Cobalt oxynitride films formed by magnetron sputter deposition of a Co target in N2 or NH3 plasma or, alternatively, by NH3 plasma nitridation of a Co film deposited on Si(100), show a divergence of properties arising from (a) N and O interactions for N and O atoms bonded to each other or through a common metal center and (b) the oxophilicity of the metal center itself. Core and valence band X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and plane wave density functional theory (DFT) calculations have been used to probe chemical and electronic interactions of nitrogen-rich cobalt oxynitride CoO1-xNx (x > 0.7) films. DFT-based calculations supervised by the Cundari group show the zinc blende (ZB) structure is found to be energetically favored over the rocksalt (RS) structure for x > ~ 0.2, with an energy minimum observed in the ZB structure for x ~ 0.8 - 0.9. There is also agreement with experiment for core level binding energies obtained for DFT calculations based on the ZB structure and this forms the basis of a predictive model for understanding how N and O interactions impact the electronic and catalytic properties of these materials. Vanadium oxynitride films were deposited in a mixture of O2/Ar/N2 environments on α-Al2O3(0001) or SiO2/Si(100) substrates to obtain films with varied N/O stoichiometries via magnetron sputter deposition using a vanadium target. Films deposited on the Al2O3(0001) substrates generally, though not always, exhibited a (111) orientation, which is consistent with a rock salt structure. The enhancement of the surface properties of vanadium oxynitride was explored to improve its catalytic properties.

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Philosophiae Doctor - PhD / Titanium oxynitride has attracted research interest for the fact that it is a bioactive non-toxic material. It is suitable for surface coating of biomaterials and in other applications such as anti-reflective coatings, while oxygen-rich titanium oxynitride has been applied in thin film resistors and photocatalysis. Two common types of titanium oxynitrides are TiOₓNᵧ. and TiO₂-ₓNᵧ. In this work, titanium mixed metals oxynitrides (Ti-TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂) were synthesized for the first time using ball milling (BM) and annealing processes. Their structural, morphological, surface, optical, and gas sensing properties were studied in detail. Structural analyses showed that upon milling a pure TiO₂ phase, tetragonal to orthorhombic phase transformation occurred. However, when milling TiO₂ mixed with Mg, Sn and Ti no evidence of the transformation was observed. Furthermore, scanning electron microscopy, transmission electron microscopy and atomic force microscopy showed that the milling process promotes particle refinement. The gas sensing analyses also demonstrated that the sensing response of the TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂ materials improved upon milling. Moreover, the Mg-TiO₂ showed improved sensing compared to pure TiO₂ due to incorporation of Mg, which might have resulted in a decrease of charge carrier concentration. The Mg-TiO₂ sensing materials showed fast response-recovery time of ~32 s and ~48 s, respectively, as well as high selectivity to NH₃ gas compared to other gases (H₂, and CH₄). In addition, the improved response observed for the milled samples is due to increased surface area and pore diameter, providing more active sites for the target gas and allowing more gas adsorption with an increase in point defects related to oxygen vacancies (Vo), which are the most favorable adsorption sites for oxygen species and thus can enhance the possibility of interaction with gas molecules. A combination of photoluminescence, x-ray photoelectron spectroscopy, vibrating sample magnetometer and sensing analyses demonstrated that a direct relation exists between the magnetization, sensing and the relative occupancy of the Vo present on the surface of TiO₂ nanoparticles. Therefore, based on these finding we conclude that the milling process promotes particle refinement, resulting in an increased BET surface and partial breaking of Ti–O bonds on the TiO₂ surface layer, which results in the formation of oxygen vacancies in the TiO₂ lattice, therefore anticipating improved sensing response. / National Research Foundation

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As the minimum feature size in complementary metal-oxide-semiconductor (CMOS) devices shrinks, the leakage current through the gate insulator (silicon oxide) will increase sufficiently to impair device operation. A high dielectric constant (k) insulator is needed as a replacement for silicon oxide in order to reduce this leakage. Hafnium-based materials are among the more promising candidates for the gate insulator, however, it is hampered by material quality and thus has been slow to be introduced into high volume integrated circuit production. Hafnium oxynitride films are deposited by Metalorganic Chemical Vapor Deposition (MOCVD) and downstream microwave Plasma Enhanced Chemical Vapor Deposition (PECVD) employing different oxidants including O2, N2O, O2 plasma, N2O plasma, N2O/N2 plasma, and O2/He plasma in the current research. The effects of oxidants on deposition kinetics, morphology, composition, bonding structure, electrical properties and thermal stability of the resultant films each are investigated. The possible chemical/physical causes of these observations are developed and some mechanisms are proposed to explain the experimental results. Oxygen radicals, which are known of present in oxidizing environments are determined to play an essential role in defining both structures and the resultant electronic properties of deposited hafnium oxynitride films. This systematic investigation of oxidant effects on CVD grown hafnium oxide/oxynitride layers, in the absence of post-deposition annealing, provides new understanding to this area with potential importance to the integrated circuit industry.

