En spintronique, l’étude des hétérostructures multicouches composées d'une électrode ferromagnétique et d'une couche isolante mince, c'est-à-dire des jonctions tunnel magnétiques (JTM), est particulièrement importante. Le système canonique est le Fe/MgO/Fe où les hautes valeurs du rapport de la magnétoresistance tunnel (TMR) ont été mesurées. Le facteur crucial définissant la performance de la jonction est l’imperfection structurelle dans un dispositif réel. Dans notre travail, nous nous sommes concentrés sur des lacunes d'oxygène dans MgO. Au moyen de la théorie de la fonctionnelle de densité, nous avons étudié les propriétés électroniques de l'état fondamental des lacunes d'oxygène simples et doubles dans MgO massif, appelées respectivement centres F et M. Nous avons ensuite étudié l'impact de ces lacunes sur le transport balistique dans les jonctions magnétiques. Nous avons démontré le rôle supérieur joué par les centres M et nous avons prouvé qu'un transport cohérent, préservant le spin et la symétrie des électrons, est possible en présence de centres M. / In sprintronics, the study of multilayer heterostructures composed of a ferromagnetic electrodes and a thin insulating layer, i.e. magnetic tunnel junctions (MTJs), is of special importance. The canonical systems are MTJs made of Fe/MgO/Fe where hight tunneling mangetoresistance ratio (TMR) values were measured. The crucial factor defining the junction performance is the structural imperfection appearing in a real devices. In our work we focused in particular on oxygen vacancies in MgO. By means of density functional theory we studied ground state electronic properties of single and double oxygen vacancies, referred as F and M centers, respectively, in bulk MgO. We then switched to full junctions where we investigated the impact of vacancies on the ballistic transport. We demonstrated that M centers played a superior role and proved that coherent transport, preserving electrons spin and symmetry, is possible in presence of paired vacancies.

L'objectif de ce travail est l'étude des interactions entre des nanoparticules de platine et la zircone yttriée (YSZ pour Yttria-Stabilized Zirconia), oxyde conducteur ionique. Il s'agissait de transposer les effets de promotion électrochimique de la catalyse mis en évidence sur des films polarisés de platine de faible dispersion déposés sur des membranes denses de YSZ à des systèmes catalytiques conventionnels à base de nanoparticules métalliques dispersées sur des poudres de YSZ. La migration des ions oxydes promoteurs n'est plus contrôlée par une polarisation électrique mais induite thermiquement. Ces travaux ont permis de mettre au point une méthode de mesure de la dispersion du Pt déposé sur la zircone yttriée. Les interactions Pt/YSZ et notamment celles entre les lacunes d'oxygène de YSZ et les nanoparticules de Pt ont été étudiées par réduction en température programmée et spectroscopie infrarouge. L'importance des lacunes d'oxygène du support YSZ sur les propriétés de chimisorption du Pt et sur son activité catalytique pour l'oxydation du propane a été clairement montrée. La migration thermique des ions oxydes a été étudiée par échange isotopique 18O/16O. Un mécanisme de la réaction de combustion du propane a été proposé incluant le rôle prépondérant des oxydes de réseau de YSZ contrairement aux supports conventionnels en silice et en zircone non substituée. Finalement, les paramètres pouvant influencer les interactions Pt/YSZ comme la surface spécifique de YSZ, le taux d'oxyde d'yttrium, la méthode de préparation de YSZ ainsi que la teneur et la taille des nanoparticules de Pt ont été évalués. Les résultats ont mis en évidence la migration thermique des ions oxydes du support vers le Pt dès 100 °C. D'autre part, l'échange entre les oxygènes du réseau et ceux de la phase gaz est extrêmement rapide dès 100°C. L'activité catalytique du Pt semble promue par la mobilité des oxygènes du support / The aim of this work is to investigate the interactions between Pt nanoparticles and Yttria-Stabilized Zirconia (YSZ), an ionically conducting support. The idea was to overcome the effects of electrochemical promotion of catalysis (EPOC) observed on Pt/YSZ electrochemical catalysts which present low metal dispersion to conventional catalytic systems based on metallic nanoparticles finely dispersed on YSZ powered support. In that configuration, the migration of the oxygen ions from YSZ toward the Pt surface is not electrically controlled but thermally induced without any polarisation. First, we have established a new procedure to measure the Pt dispersion over YSZ. The metal support interactions between Pt and YSZ were characterized by Temperature Programmed Reduction and Infrared Spectroscopy. The importance of the YSZ oxygen vacancies on the chemisorptive behaviours of Pt as well as its catalytic for the propane oxidation was clearly demonstrated. The thermal migration of oxygen ions was validated by using the Isotopic Exchange procedure 18O/16O. The impact of these vacancies was evaluated and a mechanism of the propane deep oxidation on Pt/YSZ was proposed including the important role of bulk YSZ oxygen species in opposition with conventional supports such as silica and non-substituted zirconia. Finally, the key parameters that can influence the Pt/YSZ interactions such as the YSZ specific surface area, the yttria content, the YSZ preparation route as well as the loading and size of Pt nanoparticles were investigated. Our results point out that the thermal migration of oxygen ions from YSZ toward Pt surface occurs from 100 °C. In addition, the exchange between oxygen species from YSZ bulk and those from the gas phase is extremely fast starting from 100 °C. The Pt catalytic activity for the propane deep oxidation seems to be promoted by the mobility of the bulk YSZ oxygen species

