No description available.

The past few decades have witnessed a race towards developing smaller, faster, cheaper and ultra high capacity data storage technologies. In particular, this race has been accelerated due to the emergence of the internet, consumer electronics, big data, cloud based storage and computing technologies. The enormous increase in data is paving the path to a data capacity gap wherein more data than can be stored is generated and existing storage technologies would be unable to bridge this data gap. A novel approach could be to shift away from current two dimensional architectures and onto three dimensional architectures wherein data can be stored vertically aligned on a substrate, thereby decreasing the device footprint. This thesis explores a data storage concept based on vertically aligned cylindrical magnetic nanowires which are promising candidates due to their low fabrication cost, lack of moving parts as well as predicted high operational speed. In the proposed concept, data is stored in magnetic nanowires in the form of magnetic domains or bits which can be moved along the nanowire to write/read heads situated at the bottom/top of the nanowire using spin polarized current. Cylindrical nanowires generally exhibit a single magnetic domain state i.e. a single bit, thus for these cylindrical nanowire to exhibit high density data storage, it is crucial to pack multiple domains within a nanowire. This dissertation demonstrates that by introducing compositional variation i.e. multiple segments along the nanowire, using materials with differing values of magnetization such as cobalt and nickel, it is possible to incorporate multiple domains in a nanowire. Since the fabrication of cylindrical nanowires is a batch process, examining the properties of a single nanowire is a challenging task. This dissertation deals with the fabrication, characterization and manipulation of magnetic domains in individual nanowires. The various properties of are investigated using electrical measurements, magnetic microscopy techniques and micromagnetic simulations. In addition to packing multiple domains in a cylindrical nanowire, this dissertation reports the current assisted motion of domain walls along multisegmented Co/Ni nanowires, which is a fundamental step towards achieving a high density cylindrical nanowire-based data storage device.

