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Facile preparation of graphene sheets from synthetic graphite
Materials Letters 70 (2012) 181–184
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile preparation of graphene sheets from synthetic graphite Xiaobo Fu, Xueli Song, Yuanming Zhang ⁎ Department of Chemistry, Jinan University, Guangzhou 510632, China
a r t i c l e
i n f o
Article history: Received 27 November 2011 Accepted 1 December 2011 Available online 8 December 2011 Keywords: Carbon materials Microstructure Oxidation
a b s t r a c t Graphene sheets were prepared from the raw graphite by 67–70% HNO3 oxidation under refluxing at 140 °C for 4 h, followed by the treatment with KOH powder in an automatic mortar at room temperature for 1 h and subjecting to the 1-methyl-2-pyrrolidinone (NMP) solution for exfoliation. The obtained graphene sheets were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, atom force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The KOH in the process was found to play a key role in the preparation of the graphene sheets and can be regarded as a reducing agent. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a one-atom-thick with sp 2-hydridized carbon structure and two-dimensional individual sheet morphology which resulting in its extraordinarily remarkable electronic, thermal and mechanical properties, has been received growing number of attentions [1]. Many efforts have been devoted to prepare the graphene with individual or few layers, including micromechanical exfoliation of highly ordered pyrolytic graphite oxide [2], epitaxial growth [3], chemical vapor deposition [4,5], thermal exfoliation [6], bottom–up assembly [7], electrostatic deposition[8], liquid phase exfoliation of graphite [9–11], arc-discharging [12] and solvothermal method [13]. Among these methods, micromechanical cleavage is currently the most effective and reliable method to produce high-quality graphene sheets. However, it is unsuitable for large scale production of graphene sheets because of its low productivity. Therefore, working with chemically modified forms of graphene may provide a powerful solution. It is demonstrated that exfoliation of graphite oxide either by rapid thermal expansion or ultrasonic dispersion has been one of the best approaches to obtain graphene in bulk. Herein, in this letter, we present a facile method by liquid phase exfoliation of synthetic graphite to prepare graphene sheets from raw graphite.
was ground with 3.5 g KOH powder (RDH) in an automatic mortar at room temperature for 1 h. Then, the mixed graphite powder and KOH were put into 200 mL distilled water in a beaker. After the filtration, the filter cake was dried in an oven at 70 °C for 24 h. The obtained product was subjected to a final exfoliation to obtain graphene sheets by ultrasonication and centrifugation of a NMP solution (0.1 mg/mL) of the sample. Ultrasonication for 1 h yielded a uniform dispersion which was then centrifuged at 4000 rpm for 10 min. The top half of the dispersion was collected for further characterization. The microstructure and morphology of the as-synthesized graphene sheets were characterized by scanning electron microscopy (SEM, JEOL-6700F), atomic force microscopy, and high resolution transmission electron microscopy (HRTEM, JEOL-2010F operated at 200 kV). The specimens were prepared by drop casting from the NMP dispersion onto a carbon film coated copper grid and a silicon wafer. AFM measurement was performed on a NanoscopeMultiMode/Dimension (Digital Instruments). Raman spectra were recorded with a Renishaw RM3000 Raman microscope (laser: 514 nm with a 2 μm spot size). XPS measurement was performed using PHI 5600 (Physical Electronics) with a monochromated Al Ka radiation (hm = 1486.6 eV), calibrated internally by carbon deposit C 1 s (285.0 eV). 3. Results and discussion
2. Experimental The synthesis of graphene sheets was carried out by oxidation of 1.0 g synthetic graphite (purchased from Aldrich) in 100 mL 67–70% HNO3 at 140 °C by refluxing for 4 h. Then, the reaction solution was cooled down to room temperature. The graphite powder was filtered, washed, and dried in an oven. After that, 0.5 g dried graphite powder ⁎ Corresponding author. Tel./fax: + 86 2085225036. E-mail address: [email protected] (Y. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.12.002
As shown in Fig. 1a, the raw synthetic graphite exhibits an irregular morphology with a stack of carbon layers. The size of the graphite is about 1 μm. And we found that the flakes are dark, thick and large, showing the original graphite structure. The inset of the Fig. 1a is the selected area electron diffraction pattern of the graphite. The clearly diffraction rings corresponded to its typical crystal structure of raw graphite. Fig. 1b shows the obtained dispersions upon ultrasonication and centrifugation. The homogenous solution without precipitate indicated that the graphene sheets of monolayer or a few layers may be
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X. Fu et al. / Materials Letters 70 (2012) 181–184
Fig. 1. Graphene from liquid phase exfoliation of raw graphite. (a) TEM picture of raw graphite and the inset is the SAED pattern of the raw graphite. (b) Uniform dispersions upon centrifugation. (c) TEM image of the graphene sheets and the inset is its corresponding SAED pattern. (d) SEM image of the graphene sheets. (e) AFM image of the graphene sheets. (f) The corresponding section analyses of the AFM image.
