Surface-Based Point Defect Manipulation in Semiconducting Oxides

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NON-TECHNICAL DESCRIPTION: Although ceramic semiconductors such as titanium dioxide and zinc oxide often find use in consumer sunscreens, they also have other important applications such as ultraviolet-emitting optical devices and gas sensors. For device and sensor applications, the material's performance is strongly influenced by defects in the regularly-ordered atomic structure. These defects can be mobile, and nearby surfaces can inject or withdraw them from the solid bulk. Experiments and computations are being combined to examine how this chemistry is affected by the inherent tendency of the surface to accumulate excess positive or negative charge, and by the presence of liquids such as water. Experiments entail the exposure of titanium dioxide or zinc oxide to a gas or liquid containing isotopically-labeled atoms specially by weight, and subsequent measurement of the buildup of the label within the solid. Computations employ a range of modeling techniques rooted in quantum calculations. The project opens rich new possibilities for manipulating defect behavior in applications. Using liquids near room temperature is especially exciting because of greatly reduced cost and ease of material manufacturing compared to conventional methods that often employ gases and heating to hundreds of degrees. The project also develops educational materials for middle school and high school students. A significant number of undergraduate researchers participate in the research, and several activities to promote the importance of ethics in science and engineering are being pursued. TECHNICAL DETAILS: Reactions of ceramic semiconductor surfaces with bulk point defects such as interstitial atoms exhibit rich chemistry that is poorly understood despite that it helps to regulate defect types and concentrations. Experiments and computations are combined to examine how that chemistry is affected by surface polarity, which describes the tendency of the surface to accumulate excess positive or negative charge. The project also examines liquid-enhanced defect injection from the surface into the bulk. Investigations include zinc oxide of varying polar and nonpolar crystallographic orientations as well as nonpolar titanium dioxide. Experiments entail exposure of the solid to a gas or liquid containing isotopically labeled atoms, and subsequent measurement of the spatial distribution of the label within the solid. For gaseous ambients, interstitial atoms of both oxygen and titanium or zinc are being examined in the presence of foreign adsorbates such as nitrogen, sulfur and chlorine. For aqueous liquids, oxygen interstitial defects are being examined under varying pH, photostimulation intensity and solution additives. Computations employ a hierarchy of mathematical modeling rooted in quantum calculations to provide useful atomistic insights. The project opens a rich new domain of surface behavior, and provides new means for manipulating defect behavior to improve photocatalysts, ultraviolet emitters and gas sensors.