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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
111

Crescimento de In0.52Al0.48As e In0.53Ga0.47As sobre InP por MBE / Growth of the In0.53Ga0.47As/ In0.52Al0.48As on InP by MBE

Charcape, Galo Emilio Sisniegas 24 June 1997 (has links)
Neste trabalho a epitaxia por feixes moleculares (Molecular Beam Epitaxy: MBE) foi utilizada para crescer estruturas de gases bidimensionais a base de In0.53Ga0.47As/ In0.52Al0.48As sobre substrato de fosfeto de índio (InP) e analisar suas propriedades estruturais. elétricas e óticas através de técnicas de caracterização, tais como efeito Hall, fotoluminescência (FL). Shubnikov-de Haas (SdH) e microscopia eletrônica de varredura (MEV). A calibração dos parâmetros de crescimento tais como a taxa de crescimento e a composição das ligas In0.53Ga0.47As/ In0.52Al0.48As foi feita através da observação das oscilações da da difração de elétrons de alta energia observado em reflexão (Reflection High-Energy Electron Difraction : RHEED) A densidade de dopante tipo n (em nosso caso. 0 silício) foi determinada posteriormente através de medidas de efeito Hall usando a técnica de Van der Pauw. Os resultados da Fotoluminescência mostram uma transição doador-banda com largura a meia altura (Full Width at Half Maximum : FWHM) de aproximadamente 23 meV, para a liga de In0.52Al0.48As. A formação de gás bidimensional nestas mesmas amostras tem sido observado indiretamente pela ocorrência de oscilações SdH. / In this work Molecular Beam Epitaxy (MBE) was used to growth bidimensional gas structures based on In0.53Ga0.47As/ In0.52Al0.48As on Indium Phosfate substrate (lnP) Structural. electrical and optical properties were analyzed by characterisation techniques such as Hall eflect, photoluminescence (PL), Shubnikov-de Haas (SdH) and Scanning Electron Microscope (SEM) Growth parameter calibration such as growth rate and alloy composition of In0.53Ga0.47As/ In0.52Al0.48As was determined through observation of Reflexion High-Energy Electron Diffraction (RHEED) oscillation. Density of type dopant (in our case silicon Si) was obtained by. Hall effect measurements using Van der Pauw geometry Photoluminescence results show donor to band transition with Full Width at Half Maxima (FHWM) of approximatly 23meV, for the In0.52Al0.48As alloy. Bidimensional gas formation was indirectly observed in these samples through SdH oscillations.
112

Otimização de estruturas de aço treliçadas planas com variáveis discretas / Optimization of two-dimensional trusses composed by steel available sections

Prudente, Mauro 25 September 1998 (has links)
Apresenta-se neste trabalho um processo de busca da solução de mínimo peso, para estruturas de aço treliçadas planas utilizando-se seções formadas por perfis comerciais comuns, dimensionadas segundo a norma brasileira para projeto e execução de estruturas de aço de edifícios. Como base do processo de otimização foi utilizado o método do gradiente inteiro, alterando-se alguns critérios, no sentido de melhorar o desempenho e levar em conta a relação não linear entre as características geométricas dos perfis. São abordados os conceitos matemáticos básicos envolvidos no processo de otimização, os de análise de estruturas treliçadas planas, bem como a formulação para o dimensionamento de barras sujeitas a tração ou compressão. São apresentadas ainda as listagens dos programas envolvidos no processo e também exemplos de aplicação para demonstrar a eficácia dos procedimentos utilizados. / This work presents a process to find a least weight solution for two-dimensional trusses composed of available steel sections, according to brazilian code for design and constructions or steel buildings structures. The optimization process is based on the method of integer gradient directions. Some modifications were introduced in order to improve the algorithm and take into account the non-linear relationship between geometric characteristics for steel cross-section. It is also presented the basic concepts of mathematics optimization to design two-dimensional trusses, as well as the formulation for designing members in tension or compression. The program listing used in this process is also presented, as well as some examples, to demostrate the efficacy of the process.
113