obtained by exfoliation. Fig. 1c shows a TEM picture of typical graphene sheets. It can be observed that a graphene sheets with size of about 1 × 0.5 μm were situated on the top of the grid. The actual size is bigger for its folded structure. The inset in Fig. 1c is the selected area electron diffraction pattern taken from the graphene sheets, which showing the typical hexagonally arranged lattice of carbon in graphene. The intensity ratio of I{1100/I{2110} > 1 is a unique feature for monolayer graphene. Fig. 1d shows the SEM image of a graphene sheets. It also can be seen that the graphene sheets were folded and corrugated, which was consistent with the TEM results. AFM characterization has been
one of the most direct methods of quantifying the degree of exfoliation to graphene level after dispersion of the powder in a solution. Fig. 1d shows a typical large area AFM image of graphene sheets that were spin coated onto a mica surface from the diluted solution in Fig. 1b. The measured height of the flat graphene sheets is between 1 and 1.3 nm, as showing in Fig. 1d and f, indicating graphene was comprised of few layers. Based on the observations above, the graphene sheets with high quality can be obtained by this facile process. Raman scattering is strongly sensitive to the electronic structure and it has been proved to be an essential tool to characterize graphite
X. Fu et al. / Materials Letters 70 (2012) 181–184
G
Intensity (a.u.)
2D
(a) Intensity (a.u.)
C-C (284.8 eV)
C-C (285.2 eV) C-O (286.7 eV)
290
288
286
284
282
Binding Energy (eV)
(b) C-N (285.8 eV)
Intensity (a.u.)
and graphene materials. Fig. 2 shows typical Raman spectra of raw graphite and graphene sheets obtained from raw graphite. As can be seen from the spectra, three peaks were appeared. The peak at 1580 cm-1(G-band) corresponded to an E2g mode of graphite and is related to the vibration of sp 2-bonded carbon atoms [14]. The peak at 1350 cm − 1 (D-band) is associated with vibrations of disordered graphite. The second-order band (2D) is observed around 2700 cm − 1. In comparison with raw graphite, the ratio of intensities (ID/IG) for graphene sample is markedly increased from 0.12 to 0.31, indicating of the formation of some sp 3 carbon by HNO3 oxidation and then by KOH grinding. The ID/IG of the graphene sample is lower than the chemical converted graphene sheets obtained from hydrazine hydrate-reduced graphite oxide, which can be attributed to the significant decrease size of the in-plane sp 2 domains due to oxidation and ultrasonic exfoliation and partially disordered graphite crystal structure of graphene nanoplatelets. Fig. 3 shows the C1s XPS spectra of the sample before exfoliation and the graphene sheets. In Fig. 3a, after deconvolution, the C1s spectrum of the sample before exfoliation clearly shows the lower binding energy feature at 248.8 eV corresponding to C–C carbon and the higher binding energy feature at 285.2 eV followed by a shoulder at 286.7 eV, which is assigned to C–C sp 3 [14] and oxygen containing groups arising from hydroxyl, carbonyl, carboxyl functionalities. The content of the oxygen (atom weight 2.9%) is lower than that of the graphite oxide reported. After exfoliation, the C1s XPS spectrum of the graphene sheets show different peaks compared to the sample before exfoliation. The binding energy at 285.8 eV which can be assigned to C–N bond demonstrated the existence of the solvent, owing to the incomplete volatilization of NMP. Moreover, two peaks appeared at 287.2 eV and 