STM study of the electronic structure of PTCDA on Ag/Si(1 1 1)-

PTCDA molecules were evaporated onto the Ag/Si(1 1 1)- 3 × 3 surface. Simple HOMO and LUMO models have been made to explain the STM images of the square and herringbone phases. The charge transfer from the substrate and the H-bonding between the molecules play crucial roles for the formation of these phases. We found that their electronic structures are strongly modified by the molecule–substrate and intermolecular interactions. As a result, the HOMO–LUMO gaps of the PTCDA films are different for different phases and thicknesses. The increase in the HOMO–LUMO gaps with film thicknesses is discussed based on the surface and the interface polarization effect.