Chemical reactivity on Si(111)-at the atomic scale as it occurs spontaneously, or as it is induced by phenomena occurring between the tip and the Si substrate in a scanning tunneling microscope (STM), is investigated in the presence ofas a reactant gas. The STM has been modified for this purpose to operate in the corrosive environment and in temperatures in the range of 250 - 300 K.Reactant molecules are adsorbed on the surface at a low temperature which prevents desorption, diffusion, or spontaneous reactions during the time span necessary for STM-induced experiments. A voltage pulse is applied to the STM tip located above the adsorbed molecule to generate a highly localized and strong electric field. The adsorbed molecule undergoes a chemical reaction to form volatile fluoride resulting in removal of a silicon atom from the surface. The volatile product species is identified as. The Si atoms neighboring the reaction site undergo minimum displacement as a result of the process.The spontaneous reaction kinetics ofon the Si surface at low temperatures, after low coverage deposition, is also studied. Low temperatures provide the necessary time to acquire STM information on the number of adsorbed molecules before any substantial reaction takes place. In time, the adsorbed molecules either desorb or react with Si surface atoms to produce volatile products and therefore leave vacancies behind. Once the surface is free of adsorbed fluorinated molecules, excess vacancies resulting from the reaction are counted and the true removal probability is calculated.
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