Electronic properties of a pure and sodium-doped $C70$ 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.