Crystalline indium oxide nanowires were synthesized following optimization of growth parameters. Oxygen vacancies were found to impact the optical and electronic properties of the as-grown nanowires. Photoluminescence measurements showed a strong U.V emission peak at 3.18 eV and defect peaks in the visible region at 2.85 eV, 2.66 eV and 2.5 eV. The defect peaks are attributed to neutral and charged states of oxygen vacancies. Post-growth annealing in oxygen environment and passivation with sulphur are shown to be effective in reducing the intensity of the defect induced emission. The as-grown nanowires connected in an FET type of configuration shows n-type conductivity. A single indium oxide nanowire with ohmic contacts was found to be sensitive to gas molecules adsorbed on its surface.

No description available.

Neste trabalho se realiza a síntese e caracterização de nanofios de óxido de zinco. Adicionalmente se apresenta o processo de montagem de um dispositivo para medidas elétricas deste material. Estuda-se complementarmente o efeito do tratamento de plasma sobre as propriedades de fotoluminescência do material. Nanofios foram sintetizados pelo mecanismo vapor-líquido-sólido (VLS), utilizando ouro como catalizador e safira c-plane como substrato. As amostras foram caracterizadas utilizando microscopia eletrônica de varredura, fotoluminescência a temperatura ambiente, difração de raios X, e microscopia eletrônica de transmissão. Os nanofios obtidos têm seção transversal com formato quase hexagonal, e larguras de aproximadamente 46 nm. O comprimento deles varia de 3 a 10 μm. Os resultados de difração de raios x e microscopia eletrônica de transmissão mostram que eles são monocristalinos com rede cristalina tipo wurtzita, e com direção de crescimento no eixo c. Foram estudados os efeitos da potência de plasma de oxigênio (O2) na fotoluminescência dos nanofios a temperatura ambiente. A diferença na fotoluminescência após diferentes tratamentos de plasma de O2 mostra que a razão entre a emissão da região do band gap e da banda do visível pode ser modificada pelo tratamento. Este efeito corrobora com a hipótese de que a banda verde de luminescência está relacionada às vacâncias de zinco. A variação percentual da razão entre as duas regiões apresenta uma dependência linear com a potência do plasma. / In this work, we performed the synthesis and characterization of zinc oxide nanowires. We also report an assembly process to measure the electrical properties of this material. We study the plasma treatment effect on the photoluminescence spectra of the nanowires. Nanowires were synthesized via vapor-liquid-solid mechanism, using gold as catalyst and c-plane sapphire as substrate. The samples were characterized using scanning electron microscopy, room temperature photoluminescence, x-rays diffraction and transmission electron microscopy. Our nanowires show a quasi-hexagonal cross section, with diameters of approximately 46 nm. Their lengths ranged from 3 to 10 μm. Our results show monocrystalline wurtzite crystal nanowires with c growth direction. We also study the plasma power effect of oxygen (O2) plasma treatment on the room temperature photoluminescence spectra of the nanowires. Our results show that the deep level emission to near band emission ratio decreases with the plasma treatment. This effect supports the hypothesis that claims the green band luminescence is related to the oxygen vacancies. Furthermore, the relative ratio change depends linearly on the plasma power.

