A systematic STM study of planar aromatic molecules on inorganic substrates - I. Submolecular image contrast

High resolution images of planar aromatic molecules exhibit a characteristic intramolecular tunneling current distribution in the STM. In this study we have investigated the correlation of this image contrast with the specific chemical nature of the molecules. For weakly bonded molecules on graphite and MoS2 the image contrast is dominated by the wavefunction of the specific frontier orbital being closest to the Fermi level of the substrate. For electron acceptor type molecules this is the lowest unoccupied molecular orbital (LUMO), whereas for electron donor type molecules the highest occupied molecular orbital (HOMO) has been observed. On the stronger binding: Ag(111) surface we found acceptor type molecules showing an unchanged image contrast compared with graphite and donor type molecules where the image contrast changes from HOMO on graphite to LUMO on Ag(111). (C) 1998 Elsevier Science B.V. All rights reserved.