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Metastable layered perovskites containing interlayer transition metals can readily be obtained by simple ion exchange reactions on receptive hosts, such as those of the Dion-Jacobson and Ruddlesden-Popper structure types. In this work, we focused on adding to the library of layered perovskites by not only creating new compounds, but by also showing their ability to be further manipulated, and by studying the stability of the series through thermal behavior studies. The reactions with transition metal halides are particularly interesting since they often lead to novel architectures and magnetic behavior. On subsequent heat treatment, these exchange products typically decompose to thermodynamically more stable phases. The newly synthesized spin glass-like material, FeLa2Ti3O10, obtained by ion exchange of Li2La2Ti3O10 with FeCl2 at 350 °C, behaves differently. When heated to 700 °C, the compound undergoes a significant cell contraction (Δc ≈ -2.7 Å) with an increase in the oxidation state of iron present in the interlayer that not been observed before in such compounds. Efforts were also made to synthesize new series of compounds, here MSrTa2O7 (M= Co and Zn), with vacancies in the interlayer that could lead to future topochemical manipulations. The ability to vary the composition of different phases to form solid-solutions through atomic substitution at the A or B sites with ions of similar or different charge can lead to new structures as well an enhancement of the properties of the original compound or new ones. The synthesis and characterization of the new mixed A-cation containing layered perovskite RbLaNaNb3O10, where La3+ and Na+ share the same site in the perovskite slab and RbLaCaNb2MnO10 that exhibits an ordering of the B site with Mn in the center of the perovskite slab will be presented. Further topochemical manipulation of these phases via ion exchange reactions at low temperatures (< 500 °C), lead to the new series A’LaNaNb3O10 and A’LaCaNb2MnO10 where A’= H, Li, Na, K and CuCl.

This thesis explores a range of photoactive metal halide perovskite materials for use in photovoltaic applications. These materials are of huge interest due to their outstanding optoelectronic properties which result in high photovoltaic power conversion efficiencies. In particular, this thesis discusses perovskites with stoichiometry ABX₃ where A is a singly charged cation, for example methylammonium (MA), B is predominantly lead (Pb²⁺), and X is iodide (I-) and/or bromide (Br^-). At present the commercial applications of these materials are limited by the chemical instability of the A-site cation. In this thesis, the effect of chemical substitution of the A-site is investigated as a way to increase the stability of the perovskite material. Full replacement with the inorganic cation caesium (Cs⁺) is shown to significantly improve the chemical stability. However, the inorganic lead halide perovskites with ideal bandgaps for photovoltaic applications exhibit structural instability. Routes to achieve both chemical and structural stability for these perovskites are discussed. Consequently, this thesis represents pioneering work in the field of inorganic halide perovskites and will greatly assist the development of stable inorganic perovskite materials for optoelectronic applications such as tandem photovoltaics and LEDs. Chapters 1 and 2 of this thesis present the motivation for perovskite materials to be used in solar cells, along with relevant background information about these materials and solar cell operation in general. Chapter 3 details the methods utilised in the experimental results chapters which follow. The first experimental results chapter, Chapter 4, shows how incorporation of Br^- in place of I^- in CsPbI₃ leads to increased ambient stability of the perovskite structure, and the first solar cells with CsPbI₂Br as the absorbing photovoltaic material are reported. Chapter 5 remedies the deficit of information about the optoelectronic properties of the CsPbI_3-xBr_x (0 &le; x &le; 3) perovskites through magneto-optical measurements on thin-films. These measurements raise questions about the room temperature perovskite structure of the CsPbI_3-xBrx compositions with small x, previously thought to be cubic perovskite, which is shown in Chapter 6 to be an orthorhombic perovskite polymorph. This finding motivates preliminary work presented in Chapter 7 aimed at chemical stabilisation of this orthorhombic perovskite polymorph. Finally, Chapter 8 summarises the work presented in this thesis, and recommends further research for the development of stable perovskite materials for photovoltaics.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Temos como objetivo neste trabalho estudar as propriedades dielÃtricas em rÃdio frequÃncia (RF) e micro-ondas do composto Sr2CoNbO6 (SCNO) adicionando com o Ãxido de titÃnio (TiO2) em porcentagens variando de 15% a 22% em massa. O SCNO à uma perovskita dupla, que foi preparado atravÃs do mÃtodo de reaÃÃo do estado sÃlido. As amostras foram caracterizadas estruturalmente atravÃs da difraÃÃo de raio â X (DRX). O mÃtodo utilizado para obtenÃÃo da fase desejada foi realizada atravÃs do refinamento de Rietveld, que mostrou estrutura de simetria cÃbica do tipo pm3m. As propriedades dielÃtricas: constante dielÃtrica (&#949;â) e fator de perda dielÃtrica (tg&#948;) foram medidas à temperatura ambiente na faixa de frequÃncia de 100KHz a 2GHz e mostraram que a permissividade diminui com a frequÃncia. Abaixo de 50MHz a constante dielÃtrica se apresenta alta e acima de 1GHz se apresenta baixa. A picnometria realizada mostrou que as densidades relativas ficaram todas acima de 80%. Para que o estudo de micro-ondas ficasse completo aplicamos o mÃtodo de Hakki Coleman, que mostrou crescimento da permissividade dielÃtrica com o aumento da concentraÃÃo de TiO2 (14 â 52)e perdas dielÃtricas variando de 10-2 - 10-4 . O coeficiente de temperatura da frequÃncia de ressonÃncia (&#964;f) mostrou que o SCNO apresenta &#964;f negativo (-634ppm/oC). Com a adiÃÃo do TiO2 o &#964;f passou a apresentar valores positivos. A simulaÃÃo numÃrica de uma antena ressoadora dielÃtrica (DRA) baseada no SCNO e adicionada com TiO2 foi realizada atravÃs do software HFSS e os resultados se apresentaram bastante concordantes com os experimentos. O baixo ganho e uma pequena eficiÃncia apresentada para o SCNO puro foi melhorado substancialmente com a adiÃÃo do TiO2. Estas propriedades podem ser melhoradas com uma proposta de uma nova geometria para a (DRA) e com adiÃÃo do TiO2 em novas porcentagens para anÃlises de propriedades dielÃtricas e parÃmetros de antenas.