Neste trabalho se realiza a síntese e caracterização de nanofios de óxido de zinco. Adicionalmente se apresenta o processo de montagem de um dispositivo para medidas elétricas deste material. Estuda-se complementarmente o efeito do tratamento de plasma sobre as propriedades de fotoluminescência do material. Nanofios foram sintetizados pelo mecanismo vapor-líquido-sólido (VLS), utilizando ouro como catalizador e safira c-plane como substrato. As amostras foram caracterizadas utilizando microscopia eletrônica de varredura, fotoluminescência a temperatura ambiente, difração de raios X, e microscopia eletrônica de transmissão. Os nanofios obtidos têm seção transversal com formato quase hexagonal, e larguras de aproximadamente 46 nm. O comprimento deles varia de 3 a 10 μm. Os resultados de difração de raios x e microscopia eletrônica de transmissão mostram que eles são monocristalinos com rede cristalina tipo wurtzita, e com direção de crescimento no eixo c. Foram estudados os efeitos da potência de plasma de oxigênio (O2) na fotoluminescência dos nanofios a temperatura ambiente. A diferença na fotoluminescência após diferentes tratamentos de plasma de O2 mostra que a razão entre a emissão da região do band gap e da banda do visível pode ser modificada pelo tratamento. Este efeito corrobora com a hipótese de que a banda verde de luminescência está relacionada às vacâncias de zinco. A variação percentual da razão entre as duas regiões apresenta uma dependência linear com a potência do plasma. / In this work, we performed the synthesis and characterization of zinc oxide nanowires. We also report an assembly process to measure the electrical properties of this material. We study the plasma treatment effect on the photoluminescence spectra of the nanowires. Nanowires were synthesized via vapor-liquid-solid mechanism, using gold as catalyst and c-plane sapphire as substrate. The samples were characterized using scanning electron microscopy, room temperature photoluminescence, x-rays diffraction and transmission electron microscopy. Our nanowires show a quasi-hexagonal cross section, with diameters of approximately 46 nm. Their lengths ranged from 3 to 10 μm. Our results show monocrystalline wurtzite crystal nanowires with c growth direction. We also study the plasma power effect of oxygen (O2) plasma treatment on the room temperature photoluminescence spectra of the nanowires. Our results show that the deep level emission to near band emission ratio decreases with the plasma treatment. This effect supports the hypothesis that claims the green band luminescence is related to the oxygen vacancies. Furthermore, the relative ratio change depends linearly on the plasma power.

Neste trabalho se realiza a síntese e caracterização de nanofios de óxido de zinco. Adicionalmente se apresenta o processo de montagem de um dispositivo para medidas elétricas deste material. Estuda-se complementarmente o efeito do tratamento de plasma sobre as propriedades de fotoluminescência do material. Nanofios foram sintetizados pelo mecanismo vapor-líquido-sólido (VLS), utilizando ouro como catalizador e safira c-plane como substrato. As amostras foram caracterizadas utilizando microscopia eletrônica de varredura, fotoluminescência a temperatura ambiente, difração de raios X, e microscopia eletrônica de transmissão. Os nanofios obtidos têm seção transversal com formato quase hexagonal, e larguras de aproximadamente 46 nm. O comprimento deles varia de 3 a 10 μm. Os resultados de difração de raios x e microscopia eletrônica de transmissão mostram que eles são monocristalinos com rede cristalina tipo wurtzita, e com direção de crescimento no eixo c. Foram estudados os efeitos da potência de plasma de oxigênio (O2) na fotoluminescência dos nanofios a temperatura ambiente. A diferença na fotoluminescência após diferentes tratamentos de plasma de O2 mostra que a razão entre a emissão da região do band gap e da banda do visível pode ser modificada pelo tratamento. Este efeito corrobora com a hipótese de que a banda verde de luminescência está relacionada às vacâncias de zinco. A variação percentual da razão entre as duas regiões apresenta uma dependência linear com a potência do plasma. / In this work, we performed the synthesis and characterization of zinc oxide nanowires. We also report an assembly process to measure the electrical properties of this material. We study the plasma treatment effect on the photoluminescence spectra of the nanowires. Nanowires were synthesized via vapor-liquid-solid mechanism, using gold as catalyst and c-plane sapphire as substrate. The samples were characterized using scanning electron microscopy, room temperature photoluminescence, x-rays diffraction and transmission electron microscopy. Our nanowires show a quasi-hexagonal cross section, with diameters of approximately 46 nm. Their lengths ranged from 3 to 10 μm. Our results show monocrystalline wurtzite crystal nanowires with c growth direction. We also study the plasma power effect of oxygen (O2) plasma treatment on the room temperature photoluminescence spectra of the nanowires. Our results show that the deep level emission to near band emission ratio decreases with the plasma treatment. This effect supports the hypothesis that claims the green band luminescence is related to the oxygen vacancies. Furthermore, the relative ratio change depends linearly on the plasma power.

