We present a model for exciton-mediated first-order Raman scattering by longitudinal optical phonons in the presence of surfaces and point defects. It is consistent with the experimental data for all wurtzite structure materials investigated and reviewed here (GaN, InN, ZnO and CdS) and also explains not yet understood observations in the literature. We distinguish between the involvement of elastic scattering by the surface and by point defects in the scattering process. Surface scattering causes the dependence of the line position on the crystal orientation of the excited surface in pure crystals. Point defect scattering is independent of the crystal orientation and appears as an additional contribution in defect-rich crystals. We postulate the polarization properties of these distinct processes which are in good agreement with the experiments and allow us to identify and separate the contributions of these two effects from the polarized spectra.

In broadband antenna applications, the antenna's cavity is usually loaded with absorbers to eliminate the backward radiation, but in doing so the radiation efficiency of the antenna is decreased. To enhance the radiation efficiency of the antennas EBG structures are used, but they operate over a narrow band. Uniform electromagnetic band gap (EBG) structures are usually periodic structures consisting of metal patches that are separated by small gaps and vias that connect the patches to the ground plane. The electrical equivalent circuit consists of a resonant tank circuit, whose capacitance is represented by the gap between the patches and inductance represented by the via. EBG structures are equivalent to a magnetic surface at the frequency of resonance and thus have very high surface impedance; this makes the EBG structures useful when mounting an antenna close to conducting ground plane, provided the antenna's currents are parallel to the EBG structure. Because EBG structures are known to operate over a very narrow band, they are not useful when used with a broadband antenna. Mushroom-like uniform EBG structures (that use vias) are compact in size have low loss, can be integrated into an antenna to minimize coupling effects of ground planes and increase radiation efficiency of the antenna. The bandwidth of an EBG structure is defined as the band where the reflection-phase from the structure is between +900 to -900. In this dissertation analysis of EBG structures is established using circuit analysis and transmission line analysis. Methods of increasing the bandwidth of EBG structures are explored, by cascading uniform EBG structures of different sizes progressively and vertically (stacked), and applications with different types of antennas are presented. Analyses in this dissertation are compared with previously published results and with simulated results using 3D electromagnetic tools. Validation of applications with antennas is carried by manufacturing prototypes and comparing measured performance with analysis and 3D electromagnetic simulations. The improvements in performance by using wideband progressive EBG and wideband stacked EBG structures are noted. / Ph. D.

Semiconductors play an integral part in modern society. From computing to LEDs their use is ubiquitous, and no field is more reliant on them than that of power generation. Current political movements have seen a push to decrease reliance on traditional forms of power generation, which relies on fossil fuels, to renewable sources such as solar power. However, current commercial solar panels, based on silicon, are lacking in efficiency, only reaching between 18% and 22% efficiency.1 In recent years, materials called perovskites have been garnering significant attention as possible replacements for silicon cells due to their favorable optoelectronic properties.2 However, the most widely researched perovskite, lead-halide perovskites, have obvious problems with stability.3 This instability, coupled with their use of lead, can lead to the leaching of lead into the environment. Several ideas have been proposed to mitigate this problem including better encapsulation of the lead halide perovskite, substitution of lead with other elements, and non-perovskite structures. In this work, we explore the synthesis and ion exchange of ternary bismuth chalcogenides with the goal of creating a split-anion perovskite.4 To accomplish this, we first synthesize inorganic bismuth chalcogenide ABiS2 nanocrystals, where A is a +1 cation. Following the synthesis of these nanocrystals, we suspend them in solution and add trimethylsilyliodide (TMSI) which reacts with the sulfur in the nanocrystal replacing it with iodide. By modulating the amount of TMSI in the reaction, we believe we can create a perovskite-like nanocrystal that incorporates nanocrystals with anions of differing oxidation states, enabling a greater variety of elements that could adopt the perovskite phase. In this work we focused on crystals where A = Ag+ and Cs+, with which we were able to demonstrate the ion exchange.

