This thesis studies the application of silver nanowire transparent conductors in PbS quantum dot photovoltaics. Silver nanowires were synthesized using a colloidal method and characterized using scanning electron microscopy. Nanowires were deposited on glass substrates by a stamp transfer process to generate a low density continuous network of conductive nanowires. This resulted in a highly conductive and transparent film appropriate for optoelectronic applications. Nanowire synthesis, deposition, and processing were optimised to produce transparent conductors suitable for thin film photovoltaics. These nanowire films were used to fabricate lead sulphide (PbS) colloidal quantum dot solar cells. In this structure, p-type PbS quantum dots form a junction with a n-type ZnO nanoparticle layer. A variety of fabrication and processing treatments were developed in order to reduce short-circuiting of devices and to enhance cell performance. Moderate nanowire density, improved ZnO adherence, slight device aging, and increased PbS film thickness proved to result in the highest quality devices. The champion device developed in this thesis achieved a power conversion efficiency of 2.2%.

Dans le contexte actuel d'optimisation des performances des dispositifs de microélectronique, le transistor MOSFET, brique de base, est soumis à des contraintes géométriques telles que son architecture même est remise en cause. L'augmentation du nombre de grille afin d'accentuer le contrôle électrostatique de la grille sur le canal a mis en avant des architectures ultimes telles que le nanofil dont la grille enrobe totalement le canal. Dans ce travail, une étude du nanofil de silicium a été réalisée afin d'estimer les potentialités de cette architecture au niveau transistor jusqu'à l'étude de petits circuits. Pour cela, un modèle analytique en courant a été mis en place et implémenté en Verilog-A afin de simuler des petits circuits dans un environnement de type ELDO. Toutefois, les paramètres du modèle telles que les masses effectives de transport (ou de confinement) ou le transport dans le film sont la clé de la prédictibilité au niveau circuit. C'est pourquoi des simulations avancées de type liaisons fortes ou Kubo-Greenwood ont été développées afin d'étudier finement l'évolution des caractéristiques du nanofil notamment vis-à-vis de son intégration géométriques. Issues de ces approches numériques, des expressions analytiques ont été établies afin d'inclure dans le modèle toute la physique observée en amont. Des effets comme l'évolution de la structure de bande ou l'impact des mécanismes d'interaction ont ainsi pu être apportés jusqu'au niveau circuit. Les résultats en courant acquièrent une certaine pertinence en créant un lien entre simulations numériques et données expérimentales. / The microelectronics industry is extremely competitive in the increase of performances for devices or circuits. Nanowire architectures are now considered for the integration of strongly scaled devices as predicted for advanced technology nodes. The particular shape of nanowires combined to the reduction of geometrical dimensions (diameter of several atomic layers) leads to the emergence of physical phenomena on the MOSFET electrostatics characteristics such as quantum confinement (2D effects), short channel or band structure effects as well as the electronic transport with quasi-ballistic effect. In this work, we propose an analytical model including these last mechanisms for silicon GAA nanowires. In order to guarantee the pertinence of the model, numerical code have been developed such as a Schrödinger-Poisson solver for the band structure and a mobility calculation based on the Kubo-Greenwood formula including phonons, surface roughness and remote Coulomb scattering. The different results have been gathered in a continuous model, validated on numerical simulations and experimental data. Finally, a complete chain has been built to study the impact of last phenomena from the atomistic study, the MOSFET device characteristics up to small circuit performances.

