Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-02-02Bitstream added on 2014-06-13T18:50:38Z : No. of bitstreams: 1 leite_dmg_me_bauru.pdf: 1278982 bytes, checksum: f2b696ae90b2738246ee3c7750b37751 (MD5) / A recente descoberta de propriedades ferromagnéticas em alguns semicondutores magnéticos diluídos (DMS) trouxe a esta classe de materiais um grande potencial para aplicações em dispositivos de controle de spin. Um DMS é basicamente formado por um semicondutor dopado por íons magnéticos, os quais têm o papel de criar um momento magnético local e também, em algumas situações, de introduzir portadores livres no material. Entre os DMSs conhecidos, o 'GA IND.1-x'MN IND.XN' surge como o mais forte candidato a aplicações práticas por apresentar até o momento a mais alta temperatura de transição ferromagnética ('T IND.C' 'DA ORDEM DE' 400 k). Até o presente, os filmes de 'GA IND.1-x'MN IND.XN' com propriedades ferromagnéticas relatados na literatura foram preparados por epitaxia por feixe molecular (MBE). Neste trabalho, descrevemos a preparação de filmes nanocristalinos de 'GA IND.1-x'MN IND.XN' com diferentes conteúdos de Mn (0,00 'MENOR' x 'MENOR' 0,18) pela técnica de RF-magnetron sputtering reativo. Analisamos os efeitos da incorporação de Mn na estrutura e nas propriedades ópticas destes filmes através de medidas de difração de raios-X e de absorção óptica entre o ultravioleta (6,5 eV) e infravermelho próximo (1,4 eV). Os resultados apontam um aumento do parâmetro de rede e do índice de refração, uma diminuição do gap ótico e um aumento da densidade de estados de defeitos no interior do gap conforme se aumenta o conteúdo de Mn nos filmes de 'GA IND.1-x'MN IND.XN' preparados por sputtering. Estes resultados são semelhantes aos reportados para a incorporação de Mn em filmes monocristalinos de 'GA IND.1-x'MN IND.XN' com propriedades ferromagnéticas preparados por MBE. / The recent discoveries related to the ferromagnetic properties in some diluted magnetic semiconductors (DMS) have attracted considerable attention on this class of material due to their potential application on spin control devices. A DMS is basically formed by a semiconductor doped with magnetic ions with the purpose of creating local magnetic moments and, in some situations, to introduce free carriers in the material. Among the known DMSs, 'GA IND.1-x'MN IND.XN' is the one with the highest ferromagnetic transition temperature ('T IND.C' 'DA ORDEM DE' 400 k), and it is consequently on of the stronger candidates for practical applications. Until now, the 'GA IND.1-x'MN IND.XN' films with ferromagnetic properties described in the literature were prepared by molecular beam epitaxy (MBE). In this work, we report the preparation of nanocrystalline 'GA IND.1-x'MN IND.XN' films (0,00 'MENOR' x 'MENOR' 0,18) by reactive RF-magnetron sputtering technique. We analyzed the Mn incorporation effects on structure and optical properties of the films by X-ray diffraction measurements and optical absorption between UV (6,5 eV) and near infrared (1,4 eV). The results show the increase of the lattice parameters and of the refractive index, a decrease of the optical gap and a increase of defect states in the gap when Mn concentration is increased. These results are similar to those reported for Mn incorporation in monocrystalline 'GA IND.1-x'MN IND.XN' films prepared by MBE with ferromagnetic properties.