289.0 eV corresponded to the carbonyl and carboxyl functionalities [15]. The results indicated that the graphene sheets containing oxygen groups can be exfoliated by the NMP solution. The analysis obtained from XPS can also support the Raman results. The formation mechanism of the graphene sheets from the raw graphite is still unclear, but we tentatively speculate that the whole process contains three steps: (1) the HNO3 oxidation can introduce some functional groups onto the edge or the surface of the raw graphite, such as –OH, –CO, –COOH. The main function of this step was to make the raw graphite expanded. It is reported that these oxygen functionalities will alter the van der Walls interactions between the layers and make them hydrophilic, which facilitating their exfoliation [15]. (2) The KOH grinding process can make the acid treated graphite with a large negative charge through reaction with hydroxyl, epoxy and carboxylic acid groups, resulting in reduced graphene oxide sheets, which is accordant with the previous results reported on the purification of nitric acid treated single walled carbon nanotubes by washing with alkaline hydroxide solution [16]. This can be
183
C-C (284.8 eV)
C=O (287.2 eV) C(O)O (289.0 eV)
292
290
288
286
284
282
280
Binding Energy (eV) Fig. 3. XPS spectra of (a) sample after KOH grinding from raw graphite and (b) graphene sheets.
supported by the XPS results. At the same time, it is reported that alkaline hydroxide can intercalates π-stacked structures of the graphite which can facilitate their exfoliation in solvent [16]. (3) The ultrasonication process can provide high energy environment to make the graphene sheets exfoliated by generating some gaseous species. 4. Conclusions In conclusion, we have demonstrated an effective and facile liquid phase exfoliation of graphite oxide to make graphene sheets. The KOH was found to be a key reactant to produce the graphene sheets and make the graphene reduced. We believe in that this effective synthetic method presented here may provide the way for successfully employing graphene for microelectronics, photovoltaic, composites and energy storage and conversion applications. Acknowledgement
(b)
D
The authors would like to thank for the support from “The Fundamental Research Funds for the Central Universities”. References
(a)
1000
1500
2000
2500
3000
Raman shift (cm-1) Fig. 2. Raman spectra of (a) raw graphite and (b) graphene sheets.
[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;306:666–9. [2] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Proc Natl Acad Sci U S A 2005;102:10451–3. [3] Berger C, Song ZM, Li TB, Li XB, Ogbazghi AY, Feng R, et al. J Phys Chem B 2004;108:19912–6. [4] Dato A, Radmilovic V, Lee ZH, Phillips J, Frenklach M. Nano Lett 2008;8:2012–6. [5] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Ahn JH, et al. Nature 2009;457:706–10.
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X. Fu et al. / Materials Letters 70 (2012) 181–184
[6] Osvath Z, Darabont A, Nemes-Incze P, Horvath E, Horvath ZE, Biro LP. Carbon 2007;45:3022–6. [7] Wu JS, Pisula W, Mullen K. Chem Rev 2007;107:718–47. [8] Sidorov AN, Yazdanpanah MM, Jalilian R, Ouseph PJ, Cohn RW, Sumanasekera GU. Nanotechnology 2007;18:135301–4. [9] Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun ZY, De S, et al. Nat Nanotechnol 2008;3:563–8. [10] Li XL, Wang XR, Zhang L, Lee SW, Dai HJ. Science 2008;319:1229–32. [11] Li XL, Zhang GY, Bai XD, Sun XM, Wang XR, Wang E, et al. Nat Nanotechnol 2008;3:538–42.
