Thesis (M.S.)--California Polytechnic State University, 2008. / Major professor: Fei Wang, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Electrical Engineering." "May 2008." Includes bibliographical references (leaves 53-56). Also available online. Also available on microfiche ( sheet).

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Edmund R. Nowak, Dept. of Physics & Astronomy. Includes bibliographical references.

As Células a combustível são conversores de energia química em energia elétrica. As Células do tipo PEM que funcionam com metanol como combustível tem uma ampla variedade de aplicações. Os materiais utilizados como eletrocatalisadores nas células são responsáveis por uma grande parte do custo das mesmas. Outros problemas, que provocam diminuição da eficiência da célula, são a cinética lenta da reação de redução de oxigênio (RRO) e o potencialmisto gerado devido ao cruzamento de metanol através da membrana. Neste trabalho apresenta-se um estudo de catalisadores do tipo M-MxSy (M = Pt, Rh) para a RRO em meio ácido, com diferentes relações M:MxSy. Os materiais preparados a partir da modificação do método do ácido fórmico (MAF) com tiouréia (TU) foram caracterizados mediante XRD, XPS e XAS. Foi determinado mediante estas técnicas que os catalisadores consistem numa mistura de fases: Pt ou PtRh, PtS, RhxSy e PtS2. O efeito de um tratamento térmico em H2/Ar foi reduzir completamente o PtS2 e parcialmente o PtS. A fase de RhxSy mostrou ser mais estável nas condições do tratamento. Todos os materiais mostraram ter atividade para a RRO e alta seletividade na presença de metanol. Foi determinado que para que a RRO ocorra via 4 elétrons, deve haver sítios metálicos na superfície das nanopartículas. Determinou-se que os materiais contendo maior quantidade de fase MxSy podem ser ativados mediante tratamento térmico ou eletroquímico, melhorando a atividade catalítica frente a RRO e conservando a seletividade na presença de metanol. / Fuel cells are dispositives that convert chemical energy into electricity. The PEM fuel cell types that function with methanol as fuel have a wide variety of applications. The materials used as electrocatalysts in the cells are responsible for the major part of their cost. Other problems, that cause decrease in efficiency of the cell, are the slow kinetics of oxygen reduction reaction (ORR) and the mixed potential generated due to methanol crossover through the membrane. This thesis presents a study of M-MxSy-type catalysts (M = Pt, Rh) for the ORR in acid medium, with different M:MxSy ratios. The materials prepared from the modification of the formic acid method (FAM) with thiourea (TU) were characterized by XRD, XPS and XAS. It was determined by these techniques that the catalysts consist of a mixture of phases: Pt or PtRh, PtS, RhxSy and PtS2. The effect of heat treatment in H2/Ar atmosphere was to reduce PtS2 completely and PtS partially. The RhxSy phase proved to be more stable under the treatment conditions. All materials showed to have activity for the ORR and high selectivity in the presence of methanol. It was determined that for the ORR to occur via four electrons, there must be metallic sites at the surface of the nanoparticles. It was determined that the materials containing higher amount of MxSy phase can be activated by thermal or electrochemical treatment, improving the ORR catalytic activity and retaining the selectivity in the presence of methanol.

Quantum dots (QDs) have the potential to produce more than one exciton per incident photon, if the photon energy is greater than twice the band gap energy. This process of multiple exciton generation (MEG) has the potential to lead to a step change in the efficiency of solar panels, by utilising energy commonly wasted as heat in conventional solar cells. A wide range of CdSe/CdTe and CdTe/CdSe quantum dots with and without a CdS shell were synthesised with varying core sizes and shell thicknesses. The excited state dynamics of these samples were studied with transient absorption and photoluminescence studies, with their MEG efficiencies obtained. Record MEG efficiencies were obtained with values reaching 142 ± 9 % achieved. The charge separation afforded by the type-II electronic configuration, allowed the first attractive biexciton interaction for a type-II QD system, with the potential for reducing the creation energy for a second exciton this affords. Efficient surface passivation of QDs was achieved through the reaction of CdCl2 with CdTe QDs, with near unity photoluminescence quantum yields (PLQYs) achieved. The suppression of surface trap states resulted in mono-exponential photoluminescence decay traces, with a resultant increase in exciton lifetime. Further CdCl2 treatment was carried out on CdSe/CdTe quasi-type-II QDs with alternating ‘Cd rich’ and ‘Te rich’ surfaces to elucidate the processes involved in surface treatment. It is shown that Te surface atoms are preferentially etched upon treatment, with the reaction being more aggressive when ‘Te rich’ surfaces are treated. The importance of surface composition is studied with trap states associated with chalcogen dangling bonds more prevalent and hence the increased requirement for their passivation is outlined. Control of the core/shell interface is also shown to be important in reducing trap states and ultimately increasing PLQYs, which is desirable for many optoelectronic applications.

