Oxide heterostructures are attracting huge interest in recent years due to the special functionalities of quasi two-dimensional quantum gases. In this thesis, the electronic states at the interface between perovskite oxides and pyrite compounds have been studied by first-principles calculations based on density functional theory. Optimization of the atomic positions are taken into account, which is considered very important at interfaces, as observed in the case of LaAlO3/SrTiO3. The creation of metallic states at the interfaces thus is explained in terms of charge transfer between the transition metal and oxygen atoms near the interface. It is observed that with typical thicknesses of at least 10-12 °A the gases still extend considerably in the third dimension, which essentially determines the magnitude of quantum mechanical effects. To overcome this problem, we propose incorporation of highly electronegative cations (such as Ag) in the oxides. A fundamental interest is also the thermodynamic stability of the interfaces due to the possibility of atomic intermixing in the interface region. Therefore, different cation intermixed configurations are taken into account for the interfaces aiming at the energetically stable state. The effect of O vacancies is also discussed for both polar and non-polar heterostructures. The interface metallicity is enhanced for the polar system with the creation of O vacancies, while the clean interface at the non-polar heterostructure exhibits an insulating state and becomes metallic in presence of O vacancy. The O vacancy formation energies are calculated and explained in terms of the increasing electronegativity and effective volume of A the side cation. Along with these, the electronic and magnetic properties of an interface between the ferromagnetic metal CoS2 and the non-magnetic semiconductor FeS2 is investigated. We find that this contact shows a metallic character. The CoS2 stays quasi half metallic at the interface, while the FeS2 becomes metallic. At the interface, ferromagnetic ordering is found to be energetically favorable as compared to antiferromagnetic ordering. Furthermore, tensile strain is shown to strongly enhance the spin polarization so that a virtually half-metallic interface can be achieved, for comparably moderate strain. Our detailed study is aimed at complementing experiments on various oxide interfaces and obtaining a general picture how factors like cations, anions, their atomic weights and elecronegativities, O vacancies, lattice mismatch, lattice relaxation, magnetism etc play a combined role in device design.

Perovskite oxides have the general chemical formula ABO3, where A is a rare-earth or alkali-metal cation and B is a transition metal cation. Perovskite oxides can be formed with a variety of constituent elements and exhibit a wide range of properties ranging from insulators, metals to even superconductors. With the development of growth and characterization techniques, more information on their physical and chemical properties has been revealed, which diversified their technological applications. Perovskite manganites are widely investigated compounds due to the discovery of the colossal magnetoresistance effect in 1994. They have a broad range of structural, electronic, magnetic properties and potential device applications in sensors and spintronics. There is not only the technological importance but also the need to understand the fundamental mechanisms of the unusual magnetic and transport properties that drive enormous attention. Manganites combined with other perovskite oxides are gaining interest due to novel properties especially at the interface, such as interfacial ferromagnetism, exchange bias, interfacial conductivity. Doped manganites exhibit diverse electrical properties as compared to the parent compounds. For instance, hole doped La0.7Sr0.3MnO3 is a ferromagnetic metal, whereas LaMnO3 is an antiferromagnetic insulator. Since manganites are strongly correlated systems, heterojunctions composed of manganites and other perovskite oxides are sunject to complex coupling of the spin, orbit, charge, and lattice degrees of freedom and exhibit unique electronic, magnetic, and transport properties. Electronic reconstructions, O defects, doping, intersite disorder, magnetic proximity, magnetic exchange, and polar catastrophe are some effects to explain these interfacial phenomena. In our work we use first-principles calculations to study the structural, electronic, and magnetic properties of manganite based superlattices. Firstly, we investigate the electronic structure of bulk CaMnO3 and LaNiO3. An onsite Coulomn interaction term U is tested for both the Mn and Ni atoms. G-type antiferromagnetism and insulating properties of CaMnO3 are reproduced with U = 3 eV and ferromagnetic ordering is favorable when CaMnO3 is strained to the substrate lattice constant. This implies that the CaMnO3 magnetism is sensitive to both strain and the U parameter. Antiparallel orientation of the Mn and Ti moments has been found experimentally in the BiMnO3/SrTiO3 superlattice. By introducing O defects at different layers, we find similar patterns when the defect is located in the BiO layer. The structural, electronic and magnetic properties are analysed. Strong hybridization between the d3z2−r2 orbitals of the Mn and Ti atoms near the O defect is found. The effect of uniaxial strain for the formation of a two-dimensional electron gas and the interfacial Ti magnetic moments of the (LaMnO3)2/(SrTiO3)2 superlattice are investigated. By tuning the strain state from compressive to tensile, we predict under which conditions the spin-polarization of the electron gas is enhanced. Since the thickness ratio of the superlattice correlates with the strain state, we also study the structural, electronic and magnetism trends of (LaMnO3)n/(SrTiO3)m superlattices with varying layer thicknesses. The main finding is that half-metallicity will vanish for n, m > 8. Reduction of the minority band gaps with increasing n and m originates mainly from an energetic downshift of the Ti dxy states. Along with these, the interrelation between the interface geometry and the electronic properties of the antiferromagnetic/ferromagnetic superlattice BiFeO3/ La0.7Sr0.3MnO3 is investigated. The magnetic and optical properties are also analysed by first principles calculations. The half-metallic character of bulk La0.7Sr0.3MnO3 is maintained in the superlattice, which implies potential applications on spintronics and memory devices.