Made available in DSpace on 2014-12-17T15:15:00Z (GMT). No. of bitstreams: 1 CarlosHOC_TESE.pdf: 15589268 bytes, checksum: 5a0a25bd59fcc76f4d53ba73163991d0 (MD5) Previous issue date: 2013-12-12 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / In this paper we investigate the spectra of band structures and transmittance in magnonic quasicrystals that exhibit the so-called deterministic disorders, specifically, magnetic multilayer systems, which are built obeying to the generalized Fibonacci (only golden mean (GM), silver mean (SM), bronze mean (BM), copper mean (CM) and nickel mean (NM) cases) and k-component Fibonacci substitutional sequences. The theoretical model is based on the Heisenberg Hamiltonian in the exchange regime, together with the powerful transfer matrix method, and taking into account the RPA approximation. The magnetic materials considered are simple cubic ferromagnets. Our main interest in this study is to investigate the effects of quasiperiodicity on the physical properties of the systems mentioned by analyzing the behavior of spin wave propagation through the dispersion and transmission spectra of these structures. Among of these results we detach: (i) the fragmentation of the bulk bands, which in the limit of high generations, become a Cantor set, and the presence of the mig-gap frequency in the spin waves transmission, for generalized Fibonacci sequence, and (ii) the strong dependence of the magnonic band gap with respect to the parameters k, which determines the amount of different magnetic materials are present in quasicrystal, and n, which is the generation number of the sequence k-component Fibonacci. In this last case, we have verified that the system presents a magnonic band gap, whose width and frequency region can be controlled by varying k and n. In the exchange regime, the spin waves propagate with frequency of the order of a few tens of terahertz (THz). Therefore, from a experimental and technological point of view, the magnonic quasicrystals can be used as carriers or processors of informations, and the magnon (the quantum spin wave) is responsible for this transport and processing / Neste trabalho investigamos espectros de estruturas de banda e de transmit?ncia em quasicristais magn?nicos que apresentam as chamadas desordens determin?sticas, especificamente, sistemas de multicamadas magn?ticas que s?o constru?dos obedecendo as sequ?ncias substitutionais de Fibonacci generalizada (apenas os casos golden mean (GM), silver mean (SM), bronze mean (BM), copper mean (CM) e nickel mean (NM)) e k-componente de Fibonacci. O modelo te?rico ? baseado no hamiltoniano de Heisenberg para o regime de troca, juntamente com o poderoso m?todo da matriz transfer?ncia, e levando em conta a aproxima??o RPA. Os materiais magn?ticos considerados s?o ferromagnetos c?bicos simples. O principal interesse deste estudo ? investigar o efeito da quasi-periodicidade nas propriedades f?sicas dos sistemas citados analisando o comportamento da propaga??o de ondas de spin por meio dos espectros de dispers?o e de transmiss?o dos magnons nestas estruturas. Entre os resultados destacamos: (i) a fragmenta??o das bandas de volume que, no limite de altas gera??es, se tornam conjuntos de Cantor, e a presen?a da frequ?ncia de mid-gap na transmit?ncia das ondas de spin, na sequ?ncia de Fibonacci generalizada; e (ii) a forte depend?ncia do band gap magn?nico com rela??o aos par?metros k, que determina a quantidade de materiais magn?ticos diferentes presentes no quasi-cristal, e n, que ? o n?mero da gera??o da sequ?ncia k-componente de Fibonacci. Neste ?ltimo caso, verificamos que o sistema apresenta uma banda magn?nica proibida, cuja largura e regi?o de frequ?ncia podem ser controladas variando k e n. No regime de troca, as ondas de spin propagam-se com frequ?ncia da ordem de algumas dezenas de terahertz (THz). Portanto, do ponto de vista experimental e tecnol?gico, os quasi-cristais magn?nicos podem ser utilizados como transportadores ou processadores de informa??es, sendo o magnon (o quantum da onda de spin) o respons?vel por esse transporte e processamento

This thesis studies a new class of high gain planar resonant cavity antennas based on metamaterial surfaces. High-gain planar antennas are becoming increasing popular due to their significant advantages (e.g. low profile, small weight and low cost). Metamaterial surfaces have emerged over the last few years as artificial structures that provide properties and functionalities not readily available from existing materials. This project addresses novel applications of innovative metamaterial surfaces on the design of high-gain planar antennas. A ray analysis is initially employed in order to describe the beamfonning action of planar resonant cavity antennas. The phase equations of resonance predict the possibility of low-profile/subwavelength resonant cavity antennas and tilted beams. The reduction of the resonant cavity profile can be obtained by virtue of novel metamaterial ground planes. Furthermore, the EBG property of metamaterial ground planes would suppress the surface waves and obtain lower backlobes. By suppressing the TEM mode in a resonant cavity, a novel aperture-type EBG Partially Reflective Surface (PRS) is utilized to get low sidelobes in both planes (E-plane and H-plane) in a relatively finite structure. The periodicity optimization of PRS to obtain a higher maximum directivity is also investigated. Also it is shown that antennas with unique tilted beams are achieved without complex feeding mechanism. Rectangular patch antennas and dipole antennas are employed as excitations of resonant cavity antennas throughout the project. Three commercial electromagnetic simulation packages (Flomerics Microstripes ™ ver6.S, Ansoft HFSSTM ver9.2 and Designer ™ ver2.0) are utilized during the rigorous numerical computation. Related measurements are presented to validate the analysis and simulations.

