Adsorption structure of a single C60 molecule on Si(1 1 1)-(7 × 7): Density-functional calculations

We have studied the structure and bonding nature of a single C60 molecule adsorbed on the Si(1 1 1)-(7 × 7) surface using density-functional theory calculations. Based on total-energy calculations and scanning-tunneling-microscopy (STM) simulations, we propose a structural model for the most representative adsorption configuration reported by STM experiments. In this model, C60 adsorbs at a position about 1 Å apart from the middle of the half unit cell with a C-C bond shared by pentagon and hexagon rings facing down toward the surface. We find in the calculated surface density of states that the highest occupied molecular orbital (HOMO) peak of C60 is resolved by the interaction with the surface into two levels, in good agreement with photoelectron spectroscopy data. It is shown in the charge characters of the affected electronic states that the nature of the molecule-surface interaction is basically the C-Si covalent bondings.