Problemas bidimensionais de carga espacial em geometrias plano-plano e assemelhadas / Two-dimension space-charge problems in plane-plane geometry and alike

Dantas, Ismael Francisco 14 September 1992 (has links)
A finalidade principal deste trabalho é o da obtenção de soluções numéricas de movimento de carga espacial livre em que as grandezas pertinentes dependem de mais de uma coordenada espacial (casos não unidimensionais). Mais especificamente, os problemas estudados versam sobre casos em que um eletródio plano a um dado potencial emite carga não uniformemente ao longo de sua extensão e procura-se obter a deformação das linhas de campo daí resultantes, no espaço até um outro eletródio aterrado disposto paralelamente ao primeiro. Um método baseado em soluções tentativas bem como um outro global usando a \"transformação hodográfica\" recentemente formulada por Budd e Wheeler são apresentados. E também realizado um estudo breve sobre a validade da muito empregada \"suposição de Deutsch\" / The aim of the present work is to obtain numerical solutions of free space charge motion in cases in which the pertinent quantites depend on more than one space coordinate (non uni-dimensional cases). More specifically, the problems under concern here are those in which a plane electrode, at a constant potential, emit charge in a non-uniform way along them and the resultant deformation of the field lines, in the space between it and another plane grounded electrode parallelly disposed, is looked for. A method based on guessed solutions and also a global one using the hodographic transformation recently developed by Budd and Wheeler are presented. A brief study of the validity of the often employed \"Deutsch assumption\" is also carried out
114

Caracterização protéica do micro-ambiente uterino durante o período crítico em bovinos / Characterization of the protein profile of the uterine environment during the critical period in cattle

