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

High resolution images of planar aromatic molecules exhibit a characteristi c intramolecular tunneling current distribution in the STM. In this study w e have investigated the correlation of this image contrast with the specifi c chemical nature of the molecules. For weakly bonded molecules on graphite and MoS2 the image contrast is dominated by the wavefunction of the specif ic frontier orbital being closest to the Fermi level of the substrate. For electron acceptor type molecules this is the lowest unoccupied molecular or bital (LUMO), whereas for electron donor type molecules the highest occupie d molecular orbital (HOMO) has been observed. On the stronger binding: Ag(1 11) surface we found acceptor type molecules showing an unchanged image con trast compared with graphite and donor type molecules where the image contr ast changes from HOMO on graphite to LUMO on Ag(111). (C) 1998 Elsevier Sci ence B.V. All rights reserved.