Fundamental advances in cathode materials are key to lowering the operating temperature of solid oxide fuel cells (SOFCs). Detailed understanding of the structural, electronic and defect formation characteristics are essential for rational design of cathode materials. In this thesis we employ first principles methods to study La(Mn/Co)O3 and LnBaCo2O5+δ (Ln = Pr, Gd; δ = 0.5, 1) as cathode for SOFCs. Specifically, factors affecting the O vacancy formation and migration are investigated. We demonstrate that for LaMnO3 the anisotropy effects often neglected at high operating temperatures become relevant when the temperature is lowered. We show that this fact has consequences for the material properties and can be further enhanced by strain and Sr doping. Tensile strain promotes both the O vacancy formation and migration in pristine and Sr doped LaMnO3, while Sr doping enhances the O vacancy formation but not the migration. The effect of A-site hole doping (Mg2+, Ca2+ or Ba2+) on the electronic and magnetic properties as well as the O vacancy formation and migration in LaCoO3 are studied. All three dopants are found to facilitate O vacancy formation. Substitution of La3+ with Ba2+/Mg2+ yields the lowest O vacancy formation energy for low/intermediate spin Co, implying that not only the structure, but also the spin state of Co is a key parameter. Only for low spin Co the ionic radius is correlated with the O migration barrier. Enhanced migration for intermediate spin Co is ascribed to the availability of additional space at the transition state. For LnBaCo2O5+δ we compare the O vacancy formation in GdBaCo2O5.5 (Pmmm symmetry) and GdBaCo2O6 (P4/mmm symmetry), and the influence of Sr doping. The O vacancy formation energy is demonstrated to be smaller in the already O deficient compound. This relation is maintained under Sr doping. It turns out that Sr doping can be utilized to significantly enhance the O vacancy formation in both compounds. The observed trends are explained on a microscopic level. Furthermore, we consider antisite defects as they may modify the electronic and O migration properties but are rarely studied in double perovskite oxides. It turns out that O vacancy formation is significantly easier in PrBaCo2O5.5 than in GdBaCo2O5.5, the difference in formation energy being hardly modified by antisite defects. Finally, having established that the O vacancy formation energy is significantly lower in PrBaCo2O5.5 than in GdBaCo2O5.5, we study the O Frenkel energy and migration of O ions in PrBa(Co/Fe)2O5.5. The electronic structure and charge redistribution during defect formation are analyzed. We demonstrate that Co↔Fe substitution strongly affects the formation of defects and, consequently, the O migration. The low O Frenkel energy points to a high concentration of O vacancies. The migration of the O ions shows a distinct anisotropy.

