Electronic structure at highly ordered organic/metal interfaces: Pentacene on Cu(110)

The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110). These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about -0.9 eV by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald , Surf. Sci. 600, 1909 (2006)]. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.