Surface Diffusion of Simple Organic Molecules on Graphene on Pt(111)

Studying the behavior of organic molecules adsorbed on graphene is an issue of special importance to exploit the full potential of graphene in future technology. Here we report on a scanning tunneling microscopy study in ultra-high vacuum of the surface diffusion and initial stages of growth of a simple organic molecule, the azabenzene 1,3,5-triazine, on epitaxial graphene on Pt(111) at low temperatures. Our study reveals the formation of fractal shape islands below 100 K; inside the islands, the molecules are arranged in a well-ordered hexagonal structure with the molecular plane parallel to the surface. The orientation of the molecules in this lattice is determined by the interpretation of STM images exhibiting intramolecular features via the electrostatic potential map of the 1,3,5-triazine molecule. From nucleation experiments we have measured; for the first time, the diffusion barrier for single molecules adsorbed on the graphene/Pt(111) surface. This energy barrier (68 +/- 9 meV) is higher than that previously found for 1,3,5-triazine on graphite surfaces. This important finding shows that even on the graphene/Pt(111) system, which is one of the most weakly coupled graphene-metal systems, dynamic processes such as surface diffusion, which is a fundamental process involved in the growth of monolayers of organic molecules, is affected by the interaction of graphene with the underlying metal.