Photodetectors are semiconductor devices capable of converting optical signals into electrical signals. There is a wide range of applications for photodetectors such as fiber optics communication, infrared heat camera sensors, as well as in medical and military equipment.Nanowires are thin needle-shaped structures consisting of semiconductor materials such as gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si). They are ideally suited for sensitive photodetectors with low noise due to their small size, well-controlled crystal structure, and composition tunability, as well as the possibility to fabricate them monolithically on silicon.In this thesis, Fourier Transform Infrared (FTIR) Spectroscopy was used to investigate the optical characteristics of InP nanowire-based n+-i-n+ photodetectors with 20 embedded InAsP quantum discs in each InP nanowire. The spectrally resolved photocurrent was measured and analyzed at different angles of incidence. Also, detailed current-voltage characteristics in dark and under illumination were recorded and analyzed.Summarized, the samples showed very good I-V performance with low dark leakage currents. The photocurrent scales with the numbers of nanowires, from which we conclude that most of the photocurrent is generated in the nanowires. Spectrally resolved photocurrent data, recorded at room-temperature, shows strong absorption in the near-infrared region with interesting peaks that reveal, the underlying optical processes in the substrate and nanowires.

Thesis advisor: Dunwei Wang / Semiconductor devices offer promise for efficient conversion of sunlight into other useful forms of energy, in either photovoltaic or photoelectrochemical cell configurations to produce electrical power or chemical energy, respectively. This dissertation examines ionic and electronic phenomena in some candidate semiconductors and seeks to understand their implications toward solar energy conversion applications. First, copper sulfide (Cu₂S) was examined as a candidate photovoltaic material. It was discovered that its unique property of cation diffusion allows the room-temperature synthesis of vertically-aligned nanowire arrays, a morphology which facilitates study of the diffusion processes. This diffusivity was found to induce hysteresis in the electronic behavior, leading to the phenomena of resistive switching and negative differential resistance. The Cu₂S were then demonstrated as morphological templates for solid-state conversion into different types of heterostructures, including segmented and rod-in-tube morphologies. Near-complete conversion to ZnS, enabled by the out-diffusion of Cu back into the substrate, was also achieved. While the ion diffusion property likely hinders the reliability of Cu₂S in photovoltaic applications, it was shown to enable useful electronic and ionic behaviors. Secondly, iron oxide (Fe₂O₃, hematite) was examined as a photoanode for photoelectrochemical water splitting. Its energetic limitations toward the water electrolysis reactions were addressed using two approaches aimed at achieving greater photovoltages and thereby improved water splitting efficiencies. In the first, a built-in n-p junction produced an internal field to drive charge separation and generate photovoltage. In the second, Fe₂O₃ was deposited onto a smaller band gap material, silicon, to form a device capable of producing enhanced total photovoltage by a dual-absorber Z-scheme mechanism. Both approaches resulted in a cathodic shift of the photocurrent onset potential, signifying enhanced power output and progress toward the unassisted photoelectrolysis of water. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Submitted by PPG Engenharia e Tecnologia de Materiais (engenharia.pg.materiais@pucrs.br) on 2018-06-28T19:58:26Z No. of bitstreams: 1 FICHA_CATALOGR?FICA_EMBUTIDA_Disserta??o Cristiano_Jaeger Stradolini_Processo Poliol.pdf: 3080490 bytes, checksum: 6539be3aad396dd1f8b30cd3389b1469 (MD5) / Approved for entry into archive by Sheila Dias (sheila.dias@pucrs.br) on 2018-07-06T13:07:04Z (GMT) No. of bitstreams: 1 FICHA_CATALOGR?FICA_EMBUTIDA_Disserta??o Cristiano_Jaeger Stradolini_Processo Poliol.pdf: 3080490 bytes, checksum: 6539be3aad396dd1f8b30cd3389b1469 (MD5) / Made available in DSpace on 2018-07-06T13:12:44Z (GMT). No. of bitstreams: 1 FICHA_CATALOGR?FICA_EMBUTIDA_Disserta??o Cristiano_Jaeger Stradolini_Processo Poliol.pdf: 3080490 bytes, checksum: 6539be3aad396dd1f8b30cd3389b1469 (MD5) Previous issue date: 2018-03-22 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / During the past few years, nanosized noble metals have attracted much attention due to their unique electrical and physical properties. Among them, silver has been the subject of several studies, by having the highest electrical and thermal conductivity between all metals. For that reason, its one-dimensional nanometric structure (nanowires) has been receiving a