Moiré-regulated distribution of Co adatoms and nanoclusters on epitaxial\\ graphene.

Graphene(G) grown on Ni(100) is periodically corrugated due to the lattice mismatch with the substrate, resulting in an alternate arrangement of strongly and weakly interacting G regions. In striped Moiré patterns, larger concentration of Co adatoms and nanoclusters on G's ridges as compared to valleys is experimentally observed. We performed density functional theory calculations and predicted that Co prefer to bind to ridge much stronger than on valley. Similar results are obtained for small Co clusters. The analysis of charge redistribution upon Co adsorption on G/Ni(100) shows that an electron rearrangement takes place, originating a repulsive Co-Ni interaction mediated by G and modulated by its corrugation: the farther the Co from Ni is, the weaker the repulsion is and the higher the binding energy on G is. Moreover, the repulsive nature of this interaction increases the adsorbate mobility, allowing Co atoms to drift more easily from the valley to the ridge than in any other direction, as suggested by the calculated diffusion barriers. These findings are in line with experimental scanning tunneling microscopy images that show small Co clusters sitting exclusively on the ridges.