Scanning tunneling microscopy of graphene on Ru(0001)

After prolonged annealing of a Ru(0001) sample in ultrahigh vacuum a superstructure with a periodicity of similar to 30 A was observed by scanning tunneling microscopy (STM). Using x-ray photoelectron spectroscopy it was found that the surface is covered by graphitic carbon. Auger electron spectroscopy shows that between 1000 and 1400 K carbon segregates to the surface. STM images recorded after annealing to increasing temperatures display islands of the superstructure, until, after annealing to T >= 1400 K, it covers the entire surface. The morphology of the superstructure shows that it consists of a single graphene layer. Atomically resolved STM images and low-energy electron diffraction reveal an (11x11) structure or incommensurate structure close to this periodicity superimposed by 12x12 graphene cells. The lattice mismatch causes a moire pattern. Unlike the common orientational disorder of adsorbed graphene, the graphene layer on Ru(0001) shows a single phase and very good rotational alignment. Misorientations near defects in the overlayer only amount to similar to 1 degrees, and the periodicity of similar to 30 A is unaffected. In contrast to bulk graphite both carbon atoms in the graphene unit cell were resolved by STM, with varying contrast depending on the position above the Ru atoms. The filled and empty state images of the moire structure differ massively, and electronic states at -0.4 and +0.2 V were detected by scanning tunneling spectroscopy. The data indicate a significantly stronger chemical interaction between graphene and the metal surface than between neighboring layers in bulk graphite. The uniformity of the structure and its stability at high temperatures and in air suggest an application as template for nanostructures.