Orientador: José Humberto Dias da Silva / Banca: Jair Scarminio / Banca: Alexys Bruno Alfonso / Banca: Pascoal José Giglio Pagliuso / Banca: Luis Vicente de Andrade Scalvi / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: A recente busca por semicondutores magnéticos diluídos com propriedades magnéticas de interesse motivou este trabalho de crescimento de filmes de Ga1-xMnxN pelas técnicas de sputtering e epitaxia da fase de vapor de organometálicos (MOVPE). Os filmes são caracterizados estruturalmente por medidas de difração de raio X e microscopia eletrônica de transmissão, opticamente por transmitância óptica e espalhaçamento Raman, e magneticamente por medidas magnetização versus campo aplicado e versus temperatura. As principais diferenças entre os filmes de GaMnN preparados por sputerring e MOVPE referem-se à microestrutura e ao conteúdo de Mn: os primeiros são policristalinos e apresentam conteúdo de Mn até 9%, enquanto os últimos são monocristalinos com concentração de Mn até 1%. A alta concentraçao de Mn nos filmes crescidos por sputtering é possivelmente responsável pelo surgimento coletivo destes íons de Mn nas medidas magnéticas. Esse comportamento coletivo se identifica a partir de contribuição paramagnética de domínios isolados com alto vapor de momento magnético, o que se mostra consistente com a microestrutura apresentada por estes filmes. A alta concentração de Mn nos filmes preparados por sputtering também se mostra responsável por intensa absorção óptica abaixo da energia do gap, sendo esta relacionada a transições eletrônicas entre os estados localizados do Mn e as bandas de valência e condução do GaN. O contraste entre as propriedades dos filmes de GaMnN produzidos por sputtering e por MOVPE possibilita então um entendimento mais abrangente dos aspectos da incorporação de Mn no GaN e suas respectivas características estruturais, ópticas e magnéticas. Esse entendimento é importante para delinear a otimização deste material visando propriedades magnéticas de interesse / Abstract: The current search for dilluted magnetic semiconductors with interesting magnetic properties has motivated the present work on growing GaMnN films by sputerring and metalorganic vapor phase epitaxy (MOVPE) techniques. The films are characterized by X-ray diffraction, transmission electron microscopy, optical transmission, Raman scattering, and by magnetization measurements. The main differece between the GaMnN grown by sputtering and those grown by MOVPE relates to their microstructure (polycrystalline/monocrystalline) and Mn content (up to 9%/1% respectively). The high Mn content in GaMnN samples grown by sputtering is probably responsible for a collective response on the magnetic measurements. This collective Mn response is identified as a high magnetic moment contribution which is consistent with sample microstructure. In the sputtered samples, the high Mn content is also responsible for strong subbandgap optical absorption related to eletronic transitions involving Mn states and the valence and conduction bands of GaN. The comparison between the properties of GaMnN films grown by different techniques is important in order to get a better understanding about the Mn incorporation in GaN. This understanding been important to define the next steps regarding the optimization of this material / Doutor

Submitted by Maria Medeiros (maria.dilva1@ufcg.edu.br) on 2018-06-25T21:09:22Z No. of bitstreams: 1 LUCIUS VINICIUS ROCHA MACHADO -TESE (PPGCEMat) 2015.pdf: 3378243 bytes, checksum: 8988e719f1f296de2f74c587ed4f5ba8 (MD5) / Made available in DSpace on 2018-06-25T21:09:22Z (GMT). No. of bitstreams: 1 LUCIUS VINICIUS ROCHA MACHADO -TESE (PPGCEMat) 2015.pdf: 3378243 bytes, checksum: 8988e719f1f296de2f74c587ed4f5ba8 (MD5) Previous issue date: 2015-12-18 / Esse trabalho teve como objetivo, avaliar a influência do tipo de precursor, fonte de íons de ferro, e sua concentração na dopagem do sistema Zn1-xFexO de modo a se obter um produto com ferrimagnetismo a temperatura ambiente para uso como semicondutor magnético diluído. Para esse fim, inicialmente avaliou-se a influência do tipo de precursor (nitrato de ferro III, sulfato de ferro II e acetato de ferro II) sobre a estrutura, morfologia, propriedades térmicas e magnéticas do sistema Zn1-xFexO com concentração de íons de Fe2+ e Fe3+ de 0,4 mol. Posteriormente avaliou-se o efeito da concentração de íons de ferro III variando de 0,05 a 0,4 mol sobre a estrutura e magnetismo do sistema Zn1- xFexO. Durante as reações para obtenção do produto foram feitas medições de temperatura e do tempo de reação. As amostras foram caracterizadas por: difração de raios X, análise química por fluorescência de raios X por energia dispersiva, microscopia eletrônica de varredura com mapeamento por EDS, distribuição granulométrica, análise por adsorção de nitrogênio, magnetometria de amostra vibrante e análise termogravimétrica. Os resultados mostraram que o tipo de precursor influenciou diretamente na estrutura, morfologia e magnetismos das amostras, sendo o precursor nitrato de ferro III o que possibilitou à formação de um material ferrimagnético a temperatura ambiente. Para as amostras dopadas, os espectros de DRX mostraram que até a concentração de 0,20 mol de íons ferro III resultou num sistema monofásico com comportamento ferrimagnético à temperatura ambiente, o que caracterizou a formação de um semicondutor magnético diluído. Para demais concentrações foi observado traços da fase FeFe2O4 e que às interações de troca entre os íons Fe - Fe e possivelmente o aumento da concentração de vacância de oxigênio na rede do ZnO suprimiu o comportamento ferrimagnético pela competição do comportamento ferrimagnético/paramagnético. Portanto, pode-se concluir que o precursor nitrato de ferro III com concentração de até 0,20 mol foi a melhor condição para obtenção de produto com característica para uso como semicondutor magnético diluído usando a técnica de síntese por reação de combustão. / The objective of this study is to evaluate the influence the type of precursor, source of iron ions, and its concentration in the doping Zn1-xFexO system in order to obtain a product with ferromagnetism at room temperature for use as magnetic semiconductor diluted. For this purpose, it was firstly evaluated the influence of the type of precursor (iron III nitrate, iron sulfate II, iron acetate II) on the structure, morphology, thermal and magnetic properties of Zn1-xFexO system concentration of Fe2+ and Fe3+ ions of 0.4 mol. After that, it was evaluated the effect of concentration of iron III ions ranging from 0.05 to 0.4 mol on the structure and magnetism of Zn1-xFexO system. During the reactions, there were made measurements of temperature and time. The samples were characterized by: X-ray diffraction, chemical analysis by fluorescence X-ray energy dispersive, scanning electron microscopy, with mapping by EDS, particle size analysis, analysis by nitrogen adsorption, vibrating sample magnetometer and thermal gravimetric analysis. The results have shown that the type of precursor influenced directly the structure, morphology and magnetism of the samples and the precursor of iron nitrate III was the one which favored the obtention of the ferromagnetism material monophasic at room temperature. For the doped samples, the XRD spectra showed that the concentrations until 0.20 mol of iron III ions resulted in a monophasic system with ferromagnetic behavior at room temperature, which characterized the formation of a diluted magnetic semiconductor. For the other concentrations, it was observed traces of MnFe2O4 phase and that the exchange interactions between the ionsFe - Fe and possibly the increasing of oxygen vacancy concentration in ZnO network suppressed the ferromagnetic behavior by the competition of ferromagnetic / paramagnetic one. Therefore, it can be concluded that the precursor of iron III nitrate concentration to 0.20 mol was the best condition for obtaining a product with characteristics for use as a dilute magnetic semiconductor using the synthetic technique by combustion.

66 f. / Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2013-04-05T15:41:01Z No. of bitstreams: 1 Dissertação_definitiva_Taiana Tarantino.pdf: 1712573 bytes, checksum: 77c5b3c337efd24e474e48df29eef98a (MD5) / Rejected by Ana Hilda Fonseca(anahilda@ufba.br), reason: tirar tese e colocar dissertação on 2013-06-06T12:41:22Z (GMT) / Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2013-06-06T12:43:14Z No. of bitstreams: 1 Dissertação_definitiva_Taiana Tarantino.pdf: 1712573 bytes, checksum: 77c5b3c337efd24e474e48df29eef98a (MD5) / Rejected by Ana Hilda Fonseca(anahilda@ufba.br), reason: trocar tese por dissertação on 2013-06-06T15:42:43Z (GMT) / Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2013-06-06T15:44:50Z No. of bitstreams: 1 Dissertação_definitiva_Taiana Tarantino.pdf: 1712573 bytes, checksum: 77c5b3c337efd24e474e48df29eef98a (MD5) / Approved for entry into archive by Ana Hilda Fonseca(anahilda@ufba.br) on 2013-06-07T14:45:02Z (GMT) No. of bitstreams: 1 Dissertação_definitiva_Taiana Tarantino.pdf: 1712573 bytes, checksum: 77c5b3c337efd24e474e48df29eef98a (MD5) / Made available in DSpace on 2013-06-07T14:45:02Z (GMT). No. of bitstreams: 1 Dissertação_definitiva_Taiana