Raman Scattering Study of the Graphene-Substrate Interaction.

We report on Raman scattering studies of graphene and few graphene layer films (i.e., $n$GLs, where $n $is the number of graphene layers in the film). $n$GL films (n=1-3, 25) were prepared by mechanical transfer from HOPG to various substrates (SiO2:Si, Au, Ag, cleaved Mica, and free-standing films). For metallic substrates we observed a clear G-band frequency downshift relative to that observed when the $n$GL is on SiO2:Si. This downshift is interpreted in terms of a chemical charge transfer of electrons from the metallic substrate to the nGL. Interestingly, the position and shape of the 2D' (or G') band at $\sim $ 2700 cm$^{-1}$ is found insensitive to the substrate interaction.