This work presents the development of a GaN-based dilute magnetic semiconductor (DMS) by metal organic chemical vapor deposition (MOCVD) that is ferromagnetic at room temperature (RT), electrically conductive, and possesses magnetic properties that can be tuned by n- and p-doping. The transition metal series (TM: Cr, Mn, and Fe) along with the rare earth (RE) element, Gd, was investigated in this work as the magnetic ion source for the DMS. Single- phase and strain-free GaTMN films were obtained. Optical measurements revealed that Mn is a deep acceptor in GaN, while Hall measurements showed that these GaTMN films were semi-insulating, making carrier mediated exchange unlikely. Hysteresis curves were obtained for all the GaTMN films, and by analyzing the effect of n- and p-dopants on the magnetic properties of these films it was determined that the magnetization is due to magnetic clusters. These findings are supported by the investigation of the effect of TM dopants in GaN nanostructures which reveal that TMs enhance nucleation resulting in superparamagnetic nanostructures. Additionally, this work presents the first report on the development of GaGdN by MOCVD providing an alternate route to developing a RT DMS. Room temperature magnetization results revealed that the magnetization strength increases with Gd concentration and can be enhanced by n- and p-doping, with holes being more efficient at stabilizing the ferromagnetic signal. The GaGdN films obtained in this work are single-phase, unstrained, and conductive making them suitable for the development of multifunctional devices that integrate electrical, optical, and magnetic properties.

Spectrally-resolved differential reflectivity experiments on GaMnAs over a broad spectral range (1.4-2.0 eV) are presented, representing the first such measurements in a III-Mn-V diluted magnetic semiconductor. Comparison of the measured nonlinear spectra with results in GaAs and LT-GaAs, together with calculations of the pump probe signal contributions, has allowed an unambiguous identification of the relevant scattering and relaxation processes for optically-excited carriers in this material system. The measured spectra indicate a clear blue shift in the nonlinear optical response, providing support for the valence band model of ferromagnetism in III-Mn-V diluted magnetic semiconductors. / Spectrally-resolved differential reflectivity experiments on GaMnAs over a broad spectral range (1.4-2.0 eV) are presented, representing the first such measurements in a III-Mn-V diluted magnetic semiconductor. Comparison of the measured nonlinear spectra with results in GaAs and LT-GaAs, together with calculations of the pump probe signal contributions, has allowed an unambiguous identification of the relevant scattering and relaxation processes for optically-excited carriers in this material system. The measured spectra indicate a clear blue shift in the nonlinear optical response, providing support for the valence band model of ferromagnetism in III-Mn-V diluted magnetic semiconductors.