As the internet-of-things hardware integration continues to develop and the requirements for electronics keep diversifying and expanding, the necessity for specialized properties other than the classical semiconductor performance becomes apparent. The success of emerging semiconductor materials depends on the manufacturability and cost as much as on the properties and performance they offer. Solution-based semiconductors are an emerging concept that offers the advantage of being compatible with large-scale manufacturing techniques and have the potential to yield high-quality electronic devices at a lower cost than currently available solutions. In this work, patterns of high-quality MAPbBr3 perovskite single crystals in specific locations are achieved through the modification of the substrate properties and solvent engineering. The fabrication of the substrates involved modifying the surface adhesion forces through functionalization with self-assembled monolayers and patterning them by photolithography processes. Spin coating and blade coating were used to deposit the perovskite solution on the modified silicon substrates. While single crystal perovskites were obtained with the modification of substrates alone, solvent engineering helped with improving the Marangoni flows in the deposited droplets by increasing the contact angle and lowering the evaporation rate, therefore controlling and improving the shape of the grown perovskite crystals. The methodology is extended to other types of perovskites such as the transparent MAPbCl3 and the lead-free MABi2I9, demonstrating the adaptability of the process. Adapting the process to electrode arrays opened up the path towards the fabrication of optoelectronic devices including photodetectors and field-effect transistors, for which the first iterations are demonstrated. Overall, manufacturing and integration techniques permitting the fabrication of single crystalline devices, such as the method in this thesis work, are fundamental in pushing hybrid perovskites towards commercialization.

The Inverse Temperature Crystallization (ITC) and seed-induced method were used to grow high-quality cubic samples within hours using a 1M solution of methylammonium lead bromide (MAPbBr3) samples. The current-voltage (I-V) hysteresis observed within the MAPbBr3 perovskite sample demonstrates anomalous dependence on scan rate and various preconditioning pulses. We investigate this dependence and the relationship of current-transient with slow migrating ions. The current transients fitted using a bi-exponential decay model produced two distinct time constants t1 = 38.4 s and t2 = 6.49 s associated with migrating ionic species. From the Arrhenius plot an activation energy of Ea = 0.410 associated with migrating Br ions was extracted. Future research is required towards the understanding of I-V hysteresis and the link to ion migration in MAPbBr3 perovskite. / Dissertation (MSc (Physics))--University of Pretoria, 2020. / NRF Funding / Physics / MSc (Physics) / Restricted

>Magister Scientiae - MSc / Perovskite solar cells have gained tremendous attention within the past decade, due to its rapid improvement in power conversion e ciency (PCE), with the current record cell at 25%. The aim of this study is to create a repeatable and scalable chemical vapour deposition technique that can be used to construct perovskite solar cells with a high PCE while maintaining long-term stability. The technique requires the formation of a uniform and compact lead halide layer, either PbI2 or PbCl2 that is sequentially converted into the perovskite structure with the exposure of Methylammonium iodide (MAI) vapour. The use of CVD with a 5 cm diameter quartz tube was successfully used to deposit uniform thin lms of both PbI2 and PbCl2 over an area of 6 cm2 with a thickness deviation of 5%. Thickness control was obtained by varying the amount of source material which allows for repeatable control within 5% error, without the need for a crystal thickness monitor.