[12] Subrahmanyam KS, Panchakarla LS, Govindaraj A, Rao CNR. J Phys Chem C 2009;113:4257–9. [13] Choucair M, Thordarson P, Stride JA. Nat Nanotechnol 2009;4:30–3. [14] Diaz J, Paolicelli G, Ferrer S, Comin F. Phys Rev B 1996;54:8064–9. [15] Shen JF, Hu YZ, Shi M, Lu X, Qin C, Li C, et al. Chem Mater 2009;21:3514–20. [16] Salzmann CG, Llewellyn SA, Tobias G, Ward MAH, Huh Y, Green MLH. Adv Mater 2007;19:883–7.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile preparation of graphene sheets from synthetic graphite Xiaobo Fu, Xueli Song, Yuanming Zhang ⁎ Department of Chemistry, Jinan University, Guangzhou 510632, China
a r t i c l e
i n f o
Article history: Received 27 November 2011 Accepted 1 December 2011 Available online 8 December 2011 Keywords: Carbon materials Microstructure Oxidation
a b s t r a c t Graphene sheets were prepared from the raw graphite by 67–70% HNO3 oxidation under refluxing at 140 °C for 4 h, followed by the treatment with KOH powder in an automatic mortar at room temperature for 1 h and subjecting to the 1-methyl-2-pyrrolidinone (NMP) solution for exfoliation. The obtained graphene sheets were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, atom force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The KOH in the process was found to play a key role in the preparation of the graphene sheets and can be regarded as a reducing agent. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a one-atom-thick with sp 2-hydridized carbon structure and two-dimensional individual sheet morphology which resulting in its extraordinarily remarkable electronic, thermal and mechanical properties, has been received growing number of attentions [1]. Many efforts have been devoted to prepare the graphene with individual or few layers, including micromechanical exfoliation of highly ordered pyrolytic graphite oxide [2], epitaxial growth [3], chemical vapor deposition [4,5], thermal exfoliation [6], bottom–up assembly [7], electrostatic deposition[8], liquid phase exfoliation of graphite [9–11], arc-discharging [12] and solvothermal method [13]. Among these methods, micromechanical cleavage is currently the most effective and reliable method to produce high-quality graphene sheets. However, it is unsuitable for large scale production of graphene sheets because of its low productivity. Therefore, working with chemically modified forms of graphene may provide a powerful solution. It is demonstrated that exfoliation of graphite oxide either by rapid thermal expansion or ultrasonic dispersion has been one of the best approaches to obtain graphene in bulk. Herein, in this letter, we present a facile method by liquid phase exfoliation of synthetic graphite to prepare graphene sheets from raw graphite.
was ground with 3.5 g KOH powder (RDH) in an automatic mortar at room temperature for 1 h. Then, the mixed graphite powder and KOH were put into 200 mL distilled water in a beaker. After the filtration, the filter cake was dried in an oven at 70 °C for 24 h. The obtained product was subjected to a final exfoliation to obtain graphene sheets by ultrasonication and centrifugation of a NMP solution (0.1 mg/mL) of the sample. Ultrasonication for 1 h yielded a uniform dispersion which was then centrifuged at 4000 rpm for 10 min. The top half of the dispersion was collected for further characterization. The microstructure and morphology of the as-synthesized graphene sheets were characterized by scanning electron microscopy (SEM, JEOL-6700F), atomic force microscopy, and high resolution transmission electron microscopy (HRTEM, JEOL-2010F operated at 200 kV). The specimens were prepared by drop casting from the NMP dispersion onto a carbon film coated copper grid and a silicon wafer. AFM measurement was performed on a NanoscopeMultiMode/Dimension (Digital Instruments). Raman spectra were recorded with a Renishaw RM3000 Raman microscope (laser: 514 nm with a 2 μm spot size). XPS measurement was performed using PHI 5600 (Physical Electronics) with a monochromated Al Ka radiation (hm = 1486.6 eV), calibrated internally by carbon deposit C 1 s (285.0 eV). 3. Results and discussion
2. Experimental The synthesis of graphene sheets was carried out by oxidation of 1.0 g synthetic graphite (purchased from Aldrich) in 100 mL 67–70% HNO3 at 140 °C by refluxing for 4 h. Then, the reaction solution was cooled down to room temperature. The graphite powder was filtered, washed, and dried in an oven. After that, 0.5 g dried graphite powder ⁎ Corresponding author. Tel./fax: + 86 2085225036. E-mail address: [email protected] (Y. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.12.002
As shown in Fig. 1a, the raw synthetic graphite exhibits an irregular morphology with a stack of carbon layers. The size of the graphite is about 1 μm. And we found that the flakes are dark, thick and large, showing the original graphite structure. The inset of the Fig. 1a is the selected area electron diffraction pattern of the graphite. The clearly diffraction rings corresponded to its typical crystal structure of raw graphite. Fig. 1b shows the obtained dispersions upon ultrasonication and centrifugation. The homogenous solution without precipitate indicated that the graphene sheets of monolayer or a few layers may be
182
X. Fu et al. / Materials Letters 70 (2012) 181–184
Fig. 1. Graphene from liquid phase exfoliation of raw graphite. (a) TEM picture of raw graphite and the inset is the SAED pattern of the raw graphite. (b) Uniform dispersions upon centrifugation. (c) TEM image of the graphene sheets and the inset is its corresponding SAED pattern. (d) SEM image of the graphene sheets. (e) AFM image of the graphene sheets. (f) The corresponding section analyses of the AFM image.