Fedozzi, Filipe 30 April 2008 (has links)
Em bovinos, o sucesso no estabelecimento da gestação depende de interações entre o concepto e o útero. Tais interações ocorrem através de moléculas presentes no ambiente uterino. Falhas nesse processo causam 30-40% de perdas de gestação. As proteínas são constituintes desse ambiente e possuem um importante papel nesse processo. Objetivou-se na presente dissertação caracterizar o perfil protéico do ambiente uterino de fêmeas bovinas, ciclando ou prenhes, durante o período crítico, utilizando eletroforese bidimensional. O objetivo específico foi comparar a composição protéica de lavados uterinos obtidos in vivo, por sonda de folley ou post-mortem no dia 18 após o estro de vacas não inseminadas ou gestantes. Para a obtenção das amostras, 48 fêmeas bovinos tiveram seu ciclo estral sincronizado e foram alocadas aleatoriamente para receberem lavagens in vivo por sonda de folley (12 inseminadas e sete não inseminadas) ou para receberem lavagens post-mortem (12 inseminadas e cinco não inseminadas) no dia 18 após o estro. Após a coleta, os lavados foram processados e submetidos à eletroforese bidimensional. Os géis foram analisados utilizando o programa Image Master-2D ELITE V. 4.01 (GE Healthcare) para determinação do ponto isoelétrico (PI) e peso molecular relativo (PMR) das proteínas nos diferentes grupos e análise da densidade óptica. Foram selecionadas 178 proteínas presentes nos lavados. Em relação à distribuição das proteínas de acordo com o ponto isoelétrico, 74% (132/178) apresentaram PI entre 5,5 e 8, em relação ao peso molecular, 44% (79/178) das proteínas possuíam PMR menor que 50KDa enquanto 54% (95/178) entre 50 e 100KDa. Nos lavados realizados por sonda de folley (FC ou FP) foram selecionadas 151 proteínas, sendo 96 nos lavados de animais cíclicos e 86 nos prenhes. Os lavados realizados post-mortem (PMC ou PMP) apresentaram apenas 54 proteínas, das quais 24 estiveram presentes nos lavados PMC e 36 nos lavados PMP. Em relação ao estado reprodutivo 115 proteínas foram encontradas nos lavados dos animais cíclicos (FC ou PMC), enquanto 104 proteínas foram selecionadas nos lavados de animais prenhes (FP e PMP). Entre os lavados realizados por sonda de folley (FC e FP) 31 proteínas foram comuns entre os estados, enquanto entre os lavados realizados post-mortem (PMC e PMP) apenas seis proteínas estiveram presentes em ambos os lavados. Nos lavados realizados em animais ciclando (FC e PMC) cinco proteínas estiveram presentes em ambos os tipos de lavados, enquanto nos lavados realizados em animais prenhes (FP e PMP) 18 proteínas foram comuns em ambos os tipos de lavado. Entre os lavados FC e PMP ou FP e PMC, 15 e 6 proteínas, respectivamente, foram comuns. As proteínas presentes exclusivamente nos lavados de vacas prenhes, possivelmente estão envolvidas no diálogo materno-embrionário durante o processo de estabelecimento da gestação. Proteínas detectadas exclusivamente nos lavados obtidos via sonda de folley podem ser devidas ao influxo de proteínas do soro sanguíneo em resposta à manipulação do trato reprodutivo durante a obtenção dos lavados. Esperava-se uma maior efetividade dos lavados realizados post-mortem em relação aos realizados por sonda de folley. Entretanto os lavados obtidos por sonda de folley foram melhores por identificar um maior numero de proteínas, permitindo identificar diferenças maiores entre os lavados de animais ciclando e prenhes. Concluiu-se que a eletroforese bidimendional constitui um método adequado para avaliar quantitativamente o perfil protéico do ambiente uterino bovino e que tanto o estado reprodutivo quanto o método de obtenção de lavados uterinos modificam sua composição protéica. Novos estudos são necessários para se obter a seqüência de aminoácidos das proteínas localizadas para que se conheça sua identidade e função na gestação inicial de bovinos. / In the cattle, the outcome for the establishment of pregnancy depends on interactions between conceptus and uterus. These interactions occur through molecules presents in the uterine environment. Failure in this process causes 30-40% of loss in gestation. The proteins are component of this environment and have a important role in this process. The aim of this dissertation was to characterize the protein profile of the uterine environment in cows, cyclic or pregnant, during the critical period, using two-dimensional electrophoresis. The specific aim was to compare the protein composition of uterine washings recovered in vivo, through a folley catheter, with washings recovered post-mortem at day 18 after estrus of not inseminated or pregnant cows. To obtain the samples 48 cows had their estrous synchronized and here aleatoric put in the group to receive washings in vivo through the folley catheter (12 inseminated and seven not inseminated) or to be washed post-mortem (12 inseminated and five not inseminated) at day 18 after estrus. After the recover, the washings were proceeded and submitted to two-dimensional electrophoresis. The gels were analyzed using the software Image Master-2D ELITE V. 4.01 (GE Heathcare) to determine the isoelectric point (PI) and relative molecular weight (PMR) of the proteins presents in the different groups and analysis of the optical density. One hundred and seventy eight proteins were presents in the washings. The distribution of the proteins according to the PI, 74% (132/178) had PI between 5,5 and 8 and according to the PMR 44% (79/178) of the proteins showed PMR lower than 50KDa, while 54% (95/178) had PMR between 50 and 100KDa. In the washings recovered through the folley catheter (FC and FP) 151 proteins were selected, being 96 from the cyclic cows and 86 from the pregnant cows. The washings recovered post-mortem (PMC and PMP) showed only 54 proteins, from those 24 were present in the cyclic animals and 36 in the pregnant animals. According to the reproductive status 115 proteins were found in the washings from the cyclic animals (FC and PMC), while 104 proteins were selected from the washings of the pregnant animals (FP and PMP). Among the washings recovered through the folley catheter (FC and FP) 31 proteins were the same between the reproductive status. While in the washings recovered post-mortem (PMC and PMP) only six proteins were present in both washings. In washings done in the cyclic animals (FC and PMC) five proteins were presents in both kind of washings, while in washings done in the pregnant animals (FP and PMP) 18 proteins were the same in both kind of washings. Among the washings FC and PMP or FP and PMC, 15 and six proteins, respectively were the same. The proteins presents exclusively in the washings of the pregnant animals are probably involved with the embryo-maternal interplay during the process of pregnancy establishment. Proteins detected exclusively in the washings recovered through the folley catheter may be inflow of proteins from the serum, in response to the uterine manipulation during the process of washing. It was expected a better effect from the washings post-mortem than the ones through the folley catheter. However the washings from the folley catheter had a better result, because they identified a greater number of proteins which allowed identify greater differences between the washings from cyclic and pregnant animals. In conclusion, the two-dimensional electrophoresis is a adequate method to analyze the quantitatively proteins profile of the uterine environment in the cow and that both the reproductive status and the method to obtain the uterine washings modify the proteins composition. New studies are necessary to obtain the amino acids sequence of the proteins found so that we know their identity and function in the beginning of pregnancy in the cow.
115

