Author: Karol Cieslik

Publisher: Karol Cieslik

Published:

Total Pages: 134

ISBN-13:

The climate crisis that we are facing has galvanized the scientific and engineering communities and has led to the rapid development of new, efficient, environmentally-friendly devices. One of the most promising classes of materials for such applications are transition metal oxides. This is due to the fact that by controlling the oxygen content in these crystals by means of reduction and oxidation, the material properties can be tuned in a wide range of values. Thus, the transition metal oxides, such as the model crystals, titanium dioxide (TiO2) and strontium titanate (SrTiO3), find use in so many different fields, from photocatalysis, to energy storage (solid oxide fuel cells), information technology (memristors) and even healthcare (antibacterial films). This PhD thesis is an investigation into the effect of reduction and oxidation on the electronic properties of transition metal oxides. These processes were studied at nanoscale using a multitude of techniques to provide a thorough characterization of the changes that occur in the studied systems, i.e. TiO2 and SrTiO3. Moreover, the experiments were performed in both ultra high vacuum (UHV) conditions, as well as in oxygen, and even in atmospheric air, in order to comprehensively describe the changes in properties and to bring the results closer to applications. The goal of the dissertation was to study the evolution of the electronic properties, i.e. the work function and conductivity, due to redox processes, and to add to the general understanding of these processes. The experiments revealed that the electronic properties may be tuned. In case of using reduction by means of annealing in UHV, ion sputtering, and repeated ion sputtering and annealing, and for oxidation by exposure to oxygen or air at room temperature, and annealing in oxygen. Using this range of methods, the conductivity of TiO2 can be changed from semiconductive-like to metallic-like. Furthermore, the work function of the transition metal oxides can be tuned in a wide range, from 3.4 eV to 5.0 eV for TiO2, and from 2.9 eV to 4.5 eV for SrTiO3. This is associated with changes in surface and subsurface composition, crystallography, morphology and even with the growth of new oxide phases. The key findings in the field of surface science were the description of the changes in electronic properties due to repeated sputtering and annealing, and the presence of oxygen getter substances. These results are important, because they touch upon the very basis of every experiment in the field, i.e. the preparation of crystals. This work can be used to foster greater reproducibility of experiments, as well to provide new means of designing experiments. Another object of the study was the technologically interesting system of conductive nanowires on semiconductive SrTiO3 substrate. It was shown that the nanostructures are composed of a TiO core covered with a layer of Ti3O5. The evolution of the system, starting from atomically flat strontium titanate, through nanowire-covered substrate to a crystal with a layer of porous titanium suboxides was described. The effect of annealing in oxygen on wire-covered surface was been investigated.