Tarantino.pdf: 1712573 bytes, checksum: 77c5b3c337efd24e474e48df29eef98a (MD5) Previous issue date: 2012 / CAPES / O arroz (Oryza sativa) é um dos cereais mais produzidos e consumidos, sendo o alimento básico na dieta de mais da metade da população mundial. A digestão ácida assistida por radiação micro-ondas usando ácidos diluídos vem sendo bastante utilizada, principalmente, por reduzir a concentração ácida mantendo a eficiência da digestão, gerando menor volume de resíduo e diminuindo o fator de diluição antes da análise. A determinação dos teores de nutrientes é de grande relevância devido à capacidade nutricional do arroz, que pode conter espécies essenciais ou contaminantes a saúde humana. O objetivo deste trabalho foi propor um procedimento de digestão com HNO3 diluído empregando forno de micro-ondas com cavidade para determinação de macro, micro e elementos traço (As, Ca, Cd, Fe, K, Mg, Mn, Mo, P, Se, Zn) em amostras de arroz por espectrometria de emissão óptica com plasma indutivamente acoplado (ICP OES) e espectrometria de massas com plasma indutivamente acoplado (ICP-MS). Foi realizado um planejamento fatorial 32 e Box-Behnken e as variáveis investigadas foram: volume de HNO3 destilado, temperatura e tempo, ambos no último patamar do programa de aquecimento. As condições otimizadas foram 2 mL de HNO3 e temperatura de 200 ºC durante o tempo de 20 minutos. O volume de H2O2 30%, a massa e a granulometria da amostra foram fixados em 1,0 mL, 250 mg e 500 m, respectivamente. A exatidão foi avaliada com o material de referência certificado NIST 1568a e do teste de recuperação para os analitos investigados, que variaram entre 80,1 e 102,1 % e 96,6 e 109,8 %, respectivamente. A precisão foi verificada pelo desvio padrão relativo, sendo sempre inferior a 10%, inclusive no teste de repetitividade. O procedimento proposto foi aplicado na determinação dos elementos em 23 amostras de arroz submetidas a três tipos de processamentos (polido, parboilizado e integral). As faixas de concentração para os elementos traço, em μg g-1, foram: Al (<2,4-15,3); As (<0,2-0,32); Cd (<0,004–0,017); Cu (0,2-2,2); Fe (<0,2- 17,4); Mn (2,8-34,9); Mo (<0,20-0,81); Se (<0,2-0,271) e Zn (4,8-14,9). De modo geral, os resultados obtidos encontram-se concordantes com as faixas de concentração descritas na literatura. / Salvador

Nous avons étudié la dynamique de spin d'un atome de Mn inséré dans une boite quantique CdTe. Nos résultats montrent que la relaxation de spin du Mn est plus rapide lorsque la boite quantique contient un exciton. Ceci peut permettre une orientation optique du spin du Mn. Le mélange de bande de valence est le paramètre essentiel permettant la relaxation rapide du spin du Mn dans le champ d'échange de l'exciton. Ce mélange de bande de valence est controlé par la forme et les contraintes dans la boite quantique. L'influence de ces paramètres sur la dynamique du pompage optique a été analysée en détail. Nos simulations du pompage optique sont en bon accord avec les expériences. La dynamique cohérente d'un Mn individuel a aussi été étudiée. L'influence sur le pompage optique de la dynamique cohérente du spin électronique et nucléaire est discutée. Nous avons montré que le couplage entre spin électronique et nucléaire peut être contrôlé optiquement permettant une manipulation du spin du Mn. Nous avons finalement montré que la combinaison d'une excitation résonante optique et micro-onde peut être utilisée pour détecter optiquement la résonance magnétique d'un Mn dans une boite quantique CdTe. / We have studied the spin dynamics of an individual Mn atom embedded a CdTe quantum dot. Our results show that the Mn spin relaxation is faster when the quantum dot contains an exciton. This can result in an optical orientation of the Mn spin. The valence band mixing is the critical parameter for the fast relaxation rates of the Mn spin in the exchange field of the exciton. This valence band mixing is controlled by the shape and strain of the quantum dot. The influence of these parameters on the optical pumping dynamics were analyzed in detail. Our simulation of optical pumping are in good agreement with experiments. The coherent dynamics of an individual Mn spin was also investigated. We discussed the influence of the coherent dynamics of the coupled electronic and nuclear spins on the optical pumping. We have shown that optically controlled coupling between electronic and nuclear spins could be used for Mn spin switching. We finally demonstrated that the combination of resonant laser and microwave fields can be used to optically detect the magnetic resonance of a Mn spin in a CdTe quantum dot.