lot of attention, due to its potential in the manufacture of devices that offer good electrical conduction and optical transparency. There are several methods of synthesis for the production of these silver nanowires, however the most widely used is the polyol process, due to its simplicity and low cost. Several studies show the influence of different parameters of the polyol process (reaction time, temperature and etc.) on the final morphology of the nanowires. Thus, a well-defined protocol of good practice for obtaining long and thin silver nanowires is required. The main objective of this work is the development of a low cost protocol capable of generating nanowires with great lengths by modifying the polyol process parameters. Variations were also performed in the method of addition of the main synthesis reagent, silver nitrate, and its influence on the final morphology of the silver nanowires was studied. In this work, by using the polyol process, nanowires up to 40 ?m in length were reported, demonstrating the effectiveness of the developed technique and conclusions about the final results were presented. / Ao decorrer dos ?ltimos anos, metais nobres na escala nanom?trica t?m atra?do muita aten??o, devido as suas propriedades el?tricas e f?sicas ?nicas. Dentre eles, a prata vem sendo alvo de diversos estudos, por possuir a maior condutividade el?trica e t?rmica entre todos os metais. Por conta disto, sua estrutura nanom?trica unidimensional (nanofios) vem recebendo muita aten??o, pelo seu potencial na fabrica??o de dispositivos que exijam alta condutividade el?trica e transpar?ncia ?ptica. Atualmente, existem diversos m?todos de s?nteses para a produ??o destes nanofios de prata, por?m o mais utilizado ? o processo poliol, por ser simples e de baixo custo. Diversos estudos demonstram a influ?ncia de diferentes par?metros do processo poliol (tempo de rea??o, temperatura e etc.) sobre a morfologia final dos nanofios. Assim, se faz necess?rio um protocolo bem definido de boas pr?ticas para a obten??o de fios longos e finos. O principal objetivo deste trabalho ? o desenvolvimento de um protocolo de baixo custo capaz de gerar nanofios com grandes comprimentos, atrav?s de altera??es nas vari?veis do processo poliol, como, por exemplo, varia??es no m?todo de adi??o do principal reagente da s?ntese, o nitrato de prata. Foi estudada a sua influ?ncia na morfologia final dos nanofios. Neste trabalho, os resultados finais apresentaram que nanofios de at? 40 ?m de comprimento foram produzidos pelo processo poliol, demonstrando a efic?cia da t?cnica desenvolvida e foram apresentadas as conclus?es acerca dos resultados atingidos.

This thesis studies networks of silver nanowires as a transparent conducting electrode material and presents an investigation into the relationship between electrical and optical properties in the networks. The work focusses on two main aspects: the production of networks via different deposition methods; and the development of a predictive model based on theory that relates the sheet resistance to the optical transmittance. The deposition methods of drop-casting, bar-coating and spray-coating are used to create networks and the randomness of these networks is compared using image analysis in ImageJ, a public domain image processing program, and Wolfram Mathematica, a computer algebra program. It is determined that spray-coating results in the most random networks, therefore all subsequent experiments are carried out using this as the deposition method. Annealing condition tests are carried out on the nanowire networks to determine the optimal annealing conditions required to burn off poly(vinyl pyrrolidone) (PVP) remaining from the nanowire synthesis process and sinter the nanowire junctions to improve network conductivity. The sheet resistances and optical transmittances of the networks are measured and compared to networks created by other research groups. It is found that the networks created in this study exhibited similar optical and electrical properties to those in the literature, obtaining Rs = 100 Ω/sq for T = 81%.The developed model is based on theory and relates the sheet resistance to the optical transmittance using only the length and width of the nanowires used in the network and the mean network coverage as variables. The model can be used to predict the properties of a network if these factors are known. The model is compared with experimental data both from this study and from the literature, along with simulated data from the literature that was obtained by Monte Carlo methods. It is shown that there is an excellent fit between the model and all data that it is compared with. It is demonstrated that < 1% of the network coverage is greater than 2 for typical nanowire networks, proving that the networks are two-dimensional and therefore do not require a bulk regime to describe the relationship, as has been suggested in prior work.

