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A simple method for achieving surface-enhanced Raman scattering of single-layer and few-layer graphene
Journal of Molecular Structure 1040 (2013) 213–215
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
A simple method for achieving surface-enhanced Raman scattering of single-layer and few-layer graphene Yu Ouyang ⇑ Experimental and Managing Center, Lin Yi University, 276005, China
h i g h l i g h t s We make it easier for detecting SERS of single-layer and few-layer graphene. The D0 band is split into four peaks which are concordant with the electron dispersion. The splitting of D0 band can provide more information about the defect structure. This method has much potential to be applied in the studies of defect structure in graphene.
a r t i c l e
i n f o
Article history: Received 5 March 2013 Accepted 5 March 2013 Available online 13 March 2013 Keywords: Silver Graphene Raman spectroscopy Surface-enhanced Raman scattering
a b s t r a c t We detect significant enhancements for single-layer and few-layer graphene on a new cutting cross section of 50-lm thick silver strip at 785 nm. Besides the G and 2D bands are greatly enhanced, for few-layer graphene, the D0 band (1620 cm1) is split into four peaks and the results are concordant with the electron dispersion. This method is easy for operation and observation, and can provide more information about the D0 band, has much potential to be applied in the studies of defect structure in graphene. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Raman spectroscopy has been utilized as a useful tool for the characterization of graphene since it was discovered in 2004 [1]. Raman spectroscopy can identify the number of layers, the electronic structure, the edge structure, the type of doping and defects in the graphene [2–7]. However, Raman signals of graphene are very weak, even undetectable on many substrates, and fine structural characteristics of graphene can not be sensitively probed and well-distinguished [8]. Therefore, enhancing the Raman signals of graphene is becoming the research hotspot. Fist, many researchers found the Si substrate with a metal oxide layer of a specific thickness can employed to enhance the Raman signal of single- and few-layer graphene, which is called interference-enhanced Raman scattering (IERS) [9–11]. And then, an Ag layer is inserted between the layers of metal oxide (Al2O3) and Si further to enhance the Raman signal of graphene. This is ascribed to a combi-
⇑ Tel.: +86 539 2060903; fax: +86 539 2060903. E-mail addresses: [email protected], [email protected] 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.03.011
nation of IERS and surface-enhanced Raman scattering (SERS) [8]. Now, gold disk, rods, film, and nanoparticles are deposited on graphene [12–14], results about SERS of single- and few-layer graphene have been obtained. The present study reports a simple method for achieving surface-enhanced Raman scattering of single-layer and few-layer graphene. Only need a stick of HOPG, one roll of scotch tape, one sheet of silver strip, a pair of scissors and a Raman microscope system. Graphene flakes are prepared by mechanical exfoliation from HOPG using scotch tape. A new cutting cross section of silver strip serves as substrate for surface-enhanced Raman scattering. Graphene flakes move onto the cutting cross section by simple physical contact. Because the silver strip is only 50lm thick, and the new cutting cross section is smooth, clean, non-dusty and non-oily. Under a microscope, the cutting cross became one 50-lm wide smooth surface, we can found f graphene flakes very easy. With this system, we detect SERS of singlelayer and few-layer graphene at 785 nm. Besides the G and 2D bands are greatly enhanced, for few-layer graphene, the D0 band (1620 cm1) is split into four peaks, the results are concordant with the electron dispersion. This provides more information about the defect structure of graphene.
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Y. Ouyang / Journal of Molecular Structure 1040 (2013) 213–215
2. Experimental Graphene flakes are prepared by mechanical exfoliation from HOPG using scotch tape. The 50-lm thick silver is cut into stripe of 1 cm Long and 2 mm wide. Use the cutting cross section (50 lm 2 mm) to touch graphene flakes which attached to scotch tape. Then, examine the cutting cross section under the microscope, search for graphene which move onto the cutting cross section from scotch tape. The Raman spectra were recorded by a Renishaw inVia spectrophotometer with a Leica DMLB microscope at room temperature and in a backscattering geometry, with 785 nm excitation. The output laser power was about 1mW. During the measurement, light from the laser was directed and focused onto the sample on a microscope stage through a 50 objective. Raman scattering signals were detected by a 578 385 pixels CCD array detector. Spectral data were collected by the WiRE 2.0 software and Raman band was fitted by Origin 6.0.
