Recognition of Individual Tail Groups

Self-assembled monolayers of alkanethiols and dialkyl disulfides having different tail groups coadsorbed onto gold(111) were studied with high gap-impedance scanning tunneling microsocpy. 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 remaied 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 of 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.

Figure 1: (a) STM image of a monolayer adsorbed from a solution containing dodecanethiol and azobenzene, each at concentrations of 5 micromolar in ethanol for 24 h and subsequently annealed at 100 degrees centigrade for 5 h in air. Protrusions 0.3 nm higher than the hexagonal lattice plane, randomly distributed in this scan, correspond to single azobenzene molecules. The packing of dodecanethiol molecules is not disturbed by neighboring azobenzenes. (b) STM image of a monolayer prepared by adsorption of an asymmetric disulfide with methyl and hydroxyl end groups for 3 h at room temperature. Two depressions occur on the upper left part of this gold terrace. Methyl and hydroxyl end groups, individually resolved, populate this image equally and define meandering patches with a height difference of 0.1 nm. Hydroxyl end groups constitute lower patches. Halves of the asymmetric disulfide still occupy sites of the hexagonal lattice as observed in the simple case of dodecanethiol.

Bruno Michel <bmi@zurich.ibm.com>

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Last modified: Wed, 15 May 1996 11:34