>Magister Scientiae - MSc / One-dimensional silicon nanowires (SiNWs) are promising building blocks for solar cells as they provide a controlled, vectorial transport route for photo-generated charge carriers in the device as well as providing anti-reflection for incoming light. Two major approaches are followed to synthesise SiNWs, namely the bottom-up approach during vapour-liquid-solid mechanism which employs chemical vapour deposition techniques. The other method is the top-down approach via metal assisted chemical etching (MaCE). MaCE provides a simple, inexpensive and repeatable process that yields radially and vertically aligned SiNWs in which the structure is easily controlled by changing the etching time or chemical concentrations. During MaCE synthesis, a crystalline silicon (c-Si) substrate covered with metal nanoparticles (catalyst) is etched in a diluted hydrofluoric acid solution containing oxidising agents. Since the first report on SiNWs synthesised via MaCE, various publications have described the growth during the MaCE process. However lingering questions around the role of the catalyst during formation, dispersion and the eventual diameter of the nanowires remain. In addition, very little information pertaining to the changes in crystallinity and atomic bonding properties of the nanowires post synthesis is known. As such, this study investigates the evolution of vertical SiNWs from deposited silver nanoparticles by means of in-depth electron microscopy analyses. Changes in crystallinity during synthesis of the nanowires are probed using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Deviations in the optical properties are quantified using optical reflectivity measurements by employing ultraviolet-visible (UV-Vis) spectroscopy, whereas the bonding configurations of the nanowires are probed by Raman and Fourier transforms infrared spectroscopy. Diameters of 50 – 200 nm vertical SiNWs were obtained from scanning electron micrographs and nanowires lengths linearly increased with etching time duration from about 130 nm after 30 seconds to over 15 μm after 80 minutes. No diameter modulations along nanowires axial direction and rough nanowires apexes were observed for nanowires obtained at longer etching times. These SiNWs remained crystalline as their bulk single crystalline Si wafers but had a thin amorphous layer on the surface, findings confirmed by TEM, XRD and Raman analysis. Nanowires were found to be partially passivated with oxygen with small traces of hydrogen termination, confirmed with infrared absorption studies. Finally, low optical reflection of less than 10% over visible range compared to an average of 30% for bulk Si were measured depicting an antireflective ability required in silicon solar cells.

xii, 106 p. : ill. (some col.) / Quantum dots are systems in which all three spatial sizes are comparable to the Fermi wavelength. The strong confinement leads to a discrete energy spectrum. A goal of thermoelectric research is to find a system with a high thermoelectric figure of merit, which is related to the efficiency of solid-state heat engines. The delta-like density of states of quantum dots has been predicted to boost this figure of merit. This dissertation addresses some thermoelectric properties relevant to the thermal-to-electric energy conversion using InAs/InP quantum dots embedded in nanowires. In thermoelectric experiments, a temperature difference must be established and its value needs to be determined. A novel technique for measuring electron temperature across the dot is presented. A strong nonlinearity of the thermocurrent as a function of temperature difference is observed at a small ratio of temperature gradient and cryostat temperature. At large heating currents, a sign reversal is observed. Numerical calculations explore the contribution of the energy dependence of the transmission function to this effect. Depending on the relative contributions from sequential tunneling and co-tunneling, thermovoltages of quantum dots generally have one of two different lineshapes: a sawtooth shape or a shape similar to the derivative of the conductance peak. Here a simple picture is presented that shows that thermovoltage lineshape is accurately predicted from the energy level spacing inside the dot and the width of the transmission function. An important figure of merit of all heat engines is the efficiency at maximum power. Here the thermoelectric efficiency at maximum power of quantum dots is numerically compared to that of two other low-dimensional systems: an ideal one-dimensional conductor (1D) and a thermionic power generator (TI). The numerical calculations show that either 1D or TI systems can produce the highest maximum power depending on the operating temperature, the effective mass of the electron, and the effective area of the TI system. In spite of this, 1D systems yield the highest efficiency at maximum power. / Committee in charge: Dr. Richard Taylor, Chair; Dr. Heiner Linke, Research Advisor; Dr. Dietrich Belitz; Dr. David Johnson; Dr. David Strom