A family of oxygen-deficient perovskite phases with compositions, Ca₂Mn_2-xNb_xO_γ has been synthesised and characterised using X-ray Powder Diffraction and Thermogravimetry. Property characterisation has included the study of electrical properties by Impedance Spectroscopy and the investigation of magnetic properties for one composition, x = 1.0. Some compositions were tested for possible application as electrode materials in gas sensors. The Ca₂Mn_2-xNb_xO_γ system 0 ≤ x ≤ 1.2, with variable oxygen content, γ, can accommodate up to 20% vacancies at oxygen sites and Mn occurs in valence states ranging from +2 to +4. Depending on the B-cation ratio and oxygen content, two solid solution form: an extensive GdFeO₃-type solid solution over the entire range of cation content, 0 ≤ x ≤ 1.2, with zero or small oxygen deficiencies and a closely-related, grossly oxygen-deficient solid solution over the range 0.3 ≤ x ≤ 0.8 with a simple cubic perovskite structure. Unit cell volume and orthorhombic GdFeO₃-type distortion vary greatly with Mn valency, oxygen content and B-cation content. The orthorhombic GdFeO₃-type structure of composition x = 1.0 was refined by the Rietveld method. Mn and Nb are disordered over the B-sites. Jahn-Teller activity of Mn³⁺ does not result in cooperative distortions of (Mn,Nb)O₆ octahedra, but it was observed to effect the oxygen stoichiometry. The Nb-rich composition, x = 1.0, can accommodate large amounts of Ca-vacancies (up to 10%). Electrical conductivity varies greatly with the composition. Results suggest that t_2g electrons are responsible for a variation of conductivity and activation energy rather than e_g electrons. Conductivities are believed to depend mainly on the degree of π-orbital overlap between Mn and oxygen and hence, on interatomic distances. At low temperatures, the material exhibits spin glass-like behaviour.

