Tuning electronic properties of transition metal oxides at nanoscale by means of redox processes
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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.
