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

We report on investigations of the vibrational and electronic properti es of an adsorbed C60 monolayer by using high-resolution electron ener gy loss spectroscopy (HREELS) and electron energy loss spectroscopy (E LS). Spectra were obtained for a C60 monolayer and thick multilayers o n a Rh(111) single crystal surface. We also investigated the growth me chanism 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 decompos ition occurs and graphite-like carbon is formed. Spectra from ultravio let photoelectron spectroscopy (UPS) and HREELS of a C60 monolayer are consistent with molecular adsorption on Rh(111) at or below 300 K. Th e 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 interaction s. Electronic transitions in the near-IR to vacuum-UV region from a mo nolayer of C60 on Rh(111) examined by ELS show a new, low-energy excit ation at 1.2 eV and considerable broadening of other transitions compa red to the case of thick C60 films. Adsorption of a monolayer of C60 o n Rh(111) lowers the work function of the surface by 0.35 eV. This wor k function change is associated with a positive dipole on the surface implying that C60 is a net donor ligand on Rh(111).