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2 Chemical Physics Letters 556 (2013) Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: The SERS study of graphene deposited by gold nanoparticles with 785 nm excitation Peijie Wang, Duan Zhang, Lisheng Zhang, Yan Fang The Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, Beijing , China article info abstract Article history: Received 21 July 2012 In final form 7 November 2012 Available online 24 November 2012 Surface enhanced Raman scattering (SERS) of graphene deposited by the Au nanoparticles were investigated with the 785 nm excitation. The SERS spectra are used to extract information of graphene. It was shown that the artificial defects were effectively induced by depositing Au nanoparticles on graphene surface. Meanwhile, the Au nanoparticle can quench the fluorescence of substrate and enhance the Raman signals of graphene drastically. The adatom-induced defects in graphene cause a significant Raman intensity in D and D 0 band. This shed light on the understanding of the structural characteristics of graphene-based nanocomposite and the interaction between graphene and metal adatoms. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Graphene as a prototype is a two dimensional carbon system, and has brought out many exciting properties since its discovery by Novoselov et al. [1 5]. Graphene has potential application in nanoelectronics for its unique properties such as the quantum Hall effect [1] at room temperature, ballistic transport properties [6,7] and high electron mobility [2,8]. However, all of these electronic transport properties depend strongly on the defects of graphene [9]. Therefore, the research of the defects in graphene and its interaction between graphene and the supporting substrate is very important. Furthermore, the surface modification materials of graphene [10] are also crucial in changing the graphene property. Raman spectroscopy is a powerful tool in the structural characterization of graphitic materials [11,12]. It has been utilized for understanding the behavior of electron and phonon in graphene and other phenomena such as monitoring of doping [13 17], defects [18,19], strain [20] disorder [21], chemical modifications [22] and edges [23]. The Raman bands of carbon materials are clearly distinguishable which can then be used accurately to observe the subtle changes in the crystal structure. For example, the characteristic Raman bands of graphene on the silicon substrate under 514 nm excitation only include G band, (1580 cm 1 ), T + D band (2400 cm 1 ), 2D band (2680 cm 1 ) and 2D 0 band (3250 cm 1 ) [23]. However, Raman bands, such as the T band (1087 cm 1 ), D band (1350 cm 1 ) and D 0 band (1620 cm 1 ), etc. [24], which belongs to the inner layer Raman active modes of sp 2 carbon material cannot be observed in intrinsic graphene (except for at edges of graphene). Most of these bands are related to defects in graphene which may change the electronic properties of graphene [25] and has great potential applications in electronic devices Corresponding author. address: yanfang@cnu.edu.cn (Y. Fang). [9]. Hence, the study of lattice defects in graphene is very important and has generated lot of theoretical interest in this subject [26]. Meanwhile, these Raman modes of defects are dispersive with the laser energy of excitation. Due to the affection of silicon substrates, their dispersive behaviors have not been studied systemically. Up to now, only the dispersive relation under narrow energy range were obtained which can be inferred the by DR theory. No related data has been reported in the infrared region. Generally, the 785 nm line is widely used in Raman spectra for its merit of minimizing fluorescence and maximizing sensitivity of the samples. Especially for the Raman spectroscopic studying of graphene, the 785 nm excitation wavelength was chosen to suppress the fluorescence of the polymer matrix embedding the graphene flakes. Also it helps in suppressing the fluorescence of the substrate supporting graphene[27]. Surface-enhanced Raman scattering (SERS), for its nanometer scale and high surface selective sensitivity, is widely used in interaction signal detection of the interface [28,29]. It not only provides a high surface sensitivity and a signal enhancement, but also can quench the fluorescence of substrate to achieve high quality Raman signal measurement. Recently, the SERS studies of graphene has become an important theme of research which include the research on graphene/metal composites and the interaction between graphene and metals [22,30,31]. Also, the n or p-doping effects induced by Ag and Au deposition as a function of the layer number [32] is an important theme of study. The interactions between gold nanoparticles and graphene may confer a unique electron or energy transfer mechanism subject to the SERS enhancement. Decoration of graphene sheets with nanoparticles has been demonstrated to reveal special features in new hybrids materials that can be used in catalysts [33], supercapacitors [34], biosensors [35,36], etc. In this study, the Au nanoparticles were deposited on a singlelayer graphene to form Au island films. The effect of Au island films on the Raman spectroscopy of graphene was studied by using /$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

