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 C70 layer adsorbed on polycrystalline Al (1 ML-C70/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 C70 molecules tend to orient themselves with the C5v axis perpendicular to the surface in analogy to what observed for 1 ML-C70/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 C70 molecules takes place, as underlined by the gradual increasing intensity of the C70 LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step