Graphene Moire patterns observed by umklapp double-resonance Raman scattering

Abstract

This work reports a Raman study of graphene bilayer samples grown by chemical vapor deposition on a copper foil, using laser lines in the UV range. The Raman spectra show a number of extra peaks, classified in different families, which appear nonuniformly across the Cu surface, in regions with sizes of several mu m. We interpret these new extra modes as due to Moire patterns of twisted layers of graphene, each family of peaks being associated with different twist rotational angles. We theoretically analyze the results, introducing the concept of umklapp double-resonance Raman processes associated with reciprocal lattice vectors of the Moire pattern supercells.