3 P. Wang et al. / Chemical Physics Letters 556 (2013) nm and 785 nm excitation, respectively. Our experiments showed that the single-layer graphene structure is preserved through controlling of the deposition of Au island film on the surface of graphene. Meanwhile, the deposition of the Au nanoparticles changes the surface of graphene bending, forming a new extrinsic deformation of the graphene. The interaction between the Au nanoparticles and the hexagonal rings of graphene result in the occurrence of a series of new, important Raman bands. Furthermore, the surface plasmon resonance of Au nanoparticles leads to a significant enhancement of these Raman bands. The new Raman bands due to the defects induced by Au nanoparticles as adatoms are discussed. 2. Experimental The high intrinsic strength of carbon carbon sp2 bonds of graphene make possible the isolation of single atomic layers [37]. Also it results in a very low density of lattice defects in graphene prepared by mechanical exfoliation [38]. The single-layer graphene on top of a SiO 2 /Si wafer with a carefully chosen thickness of SiO 2 (which is 315 nm) was prepared by the traditional mechanical exfoliation method. Then, an island film of Au was deposited on the single layer graphene by magnetron sputtering (MS). The sputtering voltage was 0.3 kv. An argon pressure of 0.45 Pa was maintained and Au was sputtered for 45 s. The Raman spectra were recorded with a microprobe Raman system (RENISHAW H spectrophotometer). A 50 objective was used to perform an 180 backward scattering configuration. The excitation line was nm of Ar+ laser and 785 nm of infrared semiconductor laser. The entrance-slit width was 50 lm and the integral time was 20 s. Due to our space resolution of the Raman measurements, it is difficult to distinguish the Au nanoparticles. However, we estimated the number of Au nanoparticles by atomic force microscopy (AFM, Seiko SPI3800N) by scanning an area of lm. From the AFM analysis, we found about 45 Au nano-particles having an average diameter of 20 nm. A metallograph of the graphene was collected by the German-based Carl Zeiss Axio Imager microscope. 3. Results and discussion Figure 1a shows the optical image of a typical micromechanical cleavage graphene (MCG) sheet with 100 objective. Figure 1b shows the same graphene after depositing Au nanoparticles wherein the color of the graphene sample has faded. Figure 1 c shows the 3D AFM image of the same graphene in Figure 1b where the deposited Au nanoparticles show bright color contrast compared to the substrate of SiO 2 and graphene. There are about 45 Au nanoparticles with the average diameter of 20 nm in the area of lm. Figure 2a shows the Raman spectrum of the graphene sample with a laser excitation at 514 nm. The most prominent features in the normal Raman spectra of monolayer graphene are the G band appearing at 1586 cm 1 and the 2D band at about 2683 cm 1 by using laser excitation at 514 nm. Here we measure a single, sharp 2D peak in graphene, roughly 5.5 times more intense than the G peak. Also the 2D band is formed by one Lorentzian peak with a good symmetry and has a full width half maximum at 28 cm 1 which confirms the single layer of graphene. The G band is associated with the doubly degenerate (ito and LO) phonon mode (E2g symmetry) at the Brillouin zone center. This band corresponds to the normal first order Raman scattering process in graphene and is independent of the excitation energy. Figure 3 shows the deconvolution of the G and D 0 bands in the SERS spectra in Figure 2c and d. It is evident that no splitting in the G band of the graphene with the presence of Au islands has taken place. This demonstrates that the phonon symmetry near the U point was not broken. From the molecular view of the point, the G band is attributed to the CC double stretching. The Lorentzian lineshape of the G band confirms that the main structure of the monolayer graphene is still preserved even after the deposition of Au nanoparticles. However, a Raman