Ca(1-x)Nd2x/3TiO3 and MgTiO3-Ca0.61Nd0.26TiO3 composite ceramics were prepared by the mixed oxide route and characterised in terms of their structure, microstructure and properties. Ceramics sintered at 1450-1500oC achieved better than 95% of the theoretical density. X-Ray diffraction (XRD) revealed that Ca(1-x)Nd2x/3TiO3 ceramics were single phase for all compositions. For x ≤ 0.39 the structure was Pbnm with lattice parameters of a = b = √2ac and c = 2ac and a tilt system of a-a-c+. Compositions with x ≥ 0.48 could be better described by a C2/m structure with lattice parameters of a = b = c = 2ac. Scanning electron microscopy (SEM) revealed that the ceramics had grain sizes in the 5-70 μm range with abnormal grain growth for Nd3+ rich compositions. Images revealed that the twin domains in CaTiO3 were needle shaped and on addition of Nd3+ the domain morphology becomes more complex. The needle domain morphology returns for Ca0.43Nd0.38TiO3. High resolution electron microscopy (HAADF-STEM and electron diffraction) was used to probe cation-vacancy ordering (CVO) in the lattice. It was found that there was no CVO for x < 0.48 whilst at x = 0.48 there was evidence of a transition to a short range CVO. A transition to long range ordering is almost complete for the Ca0.1Nd0.6TiO3. The structural characteristics of Ca(1-x)Nd2x/3TiO3 ceramics as a function of temperature were investigated using in-situ XRD and Raman spectroscopy. All compositions were found to have the same structure across the entire temperature range. The Raman spectroscopy as a function of temperature indicated a possible transition with similar characteristics to a Curie temperature in a ferroelectric ceramic. The transition temperature was dependent on the cation ordering with the ceramics with greatest degree of disorder having the lowest transition temperature. The microwave dielectric properties of the samples were measured by a cavity resonance method in the 2-4GHz range. The relative permittivity (εr) was found to decrease from 180 for CaTiO3 to approximately 80 for Ca0.1Nd0.6TiO3 with an exponential dependence between the composition and the property. The temperature coefficient of resonant frequency (τf) ranged from +770ppmK-1 for CaTiO3 to +200ppmK-1 for Ca0.1Nd0.6TiO3. The Q x f for CaTiO3 was found to be 6000GHz and this increased to a maximum of 13000GHz for Ca0.7Nd0.2TiO3. After the Ca0.7Nd0.2TiO3 composition, the Q x f decreased to approximately 1100GHz for Ca0.1Nd0.6TiO3. The εr and τf were found to be mainly dependent on the composition of the ceramics whilst the Q x f value was more complex being dependent on the width of the twin domains in the grains. CaTiO3 samples fabricated by spark plasma sintering at 1150oC and above achieved better than 95% of the theoretical density. XRD revealed only a single phase with an orthorhombic Pbnm structure at room temperature and a tilt system of a-a-c+. SEM confirmed that the samples were single phase with grain size between 500nm-5μm. Transmission electron microscopy (TEM) of specimens sintered at 1150oC showed evidence of both (011) and (112) type domains. The τf of the ceramics was shown to be dependent on the volume of the unit cell, in agreement with the Bosman-Havinga equations. The ceramic sintered at 1150oC showed improvement in the Q x f value compared to samples prepared by conventional sintering. The structure, microstructure and properties of composite ceramics based on the MgTiO3-Ca0.61Nd0.26TiO3 system were investigated. Optimum properties were achieved at a composition of 0.8MgTiO3-0.2Ca0.61Nd0.26TiO3 with τf = -0.1ppmK-1, Q x f of 39000GHz and εr of 25.4. XRD revealed the presence of 3 phases including Ca0.61Nd0.26TiO3, MgTiO3 and MgTi2O5. The grain size of the ceramics was typically 5μm. The Q x f value was sensitive to the cooling rate and these changes could be related to changes in the vibrational properties of the lattice through changes in the lattice parameters.

