Photoluminescence and surface photovoltage studies of defects at nanoscale surfaces and interfaces in thin films of ZnO, TiO2 and Diamond

Abstract

The electronic properties of nanomaterials are greatly affected by their surfaces, which often contain significant numbers of defects that induce localized bandgap states. These localized states may have a significant impact on a material's optical absorption spectrum, conductivity, and charge dynamics - characteristics that are important in applications. The focus of the present work was on the surface defect properties of thin films of titanium dioxide, zinc oxide and diamond. Experimental methods sensitive to the spectral signatures of surface and bulk defects (photoluminescence and surface photovoltage spectroscopy were used to estimate the electronic structure of each of these materials. Band diagrams of TiO2, TiO2/Au, TiO2/Ag, and TiO2/ZrO2 thin films have been obtained. The results suggest that all TiO2-based films contain a significant number of native defect-related gap states.^^In addition, a new energy level at ~1.8 eV was detected in TiO2/Ag and specifically attributed to the added Ag. The electronic structure of TiO2/Au did not differ significantly from that of TiO2. After the addition of zirconium, the number of native defect-related states increased. The TiO2-based samples were also subjected to low energy Ti+-irradiation followed by similar gap structure studies. The results are discussed relative to photocatalytic applications of the studied materials. The optoelectronic properties of homoepitaxial ZnO thin films synthesized by the atomic layer deposition method were also studied. The films were grown at different temperatures in order to study the effects of growth conditions on the defect concentration. Surface photovoltage and temperature-dependent photoluminescence studies showed that our ZnO thin films have low concentrations of defects and excellent crystallinity.^^The growth temperature, however, has only a slight effect on the overall quality of the films. Lastly, diamond thin films grown by the chemical vapor deposition method were studied. The films were doped with different amounts of boron and some were gamma irradiated. The mechanism for conductivity in diamond thin films with different boron concentrations was also studied. The impact of gamma irradiation on the films' conductive properties was investigated.