Evolution of the electronic structure at the interface between a thin film of halogenated phthalocyanine and the Ag(111) surface

The electronic structure of a thin film of chlorine-substitUted Zn-phthalocyanine (ZnPcCl8) deposited on Ag(111) was studied by valence band photoemission and near-edge X-ray absorption fine structure spectroscopy. At the first stages of molecular adsorption the interface formation is accompanied by the promotion of intense interface states. A charge transfer from the substrate to lowest unoccupied molecular orbital (LUMO) of the molecules of the first layer appears as a new density of states close to the Fermi level in photoemission. The central role of the LUMO is corroborated by the reduction of the LUMO-derived transition observed in the low-coverage X-ray absorption spectrum taken at the N 1s edge. However, the lack of a sizable density of states at E-F as well as the photon energy dependence of the interface state peak intensity suggest that more complicated mechanisms of intra- or extramolecular charge redistribution may also play a role. The Ag 4d-derived valence band region also appears strongly modified upon molecular adsorption. In the paper it is shown that these modifications are merely due to the vanishing of the substrate surface states and the emerging of a bulk density of states at the interface. It is suggested that such behavior should be common for other organic monolayers adsorbed on noble metal surfaces. Finally, the study of the work function as a function of coverage is presented. Its behavior is interpreted in terms of coexistence of different structural phases and compared to a local-probe work function study on the same system.