Cheung Lok Ying Teresa = 竹炭模板上形成的二氧化硅與碳化硅納米線及其表徵 / 張樂影. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Cheung Lok Ying Teresa = Zhu tan mo ban shang xing cheng de er yang hua gui yu tan hua gui na mi xian ji qi biao zheng / Zhang Leying. / ABSTRACT --- p.i / 摘要 --- p.iii / ACKNOWLEDGEMENTS --- p.v / TABLE OF CONTENTS --- p.vii / LIST OF FIGURE CAPTIONS --- p.xi / LIST OF TABLE CAPTIONS --- p.xv / Chapter Chapter 1 --- Introduction / Chapter 1.1. --- Biomineralization process --- p.1 / Chapter 1.2. --- Structures in biological matters --- p.2 / Chapter 1.2.1. --- Bone --- p.2 / Chapter 1.2.2. --- Wood --- p.3 / Chapter 1.3. --- Biomorphic products and their potential applications --- p.4 / Chapter 1.3.1 --- Environment conscious products --- p.4 / Chapter 1.3.2 --- Properties --- p.5 / Chapter 1.3.3 --- Carbide and oxide composites --- p.5 / Chapter 1.4 --- Common fabrication approaches --- p.6 / Chapter 1.4.1 --- Gaseous infiltration --- p.7 / Chapter 1.4.2 --- Liquid infiltration --- p.7 / Chapter 1.4.3 --- Sol-gel method --- p.8 / Chapter 1.5 --- Growth of nanowires --- p.8 / Chapter 1.5.1 --- Vapor-liquid-solid (VLS) mechanism --- p.9 / Chapter 1.5.2 --- Solution-liquid-solid (SLS) mechanism --- p.9 / Chapter 1.5.3 --- Vapor-solid (VS) mechanism --- p.10 / Chapter 1.6 --- Goals of the project --- p.10 / Chapter 1.6.1 --- Feedbacks on previous works --- p.10 / Chapter 1.6.2 --- Our breakthrough approach --- p.11 / Chapter 1.7 --- Thesis layout --- p.12 / References --- p.13 / Figures --- p.17 / Chapter Chapter 2 --- Samples preparation and characterization methods / Chapter 2.1 --- Materials selections --- p.22 / Chapter 2.1.1 --- Selection of materials for biomorphic substrate --- p.22 / Chapter 2.1.2 --- Selection of solution for infiltration --- p.23 / Chapter 2.2 --- Samples preparation --- p.24 / Chapter 2.2.1 --- Pyrolysis of raw bamboo --- p.24 / Chapter 2.2.2 --- Infiltration of reactants --- p.25 / Chapter 2.2.3 --- Sintering conditions --- p.26 / Chapter 2.3 --- Characterization methods --- p.26 / Chapter 2.3.1 --- Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) --- p.27 / Chapter 2.3.2 --- Transmission electron microscopy (TEM) and EDS --- p.27 / Chapter 2.3.3 --- High-resolution transmission electron microscopy (HRTEM) --- p.27 / Chapter 2.3.4 --- X-ray diffraction (XRD) --- p.28 / Chapter 2.3.5 --- X-ray photoelectron spectroscopy (XPS) --- p.28 / Chapter 2.3.6 --- Fourier transform infrared spectroscopy (FTIR) --- p.29 / Chapter 2.4 --- Conclusions --- p.29 / References --- p.30 / Figures --- p.31 / Tables --- p.33 / Chapter Chapter 3 --- Bamboo biomorphic substrates / Chapter 3.1 --- Overview --- p.34 / Chapter 3.2 --- Volumetric shrinkage and weight loss --- p.34 / Chapter 3.3 --- General morphology --- p.35 / Chapter 3.4 --- Intrinsic impurities --- p.35 / Chapter 3.5 --- Structures --- p.36 / References --- p.38 / Figures --- p.39 / Chapter Chapter 4 --- Silica / Chapter 4.1 --- Overview --- p.43 / Chapter 4.2 --- Thermal analyses --- p.44 / Chapter 4.2.1 --- Raw biomorphic substrates --- p.44 / Chapter 4.2.2 --- Infiltrated biomorphic substrates --- p.45 / Chapter 4.3 --- Experiments --- p.46 / Chapter 4.4 --- Characterization in general --- p.47 / Chapter 4.4.1 --- General morphologies --- p.47 / Chapter 4.4.2 --- Tips of the SiO2 nanowires --- p.48 / Chapter 4.5 --- Effects of sintering duration --- p.48 / Chapter 4.6 --- Effects of sintering temperature --- p.49 / Chapter 4.6.1 --- "Different temperatures, held for one hour" --- p.50 / Chapter 4.6.2 --- "Different temperatures, held for ten hours" --- p.50 / Chapter 4.7 --- Growth mechanisms --- p.52 / Chapter 4.8 --- Summary --- p.54 / References --- p.56 / Figures --- p.58 / Tables --- p.75 / Chapter Chapter 