Perfil bidimensional de proteínas do plasma seminal e características ligadas ao desempenho reprodutivo de touros de corte / Two-dimensional seminal plasma proteins profile and traits linked to beef bulls’ reproductive performance

Romitto, Graciana Corrêa 17 December 2003 (has links)
Estudou-se o perfil de proteínas de plasma seminal de touros das raças Nelore (Bos taurus primigerus indicus) e Simental (Bos taurus primigerus taurus), criados a campo, em regime de acasalamento múltiplo e manejo extensivo, na região de Dourados/MS, a fim de analisar a relação entre estas proteínas e características ligadas ao desempenho reprodutivo de touros. Coletaram-se amostras de sêmen no período de inverno (Julho/2001) e verão (Fevereiro/2002), realizando-se inicialmente o exame andrológico nos animais, e em seguida a análise padrão do sêmen. O plasma seminal foi, então, submetido à dosagem de proteínas totais, à dosagem de substâncias reativas ao ácido tiobarbitúrico (TBARS), para avaliação da ocorrência de lipoperoxidação, e à eletroforese bidimensional de proteínas. Para análise do perfil proteico de plasma seminal da cada raça, foram utilizados os seguintes parâmetros: comparação entre os períodos de inverno e verão; comparação de grupos de amostras separados de acordo com a quantificação de características ligadas ao desempenho reprodutivo (morfologia espermática, dosagem de proteína total e dosagem de TBARS); e comparação de grupos de amostras separados de acordo com classificações para avaliação da capacidade reprodutiva, previamente descritas na literatura (Breeding Soundness Evaluations – BSEs). Os touros da raça Simental demonstraram menor adaptação às condições ambientais tropicais, com valores mais altos de defeitos espermáticos maiores, proteína total e dosagem de TBARS no período de verão. Observou-se grande variabilidade no perfil proteico entre as raças Nelore e Simental e, dentro de cada raça, encontrou-se expressiva variação individual. O perfil de proteínas de plasma seminal de touros Nelore apresentou 155 spots, entre os quais 04 se destacaram como possíveis marcadores de características ligadas ao desempenho reprodutivo. Já na raça Simental foram identificados 248 spots, com destaque para 04 spots, que demonstraram maior potencial como marcadores de características de interesse para a reprodução. Os resultados mostram que o perfil de proteínas de plasma seminal está relacionado ao desempenho reprodutivo de touros, fazendo-se necessários maiores estudos sobre as proteínas que se destacaram como marcadores, a fim de identificar as funções que elas exercem no trato reprodutivo e nos eventos ligados à fertilização. / Seminal plasma proteins profiles from Nelore (Bos taurus primigerus indicus) and Simmental (Bos taurus primigerus taurus) bulls, used for multiple-sire breeding under range conditions, at Dourados/MS region, were studied aiming to analyze the relation between these proteins and traits linked to bulls’ reproductive performance. Semen samples were collected in the winter (July/2001) and summer (February/2002). Initially, an andrological examination was performed, followed by standard semen analysis. Seminal plasma was subjected to total protein quantification, thiobarbituric acid reactive substances quantification (TBARS, to evaluate lipid peroxidation), and two-dimensional protein electrophoresis. To analyze seminal plasma protein profile of each breed, the following parameters were used: comparison between winter and summer, comparison among groups of samples divided by quantification of traits linked to reproductive performance (sperm morphology, total protein quantification, TBARS quantification) and comparison among groups of samples divided by breeding soundness evaluations (BSEs), previously reported in literature. Simmental bulls showed lower adaptability to tropical environment conditions, with higher values of major sperm defects, total protein and TBARS in the summer. A great variability was observed in the protein profile between Nelore and Simmental breeds, and, for each breed it was found great individual variability too. Nelore seminal plasma protein profile presented 155 spots, 04 of which were considered potential markers for traits linked to reproductive performance. For Simmental breed, 248 spots were identified, with special interest for 04 spots, which showed higher potential as markers to reproductive characteristics. Results show that seminal plasma protein profile is related to bulls’ reproductive performance. Further studies about those proteins which are potential markers are needed, to determine their function at bulls’ reproductive system and in the events related to fertilization.
116