Foram feitos estudos das propriedades magnéticas e estruturais em nanofios de níquel em uma lâmina de alumínio. As amostras foram obtidas através da anodização em duas etapas, seguida de uma eletrodeposição AC. Foi verificado que o aumento do potencial usado nas anodizações acarretam em um aumento do tamanho dos grãos cristalinos de níquel e dos diâmetros dos nanofios. O tamanho dos grãos de níquel e dos diâmetros dos nanofios variaram, respectivamente, entre 10 a 20 nm e 30 a 50 nm. As amostras apresentam alta anisotropia de forma, com coercividade entre 565 a 725 Oe. As interações magnéticas foram estudadas através das curvas `delta´M, estas mostraram que as interações desmagnetizantes são dominantes nestes sistemas. O modelo de Stoner-Wohlfarth, acréscido de um termo de interação foi utilizado para simular e interpretar o comportamento magnético dos nanofios. Através dos resultados numéricos e experimentais foi verificado que a componente reversível (Mrev) da magnetização independe do estado inicial do sistema, diferentemente do comportamento para a componente irreversível (Mirr). Através da análise das curvas de Mrev(Mirr)Hi , provenientes dos resultados numéricos e experimentais, foi verificado que existe um forte indicativo quanto ao modo de inversão da magnetização por curling, para nanofios / Studies of magnetic and structural properties of nickel nanowires deposited on nanoporous alumina membranes were carried out. The samples were obtained by a two-step anodization, followed by an AC electrodeposition. It was noted that the diameters of the nanowires and the crystalline grain size of the deposited nickel increase with the anodization voltage. The mean diameters and the grain sizes varied from 10 to 20 nm and from 30 to 50 nm, respectively. The samples exhibited a strong shape anisotropy, with coercivities between 565 and 725 Oe. Magnetic interactions were studied via `delta´M curves, which showed that the dominant interactions are rather demagnetizing in these systems. An interacting Stoner-Wohlfarth model was developed to simulate and reproduce the magnetic behavior of the nanowires. From the comparison between numerical and experimental results (which exhibit excellent agreement), it was noted that reversible components of magnetization (Mrev) do not depend on the initial state of the system, whereas irreversible components (Mirr) do. From the analysis of Mrev(Mirr)Hi curves of numerical and experimental results, it was noted that there is strong evidence for the curling magnetization reversal mode for these systems

Neste trabalho iniciou-se os estudos com transistores de tunelamento por efeito de campo (TFET) de silício (Si) em estruturas de nanofios (NW-TFET), analisando o efeito da redução do diâmetro dos nanofios, de 167 nm até 15 nm, através de analises baseadas em medidas experimentais e simulações numéricas. Para diâmetros maiores que 30 nm, os dispositivos são pouco influenciados pela redução do diâmetro. Para diâmetros menores que 30 nm, ao diminui-los, o tunelamento entre bandas (BTBT) passa a ser o mecanismo dominante, aumentando a corrente de dreno normalizada. Reduzindo o diâmetro em baixa condução, a maior parte da junção passa a ser dominada por BTBT, aumentando a eficiência devido ao melhor acoplamento eletrostático, reduzindo a inclinação de sublimiar (SS). A análise em diferentes temperaturas (de 10 K a 423 K) destes TFETs de estruturas de nanofios mostrou que o aumento da temperatura aumentou tanto a corrente de estado ligado (ION) quanto a de estado desligado (IOFF), sendo que o aumento de IOFF é responsável pela degradação da eficiência em baixa condução. Para melhorar o desempenho dos dispositivos TFET de Si, que possuem baixa corrente, foram utilizados dispositivos experimentais com fontes de Germânio (Ge) e de uma liga de Si e Ge (Si0,73Ge0,27). O aumento da concentração de Ge na fonte reduz a largura da banda proibida (EG), resultando em um aumento da corrente de BTBT nos dispositivos. Esse aumento da corrente de BTBT também aumenta a transcondutância (gm) e o ganho intrínseco de tensão (AV). Para melhorar ainda mais o desempenho dos TFETs, foram estudados novos dispositivos fabricado com Arseneto de Indio-Galio (InXGa1-XAs), com leiaute em anel, com comprimento de canal de 5 µm e largura de canal de 400 µm, utilizando dispositivos