Surface- and Photo-Based 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 recently-discovered 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. Controlling the defects will help improve the performance in diverse applications such as solar hydrogen production by water splitting, environmental water remediation by photocatalysis, and gas sensing. 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.TECHNICAL DETAILS: Specially synthesized isotopically-labeled structures are used in conjunction with self-diffusion measurements to determine generation and annihilation rates of interstitial atoms or vacancies at surfaces. The experiments involve observation of the isotopic profile evolution by secondary ion mass spectroscopy, with supplementary experiments by photoreflectance to measure the magnitude of near-surface electric fields. Detailed modeling of the defect diffusion-reaction networks underpins experimental interpretation. The experiments examine the effects of controllable chemical adsorption and surface crystallographic orientation upon the insertion and generation of bulk defects mediated by chemically unsaturated surface bonds ? both neutral and electrically charged. In addition, optical stimulation effects are quantified by analogous experiments performed under super-band illumination, and interpreted in terms of changes in the charge state of either the surface or the defects themselves.