Au intercalation under epitaxial graphene on Ru(0001): The role of graphene edges

The intercalation mechanism of Au at the graphene-Ru (0001) interface is studied at elevated temperature using spectromicroscopy with high lateral resolution. Above 970 K, Au is shown not to stick to graphene-covered regions as expected from the weak Au-graphene binding, which is crucial to single out the graphene edges as the only intercalation channel. The modified Stranski-Krastanov Au growth on Ru (0001) with two atomic wetting layers followed by 3D islands is observed on the Ru (0001) surface partially-covered with micron sized graphene flakes. The Au layers nucleate in and out of the graphene islands following a precise order with a direct correlation to the Au adsorption energy expected in the various configurations. In particular, the adsorption energy is found to decrease going from monolayer Au on Ru (0001) to monolayer Au intercalated at the graphene-Ru interface, and further to Au bilayer on Ru (0001). Moreover, the chemical maps at different stages during Au growth point to a pronounced kinetic barrier at the graphene edges preventing intercalation before the Au monolayer is complete outside the graphene-covered regions.