Diese Arbeit befasst sich mit der Keimbildung und den Wachstumsmechanismen von GaN-Nanodrähten (NWs), die mittels Molekularstrahlepitaxie (MBE) hergestellt wurden. Die Hauptneuheiten dieser Studie sind der intensive Gebrauch von in-situ Messmethoden und der direkte Vergleich zwischen katalysatorfreien und katalysatorinduzierten NWs. In der MBE bilden sich GaN-NWs auf Silizium ohne Katalysator. Auf Saphir dagegen wachsen NWs unter den gleichen Bedingungen nur in der Anwesenheit von Ni-Partikeln. Die Nukleationsprozesse sind für beide Ansätze fundamental verschieden. In dem katalysatorinduzierten Ansatz reagiert Ga stark mit den Ni-Keimen, deren Kristallstruktur für das Nanodraht-Wachstum entscheidend sind, während in dem katalysatorfreien Ansatz bildet N eine Zwischenschicht mit Si vor der ausgeprägten GaN-Nukleation. Mittels beider Ansätze wachsen einkristalline wurtzite GaN-NWs in Ga-polarer Richtung. Allerdings sind unter denselben Wachstumsbedingungen die katalysatorinduzierten NWs länger als die katalysatorfrei gewachsenen und enthalten viele Stapelfehler. Im Vergleich sind die katalysatorfreien größtenteils defektfrei und ihre Photolumineszenz ist viel intensiver als jene der katalysatorinduzierten NWs. Alle diese Unterschiede können auf den Katalysator zurückgefürt werden. Die Ni-Partikel sammeln die an den Nanodraht-Spitzen ankommenden Ga-Atome ef?zienter ein als die unbedeckte oberste Facette im katalysatorfreien Fall. Außerdem können Stapelfehler sowohl aus der zusätzlichen Festkörperphase des Ni-Katalysators als auch aus der Verunreinigung der NWs mit Katalysatormaterial resultieren. Solch eine Kontaminierung würde schließlich nicht-strahlende Rekombinationszentren verursachen. Somit mag die Verwendung von Katalysatorkeimen zusätzliche Möglichkeiten bieten, das Wachstum von NWs zu kontrollieren. Jedoch sind sowohl die strukturellen als auch die optischen Materialeigenschaften der katalysatorfreien NWs überlegen. / This work focuses on the nucleation and growth mechanisms of GaN nanowires (NWs) by molecular beam epitaxy (MBE). The main novelties of this study are the intensive employment of in-situ techniques and the direct comparison of self-induced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE without the use of any external catalyst seed. On sapphire, in contrast, NWs grow under identical conditions only in the presence of Ni seeds. The processes leading to NW nucleation are fundamentally different for both approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst Ni particles whose crystal structure and phases are decisive for the NW growth, while in the catalyst-free approach, N forms an interfacial layer with Si before the intense nucleation of GaN starts. Both approaches yield monocrystalline wurtzite GaN NWs, which grow in the Ga-polar direction. However, the catalyst-assisted NWs are longer than the catalyst-free ones after growth under identical conditions, and they contain many stacking faults. By comparison the catalyst-free NWs are largely free of defects and their photoluminescence is much more intense than the one of the catalyst-assisted NWs. All of these differences can be explained as effects of the catalyst. The seed captures Ga atoms arriving at the NW tip more efficiently than the bare top facet in the catalyst-free approach. In addition, stacking faults could result from both the presence of the additional solid phase constituted by the catalyst-particles and the contamination of the NWs by the catalyst material. Finally, such contamination would generate non-radiative recombination centers. Thus, the use of catalyst seeds may offer an additional way to control the growth of NWs, but both the structural and the optical material quality of catalyst-free NWs are superior.