5 --- Silicon carbide / Chapter 5.1 --- Overview --- p.76 / Chapter 5.2 --- Experiments --- p.77 / Chapter 5.3 --- Characterization in general --- p.78 / Chapter 5.3.1 --- General morphologies --- p.78 / Chapter 5.3.2 --- Stacking faults and twinning --- p.79 / Chapter 5.3.3 --- Tips of the SiC nanowires --- p.81 / Chapter 5.4 --- Growth mechanisms --- p.82 / Chapter 5.5 --- Summary --- p.83 / References --- p.84 / Figures --- p.87 / Chapter Chatper 6 --- Conclusions and future works / Chapter 6.1 --- Main results of this projects --- p.101 / Chapter 6.2 --- Future works --- p.103 / References --- p.104

Orientador: Mônica Alonso Cotta / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-02T10:59:50Z (GMT). No. of bitstreams: 1 Nica_MarianaZavarize_M.pdf: 75724631 bytes, checksum: 1480ea9eb35b1740d4b908e32d13c9c4 (MD5) Previous issue date: 2017 / Resumo: Neste trabalho, estudamos o crescimento de nanofios planares de InP pelo mecanismo Vapor-Líquido-Sólido (VLS), com o objetivo de entender a dinâmica da nanopartícula metálica catalisadora durante o processo. Para isso utilizamos substratos de GaAs (111)A e o sistema de Epitaxia de Feixe Químico (CBE). O óxido nativo não foi totalmente removido termicamente antes do crescimento, com o objetivo de manter o nanofio isolado eletricamente do substrato. Como um dos objetivos do trabalho, estudamos a possibilidade de controle da direção de crescimento do nanofio planar através de diferentes tratamentos de superfície, e de modo independente da cristalografia do substrato utilizado. As amostras processadas e/ou crescidas foram caracterizadas por técnicas de microscopia eletrônica (varredura e transmissão) e microscopia de força atômica. Investigamos inicialmente como a camada de óxido influencia as direções de crescimento dos nanofios planares no substrato não tratado. Posteriormente, processamos padrões de linhas com rugosidade ligeiramente diferente da mostrada pelo substrato, utilizando técnicas como Litografia por Feixe de Elétrons (EBL), Corrosão por Feixe de Íons Focalizados (FIB) e Ataque por Íons Reativos (RIE). Os padrões gravados eram compostos por linhas perpendiculares com várias micra de comprimento e larguras de dezenas de nm. Observamos que existe uma relação direta do diâmetro do nanofio com a orientação que este assume ao chegar à região onde se encontra a linha (se segue alinhado à linha ou se a ignora; ou se muda sua orientação). Nossos resultados podem ser explicados pelas diferentes energias de superfície presentes no problema, que afetam a dinâmica da nanopartícula catalisadora. Nosso trabalho também mostra que é possível obter maior controle da orientação espacial do nanofio planar crescido, controlando o processamento da superfície e o diâmetro da nanopartícula / Abstract: In this work, we studied the growth of InP planar nanowires by the vapor-liquid-solid (VLS) mechanism, in order to understand the metallic catalyst nanoparticle dynamics during this process. In our studies, we used GaAs (111)A substrates and the Chemical Beam Epitaxy (CBE) system. The native oxide layer was not completely thermally desorbed, in order to keep the nanowire electrically isolated from the substrate. As one of the goals of this work, we study the possibility to control nanowire growth direction via different surface treatments, independently of the substrate crystallography. Our processed and/or grown samples were characterized by electron (scanning and transmission) and atomic force microscopy. We first investigated how the oxide layer influences the growth directions of planar nanowires on unprocessed substrates. Subsequently, patterns of lines with roughness slightly different from those shown by the substrate were patterned using techniques such