Vibrational properties and charge transfer of C-60 adsorbed on Si(111)-(7x7) and Si(100)-(2x1) surfaces

We report here the measurements of vibrational excitation spectra and the temperature dependence of C-60 molecules adsorbed on Si(111)-(7 x7) and Si(100)-(2 x 1) surfaces using high-resolution electron-energy-loss spectroscopy in combination with scanning tunneling microscopy. A quantitative determination of the molecular charge state is obtained by the lower energy shifts of vibrational modes of C-60 upon adsorption. On the Si(111)-(7 x7) surface, the amount of charge transfer from the surface, to a C-60 molecule is estimated to be (3+/-1) electrons at the coverage lower than 0.25 monolayer (ML) and (0.7+/-1) electrons as an average at 1 ML. On the contrary, no indication of charge transfer is observed on the Si(100)-(2 x I) surface at a coverage lower than 1 ML. The difference in the charge state on both surfaces is attributed to the difference in the nature of surface states. After annealing 1 ML C-60 film at 500 K, the strong interaction between the surface Si atom and a C-60 molecule is indicated by the softening of vibrational modes on both surfaces.