In dieser Dissertation wird die spektrale und örtliche Verteilung der Lumineszenz von Heterostrukturen in selbstorganisierten Nanodrähten (ND) mit Hilfe von Kathodolumineszenz-Spektroskopie (KL) im Rasterelektronenmikroskop untersucht. Diese Methode wird ergänzt durch Messungen der kontinuierlichen und zeitaufgelösten Mikro-Photolumineszenz. Drei verschiedene Strukturen werden behandelt: (i) GaAs-ND bestehend aus Segmenten der Wurtzit (WZ) bzw. Zinkblende (ZB) Kristallstrukturen, (ii) auf GaN-ND überwachsene GaN-Mikrokristalle und (iii) (In,Ga)N Einschlüsse in GaN-ND. Die gemischte Kristallstruktur der GaAs-ND führt zu komplexen Emissionsspektren. Dabei wird entweder ausschließlich Lumineszenz bei Energien unterhalb der ZB Bandlücke, oder aber zusätzlich bei höheren Energien, gemessen. Diese Differenz wird durch unterschiedliche Dicken der ZB und WZ Segmente erklärt. Messungen bei Raumtemperatur zeigen, dass die Bandlücke von WZ-GaAs mindestens 55 meV größer als die von ZB-GaAs ist. Die Lumineszenz-Spektren der GaN-Mikrokristalle enthalten verschiedene Emissionslinien, die auf Stapelfehler (SF) zurückzuführen sind. SF sind ZB Quantentöpfe verschiedener Dicke in einem WZ-Kristall und es wird gezeigt, dass ihre Emissionsenergie durch die spontane Polarisation bestimmt wird. Aus einer detaillierten statistischen Analyse der Emissionsenergien der verschiedenen SF-Typen werden Emissionsenergien von 3.42, 3.35 und 3.29 eV für die intrinsischen (I1 und I2) sowie für extrinsische SF ermittelt. Aus den entsprechenden Energiedifferenzen wird -0.022C/m² als experimenteller Wert für die spontane Polarisation von GaN bestimmt. Die Bedeutung sowohl der piezoelektrischen Polarisation als auch die der Lokalisierung von Ladungsträgern wird für (In,Ga)N-Einschlüsse in GaN-ND gezeigt. Hierbei spielt nicht nur die Lokalisierung von Exzitonen, sondern auch die individueller Elektronen und Löcher an unterschiedlichen Potentialminima eine Rolle. / In this thesis, the spectral and spatial luminescence distribution of heterostructures in self-induced nanowires (NWs) is investigated by cathodoluminescence spectroscopy in a scanning electron microscope. This method is complemented by data from both continuous and time-resolved micro-photoluminescence measurements. Three different structures are considered: (i) GaAs NWs containing segments of the wurtzite (WZ) and zincblende (ZB) polytypes, (ii) GaN microcrystals overgrown on GaN NWs, and (iii) (In,Ga)N insertions embedded in GaN NWs. The polytypism of GaAs NWs results in complex emission spectra. The observation of luminescence either exclusively at energies below the ZB band gap or also at higher energies is explained by differences in the distribution of ZB and WZ segment thicknesses. Measurements at room temperature suggest that the band gap of WZ GaAs is at least 55 meV larger than that of the ZB phase. The luminescence spectra of the GaN microcrystals contain distinct emission lines associated with stacking faults (SFs). SFs essentially constitute ZB quantum wells of varying thickness in a WZ matrix and it is shown that their emission energy is dominated by the spontaneous polarization. Through a detailed statistical analysis of the emission energies of the different SF types, emission energies of 3.42, 3.35 and 3.29 eV are determined for the intrinsic (I1 and I2) as well as the extrinsic SFs, respectively. From the corresponding energy differences, an experimental value of -0.022C/m² is derived for the spontaneous polarization of GaN. The importance of both carrier localization and the quantum confined Stark effect induced by the piezoelectric polarization is shown for the luminescence of (In,Ga)N insertions in GaN NWs. Not only localized excitons, but also electrons and holes individually localized at different potential minima contribute to the observed emission.

Este trabalho teve como objetivo estudar os transistores de tunelamento por efeito de campo em estruturas de nanofio (NW-TFET), sendo realizado através de analises com base em explicações teóricas, simulações numéricas e medidas experimentais. A fim de avaliar melhorar o desempenho do NW-TFET, este trabalho utilizou dispositivos com diferentes materiais de fonte, sendo eles: Si, liga SiGe e Ge, além da variação da espessura de HfO2 no material do dielétrico de porta. Com o auxílio de simulações numéricas foram obtidos os diagramas de bandas de energia dos dispositivos NW-TFET com fonte de Si0,73Ge0,27 e foi analisada a influência de cada um dos mecanismos de transporte de portadores para diversas condições de polarização, sendo observado a predominância da influência da recombinação e geração Shockley-Read-Hall (SRH) na corrente de desligamento, do tunelamento induzido por armadilhas (TAT) para baixos valores de tensões de porta (0,5V > VGS > 1,5V) e do tunelamento direto de banda para banda (BTBT) para maiores valores tensões de porta (VGS > 1,5V). A predominância de cada um desses mecanismos de transporte foi posteriormente comprovada