as Electron-beam Lithography (EBL), Focused Ion-beam Corrosion (FIB) and Reactive Ion Etching (RIE). These patterns were composed of perpendicular lines with several micra in length and tens of nanometers wide. We observed that there is a direct relation between the nanowire diameter and the orientation that the nanowire assumes when it reaches the region where the line is located (if the nanowire aligns with the line or ignores it, or if its orientation changes). Our results can be explained by the different surface energies present in the problem, which affect the dynamics of the catalytic nanoparticle. Our work also shows that it is possible to obtain better control of the spatial orientation of the grown planar nanowire, by controlling the surface processing and the nanoparticle diameter / Mestrado / Física / Mestra em Física / 132655/2015-2 / CNPQ

Leung Yee Pan = 利用有機金屬化學氣相沉積方法生長一維硒化鋅鈉米結構 / 梁懿斌. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 80-82). / Text in English; abstracts in English and Chinese. / Leung Yee Pan = li yong you ji jin shu hua xue qi xiang chen ji fang fa sheng chang yi wei xi hua xin na mi jie gou / Liang Yibin. / Acknowledgements --- p.ii / Abstract --- p.iii / Chapter Chapter 1 - --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Motivation --- p.3 / Chapter 1.2.1 --- ZnSe --- p.3 / Chapter 1.2.2 --- MOCVD --- p.3 / Chapter 1.3 --- Our Work --- p.4 / Chapter Chapter 2 - --- Experiment --- p.5 / Chapter 2.1 --- MOCVD System --- p.5 / Chapter 2.2 --- Metalorganic Sources --- p.5 / Chapter 2.3 --- Substrates --- p.7 / Chapter 2.4 --- Substrate Preparations --- p.7 / Chapter 2.5 --- Preheating (Applied Only when Using GaAs Substrates) --- p.7 / Chapter 2.6 --- Growth of Epi-layer (Applied Only when Using GaAs Substrates) --- p.8 / Chapter 2.7 --- Growth of ZnSe Nanowires on Si(100) and ZnSe/GaAs(100) --- p.8 / Chapter 2.8 --- The Samples --- p.9 / Chapter Chapter 3 - --- Characterization --- p.10 / Chapter 3.1 --- Surface Morphologies --- p.10 / Chapter 3.1.1 --- Scanning Electron Microscopy --- p.10 / Chapter 3.1.2 --- Atomic Force Microscopy --- p.12 / Chapter 3.2 --- Structural Properties - X-Ray Diffraction --- p.13 / Chapter 3.3 --- Optical Properties - Photoluminescence --- p.15 / Chapter 3.4 --- Other Techniques --- p.16 / Chapter Chapter 4 - --- Results --- p.17 / Chapter 4.1 --- ZnSe Nanowires Grown on Si(100) --- p.17 / Chapter 4.1.1 --- Effect of Growth Temperature --- p.17 / Chapter 4.2 --- Growth of ZnSe Nanowires on GaAs( 100) - The First Trial --- p.20 / Chapter 4.3 --- Optimizing the ZnSe Epi-layer --- p.21 / Chapter 4.3.1 --- Surface of GaAs(100) --- p.21 / Chapter 4.3.2 --- ZnSe Epi-layer Grown at Different Reactor Pressures --- p.22 / Chapter 4.4 --- Importance of Au --- p.26 / Chapter 4.5 --- Growth of ZnSe Nanowires on GaAs(lOO) - A Systematic Study --- p.28 / Chapter 4.5.1 --- Growth Rates --- p.28 / Chapter 4.5.2 --- Overall Morphologies --- p.32 / Chapter 4.5.3 --- Classifying the Morphologies --- p.37 / Chapter 4.5.4 --- Abundances of Different Morphologies of Different Samples --- p.40 / Chapter 4.5.5 --- Growth Direction --- p.45 / Chapter 4.5.6 --- Structure of the Nanowires --- p.50 / Chapter 4.5.7 --- Optical Properties of the Nanowires --- p.54 / Chapter Chapter 5 - --- Discussions --- p.57 / Chapter 5.1 --- Overview of the MOCVD Process --- p.57 / Chapter 5.1.1 --- Effects of Growth Temperature on Growth Rate of MOCVD --- p.58 / Chapter 5.1.2 --- Effects of Reactor Pressure on Growth Rate of MOCVD --- p.59 / Chapter 5.2 --- Effect of Reactor Pressure on the Growth Rate of the Nanowires --- p.60 / Chapter 5.3 --- Growth Mechanisms of the Nanowires --- p.64 / Chapter 5.3.1 --- VLS Mechanism --- p.64 / Chapter 5.3.2 --- Spiral Growth