VIBRATIONAL AND ELECTRONIC-PROPERTIES OF MONOLAYER AND MULTILAYER C-60 FILMS ON RH(111)

We report on investigations of the vibrational and electronic properties of an adsorbed C60 monolayer by using high-resolution electron energy loss spectroscopy (HREELS) and electron energy loss spectroscopy (ELS). Spectra were obtained for a C60 monolayer and thick multilayers on a Rh(111) single crystal surface. We also investigated the growth mechanism of vapor-deposited C60 on Rh(111) and the thermal stability of monolayer and thicker C60 films. Bulk films of C60 sublime near 500 K, but the C60 monolayer is stable up to 750 K, at which point decomposition occurs and graphite-like carbon is formed. Spectra from ultraviolet photoelectron spectroscopy (UPS) and HREELS of a C60 monolayer are consistent with molecular adsorption on Rh(111) at or below 300 K. The HREELS spectrum, however, has a much richer structure than that from the surface of a thick layer of C60, which is attributed to a further lowering of the C60 symmetry as a result of strong C60-Rh interactions. Electronic transitions in the near-IR to vacuum-UV region from a monolayer of C60 on Rh(111) examined by ELS show a new, low-energy excitation at 1.2 eV and considerable broadening of other transitions compared to the case of thick C60 films. Adsorption of a monolayer of C60 on Rh(111) lowers the work function of the surface by 0.35 eV. This work function change is associated with a positive dipole on the surface implying that C60 is a net donor ligand on Rh(111).