Les travaux de thèse ont porté sur la substitution cationique et anionique du composé LaFeO3, élaboré en couche mince par pulvérisation cathodique magnétron réactive (deux cibles métalliques et mélange gazeux Ar+O2). La première partie de la thèse est consacrée à la substitution du lanthane par d'autres terres rares plus petites : Praséodyme, Néodyme et Samarium. Les quatre pérovskites LaFeO3, PrFeO3, NdFeO3 et SmFeO3 ont donc été synthétisées et caractérisées (caractérisations optiques, électriques et structurales), et l'influence de la substitution sur les propriétés physiques est discutée. Les travaux ont montré que la substitution du lanthane par d’autres terres rares plus petites entraîne une modification des propriétés, que nous pouvons directement relier à la distorsion de la maille. Dans une deuxième partie, le dopage à l’azote du composé LaFeO3 a été réalisé, en utilisant de l'azote en tant que gaz réactif (deux cibles + mélange gazeux Ar+O2+N2). Les conditions de dépôt ont été étudiées dans le but d'optimiser la synthèse. A partir de mesures in situ des paramètres de la cible en fonction des débits de gaz, associées aux mesures post dépôt (composition, épaisseur, conductivité électrique), des conditions particulières de synthèse ont été retenues, afin d’optimiser le dopage à l’azote. L’utilisation de ces conditions de synthèses spécifiques ont permis la réalisation de plusieurs dépôts LaFeO3-xNx qui ont ensuite été caractérisés (caractérisations chimiques, optiques, électriques et structurales). Il en résulte que le dopage à l’azote du composé LaFeO3 entraîne une modification du gap de la pérovskite, ainsi que des propriétés électrique, et optique, malgré une faible teneur en azote. / The work done trough this PhD deals with cationic and anionic substitution of LaFeO3 thin films deposited by reactive magnetron sputtering (co-sputtering of two targets in Ar+O2 mixture). The first part of the word is devoted to substitution of lanthanum by smaller rare earth, Praseodymium, Neodymium, and Samarium. The four perovskites LaFeO3, PrFeO3, NdFeO3 and SmFeO3 have been synthesized and characterized (optical, electrical and structural properties), and this cationic substitution is discussed. It results in a shift of the properties which can be directly linked with lattice distortion due to smaller rare earth. In a second part, nitrogen doping of LaFeO3 has been studied, by using nitrogen as reactive gas (co-sputtering of two targets in Ar+O2+N2 mixture). Deposition conditions have been studied in order to optimize the perovskite synthesis. From in situ measurements of target voltage variation with flow rate associated to post-deposition measurement (thickness, composition, electrical conductivity), specified conditions have been found for nitrogen doping optimization. Different LaFeO3-xNx thin films have been deposited following these specified conditions, and characterized (chemical, optical, electrical and structural analysis). Nitrogen doping of LaFeO3 leads to bandgap decrease, associated to modification of optical and electrical properties, even if a small amount of nitrogen can be added to the structure.