Ces travaux de thèse portent sur l’étude des propriétés chimiques et physiques de chalcogénures polaires (CPs) à valence normal ou hypervalents. Ces composés appartiennent à la famille des intermétalliques polaires, et s’inscrivent donc dans le champ d’étude de la chimie des intermétalliques. Le but premier de cette étude est la synthèse de nouveaux composés de structure cristalline complexe, afin d’étudier la relation entre la structure cristalline, la nature des liaisons chimiques et les propriétés physiques, déterminées par des mesures expérimentales et des analyses théoriques. Les CPs ont été choisis comme matériaux d’étude car ils se situent à la frontière entre les matériaux métalliques et non-métalliques. Pour ces matériaux (les CPs), les propriétés chimiques sont gouvernées par l’interaction entre les différents types de liaisons – covalente, métallique et ionique – ouvrant la voie à l’étude des liens entre structure cristalline et liaisons chimiques. La recherche de matériaux à structure complexe permet de cibler de potentiels matériaux thermoélectriques prometteurs, puisque la complexité structurale est souvent reliée à une faible conductivité thermique, qui est une propriété clé des thermoélectriques. Les matériaux thermoélectriques transforment la chaleur en électricité, et sont donc au cœur des enjeux économiques et environnementaux actuels. La découverte de thermoélectriques à bon rendement appartenant à la famille des chalcogénures, tels que PbTe, Bi2Te3, CsBi4Te6 et le composé superionique Cu2-xSe ont orienté les recherches vers l’exploration de composés chalcogénures de type Cu- et Pn- (Pn = Sn, Bi), et ont motivé l’étude de matériaux voisins, comme BaBiTe3 (chapitre V). Une des possibilités pour induire des structures complexes est d’obtenir un transfert de charge du cation (Ba, Se) vers une structure anionique, créant ainsi des réseaux covalents anioniques complexes sous forme de chaines ou de couches, qui sont à l’origine de propriétés physiques intéressantes. Une paire d’électrons libres et stéréoactifs peut également augmenter la complexité de la structure, via une distorsion des polyèdres de coordination, ce qui justifie l’étude de matériaux contenant des éléments de type Pn comme Bi ou Sb. L’analyse des propriétés physiques ainsi que l’étude de la structure cristalline et des liaisons chimiques de chalcogénures polaires de structure complexe, certains connus et d’autres découverts au cours de ce travail de thèse, ont résulté en des découvertes prometteuses. / This thesis has its focus on the chemistry and physical properties of normal valence and hypervalent polar chalcogenides (PCs). The motivation for this study lies in the synthesis of new compounds with complex crystal structures. It aims at understanding the relationship between crystal structure, chemical bonding and physical properties through experimental and theoretical analyses. PCs are of special interest since they are at the interface between metals and nonmetals. The chemistry at this interface is governed by the interplay between covalent, metallic and ionic bonding, which makes it interesting and challenging to understand the relationship between crystal structure and chemical bonding. The main reason for aiming at structural complexity is to target new materials with low thermal conductivities – a key requirement for efficient thermoelectric materials. Thermoelectrics are capable of converting waste heat into electricity, which is of considerable economic and environmental interest. Previous discoveries of efficient, chalcogenide-based thermoelectrics such as PbTe, Bi2Te3, CsBi4Te6 and superionic Cu2-xSe motivated the exploratory search for new Cu- and Pn-chalcogenides (Pn = Sb, Bi) and to study related materials such as BaBiTe3 (cf. chapter V). One route towards complex crystal structures is to use a charge transfer from cations such as Sr or Ba to an anionic framework in order to create complex anionic, covalent networks (e.g. channels or layers) which can lead towards interesting physical properties. Stereoactive lone pairs can increase the structural complexity through distortions of the coordination polyhedra, which is one reason for studying systems containing Pn atoms such as Sb and Bi. Probing the physical properties and studying the crystal structure and chemical bonding of both, new and known polar chalcogenides with complex crystal structures resulted in interesting new discoveries, i.e. new compounds and crystal structures as well as unexpected physical properties. The thesis is separated in normal valence compounds, which can be entirely described by classical two-center two-electron (2c-2e) bonds (i.e. where the electrons are fully localized) and those, which contain hypervalent bonds and networks in which the electrons are partially delocalized. It contains four main parts: the study of 1) A0.5CuZrSe3 et ACuYSe3 (A = Sr, Ba) belonging to a family of compounds known as the “1113 family”, 2) Ba2FePnSe5 (Pn = Sb, Bi), 3) Ba4Cu8Se13 and 4) BaBiTe3-xSex (x = 0, 0.05, 1 and 3).