Catalysts based on cerium oxide are ubiquitous in industrial-scale chemical conversion. Here, a thorough study of their fundamental properties is undertaken via a model system ap- proach with the goal of furthering rational design in heterogeneous catalysis. A focus is put on understanding the behavior of oxygen vacancies in cerium oxide with respect to atomic co-ordination and electronic structure perturbations. Utilizing state-of-the-art probing tech- niques, a scalable model system framework is developed that allows for control over both the oxygen vacancy concentration and local co-ordination. High precision of the innova- tive approach facilitated observation of new phases of substoichiometric cerium oxide and lead to a first-of-a-kind investigation of the electronic structure of cerium oxide throughout isostructural transition from CeO2 to Ce2O3. The acquired results advance fundamental understanding of essential properties of cerium oxide that are relevant to its utilization in heterogeneous catalysis and open new pathways for functionalization of cerium oxide-based materials. Furthermore, the methodology developed in the thesis is transferable to other important reducible oxides. 1

This thesis investigates the impact of doping on the optical and structural characteristics of cerium oxide (ceria) nanoparticles synthesized using chemical precipitation. The dopants selected are samarium and neodymium, which have positive association energy with oxygen vacancies in the ceria host, and negative association lanthanides, holmium and erbium, as well as two metal dopants, aluminum and iron. Characteristics measured are absorption and fluorescence spectra and the diameter and lattice parameter of ceria. Analysis of the characteristics indicates qualitatively that the dopant controls the O-vacancy concentration and the ratio of the two cerium ionization states: Ce+3 and Ce+4. A novel conclusion is proposed that the negative association lanthanide dopants can act as O-vacancies scavengers in ceria while the O-vacancy concentration increases in ceria doped with positive association lanthanide elements. Doped ceria nanoparticles are evaluated in two applications: dissolved oxygen (DO) sensing and up-conversion. In the first application, ceria doped with either Sm or Nd and ceria doped with aluminum have a strong correlation between the fluorescence quenching with the DO concentration in the aqueous solution in which the ceria nanoparticles are suspended. Stern-Volmer constants (KSV) of doped ceria are found to strongly depend upon the O-vacancy concentration and are larger than some of the fluorescent molecular probes currently used to measure DO. The KSV measured between 25-50oC is found to be significantly less temperature dependent as compared to the constants of commercially-available DO molecular probes. In the second application, up-conversion, ceria nanoparticles doped with erbium and an additional lanthanide, either Sm or Nd, are exposed to IR radiation at 780 nm. Visible emission is only observed after the nanoparticles are calcinated at high temperature, greatly diminishing the concentration of O-vacancies. It is concluded that O-vacancies do not play a dominant role in up-conversion, unlike that drawn for down-conversion, where the fluorescence intensity is strongly correlated with the O-vacancy concentration. Correlations between annealing temperatures, dopant, and dopant concentrations with the power dependence of up-conversion on the pump and the origin of the intensities of the visible emission are presented. These studies show the promise of doped ceria nanoparticles. / Ph. D.

