silicon and methyl-bromide

We describe the dissociative attachment (DA) of methyl bromide to form chemisorbed CH(3) and Br on a Si(100)-2×1 surface at 270 K. The patterns of DA were studied experimentally by ultra-high vacuum scanning tunneling microscopy (STM) and interpreted by ab initio theory. The parent molecules were found to dissociate thermally by breaking the C-Br bond, attaching the resulting fragments CH(3) and Br at adjacent Si-atom sites. The observed DA resulted in three distinct attachment geometries: inter-row (IR, 88%), inter-dimer (ID, 11%), and on-dimer (OD, 1%). Ab initio computation agreed in predicting these three DA reaction pathways, with yields decreasing down the series, in accord with experiment. The three computed physisorption geometries, each of which correlated with a preferred outcome, IR, ID, or OD, exhibited similar heats of adsorption, the choice of pathway being governed by the energy barriers to DA chemisorption predicted to increase along the series: E(IR) = 0.48 eV, E(ID) = 0.57 eV, and E(OD) = 0.63 eV.

Topics: Hydrocarbons, Brominated; Models, Molecular; Quantum Theory; Silicon; Surface Properties

An earlier experimental study by scanning tunneling microscopy (STM) from this laboratory described the use of "localized reaction" as a means to the electron- or photon-imprinting of self-assembled patterns of CH(3)Br(ad) as covalently bound Br-Si(s) at Si(111)-7x7. Here we show that the thermal surface bromination reaction by CH(3)Br(ad) is also highly localized, and present a detailed ab initio dynamical model for the reaction, using DFT. Localization is seen to be due to the coexistence in the reactive transition-state of the neighboring bonds being broken (C-Br) and formed (Br-Si). Both experiment and theory are consistent with a low energy-barrier, E(a) approximately 0.2 eV, for the thermal bromination of Si(111) by CH(3)Br(ad), and also for the desorption of intact CH(3)Br(g) (E(des) approximately 0.2 eV). Two physisorbed states of CH(3)Br(ad)/Si(111) (I and II) are distinguishable by STM at 50 K by their differing displacement from the underlying Si adatom. These states can be identified with similarly displaced states in the STM images simulated by DFT. At the elevated temperature of 80 K, a markedly displaced physisorbed state (III) appears in the STM image, indicated by DFT to have a configuration encountered along the reaction path immediately prior to the transition state. The electron-induced bromination of Si(111) by CH(3)Br(ad), and also electron-induced molecular desorption, are examined as a function of the energy of the incident electron, giving for both processes a threshold energy of E(e) approximately 1.8 eV in accord with ab initio theory, and a substantial yield of 10(-6) to 10(-5) Br-Si(s)/electron.

Topics: Crystallization; Hydrocarbons, Brominated; Macromolecular Substances; Materials Testing; Molecular Conformation; Nanostructures; Nanotechnology; Particle Size; Silicon; Surface Properties