obtained by exfoliation. Fig. 1c shows a TEM picture of typical graphene sheets. It can be observed that a graphene sheets with size of about 1 × 0.5 μm were situated on the top of the grid. The actual size is bigger for its folded structure. The inset in Fig. 1c is the selected area electron diffraction pattern taken from the graphene sheets, which showing the typical hexagonally arranged lattice of carbon in graphene. The intensity ratio of I{1100/I{2110} > 1 is a unique feature for monolayer graphene. Fig. 1d shows the SEM image of a graphene sheets. It also can be seen that the graphene sheets were folded and corrugated, which was consistent with the TEM results. AFM characterization has been
one of the most direct methods of quantifying the degree of exfoliation to graphene level after dispersion of the powder in a solution. Fig. 1d shows a typical large area AFM image of graphene sheets that were spin coated onto a mica surface from the diluted solution in Fig. 1b. The measured height of the flat graphene sheets is between 1 and 1.3 nm, as showing in Fig. 1d and f, indicating graphene was comprised of few layers. Based on the observations above, the graphene sheets with high quality can be obtained by this facile process. Raman scattering is strongly sensitive to the electronic structure and it has been proved to be an essential tool to characterize graphite
X. Fu et al. / Materials Letters 70 (2012) 181–184
G
Intensity (a.u.)
2D
(a) Intensity (a.u.)
C-C (284.8 eV)
C-C (285.2 eV) C-O (286.7 eV)
290
288
286
284
282
Binding Energy (eV)
(b) C-N (285.8 eV)
Intensity (a.u.)
and graphene materials. Fig. 2 shows typical Raman spectra of raw graphite and graphene sheets obtained from raw graphite. As can be seen from the spectra, three peaks were appeared. The peak at 1580 cm-1(G-band) corresponded to an E2g mode of graphite and is related to the vibration of sp 2-bonded carbon atoms [14]. The peak at 1350 cm − 1 (D-band) is associated with vibrations of disordered graphite. The second-order band (2D) is observed around 2700 cm − 1. In comparison with raw graphite, the ratio of intensities (ID/IG) for graphene sample is markedly increased from 0.12 to 0.31, indicating of the formation of some sp 3 carbon by HNO3 oxidation and then by KOH grinding. The ID/IG of the graphene sample is lower than the chemical converted graphene sheets obtained from hydrazine hydrate-reduced graphite oxide, which can be attributed to the significant decrease size of the in-plane sp 2 domains due to oxidation and ultrasonic exfoliation and partially disordered graphite crystal structure of graphene nanoplatelets. Fig. 3 shows the C1s XPS spectra of the sample before exfoliation and the graphene sheets. In Fig. 3a, after deconvolution, the C1s spectrum of the sample before exfoliation clearly shows the lower binding energy feature at 248.8 eV corresponding to C–C carbon and the higher binding energy feature at 285.2 eV followed by a shoulder at 286.7 eV, which is assigned to C–C sp 3 [14] and oxygen containing groups arising from hydroxyl, carbonyl, carboxyl functionalities. The content of the oxygen (atom weight 2.9%) is lower than that of the graphite oxide reported. After exfoliation, the C1s XPS spectrum of the graphene sheets show different peaks compared to the sample before exfoliation. The binding energy at 285.8 eV which can be assigned to C–N bond demonstrated the existence of the solvent, owing to the incomplete volatilization of NMP. Moreover, two peaks appeared at 287.2 eV and 289.0 eV corresponded to the carbonyl and carboxyl functionalities [15]. The results indicated that the graphene sheets containing oxygen groups can be exfoliated by the NMP solution. The