abstract: Nanowires are 1D rod-like structures which are regarded as the basis for future technologies. III-V nanowires have attracted immense attention because of their stability, crystal quality and wide use. In this work, I focus on the growth and characterization of III-V semiconductor nanowires, in particular GaP, InP and InGaP alloys. These nanowires were grown using a hot wall CVD(Chemical Vapor Deposition) setup and are characterized using SEM (Scanning Electron Microscope), EDX (Energy Dispersive X-ray Spectroscopy) and PL (Photoluminescence) techniques. In the first chapter, Indium Phosphide nanowires were grown using elemental sources (In and P powders). I consider the various kinds of InP morphologies grown using this method. The effect of source temperature on the stoichiometry and optical properties of nanowires is studied. Lasing behavior has been seen in InP nanostructures, showing superior material quality of InP. InGaP alloy nanowires were grown using compound and elemental sources. Nanowires grown using compound sources have significant oxide incorporation and showed kinky morphology. Nanowires grown using elemental sources had no oxide and showed better optical quality. Also, these samples showed a tunable alloy composition across the entire substrate covering more than 50% of the InGaP alloy system. Integrated intensity showed that the bandgap of the nanowires changed from indirect to direct bandgap with increasing Indium composition. InGaP alloy nanowires were compared with Gallium Phosphide nanowires in terms of PL emission, using InGaP nanowires it is possible to grow nanowires free of defects and oxygen impurities, which are commonly encountered in GaP nanowires. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2016

abstract: Zinc telluride (ZnTe) is an attractive II-VI compound semiconductor with a direct bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared to the two dimensional (2D) thin-film semiconductors, one-dimensional (1D) nanowires can have different electronic properties for potential novel applications. In this work, we present the study of ZnTe nanowires (NWs) that are synthesized through a simple vapor-liquid-solid (VLS) method. By controlling the presence or the absence of Au catalysts and controlling the growth parameters such as growth temperature, various growth morphologies of ZnTe, such as thin films and nanowires can be obtained. The characterization of the ZnTe nanostructures and films was performed using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), high- resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy and light scattering measurement. After confirming the crystal purity of ZnTe, two-terminal diodes and three-terminal transistors were fabricated with both nanowire and planar nano-sheet configurations, in order to correlate the nanostructure geometry to device performance including field effect mobility, Schottky barrier characteristics, and turn-on characteristics. Additionally, optoelectronic properties such as photoconductive gain and responsivity were compared against morphology. Finally, ZnTe was explored in conjunction with ZnO in order to form type-II band alignment in a core-shell nanostructure. Various characterization techniques including scanning electron microscopy, energy-dispersive X-ray spectroscopy , x-ray diffraction, Raman spectroscopy, UV-vis reflectance spectra and photoluminescence were used to investigate the modification of ZnO/ZnTe core/shell structure properties. In PL spectra, the eliminated PL intensity of ZnO wires is primarily attributed to the efficient charge transfer process occurring between ZnO and ZnTe, due to the band alignment in the core/shell structure. Moreover, the result of UV-vis reflectance spectra corresponds to the band gap energy of ZnO and ZnTe, respectively, which confirm that the sample consists of ZnO/ZnTe core/shell structure of good quality. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

abstract: The larger tolerance to lattice mismatch in growth of semiconductor nanowires (NWs) offers much more flexibility for achieving a wide range of compositions and bandgaps via alloying within a single substrate. The bandgap of III-V InGaAsP alloy NWs can be tuned to cover a wide range of (0.4, 2.25) eV, appealing for various optoelectronic applications such as photodetectors, solar cells, Light Emitting Diodes (LEDs), lasers, etc., given the existing rich knowledge in device fabrication based on these materials. This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials. The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2018

La technologie de piles à combustible PEM (Proton Exchange Membrane) voit encore sa commercialisation limitée du fait de son coût élevé. L'un des éléments les plus coûteux est le catalyseur, constitué de platine, métal noble, représentant 25 % du coût global. L'étude mise en place dans le cadre de cette thèse s'oriente vers l'amélioration de l'utilisation de cet élément. La voie de nanostructuration s'avère d'un intérêt majeur afin de maintenir des tailles de structure proposant des propriétés électrocatalytiques intéressantes. De plus, l'élaboration électrochimique de catalyseurs présente l'avantage majeur de remplir l'une des conditions nécessaires en pile à combustible : le contact électronique. La réunion des deux précédents points nous a permis de mettre en place un procédé d'élaboration électrochimique de nanostructures, ensuite charactérisées par méthodes électrochimiques et physiques afin d'évaluer et de comprendre leurs propriétés catalytiques. / A key point for the Proton Exchange Membrane Fuel Cell (PEMFC) improvement and commercialization is the enhancement of mass specific electroactivity of platinum to reduce the cost (about 25% of the overall cost). The study set up as part of this thesis is directed towards improving the use of this element. Nanostructuration is of major interest to maintain the size structure offering interesting electrocatalytic properties. In addition, the development of electrochemical catalysts has the major advantage of satisfying the necessary conditions in fuel cell: the electronic contact. We propose an innovative process to elaborate a metal nanowires array on microporous substrate which allows gas diffusion by a simple electrodeposition method. As-made structures had been physically and electrochemically characterized to evaluate and understand their electrocatalytic activity.