com a utilização do método de Arrhenius, sendo este método adotado em todas as análises do trabalho. O comportamento relativamente constante da corrente dos NW-TFETs com a temperatura na região de BTBT tem chamado a atenção e por isso foi realizado o estudo dos parâmetros analógicos em função da temperatura. Este estudo foi realizado comparando a influência dos diferentes materiais de fonte. O uso de Ge na fonte, permitiu a melhora na corrente de tunelamento, devido à sua menor banda proibida, aumentando a corrente de funcionamento (ION) e a transcondutância do dispositivo. Porém, devido à forte dependência de BTBT com o campo elétrico, o uso de Ge na fonte resulta em uma maior degradação da condutância de saída. Entretanto, a redução da espessura de HfO2 no dielétrico de porta resultou no melhor acoplamento eletrostático, também aumentando a corrente de tunelamento, fazendo com que o dispositivo com fonte Ge e menor HfO2 apresentasse melhores resultados analógicos quando comparado ao puramente de Si. O uso de diferentes materiais durante o processo de fabricação induz ao aumento de defeitos nas interfaces do dispositivo. Ao longo deste trabalho foi realizado o estudo da influência da densidade de armadilhas de interface na corrente do dispositivo, demonstrando uma relação direta com o TAT e a formação de uma região de platô nas curvas de IDS x VGS, além de uma forte dependência com a temperatura, aumentando a degradação da corrente para temperaturas mais altas. Além disso, o uso de Ge introduziu maior número de impurezas no óxido, e através do estudo de ruído foi observado que o aumento na densidade de armadilhas no óxido resultou no aumento do ruído flicker em baixa frequência, que para o TFET, ocorre devido ao armadilhamento e desarmadilhamento de elétrons na região do óxido. E mais uma vez, o melhor acoplamento eletrostático devido a redução da espessura de HfO2, resultou na redução desse ruído tornando-se melhor quando comparado à um TFET puramente de Si. Neste trabalho foi proposto um modelo de ruído em baixa frequência para o NW-TFET baseado no modelo para MOSFET. Foram realizadas apenas algumas modificações, e assim, obtendo uma boa concordância com os resultados experimentais na região onde o BTBT é o mecanismo de condução predominante. / This work aims to study the nanowire tunneling field effect transistors (NW-TFET). The analyses were performed based on theoretical explanations, numerical simulations and experimental data. In order to improve the NW-TFET performance, it was used devices with different source compositions, such as Si, SiGe alloy and Ge, besides different thicknesses of HfO2 for the gate dielectric. With the aid of numerical simulations it was obtained the NW-TFET energy band diagrams and analyzed the influence of recombination and generation Shockley-Read-Hall (SRH) on the off current, the influence of the trap assisted tunneling (TAT) at low gate voltage bias (0,5V > VGS > 1,5V) and the direct band to band tunneling (BTBT) at higher gate voltage bias(VGS > 1,5V). The predominance of each conduction mechanisms was confirmed by the Arrhenius plot method, being this method adopted in all analysis in this work. The constant current with the temperature in the BTBT region has drawn attention and due to that, this work have studied the NW-TFET analog performance as function of temperature and also the influence of the source composition. The Ge source device shows an improved tunneling current, related to the bandgap narrowing, which leads to higher ION and transconductance. However, due to the strong BTBT dependence with the electric field, the use of Ge as source results in further ION/IOFF degradation. Despite this, the reduced HfO2 thickness in the gate dielectric, results in better electrostatic coupling, which also increases the tunneling current, making this device to present better analog performance when compared to devices with Si source. The use of different materials during the device fabrication leads to an increase of the interface defects. This work presented the influence of the interface trap density on the current, showing a direct relation with TAT and appearance of a plateau region in the IDS x VGS curves. In addition it was shown a strong temperature dependence increasing the current degradation at higher temperatures. Furthermore, the use of Ge has shown an increase of impurities in the oxide, and through the noise study it was observed the flicker noise increase at low frequency, which for TFETs, occurs due to the electrons trapping and detrapping in the oxide region. Once again, the reduced HfO2 thickness leads to better electrostatic coupling, resulting in noise reduction and becoming better when compared to a devices with Si source. In this work was proposed a low frequency noise model for a NW-TFET based on MOSFET models. Minor changes have been done, and thus a good agreement with the experimental results in the region where the BTBT is predominant conduction mechanism was obtained.