Our reliance on semiconductors is on the rise with the ever growing use of electronics in our daily life. Organic-inorganic hybrid lead halide perovskites have emerged as a prime alternative to current standard and expensive semiconductors because of its use of abundant elements and the ease of solution processing. This thesis has shed light on the ink-to-solid conversion during the one-step solution process of hybrid perovskite formulations from DMF. We utilize a suite of in situ diagnostic probes including high speed optical microscopy, optical reflectance and absorbance, and grazing incidence wide angle x-ray scattering (GIWAXS), all performed during spin coating, to monitor the solution thinning behavior, changes in optical absorbance, and nucleation and growth of crystalline phases of the precursor and perovskite. The starting formulation experiences solvent-solute interactions within seconds of casting, leading to the formation of a wet gel with nanoscale features visible by in situ GIWAXS. The wet gel subsequently gives way to the formation of ordered precursor solvates (equimolar iodide and chloride solutions) or disordered precursor solvates (equimolar bromide or 3:1 chloride), depending upon the halide and MAI content. The ordered precursor solute phases are stable and retain the solvent for long durations, resulting in consistent conversion behavior to the perovskite phase and solar-cell performance. In this thesis, we develop a firm understanding of the solvent engineering process in which an anti-solvent is used during the coating process through the solvent mixture of GBL and DMSO in different ratios. It has been shown that solvent engineering produce pin hole-free films, justifying its wide adoption across the field. We then translate our learnings from the lab scale spin coating process to the industrial friendly blade coating process. Here we compare the ink solidification and film formation mechanisms of CH3NH3PbI3 in solutions we used to understand the key scientific insights through spin coating. We observe high-quality film formation for T > 100oC, namely in conditions which inhibit the formation of the crystalline intermediate complex phases. In doing so, we achieve fast and direct formation of the perovskite phase with solar cells yielding PCE > 17%.

Les travaux menés dans le cadre de cette thèse ont porté sur l’ingénierie de la croissance épitaxiale (par épitaxie par jets moléculaires, MBE) et des propriétés d’oxydes fonctionnels pérovskites ABO3. Ces matériaux suscitent un intérêt fort, du fait notamment de leur grande flexibilité chimique et structurale donnant accès à de nombreuses propriétés physiques : ferroélectricité, piézoélectricité, supraconductivité à hautes températures, thermoélectricité, etc…. Dans ce contexte, l’objectif de cette thèse était d’utiliser la souplesse et le contrôle ultime de la croissance offerts par l’MBE pour exploiter au mieux cette flexibilité chimique et structurale et démontrer le potentiel des couches minces, des superréseaux et des phases de Ruddlesden-Popper basés sur le SrTiO3 et le LaSrTiO3 pour des applications en photonique. Nous avons en particulier démontré que de telles structures, si elles sont contrôlées, ouvrent des perspectives importantes pour la plasmonique, pour la réalisation de métamatériaux optiques aux performances inégalées, et pour la fabrication de couches minces fortement anisotropes. Plus spécifiquement, les principaux résultats de cette thèse sont (i) la mise en évidence des excellentes propriétés plasmoniques dans l’infrarouge des couches minces conductrices de LaxSr1-xTiO3 (SrTiO3 dopé au La), et leur forte accordabilité obtenue en contrôlant la concentration en La, (ii) la démonstration d’une nouvelle classe de métamatériaux hyperboliques dans l’infrarouge constitués de superréseaux LaxSr1-xTiO3/SrTiO3, dont l’hyperbolicité peut être largement accordée, et dont les performances surpassent celles des meilleurs métamatériaux hyperboliques basés sur du ZnO dopé au Ga ou à l’Al, et (iii) l’exploration de la croissance et de l’anisotropie optique potentiellement très forte des phases homologues de Ruddlesden-Popper à base de SrTiO3 (Srn+1TinO3n+1). Ces résultats originaux ouvrent des perspectives intéressantes pour l’utilisation de couches minces et de superréseaux structurés jusqu’à l’échelle de la monocouche à base d’oxydes pérovksites pour des applications à la photonique. / The work conducted during this thesis focused on the epitaxial growth (by molecular beam epitaxy, MBE) and the optical properties engineering of perovskite functional oxides ABO3. These materials arouse strong interest, in particular thanks to their strong chemical and structural flexibility giving access to many physical properties: ferroelectricity, piezoelectricity, high temperature superconductivity, thermoelectricity, etc.... In this context, the objective of this thesis was to use the flexibility and ultimate growth control offered by the MBE to exploit this strong chemical and structural flexibility and to demonstrate the potential of thin layers, superlattices and Ruddlesden-Popper phases based on SrTiO3 and LaSrTiO3 for photonics applications. In particular, we have demonstrated that such structures, if they are controlled, open important prospects for plasmonics, for the production of optical metamaterials with unrivalled performances, and for the production of highly anisotropic thin films. More specifically, the main results of this thesis are (i) the demonstration of the excellent infrared plasmonic properties of the LaxSr1-xTiO3 conductive thin films (La-doped SrTiO3), and their strong tunability obtained by controlling the La concentration, (ii) the demonstration of a new class of hyperbolic metamaterials in the infrared consisting of LaxSr1-xTiO3 / SrTiO3 superlattices, whose hyperbolicity can be broadly adjusted, and whose performances surpass those of the best hyperbolic metamaterials based on Ga or Al doped ZnO, and (iii) the exploration of the growth and potentially very strong optical anisotropy of Ruddlesden-Popper homologous phases based on SrTiO3 (Srn+1TinO3n+1). These original results open interesting perspectives for the use of thin layers and structured superlattices up to the monolayer scale based on perovskite oxides for photonics applications.

