Low-coverage decanethiolate structure on Au(111): Substrate effects

The effects that a reconstructed (herringbone) and stepped Au(111) surface have on the structure of submonolayers of decanethiolate were studied by variable-temperature scanning tunneling microscopy (VT-STM) between 25 and 60 degreesC. At 25 degreesC, formation of lattice gas (alpha) species at low coverages alters the herringbone structure by shortening the periodicity of the elbows from 25 to 15 nm. In addition, small beta-phase islands nucleate and grow anisotropically in regions of fcc stacking. These domains grow by incorporating nearby lattice gas species, consuming herringbone ridges and altering the remnant ridges that surround them. In a given domain, the beta-phase rows take one of the three [121] directions of the Au(111) surface. No beta-phase is found in regions of hcp stacking or in the bridge atom regions separating the fcc and hcp stacked regions. Increasing the coverage increases the beta-phase island size at the expense of herringbone ridges that bound the domains. For a coverage that saturates the beta-phase (similar to0.25 of the closest packing achievable), raising the temperature to 30 or 40 degreesC increases the average size of the beta-phase islands by condensation of neighboring islands with no evidence, at the selected coverage, for the presence of any other thiolate phase. At 60 degreesC, well above the thiolate melting point to form the epsilon-phase, small beta-phase domains remain. These are stabilized by boundaries of two types remnant herringbone and step edges and for a given domain, fluctuations of the distribution between the epsilon- and beta-phases were observed on a time scale of minutes.