Este trabalho teve como objetivo estudar os transistores de tunelamento por efeito de campo em estruturas de nanofio (NW-TFET), sendo realizado através de analises com base em explicações teóricas, simulações numéricas e medidas experimentais. A fim de avaliar melhorar o desempenho do NW-TFET, este trabalho utilizou dispositivos com diferentes materiais de fonte, sendo eles: Si, liga SiGe e Ge, além da variação da espessura de HfO2 no material do dielétrico de porta. Com o auxílio de simulações numéricas foram obtidos os diagramas de bandas de energia dos dispositivos NW-TFET com fonte de Si0,73Ge0,27 e foi analisada a influência de cada um dos mecanismos de transporte de portadores para diversas condições de polarização, sendo observado a predominância da influência da recombinação e geração Shockley-Read-Hall (SRH) na corrente de desligamento, do tunelamento induzido por armadilhas (TAT) para baixos valores de tensões de porta (0,5V > VGS > 1,5V) e do tunelamento direto de banda para banda (BTBT) para maiores valores tensões de porta (VGS > 1,5V). A predominância de cada um desses mecanismos de transporte foi posteriormente comprovada com a utilização do método de Arrhenius, sendo este método adotado em todas as análises do trabalho. O comportamento relativamente constante da corrente dos NW-TFETs com a temperatura na região de BTBT tem chamado a atenção e por isso foi realizado o estudo dos parâmetros analógicos em função da temperatura. Este estudo foi realizado comparando a influência dos diferentes materiais de fonte. O uso de Ge na fonte, permitiu a melhora na corrente de tunelamento, devido à sua menor banda proibida, aumentando a corrente de funcionamento (ION) e a transcondutância do dispositivo. Porém, devido à forte dependência de BTBT com o campo elétrico, o uso de Ge na fonte resulta em uma maior degradação da condutância de saída. Entretanto, a redução da espessura de HfO2 no dielétrico de porta resultou no melhor acoplamento eletrostático, também aumentando a corrente de tunelamento, fazendo com que o dispositivo com fonte Ge e menor HfO2 apresentasse melhores resultados analógicos quando comparado ao puramente de Si. O uso de diferentes materiais durante o processo de fabricação induz ao aumento de defeitos nas interfaces do dispositivo. Ao longo deste trabalho foi realizado o estudo da influência da densidade de armadilhas de interface na corrente do dispositivo, demonstrando uma relação direta com o TAT e a formação de uma região de platô nas curvas de IDS x VGS, além de uma forte dependência com a temperatura, aumentando a degradação da corrente para temperaturas mais altas. Além disso, o uso de Ge introduziu maior número de impurezas no óxido, e através do estudo de ruído foi observado que o aumento na densidade de armadilhas no óxido resultou no aumento do ruído flicker em baixa frequência, que para o TFET, ocorre devido ao armadilhamento e desarmadilhamento de elétrons na região do óxido. E mais uma vez, o melhor acoplamento eletrostático devido a redução da espessura de HfO2, resultou na redução desse ruído tornando-se melhor quando comparado à um TFET puramente de Si. Neste trabalho foi proposto um modelo de ruído em baixa frequência para o NW-TFET baseado no modelo para MOSFET. Foram realizadas apenas algumas modificações, e assim, obtendo uma boa concordância com os resultados experimentais na região onde o BTBT é o mecanismo de condução predominante. / This work aims to study the nanowire tunneling field effect transistors (NW-TFET). The analyses were performed based on theoretical explanations, numerical simulations and experimental data. In order to improve the NW-TFET performance, it was used devices with different source compositions, such as Si, SiGe alloy and Ge, besides different thicknesses of HfO2 for the gate dielectric. With the aid of numerical simulations it was obtained the NW-TFET energy band diagrams and analyzed the influence of recombination and generation Shockley-Read-Hall (SRH) on the off current, the influence of the trap assisted tunneling (TAT) at low gate voltage bias (0,5V > VGS > 1,5V) and the direct band to band tunneling (BTBT) at higher gate voltage bias(VGS > 1,5V). The predominance of each conduction mechanisms was confirmed by the Arrhenius plot method, being this method adopted in all analysis in this work. The constant current with the temperature in the BTBT region has drawn attention and due to that, this work have studied the NW-TFET analog performance as function of temperature and also the influence of the source composition. The Ge source device shows an improved tunneling current, related to the bandgap narrowing, which leads to higher ION and transconductance. However, due to the strong BTBT dependence with the electric field, the use of Ge as source results in further ION/IOFF degradation. Despite this, the reduced HfO2 thickness in the gate dielectric, results in better electrostatic coupling, which also increases the tunneling current, making this device to present better analog performance when compared to devices with Si source. The use of different materials during the device fabrication leads to an increase of the interface defects. This work presented the influence of the interface trap density on the current, showing a direct relation with TAT and appearance of a plateau region in the IDS x VGS curves. In addition it was shown a strong temperature dependence increasing the current degradation at higher temperatures. Furthermore, the use of Ge has shown an increase of impurities in the oxide, and through the noise study it was observed the flicker noise increase at low frequency, which for TFETs, occurs due to the electrons trapping and detrapping in the oxide region. Once again, the reduced HfO2 thickness leads to better electrostatic coupling, resulting in noise reduction and becoming better when compared to a devices with Si source. In this work was proposed a low frequency noise model for a NW-TFET based on MOSFET models. Minor changes have been done, and thus a good agreement with the experimental results in the region where the BTBT is predominant conduction mechanism was obtained.

