Effect of doping on the Raman lineshape and intensity of graphene

Graphene can be doped by applying a gate voltage [1-2]. Doping strongly affects the G and 2D Raman peaks: i) the G peak upshifts for increasing doping, while its width decreases [1]; ii) the 2D upshifts for p-doping, while it downshifts for n-doping [2]. iii) the ratio between the 2D and G peaks intensity decreases for increasing doping [2]. The 2D intensity is strongly affected by the electron-electron scattering rate, which increases with doping [3]. Similar Raman peaks variations were observed for non-gated samples, as an effect of charged impurities [4]. Here we use the 2D peak intensity variation with doping to extract the electron-phonon scattering rate [3,5]. We note that in non-gated samples, where the Fermi level shift is induced by charged impurities, we can probe the 2D peak dependence much closer to the Dirac point than in gated ones [5]. We find an electron-phonon coupling scattering rate of 60 ps$^{-1}$ at 2.41 eV excitation energy [3,5]. \\[4pt] [1] S. Pisana et al, Nature Mat. 6, 198 (2007)\\[0pt] [2] A. Das et al., Nature Nanotech. 3, 210 (2008)\\[0pt] [3] D. M. Basko et al. PRB 80, 165413 (2009)\\[0pt] [4] C. Casiraghi et al., APL 91, 233108 (2007)\\[0pt] [5] C. Casiraghi, arXiv:0908.4480