Nanotechnology is a technology of the design and the applications of nanoscale materials with their fundamentally new properties and functions. Nanosensor devices based on nanomaterials provide very fast response, low-cost, long-life time, easy to use for unskilled users, and provide high-efficiency. 1-D ZnO nanostructures materials have great potential applications in various sensing applications. ZnO is a wide band gap (3.37 eV at room temperature) semiconductor materials having large exciton binding energy (60 meV) and excellent chemical stability, electrical, optical, piezoelectric and pyroelectric properties. By doping the transition metals (TM) into ZnO matrix, the properties of ZnO nanostructures can be tuned and its room  temperature ferromagnetic behavior can be enhanced, which provide the TM-doped ZnO nanostructures as promising candidate for optoelectronic, spintronics and high performance sensors based devices. The synthesis of ZnO and TM-doped ZnO nanostructures via the low temperature hydrothermal method is considered a promising technique due to low cost, environmental friendly, simple solution process, diverse 1-D ZnO nanostructures can be achieved, and large scale production on any type of substrate, and their properties can be controlled by the growth parameters. However, to synthesize 1-D ZnO and TM-doped ZnO nanostructures with controlled shape, structure and uniform size distribution on large area substrates with desirable properties, low cost and simple processes are of high interest and it is a big challenge at present. The main purpose of this dissertation aims to develop new techniques to synthesize 1-D ZnO and (Fe, Mn)-doped ZnO nanostructures via the hydrothermal method, to characterize and to enhance their functional properties for developing sensing devices such as biosensors for clinical diagnoses and environmental monitoring applications, piezoresistive sensors and UV photodetector. The first part of the dissertation deals with the hydrothermal synthesis of ZnO nanostructures with controlled shape, structure and uniform size distribution under different conditions and their structural characterization. The possible parameters affecting the growth which can alter the morphology, uniformity and properties of the ZnO nanostructures were investigated. Well-aligned ZnO nanorods have been fabricated for high sensitive piezoresistive sensor. The development of creatinine biosensor for clinical diagnoses purpose and the development of glucose biosensor for indirect determination of mercury ions for an inexpensive and unskilled users for environmental monitoring applications with highly sensitive, selective, stable, reproducible, interference resistant, and fast response time have been fabricated based on ZnO nanorods. The second part of the dissertation presents a new hydrothermal synthesis of (Fe, Mn)-doped-ZnO nanostructures under different preparation conditions, their properties characterization and the fabrication of piezoresistive sensors and UV photodetectors based devices were demonstrated. The solution preparation condition and growth parameters that influences on the morphology, structures and properties of the nanostructures were investigated. The fabrication of Mn-doped-ZnO NRs/PEDOT:PSS Schottky diodes used as high performance piezoresistive sensor and UV photodetector have been studied and Fe-doped ZnO NRs/FTO Schottky diode has also been fabricated for high performance of UV photodetector. Finally, a brief outlook into future challenges and relating new opportunities are presented in the last part of the dissertation.

No description available.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 114-127).

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho abordamos algumas propriedades físicas associadas ao transporte spintrônico de nanoestruturas formadas por camadas de semicondutor diluído magnético (SDM) e semicondutor convencional submetidas a campos elétrico e magnético cruzados. O campo elétrico é aplicado na direção de crescimento da nanoestrutura e o campo magnético é aplicado perpendicularmente a essa direção. Estuda mos duas configurações de nanoestruturas onde o SDM localiza-se no poço quântico ou nas barreiras. Mostramos que é possível encontrar um potencial efetivo tipo poço de potencial duplo para um intervalo de intensidades de campos externos, altura da barreira de potencial e largura de poço quântico parabólico. Em tal condição esse sistema pode ser visto como um dispositivo spintrônico chamado filtro de spin, pois consegue selecionar polarizações de spin em diferentes regiões da nanoestrutura. / In the present work we studied properties of the spintronic transport of nanostructures formed by layers of diluted magnetic semiconductors (DMS) and conventional semiconductors with crossed fields applied. The electric field is in the growth direction and the magnetic field is perpendicular t o this one. We studied two configurations of nanostructures where the DMS is located in the barriers or in the well. We will show the possibility of the formation of a double quantum well like effective potential for different values of the applied fields intensities, barriers height and quantum well width. In this situation the system can be seen as a spintronic device called spin filter, since it can control the spin polarization in different regions of the nanostructure.