BARBOSA, R. de L. Propriedades dielétricas intrínsecas de perovskitas duplas RE2CoMnO6 (RE = íon terra-rara). 2017. 69 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-08-23T16:49:17Z No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) / Approved for entry into archive by Giordana Silva (giordana.nascimento@gmail.com) on 2017-08-23T18:16:47Z (GMT) No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) / Made available in DSpace on 2017-08-23T18:16:47Z (GMT). No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5) Previous issue date: 2017 / Dielectric ceramics with ordered double perovskite structure and RE2CoMnO6 stoichiometry, in which RE is a rare-earth ion, are usually multiferroic with critical magnetic and electrical transition temperatures depending on the RE ion. This family of materials crystallizes in the P21/n symmetry when there is ordering of the Mn and Co ions. This monoclinic structure has a distortion directly dependent on the RE ionic size, influencing directly the properties of these materials. In this work, we investigate the intrinsic dielectric properties of these samples for RE = Tb, Dy, Ho, Yb and Tm. These properties were investigated by infrared reflection spectroscopy, allowing to estimate both the intrinsic static dielectric constant and the dielectric losses due to the polar phonons, which allows estimating the maximum microwave quality factor obtained when a resonator is formed with the respective materials. Our results show that, as obtained for La2CoMnO6 (LaCMO), the main compound of this family, the intrinsic static dielectric constant is reduced. In the case of LaCMO, dielectric measurements performed by other authors showed a colossal effect, whose origin was extrinsic effects. Therefore, our result allows to predict that any effect that leads to very high dielectric constants is extrinsic in this family of materials. In addition, the analysis of the quality factor showed that such ceramics have a quality factor compatible with those employed in microwave dielectric resonators since the low dielectric constant is not a problem. / As cerâmicas dielétricas com estrutura perovskita dupla ordenada com estequiometria RE2CoMnO6, na qual RE é um íon terra-rara, são usualmente multiferróicas com temperaturas críticas de transição magnética e elétrica dependendo do íon RE. Esta família de materiais cristaliza-se na simetria P21/n quando há o ordenamento dos íons Mn e Co. Esta estrutura monoclínica, tem um distorção diretamente ligada ao tamanho do íon RE, o que influencia diretamente suas propriedades. Neste trabalho investigamos as propriedades dielétricas intrínsecas destas amostras para RE = Tb, Dy, Ho, Yb e Tm. Tais propriedades foram investigadas por espectroscopia de reflexão especular no infravermelho, permitindo estimar tanto a constante dielétrica estática intrínseca devido aos fônons polares, como as perdas dielétricas devido a estes fônons, o que permite estimar o fator de qualidade em micro-ondas máximo obtido quando se conforma um ressonador dielétrico com o respectivo material. Nossos resultados mostram que, assim como obtido para o La2CoMnO6 (LaCMO), principal composto desta família, a constante dielétrica estática intrínseca é reduzida. No caso do LaCMO , medidas dielétricas realizadas por outros pesquisadores mostraram efeito colossal, cuja origem eram efeitos extrínsecos. Portanto, nosso resultado permite prever que qualquer efeito que leve a obter constantes dielétricas muito altas tem natureza extrínseca nesta família de materiais. Além disso, a análise do fator de qualidade mostrou que tais cerâmicas tem fator de qualidade compatível com aqueles empregados em ressonadores dielétricos para micro-ondas desde que a constante dielétrica baixa não seja um problema.

With the soaring advancement of organolead halide perovskite solar cells rising from a power conversion efficiency of merely 3% to more than 22% shortly in five years, researchers’ interests on this big material family have been greatly spurred. So far, both in-depth studies on the fundamental properties of organolead halide perovskites and their extended applications such as photodetectors, light emitting diodes, and lasing have been intensively reported. The great successes have been ascribed to various superior properties of organolead halide hybrid perovskites such as long carrier lifetimes, high carrier mobility, and solution-processable high quality thin films, as will be discussed in Chapter 1. Notably, most of these studies have been limited to their polycrystalline thin films. Single crystals, as a counter form of polycrystals, have no grain boundaries and higher crystallinity, and thus less defects. These characteristics gift single crystals with superior optical, electrical, and mechanical properties, which will be discussed in Chapter 2. For example, organolead halide perovskite single crystals have been reported with much longer carrier lifetimes and higher carrier mobilities, which are especially intriguing for optoelectronic applications. Besides their superior optoelectronic properties, organolead halide perovskites have shown large composition versatility, especially their organic components, which can be controlled to effectively adjust their crystal structures and further fundamental properties. Single crystals are an ideal platform for such composition-structure-property study since a uniform structure with homogeneous compositions and without distraction from grain boundaries as well as excess defects can provide unambiguously information of material properties. As a major part of work of this dissertation, explorative work on the composition-structure-property study of organic-cation-alloyed organolead halide perovskites using their single crystals will be discussed in Chapter 3 and 4. Despite their outstanding charge transport characteristics, organolead halide perovskite single crystals grown by hitherto reported crystallization methods are not suitable for most optoelectronic devices due to their small aspect ratios and free standing growth. As the other major part of work of this dissertation, explorative work on growing organolead halide perovskite monocrystalline films and further their application in solar cells will be discussed in Chapter 5.