Le numérique prend une place de plus en plus importante dans la vie de tous les jours et les quantités de données échangées explosent ce qui impose de développer des mémoires de plus en plus performantes, enjeu majeur du secteur de la microélectronique. Parmi les mémoires non-volatiles émergentes, les mémoires OxRRAM à base d'oxyde résistif sont particulièrement attrayantes et représentent un candidat potentiel au remplacement des mémoires FLASH (compatibles avec la technologie CMOS, faibles tensions de programmation). Leur structure est simple (Métal-Isolant-Métal) et leur fonctionnement est basé sur une commutation de résistance sous l'effet d'un champ électrique. Si le mécanisme de formation/dissolution d'un filament conducteur de taille nanométrique est reconnu par la communauté, un débat subsiste encore sur la nature et les caractéristiques du/des filaments dans le cas de l'oxyde HfO2 (lacunes d'oxygène, élément métallique). En nous appuyant sur des méthodes de la microscopie électronique en transmission - STEM-HAADF et STEM-EELS - cette thèse apporte des éléments de compréhension par rapport aux modifications d'état physico-chimique qui s'opèrent lors des différentes étapes du fonctionnement d'une mémoire (FORMING, RESET) et ceci à l'échelle nanométrique définie par la taille du filament conducteur. L'empilement TiN/Ti/HfO2/TiN, préparé selon les procédés de la microélectronique, a été intégré dans différentes architectures (1R, 1T1R) avec une électrode supérieure structurée (50 à 200 nm) pour confiner la zone de conduction dans un volume fini compatible avec la MET puis polarisé selon différentes méthodes (C-AFM, banc de mesure et TEM in-situ). Lorsque les effets thermiques sont contrôlés, l'analyse des cartographies chimiques élémentaires montre que le titane de l'électrode supérieure participe au mécanisme de commutation (migration localisée dans la couche HfO2) en plus de la déplétion en oxygène à l'interface HfO2/électrode inférieure et probablement aux joints de grains dans HfO2. / Digital technology is invading our day life and the amount of data is exploding. This implies to develop memories which perform better and better. This is a major issue in microelectronics. Among non-volatile memories, Oxide based resistive RAM are particularly attractive (compatible with CMOS technology, low programming voltage) and are considered as promising candidate for replacing FLASH memories. The stack is simple (M-I-M) and the switching is based on resistance changes under an applied electrical stress. If forming and breaking a nanometer-sized conductive area is commonly accepted as the physical phenomenon involved in the switching mechanism, a debate remains about the nature and the characteristics of the filamentary area (oxygen vacancies, metallic element). Based on transmission electron microscopy methods - STEM-HAADF and STEM-EELS - this thesis work provides, at the scale of the filament (nm), a further understanding about the physico-chemical modifications of the memory cell induced by the operating step (FORMING, RESET). The TiN/Ti/HfO2/TiN stack, processed with microelectronic techniques, was incorporated into different architectures (1R, 1T1R) with a shaped top electrode (diameter 50 to 200 nm) to confine the filament in a volume compatible with TEM and then biased with different methods (C-AFM, measuring bench, in-situ TEM). When thermal effects are under control, the analysis of the EELS elementary maps shows that titanium from the top electrode plays a role in the switching mechanism (local migration in the HfO2 layer) in addition to the oxygen depletion at the HfO2/bottom electrode interface and probably at grain boundaries in HfO2.

No description available.

In this dissertation, we used a combination of several synchrotron-based x-ray spectroscopic techniques to investigate the effects of strain, ionic defect formation, and heteroengineering in strongly-correlated electronic systems. First, we introduce a method to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. In our approach, we utilized atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single crystalline CaMnO3 films under coherent tensile strain, varying systematically from +0.8% to +4%. An increase of the oxygen vacancy content in the single-crystalline CaMnO3 thin films with applied in-plane strain was experimentally observed using high-resolution soft x-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES). Our experimental results were verified using first-principles theory and atomic core-hole multiplet calculations. Furthermore, our results highlight the importance of protecting the surfaces of CaMnO3 thin-films with thin Pt layers in-situ in order to stabilize the oxygen vacancy content. Next, we discuss the role of oxygen vacancies in driving the metal-insulator transition in LaNiO3 thin films. Here, we also use atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single-crystalline LaNiO3 films with systematically varying thicknesses, ranging from 1 u.c. to 50 u.c. An increase in the oxygen vacancy content was observed with the decreasing LaNiO3 film thickness using XAS. A higher concentration of oxygen vacancies was observed for the ultrathin insulating films (<1.5 u.c.). The experimental results were compared to first-principles theoretical calculations. We found that LaNiO3 exhibits room-temperature metallic behavior for thicknesses down to 1.5 u.c., which is the lowest value reported to date. Finally, we have investigated an atomically-abrupt interface between the paramagnetic LaNiO3 and the antiferromagnetic CaMnO3 thin films. The interface between these two complex oxides exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Here, we used depth-resolved standing-wave photoemission spectroscopy (SW-XPS), scanning transmission electron microscopy (STEM), and XAS to observe a depth-dependent charge reconstruction occurring at the LaNiO3/CaMnO3 interface. Our elemental standing-wave rocking-curve analysis revealed the depth-dependent changes of the Mn and Ni valence states at the interface, yielding increased amounts of Mn3+ and Ni2+ cations at the interface. These results suggest Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-Mn4+ superexchange as possible underlying causes of the emergent interfacial ferromagnetism. / Physics