Chemical origin of a graphene moiré overlayer on Ru(0001).

Epitaxial graphene on Ru(0001) was studied by means of large-scale density functional theory (DFT) calculations. The results agree well with scanning tunneling microscopy experiments. In contrast to the current understanding, we show that the measured corrugation originates mainly from a geometric buckling of the graphene sheet, induced by alternating weak and strong chemical interactions with the Ru support. In the strong contact regions, charge transfer is evidenced and the opening of a considerable band gap in the graphene is found.