La conversion thermoélectrique a suscité un regain d'intérêt en raison des possibilités d'augmenter l'efficacité tout en exploitant les effets de taille. Par exemple, les nanofils montrent théoriquement une augmentation des facteurs de puissance ainsi qu'une réduction du transport des phonons en raison d'effets de confinement et/ou de taille. Dans ce contexte, le diamètre des nanofils devient un paramètre crucial à prendre en compte pour obtenir des rendements thermoélectriques élevés. Une approche habituelle consiste à réduire la conductivité thermique phononique dans les nanofils en améliorant la diffusion sur les surfaces tout en réduisant les diamètres.Dans ce travail, la caractérisation thermique d'une forêt dense de nanofils de silicium, germanium, silicium-germanium et alliage Bi2Te3 est réalisée par une méthode 3-omega très sensible. Ces forêts de nanofils pour le silicium, le germanium et les alliages silicium-germanium ont été fabriqués selon une technique "bottom-up" suivant le mécanisme Vapeur-Liquide-Solide en dépôt chimique en phase vapeur. La croissance assistée par matrice et la croissance par catalyseurs en or des nanofils à diamètres contrôlés ont été réalisés à l'aide d'alumine nanoporeuse comme matrice. Les nanofils sont fabriqués selon la géométrie interne des nanopores, dans ce cas le profil de surface des nanofils peut être modifié en fonction de la géométrie des nanopores. Profitant de ce fait, la croissance à haute densité de nanofils modulés en diamètre a également été démontrée, où l'amplitude et la période de modulation peuvent être facilement contrôlées pendant la fabrication des matrices. Même en modulant les diamètres pendant la croissance, les nanofils ont été structurellement caractérisés comme étant monocristallins par microscopie électronique à transmission et analyse par diffraction des rayons X.La caractérisation thermique de ces nanofils a révélé une forte diminution de la conductivité thermique en fonction du diamètre, dont la réduction était principalement liée à une forte diffusion par les surfaces. La contribution du libre parcours moyen à la conductivité thermique observée dans ces matériaux "bulk" varie beaucoup, Bi2Te3 ayant une distribution en libre parcours moyen (0,1 nm à 15 nm) très faible par rapport aux autres matériaux. Même alors, des conductivités thermiques réduites (~40%) ont été observées dans ces alliages attribuées à la diffusion par les surfaces et par les impuretés. D'autre part, le silicium et le germanium ont une conductivité thermique plus élevée avec une plus grande distribution de libre parcours moyen. Dans ces nanofils, une réduction significative (facteur 10 à 15 ) a été observée avec une forte dépendance avec la taille des nanofils.Alors que les effets de taille réduisent la conductivité thermique par une meilleure diffusion sur les surfaces, le dopage de ces nanofils peut ajouter un mécanisme de diffusion par différence de masse à des échelles de longueur atomique. La dépendance en température de la conductivité thermique a été déterminée pour les nanofils dopés de silicium afin d'observer une réduction de la conductivité thermique à une valeur de 4,6 W.m-1K-1 dans des nanofils de silicium fortement dopés avec un diamètre de 38 nm. En tenant compte de la conductivité électrique et du coefficient Seebeck calculé, on a observé un ZT de 0,5. Avec l'augmentation significative de l'efficacité du silicium en tant que matériau thermoélectrique, une application pratique réelle sur les appareils n'est pas loin de la réalité. / Thermoelectric conversion has gained renewed interest based on the possibilities of increasing the efficiencies while exploiting the size effects. For instance, nanowires theoretically show increased power factors along with reduced phonon transport owing to confinement and/or size effects. In this context, the diameter of the nanowires becomes a crucial parameter to address in order to obtain high thermoelectric efficiencies. A usual approach is directed towards reducing the phononic thermal conductivity in nanowires by achieving enhanced boundary scattering while reducing diameters.In this work, thermal characterisation of a dense forest of silicon, germanium, silicon-germanium and Bi2Te3 alloy nanowires is done through a sensitive 3ω method. These forest of nanowires for silicon, germanium and silicon-germanium alloy were grown through bottom-up technique following the Vapour-Liquid-Solid mechanism in Chemical vapour deposition. The template-assisted and gold catalyst growth of nanowires with controlled diameters was achieved with the aid of tuneable nanoporous alumina as templates. The nanowires are grown following the internal geometry of the nanopores, in such a case the surface profile of the nanowires can be modified according to the fabricated geometry of nanopores. Benefiting from this fact, high-density growth of diameter-modulated nanowires was also demonstrated, where the amplitude and the period of modulation can be easily tuned during the fabrication of the templates. Even while modulating the diameters during growth, the nanowires were structurally characterised to be monocrystalline through transmission electron microscopy and X-ray diffraction analysis.The thermal characterisation of these nanowires revealed a strong diameter dependent decrease in the thermal conductivity, where the reduction was predominantly linked to strong boundary scattering. The mean free path contribution to the thermal conductivity observed in the bulk of fabricated nanowire materials vary a lot, where Bi2Te3 has strikingly low mean free path distribution (0.1 nm to 15 nm) as compared to the other materials. Even then, reduced thermal conductivities (~40%) were observed in these alloys attributed to boundary and impurity scattering. On the other hand, silicon and germanium have higher thermal conductivity with a larger mean free path distribution. In these nanowires, a significant reduction (10-15 times) was observed with a strong dependence on the size of the nanowires.While size effects reduce the thermal conductivity by enhanced boundary scattering, doping these nanowires can incorporate mass-difference scattering at atomic length scales. The temperature dependence of thermal conductivity was determined for doped nanowires of silicon to observe a reduction in thermal conductivity to a value of 4.6 W.m-1K-1 in highly n-doped silicon nanowires with 38 nm diameter. Taking into account the electrical conductivity and calculated Seebeck coefficient, a ZT of 0.5 was observed. With these significant increase in the efficiency of silicon as a thermoelectric material, a real practical application to devices is not far from reality.

A highly self-ordered hexagonal array of cylindrical pores has been fabricated by anodizing a thin film of Al on substrate and subsequent growth of GaN and InGaN in these nanoholes has been performed. This AAO template-based synthesis method provides a low cost process to fabricate GaN-based nanomaterials fabrication. / Singapore-MIT Alliance (SMA)

