Structure and bonding nature of C60/Si(100)-c(4x4):: Density-functional theory calculations

We have studied the adsorption structure and bonding nature of the C-60/Si(100)-c(4x4) surface using density-functional theory calculations. Based on energetics and scanning-tunneling-microscopy simulations, we propose a structural model where C-60 adsorbs in between Si dimer rows with a C-C bond shared by two hexagon faces headed down and aligned perpendicular to Si dimer rows. This structure shows a semiconducting band structure, and the modified occupied and unoccupied molecular-orbital levels are resolved. Our charge character analysis figures out the bonding nature as covalent by the evidence of C-Si sigma bond formations. The calculated surface density of states, well reflecting the modified molecular levels, explains well the position and origin of the peaks measured by photoelectron spectroscopy study.