Die Realstruktur des perowskitischen Modellsystems SrTiO3, welches in der Raumgruppe Pm-3m kristallisiert, wird durch die Sauerstoffvakanz als wichtigstem Defekt dominiert. Durch Temperaturbehandlung unter reduzierenden Bedingungen können Sauerstoffvakanzen in die Kristallstruktur eingebracht werden. Aufgrund ihrer positiven Ladung relativ zum Kristallgitter bewegen sie sich im elektrischen Feld entlang des TiO6-Oktaedernetzwerkes. Die Elektroformierung folgt dabei einem Arrheniuszusammenhang, wobei sowohl die Aktivierungsenergie als auch die Mobilität eine deutliche Abhängigkeit von der Kristallorientierung zeigen. Die Umverteilung der Sauerstoffvakanzen führt zu lokalen reversiblen Strukturänderungen, welche die Ausbildung einer neuen migrationsinduzierten feldstabilisierten polaren (MFP) Phase verursachen. In Abhängigkeit von der elektrischen Feldstärke zeichnet sie sich strukturell durch eine tetragonale Verzerrung der ursprünglich kubischen Elementarzelle aus und geht mit dem Verlust der Inversionssymmetrie einher. Die Polarisation in der Struktur wird durch die erwiesene Pyroelektrizität bestätigt und gilt als Grundlage, um anhand eines kristallographischen Symmetrieabstieges die Herleitung der Raumgruppe P4mm zu ermöglichen. Der durch die Migration hervorgerufene intrinsische Defektkonzentrationsgradient ruft eine elektromotorische Kraft hervor, deren Verwendung in einem elektrochemischen Energiespeicher experimentell nachgewiesen wurde. Diese neuen Funktionalitäten sind durch die Anwendung defektchemischer und kristallphysikalischer Konzepte infolge gezielter Materialmodifizierung unter Einfluss von Temperatur und elektrischem Feld zu verstehen. / The real structure of the perovskite-type model system SrTiO3, crystallizing in space group Pm-3m, is dominated by oxygen vacancies as most important defects. They are introduced in the crystal structure through heat-treatment under reducing conditions. Because of their positive charge relative to the crystal lattice, oxygen vacancies move in an electric field along the TiO6 octahedron network. This electroformation process follows an Arrhenius behavior. Both the activation energy and the mobility show an obvious dependence on the crystal orientation. Redistribution of oxygen defects causes local reversible structural changes, which involve the formation of a migration-induced field-stabilized polar (MFP) phase. In dependence on the electric field strength, this is structurally marked by a tetragonal distortion of the original cubic unit cell and accompanied by a loss of inversion symmetry. The polarisation in the crystal structure is confirmed by the proven pyroelectricity and serves for the argumentation to derive the space group P4mm by means of a crystallographic symmetry descent. The migration-induced intrinsic concentration gradient of oxygen vacancies leads to an electromotive force, whose application as electrochemical energy storage was proven experimentally. These new functionalities are explainable using defect chemistry and crystal physics in consequence of specific material modifications under the influence of temperature and external electric fields.

