Functional Group Contrast in Scanning Tunneling Microscopy Images of Substituted Phenylethers

Abstract

Numerous scanning tunneling microscopy (STM) studies of adsorbates at gas/solid and liquid/solid interfaces have been reported. Although early STM experiments of these systems were concerned primarily with visualizing molecules at the atomic level, the focus has shifted to extracting chemical information from STM images, including the identity of atoms or of functional groups within an adsorbed molecule. However, STM image interpretation continues to be an immense challenge and one currently debated issue of critical importance is the mechanism(s) by which the image contrast reveals atomic and molecular structure. Recently, a combination of electronic and geometric factors was proposed. The electronic factor addresses the coupling between the energy levels of the adsorbate and the Fermi level of the surface whereas the geometric factor is related to the spatial overlap between the STM tip and the functional group.A previous study in our laboratory of a homologous series of para-halogenated phenyloctadecyl ethers (X-POEs, where X = H, CI, Br, I), physisorbed onto highly oriented pyrolytic graphite (HOPG), revealed a bias-dependent contrast in STM images resembling calculated (HyperChem) electron density contours of bonding molecular orbitals.