Abstract

This thesis contains studies of the nature of the adsorption and thermally or electrochemically-induced evolution of metallic overlayers on fcc(111) metal surfaces employing scanning tunneling microscopy (STM) as a primary analytical tool. In the first study, the formation and thermal evolution of the overlayer structures formed through silicon chemical vapor deposition (CVD) using silane as the silicon source onto Pt(111) followed by a flash anneals was analyzed. Two platinum silicide phases were found for saturation silane doses which form (√7×√7)19.1° and (√19×√19)23.4° overlayers, respectively. The thermal evolution of this system and the processes involved in CO adsorption onto these overlayers are described.

A subsequent study employing STM and Auger electron spectroscopy extended the scope of the investigation to sub-saturation doses and elevated temperature exposures resulting in the observation of five other atomically-ordered overlayers. These overlayers were found to be multilayers and to reflect the substrate lattice's symmetry without being correlated to known platinum silicide bulk phases. The surface structures were found to be strongly dependent on the local silicon concentration and a result of the competition between platinum-silicon chemical reactions and bulk silicon diffusion as evidenced by silicide formation and degradation behaviors. Coexistent phases were also observed in contrast to accepted models.

Further investigation of the platinum-silicon system was through a similar set of studies on Pt(100) for saturation exposures to silane. These studies resulted in (√2×√2)45°, (√5×√5)26.6°, and (√17×√17)14.0° atomically-ordered overlayers as well as a quasihexagonal overlayer of lower order which converts to the (√17×√17)14.0° overlayer at higher temperatures. These overlayers are also multilayer and generally reflect the substrate's lattice symmetry but, in contrast to the Pt(111) results, do reflect known bulk structures in two cases. Together the silicon-on-platinum. results establish substrate direction as the primary, but not sole, structure-determining interaction in this system.

Finally, the underpotential deposition (UPD) of cadmium onto Au(111) in aqueous solution was studied with STM and cyclic voltammetry. This system resulted in three linear overlayer structures which do not directly reflect the substrate symmetry. This result is in direct contrast to the majority of the UPD literature.