Density functional theory (DFT) provides an affordable computational tool to understand electronic structure of various molecules and solids. However, the use of DFT is still challenging to investigate nanomaterials of intermediate size that are too small to assume translational symmetry and too large to be considered as molecules. This thesis focuses on developing cost-effective but accurate computational methods for nanomaterials and using the methods to rationalize and predict experimental behaviors. A notable difference of a nanomaterial from its bulk counterpart is that its properties are exceptionally sensitive to the dielectric environment, requiring a proper treatment of the surrounding dielectrics for an accurate understanding. The consequences of heterogeneous dielectric screening on transition metal dichalcogenides are studied by developing a new theory based on classical electrostatics, which closely reproduced the band gaps and optical gaps calculated by the ab initio GW approximation and the Bethe-Salpeter equation (BSE). The relative insensitivity of the first optical transition energy observed by experiments was explained for the first time in terms of the cancellation effect of changes of the band gap and the exciton binding energy. The theory of heterogeneous dielectric environments is further developed to be used in an atomistic calculation of layered hybrid organic-inorganic lead halide perovskites via a tight-binding GW-BSE method. The binding energies of trions and biexcitons were also calculated using the stochastic variational method to give spectrum peak energies that show a good agreement with reported experimental measurements. Lastly, the tight-binding GW-BSE method is generalized into an atomistic, semiempirical approach to calculate the electronic structure and optical spectra of arbitrary nanomaterials, termed semiempirical GW (sGW) and BSE (sBSE).

Optical wireless communication, taking advantage of the unlicensed ultraviolet-to visible wavelength region of the electromagnetic spectrum, had been coined as the next-generation wireless communication technology and holds promises to deliver a high-speed, reliable, and secured broadband experience. The push towards the optical-based medium is manifested by the demand for additional channel bandwidth to accommodate the rapid growth of the Internet-of-Things (IoT) and Internet-of-Underwater-Things (IoUT). Therefore, high-bitrate optoelectronics devices and components forming the transceiver units used in an optical wireless communication system require substantial progression to accelerate the development of this paradigm-shifting technology. In this dissertation, we demonstrated a plethora of optical detection platforms to circumvent the existing long-standing issues related to modulation bandwidth, wavelength-selectiveness, and solar-blind ultraviolet-C detection found in conventional planar silicon-based optical detectors. Herein, we presented the semipolar group-III-nitride-based micro-photodiodes for enabling up to Gbit/s optical detection in the ultraviolet-to-violet domain. The wavelength-selectiveness nature of the micro-photodiodes enabled a bitrate of up to 1.5 Gbit/s based on a power-saving on-off-keying modulation scheme. While it offers a high bitrate for the optical communication link, it restricts its detection size and angle-of-view due to the conventional resistance-capacitance and étendue limits. Therefore, we also explored using polymer-based scintillating fibers as a high-speed and near-omnidirectional optical detection platform to cater to various dynamic scenarios in optical wireless communication. The detection platform formed by the scintillating fibers enabled near-omnidirectional and large-area optical detection without sacrificing the modulation bandwidth. These investigations paved the way towards relieving the resistance-capacitance limit while addressing the pointing, acquisition, and tracking issue in underwater wireless optical communication. Subsequently, we also presented a novel wavelength-converting mechanism based on halide-perovskite nanocrystals and a conventional silicon-based platform. This demonstration addressed the lack of ultraviolet-C optical detectors in the existing market and enabled future solar-blind optical communication links. Finally, we also presented on halide-perovskite polymer-based scintillating fibers as the high-bitrate and near-omnidirectional optical detection platform. Our studies successfully addressed the existing inadequacy for high-bitrate photodetection. These works could play a significant role in progressing the technology forward, based on bottom-up material and devices innovation, to offer a reliable internet connection to the future highly interconnected society.

