Electron states of mono- and bilayer graphene on SiC probed by scanning-tunneling microscopy

We present a scanning-tunneling microscopy (STM) study of a gently graphitized $6H\text{−}\mathrm{Si}\mathrm{C}\left(0001\right)$ surface in ultrahigh vacuum. From an analysis of atomic scale images, we identify two different kinds of terraces, which we attribute to mono- and bilayer graphene capping a C-rich interface. At low temperature, both terraces show $(\sqrt{3}\times \sqrt{3})$ quantum interferences generated by static impurities. Such interferences are a fingerprint of $\pi$-like states close to the Fermi level. We conclude that the metallic states of the first graphene layer are almost unperturbed by the underlying interface, in agreement with recent photoemission experiments [Bostwick et al., Nat. Phys. 3, 36 (2007)].