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, su mmation formula C20H19N5) on the basal plane of highly oriented pyroly tic graphite (HOPG). Monolayers of the dye were prepared by vapour dep osition or by dissolving the molecules in the liquid crystal octylcyan obiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resoluti on images allow the identification of individual molecules and the obs ervation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules wit h the substrate. A model for the unit cell is proposed.