Submitted by Alison Vanceto (alison-vanceto@hotmail.com) on 2017-05-22T12:58:51Z No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T20:35:23Z (GMT) No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T20:35:31Z (GMT) No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Made available in DSpace on 2017-05-25T19:51:05Z (GMT). No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) Previous issue date: 2016-03-09 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / We report a systematic study on the structural, morphological and optical properties of Zn1−xCoxO thin films grown by spray pyrolysis. The employed nominal concentrations were up to 15 Co %. The films were deposited on glass substrates between 220-300 o C using precursors such as zinc acetate and cobalt acetate in aqueous solutions. The effects of Co-doping on structural, morphological and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance and absorbance as well as photoluminescence (PL). The XRD results indicated that all films are polycrystalline with wurtzite crystal structure, a preferential growth direction in c-axis, and no secondary phases. SEM images generally showed uniform surface, and few regions have irregularities due to effects of the inclusion of cobalt as well as substrate deposition temperature. The spectra of transmittance and absorbance exhibited the transitions d-d due to Co2+ ion, a redshift of the absorption-edge and changes in the absorption band intensity with increasing of nominal Co-doping. These observations revealed the substitution of Zn ions Zn2+ and the presence of Co in the state 2+ in tetrahedral coordination in the ZnO lattice. Furthermore, there was a narrowing in band gap of 28 meV by percentage of Co inserted into the lattice. The PL spectra of ZnCoO samples presented three bands associated to near-band-edge absorption (NBE), structural defects and intratomic emissions of Co2+. Their optical emissions were characterized by the presence of traps which capture carriers and affect significantly the emissions. Through a qualitative model in a temperature range we interpreted these traps as potential fluctuations up to 17 meV. We also discussed the annealing in selected samples. / Apresentamos um estudo sistemático sobre as propriedades estruturais, morfológicas e ópticas de filmes Zn1−xCoxO com concentrações nominais de Co até 15% obtidos por spray pirólise. Os filmes foram depositados sobre substratos de vidro entre 220 - 300 oC, utilizando como precursores acetato de zinco e acetato de cobalto em solução aquosa. Os efeitos da dopagem com Co nas propriedades estruturais, morfológicas e ópticas foram estudados por difração de raios-X (DRX), microscopia eletrônica de varredura (MEV), transmitância e absorbância óptica, e fotoluminescência (PL). Os resultados de DRX indicaram que todos os filmes são policristalinos, com estrutura cristalina do tipo wurtzita e direção de crescimento preferencial no eixo c, com ausência de fases secundárias. As imagens do MEV, em geral, mostraram que as superfícies são uniformes, com irregularidades em algumas regiões devido aos efeitos da inserção de Cobalto e da temperatura de deposição do substrato. Nos espectros de transmitância e absorbância, observaram-se a presença das transições d-d do íon de Co2+, o deslocamento da borda de absorção para baixas energias, e a variação da intensidade da banda de absorção das transições d-d com o aumento da dopagem nominal de Co. Estas observações revelaram a substituição dos íons de Zn2+ e a presença de Co no estado +2 na coordenação tetraédrica na rede do ZnO. Além disso, houve uma redução da energia do bandgap de 28 meV por porcentagem de Co inserido na rede. Os espectros de PL das amostras de ZnO dopado com Co apresentaram três bandas associadas à emissão próxima à borda de absorção óptica, defeitos estruturais e emissão intratômica do Co2+. A emissão óptica dos filmes foi caracterizada pela presença de armadilhas que localizam os portadores de carga e afetam significativamente as bandas de emissão no intervalo de temperatura estudado. Através de um modelo qualitativo interpretamos estas armadilhas como flutuações de potencial com profundidades de até 17 meV. Discutimos também o tratamento térmico em amostras selecionadas.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho abordamos algumas propriedades físicas associadas ao transporte spintrônico de nanoestruturas formadas por camadas de semicondutor diluído magnético (SDM) e semicondutor convencional submetidas a campos elétrico e magnético cruzados. O campo elétrico é aplicado na direção de crescimento da nanoestrutura e o campo magnético é aplicado perpendicularmente a essa direção. Estuda mos duas configurações de nanoestruturas onde o SDM localiza-se no poço quântico ou nas barreiras. Mostramos que é possível encontrar um potencial efetivo tipo poço de potencial duplo para um intervalo de intensidades de campos externos, altura da barreira de potencial e largura de poço quântico parabólico. Em tal condição esse sistema pode ser visto como um dispositivo spintrônico chamado filtro de spin, pois consegue selecionar polarizações de spin em diferentes regiões da nanoestrutura. / In the present work we studied properties of the spintronic transport of nanostructures formed by layers of diluted magnetic semiconductors (DMS) and conventional semiconductors with crossed fields applied. The electric field is in the growth direction and the magnetic field is perpendicular t o this one. We studied two configurations of nanostructures where the DMS is located in the barriers or in the well. We will show the possibility of the formation of a double quantum well like effective potential for different values of the applied fields intensities, barriers height and quantum well width. In this situation the system can be seen as a spintronic device called spin filter, since it can control the spin polarization in different regions of the nanostructure.

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.