Insulating state of electron-doped Cu-phthalocyanine layers

A copper-phthalocyanine (CuPc) thin film and a highly ordered CuPc single layer prepared on the Au(110) surface are electron doped by exposition to alkali metal. The CuPc thin film remains insulating when the donated electrons flow into the unoccupied molecular orbitals. The CuPc/Au interface is metallic in the absence of doping, and transition to an insulating state is observed by the filling up of the former empty states. In particular, two occupied states are generated by filling the lowest unoccupied molecular orbitals with the doping electrons. Sweeping of the former empty states through the Fermi energy does not induce any enhancing of the electronic spectral density, as detected by high-resolution photoelectron spectroscopy. Electron correlation and Jahn-Teller effects act to determine the insulating response of electron-doped CuPc.