Effect of Alkyl Chain-Length on Dissociative Attachment: 1-Bromoalkanes on Si(100)-c(4×2)

Abstract

Chemical reactivity as a function of chain-length was investigated for three 1-bromoalkanes on silicon. The physisorption and subsequent thermal dissociative attachment of bromoethane (EtBr), 1-bromopropane (PrBr), and 1-bromobutane (BuBr) on Si(100)-c(4×2) were examined by scanning tunneling microscopy in ultrahigh vacuum from 50 to 180 K, and interpreted by ab initio theory. These 1-bromoalkanes were found to physisorb and react exclusively over the inter-row sites of Si(100)-c(4×2), with activation barriers, Ea, increasing with alkyl chain-length: Ea = 343 ± 5 meV for EtBr, Ea = 410 ± 6 meV for PrBr, and Ea = 536 ± 2 meV for BuBr. Extensive ab initio calculations gave increasing barriers along the series: Ec = 317 meV for EtBr, Ec = 406 meV for PrBr, and Ec = 430 meV for BuBr. On the basis of our calculated geometries, we interpret this dependence of thermal barrier on chain-length as due to the additional energy required with increasing chain-length in order to lift the alkyl chain away from the surface, in going from the initial physisorbed state to the reactive transition state. For BuBr, the measured Ea significantly exceeded the calculated value. This increase in effective barrier-height could be due to a “dynamical delay” in optimizing the configuration of the alkyl chain.