Dissociative Attachment and Reaction on Silicon

Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study two types of reaction at a crystal surface: long-range migration and dissociative attachment. We developed the first dynamical model for the observation of long-range migration of ethylene across the rough Si(100) surface. Ab initio molecular dynamics revealed two requirements for this long-range migration: first the molecule must leave the surface through ‘ballistic’ motion, traveling out of range of its roughness, second it must ‘bounce’ like a skipping-stone to reach the observed distances. Additionally, comparable behavior was predicted for atomic chlorine following electron-induced dissociation of chlorophenyl at Cu(110). In a following chapter we examined the factors governing the choice of pathway in the dissociative attachment (DA) of H2 and HCl at Si(100). In a computational study, we correlated the location of the energy barrier along the minimum-energy pathway with the separation of chemisorbed products: an ‘early’ barrier for reaction at closely-separated silicon atoms and a ‘late’ barrier for more-widely separated silicons, suggesting a means of 'steering' the reaction by reagent translation or vibration. In the remainder of the thesis DA at Si(100) was examined for two families of molecules, a series of hydrogen halides and of primary alkyl alcohols. For the hydrogen halides we observed three patterns of DA, unaltered in their relative yields by temperature change. A detailed study of the molecular dynamics for hydrogen bromide linked the choice of pathway to the sequential approach to the surface of the light H followed by the heavy Br. The charge flow resulting from the attachment of H explained the preference for Br to react ‘Inter-Dimer’. For the family of alcohols, methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, we again observed a dominant 'Inter-Dimer' reaction path, rather than the previously-accepted 'On-Dimer' mechanism. Neither theory nor experiment gave evidence of a significant alteration in energy-barrier along the series. This finding contrasted with the experimental and theoretical observations by previous workers in which the energy-barrier for DA at Si(100) increased with increasing alkyl chain length along a series of 1-bromoalkanes. The difference here is the absence of ‘chain-lifting’ during the alcohols’ reactions.Ph.D.2017-02-09 00:00:0