SCANNING TUNNELING MICROSCOPY OF SEVERAL ALKYLATED MOLECULAR MOIETIES IN MONOLAYERS ON GRAPHITE

Scanning tunnelling microscopy (STM) has been employed to characterize structure, dynamics and electronic properties at interfaces with molecular materials on the length and timescale of 10 pm and 10 mus, respectively. We report specifically on bias-dependent current images of monomolecular layers of several alkylated or alkoxylated molecular segments on the basal plane of graphite, including benzene, naphthalene, triphenylene, phenylbenzoate, disulfide and diacetylene. Derivatives with sufficiently long side chains tend to form lamellar two-dimensional crystals, which typically are incommensurate with the substrate along the lamellae, and commensurate perpendicular to them. The data indicate a pronounced dependence of conductivity on the relative position of adsorbate and substrate, as well as on the bias. In a bias range around 1 V, the conductivity of the aromatic segments, the disulfide and the diacetylene is generally larger than that of methylenes, methyls, hydroxyls and carboxyls. These results are in qualitative agreement with models in which the conductivity increases as the HOMO-LUMO gap of the free molecule decreases.