Unoccupied electronic states in quaterphenyl oligomer films and at the film-gold and film-oxidized silicon interfaces

Abstract

Thin films of 4-quaterphenyl (4-QP) are thermally deposited in an ultrahigh vacuum on polycrystalline gold and oxidized silicon substrates. In the process of deposition, the structure of unoccupied electron states 5–20 eV above the Fermi level (E _F) and the surface potential are monitored by the method of total current spectroscopy (TCS) using an incident beam of low-energy electrons. During the deposition, the electron work function of the surface changes because of a change in the surface layer composition, reaching a steady-state value of 4.3 ± 0.1 eV at a 4-QP film thickness of 8–10 nm. The density of valence states (DOVS) and the density of unoccupied states (DOUS) in model 4-QP films are calculated using the linearized augmented plane wave method in the generalized gradient approximation of the density functional theory. In the model 4-QP structure, the minimal spacing between carbon atoms of neighboring 4-QP molecules is taken to be 0.4 nm in order that intermolecular interaction can be assumed to be relatively weak, which is observed in disordered 4-QP films. The TCS-measured DOUS and the DOUS predicted theoretically are in good agreement.