Electronic state of the carbon 60 adsorbed silicon surfaces

We have investigated the coverage-dependent electronic structures of C₆₀ molecules adsorbed on the Si(111)-(7×7) and Si(001)-(2×1) surfaces using photoelectron spectroscopy. The valence band spectra show that the highest occupied molecular orbital (HOMO) of a C₆₀ molecule splits into two peaks on both surfaces at a coverage lower than 0.25 monolayer. These split peaks are assigned to be the shifted HOMO and the bonding state, with the polarization-dependent measurements and the Si 2p core level spectra. The binding energies of the molecular orbitals and the C 1s core level show a small shift on the Si(001)-(2×1) surface, and no shift on the Si(111)-(7×7) surface with decreasing the coverage. These results indicate that the strong interaction between C₆₀ molecules and the surfaces has a covalent character on the Si(111)-(7×7) surface, and both covalent and ionic characters on the Si(001)-(2×1) surface. We attribute the difference in the charge states of C₆₀ molecules to the different surface structures.