Thesis (Ph.D. (Physics)) -- University of Limpopo, 2019 / LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion batteries which, despite its high average voltage of lithium intercalation, suffers crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the energy density of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of Ni incorporation and its effect on the electrochemical properties of this spinel are not fully understood. Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and LiNixMn2-xO4 have been calculated out using density functional theory employing the pseudo-potential plane-wave approach within the generalised gradient approximation, together with Virtual Cluster Approximation. The structural properties included equilibrium lattice parameters; electronic properties cover both total and partial density of states and mechanical properties investigated elastic properties of all systems. Secondly, the pressure variation of several properties was investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are consistent with the available experimental and literature results. The average Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties predicted the nature of bonding for both pure and doped systems with partial density of states showing the contribution of each metal in our systems. All systems are shown to be metallic as it has been previously observed for pure spinel LiMn2O4, and mechanical properties, as deduced from elastic properties, depicted their stabilities. Furthermore, the cluster expansion formalism was used to investigate the nickel doped LiMn2O4 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while iv maintaining the predictive power and accuracy of first-principles density functional methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31 generated various structures with different concentrations and symmetries. The findings predict that all nickel doped LMO structures on the ground state line are most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16, Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups they were further investigated by calculating the mechanical and vibrational properties, where the elastic constants and phonon vibrations indicated that the structures are stable in accordance with stability conditions of mechanical properties and phonon dispersions. Lastly, a computer program that identifies different site occupancy configurations for any structure with arbitrary supercell size, space group or composition was employed to investigate voltage profiles for LiNixMn2-xO4. The density functional theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000K, which is in excellent agreement with experiments. It was also found that the configurational entropy is much lower than the maximum entropy at 1000K, indicating that higher temperatures are required to reach a fully disordered solid solution. The maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the experimental value. The temperature has a negligible effect on the Li intercalation voltage of the most stable composition. The approach presented here shows that moderate Ni doping of the LiMn2O4 leads to a substantial change in the average voltage of lithium intercalation, suggesting an attractive route for tuning the cathode properties of this spinel. / National Research Foundation (NRF)

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Zircônia 3YTZP apresenta propriedades excelentes à temperatura ambiente, mas estas propriedades são afetadas pelo aumento da temperatura pois esta age negativamente sobre o mecanismo de transformação de fase induzida por tensão, que fortalece a tenacidade da matriz. A adição de Si3N4 e SiC em uma matriz de 3YTZP é muito interessante porque conduz à formação de oxinitreto de silício, melhorando as propriedades mecânicas tais como dureza e tenacidade, mas esta adição está limitada por várias dificuldades que se apresentam durante o processamento e sinterização destes materiais. Neste trabalho foi estudada a obtenção, por sinterização sem pressão, do compósito Y-TZP/Si2N2O, partindo-se da adição de 20vol%Si3N4-SiC em uma matriz de zircônia dopada com 3mol% de Y2O3 - 3YTZP, utilizando-se Al2O3 e Y2O3 como aditivos de sinterização. A mistura foi moída e moldada por prensagem isostática a frio. Amostras foram sinterizadas a 1500º, 1600º e 1700ºC por 2h sem pressão e em atmosfera ambiente, utilizando-se um leito de nitreto de silício. Após sinterização, as amostras foram caracterizadas por difração de raios-X. Foram medidas a densidade, tenacidade, dureza e resistência mecânica à flexão em temperatura ambiente. A estrutura do material foi observada em microscopia eletrônica de varredura e de transmissão, com mapeamento químico, para verificar a homogeneidade e morfologia das fases do compósito. A formação de Si2N2O foi observada no material sinterizado devido à reação entre os pós adicionados. O material obtido apresentou aumento de tenacidade e dureza com o aumento de temperatura de sinterização. As amostras apresentaram boa resistência à oxidação a 1000ºC. / Zirconia 3YTZP presents excellent properties at room temperature. These properties decrease as the temperature increases because high temperature acts negatively over the stress induced transformation toughening in the matrix. The addition of Si3N4 and SiC in a Y-TZP matrix is very interesting because leads to formation of silicon oxynitride and it increases the mechanical properties like toughness and hardness. Certainly the mechanical properties increment is limited by several difficulties which have appeared during processing and heating of these materials. This paper studies the Y-TZP/Si2N2O pressureless sintered composite, under different temperatures, showing the behavior of 20vol%Si3N4-SiC when added in YTZP matrix and heated under no pressure system. Al2O3 and Y2O3 were used as sintering aids. The mixture was milled and molded by cold isostatic pressure. Samples were heated at 1500º, 1600º and 1700ºC x 2h without pressure under atmospheric conditions using Si3N4 bed-powder. Samples were characterized by XRD and density, hardness, toughness, bending strength were measured. The structure of the material was observed in SEM/TEM/EPMA to verify the distribution and composition of the materials in the composite and the contact between filler surface and the matrix. The formation of SiON2 was observed in the sintered material due to reaction between both nitride and carbide with Y-TZP matrix. Furthermore the material showed an increment of both hardness and toughness as temperature increases. The samples presented considerable resistance to oxidation below 1000ºC.