Scanning tunneling microscopy studies of diazo dye monolayers on HOPG

We report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C₂₀H₁₉N₅) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.