Ce sujet de thèse porte sur l’élaboration et l’étude de l’effet de la substitution du fer par le cobalt sur les propriétés physiques (structurales, morphologiques et magnétiques) et particulièrement la détection des deux gaz réducteurs NH3 et CO des composés La0,8Ca0,1Pb0,1Fe1-xCoxO3 (x = 0,00 ; 0,05 ; 0,10 ; 0,15 et 0,20). La diminution du volume a été, par la suite, confirmée par l’approximation SGGA+U en utilisant la théorie fonctionnelle de la densité (DFT). De même l’étude morphologique a révélé des micrographies poreuses présentant des particules agrégées et agglomérées de taille nanométrique et de forme irrégulière. Les analyses structurales et morphologiques nous ont permis de prédire que le composé avec x = 0,05 peut être considéré comme un bon candidat pour l’application dans le domaine de la détection des gaz. Les résultats des mesures électriques ont montré que la résistance diminue pour des taux de Co inférieurs à 0,10 puis augmente avec des taux supérieurs. De même les réponses électriques sous gaz ont montré que nos composés sont capables de détecter des gaz, avec une variation de la résistance électrique aisément mesurable suite à l’exposition sous différentes concentrations des deux gaz (NH3 et CO) et de déduire que le composé La0,8Ca0,1Pb0,1Fe0,95Co0,05O3 (x = 0,05) présente la meilleure réponse envers les deux gaz testés / This thesis deals with the elaboration and study of the effect of iron substitution by cobalt on the physical properties (structural, morphological and magnetic) and particularly the detection of the two reducing gases NH3 and CO of the compounds La0.8Ca0.1Pb0.1Fe1-xCoxO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20). The decrease of valume was subsequently confirmed by the SGGA + U approximation using the Density Functional Theory (DFT). Similarly, the morphological study reveals porous micrographs presenting aggregated and agglomerated particles of nanometric size and irregular shape. Structural and morphological analyzes predicted that the compound with x = 0.05 could be considered as a good candidate for application in the field of gas detection. The results of the electrical measurements have shown that the resistance decreases for Co rate below 0.10 and then increases with higher rate. Similarly, electrical responses under gas have shown that our compounds are able to detect gases, with a variation of the electrical resistance easily measurable following exposure under different concentrations of both gases (NH3 and CO) and to deduce that the compound La0.8Ca0.1Pb0.1Fe0.95Co0.05O3 (x = 0.05) presents the best response towards the two tested gases

There are many interesting applications of proton conducting oxides, such as electrolytes in fuel cells, hydrogen sensors and catalytic membranes for hydrogenation or dehydrogenation of organic compounds. Previous work explored doping BaTiO3 with the Rare Earth Element (REE) Sc to introduce oxygen vacancies in the structure, making it a Proton Conductor (PC). PC oxides are often perovskite materials, ABX3, wherethe A- or B-site are doped in such a way that less oxygen can be contained, creating oxygen vacancies. When these materials are then hydrated so that water molecules occupy the vacancies, hydrogen is essentially added to the system in the form of protons. This study expanded on those results, exploring possible dopants that are non-REEs, theoretically improving availability and cost. Fe and Al were chosen as B-site dopants and Sr was included as an alternative to Ba as the A-site cation, compensating for the smaller size of the Fe- and Al atoms compared to Ti. Solid state synthesis was used to manufacture the different compounds and then X-Ray Diffraction (XRD), ThermoGravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were used inorder to investigate their properties. The main focus was to explore ift he materials could be synthesised using this method, their structures and how they reacted to hydration. All Al doped samples could not be made phase pure with the synthesis parameters used. BaTi0.5Fe0.5Oy was stabilised by the inclusion of Ti, as it did not decompose during hydration, unlike pure BaFeOx. Significant hydration was achieved in a wet nitrogen atmosphere at 185°C, but its structure type was hexagonal, which is known to be unfavourable for proton conduction. SrTi0.5Fe0.5Oy had a cubic structure which is a beneficial structural trait for proton conduction, but negligible hydration was observed by TGA. A small increase in cell volume indicates that it might still have taken up some water, but the methods used for hydration were not optimal for this material. The oxygen content of the materials was not determined in this project and methods such as Mössbauer spectroscopy and iodometric titration should be included in any related future studies.