shift of 3.4 cm 1 is observed and an increase in intensity of around 77.6 times (compared with Figure 2a) of G band is evident. This can be related to the enhanced effects from the surface plasmon resonance of Au nanoparticles. On the other hand, the 2D band originate from a second-order process, involving two ito phonons near the K point without any kind of disorder or defects. It must be noted that here we focused the laser spot on the center of the graphene sample. The D-band are not observed here in normal Raman spectra due to the lack of defect on the center part of the graphene. Figure 2b shows the normal Raman spectrum of the same graphene sample with a laser excitation at 785 nm. It is evident that besides the broad peak at 2700 cm 1 which corresponds to the fluorescent band of the SiO 2, there are no additional peaks observed in the Raman spectra on using 785 nm infrared laser. Up to now, few paper reports on the Raman signal of graphene supported by silicon substrate excited by 785 nm infrared laser [27,39]. Thanks to the merit of SERS technique, it not only enhanced the normal weak Raman signals but also quenched the fluorescence of the substrate. Figure 2d shows the SERS spectrum of the graphene with Au island film excited by 785 nm infrared laser. From the spectrum, the G band is clearly observed at 1581 cm 1 and the 2D band at 2591 cm 1. For the low frequency modes, various modes are observed and enhanced. The prominent D band appeared at 1299 cm 1, T band at 1077 cm 1 and the D 0 band at 1606 cm 1. For the high frequency region, due to the SERS effect and the suppression of fluorescence, the 2D band appeared at 2591 cm 1. The D+D 0 and 2D 0 combination bands are observed at 2877 cm 1 and 3035 cm 1 respectively. Compared to Figure 2b, more new bands appeared and the SERS signal of graphene which was enhanced on excitation with 785 nm laser. These phenomena reveal that the effective interaction occurred between graphene and deposited Figure 1. (a) A metallograph of the graphene (highlighted by the inset box) with 100 objective; (b) after deposited by Au nanoparticles islands; (c) the 3D AFM image of the same graphene of (c). There are about 45 Au nano-particles with the average diameter of 20 nm in the area of lm.

4 148 P. Wang et al. / Chemical Physics Letters 556 (2013) Table 1 The dispersive relations of Raman modes (in cm 1 ) by two different visible and infrared exciting lasers. Raman bands (cm 1 ) T D G D 0 2D D + D 0 2D 0 SERS (by 514 nm) SERS (by 785 nm) Dx Dx/DE Laser Figure 2. (a) The normal Raman spectrum of graphene with a laser excitation at 514 nm; (b) the normal Raman spectrum of graphene with a laser excitation at 785 nm; (c) the SERS of the monolayer graphene deposited by Au nanoparticles with a laser excitation at 514 nm; and (d) the SERS of the monolayer graphene deposited by Au nanoparticles with a laser excitation at 785 nm. Raman Intensity(a.u.) Raman Shift (cm -1 ) Figure 3. Deconvolution of the G and D 0 bands in the SERS spectra of Figure 2c and d. Au nanoparticles. Here, the most prominent D band was observed profoundly at 1299 cm 1 in the SERS spectrum, which is attributed to the artificial defects on graphene induced by the Au nanoparticles. The most striking feature of the SERS results presented here is the qualitatively different behavior of the D and 2D mode for the 785 and nm excitation. It is noted that the D and 2D modes are well explained by a double resonance (DR) Raman scattering mechanism [40,41]. Both modes show dispersive spectral features, that is, their frequencies vary as a function of the energy of the incident laser [42]. For the D band, its intensity is more enhanced by 785 nm (1.58 ev) excitation than that by 514 nm (2.41 ev) and the linewidth of both bands broadens at 785 nm excitation. The band intensity enhancement and linewidth broadening effect can be inferred by the electronic equi-excitation-energy contours for 1.58 and 2.41 ev excitation energies, derived by means of a simple nearest neighbor tight binding theory (as shown in the Fig. 5 of Ref. [43]). There the trigonal warping effect becomes stronger and the Dirac cones more deformed as the excitation energy increases. It is demonstrated that the predicted highest Raman intensity occurs for the outer process, and the inner process is weaker for 514 nm excitation. But for the 785 nm excitation, the Raman intensity is d c due to the contribution of two distinct processes (inner and outer) to the double resonance signal. For the intensity of 2D band, there is a discrepancy that its intensity is not enhanced by 785 nm excitation. This indicates the complex substrate effect of the graphene supported