experimentais e simulados. O uso desse material gera um grande aumento de ION devido ao aumento considerável de BTBT, alcançando valores de SS próximos a 60mV/dec, valor muito menor que 200mV/dec obtido nos dispositivos de Si. Os dispositivos com InXGa1-XAs apresentaram alto AV (~50 dB) mesmo em baixas polarizações, sendo promissores em aplicações de baixa tensão e baixa potência. Aumento da concentração de In (In0,7Ga0,3As) reduz EG, aumentando BTBT. O aumento de BTBT aumenta gm, porém, aumenta também a condutância de saída (gD), aumentando AV para alto VGS e reduzindo para baixos VGS. A redução da espessura de HfO2, de 3nm para 2nm, resultou em melhoria em todos os dispositivos devido ao melhor acoplamento eletrostático, onde o dispositivo de In0,53Ga0,47As apresentou um SS de 56mV/dec. A temperatura influencia mais gD que gm, aumentando AV em baixas temperaturas. O uso de fonte gasosa na difusão de Zinco (Zn), no lugar de fonte sólida, resultou em uma junção mais abrupta, aumentando ION e melhorando SS. Pode-se obter um dispositivo otimizado utilizando In0,7Ga0,3As utilizando difusão de Zn na fonte por fase gasosa, para dispositivos que vão atuar em aplicações digitais, ou utilizando difusão de Zn na fonte por fonte sólida, para dispositivos que vão atuar em aplicações analógicas, ambos à 520ºC por 1 minuto, utilizando 2 nm de HfO2 na porta. / In this work, initially it was studied Silicon (Si) n type tunnel field effects transistors (TFET) in nanowire structures (NW-TFET), analyzing the diameter reduction effect of the nanowires, from 167 nm to 15 nm, using experimental measurements and numerical simulations. For diameters higher than 30 nm, the devices are less influenced by the diameter reduction. For diameters lower than 30 nm, decreasing the diameter, band-to-band tunneling (BTBT) start to become the dominant mechanism, increasing the normalized drain current. Reducing the diameter, in low conduction, the most of the junction becomes dominated by BTBT, increasing the transistor efficiency due to the better electrostatic coupling, reducing the subthreshold swing (SS). The analysis of this nTFETs at different temperatures (from 10 K to 423 K) showed that at high temperatures both the on and the off state current (ION and IOFF) of these NW-TFETs have raised, degrading SS, and consequently the efficiency at low conduction. In order to improve ION, which is very low in pure Si nTFETs, experimental devices using source made by Ge and Si0.73Ge0.27 was studied. The increase of the Ge concentration in the source reduces the bandgap results in higher BTBT current. This high BTBT current also lead the transconductance (gm) and the intrinsic voltage gain (AV) to increase. To further improve the TFETs performance, new devices made of InGaAs with ring layout, with channel length of 5 µm and channel width of 400 µm was studied, using experimental and simulated data. The use of InGaAs generates a large increase of ION due to its low bandgap, enabling to reach values of SS near 60 mV/dec, much steeper than the 200mV/dec obtained on Si nTFETs. These InGaAs nTFETs have presented high AV (~50 dB), even at low bias, being promising devices in low power low voltage applications. When increasing the In concentration in the InXGa1-XAs TFET the bandgap is reduced, improving the BTBT current. The BTBT raise leads both gm and the output conductance (gD) to increase, improving AV for high VGS bias and degrading it at low VGS bias. The reduction of the HfO2 thickness, from 3 nm to 2 nm, have resulted in improvement all devices due to the better electrostatic coupling, where the In0.53Ga0.47As device have presented SS of 56mV/dec. As the temperature have more influence in gD than gm, AV is improved at low temperatures. The use of gas phase Zn diffusion at the source doping, instead of solid source Zn diffusion, have increased ION and improved SS. The possibly reason to this behavior is the higher abruptness of the source/channel junction when using gas phase Zn diffusion. An optimized device can be obtained using a device with In0,7Ga0,3As with the source diffusion made by gas phase, for devices to be used in digital applications, or with the source diffusion made by solid source, for devices to be used in analog applications. Both diffusion process made at 520 ºC, using 2 nm of HfO2 in the gate stack.

