Delicate interactions of PTCDI molecules on Ag/Si(111)-3×3

PTCDI molecules were evaporated onto a Ag/Si(111)3×3 surface and studied using scanning tunneling microscopy (STM) and angular resolved ultraviolet photoelectron spectroscopy (ARUPS). High resolution STM images are used to identify the delicate molecule/molecule and molecule/substrate interactions and the shapes of the molecular orbitals. The results show that the substrate/molecule interaction strongly modifies the electronic configuration of the molecules as their orbital shapes are quite different at 1 and 2 monolayer (ML) coverage. Simple models of molecular HOMO/LUMO levels and intermolecular hydrogen-bondings have been made for 1 and 2 ML PTCDI coverages to explain the STM images. Changes due to the interaction with the substrate are also found in ARUPS as extra states above the regular HOMO level at 1 ML PTCDI coverage. The ARUPS data also show that the electronic structure of the substrate remains unchanged after the deposition of molecules as the dispersion of the substrate related bands is unchanged. The changes in electronic structure of the molecules are discussed based on a HOMO/LUMO split.