The work presented in this thesis is concerned with the propagation of acoustic waves through phononic crystal systems and their ability to attenuate sound in the low frequency regime. The plane wave expansion method and finite element method are utilised to investigate the properties of conventional phononic crystal systems. The acoustic band structure and transmission measurements of such systems are computed and verified experimentally. Good agreement between band gap locations for the investigative methods detailed is found. The well known link between the frequency range a phononic crystal can attenuate sound over and its lattice parameter is confirmed. This leads to a reduction in its usefulness as a viable noise barrier technology, due to the necessary increase in overall crystal size. To overcome this restriction the concept of an acoustically resonant phononic crystal system is proposed, which utilises acoustic resonances, similar to Helmholtz resonance, to form additional band gaps that are decoupled from the lattice periodicity of the phononic crystal system. An acoustically resonant phononic crystal system is constructed and experimental transmission measurements carried out to verify the existence of separate attenuation mechanisms. Experimental attenuation levels achieved by Bragg formation and resonance reach 25dB. The two separate attenuation mechanisms present in the acoustically resonant phononic crystal, increase the efficiency of its performance in the low frequency regime, whilst maintaining a reduced crystal size for viable noise barrier technology. Methods to optimise acoustically resonant phononic crystal systems and to increase their performance in the lower frequency regime are discussed, namely by introducing the Matryoshka acoustically resonant phononic crystal system, where each scattering unit is composed of multiple concentric C-shape inclusions.

This thesis reports the study of metal oxide semiconductors for the application of photoelectrochemical water splitting with a particular emphasis on both anion and cation-doped zinc oxides. A study of the mechanisms of visible light absorption in both anion and cation-doped ZnO semiconductors, the potentials of metal oxide materials modified by impurities as one of the ideal photocatalysts in harvesting solar light has been explored. X-ray photoelectron spectroscopy (XPS) and UV-Vis spectroscopes have been performed to establish the electronic structures of anion and cation-doped ZnO. Aluminium impurities in ZnO thin films reveal the relationship between the bandgap broadening and the so-called Burstein Moss effect. Both cadmium and sulphur dopants were incorporated in ZnO either as powders by the solid state synthesis or as thin films by spray pyrolysis technique. Cadmium and sulphur dopants demonstrate effective electronic bandgap reduction and an increasing absorption of visible light. Furthermore, the incorporation of cadmium and sulphur in ZnO were prepared as photoanodes and evaluated in a custom-built photoelectrochemical workstation for the measurement of photon energy conversion efficiencies.

The demand for wide bandgap semiconductors for radiation detector applications has significantly increased in recent years due to an ever-growing need for safeguard measures and medical imaging systems amongst other applications. The need for these devices to be portable and efficient, and to operate at room temperature is important for practical applications. For radiation detectors, the semiconductor materials are mainly required to have an optimal energy gap, high average atomic number, good electrical resistivity and charge transport properties as well as purity and homogeneity. Cadmium zinc telluride (CZT) distinctly stands out among the other choices of semiconductor materials for radiation detector applications, due to its attractive material properties and the room temperature operation possibility. A tremendous amount of research is being conducted to improve CZT technology and its implementation into more commercial systems. Applications of CZT detector technology in national security, high energy physics, nuclear spectroscopy, and medical imaging systems are of special interests. However, CZT devices still face challenges that need to be understood and overcome in order to have more efficient radiation detector systems. One such challenge lies in the understanding of the surfaces of CZT detectors and surface recombination effects on charge transport, charge collection efficiency, and detector performance. Another common issue is the degradation of CZT detectors due to the presence of defects which can act as traps for the charge carriers and cause incomplete charge collection from the detectors. Thus, a major challenge is that, the commercial CZT crystals have large concentrations of defects and impurities that need to be characterized, and their effects on the detector performance should be studied. Photoluminescence (PL) spectroscopy is a sensitive, non-contact and non-destructive method, suitable to characterize lower concentrations of point defects, such as substitutional impurities (donors, acceptors) and native defects in CZT crystals. A PL spectrum provides information regarding the defect nature of the crystal by determining the presence and the type of vacancies, interstitials, and impurities in the lattice. The main objective of this thesis is to address the presence of the defects in CZT crystals, identify their types, and study their roles in the performance of x-ray radiation detectors using PL spectroscopy. Additionally, using PL method and different excitation sources including UV excitation, this thesis studies the surface of CZT samples and investigates the PL signature of the surface oxide of the samples, in an effort to optimize the surface processing and thereby improve CZT detector performance. / Graduate