Kyle Heller, for the Master of Science degree in Mechanical Engineering, presented on October 2020, at Southern Illinois University Carbondale.MAJOR PROFESSOR: Dr. Kanchan Mondal Using powder X-ray diffraction, a material can have its crystalline structure identified. Powder X-ray diffraction alone is not enough if a material is unknown. Usually the exact chemical formula of the material is known, or a secondary analytical method is used to extract additional data in order to analyze the crystalline structure using Bragg’s law and the interplanar relationships. Secondary analytical methods are not as easy or inexpensive though. Generic values could be placed into the more basic structure types to obtain a rough idea of potential crystal types including space groups for the material based on its diffraction peaks. However, with a material that has an unknown spacing between its atoms (d-spacing) this is harder to implement. Thus, the use of a secondary software was employed to further analyze the possibilities. In this thesis, the software used for data extraction and refining were Expo 2014 and CrystalMaker X paired with CrystalDiffract and the final visualization was achieved using Endeavor. Two different titanium based organic inorganic halide perovskites (Dye 3 and Dye 4) prepared at different temperatures were evaluated to identify the crystallographic structure using only x-ray data available. The crystal parameters were calculated, and potential unit cells were visualized. Both the materials were found to be 4 (ABX3) type perovskites. The organic component for Dye 3 was methyl ammonium ion and that of Dye 4 was formamidinium ion. These perovskites have shown potential for use as sensitizers in visible light photovoltaic cells. It was concluded that Dye 4 was orthorhombic with a space grouping of C m c a (space group 64). The associated values were a = b =7.94 Å and c =11.55 Å. Dye 3 was also found to be orthorhombic with space grouping of P c c n (space group 56) being a better fit than C m c a. The associated values were a=b=16 Å and c=11 Å.

Over the last decade, impressive development in lead halide perovskites (LHPs) have made them leading candidate materials for photovoltaics (PVs), X-ray scintillators, and light-emitting diodes (LEDs). The success of LHPs NCs in lighting and display applications is mainly originated from their high photoluminescence quantum yield (PLQY), narrow emission, sizable bandgap, and cost-effective fabrication. Consequently, a comprehensive understanding of the design principles of LHP NCs will fuel further innovations in their optoelectronic applications. This dissertation centers on the synthesis and self-assembly of LHP NCs. At first, we investigate the capability of colloidal synthetic routine to engineer the shape, size, and dimensionality of the resulting LHPs NCs (chapter 2), including 0D nanospheres, 2D nanoplates, and 3D nanocubes. Starting from the LHPs NCs, nanoplates (chapter 3), nanowires (chapter 4), and superstructures (chapter 5) are successfully achieved via various self-assembly strategies. In chapter 3, we present a liquid-air interfaces-assisted self-assembly technique to obtain micro-scale CsPbBr3 nanoplates from as-synthesized nanoscale NCs. The AC-HRTEM offered an atomic-level observation during the structural evolution and revealed an oriented attachment-mediated assembly mechanism. The assembled CsPbBr3 nanoplates exhibited ultrahigh stability under X-ray energy dispersive spectroscopy (EDS) mapping conditions (300-kV electron beam), and the first atomic-resolution EDS elemental mapping data of LHP NCs were acquired. In chapter 4, we demonstrate an efficient green-chemistry approach for the self-assembly of CsPbBr3 NCs into 1D nanowires and nanobelts via the light induction. As an elegant and promising green-chemistry approach, light-induced self-assembly represents a rational method for designing perovskites. In chapter 5, we will explore the self-assembly of CsPbBr3 NCs into superstructures to overcome the ‘green gap’ to achieve a pure green emission with high PLQY for realizing next-generation vivid displays. In summary, we systematically investigated the mechanisms of LHP NC self-assembly, the kinetics of their morphological evolution and phase transitions, and driving forces that govern the self-assembly process. The assembled LHP NCs manifest desirable properties (e.g., superfluorescence, improved photoluminescence lifetime, enhanced stability against moisture, light, electron-beam irradiation, and thermal-degradation) that translate into dramatic improvements in device performance.