Die Realstruktur des perowskitischen Modellsystems SrTiO3, welches in der Raumgruppe Pm-3m kristallisiert, wird durch die Sauerstoffvakanz als wichtigstem Defekt dominiert. Durch Temperaturbehandlung unter reduzierenden Bedingungen können Sauerstoffvakanzen in die Kristallstruktur eingebracht werden. Aufgrund ihrer positiven Ladung relativ zum Kristallgitter bewegen sie sich im elektrischen Feld entlang des TiO6-Oktaedernetzwerkes. Die Elektroformierung folgt dabei einem Arrheniuszusammenhang, wobei sowohl die Aktivierungsenergie als auch die Mobilität eine deutliche Abhängigkeit von der Kristallorientierung zeigen. Die Umverteilung der Sauerstoffvakanzen führt zu lokalen reversiblen Strukturänderungen, welche die Ausbildung einer neuen migrationsinduzierten feldstabilisierten polaren (MFP) Phase verursachen. In Abhängigkeit von der elektrischen Feldstärke zeichnet sie sich strukturell durch eine tetragonale Verzerrung der ursprünglich kubischen Elementarzelle aus und geht mit dem Verlust der Inversionssymmetrie einher. Die Polarisation in der Struktur wird durch die erwiesene Pyroelektrizität bestätigt und gilt als Grundlage, um anhand eines kristallographischen Symmetrieabstieges die Herleitung der Raumgruppe P4mm zu ermöglichen. Der durch die Migration hervorgerufene intrinsische Defektkonzentrationsgradient ruft eine elektromotorische Kraft hervor, deren Verwendung in einem elektrochemischen Energiespeicher experimentell nachgewiesen wurde. Diese neuen Funktionalitäten sind durch die Anwendung defektchemischer und kristallphysikalischer Konzepte infolge gezielter Materialmodifizierung unter Einfluss von Temperatur und elektrischem Feld zu verstehen. / The real structure of the perovskite-type model system SrTiO3, crystallizing in space group Pm-3m, is dominated by oxygen vacancies as most important defects. They are introduced in the crystal structure through heat-treatment under reducing conditions. Because of their positive charge relative to the crystal lattice, oxygen vacancies move in an electric field along the TiO6 octahedron network. This electroformation process follows an Arrhenius behavior. Both the activation energy and the mobility show an obvious dependence on the crystal orientation. Redistribution of oxygen defects causes local reversible structural changes, which involve the formation of a migration-induced field-stabilized polar (MFP) phase. In dependence on the electric field strength, this is structurally marked by a tetragonal distortion of the original cubic unit cell and accompanied by a loss of inversion symmetry. The polarisation in the crystal structure is confirmed by the proven pyroelectricity and serves for the argumentation to derive the space group P4mm by means of a crystallographic symmetry descent. The migration-induced intrinsic concentration gradient of oxygen vacancies leads to an electromotive force, whose application as electrochemical energy storage was proven experimentally. These new functionalities are explainable using defect chemistry and crystal physics in consequence of specific material modifications under the influence of temperature and external electric fields.

Thin film metal–insulator–metal capacitors with undoped HfO₂ as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10–50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO₂ with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO₂ film during deposition and annealing is correlated to the phase formation process.

This article corrects the following: 'Origin of Ferroelectric Phase in Undoped HfO2 Films Deposited by Sputtering' Advanced Materials Interfaces 6(11) 2019, first Published online: April 29, 2019

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.

