Inhomogeneous electronic properties of monolayer graphene on Ru(0001)

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at low temperatures were applied to prove lateral inhomogeneities in the electronic properties of a single layer of graphene on the Ru(0001) surface. We found two different types of STS curves depending on the location within the moire pattern of graphene, providing direct proof for coupled and uncoupled parts of monolayer graphene on Ru(0001). Moreover, we observed differences in dI/dU curves between hills, valleys, and rims of hills, and we discuss them in relation to the electronic bulk states of Ru(0001). A detailed analysis revealed that graphene's dominating STS peak at -0.4 eV originates from a d-like Ru bulk state.