Mechanism --- p.66 / Chapter 5.3.3 --- Reentrant Corner Mechanism --- p.67 / Chapter 5.3.4 --- Roles of Au Particles and ZnSe Epi-layer --- p.68 / Chapter 5.3.5 --- Growth Mechanisms of Different Types of Nanowires --- p.69 / Chapter 5.3.6 --- Effect of Growth Temperature --- p.71 / Chapter 5.4 --- Quality of the Nanowires --- p.72 / Chapter 5.5 --- "Remarks of the AFM Experiments and the ""Transferred"" Samples" --- p.72 / Chapter Chapter 6 - --- Conclusions --- p.75 / Appendices --- p.77 / Chapter I - --- "Estimation of the mass, other than the nanowires, contributed to the sample" --- p.77 / Chapter II - --- Calculation of the growth angle with respect to the surface normal --- p.78 / References --- p.80

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-08-06T12:19:22Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) udson cabral.pdf: 2045459 bytes, checksum: 0c5126468995368f097978ee97cb41f5 (MD5) / Made available in DSpace on 2014-08-06T12:19:22Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) udson cabral.pdf: 2045459 bytes, checksum: 0c5126468995368f097978ee97cb41f5 (MD5) Previous issue date: 2010 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / We investigate core-shell nanowires of diluted magnetic semiconductors (DMS) with remote n-type modulation doping. The incorporation of Mn2 ions acting as spin 5/2 impurities in the core region of the wire gives rise to a strong s-d exchange coupling between electrons in the wire and those of the d levels of the Mn2 ions. Applying an external magnetic eld along the axis of the wire, within the mean eld approximation, the s-d exchange generates a spin-dependent core potential. A gate voltage is applied radially to wire, to obtain some control over the density of the wire. Electronic strucutre of the wire was calculated within the e?ective mass approximation, in both approximations Hartree and spin density functional theory. We calculated the conductance of wire using the Landauer-B?uttiker formulation in the linear response regime, which generally results in a total conductance with well-de ned plateaus in GT = 2; 6; 10G0 (G0 = e2=h is the quanta of conductance), which occurred because in the system investigated the rst level is twofold degenerated (spin degenerescence) and the others are fourfold degenerated (spin degenerescence and orbital angular momentum). In the absence of a magnetic eld we observe that when we take into account the e?ects of exchange and correlation, the states with eigenvalues of Lz nonzero will be polarized while those with l = 0 isn't polarized. This unpolarized level with eigenvalue of Lz null suggests that, perhaps, the 0.7 anomaly (the emergence of two plateau at G = 0:7G0 and the other in G = G0) quantum wires on existing geometry of split-gate is related to the geometry of the wire. The results for total energy show that there are a competition between the ferromagnetic and paramagnetic states. / Investigamos nano fios de semicondutores magnéticos dilu??dos (DMSs - Diluted Magnetic Semiconductors) do tipo caroço-casca com dopagem remota tipo-n. A incorporação dos íons de Mn+2, que atuam como impurezas de spin 5/2 no caroço do fi o, faz surgir um forte acoplamento de trocas dentre os eletrons do fio e aqueles dos níveis d do íon Mn+2. Com a aplicação de um campo magnético externo ao longo do eixo do fi o, na aproximação de campo médio, a interação de troca s-d gera um potencial dependente do spin na região do caroço do fi o. Um potencial de gate é aplicado radialmente ao nanofi o, para obtermos um certo controle sobre a densidade eletrônica do fi o. Calculamos a estrutura eletrônica do nanofi o de DMSs usando o modelo da massa efetiva, tanto na aproximação de Hartree quanto na teoria do funcional da