This thesis is dealing with nanowires of zinc oxide and of cesium lead bromide. Main goal was a preparation of ZnO nanowires using MBE. This goal was partially achieved and growth of needle-like structures was observed. Another goal was characterization of already prepared ZnO nanowires. We successfully determined polarity of their surfaces, examined the influence of lattice defects and assessed the result of their doping. Side goal of this work was characterization of nanostructures of cesium lead bromide. However, preparation of cesium lead bromide nanowires remains an open issue.

Finding luminescent materials with narrow-band emissions, high stability, and high photoluminescence quantum yield (PLQY), yet relatively fast radiative decay rates, has been an outstanding research challenge. This thesis aims to develop different luminescent materials and examine their structural and optical properties for light-emitting applications. The first part of this thesis covers the controversy regarding the origin of emission in zerodimensional perovskites (0D), Cs4PbBr6 and Cs4PbI6, through a comparative analysis between 0D and three-dimensional (3D) perovskites. A series of optical studies excluded the 3D phases as the origin of emission in these materials. In parallel, the results from the DFT proposed the defects as a possible origin of emission. The second part of the thesis addresses the shortcoming of lead-based perovskites in terms of toxicity and stability, motivated by the high demand for sustainable materials with analogous electrical and structural properties. Thus, a series of solid-state Zn-based and Mn-based ternary compounds were investigated with and without doping. The compounds' photoluminescence peaks were between 520 nm – 525 nm, with PLQY between 34% -60%. Finally, CsMnBr3 NCs were synthesized revealing an intense red PL peak centered at 650 nm and a PLQY as high as 54% along with a remarkable excitation spectrum and surprisingly short lifetimes. The single crystals of this composition were also reported with a PL peak at 640 nm and a relatively high PLQY of 6.7% under the excitation at 360 nm. Further, a combination of structural and optical analysis attributed the green and red luminescence to the tetrahedral and octahedral environment of Mn2+, respectively. These materials represent a milestone towards lead-free luminescent materials with interesting optical properties. This dissertation aims at engineering different materials to address critical aspects in the field including stability and good luminescence properties while simultaneously examining the photophysics and mechanisms of the corresponding properties. This work paves the way for finding sustainable light-emitting materials for the next generation of light-emitting applications.

Many countries around the world have decided to play a positive role in combating climate change and reduce carbon dioxide in the atmosphere. In addition to reducing emissions, initiatives include the capture, storage and utilization of CO2. Converting it to valuable products through reforming of methane not only utilizes major greenhouse gasses, but can also be a means for energy from biogas. The main challenge hindering this process is developing a scalable active catalyst that can resist deactivation. To address this challenge, focus has shifted from simple metal oxides towards metal nanoparticles dispersed and organized in complex well defined structures. Oxide perovskites have the potential to contain metal and support in a single structure as the case of LaNiO3. Metal-organic frameworks are another type of materials that can be used as sacrificial agents to produce the type of complex metal oxides required. Three synthesis approaches were studied for the synthesis of La-Ni materials. Combustion synthesis is a cost and time efficient method. However, it becomes challenging to accurately predict the outcomes. Hydrothermally synthesized perovskites give pure phase materials but are sensitive to synthesis variables. MOF based materials showed conversions of 94% and 83% for CO2 and CH4, respectively, with stable performance for +100 hours and can be a promising future route in heterogeneous catalysis.