Le travail de thèse implique l'étude de matériaux conducteurs transparents sans indium (TCM), composants essentiels de nombreux dispositifs optoélectroniques, utilisant le dépôt spatial de couches atomiques sous pression atmosphérique (AP-SALD). Cette nouvelle technique partage les avantages principaux de l'ALD classique, mais en plus permet le dépôt de couches minces de haute qualité sur de grandes surfaces avec un contrôle précis à l’échelle nanométrique. Ce travail est focalisé sur l'optimisation des propriétés électriques des films d'oxyde de zinc dopé à l'aluminium (ZnO: Al), l'un des oxydes conducteurs les plus étudiés (TCOs). L'influence de plusieurs paramètres expérimentaux sur les propriétés physiques des films a été étudié. Le mécanisme de transport des porteurs de charge au niveau des joints de grains a été identifié comme étant l'émission tunnel plutôt que l’émission thermoïonique dans le ZnO fortement dopé, grâce à un nouveau modèle que nous avons développé en utilisant la méthode de la matrice de transfert à fonction Airy (AFTMM). En résumé, la densité du piège à électrons aux joints de grains pour les échantillons de ZnO:Al (2,2 × 10^20 cm-3) préparés par AP-SALD a été estimée à environ 7,6 ×10^13 cm-2. Notre modèle montre que la diffusion par les joints de grains est le mécanisme de diffusion dominant dans nos films fabriqués par AP-SALD. Nous avons trouvé que le recuit assisté par UV (~ 200 ° C) sous vide était une méthode efficace pour réduire les pièges aux joints de grains, entraînant une amélioration de la mobilité de 1 cm2V-1s-1 à 24 cm2V-1s-1 pour ZnO et à 6 cm2V -1s-1 pour ZnO:Al. Nous avons également utilisé AP-SALD pour fabriquer des TCM performants, stables et flexibles basés sur un réseau de nanofils métalliques. Pour cela, nous avons développé des électrodes composites en revêtant des nanofils argent ou cuivre (AgNWs ou CuNWs) avec ZnO, Al2O3, ou ZnO: Al. Un revêtement très conforme d’une épaisseur de quelques dizaines de nanomètres déposé par la technique AP-SALD améliore considérablement les stabilités thermique et électrique du réseau AgNWs ou CuNWs. Les propriétés optoélectroniques élevées (résistance de surface 10 ohms/carré, transmittance ~ 90%) du composite AgNW / ZnO: Al les rendent très appropriés pour une application en tant que TCM, en particulier pour les dispositifs flexibles.Enfin, en tant que technique de dépôt versatile, AP-SALD est bien compatible avec la technologie des cellules solaires à hétérojonction de silicium (Si-HET) en termes de passivation d'interface. L'intégration de TCM ZnO: Al et AgNWs à la cellule Si-HET a également été explorée. / The thesis work involves the study of Indium-free Transparent Conductive Materials (TCMs), key components of many optoelectronic devices, using Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD). This new approach shares the main advantages of conventional ALD but allows open-air, very fast deposition of high-quality nanometer-thick materials over large surfaces. We focused on the optimization of the electrical properties of Aluminum doped Zinc Oxide (ZnO:Al) films, one of the most studied Transparent Conductive Oxides (TCOs). The effect of several experimental parameters on the physical properties of the deposited films has been evaluated. The carrier transport mechanism at grain boundaries was identified to be tunneling rather than thermionic emission in highly doped ZnO, thanks to a new model we have developed using the Airy Function Transfer Matrix Method. Accordingly, the electron trap density at grain boundaries for ZnO:Al samples (2.2×1020 cm-3) prepared by AP-SALD was estimated to be about 7.6×1013 cm-2. Our model shows that grain boundary scattering is the dominant scattering mechanism in our films. We found that UV assisted annealing (~ 200 °C) under vacuum was an efficient method to reduce grain boundary traps, resulting in an improvement of mobility from 1 cm2V-1s-1 to 24 cm2V-1s-1 for ZnO and to 6 cm2V-1s-1 for ZnO:Al. We have also used AP-SALD to fabricate high-performance, stable and flexible TCMs based on metallic nanowire network. For that, we developed composite electrodes by coating silver/copper nanowires (AgNWs/CuNWs) with ZnO, Al2O3, or ZnO:Al. A thin conformal coating deposited by AP-SALD technique enhanced drastically the thermal/electrical stability of the AgNWs/CuNWs network. High optoelectronic properties (resistivity ~ 10-4 Ωcm, transmittance ~ 90 %) of the AgNW/ZnO:Al composite make them very appropriate for application as TCM, especially for flexible devices.Finally, as a soft deposition technique, AP-SALD is completely compatible to the Silicon heterojunction (Si-HET) solar cell technology in terms of interface passivation. The integration of ZnO:Al and AgNWs based TCMs to Si-HET cell has also been explored.

