Modification of electronic band structure in mL + nL (m = 1, 2; N = 1-5) free-stacking graphene

In this paper, we studied stacked mL + nL graphene layers using Raman scattering spectroscopy. Our results indicate that the 2D band from stacked graphene can be considered as a superposition of those from the constituent nL and mL graphene layers, and a blueshift in the 2D band is observed when n or m = 1. The blueshift increases with the number of stacked layers and can be well understood by the reduction of Fermi velocity in the single layer graphene, as studied in the 1L + 1L (or twisted bilayer) case. As the number of stacked layers changes from 1 to 5, the Fermi velocity in the single layer graphene reduces to about 85% of its initial value. This study shows a convenient way to realize the modification of the Fermi velocity in free-stacking graphene and is of significance to the applications of graphene-based heterostructures.