Dependence of the Raman spectrum characteristics on the number of layers and stacking orientation in few-layer graphene

Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO₂/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A_2D/A_G) and of the 2D band width (Γ_2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A_2D/A_G decreases and Γ_2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (θ), the values of the A_2D/A_G ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A_2D/A_G ratio decreases (respectively, increases) and, conversely, Γ_2D increases (respectively, decreases) as a function of N for small (respectively, large) θ. An intermediate behavior has also been found and is interpreted as the presence of both small and large θ within the studied area. These results confirm that neither A_2D/A_G nor Γ_2D are definitive criteria to identify single-layer graphene, or to count N in FLG.