3. Results and discussion Fig. 1 shows the optical images of the cutting cross section of silver before (Fig. 1a) and after (Fig. 1b) adsorbing graphene. The cutting cross section of silver is about 50 lm wide. Parallel tearing ridges could be seen on the tensile fracture surfaces (Fig. 1a). The tearing ridge is about 1 lm wide. Through adjusting image contrast, we can faintly see very thin graphene chips which disperse on the cutting cross section of silver (Fig. 1b), such as the rectangular area (;) and triangular area ( ). At the same condition (incident power of 1 mW and an exposure time of 10 s), we detect the cutting cross section of silver, the rectangular area (;), triangular area ( ) and HOPG respectively. Results are shown in Fig. 2. The cutting cross section of silver shows no Raman peaks (Fig. 2a). On the region (;) (Fig. 2b), it shows a sharp and single 2D band at 2612 cm1 with a full width at half maximum (FWHM) of 32 cm1. The G band is at 1580 cm1 with a FWHM of 9.6 cm1. The I2D/IG is about 5.8. According to the above spectral features, we think it is a single-layer graphene. It has been shown by experimentation that single-layer graphene shows no band between 1250 and 2850 cm1 in the normal Raman
Fig. 1. Optical images of the cutting cross section of silver before (a) and after (b) adsorbing graphene.
Fig. 2. Raman spectra of Ag (a), HOPG (d) and SERS of single-layer (b) and few-layer (c) graphene, with 785 nm excitation, at the same condition (incident power of 1 mW and an exposure time of 10 s).
spectrum with 785 nm laser [15]. So, on the cutting cross section of silver, it is SERS of single-layer graphene and the SERS enhancement factor is expected to be large. On the region ( ) (Fig. 2c), the 2D band locate in 2612 cm1 with a shoulder at about 2646 cm1. The G band is at 1580 cm1 with a FWHM of 20 cm1. The I2D/IG is about 1.09. From the shape and position of 2D band, we can judge it is a few-layer graphene on the region ( ). Cançado et al. compared the Raman spectra in the 2D band region for different layers graphene and HOPG [16]. They found that for 10-LG, the shape of 2D band is almost the same as HOPG’s, with a sharp peak at high frequency side of the 2D band. When the number of graphene layers is five, the intensity of high frequency side of the 2D band decreases significantly. For fourlayer graphene, the sharp peak at high frequency side of 2D band disappears almost completely. So Cançado et al. pointed out fewlayer graphene could be distinguished from HOPG by the sharp
Fig. 3. G0 band region in SERS of few-layer graphene and fitted with four Gaussian lines.
Y. Ouyang / Journal of Molecular Structure 1040 (2013) 213–215
215
Fig. 4. The DR process of the D0 band for single-layer (a) [17] and bilayer graphene (b–e).