Les nano-objets unidimensionnels alliés présentent des propriétés physiques spécifiques qui résultent à la fois de leur morphologie, de leur taille et de la répartition chimique des atomes. Nous exploitons un modèle d’Ising sur réseau qui rend compte en particulier des effets de ségrégation au sein de nanofils bimétalliques pour obtenir une compréhension fine des effets gouvernant cette répartition à l’équilibre.Dans une première section, nous détaillons l’équilibre d’un nanofil en fonction de sa taille et de sa composition, de manière à mettre en évidence le rôle des effets de taille finie sur la thermodynamique d’équilibre d’objets bimétalliques 1D. Contrairement aux systèmes infinis, l’équilibre dépend de l’ensemble statistique considéré. Ainsi la ségrégation est plus marquée dans l’ensemble canonique, où la concentration du nanofil est imposée, que dans l’ensemble pseudo-Grand Canonique (p-GC) où le nanofil est en équilibre avec un réservoir qui fixe la différence de potentiel chimique entre les espèces. De même, la contrainte de composition dans l’ensemble canonique induit des corrélations chimiques d’occupation des sites qui favorisent davantage les paires hétéroatomiques. Nous montrons que l’écart observé entre les isothermes des deux ensembles croît avec la courbure de l’isotherme canonique et avec l’amplitude des fluctuations de la concentration nominale dans l’ensemble p-GC. Ces fluctuations diminuant avec la taille du nanofil considéré, l’écart entre les ensembles s’annule à la limite thermodynamique. Les effets de taille finie se traduisent par ailleurs par l’apparition, à basse température et pour de petits nanofils, d’une coexistence d’un mode pur en l’espèce ségrégeante et d’un mode de faible concentration nominale constitué principalement de configurations de type cœur-coquille et Janus. Nous développons alors un formalisme permettant de caractériser cette bimodalité.Alors que les résultats évoqués précédemment concernent un nanofil considéré seul, nous étudions dans la deuxième section l’équilibre de l’ensemble des nanofils formant un co-dépôt unidimensionnel inférieur à la mono-couche. Nous montrons que la distribution en taille de ces nanofils varie globalement selon une loi de puissance, quelle que soit la composition du codépôt, de sorte que la ségrégation n’a que peu d’influence sur la microstructure observée. Par contre, en raison du rapport surface/volume et des corrélations chimiques dans ces objets, la composition des nanofils du co-dépôt varie très fortement selon leur taille, les petits nanofils étant plus riches en l’espèce ségrégeante que les plus grands. Enfin, nous étendons le diagramme de bimodalité d’un nanofil seul à l’ensemble des nanofils du co-dépôt et montrons que cette bimodalité est difficilement observable car elle ne concerne que des amas de petite taille qui sont très minoritaires du fait de la cohésion atomique. / The chemical configuration and the specific shape of 1D bimetallic nano-objects endow them with physical properties (such as magnetic ones) that strongly differ from their bulk counterparts. To get a deep insight of the parameters that govern the equilibrium configuration, we consider a rigid lattice-gas Ising model that accounts for segregation effects within bimetallic nanowires that decorate a step edge.In a first section, we detail the equilibrium of a nanowire as a function of both its size and composition in order to specify the role of finite-size effects onto the equilibrium thermodynamics of 1D bimetallic objects. Contrary to infinite systems, this equilibrium depends on the statistical ensemble to be considered. The segregation profile is indeed stiffer in the canonical ensemble where the nanowire concentration is imposed, than in the semi-grand-canonical ensemble (s-GC) where the nanowire is in equibrium with a reservoir that sets the difference of chemical potentials between the species. Moreover, the composition constraint in the canonical ensemble yields chemical correlations between occupation sites that favor heteroatomic pairs. We show that the deviation observed between the isotherms related to the two ensembles increases with the curvature of the canonical isotherm and with the amplitude of the fluctuations of the nominal concentration within the s-GC ensemble. As these fluctuations decrease with the nanowire size, the deviation between ensembles vanishes at the thermodynamical limit. The finite-size effects also imply at low temperature for small nanowires, that a pure mode of the segregating species coexists with a low-concentration mode that mainly corresponds to core-shell and Janus configurations. We develop a framework to characterize the resulting two-mode density of compositions.While the abovementioned results deal with a fixed-size nanowire, we study in the second section the equilibrium of the set of nanowires that forms a submonolayer 1D-codeposit. We show that the size distribution of these nanowires globally varies as a power law, whatever the codeposit composition, so that segregation has a slight influence onto the observed microstructure. However, due to the surface/volume ratio and chemical correlations within these objects, the composition of the nanowires of the codeposit varies strongly with their size, the smaller the richer in the segregating species. Finally we extend the two-mode diagram of the single nanowire to the set of nanowires forming the codeposit and show that this two-mode distribution is hardly visible as it concerns only short nanowires which are very rare, mainly due to atomic cohesion that is reinforced at low temperature.