Cette thèse se situe dans le cadre du projet CANOE, piloté par GDF SUEZ et l'ADEME, qui vise à étudier la faisabilité et la viabilité du régime de combustion sans flamme dans les chaudières industrielles. Il est maintenant établi que le préchauffage des réactifs permet d'améliorer le rendement thermique et de diminuer la consommation de combustible d'une configuration. Pour contourner la formation d'oxydes d'azote résultant de l'augmentation de température des réactifs, ces derniers peuvent être massivement dilués par des produits de combustion. Cela permet d'éviter la formation de points chauds et d'homogénéiser les gradients de température: c'est le principe de la combustion sans flamme. L'objectif de cette thèse est de développer un modèle de combustion turbulente adapté à ce type de régime. La cinétique chimique complexe et le contrôle des pertes thermiques est d'une importance capitale dans l'établissement et la stabilisation du processus de combustion sans flamme. Ici, ces effets sont considérés dans une approche de tabulation de la cinétique chimique de type FPV. Pour discriminer les effets associés aux évolutions suivant les paramètres de contrôle de nos bases de données, on effectue une analyse des réponses de flammes laminaires à différents niveaux de dilution et de pertes thermiques. De plus, nous évaluons l'importance de l'utilisation de tabulations de dimensions élevées, et les capacités prédictives des méthodes de tabulation développées sont mises en exergue. Puis, des simulations aux grandes échelles de la turbulence de configurations adiabatique et à parois refroidies sont effectuées. On compare des tabulations de nombre de degrés de libertés variés aux données expérimentales. Les résultats obtenus sont en très bon accord avec ces dernières pour les tabulations les plus complexes, alors que des limitations significatives apparaissent pour des tabulations de dimensions inférieures. Les simulations proposées indiquent la capacité de nos modèles à reproduire des structures de flammes réalistes. / This thesis is within a framework of the CANOE project, under the responsibility of GDF SUEZ and the ADEME, and aims at studying the feasibility of the flameless combustion regime in industrial boilers. It is now well established that reactants preheating allows an improvement on thermal efficiency as well as fuel savings. To avoid an increase in nitrogen oxides emissions arising from reactants temperature augmentation, massive dilution of the reactants by burnt gases can be used. While doing so, hot spots are averted and temperature gradients are smoothed: it’s the principle of flameless combustion. Even though this combustion regime is a subject of increasing interest to the industry, its mechanisms are not yet fully understood. The objective of this work is to develop and validate a new turbulent combustion model adapted to this kind of regimes. Complex chemistry as well as heat losses control is of paramount importance in the establishment and stabilization of the flameless combustion process. Here, these effects are taken into account in an FPV-type chemistry tabulation approach. To discriminate the effects associated with evolutions along the database control parameters, we analyze laminar flame responses to different levels of dilution and heat losses. Moreover, we assess the importance of high-order tabulations, and predictive capabilities of our approaches are highlighted. Then, large-eddy simulations of laboratory scale experiments (an adiabatic and a cooled wall configuration) are carried out. To this end, we compare databases featuring different number of dimensions to experimental data on temperature and species distributions available in the literature. Simulation results are in very good agreement with experimental data for complex tabulations, while discrepancies arise for lower order tabulations. Simulation results show that flameless combustion features a wide variety of flame structures, and that our models are able to tackle realistic flame structures.