by Si/SiO2 and the interaction between Au nanoparticle and graphene (i.e., the SERS effect). For the investigation of the dispersive behaviors of the graphene, the SERS spectrum of the single layer graphene with Au island film excited by 514 nm (2.41 ev) visible laser was measured as shown in Figure 2c. Here, the D, G, 2D bands which are clearly observed are enhanced. Both the D and 2D bands exhibit a dispersive behavior comparison with the Figure 2d SERS spectrum excited by 785 nm (1.58 ev). As shown in Table 1, it gives the dispersive relations of Raman modes (in cm 1 ) by two different visible and infrared exciting lasers. The first line is the different Raman modes. The second and the third line describe the SERS Raman shifts by 514 and 785 nm excitation. The forth line demonstrates the dispersive relations of different Raman modes. Since the frequencies of the Raman bands change with the energy of the incident laser (E laser ), the D-band frequency shifts upwards with increasing Elaser over a wide energy range. The slope D =@E laser were found to be about 67 cm 1 /ev. The slope 2D =@E laser is about twice that of the D-band, i.e., around 135 cm 1 /ev. This dispersive behavior are consistent with the literature reported for sp2 carbon material [44]. For the pristine monolayer graphene, this dispersive slope of 2D by normal Raman spectrum is of around 88 cm 1 /ev [45]. Here, the dispersive behavior of 2D bands in SERS spectrum is profound. Considering the interaction of the metal and the graphene, it can be said that the SPR of the Au nanoparticles assists in enhancing the nonlinearity of the 2D bands. On the other hand, it may also be related to the doping effect by Au deposition in which the Au deposition induced p-doping in graphene [32]. The origin of the D and 2D frequency bands of the monolayer graphene can be well interpreted by the double resonance (DR) theory [40,41] In this DR process, the wave-vectors q of the phonons associated with the D and 2D bands (measured from the K point) would couple preferentially to the electronic states with wave-vectors k (measured from the K point), such that q 2 k. This DR Raman process begins with an electron of wave-vector k around K absorbing a photon of energy E laser. The electron is inelastically scattered by a phonon of wave vector q and energy to a point belonging to K 0 which is inequivalent to K point. The electron is then scattered back to a k state, and emits a photon by recombining with a hole at a k state. In the case of the D band, the two scattering processes consist of one inelastic scattering event by emitting (or absorbing) a phonon and another elastic scattering event by defects of the crystal. In the case of the 2D-band, both processes are inelastic scattering events and two phonons are involved. This means that, although in the center of the graphene there are no defects, the 2D-band can be observed, but the D-band will not appear. In this SERS experiment, we focused the laser spot on the center of the graphene sample. It is evident that the D-band cannot be observed here in normal Raman spectra because there is lack of defect on the center part of the graphene. After being deposition of the Au nanoparticles, the artificial defect on the graphene

5 P. Wang et al. / Chemical Physics Letters 556 (2013) Figure 4. Schematic illustration of the SERS effects of graphene deposited by gold nanoparticle islands. are induced efficiently and D-band is observed profoundly in the SERS spectrum as shown in Figure 2c and d. In a DR Raman process, the Raman process can also occur by scattering of holes which may be caused by the triple resonance (TR) Raman process [46]. The TR process occurs in the case of graphene where the valence and conduction bands are almost mirror bands of one another relative to the Fermi energy. This TR process may explain why the 2D band is more intense than the G-band in monolayer graphene as shown Figure 2a. However, if Au nanoparticles are deposited on the surface of graphene, the mirror symmetry of valence and conduction bands is lowered due to the charge doping of Au nanoparticles. This in-turn suppresses the TR which might account for the lowering of the ratio of I 2D /I G (I 2D : the Raman intensity of 2D band; I G : the Raman intensity G band) in SERS experiment in Figure 2c and d. Besides the D band, another kind of defect band, which is due to the intravelly transition [40] defect of D 0 are also observed at 1606 cm 1 (by 785 nm) and 1622 cm 1 (by 514 nm). The appearance of such band as the shoulder of G band is due to the enhancement by the Au nanoparticles in both spectrum of Figure 3c and d. Comparing with Figure 2a, more