comparison with a graphene/PANI nanocomposite H2 gas sensor. There is growing interest in the scientific community on the development of sensors based on graphene for its unique properties.14 Novoselov et al. investigated mechanically exfoliated graphene for the detection of individual gas molecules.15 Concentrations as low as 1 ppm (ppm) of NO2, NH3, H2O, and CO gases were detected. Thermally reduced graphene oxide has also been tested for NO2 and NH3 gas sensing.16 A sensitivity of ∼1.56 for NO2 gas was recorded. However, the NH3 gas sensor did not produce a coherent response when it was tested after 2 months. Further analysis suggested nonohmic contact between the gold electrode and the graphene oxide due to an asymmetric I-V curve, which was likely responsible for the alteration of the response through variations in the absorbateinduced Schottky barrier. In contrast, gas sensors based on graphene from the CMG approach maintained ohmic contacts between graphene and metal electrodes during interaction with NO2 and NH3.17 In another report, a surface acoustic wave gas sensor based on chemically modified graphenelike nanosheets was fabricated and used to detect H2 and CO gases.18 Conjugated polymers such as PANI, polypyrrole, and polythiophene have been extensively studied for the development of gas sensors based on their high sensitivities, fast response times, and room temperature operation.19–22 Their electronic structure is comprised of conjugated π-bonds that can undergo changes under the influence of chemical species adsorbed onto their surfaces due to a redox or acid-base type interaction between the polymer and the chemical species.

10.1021/jp103134u  2010 American Chemical Society Published on Web 09/08/2010

Graphene/PANI Nanocomposite for Hydrogen Sensing Recently, several reports have been made on incorporating graphene into polymer matrices to produce novel nanocomposite materials.23,24 It was found that a homogeneously dispersed graphene nanofiller in a polymer matrix provided nanocomposites with enhanced mechanical, electrical, thermal, and other properties due to the high aspect and surface-to-volume ratios of the nanofiller. It was also suggested that graphene nanosheets can provide more active nucleation sites for PANI as well as excellent electron transfer pathways.24 Here we report, for the first time to the best of our knowledge, a hydrogen gas sensor featuring a graphene/PANI nanocomposite as the sensitive layer. Because of the high surface-to-volume ratio of the incorporated graphene nanosheets, the developed sensor has higher sensitivity toward H2 gas than that of only graphene and PANI nanofibersbased gas sensors. Experimental Section The graphene/PANI nanocomposite was prepared through chemical polymerization. Graphite oxide (GO) was synthesized through graphite oxidation with sulfuric acid and potassium permanganate (H2SO4-KMnO4). Reduction of GO to graphene was achieved via refluxing GO with hydrazine in N,Ndimethylformamide (DMF). The graphene was sonicated for one hour in 10 mL of ethylene glycol in the presence of 1 mmol of aniline and 0.05 mmol of N-phenyl-1,4-phenylenediamine. Then 0.25 mmol of APS in 10 mL of 1.0 M HCl was rapidly added to the mixture. The resulting solution was magnetically stirred overnight to yield a graphene/PANI nanocomposite. For comparison, graphene nanosheets were also synthesized, characterized, and employed as a sensitive material for H2 gas. Aqueous graphene dispersions were synthesized from graphite oxide via a CMG approach as reported by Li et al.13 Graphene nanosheets were obtained through hydrazine reduction. The optimum hydrazine/GO ratio of 7:10 by weight was employed to obtain a stable solution, since an excessive hydrazine concentration may cause restacking of the graphene sheets. Further experimental details on graphene nanosheet synthesis can be found in the methods section of ref 13. To obtain a complete evaluation of the graphene/PANI based H2 sensor performance, a sensor based on doped PANI nanofibers was also tested toward H2 gas. The synthesis conditions for the PANI nanofibers were identical to that of the PANI in the graphene/PANI nanocomposite. Hence the same doping degree of PANI for both materials was carefully maintained by utilizing an identical type and concentration of the doping acid during synthesis, which was 1.0 M HCl. Further details on the synthesis process, characterization, and gas sensing of PANI nanofibers are provided elsewhere.19 The morphologies of the developed nanomaterials were assessed using an FEI Nova NanoSEM and an FEI CM120 transmission electron microscopy (TEM) which has a LaB6 filament operating at 120 KeV. Raman spectra were recorded with a Ramanstation 400F Dispersive Raman Spectrometer (Perkin-Elmer) using a 350 mW near-infrared laser operating at 785 nm with a cooled CCD detector. The elemental composition of graphene/PANI was determined by X-ray photoemission spectroscopy (XPS) measurements, which were carried out using a Thermo K-Alpha spectrometer using an Al-KR source with a spot size of 400 µm. Charging was minimized by a low energy electron ion flood gun. Individual peaks were scanned at 200 eV pass energy with an energy step of 1.0 eV. Conductometric transducers were fabricated by depositing metal thin films on quartz substrates through a Balzer electron