Neste trabalho e apresentado um estudo, via teoria de massa efetiva multibanda autoconsistente de heteroestruturas de semicondutores magnéticos diluídos, generalizada para incluir parâmetros de diferentes materiais. A interacao magnética e descrita por um modelo de campo médio baseado no mecanismo de troca indireta, com a possibilidade de inclusão de diferentes íons magnéticos. As equacoes de massa efetiva são resolvidas de forma autoconsistente com o auxílio da equacao de Poisson. As interacoes de spin-órbita e de troca-correlacao, na aproximacao de densidade local, são incluídas no cálculo. O método e aplicado para o estudo das estruturas de bandas e densidades de carga com separacao por spin do portador de heteroestruturas com dopagem tipo-n e tipo-p, variando a geometria dos pocos magnéticos e também o período da super-rede, as densidades de portadores e as concentracoes de íons magnéticos. Solucoes autoconsistentes da equacao de massa efetiva são encontradas para o oxido semicondutor (Zn,Co)O. Será mostrada a separacao de portadores por spin em funcao dos parâmetros variados, simulando diversas concentracoes possíveis, utilizadas em sistemas descritos na literatura, e será analisado o comportamento dos perfis de potencial. Usando os dados obtidos, um diagrama de fases será traçado com base na polarizacao total ou parcial dos portadores, e o seu comportamento será discutido. Também serão mostradas as estruturas de bandas, os perfis de potencial e as distribuicoes de carga do semicondutor (GaMn)As, variando as densidades de portadores e a direcao do campo magnético intrínseco, gerado pela dopagem com íons magnéticos. Os resultados obtidos neste trabalho podem servir de guia para futuras experiências e para o desenvolvimento de dispositivos com semicondutores magnéticos diluídos baseados em (Zn,Co)O e (Ga,Mn)As. Os métodos aqui descritos são gerais e podem ser utilizados para outros materiais. / This work presents a self-consistent multiband effective mass theory applied to diluted magnetic semiconductor heterostructures, generalized to include parameters of different ma- terials. The magnetic interaction is described by a mean-field approximation based on indirect- exchange mecanism, with the possibility of inclusion of different magnetic ions. The effective mass equations are solved self-consistently with the help of the Poisson equation. Spin-orbit and exchange-correlation interactions are included in the simulation in the local density appro- ximation. The method is used to study band structures and charge densities separated by spin in n- and p-type heterostructures. The magnetic well\'s geometry, the superlattice period, the carrier density and the magnetic ion concentration are changed. Self-consistent solutions of the effective mass equation are found for the semiconductor oxide (Zn,Co)O. Charge separation by spin will be show in function of the variation of the simulation parameters, simulating several ion concentrations and charge densities used in systems described in literature, and the potenti- als profiles will be analised. Using the data obtained a phase diagram will be plotted, based on the carrier total or partial carrier polarization, and a model for the behavior of the phase diagram will be discussed. It will also be shown band structures, potential profiles and charge densities of the (Ga,Mn)As semiconductor, varying it carrier density and the direction of the intrinsic magnetic field, generated by the magnetic ions that doped the heterostructure. The results ob- tained in this work can be used as a guide in future experiences and development of devices with diluted magnetic semiconductors based on (Zn,Co)O and (Ga,Mn)As. The methods here described are general and can be used for other materials.

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Nous avons étudié la dynamique de spin d'un atome de Mn inséré dans une boite quantique CdTe. Nos résultats montrent que la relaxation de spin du Mn est plus rapide lorsque la boite quantique contient un exciton. Ceci peut permettre une orientation optique du spin du Mn. Le mélange de bande de valence est le paramètre essentiel permettant la relaxation rapide du spin du Mn dans le champ d'échange de l'exciton. Ce mélange de bande de valence est controlé par la forme et les contraintes dans la boite quantique. L'influence de ces paramètres sur la dynamique du pompage optique a été analysée en détail. Nos simulations du pompage optique sont en bon accord avec les expériences. La dynamique cohérente d'un Mn individuel a aussi été étudiée. L'influence sur le pompage optique de la dynamique cohérente du spin électronique et nucléaire est discutée. Nous avons montré que le couplage entre spin électronique et nucléaire peut être contrôlé optiquement permettant une manipulation du spin du Mn. Nous avons finalement montré que la combinaison d'une excitation résonante optique et micro-onde peut être utilisée pour détecter optiquement la résonance magnétique d'un Mn dans une boite quantique CdTe. / We have studied the spin dynamics of an individual Mn atom embedded a CdTe quantum dot. Our results show that the Mn spin relaxation is faster when the quantum dot contains an exciton. This can result in an optical orientation of the Mn spin. The valence band mixing is the critical parameter for the fast relaxation rates of the Mn spin in the exchange field of the exciton. This valence band mixing is controlled by the shape and strain of the quantum dot. The influence of these parameters on the optical pumping dynamics were analyzed in detail. Our simulation of optical pumping are in good agreement with experiments. The coherent dynamics of an individual Mn spin was also investigated. We discussed the influence of the coherent dynamics of the coupled electronic and nuclear spins on the optical pumping. We have shown that optically controlled coupling between electronic and nuclear spins could be used for Mn spin switching. We finally demonstrated that the combination of resonant laser and microwave fields can be used to optically detect the magnetic resonance of a Mn spin in a CdTe quantum dot.

