A role of metal d-band in the interfacial electronic structure at organic/metal interface: PTCDA on Au, Ag and Cu

We analyzed the vacuum level shift (Δ) induced by the dipole layer at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and noble metals (Au, Ag and Cu). The variation of Δ observed by ultraviolet photoelectron spectroscopy does not show a simple dependence on the metal work function, which contradicts the prediction by the induced density of interface states (IDIS) model proposed by Vázquez et al. [H. Vázquez, F. Flores, R. Oszwaldowski, J. Ortega, R. Pérez and A. Kahn, Appl. Surf. Sci 234 (2004) 107]. We found that two factors, (1) the energy separation between the lowest unoccupied molecular orbital (LUMO) of PTCDA and the metal d-band states, which results in the attractive effect due to the orbital hybridization, and (2) the coupling matrix element between the adsorbate states and the metal d-band states, which result in the repulsive effect due to the orbital orthogonalization between the adsorbate states and the metal d-band states, have a clear correlation with the Δ formation. Our results indicate that the interactions between the molecular orbitals of PTCDA and the metal d-band states play an important role in determining the interfacial electronic structure, which has not been taken into account within the framework of the IDIS model.