Binding nature of adenine and C60 on graphene: a van der Waals density functional analysis

Based on van der Waals density functional theory (using vdW-DF1 and vdW-DF2), we study and analyze the adsorption of adenine\footnote{K. Berland et al; J. Phys.: Condens. Matter 23 135001 (2001)} and C60\footnote{A. Bergvall et al; Phys. Rev. B 84, 155451 (2011)}on graphene. Understanding molecular binding on graphene helps development of functionals because the infinite graphene shifts the balance between short-range and long-range contributions to binding as compared dimers or molecular crystals. The potential of graphene as contacts in single-molecule electronics also motivates the study of the interaction between aromatic molecules and graphene; the binding separation affects the magnitude of hopping parameters. We present results on binding energy and separation, vibrational states, overlayers, and charge transfer. We find that the hexagonal ring in C60 binds closer to the graphene sheet than what a flat molecule such as adenine does. The role played by the difference in geometry between the flat (adenine) and spherical (C60) shape is discussed.