Ripples in epitaxial graphene on the Si-terminated SiC(0001) surface

The interaction with a substrate can modify the graphene honeycomb lattice and, thus, alter its outstanding properties. This could be particularly true for epitaxial graphene where the carbon layers are grown from the SiC substrate. Extensive ab initio calculations supported by the scanning tunneling microscopy experiments demonstrate here that the substrate indeed induces a strong nanostructuration of the interface carbon layer. It generates an apparent 6x6 modulation different from the interface 6 root 3 x 6 root 3R30 symmetry used for the calculation. The top carbon layer roughly follows the interface layer morphology. This creates soft 6x6 ripples in the otherwise graphene-like honeycomb lattice. The wavelength and height of the ripples are much smaller than the one found in exfoliated graphene. Their formation mechanism also differs; they are due to the weak interaction with the interface layer and not to the roughening of the plane due to the instability of a strictly two-dimensional crystal.