peak at high frequency side of 2D band. Here, the 2D band on the region ( ) is obvious that the intensity of the sharp peaks at high frequency side, differ from HOPG’s (Fig. 2d), so we can judge it is a few-layer graphene on the region ( ). Moreover, IG of few-layer graphene is apparently higher than HOPG’s (Fig. 2c and d). This is attributed to the enhance effect of the cutting cross section of silver. More important, four new Raman bands (1600–1630 cm1) are shown in SERS of few-layer graphene (Fig. 2c). Usually the D0 band lies in this region (1620 cm1). The D0 band is a weak disorder-induced feature which originates from a double resonance (DR) Raman process. But in SERS of few-layer graphene (Fig. 2c), the D0 band splits into four bands. We fitted the D0 band with four Gaussian lines, as shown in Fig. 3. They lie at 1594, 1603, 1612 and 1626 cm1, with the full width at half maximum 5.2, 7.9, 9.2 and 5.6 cm1 respectively. For the splitting of D0 band, through analysis, graphene flakes prepared by mechanical exfoliation often has defect construction on the edge, defects easy form plenty of unsaturated chemical bonds and roughed surface, which ensure strong adsorption and interaction with Ag surface. As a result, Raman signals from defects are enhanced greatly. And according to the adsorption effect of Ag, the symmetry of graphene changes from high to low, the selective rules for the Raman spectrum of graphene becomes wider. So, in SERS of few-layer graphene, D0 band is enhanced and split. In addition, we analyze the electron dispersion of bilayer (2-LG) graphene near the K point and find the D0 band consisted with several Raman lines. The D0 band originates from a DR Raman process. For single-layer graphene, the DR process shown in Fig. 4a begins with an electron of wave-vector k around K absorbing a photon of energy Elaser. The electron is inelastically scattered by a defect of wavevector q and energy Ephonon to a point belonging to the same circle around the K point, with wavevector k + q. The electron is then scattered back to a k state, and emits a photon by recombining with a hole at a k state [17]. For 2-LG graphene with Bernal AB layer stacking, both the electronic and phonon bands split into two components with special symmetries [18]. Fig. 4b shows a schematic view of the bilayer graphene electronic structure where the upper (lower) and lower (upper) branches of the valence (conduction) band are labeled as p1 (p1) and p2 (p2), respectively [19]. Then the DR process turns into four different Pij processes as shown in Fig. 4b–e. The results are concordant with SERS in Fig. 3.
4. Conclusion It is very easy for operation and observation to use the cutting cross section of silver strip as substrate in SERS of graphene. And the enhance effect is significant, especially for the D0 band. The SERS system causes D0 band of few-layer graphene splitting and provides more information about D0 band. This method could be used in study on defect structure of graphene. Acknowledgements The authors thank the support of National Natural Science Foundation of China (10974076). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [2] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (187) (2006) 401. [3] J. Yan, Y. Zhang, P. Kim, A. Pinczuk, Phys. Rev. Lett. 98 (166) (2007) 802. [4] S. Pisana, M. Lazzeri, C. Casiraghi, K.S. Novoselov, A.K. Geim, A.C. Ferrari, F. Mauri, Nat. Mater. 6 (2007) 198. [5] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, K.S. Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari, A.K. Sood, Nat. Nanotechnol. 3 (2008) 210. [6] A.K. Gupta, T.J. Russin, H.R. Gutierresz, P.C. Eklund, ACS Nano 3 (2009) 45. [7] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, ACS Nano 2 (2008) 2301. [8] Libo Gao, Wencai Ren, Bilu Liu, Riichiro Saito, Wu Zhong-Shuai, Shisheng Li, Chuanbin Jiang, Feng Li, Hui-Ming Cheng, ACS Nano 3 (2009) 933. [9] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [10] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Nano Lett. 6 (2667) (2006) 2673. [11] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (238) (2007) 242. [12] F. Schedin, E. Lidorikis, A. Lombardo, V. G. Kravets, A. K.Geim, A. N. Grigrenko, K. S. Novoselov, A. C. Ferrari, vl. [13] Y.-K. Kim, H.-K. Na, Y.W. Lee, H. Jang, S.W. Han, D.-H. Min, Chem. Commun. 46 (2010) 3185. [14] N. Kim, M.K. Oh, S. Park, S.K. Kim, B.H. Hong, Bull. Korean Chem. Soc. 31 (2010) 999. [15] Jisook Lee, Sangdeok Shim, Bongsoo Kim, Chem. Eur. J. 17 (2011) 2381. [16] L.G. Cançado, A. Reina, J. Kong, M.S. Dresselhaus, Phys. Rev. B. 77 (2008) 245408. [17] Mildred S. Dresselhaus, Gene Dresselhaus, Ado Jorio, Group Theory: Application to the Physics of Condensed Matter, Springer, Berlin, 2008. [18] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51. [19] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Modern Phys. 81 (2009) 109.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
A simple method for achieving surface-enhanced Raman scattering of single-layer and few-layer graphene Yu Ouyang ⇑ Experimental and Managing Center, Lin Yi University, 276005, China
h i g h l i g h t s We make it easier for detecting SERS of single-layer and few-layer graphene. The D0 band is split into four peaks which are concordant with the electron dispersion. The splitting of D0 band can provide more information about the defect structure. This method has much potential to be applied in the studies of defect structure in graphene.
