Multistable Rippling of Graphene on SiC: A Density Functional Theory Study

Graphene monolayer grown by Si evaporation from the 0001 surface of SiC displays a moiré pattern of corrugation whose structure is ambiguous: different measurements and theoretical studies show either protruding bumps surrounded by valleys or, reversely, wells surrounded by crests. Here we address the fine structure of monolayer graphene on SiC by means of density functional theory, using a model including the full symmetry of the system and the substrate (1648 atoms) and therefore realistically reproducing the experimental sample. We find that the specific treatment of the vdW interactions between monolayer and the underlying substrate-bound buffer layer is crucial in stabilizing one or the opposite corrugation pattern, which explains the different results and measurement available in the literature. Our study indicates that at low temperature a state more closely following the topography of the underneath buffer layer is stabilized, while others are metastable. Because environmental conditions (e.g., temperature or doping) can influence the vdW forces and reduce the energy differences, this system is prone to externally driven switching between different (opposite) corrugation states. In turn, corrugation is related to local reactivity and to electronic properties of graphene. This opens to potentially interesting applications in nanoelectronics or tailored graphene chemical functionalization.