Bonding nature of C₆₀ adsorbed on Si(111) 7 × 7 and Si(100) 2 × 1 surfaces studied by HREELS and PES

We report here the measurements of vibrational excitation spectra, valence band spectra and C 1s core level spectra of C₆₀ molecules adsorbed on Si(111) 7 × 7 and Si(100) 2 × 1 surfaces, using high-resolution electron energy loss spectroscopy (HREELS) and photoelectron spectroscopy (PES). At a coverage between 1 and 0.25 ML, HREELS and PES measurements show that the interaction is as weak as the van der Waals interaction on both surfaces. At a coverage lower than 0.25 ML, some contradictory results are obtained by HREELS and PES. With HREELS, the softening of several vibrational modes is observed on the Si(111) 7 × 7 surface, but no indication of softening is observed on the Si(100) 2 × 1 surface. The charge transfer scheme explains the softening well. These results by HREELS indicate that the nature of the bonding is ionic on the Si(111) 7 × 7 surface and of the van der Waals type on the Si(100) 2 × 1 surface. In contrast, the bonding states between C₆₀ molecules and Si substrates are clearly observed at 2.4 eV on both surfaces with PES. The PES measurements indicate that the interaction has a covalent character on the Si(111) 7 × 7 surface, and that it is in between covalent and ionic, i.e. not purely ionic or at the covalent limit, on the Si(100) 2 × 1 surface. We will discuss the nature of the interaction manifested by HREELS and PES measurements in comparison with the spectra of C₆₀ molecules adsorbed on metal surfaces.