a r t i c l e
i n f o
Article history: Received 5 March 2013 Accepted 5 March 2013 Available online 13 March 2013 Keywords: Silver Graphene Raman spectroscopy Surface-enhanced Raman scattering
a b s t r a c t We detect significant enhancements for single-layer and few-layer graphene on a new cutting cross section of 50-lm thick silver strip at 785 nm. Besides the G and 2D bands are greatly enhanced, for few-layer graphene, the D0 band (1620 cm1) is split into four peaks and the results are concordant with the electron dispersion. This method is easy for operation and observation, and can provide more information about the D0 band, has much potential to be applied in the studies of defect structure in graphene. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Raman spectroscopy has been utilized as a useful tool for the characterization of graphene since it was discovered in 2004 [1]. Raman spectroscopy can identify the number of layers, the electronic structure, the edge structure, the type of doping and defects in the graphene [2–7]. However, Raman signals of graphene are very weak, even undetectable on many substrates, and fine structural characteristics of graphene can not be sensitively probed and well-distinguished [8]. Therefore, enhancing the Raman signals of graphene is becoming the research hotspot. Fist, many researchers found the Si substrate with a metal oxide layer of a specific thickness can employed to enhance the Raman signal of single- and few-layer graphene, which is called interference-enhanced Raman scattering (IERS) [9–11]. And then, an Ag layer is inserted between the layers of metal oxide (Al2O3) and Si further to enhance the Raman signal of graphene. This is ascribed to a combi-
⇑ Tel.: +86 539 2060903; fax: +86 539 2060903. E-mail addresses: [email protected], [email protected] 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.03.011
nation of IERS and surface-enhanced Raman scattering (SERS) [8]. Now, gold disk, rods, film, and nanoparticles are deposited on graphene [12–14], results about SERS of single- and few-layer graphene have been obtained. The present study reports a simple method for achieving surface-enhanced Raman scattering of single-layer and few-layer graphene. Only need a stick of HOPG, one roll of scotch tape, one sheet of silver strip, a pair of scissors and a Raman microscope system. Graphene flakes are prepared by mechanical exfoliation from HOPG using scotch tape. A new cutting cross section of silver strip serves as substrate for surface-enhanced Raman scattering. Graphene flakes move onto the cutting cross section by simple physical contact. Because the silver strip is only 50lm thick, and the new cutting cross section is smooth, clean, non-dusty and non-oily. Under a microscope, the cutting cross became one 50-lm wide smooth surface, we can found f graphene flakes very easy. With this system, we detect SERS of singlelayer and few-layer graphene at 785 nm. Besides the G and 2D bands are greatly enhanced, for few-layer graphene, the D0 band (1620 cm1) is split into four peaks, the results are concordant with the electron dispersion. This provides more information about the defect structure of graphene.
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Y. Ouyang / Journal of Molecular Structure 1040 (2013) 213–215
2. Experimental Graphene flakes are prepared by mechanical exfoliation from HOPG using scotch tape. The 50-lm thick silver is cut into stripe of 1 cm Long and 2 mm wide. Use the cutting cross section (50 lm 2 mm) to touch graphene flakes which attached to scotch tape. Then, examine the cutting cross section under the microscope, search for graphene which move onto the cutting cross section from scotch tape. The Raman spectra were recorded by a Renishaw inVia spectrophotometer with a Leica DMLB microscope at room temperature and in a backscattering geometry, with 785 nm excitation. The output laser power was about 1mW. During the measurement, light from the laser was directed and focused onto the sample on a microscope stage through a 50 objective. Raman scattering signals were detected by a 578 385 pixels CCD array detector. Spectral data were collected by the WiRE 2.0 software and Raman band was fitted by Origin 6.0.
