Electronic properties of a pure and sodium-doped C-70 single layer adsorbed on Al polycrystalline surface

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C-70 layer adsorbed on polycrystalline Al (1 ML-C-70/Al) (ML- monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C-70 molecules tend to orient themselves with the C-5v axis perpendicular to the surface in analogy to what observed for 1 ML-C-70/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C-70 molecules takes place, as underlined by the gradual increasing intensity of the C-70 LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step.