Interaction of C60 with Si(111)7x7 and Si(100)2x1 surfaces studied by STM, PES and HREELS:: annealing effect

We report here measurements of the valence band spectra, the C 1s core level spectra and the vibrational excitation spectra of a C-60 monolayer (ML) film on Si(111)7 x 7 and Si(100)2 x 1 surfaces, using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) in combination with scanning tunneling microscopy. 1 ML films are formed after annealing the 5 ML films at 670 K. The bonding states between C-60 molecules and Si substrates are clearly observed at 2.1 eV on both surfaces in the valence band spectra. The shift of binding energies of the molecular orbitals and the C 1s care level spectra indicate the charge transfer. The softening of several vibrational modes is observed on the Si(111)7x7 and Si(100)2x1 surfaces with HREELS. The charge transfer scheme explains the softening well. We discuss the bonding nature in terms of hybridization of C-60 molecular orbitals with the surface states. (C) 1999 Elsevier Science B.V. All rights reserved.