3. Results and discussion Fig. 1 shows the optical images of the cutting cross section of silver before (Fig. 1a) and after (Fig. 1b) adsorbing graphene. The cutting cross section of silver is about 50 lm wide. Parallel tearing ridges could be seen on the tensile fracture surfaces (Fig. 1a). The tearing ridge is about 1 lm wide. Through adjusting image contrast, we can faintly see very thin graphene chips which disperse on the cutting cross section of silver (Fig. 1b), such as the rectangular area (;) and triangular area ( ). At the same condition (incident power of 1 mW and an exposure time of 10 s), we detect the cutting cross section of silver, the rectangular area (;), triangular area ( ) and HOPG respectively. Results are shown in Fig. 2. The cutting cross section of silver shows no Raman peaks (Fig. 2a). On the region (;) (Fig. 2b), it shows a sharp and single 2D band at 2612 cm1 with a full width at half maximum (FWHM) of 32 cm1. The G band is at 1580 cm1 with a FWHM of 9.6 cm1. The I2D/IG is about 5.8. According to the above spectral features, we think it is a single-layer graphene. It has been shown by experimentation that single-layer graphene shows no band between 1250 and 2850 cm1 in the normal Raman
Fig. 1. Optical images of the cutting cross section of silver before (a) and after (b) adsorbing graphene.
Fig. 2. Raman spectra of Ag (a), HOPG (d) and SERS of single-layer (b) and few-layer (c) graphene, with 785 nm excitation, at the same condition (incident power of 1 mW and an exposure time of 10 s).
spectrum with 785 nm laser [15]. So, on the cutting cross section of silver, it is SERS of single-layer graphene and the SERS enhancement factor is expected to be large. On the region ( ) (Fig. 2c), the 2D band locate in 2612 cm1 with a shoulder at about 2646 cm1. The G band is at 1580 cm1 with a FWHM of 20 cm1. The I2D/IG is about 1.09. From the shape and position of 2D band, we can judge it is a few-layer graphene on the region ( ). Cançado et al. compared the Raman spectra in the 2D band region for different layers graphene and HOPG [16]. They found that for 10-LG, the shape of 2D band is almost the same as HOPG’s, with a sharp peak at high frequency side of the 2D band. When the number of graphene layers is five, the intensity of high frequency side of the 2D band decreases significantly. For fourlayer graphene, the sharp peak at high frequency side of 2D band disappears almost completely. So Cançado et al. pointed out fewlayer graphene could be distinguished from HOPG by the sharp
Fig. 3. G0 band region in SERS of few-layer graphene and fitted with four Gaussian lines.
Y. Ouyang / Journal of Molecular Structure 1040 (2013) 213–215
215
Fig. 4. The DR process of the D0 band for single-layer (a) [17] and bilayer graphene (b–e).