Measuring, interpreting, and translating electron quasiparticle-phonon interactions on the surfaces of the topological insulators bismuth selenide and bismuth telluride

Howard, Colin 08 April 2016 (has links)
The following dissertation presents a comprehensive study of the interaction between Dirac fermion quasiparticles (DFQs) and surface phonons on the surfaces of the topological insulators Bi2Se3 and Bi2Te3. Inelastic helium atom surface scattering (HASS) spectroscopy and time of flight (TOF) techniques were used to measure the surface phonon dispersion of these materials along the two high-symmetry directions of the surface Brillouin zone (SBZ). Two anomalies common to both materials are exhibited in the experimental data. First, there is an absence of Rayleigh acoustic waves on the surface of these materials, pointing to weak coupling between the surface charge density and the surface acoustic phonon modes and potential applications for soundproofing technologies. Secondly, both materials exhibit an out-of-plane polarized optical phonon mode beginning at the SBZ center and dispersing to lower energy with increasing wave vector along both high-symmetry directions of the SBZ. This trend terminates in a V-shaped minimum at a wave vector corresponding to 2kF for each material, after which the dispersion resumes its upward trend. This phenomenon constitutes a strong Kohn anomaly and can be attributed to the interaction between the surface phonons and DFQs. To quantify the coupling between the optical phonons experiencing strong renormalization and the DFQs at the surface, a phenomenological model was constructed based within the random phase approximation. Fitting the theoretical model to the experimental data allowed for the extraction of the matrix elements of the coupling Hamiltonian and the modifications to the surface phonon propagator encoded in the phonon self energy. This allowed, for the first time, calculation of phonon mode-specific quasiparticle-phonon coupling λⱱ(q) from experimental data. Additionally, an averaged coupling parameter was determined for both materials yielding ¯λ^Te ≈ 2 and ¯λ^Se ≈ 0.7. These values are significantly higher than those of typical metals, underscoring the strong coupling between optical surface phonons and DFQs in topological insulators. In an effort to connect experimental results obtained from phonon and photoemission spectroscopies, a computational process for taking coupling information from the phonon perspective and translating it to the DFQ perspective was derived. The procedure involves using information obtained from HASS measurements (namely the coupling matrix elements and optical phonon dispersion) as input to a Matsubara Green function formalism, from which one can obtain the real and imaginary parts of the DFQ self energy. With these at hand it is possible to calculate the DFQ spectral function and density of states, allowing for comparison with photoemission and scanning tunneling spectroscopies. The results set the necessary energy resolution and extraction methodology for calculating ¯λ from the DFQ perspective. Additionally, determining ¯λ from the calculated spectral functions yields results identical to those obtained from HASS, proving the self-consistency of the approach.
117

Eletroforese bidimensional em gel de poliocrilamida do plasma seminal equino e a sua relação com a congelabilidade do sêmen / Two-dimensional polyacrylamide gel electrophoresis of equine seminal plasma proteins and their relation with semen freezability

