The photovoltaic energy conversion system (PV cells or solar cells) has been researched over the last few decades, and new technologies have been proposed. At the same time, the synthesis of nano-scale materials has been investigated intensively from the 1990s. These new types of materials encourage the development of new PV technologies with extensive research. Dye-sensitized solar cells (DSSCs) can be a part of these efforts. Since first presented in 1991, DSSCs have become the center of attention due to their great advantages to the traditional silicon solar cells. However, it remains a challenge to develop better performing DSSCs since the efficiency of DSSCs is still much lower than that of high performance solar cells. To meet this challenge, the different types of TiO2 nanostructures in DSSCs have been studied. This thesis presents the synthesis of TiO2 hetero-structures. These structures can achieve two important factors in DSSCs. One is the electron pathway for high electron transport rate, and the other is the large surface area for the dye absorption. TiO2 hetero-structures were successfully synthesized by using a simple thermal annealing method. The synthesis method required neither a high reaction temperature nor complicated reaction processes and produced dense TiO2 nanowires and incorporating TiO2 nanoparticles with relatively short reaction time. The key parameters of growing 1-D TiO2 nanostructures were the Cu eutectic catalyst, the reaction temperatures, and the annealing time. The repetition time and the reaction temperatures were important factors for incorporating TiO2 nanoparticles. The structure and composition of as-grown samples were analyzed using an x-ray diffractometer, a scanning electron microscope, a field emission scanning electron microscope, a transmission electron microscope and an ultraviolet-visible spectroscopy. The results showed they were crystalline structures in rutile phase of TiO2. From this research, we can utilize hetero-structures as an electrode of DSSCs. We also expect that our simple and effective synthesis method can be used for growing other kinds of metal oxide nanostructures, especially for those melting temperature are high.

Complex-oxides have seen an enormous amount of attention in the realm of Condensed Matter Physics and Materials Science/Engineering over the last several decades. Their ability to host a wide variety of novel physical properties has even caused them to be exploited commercially as dielectric, metallic and magnetic materials. Indeed, since the discovery of high temperature superconductivity in the “Cuprates” in the late 1980’s there has been an explosion of activity involving complex-oxides. Further, as the experimental techniques and equipment for fabricating thin films and heterostructures of these materials has improved over the last several decades, the search for new and more exotic properties has intensified. These properties stem from the interfaces formed by depositing these materials onto one another. Whether it be interfacial strain induced by the mismatch between the crystal structures, modified exchange interactions, or some combination of these and other interactions, thin films and heterostuctures provide an invaluable tool the modern condensed matter community. Simply put, a “complex-oxide” is any compound that contains Oxygen and at least two other elements; or one atom in two different oxidation states. Transition Metal Oxides (TMO’s) are a subset of complex-oxides which are of particular interest because of their strong competition between their charge, spin and orbit degrees of freedom. As we progress down the periodic table from 3d to 4d to 5d transition metals, the crystal field, electron correlation and spin-orbit energies become more and more comparable. Therefore, TMO thin films and heterostructures are indispensable to the search for novel physical properties. KTaO3 (KTO) is a polar 5d TMO which has been investigated for its high-k dielectric properties. It is a band insulator with a cubic perovskite crystal structure which is isomorphic to SrTiO3 (STO). This is important because non-polar STO is famous for forming a highly mobile, 2-Dimensional Electron Gas (2DEG) at the hetero-interface with polar LaAlO3 (LAO) as a result of the so-called “polar catastrophe”. Here, I use this concept of polarity to ask an important question: “What happens at hetero-interfaces where two different polar complex oxides meet?” From this question we propose that a hetero-interface between two polar complex-oxides with opposite polarity (I-V/III-III) should be impossible because of the strong Coulomb repulsion between the adjacent layers. However, we find that despite this proposed conflict we are able to synthesize KTO thin films on (110) oriented GdScO3 (GSO) substrates and the conflict is avoided through atomic reconfiguration at the hetero-interface. SrRuO3 (SRO) is a 4d TMO, and an itinerant ferromagnet that is used extensively as an electrode material in capacitor and transistor geometries and proof-of-concept devices. However, in the thin film limit the ferromagnetic transition temperature, TC, and conductivity drop significantly and even become insulating and lose their ferromagnetic properties. Therefore, we ask “Are the transport properties of SRO thin films inherently inferior to single crystals, or is there a way to maintain and/or enhance the metallic properties in the thin film limit?” We have fabricated SRO thin films of various thickness on GSO substrates (tensile strain) and find that all of our samples have enhanced metallic properties and even match those of single crystals. Finally, we ask “Can these enhanced metallic properties in SRO thin films allow us to observe evidence of a topological phase without the complexity of off-stoichiometry and/or additional hetero-structural layers?” Recent reports of oxygen deficient EuO films as well as hetero-structures and superlattices of SRO mixed with SrIrO3 or La0.7Sr0.3MnO3 have suggested that a magnetic skyrmion phase may exist in these systems. By measuring the Hall resistivity, we are able to observer a topological Hall effect which is likely a result of a magnetic skyrmion. We find that of the THE exists in a narrow temperature range and the proposed magnetic skyrmions range in size from 20-120 nm. Therefore, the SRO/GSO system can provide a more viable means for investigating magnetic skyrmions and their fundamental interactions.

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Ce travail de thèse est une contribution à l'étude des propriétés de spin dans les semiconducteurs par spectroscopie de photoluminescence et par photoconductivité en vue d’applications possibles dans le domaine de l’électronique du spin.Nous avons analysé les propriétés de spin des électrons de conduction dans les matériaux semiconducteurs nitrures dilués, massif et puits quantiques (GaAsN, GaAsN/GaAs). Nous avons étudié le mécanisme de recombinaison dépendante du spin des électrons de conduction sur les centres paramagnétiques induits par l’introduction d’azote dans GaAs. Nous avons mis en évidence l’effet de « filtrage » de spin des électrons de conduction que ce mécanisme peut induire ; en particulier, nous avons mené des études détaillées en fonction de la concentration d’azote, de la puissance excitatrice, d’un champ magnétique externe et, pour les hétérostructures, de l’épaisseur des puits quantiques. L’origine chimique des centres paramagnétiques a été, de plus, identifiée par des études de résonance paramagnétique détectée optiquement (ODMR).Nous avons également complété ces études purement optiques sur la recombinaison dépendante du spin, par des expériences de photoconductivité en vue d’applications possibles liées à l’électronique du spin. Nous avons montré que la photoconductivité des matériaux nitrures dilués peut être contrôlée par la polarisation de la lumière incidente. Un détecteur électrique de la polarisation de la lumière à base de GaAsN a été ainsi fabriqué et testé.Ces résultats ont été également interprétés et simulés grâce à un système d’équations dynamiques pouvant rendre compte à la fois des résultats de photoluminescence et de transport / This thesis work is a contribution to the investigation of the spin properties of semiconductors by photoluminescence and photoconductivity spectroscopy with the aim of future applications in the spintronic field. We have studied the conduction band electron spin properties of dilute nitride semiconductors in epilayers and quantum wells (GaAsN, GaAsN/GaAs). In particular, we have investigated the spin dependent recombination of conduction band electrons on deep paramagnetic centers induced by the introduction of nitrogen into GaAs. We have also evidenced the “spin filtering” effect made possible by this spin dependent recombination mechanism. More precisely, we have carried out a systematic study of the spin filtering effect as a function of the nitrogen concentration, excitation power, external magnetic field and, for the hetero-structures, as well as a function of the quantum well thickness. The chemical origin of the deep paramagnetic centers has been also determined by optically detected magnetic resonance (ODMR). We have completed these all-optical studies on the spin dependent recombination by photoconductivity experiments in order to demonstrate a “proof of concept” system for spintronic applications. We have shown that the photoconductivity in dilute nitride semiconductors can be controlled by the polarization of the incident light: an electrical detector of the light polarization has therefore been built. These results have been as well modeled thanks to a rate equation system able to reproduced both the photoluminescence and photoconductivity experimental results