RECOGNITION OF INDIVIDUAL TAIL GROUPS IN SELF-ASSEMBLED MONOLAYERS

Self-assembled monolayers of alkanethiols and dialkyl disulfides havin g different tail groups (methyl, hydroxyl, and azobenzene) coadsorbed onto Au(111) were studied with high-gap-impedance scanning tunneling m icroscopy. Topographic differences correlated to the tail groups allow ed recognition of individual molecules in these two-component monolaye rs and established that topographic contours were generated at the mon olayer 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 distrib ution of the different tail groups allow statements concerning diffusi on processes and specific intermolecular interactions. Adsorption of m ixed disulfides showed no separation of the tail groups, either becaus e the disulfide remained intact upon chemisorption or because the ener gy barrier for lateral diffusion of the thiolates formed was too high. Individual azobenzene molecules diluted in a monolayer of methyl-term inated chains fit into the hexagonal lattice. Small clusters of azoben zenes disrupt the hexagonal lattice locally, although globally the mon olayer maintains a well-packed lattice. Using scanning tunneling micro scopy, we are able to investigate accurately the structure of these co mplex self-assembled monolayers and thereby contribute to a better und erstanding of organic interfaces.