Point defects and impurities are well known to influence the electronic and chemical properties of transition metal oxides. Rutile TiO₂ has been observed to contain oxygen deficiency (or metal excess) under experimentally attainable oxygen partial pressures. The objective of this project is to obtain a fundamental understanding of the structure configuration, formation energetics and electronic structure of intrinsic and extrinsic point defects in rutile TiO₂. In this work the first-principles plane-wave pseudopotential calculations coupled with thermodynamics calculations are performed to investigate defect complex as well as single defects in various charge states. The densities of states (DOS) are also calculated for the bulk TiO₂ structure with and without point defects. Thus, the effect of point defects on the conductivity of transition metal oxides are determined quantitatively. This work is supported by the National Science Foundation under Grant No. DMR-0303279 and is performed in collaboration with Professor Elizabeth Dickey (Penn State University) and Professor Mike Finnis (Queen's University, Belfast). Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

      Interstitital and vacancy diffusion play crucial roles in photovoltaic applications of TiO₂. Here, possible diffusion paths for Ti interstitials and oxygen vacancies in the TiO₂ bulk are investigated. In particular, different oxygen ion diffusion mechanisms are considered, including oxygen vacancy diffusion by site exchange and oxygen interstitial migration. Our DFT simulations on the O vacancy diffusion involve different oxygen sites, including the octahedral site at 0.5c and the off-center position at 0.25c. In addition, different paths ([100], [010], [001]) are considered. Various Ti interstitial diffusion mechanisms are also investigated: 1) Ti interstitial diffuses through open channel along [001] direction; 2) Ti interstitial migration from Frenkel pair.

      Ab-initio total energy calculations are performed to study the properties of relaxed metal oxide low angle grain boundary structures and metal-GaAs and metal-metal oxide heterogeneous interfaces. The calculations use density functional and pseudopotential theories in conjunction with plan wave expansions within the framework of a supercell approach. To date we have examined grain boundary structures in ZrO₂ (cubic) and in TiO₂ (rutile). The relaxed grain boundary structures are compared with experimental results obtained using a scanning transmission electron microscope (STEM) at ORNL and Penn State. The results provide details about distortions in the electron distribution around strained atoms in the grain boundaries.

      We are also investigating the stability of heterogeneous material interfaces that are important for electronic and quantum well devices. As metal-semiconductor interfaces are of special fundamental and applied interest, we have started by considering Ag/GaAs(110), a model system that has received considerable attention and for which there is intriguing experimental data. We are currently working to explain the observed growth modes of Ag on GaAs(110) which vary from Ag island growth to thin film growth depending on the coverage and deposition temperature.

      This work is performed in collaboration with Professor Elizabeth Dickey (Penn State University), Richard Wood (ORNL), Zhenyu Zhang (ORNL), and Steve Pennycook (ORNL) and This work is supported by the National Science Foundation under Grant No. DMR-0303279. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).