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Scanning Tunneling Microscopy and Adsorption Studies on Single-Crystal Metal Oxide Surfaces
Conway, Timothy James
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Natural and synthetic SnO2 samples were studied using scanning tunneling microscopy (STM). The SnO2 surface flattens considerably following high temperature treatments up to 1500 K. The conductivity of the synthetic SnO2 surface is significantly reduced following annealing at temperatures of approximately 1200-1500 K, making tunneling impossible. A decrease in conductivity was not observed for the natural SnO2 sample following similar high temperature treatments, most likely due to impurities which act as dopants. No atomic scale images were collected on the SnO2 surface which provided information regarding atomic positions and point defects on the surface. Water adsorption was studied on the stoichiometric Cr2O3 (1012) surface, using thermal desorption spectroscopy (TDS). Water was the only desorption product observed during TDS. Adsorption is primarily dissociative following exposure to water at 163 K. Approximately, 0.12 monolayers of water dissociate on the clean, nearly stoichiometric Cr2O3 (1012) surface. The first order kinetics observed for the recombination of dissociated water are not well understood. One possible explanation is that the rate limiting step for desorption involves the breaking of a Cr-O bond resulting in a freely diffusing OH species The exchange of halogen and oxygen was studied on Cr2O3 (1012) using Auger electron spectroscopy (AES) and TDS. The exchange of chlorine and oxygen is completely reversible. Chlorine is removed from the Cr2O3 (1012) surface following exposure to oxygen. Exposure of CFCl2CH2Cl reduces the surface oxygen concentration to that of the clean, nearly stoichiometric Cr2O3 (1012) surface. The exchange of chlorine with oxygen appears to involve only chemisorbed surface oxygen, not bulk lattice oxygen.
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