[pt] Esta tese investigou a síntese de metanol via hidrogenação do CO2 empregando dois conjuntos de catalisadores. O primeiro é composto pelos tradicionais catalisadores de Cu/ZnO/Al e o segundo aborda os catalisadores de In2O3 e ZrO2. Com relação ao Cu/ZnO/Al, foram preparados quatro amostras via coprecipitação. Os resultados mostraram que há um teor ótimo (3,8 por cento at.) de Al para a qual se observa uma maior taxa de formação de metanol. Os catalisadores foram caracterizados por fisissorção de N2, titulação com N2O,espectroscopia de absorção atômica, ICP, DRX, XPS, TPD-(CO2,NH3 e H2O), TPSR-CO2/H2, TEM/HRTEM/EDS. Uma correlação entre a taxa de formação de metanol e a quantidade de vacâncias de oxigênio superficiais do catalisador foi observada. Foi verificado que o Al atua como um promotor na geração de vacâncias de oxigênio. Com relação aos sistemas de In2O3, foi realizado um screening e selecionado nove catalisadores. Esses sólidos foram caracterizados pelas seguintes técnicas: DRX, TPD-NH3, TPD-CO2, TPR-H2 e TPSR-CO2/H2. Foi realizado um estudo em dinâmica molecular clássica investigando os efeitos da dopagem do In2O3 e da interação entre o In2O3 e o ZrO2 e relacionando os resultados com a performance dos catalisadores. O melhor desempenho catalítico foi obtido para o inédito catalisador de 0,6Pt-In2O3+6ZnZrO2, sendo esse desempenho associado à presença de vacâncias. Além disso, pelos cálculos teóricos de dinâmica molecular foi verificado que tanto a mistura física quanto a dopagem do In2O3 podem promovem a mobilidade de oxigênio da rede dos óxidos, o que facilita a formação de vacâncias de oxigênio. Sendo assim, os dois conjuntos de catalisadores estudados mostram que as vacâncias de oxigênio têm papel central na formação do metanol a partir da hidrogenação do CO2. As informações geradas neste trabalho contribuirão para o desenvolvimento de catalisadores promissores para a futura exploração industrial da geração de metanol a partir de CO2. / [en] This thesis investigated methanol synthesis via CO2 hydrogenation using two sets of catalysts. The first set consists of the traditional Cu/ZnO/Al catalysts and the second set involves In2O3 and ZrO2 catalysts. Concerning Cu/ZnO/Al, four samples were prepared via coprecipitation. The results showed that there is an optimum Al content (3.8 percent at.) for which a higher methanol formation rate is observed. The catalysts were characterized by N2 physisorption, titration with N2O, atomic absorption spectroscopy, ICP, XRD, XPS (CO2,NH3 and H2O)-TPD, CO2/H2-TPSR, TEM/HRTEM/EDS. A correlation was observed between the rate of methanol formation and the amount of surface oxygen vacancies on the catalyst. It was found that Al acts as a promoter in the generation of oxygen vacancies. Regarding the In2O3 systems, a screening was carried out and nine catalysts were selected. These solids were characterized using the following techniques: XRD, NH3- TPD, CO2-TPD, TPR-H2 and CO2/H2-TPSR. A classical molecular dynamics study was carried out investigating the effects of doping In2O3 and the interaction between In2O3 and ZrO2 and relating the results to the performance of the catalysts. The best catalytic performance was obtained for the new 0,6Pt-In2O3+6ZnZrO2 catalyst, and this performance was associated with the presence of vacancies. In addition, molecular dynamics calculations showed that both the physical mixture and the doping of In2O3 can promote the mobility of oxygen in the oxide lattice, facilitating the formation of oxygen vacancies. Thus, the two sets of catalysts studied show that oxygen vacancies play a central role in the formation of methanol from the hydrogenation of CO2. The information generated in this work will contribute to the development of promising catalysts for the future industrial exploitation of methanol generation from CO2.

