On a Si(100)-(2x1) surface with abundant dangling bonds, reaction of HCl molecules at room temperature is dominated by exothermic dissociative adsorption of H and Cl on two adjacent dangling bonds. This co-adsorption reaction is blocked for an isolated dangling bond, yet surprisingly endothermic H or Cl abstractive adsorption occurs, as observed by in situ scanning tunneling microscopy. On an isolated dimer dangling bond pair, co-adsorption of H and Cl is common as expected, but adsorption of a pair of abstracted Cl or H from two HCl molecules also occurs. These results, complemented by theoretical calculations, indicate that dissociative adsorption and abstractive reaction of a multi-atom gas molecule can be initiated at a single DB by forming an intermediate adsorption state.

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An atomic layer of stoichiometric NaCl was formed on a covalent Si(100) surface after two successive half-reactions at room temperature. The first half-reaction due to Cl 2 exposure generates a square array of Cl adatoms with a distance close to that in a NaCl(100) surface plane. By utilizing scanning tunneling microscopy (STM), corelevel photoemission spectroscopy, and ab initio density functional theory (DFT) calculations, it was found that progressive deposition of Na in the second-half reaction results in surface-supported Na 3 Cl clusters, onedimensional cluster chains, and (2 × 2) patches, and eventually turns the Cl-adlayer into a single-terrace, wavy NaCl layer at one monolayer Na coverage. The grown NaCl monolayer rolls over atomic steps like a carpet and covers the entire surface.The atomic and electronic structure of the topmost Si layer underneath the NaCl layer resembles that of the initial silicon surface layer with buckled dimers. Results of the comprehensive investigation together suggest that an ionic NaCl monolayer is very weakly bonded to the covalent substrate and appears nearly free-standing.

A fundamental question for the adsorption of any gas molecule on surfaces is its saturation coverage, whose value can provide a comprehensive examination for the adsorption mechanisms, dynamic and kinetic processes involved in the adsorption processes. This investigation utilizes scanning tunneling microscopy to visualize the H2O adsorption processes on the Si(100) surface with a sub-monolayers (<0.05 ML) of chemically-reactive dangling bonds remaining after exposure to (1) a hydrogen atomic beam, (2) H2O, and (3) Cl2 gases at room temperature. In all three cases, each of the remaining isolated single dangling bonds (sDB) adsorb and is passivated by either of the two dissociation fragments, the H or OH radical, to form a surface Si–H and Si–OH species. A new adsorption mechanism, termed ‘dissociative and asynchronous chemisorption’, is proposed for the observation presented herein. Upon approaching a sDB site, the H2O molecule breaks apart into two fragments. One is chemisorbed to the sDB. The other attaches to the same or the neighboring passivated dimer to form a transition state of surface diffusion, which then diffuses on the mostly passivated surface and is eventually chemisorbed to another reactive site. In other words, the chemisorption reactions of the two fragments after dissociation occur at different and uncorrelated time and places. This adsorption mechanism suggests that a diffusion transition state can be an adsorption product in the first step of the dissociative adsorption processes.

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