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 · H2O. These acids are mainly physisorbed on NaCl and KCl, whereas on MgSO4 · 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.