Bonding state of the C60 molecule adsorbed on a Si(111)-(7X7) surface

We report here the measurements of the valence spectra, the C 1s and the Si 2p core-level spectra of C60 molecules adsorbed on a Si(111)-(7X7) surface, using photoelectron spectroscopy. In the valence spectra, the highest occupied molecular orbital (HOMO) of a C60 splits into two peaks at a coverage lower than 0.25 ML. The binding energies of the split peaks are 1.8 and 2.4 eV. Taking into account the polarization-dependence of the valence spectra and the binding energy of the C 1s core-level spectra, it is found that the 2.4-eV peak observed in the valence spectra is the covalent bonding state between a C60 molecule and the Si substrate, and that the 1.8-eV peak is the shifted HOMO. The Si 2p core-level spectra suggest that the bonding site is localized at the interface. We also present the energy-level scheme of the bonding state in terms of the symmetry of the HOMO

Temperature dependence of the electronic structure of C60 films adsorbed on Si(001)-(2x1) and Si(111)-(7x7) surfaces

We report here the temperature-dependent measurements of the valence spectra, the C 1s and the Si 2p core level spectra of the one monolayer C60 film adsorbed on Si(001)-(2x1) and Si(111)-(7x7) surfaces, using photoelectron spectroscopy. At 300 K, most C60 molecules are physisorbed with the coexistence of minority chemisorbed species on both Si(001)-(2x1) and Si(111)-(7x7) surfaces. After annealing the samples at 670 K, C60 molecules change the bonding nature to a chemisorption that has both covalent and ionic characters. The covalent bonding orbital is observed at a binding energy of 2.10 eV on both Si surfaces. The amount of charge transfer is estimated to be 0.19 electrons per C60 molecule on the Si(001) surface, and to be 0.21 electrons per molecule on the Si(111) surface. We consider the origin of the change in bonding nature to the different distance between two dangling bonds that results from the rearrangement of the surface Si atoms. After annealing at 1070 K, C60 molecules decompose and the SiC formation takes progress at the interface. On the Si(001) surface, the molecular orbitals (MO's) disappear at 1120 K and the binding energies of peaks observed in the valence spectra indicate the formation of SiC islands at this temperature. On the Si(111) surface, the disappearance of MO's and the formation of SiC islands are verified at 1170 K. The difference in formation temperature is attributed to the different surface structure