Thesis advisor: Dunwei Wang / Solar energy, the most abundant and free renewable energy, holds great promise for humanity's sustainable development. How to efficiently and inexpensively capture, covert solar energy and store it for off peak usages constitutes a grand challenge for the scientific community. Photovoltaic devices are promising candidates but are too costly to be implemented in large scales. On a fundamental level, this is due to the dilemma that the length scales of the optical pathways and electrical pathways often do not match within the photovoltaic device materials. Consider traditional Si solar cell as an example, effective light absorption requires up to hundreds of microns material while the photogenerated charge carries can only diffuse less than a few microns or even shorter before recombination. Such a problem may be solved by using Si nanowires (SiNWs) because vertically aligned nanowires can orthogonalize the light absorption and charge carrier collection pathways, thereby enabling the use of low-cost materials for practically appealing solar energy conversion devices. The objective of this thesis work is to explore low-cost synthesis of vertically aligned SiNW arrays and study their performance in both solar energy conversion and storage devices. We developed a method to synthesize vertically aligned SiNW arrays in a hot-wall chemical vapor deposition system with tunable length, doping level, and diameter for systematical studies. Empowered by the synthetic control, various types of vertical SiNW arrays were characterized by both steady-state (photoelectrochemical measurement) and transient (electrochemical impedance spectroscopy) techniques in a photoelectrochemical cell platform. Additionally, SiNWs were demonstrated to be a promising candidate for photoelectrochemical aromatic ketone reduction and CO₂ fixation. The reactions studied in this thesis are in close resemblance to natural photosynthesis and the resulted product molecules are precursors to nonsteroidal anti-inflammatory drugs, ibuprofen and naproxen. Lastly, vertical transparent conductive oxide nanotubes were prepared from vertical SiNW array templates. Ultrathin hematite (Fe₂O₃) film was coated on the nanotube scaffold by atomic layer deposition to form a heteronanostructure photoelectrode for efficient solar water oxidation. Our results highlight the potential of vertically aligned SiNW arrays in solar cell, solar water splitting and artificial photosynthesis applications. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The implications of non-Gaussian interference for various communication systemsare explored. The focus is on the Kappa distribution, Generalized Gaussian distribu-tions, and the distribution of the interference in molecular communication systems.A review of how dynamic systems that are not in equilibrium are modeled by theKappa distribution and how this distribution models interference in HF communica-tions systems at sunrise is provided. The channel model, bit error rate for single andmultiple antennas, channel capacity, and polar code performance are shown.<div><br><div>Next, a review of the Generalized Gaussian distribution that has been found tomodel the interference resulting from surface activities is provided. This modeling isextended to find the secrecy capacity so that information cannot be obtained by theeavesdropper.</div><div><br></div><div>Finally, future nanomachnines are examined. The vulnerability to a receptorantagonist of a ligand-based molecule receiver is explored. These effects are consideredto be interference as in other wireless systems and the damage to signal reception isquantified.</div></div>

Neste trabalho iniciou-se os estudos com transistores de tunelamento por efeito de campo (TFET) de silício (Si) em estruturas de nanofios (NW-TFET), analisando o efeito da redução do diâmetro dos nanofios, de 167 nm até 15 nm, através de analises baseadas em medidas experimentais e simulações numéricas. Para diâmetros maiores que 30 nm, os dispositivos são pouco influenciados pela redução do diâmetro. Para diâmetros menores que 30 nm, ao diminui-los, o tunelamento entre bandas (BTBT) passa a ser o mecanismo dominante, aumentando a corrente de dreno normalizada. Reduzindo o diâmetro em baixa condução, a maior parte da junção passa a ser dominada por BTBT, aumentando a eficiência devido ao melhor acoplamento eletrostático, reduzindo a inclinação de sublimiar (SS). A análise em diferentes temperaturas (de 10 K a 423 K) destes TFETs de estruturas de nanofios mostrou que o aumento da temperatura aumentou tanto a corrente de estado ligado (ION) quanto a de estado desligado (IOFF), sendo que o aumento de IOFF é responsável pela degradação da eficiência em baixa condução. Para melhorar o desempenho dos dispositivos TFET de Si, que possuem baixa corrente, foram utilizados dispositivos experimentais com fontes