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16169 beam evaporator. A 50 nm thick layer of chromium was deposited first on quartz substrates to provide an adhesive medium for gold. Then a 100 nm gold layer was deposited over the chromium layer. A dicing saw was used to cut the metal coated quartz substrates to 8 × 12 mm2. Gold interdigital transducers were patterned through the standard microfabrication processes of photolithography and etching. The width of each gold finger and the spacing between fingers were both 50 µm. The solutions containing the nanomaterials were airbrushed onto the surface of the heated conductometric transducers. The transducer temperature was 60 °C. Nitrogen was used for airbrushing because it is an inert gas. The distance between the airbrush tip and the transducer surface was maintained at about 10 cm. The deposited film thicknesses were measured with an Ambios XP-2 Stylus profilometer and were found to be ∼500 nm. The sensors were mounted inside an enclosed environmental cell. Four mass flow controllers (MFC) were connected to form a single output that supplied gas to the cell. Teflon tubing was used to prevent atmospheric contamination. A constant flow rate of 200 SCCM was delivered via the MFCs. The sensors were exposed to a H2 gas pulse sequence with different concentrations of H2 gas diluted in synthetic air at room temperature. A computerized gas calibration system was used to vary the concentration of H2 gas in the synthetic air. The sensors were connected to a Keithley 2001 multimeter to measure the variations in film resistance during the interaction with H2 gas. The multimeter resolution for the 200 kΩ range, which was used during the measurements, was 10 mΩ. A computer was used to log the data from the multimeter. Certified gas bottles were used in this study. Any external humidity from the room environment introduced during the placement of the sensors inside the gas chamber was removed by purging the gas chamber with dry synthetic air for 1.5 h before starting the tests. The synthetic air used consisted of 79% nitrogen and 21% oxygen as specified by the supplier, while the H2 gas bottle contained 1% of H2 balanced in synthetic air. The synthetic air was used as the washing gas. All gas sensing experiments were conducted at room temperature which was 24 °C. Results and Discussions Characterization. The morphology of the developed graphene/ PANI nanocomposite was investigated by taking scanning electron microscopy (SEM) images as shown in Figure 1. It can be seen that PANI nanofibers on the order of 25-50 nm have been grown on the surface of the graphene nanosheets which has been confirmed by TEM (Figure 2). In analyzing the morphology of the graphene/PANI nanocomposite, it is important to take into consideration the electron transport properties of both graphene and aniline monomer in order to determine the molecular binding mechanism. Graphene is a good electron acceptor, analogous to other forms of carbon such as the zero-dimensional fullerene (C60) or one-dimensional single-walled carbon nanotubes.25 Aniline, on the other hand, is a very good electron donor.24 It was reported earlier that C60 and CNTs can form donor-acceptor complexes in the liquid state when dissolved in tertiary amines and substituted anilines can form stable CNT-aniline complexes.26 In the likely mechanism for the formation of nanofibers on the graphene nanosheets, the aniline monomer is first adsorbed onto the surfaces of the graphene nanosheets due to electrostatic attractions. Oxidation of aniline by APS in the presence of the aniline dimer, N-phenyl-1,4-phenylenediamine, results in a

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Figure 1. SEM images of graphene/PANI nanocomposite (A) top view, (B) tilted view (45°).

