Image states at the interface with a dipolar organic semiconductor

Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage. Above 1 ML, a new image state develops, corresponding to the screened interaction at the organic semiconductor/substrate interface. We show that the evolution of the observed image states can be understood on the basis of resonance-enhanced anion formation in the presence of strong electric fields. These data represent a step toward understanding the influence of electrostatic fields on electronic structure at organic semiconductor interfaces.