Ce manuscrit porte sur la conception d’un transistor à effet de champ destiné à l’observation de la photoluminescence de l’exciton et des complexes excitoniques chargés du ZnO sous l’influence d’un champ électrique. Pour cela, des simulations ont permis de définir un cahier des charges de la structure du transistor afin de bloquer la conductivité dans le canal de ZnO et d’appliquer un champ électrique intense. La seconde partie concerne le choix du matériau de grille et de l’électrode transparente de surface pour l’observation de la photoluminescence dans le canal. L’oxyde de gallium (-Ga2O3) a été choisi car il présente un grand gap, des propriétés d’isolant et de semi-conducteur avec dopage. Cependant les films de Ga2O3 dopés avec Ti, Sn, Zn et Mg élaborés par MOCVD n’ont pas révélé de conductivité. Les films d’alliages (Ga,Sn)2O3 n’ont pas non plus montré de conductivité et leur structure est étudiée intensivement. Des traitements plasma radiofréquence sous flux d’argon, d’oxygène ou d’hydrogène ont permis de montrer que l’implantation de l’hydrogène donne lieu à un niveau donneur avec une énergie d’activation de 7 meV. La conductivité est toutefois modulée par le dopage en Sn et les traitements s’accompagnent d’un changement de la sous-stœchiométrie en oxygène qui diminue la transparence à cause de la formation de niveau profond de lacune d’oxygène. La structure finale de la grille transparente dans l’ultraviolet pour l’observation de la photoluminescence du ZnO peut donc être élaborée par une grille diélectrique de -Ga2O3 puis une électrode conductrice transparente de (Ga,Sn)2O3 traitée superficiellement par un plasma d’hydrogène. / This manuscript covers the design of a field transistor for the observation of photoluminescence of the exciton and the charged excitonic complex of ZnO under the influence of an electric field. For this, simulations have helped to define the specifications of the transistor structure to block the conductivity in the ZnO channel and applying a strong electric field. The second part concerns the choice of gate material and the surface transparent electrode for the observation of photoluminescence in the channel. The gallium oxide (-Ga2O3) was chosen because it has a large gap, insulating properties and semiconductor properties with doping. However, Ga2O3 films doped with Ti, Sn, Zn and Mg MOCVD did not show conductivity. Films of alloys (Ga,Sn)2O3 have not shown either conductivity and their structure is studied intensively. Radio frequency plasma treatment under a flux of argon, oxygen or hydrogen have shown that implantation of hydrogen gives rise to a donor level with 7 meV activation energy. However, the conductivity is modulated by doping Sn and treatments are accompanied by a change of sub-stoichiometry in oxygen, which reduces the transparency due to the formation of deep level of oxygen vacancy. The final structure of the transparent gate in the ultraviolet for the observation of photoluminescence of ZnO can be prepared by a dielectric gate -Ga2O3 and a transparent conductive electrode of (Ga,Sn)2O3 surface treated by a plasma of hydrogen.

Material science is an interdisciplinary area of research, which in part, designs and characterizes new materials. Research is concerned with synthesis, structure, properties, and performance of materials. Discoveries in materials science have significant impact on future technologies, especially in nano-scale applications where the physical properties of nanomaterials are significantly different than their bulk counterparts. The work presented here discusses the use of ferritin, a hollow sphere-like biomolecule, which forms metal oxo-hydride nanoparticles inside its protein shell for uses as a bio-inorganic material.Ferritin is capable of forming and sequestering 8 nm metal-oxide nanoparticles within its 2 nm thick protein shell. A variety of metal-oxide nanoparticles have been synthesized inside ferritin. The work herein focuses on three distinct areas:1) Ferritin's light harvesting properties: namely band gaps. Discrepancies in the band gap energies for ferritin's native ferrihydrite mineral and non-native minerals have been previously reported. Through the use of optical absorption spectroscopy, I resolved the types of band gaps as well as the energy of these band gaps. I show that metal oxides in ferritin are indirect band gap semiconductors which also contain a direct transition. Modifications to the ferrihydrite mineral's band gaps are measured as a result of co-depositing anions into ferritin during iron loading. I demonstrate that these band gaps can be used to photocatalytically reduce gold ions in solution with titanium oxide nanoparticles in ferritin. 2) A new method for manganese mineral synthesis inside ferritin: Comproportionation between permanganate and Mn(II) forms new manganese oxide minerals inside ferritin that are different than traditional manganese oxide mineral synthesis. This reaction creates a MnO2, Mn2O3, or Mn3O4 mineral inside ferritin, depending on the synthesis conditions. 3) Ferritin as an energetic material: Ferritin is capable of sequestering various metals and anions into its interior. Perchlorate, an energetic anion, is sequestered through a co-deposition process during iron loading and is tested with energetic binding materials. Peroxide, which can be used as an oxidant, is also shown to be sequestered within apoferritin and combined with an aluminum based fuel for solid rocket propellants.