No description available.

Femtosecond laser direct writing (FLDW) is a versatile process that uses focused femtosecond pulses to modify the physical structure of a material, which can result in a shift of optical properties such as the linear and nonlinear refractive index. If the photon energy of the femtosecond pulses lies below the material bandgap, nonlinear absorption rather than linear absorption becomes the dominant mechanism of energy transfer to the material. In this manner, a focused femtosecond pulse train can be used to fabricate functional features such as optical waveguides, diffractive optical elements, or micro-fluidic elements within the volume of a transparent medium. In this dissertation, the utility of femtosecond laser processing as a fabrication technique of optical and micro-fluidic elements in chalcogenide glasses is explored. The photo-induced modifications of optical and chemical parameters of new germanium-based Chalcogenide glasses in both bulk and thin-film form are characterized for the first time and the impact of material composition and laser fabrication parameters are discussed. The glasses are found to display an increase in volume, a decrease of the linear optical refractive index, and an increase of the nonlinear refractive index when exposed to femtosecond laser pulses. A model based on avalanche ionization and multi-photon ionization is used to describe the highly nonlinear absorption of laser light in the material and correlate the photo-induced modifications to the electron density generated during irradiation. The magnitude of the induced photomodification is shown to be dependent on laser parameters such as laser dose and repetition rate. The fabrication of microfluidic elements through both direct ablation and the preferential etching of photo-modified regions is also explored. Finally, the integration of both optical elements and fluidic elements fabricated by FLDW into a single substrate is discussed. iv TABLE OF CONT