The only treatment for coeliac disease, a common autoimmune disorder, is life-long adherence to a gluten-free diet. However, the replacement of wheat gluten, a key structural and functional component in bread, poses a major technological challenge for food scientists. The use of non-wheat cereal proteins, as alternatives to gluten, shows much promise in gluten-free bread making. Literature has shown that when zein, the maize prolamin protein, is subjected to wet heat above its glass transition temperature (Tg), the protein becomes viscoelastic, rubbery and dough-like. Gluten-like fibrils are visible, which form complex protein networks similar to those found in wheat dough. The resulting zein dough has viscoelastic characteristics and can be successfully used with hydrocolloids to produce gluten-free bread. This project examined the influence of wet heat treatment and dilute organic acids (lactic acid and acetic acid) on the dough-making quality of non-wheat cereal proteins, such as kafirin and zein. Zein was the only non-wheat cereal protein to show any physical change when it was subjected to wet heat treatments, forming a dough-like substance. Acidification of the zein dough prepared at 40°C with concentrations of 0.7, 1.3 and 5.4% (v/v) organic acid in distilled water solutions, showed that the higher the concentration of acid used, the greater its effect on the dough's rheological properties. Tensile tests using a Keifer rig on zein dough showed that as the concentration of organic acid was increased from 0.7 to 1.3 and to 5.4% (v/v) the dough become softer and increasingly more extensible. The dough also exhibited less resistance to extension and reduced elasticity. CLSM revealed that the zein doughs contained a protein network, made up of fine protein fibrils, which became smoother and more homogenous as the concentration of acid was increased. Although SDS-PAGE revealed that no oligomerization took place with acid addition, iv FTIR showed that zein dough prepared with distilled water at 40°C had elevated levels of β-sheets. When organic acids were added in increasing levels, corresponding increases in the quantities of α-helices in the protein were observed. Alveography showed that zein-based doughs prepared with dilute organic acids retained gases well and that the concentration of dilute organic acids influenced dough distensibility (biaxial extensibility) and stability (the ability of the dough to retain gas). Low concentration of acids (0.7 and 1.3%) increased dough stability to levels similar to that of strong wheat flour, 103 mm H2O, but higher concentrations of acids (5.4%) led to a marked reduction in dough stability. Thus, by increasing zein dough functionality to such an extent, the apparent usefulness of the doughs and their ability to retain gases produced during fermentation is reduced. Simple distensibility tests on zein doughs showed that added organic acids promoted ‘clumping’ of the fine protein fibrils in the dough network into pronounced fibres. This would account for the decreased dough stability when high levels (5.4%) of organic acids were used. Baking trials with zein doughs were not successful as adequate leavening was impossible without an acid-tolerant leavening agent. It is believed that dilute organic acids influence the rheological properties of zein dough by creating a positively charged environment, in which the protein is partially solubilized. The higher the level of organic acid used, the greater the positive net charge and the more pronounced the effect on the protein network structure. Organic acids could also improve fluidity of the zein dough by acting as plasticizers. From this work it can be seen that although a protein network is present in all zein-based doughs, the ability of this network to retain gases is dependant on the level of organic acids present. The functional properties of zein-doughs made with low levels of organic acids (0.7 and 1.3%) shows potential in the production of gluten-free bread for individuals suffering from coeliac disease. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Food Science / unrestricted