Evidence of charge transfer at the Cu-phthalocyanine/Al(100) lnterface

Electronic and structural properties of the CuPc/Al(100) organic-inorganic interface were investigated by means of a multitechnique experimental approach based on synchrotron radiation. The chemical selectivity of X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of copper-phthalocyanine (CuPc) as a function of the molecular thickness ranging from the submonolayer to 40 angstrom. Photoemission from core levels shows a dramatic alteration of the electronic structure of molecules localized at the interface. At the lowest CuPc coverages, the complete reduction of the oxidation state of copper was observed, while C 1s and N 1s shake-up satellites were no longer visible. Both findings are explained with a sizable charge transfer from the substrate to the molecule involving the b(1g) (Cu 3d-derived) and the LUMO hybridized with the substrate conduction band. The linear polarization of the synchrotron light was employed in X-ray absorption near-edge spectroscopy (XANES) to determine the orientation of CuPc molecules. Molecular planes oriented almost perpendicular with respect to the metal surface were observed from the second layer on.