Trein, Cristina Rodrigues January 2012 (has links)
O objetivo deste estudo foi avaliar o perfil proteico do plasma seminal equino utilizando eletroforese bidimensional de gel de acrilamida (2D-PAGE) e determinar se algumas das proteínas presentes estavam relacionadas com a congelabilidade do sêmen. O plasma seminal foi coletado de dez garanhões, de alta e baixa congelabilidade de sêmen, provenientes do Haras Estatal da Baixa Saxônia, na cidade de Celle, na Alemanha e rotineiramente utilizados em programas de inseminação artificial. Vinte e cinco bandas proteicas foram encontradas nos géis bidimensionais (12%) e sete delas foram identificadas por MALDI-MS. Das 25 proteínas encontradas nas amostras de plasma seminal dos garanhões, duas bandas proteicas apresentaram densidade óptica superior (P<0,05) nas amostras de garanhões de alta congelabilidade o sêmen: as bandas 5 (80-85 kDa, pI 7,54), que foi identificada como CRISP3 e a 45 (18,2 kDa, pI 5,0-5,2) identificada como HSP-2. Contrariamente a banda 7 (75,4 kDa, pI 6,9 – 7,4), identificada como lactoferrina, a 15 (26,7 kDa, pI 5,51) identificada como calicreína, a 25 (25 kDa, pI 7,54) como CRISP3 e a 35 (13,9 kDa, pI 3,8 – 4,2) que foi identificada como HSP-1, apresentaram valores de densidade óptica superior (P<0,05) nos reprodutores de baixa congelabilidade do sêmen. As proteínas foram identificadas através de espectometria de massa MALDI-MS. As evidências encontradas neste experimento mostram que existem diferenças no perfil proteico dos reprodutores de alta e baixa congelabilidade do sêmen, sugerindo as proteínas CRISP3 e a HSP-2 como possíveis marcadores da alta congelabilidade de sêmen de garanhões. / The objective of this study was to evaluate protein profile of equine seminal plasma using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and to find if any of these proteins were related to semen freezability. Seminal plasma was collected from 10 stallions of high and low semen freezability routinely used in AI programs from the State Stud of Lower Saxony and Artificial Insemination Center. Twenty five protein spots were identified from the 2 D gel (12%), seven of which were present in all samples. Of the 25 proteins found in the research stallions, two spots showed superior relative content (P<0.05) in seminal plasma samples collected from stallions with high semen freezability: the spots 5 (80-85 kDa, pI 7.54) was identified as CRISP3 and 45 (18.2 kDa, pI 5.0-5.2) was identified as HSP-2. Conversely, the spot 7 (75.4 kDa, pI 6.9 – 7.4) was identified as lactoferrin, 15 (26.7 kDa, pI 5.51) was identified as kallikrein, 25 (25 kDa, pI 7.54) was identified as CRISP3 and 35 (13.9 kDa, pI 3.8 – 4.2) was identified as HSP-1, showed superior relative protein content (P<0.05) on seminal plasma samples from stallions with low semen freezability. The proteins were identified by MALDI-MS. There were differences in the seminal plasma protein profile between stallions with high and low semen freezability. It may be suggested that CRISP3 and HSP-2 may be considered possible seminal plasma markers of high semen freezability.
118

The physics of multilayer topological insulator heterostructures using low-energy models

Nikolic, Aleksandar January 2018 (has links)
This thesis studies the physics of multilayer heterostructures grown from topological insulators (TIs), primarily bismuth selenide and antimony telluride, and other topologically trivial materials. This is done by extending a standard low-energy 3D TI Hamiltonian and varying its associated material parameters across the simulation domain. New results arising from the position-dependent TI interface model are found. For the first time, this method is incorporated into a density-functional theory (DFT) solver in order to study the self-consistent charge density in multilayer TI heterostructures due to the interface states. The thesis is structured as follows. The introduction (Ch. 1) presents a pedagogical review of the theory of 3D TIs and low-energy Hamiltonians used to study them, as well as typical methods in solid state physics that are made use of throughout the thesis. Chapter 2 presents the position-dependent Hamiltonian, showing new evidence for topological features of bulk states including varying degrees of band mixing and inversion; also, interface state tunnelling is shown to be affected by atomic layer orbital overlap, and incomplete localisation of surface states is demonstrated for antimony telluride. Chapter 3 presents a new DFT model of TI heterostructure interfaces and shows how conduction through TI interface states can be controlled with an electric field. Chapter 4 covers the extension of the model in Ch. 1 to 2D cross-sections of TI wires and heterostructures, showing for the first time evidence of localisation of conduction almost entirely within the inner interfaces of a 2D heterostructure wire. Chapter 5 presents our work with magnetic fields, demonstrating evolution of interface and bulk states with changing magnetic field and Landau level, as well as presenting new evidence for more complex spin structures in bismuth selenide arising from Landé factor signs. Our conclusions are presented in Chapter 6.
119

