The chemical and physical properties of hydrogen terminated silicon surfaces have received much attention because these surfaces are reasonably stable and can be prepared and manipulated in air as well as in a number of organic solvents [1]. Thus, high quality materials are available without the need for expensive vacuum systems.
The hydrogenated 111 surface of silicon can be easily prepared by etching in NH4F solutions. In this way, surfaces with large terraces can be obtained. Each silicon atop atom has only one Si-H bond, which is normal to the surface. After several hours of exposure to air, the surface becomes oxidized. Incorporation of oxygen into silicon is activated and involves a multistep process. During the initial stages of oxidation, O2 molecules are incorporated into the Si–Si backbonds without removing surface hydrogen [2, 3]. The oxidation of Si(111)-H requires the presence of an oxidant, O2, and a nucleophile, H2O [2]. The mechanisms of oxidation of chlorinated and methylated Si(111) surfaces in air is not known yet. The chlorinated surfaces readily oxidize in air whereas the methylated surfaces are more stable than the hydrogenated surface.
In this work we investigated the mechanism of oxidation of Si(111)-H, Si(111)-Cl and Si(111)-CH3 by H2O and O2. Density functional theory calculations were performed to calculate reaction pathways which allowed the identification of the different elementary reaction steps, transition states and intermediates.
The successive oxidation of the silicon backbonds make the oxidation reactions more exothermic [4] and this decreases the activation energy barriers because the increasingly positive silicon atom favors the attack by nucleophiles such as water. Steric effects in the fully methylated surface greatly increase the activation energy barriers for oxidation.