Figure 2. TEM image of a graphene/PANI nanocomposite.

product with the nanostructure shown in Figures 1 and 2. It was reported earlier that a rapid reaction promotes homogeneous nucleation of growing PANI chains and thus the formation of uniform PANI nanofibers.19 It is anticipated that the N-phenyl1,4-phenylenediamine has accelerated the polymerization of the aniline monomer and results in a nanofibrillar morphology on the surface of the graphene nanosheets. Hence, the graphene nanosheets can be considered as a support material for PANI growth providing a large number of active nucleation sites.24 The Raman spectra for the developed graphene/PANI nanocomposite as well as for graphene and PANI are shown in Figure 3. The Raman spectra of pure graphene and the graphene based nanocomposite have very distinctive peaks for the D and G bands. The D band peak which can be found in Figure 3 at around 1318 cm-1 is attributed to the K-point phonons of A1g symmetry, while the G band peak at around 1592 cm-1 is usually assigned to zone center phonons of E2g symmetry.27 Characteristic vibrational peaks of PANI are also evident in the graphene/PANI nanocomposite spectrum. For the doped form of the polymer, both polaronic and bipolaronic structures are expected to coexist and hence the vibrational modes of both structures contribute to the overall spectrum. However, a slight deviation from the theoretical values is always anticipated.28 In the graphene/PANI nanocomposite spectrum presented in Figure 3, the bipolaronic N-H bending vibration can be seen at 1504 cm-1. The peak at 1372 cm-1 can be attributed to the C-N stretching vibration of the cation radical species indicating an intermediate level of doping. The peak at 1172 cm-1 can be

Figure 3. Raman spectra of (A) a graphene/PANI nanocomposite, (B) PANI, and (C) graphene with the inset showing the 2D band of the graphene/PANI nanocomposite.

attributed to the C-H bending of the quinoid ring. A minor polaronic peak at 856 cm-1 is a characteristic benzene ring deformation. The peaks observed at 806, 712, 640, and 576 cm-1 can be attributed to bipolaronic quinoid ring deformations, bipolaronic C-C ring deformations, bipolaronic benzene ring deformations, and bipolaronic amine deformations, respectively. The peak at 518 cm-1 can be correlated to a polaronic amine deformation, while the peak related to the polaronic C-N-C torsion can be found at 418 cm-1.28 It was demonstrated in earlier studies that the number of graphene layers can be determined through examining the twodimensional (2D) band of the Raman spectrum.29,30 Raman measurements for our sample could not be taken using a micro Raman system with a spot size of ∼350 nm due to the nanofibrillar morphology of our material. Instead our Raman measurements were conducted using a Raman system with a spot size of 100 µm. The inset to Figure 3 shows the 2D band of our graphene/ PANI nanocomposite spectrum. It appears that the graphene in our graphene/PANI nanoconposite consists of a variety of sheets containing from just few layers up to several tens of layers.31,32 However, because of the nanofibrillar morphology and high

Graphene/PANI Nanocomposite for Hydrogen Sensing

Figure 4. Peak deconvolution of the C(1s) XPS core level for the graphene/PANI nanocomposite.

overall coverage of the PANI nanofibers, the number of graphene layers cannot be accurately determined from either the Raman measurements or SEM images. According to a study conducted by Ferrari,33 the number of graphene layers can be verified by identifying the four components of the 2D band which were named as 2D1B, 2D1A, 2D2A, and 2D2B. For our graphene/PANI nanocomposite, these four components can be found in the inset to Figure 3. The 2D1A and 2D2A peaks have higher relative intensities than the other two peaks. The relatively strong 2D1A corresponds to the presence of single or bilayer graphene sheets in the nanocomposite.31,32 However, the intensity of 2D2A is also large, which can correspond to the presence of multilayer graphene sheets.31,32 Further complication arises from the fact that the Raman spectrum of PANI itself exhibits an elevated intensity in the same wavenumber range of the 2D band due to its aromatic structure, which further distorts the number of layers measurement using Raman spectroscopy. We have mentioned earlier that the graphene/PANI nanocomposite can function as a donor-acceptor structure from the electron transport point of view. However for a graphene/TiO2 nanocomposite, it was found that further chemical reduction occurred due to electron transfer to graphene oxide.34,35 To clarify this point and for further understanding of the graphene/ PANI nanocomposite chemical composition, XPS measurements were conducted. Figure 4 shows the C(1s) XPS spectrum of the graphene/ PANI composite. Curve fitting of the C(1s) reveals a high intensity peak centered at a binding energy of 284.6 eV that corresponds to C-C, CdC and C-H bonds. It is reported that the peaks, found at shifts of +1.5, +2.5 and +4.0 eV from the main peak, can be attributed to the functional groups C-OH, CdO, and OdC-OH, respectively.36,37 In our work, these peaks are found at 286.08, 287.38, and 288.18 eV, respectively. It is also important to examine the O(1s) XPS spectrum in order to determine the chemical reduction state of the graphene/ PANI composite as shown in Figure 5. The deconvolution of the O(1s) spectrum results in three peaks, which are comparable to those found in earlier reports.36,37 The peak with the lowest intensity at a binding energy of ∼530 eV can be attributed to the CdO and OdC-OH functional groups, while the main peak with a binding energy of ∼532 eV can be assigned to the C-OH bond. The third peak is from bound water at a binding energy of ∼533.1 eV. The relatively high intensity of the H2O bond peak can be attributed to the fact that our chemically prepared graphene/PANI nanocomposite is suspended in water and the deposited films were not exposed to any heat treatment to avoid changing or decomposing the PANI. In a recent published study, Akhavan37 prepared graphene films from graphene oxide nanosheets following two heat

