Symmetry Breaking Induced Bandgap in Epitaxial Graphene Layers on SiC

By performing density-functional calculations, we have investigated the electronic bandgap of single epitaxial and multiepitaxial graphene layers on SiC. The calculations show that a defect-free graphene layer above the carbon buffer layer is very flat and no bandgap is found in the Dirac bands. By introducing a finite density of Stone-Wales defects in the graphene layer(s), we find that a bandgap is opened and decreases as the thickness of graphene layers increases, in good agreement with experiments. The band splitting and the charge distribution vary greatly with the number of graphene layers. The bandgap opening is due to the symmetry breaking within the single graphene layer. The narrowing of the bandgap in multiple graphene layers is induced by interlayer interaction.