Orientador: Edison Zacarias da Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T22:33:42Z (GMT). No. of bitstreams: 1 Amorim_EdgardPachecoMoreira_M.pdf: 126699894 bytes, checksum: 643d8eb527747175e4bea9eb00e13e73 (MD5) Previous issue date: 2006 / Resumo: Nanofios metálicos são objetos de estudo há pelo menos quinze anos e o interesse nestes novos materiais tem aumentado continuamente devido à possibilidade de utilizá-los como contatos metálicos em nanodispositivos eletrônicos. Por exemplo, entre duas pontas de nanofios ultrafinos, é possível inserir pequenas estruturas, tais como fulerenos ou moléculas orgânicas, podendo ser utilizados como dispositivos. A produção e a evolução de nanofios metálicos têm sido investigadas experimentalmente com o uso de várias técnicas, sendo a microscopia de transmissão de alta resolução uma das mais importantes. Enquanto há vasta literatura sobre o ouro, poucos trabalhos foram feitos sobre o cobre, outro importante metal. Neste trabalho estudamos a evolução dinâmica de nanofios de cobre sob tensão nas direções cristalográficas [111], [110] e [100] de elongação até a ruptura. A fim de comparar alguns resultados do cobre com o ouro fizemos o mesmo estudo para o ouro nas direções [111] e [110], para evidenciar diferenças e similaridades entre esses dois metais. O método utilizado para este estudo é a dinâmica molecular Tight-Binding. Este método mostrou-se uma excelente ferramenta para estudar a evolução dinâmica de nano½os metálicos, porque é mais preciso do que métodos utilizando potenciais empíricos já que calcula a estrutura eletrônica a cada passo na simulação e computacionalmente, muito mais rápido do que métodos de primeiros princípios. Apresentamos a evolução temporal das estruturas, a formação dos defeitos estruturais, a formação das pontas, cadeias atômicas lineares e as forças sustentadas pelos nanofios. Observamos que as forças sustentadas pelos nanofios têm comportamento dente-de-serra e que os nanofios de cobre formam cadeias atômicas lineares muito menores do que as formadas no ouro. Constatamos também a existência de estruturas helicoidais e comparamos a geometria estrutural entre nossos nanofios e imagens de microscopia. Nossos resultados estão em boa concordância com resultados experimentais / Abstract: Metallic nanowires have been a subject of study for at least ½fteen years. The interest in these new materials is increasing continuously due to the possibility of using them as metallic contacts in electronic nanodevices. For example, between two tips of very thin nanowires, it is possible to insert small structures, such as carbon buckyballs or small organic molecules, with the possibility of use as devices. The production and evolution of metal nanowires have been investigated experimentally with many techniques, the high resolution transmission electron microscopy one of the most important. While gold has received a lot of attention, copper which is also an important metal has had few works devoted to it. In this work we study the dynamical evolution of copper nanowires under stress along the [111], [110] and [100] crystallographic directions of elongation until their ruptures. In order to compare some results between copper and gold, we have performed the same study for gold in the [111] and [110] directions, to elucidate diÿerences and similarities between these metals. The method used was the Tight-Binding Molecular Dynamics (TBMD). The TBMD calculations have been show to be an excellent tool to study evolution of gold nanowires. This method is more accurate than empirical potential methods since it explicitly solves for the electronic structure at each time step, and it is much faster than first principles method. We present the dynamical evolution of the structures, the formation of structural defects, tips, linear atomic chains and the forces sustained by the nanowires. We observed that the forces sustained by the nanowires have a sawtooth behavior and that the copper nanowires form linear atomic chains which are much smaller than the chains formed in gold nanowires. We observed also, helicoidal structures and compare the structural geometry between our nanowires and images obtained from microscopy. Our results are in good agreement with the experimental results / Mestrado / Física da Matéria Condensada / Mestre em Física