Two-dimensional Tellurium: Material Characterizations, Electronic Applications and Quantum Transport

Gang Qiu (7584812) 31 October 2019 (has links)
<div>Since the debut of graphene, many 2D materials have emerged as promising candidates for silicon alternatives to extend Moore’s Law, such as MoS<sub>2</sub> and phosphorene. However, some common shortcomings such as low mobility, instability and lack of massive production methods limit the exploration and applications of these materials. Here, we introduce a novel member to the 2D category – high-mobility air-stable 2D tellurium film (tellurene).</div><div><br></div><div>Tellurium (Te) is a narrow bandgap semiconductor with unique one-dimensional chiral structure. Recently, a hydrothermal synthesizing method was developed to produce large-area tellurene nanofilms with thickness ranging from tens of nanometers down to few layers. In this thesis, a thorough investigation of Te properties in 2D quantum region was first carried out by various material characterization techniques including TEM and Raman spectroscopy. Potential applications of Te-based electronics, optoelectronic and thermoelectric devices were explored, and high-performance Te FETs were achieved with record-high drive current over 1 A/mm via device scaling and contact engineering. Magneto-transport, including weak anti-localization and Shubnikov-de-Haas oscillations was studied at cryogenic temperature. Quantum Hall effect was observed for the first time in both 2D electron and hole gases with mobility of 6,000 and 3,000 cm<sup>2</sup>/Vs, and non-trivial Berry phase in Te 2D electron system was detected as the first experimental evidence of massive Weyl fermions. This work not only demonstrates the great potential of tellurene films for electronics and quantum device applications, but also expands the spectrum of topological matters into a new material species - Weyl semiconductors.</div>
120

Enzyme dynamics and their role in formate dehydrogenase

Guo, Qi 01 December 2016 (has links)
How the fast (femtosecond-picosecond, fs-ps) protein dynamics contribute to enzymatic function has gained popularity in modern enzymology. With multiple experimental and theoretical studies developed, the most challenging part is to assess both the chemical step kinetics and the relevant motions at the transition state (TS) on the fast time scale. Formate dehydrogenase (FDH), which catalyzes a single hydride transfer reaction, is a model system to address this specific issue. I have crystallized and solved the structure of FDH from Candida boidinii (CbFDH) in complex with NAD+ and azide. With the guidance of the structure information, two active site residues were identified, V123 and I175, which could be responsible for the narrow donor-acceptor-distance (DAD) distribution observed in the wild type CbFDH. This thesis describes studies using kinetic isotope effects (KIEs) and their temperature dependence together with two-dimensional infrared spectroscopy on the recombinant CbFDH and its V123 and I175 mutants. Those mutants were designed to systematically reduce the size of their side chain (I175V, I175A, V123A, V123G and double mutant I175V/V123A), leading to broader distribution of DADs. The kinetic experiments identified a correlation between the DAD distribution and the intrinsic KIEs. The contribution of the fs-ps dynamics was examined via two-dimensional infrared spectroscopy (2D IR) by measuring the vibrational relaxation of TS analog inhibitor, aizde, reflecting the TS environmental motions. Our results provide a test of models for the kinetics of the enzyme-catalyzed reaction that invokes motions of the enzyme at the fs-ps time scale to explain the temperature dependence of intrinsic KIEs.

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