Structure, registry and imaging mechanism of alkylcyanobiphenyl molecules by tunnelling microscopy

Abstract

GRAPHITE is a popular choice as a substrate for the study of absorption of molecules on surfaces because of its relatively simple interaction with adsorbates and the ease of preparing a clean and homogeneous surface. Here we use scanning tunnelling microscopy (STM) to probe the graphite/liquid-crystal interface and we examine what we believe to be the first monolayer of the liquid with atomic resolution. We resolve the different functional groups of the molecules and determine their orientation and packing arrangement. By decreasing the tunnel-gap resistance we examine the underlying graphite substrate and deduce the registry of the adsorbed molecules with the graphite. We thus propose a microscopic picture of the adsorption and formation of the interfacial structure. We find that aromatically bonded carbon atoms are visible in the STM images whereas tetrahedrally- and triple-bonded carbon are invisible. Hydrogen and nitrogen atoms can also be seen, although with less apparent height than the aromatic carbon. Molecular orbital calculations predict electron densities in good agreement with the observed STM contrast.