Vibrational and electronic properties of monolayer and multilayer C₆₀ 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).