de Germânio (Ge) e de uma liga de Si e Ge (Si0,73Ge0,27). O aumento da concentração de Ge na fonte reduz a largura da banda proibida (EG), resultando em um aumento da corrente de BTBT nos dispositivos. Esse aumento da corrente de BTBT também aumenta a transcondutância (gm) e o ganho intrínseco de tensão (AV). Para melhorar ainda mais o desempenho dos TFETs, foram estudados novos dispositivos fabricado com Arseneto de Indio-Galio (InXGa1-XAs), com leiaute em anel, com comprimento de canal de 5 µm e largura de canal de 400 µm, utilizando dispositivos experimentais e simulados. O uso desse material gera um grande aumento de ION devido ao aumento considerável de BTBT, alcançando valores de SS próximos a 60mV/dec, valor muito menor que 200mV/dec obtido nos dispositivos de Si. Os dispositivos com InXGa1-XAs apresentaram alto AV (~50 dB) mesmo em baixas polarizações, sendo promissores em aplicações de baixa tensão e baixa potência. Aumento da concentração de In (In0,7Ga0,3As) reduz EG, aumentando BTBT. O aumento de BTBT aumenta gm, porém, aumenta também a condutância de saída (gD), aumentando AV para alto VGS e reduzindo para baixos VGS. A redução da espessura de HfO2, de 3nm para 2nm, resultou em melhoria em todos os dispositivos devido ao melhor acoplamento eletrostático, onde o dispositivo de In0,53Ga0,47As apresentou um SS de 56mV/dec. A temperatura influencia mais gD que gm, aumentando AV em baixas temperaturas. O uso de fonte gasosa na difusão de Zinco (Zn), no lugar de fonte sólida, resultou em uma junção mais abrupta, aumentando ION e melhorando SS. Pode-se obter um dispositivo otimizado utilizando In0,7Ga0,3As utilizando difusão de Zn na fonte por fase gasosa, para dispositivos que vão atuar em aplicações digitais, ou utilizando difusão de Zn na fonte por fonte sólida, para dispositivos que vão atuar em aplicações analógicas, ambos à 520ºC por 1 minuto, utilizando 2 nm de HfO2 na porta. / In this work, initially it was studied Silicon (Si) n type tunnel field effects transistors (TFET) in nanowire structures (NW-TFET), analyzing the diameter reduction effect of the nanowires, from 167 nm to 15 nm, using experimental measurements and numerical simulations. For diameters higher than 30 nm, the devices are less influenced by the diameter reduction. For diameters lower than 30 nm, decreasing the diameter, band-to-band tunneling (BTBT) start to become the dominant mechanism, increasing the normalized drain current. Reducing the diameter, in low conduction, the most of the junction becomes dominated by BTBT, increasing the transistor efficiency due to the better electrostatic coupling, reducing the subthreshold swing (SS). The analysis of this nTFETs at different temperatures (from 10 K to 423 K) showed that at high temperatures both the on and the off state current (ION and IOFF) of these NW-TFETs have raised, degrading SS, and consequently the efficiency at low conduction. In order to improve ION, which is very low in pure Si nTFETs, experimental devices using source made by Ge and Si0.73Ge0.27 was studied. The increase of the Ge concentration in the source reduces the bandgap results in higher BTBT current. This high BTBT current also lead the transconductance (gm) and the intrinsic voltage gain (AV) to increase. To further improve the TFETs performance, new devices made of InGaAs with ring layout, with channel length of 5 µm and channel width of 400 µm was studied, using experimental and simulated data. The use of InGaAs generates a large increase of ION due to its low bandgap, enabling to reach values of SS near 60 mV/dec, much steeper than the 200mV/dec obtained on Si nTFETs. These InGaAs nTFETs have presented high AV (~50 dB), even at low bias, being promising devices in low power low voltage applications. When increasing the In concentration in the InXGa1-XAs TFET the bandgap is reduced, improving the BTBT current. The BTBT raise leads both gm and the output conductance (gD) to increase, improving AV for high VGS bias and degrading it at low VGS bias. The reduction of the HfO2 thickness, from 3 nm to 2 nm, have resulted in improvement all devices due to the better electrostatic coupling, where the In0.53Ga0.47As device have presented SS of 56mV/dec. As the temperature have more influence in gD than gm, AV is improved at low temperatures. The use of gas phase Zn diffusion at the source doping, instead of solid source Zn diffusion, have increased ION and improved SS. The possibly reason to this behavior is the higher abruptness of the source/channel junction when using gas phase Zn diffusion. An optimized device can be obtained using a device with In0,7Ga0,3As with the source diffusion made by gas phase, for devices to be used in digital applications, or with the source diffusion made by solid source, for devices to be used in analog applications. Both diffusion process made at 520 ºC, using 2 nm of HfO2 in the gate stack.

