Raman fingerprint of charged impurities in graphene

We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to similar to 10(13) cm(-2) are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1 mu m. (c) 2007 American Institute of Physics.