peak at high frequency side of 2D band. Here, the 2D band on the region ( ) is obvious that the intensity of the sharp peaks at high frequency side, differ from HOPG’s (Fig. 2d), so we can judge it is a few-layer graphene on the region ( ). Moreover, IG of few-layer graphene is apparently higher than HOPG’s (Fig. 2c and d). This is attributed to the enhance effect of the cutting cross section of silver. More important, four new Raman bands (1600–1630 cm1) are shown in SERS of few-layer graphene (Fig. 2c). Usually the D0 band lies in this region (1620 cm1). The D0 band is a weak disorder-induced feature which originates from a double resonance (DR) Raman process. But in SERS of few-layer graphene (Fig. 2c), the D0 band splits into four bands. We fitted the D0 band with four Gaussian lines, as shown in Fig. 3. They lie at 1594, 1603, 1612 and 1626 cm1, with the full width at half maximum 5.2, 7.9, 9.2 and 5.6 cm1 respectively. For the splitting of D0 band, through analysis, graphene flakes prepared by mechanical exfoliation often has defect construction on the edge, defects easy form plenty of unsaturated chemical bonds and roughed surface, which ensure strong adsorption and interaction with Ag surface. As a result, Raman signals from defects are enhanced greatly. And according to the adsorption effect of Ag, the symmetry of graphene changes from high to low, the selective rules for the Raman spectrum of graphene becomes wider. So, in SERS of few-layer graphene, D0 band is enhanced and split. In addition, we analyze the electron dispersion of bilayer (2-LG) graphene near the K point and find the D0 band consisted with several Raman lines. The D0 band originates from a DR Raman process. For single-layer graphene, the DR process shown in Fig. 4a begins with an electron of wave-vector k around K absorbing a photon of energy Elaser. The electron is inelastically scattered by a defect of wavevector q and energy Ephonon to a point belonging to the same circle around the K point, with wavevector k + q. The electron is then scattered back to a k state, and emits a photon by recombining with a hole at a k state [17]. For 2-LG graphene with Bernal AB layer stacking, both the electronic and phonon bands split into two components with special symmetries [18]. Fig. 4b shows a schematic view of the bilayer graphene electronic structure where the upper (lower) and lower (upper) branches of the valence (conduction) band are labeled as p1 (p1) and p2 (p2), respectively [19]. Then the DR process turns into four different Pij processes as shown in Fig. 4b–e. The results are concordant with SERS in Fig. 3.
4. Conclusion It is very easy for operation and observation to use the cutting cross section of silver strip as substrate in SERS of graphene. And the enhance effect is significant, especially for the D0 band. The SERS system causes D0 band of few-layer graphene splitting and provides more information about D0 band. This method could be used in study on defect structure of graphene. Acknowledgements The authors thank the support of National Natural Science Foundation of China (10974076). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [2] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (187) (2006) 401. [3] J. Yan, Y. Zhang, P. Kim, A. Pinczuk, Phys. Rev. Lett. 98 (166) (2007) 802. [4] S. Pisana, M. Lazzeri, C. Casiraghi, K.S. Novoselov, A.K. Geim, A.C. Ferrari, F. Mauri, Nat. Mater. 6 (2007) 198. [5] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, K.S. Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari, A.K. Sood, Nat. Nanotechnol. 3 (2008) 210. [6] A.K. Gupta, T.J. Russin, H.R. Gutierresz, P.C. Eklund, ACS Nano 3 (2009) 45. [7] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, ACS Nano 2 (2008) 2301. [8] Libo Gao, Wencai Ren, Bilu Liu, Riichiro Saito, Wu Zhong-Shuai, Shisheng Li, Chuanbin Jiang, Feng Li, Hui-Ming Cheng, ACS Nano 3 (2009) 933. [9] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [10] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Nano Lett. 6 (2667) (2006) 2673. [11] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (238) (2007) 242. [12] F. Schedin, E. Lidorikis, A. Lombardo, V. G. Kravets, A. K.Geim, A. N. Grigrenko, K. S. Novoselov, A. C. Ferrari, vl. [13] Y.-K. Kim, H.-K. Na, Y.W. Lee, H. Jang, S.W. Han, D.-H. Min, Chem. Commun. 46 (2010) 3185. [14] N. Kim, M.K. Oh, S. Park, S.K. Kim, B.H. Hong, Bull. Korean Chem. Soc. 31 (2010) 999. [15] Jisook Lee, Sangdeok Shim, Bongsoo Kim, Chem. Eur. J. 17 (2011) 2381. [16] L.G. Cançado, A. Reina, J. Kong, M.S. Dresselhaus, Phys. Rev. B. 77 (2008) 245408. [17] Mildred S. Dresselhaus, Gene Dresselhaus, Ado Jorio, Group Theory: Application to the Physics of Condensed Matter, Springer, Berlin, 2008. [18] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51. [19] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Modern Phys. 81 (2009) 109.