Thermoelectric semiconductors can convert temperature differences into electricity or electricity into temperature differences. This offers great potential for the use of wasted heat or cooling. These materials can be used in a variety of fields, from healthcare to space exploration. The effectiveness of the materials is evaluated by their thermoelectric properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. The aim of this PhD thesis is to investigate the electronic structure using first-principle methods for potential thermoelectric applications. Materials of interest include Copper and Tin based ternary /quaternary compounds, and monolayers of SnS2, SnSe2 and Janus SnSSe. Density functional theory, ab initio molecular dynamics and Boltzmann transport theory are used to study the electronic and phonon transport properties. In the first part of the thesis, electronic structure calculations were performed on both monoclinic and disordered cubic forms of Cu2SnS3(CTS). The impact of structural disorder on thermoelectric properties was examined through these simulations. The results, obtained through first-principle calculations, revealed the existence of band tails in the electronic density of states for the disordered structure, and low-lying optical modes in the disordered cubic structure. This was found to be caused by a significant variation in Sn bonding, leading to strong anharmonicity as measured by the Grüneisen parameter. The findings from the first principle calculations were supported by Nuclear inelastic scattering experiments. Furthermore, the effect of grain size on Cu2SnS3 was studied using first-principles calculations on various ordered and disordered surfaces. The density of states (DOS) revealed that the surface of CTS is conductive due to the presence of dangling bonds. Furthermore, calculations of the formation energy showed that the stoichiometric CTS, Cu-vacant and Cu-rich systems are energetically more favourable, while the formation of Sn-vacant and Sn-rich systems is less likely. In the subsequent study, the impact of Ag substitution at the Sn site at various concentrations was investigated. The Fermi level for Ag-substituted systems was found to lie deep within the valence band, with the shift of the Fermi level inside the valence band increasing with substitution increasing the carrier concentration. The incorporation of Ag into the system decreases the root mean squared displacement of the other cations and anions, which reduces the scattering of phonons and thereby increases the lattice thermal conductivity. A comparative study of various polymorphs of CTS, Cu2ZnSnS4 and Cu2ZnSnSe4 was done. Ab-initio molecular dynamics was performed on CTS, CZTS and CZTSe. The root mean squared displacement value for the disordered polymorph was higher than for the ordered phase, indicating increased static disorder. This corresponds to the static (temperature-independent) distortion of the crystalline lattice due to the disorder of the cations and is associated with higher anharmonicity and bond inhomogeneity in the disordered phase, which is then directly responsible for the ultra-low thermal conductivity. In the final part of the thesis, thermoelectric properties of dichalcogenide monolayer of SnS2, SnSe2 and Janus SnSSe was performed. Density functional theoretical calculations points out the hexagonal Janus SnSSe monolayer as a potential high-performing thermoelectric material. Results for the Janus SnSSe monolayer show an ultra-low thermal conductivity originating from the low group velocity of the low-lying optical modes, leading to superior zT values of 0.5 and 3 at 300 K and 700 K for the p-type doping, respectively. The successful calculation of properties for materials shows that the computational work done in this thesis can be used for further research into thermoelectricity.

The quest for efficient and cost-effective thin-film photovoltaic (PV) materials has recently zeroed in on hybrid lead halide perovskites, owing to their low cost, ease of processing, and exceptional efficiency metrics—peaking at 33.9% when combined with silicon in tandem devices. Nevertheless, there are substantial concerns about the stability, toxicity, and consequential environmental footprint of lead-based perovskites, thereby necessitating rigorous research to identify and develop alternative materials with superior stability profiles and diminished toxicity. Amongst the myriad candidates, chalcogenide perovskites and their related structures, represented by the empirical formula ABQ3 (with A = Ca, Ba, Sr; B = Zr, Hf, Ti; Q = S, Se), have emerged as particularly promising contenders. These materials are distinguished by their optimal optoelectronic properties and robust stability. Notably, barium zirconium sulfide, BaZrS3, has garnered significant attention in the scientific community due to its distinctive perovskite structure and several unique optoelectronic properties, making it a frontrunner in this domain of PV materials research. However, synthetic routes to these materials, especially as colloidal nanomaterials, remain limited, due in part to their high crystallization energy and oxophilicity. In this thesis, we have successfully devised solution-based approaches to synthesize colloidal nanomaterials of BaTiS3 and BaZrS3, including its titanium- and selenium alloyed phases. Our methodology involves utilizing reactive metal amide precursors in oleylamine, with diethylthiourea and trioctylphosphine selenide serving as sources for sulfur and selenium, respectively. Chapter I discusses the general background of current and emerging PV materials. Chapter II delves into various synthetic routes reported for inorganic ternary and binary sulfide and selenide nanomaterials, incorporating transition metals from groups 3, 4, and 5. This section also encompasses our synthetic methods for BaZrS3 and BaTiS3 colloidal nanomaterials. Chapter III provides an in-depth discussion of our developed techniques for producing nanorods and nanoparticles of barium titanium sulfide. In Chapter IV, our focus shifts to the synthesis of colloidal nanoparticles of barium zirconium sulfide perovskites. Additionally, Chapter V explores the synthesis of titanium and selenium alloyed barium zirconium sulfide. Finally, the synthesis of mixed halide lead perovskite nanocrystals, achieved through a postsynthetic anion-exchange method, is discussed in Appendix E.

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