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Abstract

The anatase phase of TiO2 is a key material in the third generation of photovoltaics and for photo-catalytic devices. The number of promising applications, where one would profit from the electronic degrees of freedom of the material, like in memristors, spintronics, thermoelectrics etc., is constantly increasing. This vivid interest for anatase, beyond the traditionally used nanoparticle form, puts forward other structures like nanowires and thin films. Despite this progress in the demand and processing of anatase the description of the electronic properties is very limited. Few of the relevant questions are: what is the best way to engineer the band-gap; what is the character of the injected charge carriers; is there a polaron formation in this ionic material; what is the best way to stabilize a metallic state; can one observe superconductivity in TiO2 like in SrTiO3; how do the electronic properties evolve on going from bulk crystals, thin films, nanowires to nanoparticles? Bringing answer to some of these questions was the leitmotif of my PhD program. The principle quantities which I have investigated were the electrical transport coefficients: resistivity and thermoelectric power. Both quantities were measured in a broad temperature range (in some cases from 30 mK to 1000 K) and as a function of hydrostatic pressure (up to 4 GPa). With the progressive emergence of a physical model about this material in time, it was necessary to perform other measurements, to go beyond the expertise of our laboratory. I initiated and participated in optical and pumped-probe optical measurements and launched angle resolved photoemission spectroscopy (ARPES) studies. The major results are the following: 1. The easiest and almost inevitable doping of anatase is by oxygen vacancies. They create a low-lying donor level. The electrons are dressed by interactions with the lattice and upon thermal excitation into the conduction band they behave as large polarons. Their lifetime depends on the density of the vacancies. Besides high temperature heat treatment high energy photons (90 eV) can create these vacancies, as well. This offers the way of finely tuning the carrier density and to study polaron-polaron interactions. High pressure transport study confirms the polaronic character of the charge carriers and shows a non-monotonic pressure dependence of their mobility. 2. In Cr, N doped and Cr-N co-doped anatase thin films the dominant doping comes from oxygen vacancies. Surprisingly, with increasing dopant concentration the donor level - conduction band separation decreases considerably while the scattering rate goes up. These measuremnts call for an extreme care in the synthesis if one wants to have high quality films. 3. The best metallicity with doping is achieved in the case of Nb replacement of Ti. The 6% doping leads to 10−4 Ωcm resistivity at room temperature. A slight cation off-stoichiometry gives a weak but noticeable Kondo scattering at sub-Kelvin temperatures which might be at the origin of the absence of superconductivity. 4. In the case of an assembly of nanowires and nanoparticles the transport mechanism is suspected to be of hopping of charge carriers. However, the samples were too resistive to be measured at room temperatures with a d.c. method. Going to high temperatures the samples loose oxygen which precludes the precise measurement of the temperature dependence of the resistivity and the extraction of the hopping parameters. This extensive program of characterizing the electronic properties of anatase in all forms was possible due to the exceptional sample preparation expertise of our laboratory. Unique pristine and Cr, Co, Ir and Nd doped single crystals, nanowires and nanoparticles were prepared in-house; the thin films were provided by Dr. G. Eres (Oak Ridge national Laboratory) and by Prof. T. Hasegawa (Tokyo University) in the frame of a collaboration.

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