Electronic structures of C60 adsorbed on Si(111)-(7×7) and Si(001)-(2×1) surfaces

We have investigated the electronic structures of C60 molecules adsorbed on the Si(111)-(7×7) and Si(001)-(2×1) surfaces using photoelectron spectroscopy. The coverage-dependent 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 of less than 0.25 monolayer. These split peaks are assigned to be the shifted HOMO and the bonding state with the polarization-dependent measurements. 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 have 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.