Methods for Defect Manipulation in Semiconducting Oxides

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NON-TECHNICAL DESCRIPTION: The technologically useful properties of a ceramic material often depend upon the atomic defects contained in its crystalline structure. The present work seeks to further develop methods to control the concentration and movement of atomic defects in semiconducting ceramics such as titanium dioxide and zinc oxide. The methods are based upon manipulation of chemical bonds at the material surface or illumination by ultraviolet light. Control via these methods opens new possibilities for manipulating oxide defects in numerous applications. As specific examples, such control improves the properties of titanium dioxide as a photocatalyst for environmental water remediation and hydrogen production for renewable energy by water splitting, and as a component of catalysts for a wide variety of high-volume chemical production processes. The use of defect-manipulated catalysts to accelerate chemical reactions and to provide routes to new chemical products could further strengthen the competitive edge of the US chemical industry. Uses for high quality zinc oxide include transparent conducting electrodes for solar cells to produce renewable energy, and inexpensive green-blue optical emitters for consumer electronic devices.TECHNICAL DETAILS: This research project uses a combination of experiments and computations to quantify and model the mechanisms for surface-based defect manipulation in single-crystal titanium dioxide and zinc oxide, and to exploit those discoveries more fully in the synthesis of defect-engineered polycrystalline titanium dioxide for thin-film applications. Experiments with single crystals focus upon the measurement of oxygen diffusion as a marker for atomic defect behavior. Parallel experiments on thin films involve carefully controlled deposition of titanium dioxide by atomic layer techniques, followed by heating in controlled atmospheres of specific chemical composition, in some cases under ultraviolet photostimulation. Computations that support all this work entail refinement of a mathematical simulator software package for modeling defect behavior in titanium dioxide and zinc oxide based upon both experimental findings as well as quantum calculations that give atomic-scale insights. In addition to a graduate student, a substantial number of undergraduate researchers are involved directly in the research, providing important skills training in materials synthesis and characterization as well as chemical reaction measurements. This research is taking place in parallel with development of an industry-supported laboratory course for upper-division undergraduates and graduate students called "Chemistry and Transport in Semiconductor Materials Synthesis." In addition, several activities to promote the importance of ethics in science and engineering are being pursued.