RECOGNITION OF INDIVIDUAL TAIL GROUPS IN SELF-ASSEMBLED MONOLAYERS

Self-assembled monolayers of alkanethiols and dialkyl disulfides having different tail groups (methyl, hydroxyl, and azobenzene) coadsorbed onto Au(111) were studied with high-gap-impedance scanning tunneling microscopy. Topographic differences correlated to the tail groups allowed recognition of individual molecules in these two-component monolayers and established that topographic contours were generated at the monolayer episurface. The constituents of the monolayer do not segregate during the adsorption or thermal treatment, and their packing remained ordered and compact. Characteristic patterns generated by the distribution of the different tail groups allow statements concerning diffusion processes and specific intermolecular interactions. Adsorption of mixed disulfides showed no separation of the tail groups, either because the disulfide remained intact upon chemisorption or because the energy barrier for lateral diffusion of the thiolates formed was too high. Individual azobenzene molecules diluted in a monolayer of methyl-terminated chains fit into the hexagonal lattice. Small clusters of azobenzenes disrupt the hexagonal lattice locally, although globally the monolayer maintains a well-packed lattice. Using scanning tunneling microscopy, we are able to investigate accurately the structure of these complex self-assembled monolayers and thereby contribute to a better understanding of organic interfaces.