new bands appeared in the SERS spectra. The new SERS feature at 2877 cm 1 (by 785 nm excitation) and 2937 cm 1 (by 514 nm excitation) generally could not be observed in normal Raman spectrum of graphene (Figure 2a). But with the deposition of Au nanoparticles on graphene, this band appeared in Figure 2c and d. Analogous to the observation of carbon ion-implanted highly oriented pyrolytic graphite (CHOPG) [47], we attribute this band to a D + D 0 combination mode. This band results from the appearance of the D band. The SERS band at 3035 cm 1 (which is appeared as a shoulder band by 785 nm excitation) and 3180 cm 1 (by 514 nm excitation) in Figure 2c and d also appears in normal Raman spectrum of monolayer graphene (Figure 2a). It is attributed to the overtone of D 0 band. The dispersive behaviors of these new SERS bands are also clear. As shown in Table 1, we have attribute the new bands and given their dispersive behaviors by two different visible and infrared exciting lasers. By comparing Figure 2b and d, it is clearly shown that the Au nanoparticles deposited on the surface of graphene play an important role in SERS investigation of graphene. Concerning the interaction of the Au nanoparticles with graphene, we considered two possible kinds of mechanisms. First, the graphene have intrinsic microscopic corrugation [48,49]. When the Au nanoparticles were deposited on the surface of graphene, it will induce more additional deformation on the graphene and meanwhile enhance the amplitude of the intrinsic ripples. The contact between the Au nanoparticles and the graphene would produce new edge defects. Also, the interaction of Au nanoparticles with graphene can break the local symmetry of the graphene and make the selection rule of Raman active mode more loose such that some new Raman bands become active and can be observed in Raman spectroscopy. Furthermore, the lowering of symmetry of the graphene surface by Au induced defects would suppress the TR process which is demonstrated by the lowering of the ratio of I 2D /I G in SERS experiment. All these artificial defects induced by depositing Au nanoparticles on the surface of graphene and their SERS effects are schematically illustrated in Figure 4. Here, graphene can also be thought as a super molecule whose Raman active modes can be enhanced due to the excitation of SPR of the deposited Au nanoparticles [50]. Second, It is generally believed that SERS arise at certain hot sites where the optical interactions are locally enhanced due to excitation of SPR. In our SERS experiments, we deposited the Au nanoparticles on the surface of the graphene which might have produced the hot sites at junctions between nanoparticles and graphene or at edges of nanoparticles. A very inhomogeneous field distribution over the Au nanoparticles, including large electromagnetic fields provides the key effect for the SERS enhancement. Also, the SERS effect of Au nanoparticles deposited on graphene can quench the florescence of substrate of SiO 2 substrate which influences the quality of the Raman signals obtained at 785 nm excitation. 4. Concluding remarks In summary, the SERS of graphene with the gold island film on SiO 2 /Si substrate by 785 nm excitation was investigated. The gold island film is formed by depositing the gold nano-particles on the monolayer graphene which can quench the florescence of the SiO 2 /Si substrate in SERS experiment. The new artificial defect bands which do not exist in the pristine graphene are induced and observed. It was shown that adatom-induced defects by gold island film in graphene cause a significant intensity in the Raman D band associated with intervalley electron scattering. Also an increase in intensity was observed in the G band, 2D and the overtone of the D band. Here we prepared the new Au island film graphene system to induce the artificial defects as to study the interaction of the adatom with graphene. The two possible kinds of mechanism of interaction were discussed and their effects on the Raman spectroscopy are analyzed. The graphene and Au nanoparticles composite will has great potential applications for surface analysis by means of chemical modifications, controlling the doping of graphene or local distortions driven catalytic action. This shed light on the understanding of the structural characteristics of graphene based nanocomposites and the interaction between graphene and metal nanoparticles. Rapid progress in nanofabrication on both graphene wafer and nanoparticle scale, show great promise for the future of graphene based nanocomposites SERS application in surface chemistry.

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