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Figure 5. Peak deconvolution of the O(1s) XPS core level for the graphene/PANI nanocomposite.

Figure 6. Peak deconvolution of the N(1s) XPS core level for the graphene/PANI nanocomposite.

treatment methods. The deconvolution of the O(1s) spectrum of graphene films obtained in his work showed a minimum intensity for the peak assigned to the CdO and OdC-OH functional groups. For the graphene/PANI nanocomposite, we have also noticed a minimum intensity for that peak as a result of chemical reduction. Notice that the shape of our O(1s) spectrum is very similar to the shapes of the O(1s) spectra of graphene films obtained by heat treatment.37 As a result, the graphene in our nanocomposite is a reduced graphene. The deconvolution of the N(1s) core level spectrum for graphene/PANI is shown in Figure 6. It consists of three components. The quinoid amine is found at a binding energy of 398.3 eV. The main benzenoid amine peak can be found at a binding energy of 399.7 eV. The nitrogen cationic radical (N+) component can be found at a binding energy of 401.1 eV. The deconvolution of the N(1s) core level spectrum presented here is comparable to the typical XPS N(1s) deconvolution of PANI reported in the literature.38 Hydrogen Gas Sensing Results. A gas sensor based on the graphene/PANI nanocomposite was fabricated and tested toward different concentrations of H2 gas balanced in synthetic air. Since PANI nanofibers are a well-known sensing material for H2 gas,19,39 we have fabricated and tested a PANI nanofiber-based sensor with the same doping degree of the PANI as in the graphene/PANI nanocomposite to benchmark the graphene/ PANI based sensor performance against a well-known polymer system. Additionally for comparison purposes, an H2 gas sensor based on graphene was also fabricated and tested under the same experimental conditions. Figure 7 displays the dynamic responses of the three different sensors. The sensor sensitivities (S) toward H2 gas were calculated according to the equation S ) |(Rgas - R0)/(R0)| × 100% and plotted in Figure 8, where Rgas is the sensing film resistance during gas exposure and R0 is the film resistance under synthetic air only.

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Figure 7. The dynamic responses of the H2 gas sensors with sensitive layers of (A) graphene/PANI, (B) PANI nanofibers, and (C) graphene.

Figure 8. The sensitivities of the H2 gas sensors with sensitive layers of (A) graphene/PANI, (B) PANI nanofibers, and (C) graphene.