Orientador: Oswaldo Luiz Alves / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-08T20:29:31Z (GMT). No. of bitstreams: 1 Romano_Ricardo_D.pdf: 7756171 bytes, checksum: bd3a8af5af06fb7ec50b3cd881d41bfd (MD5) Previous issue date: 2007 / Resumo: O desenvolvimento das nanoestruturas e suas aplicações compõem uma das áreas científicas em mais dinâmica ascensão. Grande parte dos estudos na área concentra-se nos métodos de preparação e, de uma forma geral, podem ser divididos em métodos físicos ou químicos. Os primeiros reúnem processos baseados em sistemas de feixes moleculares ou litografia, enquanto os últimos envolvem reações químicas em meios onde o crescimento dos cristais possa ser controlado e estabilizado. Nesta Tese foram preparadas nano(micro)estruturas de semicondutores II-VI e IV-VI a partir de três abordagens químicas distintas. Na primeira, foram obtidos nanocompósitos pelo encapsulamento de nanocristais de CdS e PbS no ambiente microporoso de um vidro transparente, comercialmente conhecido por Vycor®. A técnica envolvida foi a impregnação de peças do vidro com precursor single-source seguida por tratamento térmico visando a pirólise in situ do precursor. Efeitos de confinamento quântico no espectro óptico e micrografias eletrônicas de transmissão confirmaram a natureza nanométrica da fase ocluída. Na segunda, foram obtidos cristais nano e micrométricos de CdS, PbS e ZnS através da técnica conhecida por moldagem molecular por solvente coordenante, na qual precursores single-source foram tratados solvotermicamente em solventes coordenantes e levaram à formação de bastões de CdS, microestrelas de PbS e intercalatos de ZnS com etilenodiamina. Na última abordagem, nanocristais coloidais de CdSe com diferentes faixas de tamanho (2 a 7 nm) foram preparados com a finalidade de se estudar o comportamento da estrutura local e dinâmica vibracional do CdSe em função da redução no tamanho e substituição do agente de recobrimento usado na síntese / Abstract: The development of nanostructures and their applications constitute one of the most exciting scientific areas. A great number of studies in this area concern the preparation methods. Generally, they are classified in physical and chemical methods. The former class is based on molecular beam and lithography techniques, while the latter involves chemical reactions where crystal growth can be controlled and stabilized. In this Thesis, II-VI and IV-VI semiconductor nano(micro)structures were prepared according to three different approaches. In the first one, nanocomposites were obtained through the encapsulation of CdS and PbS nanocrystals into a porous and transparent commercial glass, named Vycor®. Glass pieces were impregnated with single-source precursors and, then, thermally treated in order to achieve in situ pyrolysis, making use of the porous environment as the stabilizer for the crystal growth. Quantum confinement effects in the optical spectrum and transmission electron micrographs characterized the nanometric dimensions of the occluded phase. In the second approach, a technique known as molecular templating by coordenant solvents was employed in order to obtaining CdS, PbS and ZnS nano(micro)crystals showing unusual morphologies. Single-source precursors were solvothermically treated in such solvents leading to CdS nanowires, PbS microstars and ZnS-ethylenediamine intercalates. In the last approach, colloidal CdSe nanocrystals with different size ranges (2 up to 7 nm) were prepared and employed in the study of the local structure and vibrational dynamics of CdSe as a function of crystal size reduction and substitution of the covering agent used in the synthetic procedure / Doutorado / Quimica Inorganica / Doutor em Ciências