A wide variety of tunable physio-chemical properties make ZnO nanowires a promising candidate for functional device applications. Although bottom-up grown nanowires are producible in volume, their high-throughput device integration requires control over dimensions and, more importantly, of precise placement. Thus development of top-down fabrication routes with accurate device positioning is imperative and hence pursued in this thesis. ZnO thin film transistors (TFT) were fabricated using solution based precursor zinc neodecanoate. A range of ZnO thin films were prepared by varying process parameters, such as precursor concentrations and annealing temperatures, and then analysed for their optical and electrical characteristics. ZnO TFTs prepared from a 15 % precursor concentration and annealing at 700 $^\circ$C exhibited best device performance with a saturation mobility of 0.1 cm$^2$/V.s and an on/off ratio of 10$^7$. Trap limited conduction (TLC) transport was found to be dominant in these devices. A direct-write electron beam lithography (EBL) process was developed using zinc naphthenate and zinc neodecanoate precursors for the top-down synthesis of ZnO nanowires. Nanoscale ZnO patterns with a resolution of 50 nm and lengths up to 25 $\mu$m were fabricated. A linear mobility of 0.5 cm$^2$/V.s and an on/off ratio of $\sim$10$^5$ was achieved in the micro-FETs with 50 $\mu$m channel width. Interestingly, on scaling down the ZnO channel width down to 100 nm, almost two orders of magnitude enhancement in the linear mobility was observed, which reached $\sim$33.75 cm$^2$/V.s. Such increment in the device performance was attributed to the formation of larger grains and thus reduction in the grain-boundary scattering. Six volatile organic compounds (VOCs) were sensed at room temperature using the direct-write EBL fabricated ZnO devices under UV sensitisation. As the surface-to-volume ratio increases with the decreasing channel width (from 50 $\mu$m to 100 nm), sensing response of the ZnO devices becomes more significant. Ppm level detection of various VOCs was observed; with a 25 ppm level Anisole detection being the lowest concentration. Additionally, using 100 nm device, detection of 10 ppm NO$_2$ was achieved at room temperature. The sensing response towards NO$_2$ was found to be increased with UV illumination and sensor temperature. This led to exhibit $\sim$171 % sensing response for a 2.5 ppm level of NO$_2$.

Cette thèse porte sur l’élaboration de nanocomposites anisotropes à base de nanofils de cobalt/polymères pour la fabrication d’aimants permanents qui ne contiennent pas de terres rares et l’optimisation des propriétés magnétiques de ces matériaux composites. La préparation de nanofils de cobalt mono-domaines (R~6-10 nm et L~250-350 nm) a été réalisée par voie thermique conventionnelle et par voie micro-onde. Des films de composites Co/polymère alignés ont été élaborés avec de très bonnes propriétés magnétiques (μ0Hc=0.75T, Mr/Ms=0.92). Le (BH)max est de 160 kJ/m3 qui est dans la gamme des aimants SmCo (BHmax~ 120-200 kJ/m3). Les techniques de diffusion de neutrons et de rayons-X aux petits angles (DNPA et DXPA) ont été utilisées pour la caractérisation des dispersions et des systèmes anisotropes. Les fils dans le chloroforme sont mieux dispersés par rapport aux autres solvants et forment des agrégats moins gros. Pour les films de composites, l’agrégation des nanofils est relativement plus dense dans le polystyrène que dans le poly(vinyl pyrrolidone). La qualité de l’alignement est proportionnelle à l’amplitude du champ appliqué même pour des champs très élevés. Cependant, un meilleur alignement ne conduit pas automatiquement à une meilleure coercivité. Les interactions entre des nanofils ont été caractérisées par Henkel plots. Les valeurs de ΔM sont faibles (ΔM<-0.2). En outre, la DNPA polarisée a permis de suivre le renversement magnétique à l’échelle nanométrique. Le champ coercitif Hc est défini par le renversement global de gros paquets de fils. Au-delà de Hc, il n’y a plus que des processus de retournements de fils individuels. Afin d’optimiser le Hc, l’optimisation de la microstructure (organisation des fils) est plus importante que l’optimisation des propriétés des fils individuels. / This thesis focuses on the development of nanocomposites made from anisotropic cobalt nanowires / polymer for the manufacture of permanent magnets which do not contain any rare earth and the optimization of the magnetic properties of these composite materials.The preparation of single-domain cobalt nanowires (R ~ 6-10 nm and L ~ 250-350 nm) was performed by conventional thermal route and by microwave route. The films of composite aligned Co nanowires/polymer have been elaborated with very good magnetic properties (μ0Hc = 0.75T, Mr / Ms = 0.92). The (BH)max is 160 kJ/m3 which is in the range of SmCo magnets (~ 120-200 BHmax kJ/m3). The techniques of small angles neutron and X-ray small angle scattering (SANS and SAXS) were used for the characterization of anisotropic systems and dispersions. The wires in chloroform are better dispersed compared to other solvents and form aggregates smaller. For the films of composite, the aggregation of the nanowires is relatively denser in polystyrene than in poly (vinyl pyrrolidone). The quality of alignment is proportional to the amplitude of the applied field, even for very high fields. However, a better alignment does not automatically lead to a better coercivity. The interactions between nanowires were characterized by Henkel plots. The ΔM values are pretty low (ΔM <-0.2). In addition, polarized SANS was used to track the magnetic reversal at the nanoscale. The coercive field Hc is defined by global reversal of large packets of wires. Beyond Hc, there are more processes than reversals of individual wires. To optimize Hc, optimizing the microstructure (organization of wires) is more important than optimizing the properties of the individual wires.