Functional nanomaterials are gaining attention due to their excellent shape and size dependent optical, electrical and catalytic properties. Synthesizing nanoparticles is no longer novel with the availability of a host of synthesis protocols for a variety of shapes and sizes of particles. What is currently needed is an understanding the fundamentals of shape and size controlled synthesis to produce functional nanomaterials that is simple and general. In addition to simple metallic nanostructures, synthesizing bimetallic and hybrid nanostructures are important for applications. Instead of trying to add functionality to the preformed nanomaterials, it is advantageous to look for cost effective and general synthetic protocols that can yield bimetallic, hybrid nanostructures along with the shape and size control. In this dissertation, a novel synthetic protocol for the synthesis of ultrfine single crystalline nanowires, metallic and bimetallic nanostructures and hybrid nanostructures has been investigated. The key point of the synthesis is that all different functional nanostructures are achieved by the use of noble metal intermediates in organic medium without phase transfer reagents. The roles of capping agents, oriented attachment and aggregation phenomenon have been studied in order to understand the formation mechanisms. Along with the synthesis, formation mechanisms, the optical and catalytic properties of the functional, noble metal, bimetallic and hybrid nanostructures have been studied. The entire thesis based on the results and findings obtained from the present investigation is organized as follows: Chapter I provides a general introduction to functional nanomaterials, their properties and some general applications, along with a brief description of conventional methods for size and shape-controlled synthesis. Chapter II deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter III presents a novel method of for synthesizing noble metals nanostructures starting from an intermediate solid phase. The method involves the direct synthesis of noble metal intermediates in organic medium without the use of any phase transfer reagent. Controlled reduction of these intermediates leads to the formation of ultrafine nanocrystallite building blocks. Controlled aggregation of the nanocrystallites under different conditions leads to the formation of different nanostructures ranging from single crystalline nanowires to porous metallic clusters. In this chapter, the details of synthesis of the intermediate phase of gold are presented. This intermediate phase is the rocksalt phase of AuCl that has been experimentally realized for the first time. Manipulation of the AuCl nanocubes leads to the formation of a variety of nanostructures of Au starting from hollow cubes to extended porous structures. Mechanistic details of the formation of the intermediate and the nanostructures are presented in this chapter. Chapter IV deals with the symmetry breaking of an FCC metal (gold) by oriented attachment of metal nanoparticles by the preferential removal of capping agent from certain facets and followed by the attachment of gold nanoparticles along those bare facets. This kind of oriented attachment leads to the formation of 1D nanostructures with high aspect ratios. In this chapter, the synthesis, characterisation, formation mechanism and optical properties of high aspect ratio, molecular scale single crystalline gold nanowires has been described. This represent the first ever successful method to produce ultrafine 1D metallic nanostructures approaching molecular dimensions. Chapter V deals with the formation of hybrid nanostructures by attaching the cubic intermediate phase to a substrate like carbon nanotubes followed by the reduction of the attached intermediates on the tubes. The Pt intermediates have been synthesized and attached on the wall of functionalized CNTs and reduced. The PtCNT nanocomposites been characterized by several spectroscopic and microscopic techniques. The electrocatalytic activity of these nanocomposites towards the methanol oxidation has also been investigated. The composites exhibit high catalytic activity and good long term performance. The presence of functional groups on the CNT surface overcomes some of the limitations of current single metal catalysts that suffer from CO poisoning. Chapter VI deals with the formation of palladium nanostructures ranging from nanoparticles to hierarchical aggregates by controlled aggregation of nanoparticles in an organic medium that is tuned by the dielectric constant of the system. A crystalline intermediate of palladium salt has been synthesized and this intermediate of palladium has been used as the precursor solution for the synthesis of palladium nanostructures. The formation mechanism of the nanoporous Pd cluster is investigated using the modified DLVO approach. The catalytic efficiency of the Pd nanostructures has been investigated using the reduction of pnitrophenol and electrocatalytic hydrogen storage as model reactions. Chapter VII discusses the possibility of achieving functional bimetallic alloys by simultaneous reduction of the cubic intermediate of two different metals with experimental evidences. The synergistic effect of the two different metals gives rise to better catalytic activity. This chapter mainly deals with the synthesis of bimetallic porous nanoclusters of goldpalladium and goldplatinum in an organic medium. Detailed microstructural and spectroscopic characterisation of the bimetallic nanoclusters has been carried out and their electrocatalytic performance, morphological stability also investigated.