Orientador: José Alexandre Diniz / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-24T05:49:16Z (GMT). No. of bitstreams: 1 Santos_MarcosViniciusPuydingerdos_M.pdf: 6478260 bytes, checksum: 702c164c26bda0f3d93109290d6f74a1 (MD5) Previous issue date: 2013 / Resumo: Neste trabalho é apresentado o desenvolvimento do processo de obtenção de nanofios de silício (SiNW) para aplicações em dispositivos MOS tridimensionais utilizando as técnicas de Feixe Íons Focalizados com íons de Gálio (GaFIB) e Litografia por Feixe de Elétrons (EBL). O processo completo de fabricação foi desenvolvido para a obtenção de transistores sem junção baseados em nanofios (junctionless nanowire transistors, JNT), escolhidos devido à facilidade de processamento ¿ comparativamente a outros dispositivos, como FinFETs ¿ e à ausência de efeitos de canal curto e perfuração MOS (punchthrough). Lâminas de tecnologia SOI (Silicon on Insulator) foram utilizadas como substrato. GaFIB/SEM ¿ um sistema de duplo feixe acoplado a um microscópio eletrônico de varredura -, com resolução nominal de feixe iônico de 20 nm, foi utilizado para a definição dos nanofios de silício com dopagem local por íons de Gálio (p+ - SiNW) e deposição de dielétrico de porta de SiO2 e eletrodos de fonte, dreno e porta de Platina. Para deposição dos eletrodos metálicos e do dielétrico de porta foi utilizado feixe de elétrons disponível no SEM de modo a evitar implantação iônica extra e evitar o processo de sputtering dos nanofios de silício. As dimensões do comprimento (LFin) e altura (HFin) do nanofio, comprimento (LPorta) e largura (WPorta) da porta foram, respectivamente, 6 ?m, 15 nm, 1 ?m e 35 nm. O estudo da condução de corrente elétrica no p+-SiNW foi feito por medidas elétricas em dispositivos pseudo-MOS utilizando o dióxido de silício enterrado (BOX) da lâmina SOI como dielétrico de porta para controlar a corrente através do p+-SiNW. Curvas de corrente entre fonte e dreno (IDS) versus tensão entre a porta das costas da lâmina e fonte (VBGS) indicam regime de acumulação para o p+-SiNW. Curvas IDS versus VDS indicam que o dispositivo JNT opera como um resistor controlado pela porta. Por outro lado, a técnica EBL ¿ com resolução nominal do feixe eletrônico 2 nm ¿ foi utilizada para a fabricação de dispositivos JNT do tipo nMOS - com dopagem de Arsênio (n+-SiNW) por implantação iônica -, juntamente com o sistema de deposição a partir de fase química, ECR-CVV (Electron Cyclotron Ressonance) para a definição dos nanofios utilizando o sistema de corrosão por plasma RF e formação de dielétrico de porta. Eletrodos de fonte, dreno e porta de Titânio e Alumínio foram depositados pela técnica de sputtering. As dimensões de largura (W) e comprimento (L), assim como o número de nanofios dos transistores foram variados para permitir uma excursão de até 3 ordens de grandeza da corrente elétrica do dispositivo. As dimensões mínimas obtidas para o comprimento (LFin) e altura (HFin) do nanofio, comprimento (LPorta) e largura (WPorta) da porta foram, respectivamente, 10 ?m, 15 nm, 100 nm e 50 nm O tempo médio para fabricação de um dispositivo JNT utilizando o sistema FIB é de aproxi-madamente 2 dias e seu custo médio é estimado em US$ 4,000.00. Por outro lado, a fabricação do dispositivo utilizando a técnica EBL demanda maior tempo ¿ aproximadamente 10 dias ¿, contudo custando menos de uma ordem de grandeza do valor do FIB (aproximadamente US$ 150.00). Os resultados obtidos revelam que os métodos desenvolvidos nos sistemas FIB e EBL para fa-bricação de nanofios de silício para aplicações em nanoeletrônica são inovadores no Brasil e permitem avanços consistentes em nanofabricação. Esses processos, já calibrados, contribuirão para o desenvolvimento de novos processos, como, por exemplo, transistores do tipo FinFET ou dispositivos baseados em nanofios / Abstract: This work presents the development for obtaining silicon nanowires (SiNW) for applications in 3D MOS devices using Focused Ion Beam with gallium ions (GaFIB) and Electron Beam Lithography (EBL) techniques. The complete fabrication process was developed for obtaining junctionless nanowire-based transistors, chosen due to the simplicity of processing and to the absence of short channel and punchthrough effects. Silicon on Insulator (SOI) wafers were used as substrate. GaFIB/SEM - a dual beam system coupled to a scanning electron microscope -, with nominal resolution for the ionic beam of 20 nm, was used to define silicon nanowires and dope them locally by gallium ions (p+-SiNW), in addition to deposit SiO2 dielectric gate and Pt source, drain and gate electrodes. Metal electrodes and gate dielectric deposition were taken place with the electron beam available in the SEM to avoid extra ion implantation and prevent sputtering process of silicon nanowires. The dimensions obtained for the nanowire length (LFin) and high (HFin), gate length (LGate) and width (WGate) were, respectively, 