Band gap engineering at surfaces of insulating films by adsorption of organic molecules

Adsorption of simple organic molecules on wide band gap insulators is a versatile tool to introduce unoccupied electronic states in the band gap at the surface. Relative energetic positions, varied also by intra-molecular interactions of functional side groups, determine the effective band gap. Specific examples used here are adsorbed molecules of benzoic acid and of its OH-substituted derivates (salicylic acid, etc.) on NaCl, KCl and on MgSO4 center dot H2O. These acids are mainly physisorbed on NaCl and KCl, whereas on MgSO4 center dot H2O chemical bond formation is found. Ab initio and semi-empirical calculations are used in order to identify mechanisms of surface-molecule as well as intra-molecular interactions. (c) 2006 Elsevier B.V. All rights reserved.