Vibrational study of C-60 overlayers on H/Si(111)-(1x1)

We have studied the vibrational properties of thin C60 overlayers deposited on ideally H-terminated Si(111) surfaces, by multiple infrared reflection and high-resolution electron energy-loss spectroscopies. We find that the Si-H interface remains intact upon C60 deposition: the Si-H stretching mode frequency, initially at 2083.8 cm-1, shifts downward to 2062 cm-1, and broadens while keeping the same integrated intensity. Due to the symmetry-breaking effect at the surface on the adsorbed C60 molecules, a new IR band is observed at 1445 cm-1 which is mainly s-polarised. No evidence for chemical interaction between C60 and H/Si(111)-(1 x 1) is found, pointing to van der Waals-type bonding. From the relative intensities of the dipole-active and -inactive modes in HREELS in the specular direction, and from the angular width of the scattered elastic peak, the growth of an ordered film is inferred.