densidade dependente de spin (SDFT - Spin Density Functional Theory). Calculamos a condutância do nano fio usando a formulação de Landauer-B?uttiker no regime de resposta linear, o que de modo geral, resultou numa condutância total com platôs bem de finidos em GT = 2; 6; 10G0 (G0 = e2=h ?e o quanta de condutância), o que ocorreu porque no sistema investigado a primeira subbanda ?e duplamente degenerada (degenerescência de spin) e as outras duas são quadruplamente degenerada (degenerescência de spin e de momento angular orbital). Na ausência de um campo magnético observamos que ao levarmos em conta os efeitos de troca e correlação, os estados que possuem autovalor de Lz diferente de zero se polarizam enquanto que os que possuem l = 0 não se polarizam. Essa não-polarização do nível com autovalor de Lz nulo sugere que, talvez, a anomalia 0,7 (o surgimento de dois platôs um em G = 0; 7G0 e outro em G = G0) existente em os quânticos com geometria de split-gate esteja relacionada com a geometria do o. Os resultados obtidos para a energia total mostram que há uma competição entre os estados ferromagnético e paramagnéticos.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-08-23T17:31:16Z No. of bitstreams: 1 AndreCortez.pdf: 8621619 bytes, checksum: a2c58769de7cc646bd6e13cee7259a78 (MD5) / Made available in DSpace on 2017-08-23T17:31:16Z (GMT). No. of bitstreams: 1 AndreCortez.pdf: 8621619 bytes, checksum: a2c58769de7cc646bd6e13cee7259a78 (MD5) Previous issue date: 2012-01-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / In this work we address a theoretical calculation of the electronic spectra in Si-Ge atomic chain that is arranged in a Fibonacci quasi-periodic sequence, by using semi-empirical quantum method based on Hückel extended model. We apply the Fibonacci substitutional sequences in the atomic building blocks A(Si) and B(Ge) through the in ation rule or recursion relation. In our ab initio calculations we use only a single point which is the sufi cient condition to consider all the orbitals and charge distribution across the entire system. Although the calculations presented here are more complete than the models adopted in the literature which take into account the electronic interaction, up to the second and third neighbors, an interesting property remains in their electronic spectra: the fractality (which is the main signature of this kind of system). We discuss this fractality of the spectra and we compare them with the randomic arrangement of Si-Ge atomic chain, and with previous results based on the tight-binding approximation of Schorödinger equation considering until the nearest neighbor. / Neste trabalho abordamos um cálculo teórico dos espectros eletrônicos em cadeia atômica de Si-Ge que é organizado em uma sequência Fibonacci quasi-periódica, usando o método semi-empírico com base no modelo quântico Hückel estendido. Nós aplicamos as sequências substitucional de Fibonacci nos blocos de construção atômica A(Si) e B(Ge) através da relação de recursão. Em nossos cálculos ab initio, usamos apenas um único ponto, que é a condição sufi ciente para considerar todos os orbitais e a distribuição de cargas em todo o sistema. Embora os cálculos apresentados aqui sejam mais completos do que os modelos adotados na literatura que levam em conta a interação eletrônica, até segundos e terceiros vizinhos, uma propriedade interessante permanece em seus espectros eletrônicos: a fractalidade (que é a assinatura principal deste tipo de sistema). Discutimos essa fractalidade dos espectros e as comparamos com os espectros de cadeia atômica randômica de Si-Ge, e, também, com os resultados prévios baseados na aproximação tight-binding da equação Schorödinger considerando até o vizinho mais próximo.