6 ?m, 15 nm, 1 ?m e 35 nm. The study of the driving electric current through p+-SiNW was achieved by electrical measurements in the pseudo-MOS devices using the buried silicon dioxide (BOX) of the SOI wafer as gate dielectric to control the current through the p+-SiNW. Electrical current between source and drain (IDS) versus gate voltage between the back-gate and source (VBGS) curves indicate accumulation regime for the p+-SiNW. IDS versus VDS curves indicate that the JNT device operates as a gated resistor gate. Still, the EBL technique ¿ with nominal resolution for the electronic beam of 2 nm ¿ was used to fabricate nMOS JNT devices - with arsenic dopant (n+-SiNW) - along with ECR-CVC (Electron Cyclotron Resonance) chemical phase deposition plasma system, for defining the nanowires using RF plasma etching and formation of the gate dielectric. Titanium and aluminum source, drain and gate electrodes were deposited by sputtering. The dimensions of width (W) and length (L), as well as the number of nanowire transistors were varied to allow a range of up to 3 orders of the electrical current magnitude through the device. The minimum dimensions obtained for the nanowire length (LFin) and high (HFin), gate length (LGate) and width (WGate) were, respectively, 10 ?m, 15 nm, 100 nm e 50 nm. The average time for the fabrication of one single JNT device using FIB system is 2 days, with the average cost of US$ 4,000.00. Still, the device fabrication using EBL technique is longer ¿ approximately 10 days ¿, however it costs less than one order of magnitude compared to FIB (approximately US$ 150.00). These results show that the methods developed for FIB and EBL systems for fabrication of silicon nanowires for applications in nanoelectronics are innovative in Brazil and allow consistent advances in nanofabrication. These processes, now calibrated, will contribute to the development of new processes, for example, FinFET transistors based on nanowires / Mestrado / Eletrônica, Microeletrônica e Optoeletrônica / Mestre em Engenharia Elétrica

The constant trend of device miniaturization along with ever-growing list of unusual behaviour of nanoscale materials has fuelled the recent research in fabrication and applications of ultrathin (~2 nm diameter) nanowires. Although semiconductor nanowires of this dimension is well-researched, molecular-scale single-crystalline metal nanowires have not been addressed in details. Such single crystalline Au nanowires are formed by oriented attachment of Au nanoparticles along [111] direction. A very low concentration of extended defects in these wires result in a high electrical conductivity, making them ideal for nanoscale interconnects. Other metal nanowires, e.g. Ag and Cu, have very low absorption co-efficient useful for fabrication of transparent conducting films. On the other hand, because of the reduced dimensions, there exists a tantalizing possibility of dominating quantum effects leading to their application in sensing and actuation. Also, speaking in terms of atomic structure, these systems suffer from intense surface stress, and the atomistic picture can be drastically different from bulk. Thus, although a myriad of applications are possible with ultrathin metal nanowires, a rigorous systematic knowledge of their atomic and electronic structure is not yet available. This thesis is the first one to model such computationally demanding systems with emphasis on their possible applications. In this thesis, we have explored various structural and electronic aspects of one-dimensional ultrathin nanowires with ab initio density functional theory coupled with experiments. The merit of Au nanowires has been tested as nanoscale interconnects. From atomistic point of view, these FCC Au nanowires exhibit an intriguing relaxation mechanism, which has been explored by both theory and experiment. The primary factor governing the relaxation mechanism was found to be the anisotropic surface stress of the bounding facets, and it is extended to explain the relaxation of other metallic nanowires. Our studies suggest that AuNWs of this dimension show semiconductor-like sensitivity towards small chemical analytes and can be used as nanoscale sensors. Also, we have found that further reducing the diameter of the Au-nanowires leads to opening of a band gap.