During the exposure to 1% of H2 gas balanced in synthetic air, the different sensors responded differently. The graphenebased sensor sensitivity was 0.83%. The sensitivity of the PANI nanofibers-based sensor was 9.38%, while the graphene/PANIbased sensor sensitivity was 16.57%. It can be seen from Figure 8 that for low concentrations of H2 gas (0.06, 0.12, 0.25%) the sensitivities of the graphene/PANI nanocomposite and PANI nanofibers-based sensors are essentially equal. We believe that this similarity is caused by the fact that the same doping degree of PANI was achieved for both sensors. However for higher concentrations of H2 gas, the PANI nanofibers-based sensor response saturates. An earlier study suggested that the chemisorped H2 can react with oxygen from the environment to produce H2O.40 They also showed that the presence of H2O ruins the ability of PANI to sense H2. Similar effects might also deteriorate the response of our PANI sensors at high H2 concentrations (Figure 7B) due to the presence of 21% oxygen in the synthetic air used in our experiments. However, the presence of graphene nanosheets, which are employed to enhance the surface area of the overall nanostructure, resulted in the formation of PANI nanofibers with higher porosity than the pristine PANI nanofibers. As a result, the excess H2O,

produced during the exposure to high concentrations of H2, can be more easily released from the surface and hence the response does not deteriorate. Our observation is supported by published results in an earlier paper where it was found that the diameter of the PANI nanofibers has a direct effect on the H2 gas sensing performance.41 Hydrogen gas is a reducing agent and the mechanisms of its interaction with the sensitive materials considered in this report are largely governed by the chemical properties of each material. For graphene synthesized through chemical modification, which is considered in this study, the residual epoxide and carboxylic groups in the product are electron-withdrawing and promote holes in the valence band.17 Therefore, the interaction with a reducing (electron-donating) type gas such as NH3 would cause depletion of holes from the valence band and hence an increase in the materials’ resistance.16,17 This mechanism is applicable to the graphene-based sensor where the sensitive film resistance increases during the interaction with different concentrations of H2 gas balanced in synthetic air as shown in Figure 7C. The H2 sensing mechanism for PANI is not fully understood. A chemisorption of H2 gas molecules at the charged amine nitrogen sites may occur. The dissociation of the H2 bond then leads to the formation of new N-H bonds with the amine nitrogen on the PANI chain. Subsequent charge transfer between adjacent amine nitrogens returns the PANI back to its polaronic, doped, emeraldine salt state. Removal of the gas source and exposure to synthetic air releases the H2 from the polymer chains resulting in a completely reversible reaction.39 The polaronic form of PANI is the state of high conductivity. Therefore the PANI film resistance is expected to decrease during the interaction with H2 gas as observed from Figure 7B. Hence, both graphene and PANI interact with H2 gas but with opposite effects. For the graphene/PANI nanocomposite, the H2 gas sensing mechanism will be governed by the dominant interaction. From the dynamic response of the graphene/PANI nanocomposite toward H2 gas shown in Figure 7A, it can be concluded that the PANI gas sensing mechanism is the dominant one. This idea is also supported by the SEM study of the developed graphene/PANI nanocomposite as demonstrated in

Graphene/PANI Nanocomposite for Hydrogen Sensing Figure 1. According to the SEM images, it is clear that PANI nanostructures cover the whole surface uniformly. Therefore the charged amine nitrogen sites on the PANI backbone are largely accessible to H2 gas molecules resulting in the dominant PANI H2 gas sensing mechanism. However, the sensitivity of the graphene/PANI nanocomposite toward H2 gas is noticeably higher than that of graphene and PANI individually due to the higher porosity of the graphene/PANI nanocomposite. Conclusions The synthesis and device application of a graphene/PANI nanocomposite is reported. The developed material is characterized using SEM, TEM, Raman spectroscopy, and XPS. We have investigated the H2 gas sensing performance of this composite and compared it with that of sensors based on just graphene and PANI nanofibers separately. We have found that the graphene/PANI nanocomposite-based gas sensor sensitivity is 16.57% toward 1% of H2 gas, which is larger than the sensitivities of the sensors based on only PANI nanofibers (9.38%) and much larger than that of only graphene (0.83%). Acknowledgment. This work was financially supported by: (1) the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2010-0020207), (2) the Focused Centered Research Program’s Functional Engineering Architectonic’s Center based at UCLA (R.B.K.), (3) Monash University, and (4) RMIT University. References and Notes (1) Li, D.; Kaner, R. B. Science 2008, 320, 1170–1171. (2) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385– 388. (3) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett. 2008, 8, 902–907. (4) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Kilma, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351– 355. (5) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498–3502. (6) Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201–204. (7) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (8) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312–1314. (9) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; De Heer, W. A. Science 2006, 312, 1191–1196.

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