O estudo e conhecimento do magnetismo é muito importante para a nossa sociedade, devido ao grande número de aplicações que envolvem as propriedades magnéticas. Por isso, umas das problemáticas vigentes em física do estado sólido é o estudo, conhecimento e controle deste tipo de propriedades em diferentes tipos de materiais, dentre os quais encontramse materiais de dimensões nanométrica como nanofios e nanotubos. No presente trabalho temos como objetivo a realização de estudos sobre propriedades magnéticas tais como anisotropia e magnetização em sistemas formados por nanofios de Ni eletrodepositados em alumina nanoporosa, e nanotubos de NiB crescidos em membrana porosa de policarbonato. A caracterização microestrutural das amostras de nanofios de Ni foi realizada por difração de raios X, microscopia eletrônica de varredura, microscopia de força atômica e microscopia eletrônica de transmissão de alta resolução. Os resultados mostraram que os nanofios de Ni tinham diâmetros de aproximadamente 60nm, são policristalinos com estrutura predominante fcc, embora também foram encontrados grãos minoritários de estrutura hcp. O tamanho de grão esteve na faixa entre 5 e 15 nm, aproximadamente. Por outro lado, os nanotubos de NiB estavam formados por uma liga amorfa de composição Ni80B20, com a presença de uma porção minoritária de pequenos cristalitos de NiB. Os nanotubos tinham comprimentos de 20 ?m aproximadamente, diâmetro externo de 400 nm e espessuras das paredes na faixa entre 50 e 160 nm. Todas as amostras foram caracterizadas magneticamente a partir de medidas de ciclos de histerese, curvas de remanências isotérmica e desmagnetizante (para a obtenção de curvas ?M) e curvas de inversão de primeira ordem (para o cálculo das distribuições FORC). Um novo modelo fenomenológico para estudar anisotropias, desenvolvido em nosso grupo de pesquisa, permitiu a determinação das constantes de anisotropias K1 (constante de anisotropia magnetocristalina de segunda ordem) e Ku (constante de anisotropia uniaxial) nos sistemas de nanofios de Ni. Os resultados mostraram que a constante de anisotropia K1 cresce conforme aumenta o comprimento dos nanofios, ficando próximo e até superior ao valor correspondente ao Ni massivo. O processo de nucleação e propagação de paredes de domínios transversais foi confirmado como mecanismo de inversão da magnetização nos nanofios, por meio da aplicação de um modelo analítico que determina o campo de nucleação do sistema. A importância das interações dipolares no comportamento magnético global destes sistemas foi evidenciada pela determinação das curvas ?M e das distribuições FORC. No caso dos nanotubos de NiB foi determinado que o mecanismo de inversão da magnetização corresponde à nucleação e propagação de vórtices, como acontece em outros sistemas similares. No entanto, foi observado que a espessura da parede do tubo tem um papel importante, porque foi verificado que a coercividade dos nanotubos decresce se a parede deles é suficientemente grossa em comparação com o tamanho de parede de domínio do Ni. Também para estes sistemas, as curvas ?M e das distribuições FORC mostraram a predominância de interações desmagnetizantes. / The study and knowledge of magnetism is very important for our society, due to the large number of applications involving magnetic properties. Therefore, one of the current problems in solid state physics is the study, knowledge and control of this type of properties in different types of materials, among which are materials of nanometric dimensions such as nanowires and nanotubes. In the present work, we have carried out studies on magnetic properties such as anisotropy and magnetization in systems formed by Ni nanowires electrodeposited in nanoporous alumina, and NiB nanotubes grown in polycarbonate porous membrane. The microstructural characterization of Ni nanowire samples was performed by X-ray diffraction, scanning electron microscopy, atomic force microscopy and high resolution transmission electron microscopy. The results showed that Ni nanowires had diameters of approximately 60 nm, are polycrystalline with predominant fcc structure, although a minor fraction of hcp structure was also found. The grain size was in the range between 5 and 15 nm, approximately. On the other hand, the NiB nanotube samples were formed by an amorphous alloy of Ni80B20 composition, with the presence of a minor portion of small NiB crystallites. The nanotubes had lengths of approximately 20 µm, external diameter of 400 nm and wall thicknesses in the range of 50 to 160 nm. All the samples were magnetically characterized by measurements of hysteresis cycles, isothermal and demagnetizing remanence curves (to obtain ?M curves) and first-order reversal curves (for calculating FORC distributions). A new phenomenological model to study anisotropies, developed in our research group, allowed the determination of the anisotropy constants K1 (second order magnetocrystalline anisotropy constant) and Ku (uniaxial anisotropy constant) in the Ni nanowire systems. The results showed that the anisotropy constant K1 increases as the nanowire length increases, being close to and even higher than the Ni bulk value. The process of nucleation and propagation of transverse domain walls was confirmed as the mechanism for magnetization reversal in nanowires. This result as obtained by applying an analytical model that determines the nucleation field of the system. The importance of the dipolar interactions on the global magnetic behavior for these systems was evidenced by the determination of the ?M curves and the FORC distributions. In the case of NiB nanotubes, it was determined that the magnetization reversal mechanism corresponds to the nucleation and propagation of vortices, as in other similar systems. However, it has been observed that the wall thickness of the tube plays an important role because it has been found that the coercivity of the nanotubes decreases if their walls are thick enough compared to the Ni domain wall size. Also, for these systems, the ?M curves and the FORC distributions showed the predominance of demagnetizing interactions.