Halide perovskite-based photovoltaics are the fastest-growing solar technology in nowadays. Because of the low production costs, perovskite-based photovoltaics are competitive for commercial applications in the marketplace. Additionally, due to the remarkable optoelectronic properties, perovskites are also promising for other optoelectronics, including photodetector, light emitting diode and laser. However, for commercial applications in optoelectronics, there are still several crucial obstacles: (i) a robustness patterning technique is missing for nanofabrication of perovskite devices, (ii) hysteresis effect exits in perovskite devices, and (iii) the stability issue of perovskite. To address these problems, we have performed the fundamental study on perovskite from four aspects: orthogonal patterning, metal contact, carrier transport, and light emission stability. Due to the ionic nature, halide perovskite can be easily dissolved by most of the commonly used organic solvents, which means conventional lithography patternings are not applicable for perovskite, limiting the extensive applications of perovskite electronics. To adress this, we introduced chlorobenzene and hexane and proposed an orthogonal electron beam lithography method for fabrication of perovskite nanodevices without damaging their electrical and optical properties. By this orthogonal method, we fabricated a two-dimensional single crystalline (C6H5C2H4NH3)2PbI4 photodetector with device channel length of few hundred nanometers and outstanding photosensing capability. The hysteresis effect in perovskite is highly related to the interfacial recombination and ionic transport, which requires abundant fundamental understanding on perovskite contact and transport to help to solve this issue. In this study, we performed the lithography patterning method and the transfer length measurement on cm-sized single crystalline perovskite bulk single crystal for indicating the metal contact interface and charge transport, which are requared for efficienct device design and improving the device performance. For stable light emission, we fabricated perovskite nanowires in the nanopores of anodic aluminum oxide substrate using an inkjet printing technique. Lasing behaviors and color-tunable light emission of perovskite nanowires are demonstrated in this study, and the photostability is much better than reported all-inorganic perovskite quantum dots. We believe these fundamental studies provide solutions to some critical issues in current perovskite technology, thus paving the way for future optoelectronic applications.

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With the rapid increase in energy demand and the rise of CO2 levels due to traditional energy production from fossil fuels, it is critical to the transition to a sustainable and renewable energy sources. Recently, photovoltaic technology has been raised as a promising alternative to fossil fuel energy production. Solar cells, predominately crystalline silicon technology, are currently 3.6% of electricity production. To maintain this progress, coupling the perovskite and silicon in tandem devices has enormous potential to increase the efficiency of solar energy production, where perovskite solar cells emerged as a promising technology. Textured silicon solar cells are a well-established technology; keeping the advantage of this technology, it is crucial to employ the perovskite to be a compatible top cell for silicon-based tandems. Here, we optimize the silicon bottom cell by understanding the influence of temperature, time, and etchant concentration on the optical properties and performance of the device. Then, we investigate the impact of the textured silicon on the optoelectronic properties of perovskite. Using hyperspectral imaging, we demonstrate that different texturing substrates influence the PL of perovskite, which is associated with the thickness of the perovskite. Lastly, we explored the delamination of the devices due to the weak adhesion between C60/SnO2 after the deposition of IZO and MgF2, which was found to be caused by the deposition conditions. The high temperature and power density caused a weak adhesion between C60/SnO2. Overall, these findings will help to alter the design of Perovskite/Silicon tandem devices to accelerate the commercialization of tandem technology.

Lead halide perovskite solar cells (PSCs) are considered the next generation of photovoltaic technology, reaching an outstanding certified power conversion efficiency of 25.7% in just 20 years. The best-performing PSCs are based on polycrystalline films, where the presence of grain boundaries and a tremendous number of defects limit stability and efficiency and thus further industrial development. Compared to their polycrystalline counterparts, single crystals of lead halide perovskites have been shown to possess much lower trap-state densities, long diffusion lengths, high stability, and near-IR absorption. This thesis describes the use of a confined space and inverse temperature method to grow perovskite single crystals of MAPbI3 directly on PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) films. This method allowed the fabrication of p-i-n inverted solar cells with the structure ITO/PTAA/perovskite single-crystal thin film/C60 (fullerene)/BCP (bathocuproine)/Cu (copper). A key requirement in achieving high photoconversion efficiency (PCE) is avoiding iodine oxidation, which forms triiodide impurities that function as defects in perovskites and that can seriously hinder the performance of perovskites. By suppressing the formation of triiodide, high-quality perovskite photodetectors and solar cells can be realized. For single crystals, orientation has a strong effect on device performance. Here, (100)- and (001)-facet single-crystal thin films were fabricated into solar cells. Unlike traditional (100)-facet films, which exhibit high PCE in a glovebox environment, (001)-facet single-crystal thin films show high stability under ambient conditions.