Differences in Self-Assembly of Spherical C₆₀ and Planar PTCDA on Rippled Graphene Surfaces

It was recently recognized that two-dimensional (2D) graphene exhibits nonplanar aberrations such as a rippled surface. Understanding the self-assembly of organic semiconductor molecules on monolayer 2D curved graphene surfaces is a paramount issue for ultimate application. Herein, we report on the preparation of fullerene, C₆₀ and perylenetetracarboxylic dianhydride (PTCDA) molecules adsorbed on a rippled graphene surface. We find that the C₆₀ form a quasi-hexagonal close packed (hcp) structure, while the PTCDA form a disordered herringbone structure. These 2D layer systems have been characterized by STM imaging and DFT approaches. The DFT results exhibit interaction energies for adsorbed molecule/rippled graphene complexes located in the 2D graphene valley sites that are significantly larger in comparison with adsorbed planar/molecule graphene 2D complexes. In addition, we report that the adsorbed PTCDA prefer different orientations when the rippled graphene peak regions are compared to the valley regions. This difference in orientations causes the PTCDA molecules to form a disordered herringbone structure on the rippled graphene surface.