[pt] Os catalisadores de Ni/ZrO2, Ni/CaZrO2, Ni/SiO2 e Ni/Mg(Al)O foram empregados na reação de metanação do CO2. Os catalisadores foram caracterizados por XPS, XRD e os dados foram tratados pelo método de refinamento de Rietveld, EPR, TPR, TPSR de CO2 [mais] H2 e CO [mais] H2, reação modelo de desidrogenação do ciclohexano, TPD de CO2, análise química (FRX) e área superficial (BET). Com o objetivo de investigar o papel do suporte durante a metanação do CO2, os catalisadores foram preparados de modo a se obter a mesma área metálica de níquel. Os comportamentos catalíticos desses catalisadores na metanação do CO2 foram avaliados através de testes catalíticos convencionais e da taxa da reação. O catalisador Ni/ZrO2 apresentou maior atividade e seletividade para a metanação de CO2 quando comparado com os catalisadores de Ni/Mg(Al) O e Ni/SiO2. Sugere-se que as vacâncias de oxigênio sejam responsáveis pelo melhor desempenho do catalisador Ni/ZrO2. Ca foi adicionado ao catalisador Ni/ZrO2 com o objetivo de aumentar as vacâncias oxigênio. A área da superfície metálica do catalisador contendo Ca não se alterou, enquanto a taxa de consumo de CO2 quase triplicou. As análises de DRX, XPS e EPR mostraram que principalmente Ca[mais]2, mas também ions de Ni2[mais] estão na superfície da rede de ZrO2 no catalisador Ni/CaZrO2. Esses cátions formam vacâncias de oxigênio e pares de sítios de coordenação insaturados (cus), os quais são pares de sítios básicos e ácidos fortes. Em resumo, os dados obtidos indicaram que aumentando a concentração destes pares de sítios pela adição de Ca no Ni/ZrO2, não só aumenta a quantidade de CO2 adsorvido, mas também aumenta o número de sitios ativos da etapa limitante da reação. / [en] The Ni/ZrO2, Ni/CaZrO2, Ni/SiO2 and Ni/Mg(Al)O catalysts were employed in the CO2 methanation. The catalysts were characterized by XPS (reduced in situ and passivated), XRD (reduced in situ and passivated along with Rietveld refinement), EPR, TPSR of CO2 plus H2 and CO plus H2, cyclohexane dehydrogenation model reaction, CO2-TPD and chemical analysis. The metallic area of all of these catalysts was prepared to be similar among them in order to investigate the role of the support during the methanation of CO2. The behaviors of these catalysts in the CO2 methanation were analyzed employing a conventional catalytic test. The Ni/ZrO2 catalyst exhibited the highest activity and selectivity toward the methanation of CO2 against Ni/Mg(Al)O and Ni/SiO2. Oxygen vacancies are suggested to be responsible for the excellent performance of the Ni/ZrO2 catalyst. Based on the above, Ca was added to the Ni/ZrO2 catalyst, the metallic area did not change whereas the CO2 consumption rate almost tripled. The XRD, XPS and EPR analyses showed that mainly Ca positive 2 but also some Ni2 positive are on the ZrO2 lattice surface of the Ni/CaZrO2 catalyst. These cations form oxygen vacancies and unsaturated coordination sites (cus) pairs, which are strong basic and acid sites pairs, respectively. In short, increasing the concentration of these pairs by adding Ca to Ni/ZrO2, not only does the amount of CO2 adsorbed increase, but also the number of active sites of the rate limiting step, which is enhanced leading to an increase of the Zr based catalyst activity in the CO2 methanation.

The discovery of ferroelectricity in hafnium oxide has revived the interest in ferroelectric memories as a viable option for low power non-volatile memories. However, due to the high coercive field of ferroelectric hafnium oxide, instabilities in the field cycling process are commonly observed and explained by the defect movement, defect generation and field induced phase transitions. In this work, the optical and transport experiments are combined with ab-initio simulations and transport modeling to validate that the defects which act as charge traps in ferroelectric active layers are oxygen vacancies. A new oxygen vacancy generation leads to a fast growth of leakage currents and a consequent degradation of the ferroelectric response in Hf₀.₅Zr₀.₅O₂ films. Two possible pathways of the Hf₀.₅Zr₀.₅O₂ ferroelectric property degradation are discussed.