Depuis le début des années 2000, une vaste classe de nanofils de nitrures d’éléments III et de ZnO peut être synthétisée avec un excellent contrôle des propriétés de dopage et de composition. La géométrie spécifique de ces nanofils permet de faire croître des hétérostructures radiales et axiales qui ont des propriétés optiques et de transport très avantageuses par rapport aux couches minces. Ces propriétés en font des candidats prometteurs pour la réalisation d’une nouvelle génération de dispositifs plus efficaces (LEDs, photodétecteurs,…). Pour cela, il est indispensable de comprendre les nouveaux effets induits par la géométrie particulière de ces nanostructures : c’est l’objet de cette thèse. Dans une première partie, je présente une étude des propriétés optiques de nanofils de semiconducteurs à grande énergie de bande interdite. J’analyse d’abord l’effet de la contrainte sur les propriétés d’émission des nanofils cœur-coquille GaN/AlGaN. En particulier, je mets en évidence le croisement des bandes de valence et son influence sur les propriétés optiques des nanofils. Ensuite, je me focalise sur l’effet du confinement quantique et les propriétés de polarisation dans les nanofils hétérostructurés de nitrures d’éléments III. Dans une seconde partie, je m’intéresse à la réalisation et à la caractérisation de dispositifs à base de nanofils de nitrures d’éléments III et de ZnO. J’expose tout d’abord la modélisation et l’étude expérimentale de photodétecteurs à ensemble de nanofils en mettant en avant l’influence des états de surface sur leur réponse. Je m’intéresse ensuite aux propriétés de transport dans des nanofils uniques de nitrures d’éléments III hétérostructurés. Je montre, en particulier, que ces hétérostructures sont le siège d’une résistance différentielle négative. Enfin, je présente la réalisation et la caractérisation de photodétecteurs et de LEDs utilisant des nanofils uniques InGaN/GaN cœur-coquille. Un modèle électrique équivalent permet de rendre compte du comportement observé. / Since the early 2000s, a large class of wide bandgap nanowires can be grown with an excellent control of doping and composition. The specific geometry of the nanowires leads to radial or axial heterostructures with better optical and transport properties compared to thin films. Due to these properties, they are promising candidates for a new generation of more efficient devices (LEDs, photodetectors, etc.). It is essential to understand the new effects induced by the particular geometry of these nanostructures.In the first part, I deal with the optical properties of wide bandgap semiconductor nanowires. First, I analyze the effect of the stress on the emission properties of core-shell GaN/AlGaN nanowires. I highlight the intersection of valence bands and its influence on the optical properties of nanowires. Then, I focus on the effect of quantum confinement and on the polarization properties of III-nitride heterostructured nanowires.In the second part, I describe the fabrication and characterization of III-nitride and ZnO nanowire-based devices. I first model and study photodetectors based on ensemble of nanowires. Then, I focus on the transport properties of single heterostructured nanowires of III-nitride heterostructures. I show in particular that these heterostructures exhibit a negative differential resistance. Finally, I present characterization of photodetectors and LEDs using single core-shell InGaN/GaN nanowires. An equivalent electrical circuit explains the observed behavior