analysis obtained from XPS can also support the Raman results. The formation mechanism of the graphene sheets from the raw graphite is still unclear, but we tentatively speculate that the whole process contains three steps: (1) the HNO3 oxidation can introduce some functional groups onto the edge or the surface of the raw graphite, such as –OH, –CO, –COOH. The main function of this step was to make the raw graphite expanded. It is reported that these oxygen functionalities will alter the van der Walls interactions between the layers and make them hydrophilic, which facilitating their exfoliation [15]. (2) The KOH grinding process can make the acid treated graphite with a large negative charge through reaction with hydroxyl, epoxy and carboxylic acid groups, resulting in reduced graphene oxide sheets, which is accordant with the previous results reported on the purification of nitric acid treated single walled carbon nanotubes by washing with alkaline hydroxide solution [16]. This can be
183
C-C (284.8 eV)
C=O (287.2 eV) C(O)O (289.0 eV)
292
290
288
286
284
282
280
Binding Energy (eV) Fig. 3. XPS spectra of (a) sample after KOH grinding from raw graphite and (b) graphene sheets.
supported by the XPS results. At the same time, it is reported that alkaline hydroxide can intercalates π-stacked structures of the graphite which can facilitate their exfoliation in solvent [16]. (3) The ultrasonication process can provide high energy environment to make the graphene sheets exfoliated by generating some gaseous species. 4. Conclusions In conclusion, we have demonstrated an effective and facile liquid phase exfoliation of graphite oxide to make graphene sheets. The KOH was found to be a key reactant to produce the graphene sheets and make the graphene reduced. We believe in that this effective synthetic method presented here may provide the way for successfully employing graphene for microelectronics, photovoltaic, composites and energy storage and conversion applications. Acknowledgement
(b)
D
The authors would like to thank for the support from “The Fundamental Research Funds for the Central Universities”. References
(a)
1000
1500
2000
2500
3000
Raman shift (cm-1) Fig. 2. Raman spectra of (a) raw graphite and (b) graphene sheets.
[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;306:666–9. [2] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Proc Natl Acad Sci U S A 2005;102:10451–3. [3] Berger C, Song ZM, Li TB, Li XB, Ogbazghi AY, Feng R, et al. J Phys Chem B 2004;108:19912–6. [4] Dato A, Radmilovic V, Lee ZH, Phillips J, Frenklach M. Nano Lett 2008;8:2012–6. [5] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Ahn JH, et al. Nature 2009;457:706–10.
184
X. Fu et al. / Materials Letters 70 (2012) 181–184
[6] Osvath Z, Darabont A, Nemes-Incze P, Horvath E, Horvath ZE, Biro LP. Carbon 2007;45:3022–6. [7] Wu JS, Pisula W, Mullen K. Chem Rev 2007;107:718–47. [8] Sidorov AN, Yazdanpanah MM, Jalilian R, Ouseph PJ, Cohn RW, Sumanasekera GU. Nanotechnology 2007;18:135301–4. [9] Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun ZY, De S, et al. Nat Nanotechnol 2008;3:563–8. [10] Li XL, Wang XR, Zhang L, Lee SW, Dai HJ. Science 2008;319:1229–32. [11] Li XL, Zhang GY, Bai XD, Sun XM, Wang XR, Wang E, et al. Nat Nanotechnol 2008;3:538–42.
[12] Subrahmanyam KS, Panchakarla LS, Govindaraj A, Rao CNR. J Phys Chem C 2009;113:4257–9. [13] Choucair M, Thordarson P, Stride JA. Nat Nanotechnol 2009;4:30–3. [14] Diaz J, Paolicelli G, Ferrer S, Comin F. Phys Rev B 1996;54:8064–9. [15] Shen JF, Hu YZ, Shi M, Lu X, Qin C, Li C, et al. Chem Mater 2009;21:3514–20. [16] Salzmann CG, Llewellyn SA, Tobias G, Ward MAH, Huh Y, Green MLH. Adv Mater 2007;19:883–7.