An abundance of energy in our environment exists in the form of light, thermal, mechanical (e.g., vibration, sonic waves, wind, and hydraulic), magnetic, chemical, and biological. Harvesting these forms of energy is of critical importance for solving long-term energy needs and the sustainable development of the planet. However, conversion cells for harvesting solar energy and mechanical energy are usually independent entities that are designed and built following distinct physical principles. The effective and complementary use of such energy resources whenever and wherever one or all of them are available demands the development of innovative approaches for the conjunctional harvesting of multiple types of energy using an integrated structure/material. By combining solar and mechanical energy-harvesting modules into a single package for higher energy conversion efficiency and a more effective energy recovery process, the research has designed and demonstrated a hybrid cell for harvesting solar and mechanical energy. The results of the research show that we can fully utilize the energy available from our living environment by developing a technology that harvests multiple forms of both solar and mechanical energy 24 hours a day. As the proposed research represents a breakthrough in the innovation of energy harvesting, it should pave the way toward building a new field called "multi-type hybrid" energy harvesting.

Durch Multiskalensimulationen wird der Ladungstransport in nanodrahtbasierten Schottky-Barrieren-Feldeffekt-Transistoren im Materialsystem Ni2Si/Si untersucht. Die Bedingungen an die Genauigkeit der verwendeten Eingangsparameter werden bestimmt und Vorhersagen über optimale Material- und Geräteparameter werden getroffen. Es wird die Frage beantwortet, ob die Bestimmung von physikalischen Parametern aus einzelnen gemessenen Strom-Spannungs-Kennlinie möglich ist. Der Feldeffekt wird durch Berechnungen auf Basis der Finiten-Elemente-Methode und die resultierenden Stromflüsse durch ein quantenmechanisches Transportmodell ermittelt. In der Untersuchung der geometrischen Eingangsparameter wird gezeigt, dass bis auf den Radius des Nanodrahtes die in einem Experiment zu erwartenden Messfehler keinen drastischen Einfluss auf die Strom-Spannungs-Kennlinie haben. Signifikant ist hingegen der Einfluss der Temperatur, der effektiven Ladungsträgermassen und der Höhe der Schottky-Barriere. Da diese drei Eingangsparameter des betrachteten Systems mit relativ großen Ungenauigkeiten behaftet sind, ist die Bestimmung von physikalischen Parametern aus einzelnen gemessenen Strom-Spannungs-Kennlinien auf die erhoffte Weise nicht möglich. Die Arbeit zeigt auch, dass bereits moderate Veränderungen der Arbeitstemperatur einen bedeutenden Einfluss auf die Strom-Spannungs-Kennlinie haben. Für die Konstruktion von Transistoren mit hoher Stromdichte kann anhand der ermittelten Daten die Verkleinerung der aktiven Region durch Oxidation vorgeschlagen werden. / Charge transport in nanowire-based Schottky-barrier field-effect transistors in the material system Ni2Si/Si is examined by multi-scale simulations. The requirements for the accuracy of the input parameters are determined and predictions about optimum material and device parameters are made. The question is answered, whether the determination of physical parameters from individual measured current-voltage curves is possible? The field effect is described by calculations based on the finite element method and the resulting currents are calculated with a quantum mechanical transport model. In the study of the geometric input parameters it is shown that experimental uncertainties do not drastically affect the current-voltage characteristic, except from the nanowire radius. However, significant is the influence of the temperature, the effective charge carrier mass and the height of the Schottky-barrier. Since these three input parameters are known only with low experimental accuracy for the considered system, the determination of physical parameters from individual measured current-voltage curves is not possible in the expected way. The results also show that moderate changes of the working temperature have a significant influence on the current-voltage characteristic. For the construction of transistors with high current density the reduction of the active region by oxidation is proposed.

Nanotechnology offers great promise for the development of nanodevices. Hence it becomes important to study the mechanical behavior of nanostructures for their use in such systems. MEMS (Micro ElectroMechanical Systems) provide an effective and precise method for testing nanostructures. Consequently this study focuses on the development of a MEMS thermal nanotensile tester to investigate the mechanical behavior of one-dimensional nanostructures. Extensive characterization of these MEMS devices (structural, electrical and thermal behavior) was performed using experimental as well as finite element methods. Tensile testing of nanostructures requires manipulation of individual nanostructures on the MEMS device. The study involves the development of an efficient methodology for the manipulation of nanowires and nanobeams for nanoscale testing. Furthermore, two different sensing schemes for the developed devices, namely capacitive and resistive, have been extensively investigated and the advantages and various issues related to both have been discussed. Nanocrystalline (nc) Ni nanobeams (typical dimensions of 500 nm x 200 nm x 20 µm) have been tested to failure using the MEMS devices. Improvements in the design for the MEMS nanotensile tester have been suggested to significantly enhance the device performance and to resolve the various issues involved with nano scale tests. Differential capacitive sensing for stress-strain measurements has been suggested to improve the accuracy of strain measurements.