<p>Materials prepared on the nanoscale often exhibit many different properties compared to the same materials in their bulk-state. Interest in nanostructured materials has increased because of these properties in fields such as microelectronics, catalysis, optics and sensors. This increased interest in nanostructured materials calls for new and more precise fabrication techniques.</p><p>This thesis describes how to use the porous anodic aluminium oxide as a template for the fabrication of a variety of nanostructured materials. Palladium and copper nanoparticles were deposited along the pore walls in anodic aluminum oxide using electroless deposition and atomic layer deposition. In both cases, it was possible to control the size of the nanoparticles by carefully monitoring the deposition parameters. The thesis also describes how Prussian blue nanoparticles and nanotubes can be fabricated using the anodic aluminium oxide as a template. The deposition of Prussian blue was performed by a sequential wet-chemical method. By using atomic layer deposition, it was also possible to deposit thin films of amorphous Nb2O5 on the pore walls. When the template was removed by etching, freestanding nanotubes were obtained. The anodic aluminium oxide membrane was also used as a mask for high energy (MeV) ion irradiation of an underlying substrate. The tracks produced were etched away with hydrogen fluoride. In this way, it was possible to transfer the highly ordered porous pattern from the mask onto other oxides such as SiO2 and TiO2.</p><p>All fabricated structures were characterized using a variety of analysis techniques: scanning electron microscopy for evaluating sample morphology; transmission electron microscopy for better resolved investigations of the morphology; X-ray diffraction to assess crystallinity; energy dispersive spectroscopy and X-ray fluorescence spectroscopy to determine the elemental composition and identify possible contaminants.</p><p>The general aim of the work described in this thesis has been to create a set of tools for use in the fabrication of a variety of nanostructured materials, whose dimensions composition can be tailored by selecting appropriate fabrication methods and parameters.</p>

Materials prepared on the nanoscale often exhibit many different properties compared to the same materials in their bulk-state. Interest in nanostructured materials has increased because of these properties in fields such as microelectronics, catalysis, optics and sensors. This increased interest in nanostructured materials calls for new and more precise fabrication techniques. This thesis describes how to use the porous anodic aluminium oxide as a template for the fabrication of a variety of nanostructured materials. Palladium and copper nanoparticles were deposited along the pore walls in anodic aluminum oxide using electroless deposition and atomic layer deposition. In both cases, it was possible to control the size of the nanoparticles by carefully monitoring the deposition parameters. The thesis also describes how Prussian blue nanoparticles and nanotubes can be fabricated using the anodic aluminium oxide as a template. The deposition of Prussian blue was performed by a sequential wet-chemical method. By using atomic layer deposition, it was also possible to deposit thin films of amorphous Nb2O5 on the pore walls. When the template was removed by etching, freestanding nanotubes were obtained. The anodic aluminium oxide membrane was also used as a mask for high energy (MeV) ion irradiation of an underlying substrate. The tracks produced were etched away with hydrogen fluoride. In this way, it was possible to transfer the highly ordered porous pattern from the mask onto other oxides such as SiO2 and TiO2. All fabricated structures were characterized using a variety of analysis techniques: scanning electron microscopy for evaluating sample morphology; transmission electron microscopy for better resolved investigations of the morphology; X-ray diffraction to assess crystallinity; energy dispersive spectroscopy and X-ray fluorescence spectroscopy to determine the elemental composition and identify possible contaminants. The general aim of the work described in this thesis has been to create a set of tools for use in the fabrication of a variety of nanostructured materials, whose dimensions composition can be tailored by selecting appropriate fabrication methods and parameters.

Les stents urétéraux et les sondes de néphrostomie sont constitués de silicone ou de polyuréthane thermoplastique (TPU). Afin de limiter les risques infectieux lors de leur implantation, une modification topographique par création de pores permettrait de limiter l’adhésion des bactéries et de former des réservoirs pour une libération in situ de substances antiinfectieuses. Ce travail vise à préparer des surfaces en polymère ayant un motif poreux tubulaire ordonné. Une réplication en deux temps à partir de surfaces ordonnées poreuses d’oxyde d’aluminium (PAAO) préparées par 2 méthodes (double anodisation douce et double anodisation dure/douce) a été mise en œuvre pour reproduire le motif initial sur des surfaces en silicone et TPU. Pour le moule intermédiaire trois matériaux ont été testés (acrylonitrile butadiène styrène, polystyrène et résine polyacrylate). Les surfaces ont toutes été caractérisées par des techniques microscopiques et spectroscopiques. Les surfaces en PAAO préparées par double anodisation douce possédaient des pores d’environ 50 nm de diamètre et 100 nm de profondeur, alors que celles obtenues après mise au point de la méthode dure/douce étaient de taille supérieure, d’environ 125 nm de diamètre et ayant des profondeurs de quelques centaines de nanomètres. La surface du moule intermédiaire est constituée de picots. Une adhésion latérale de ceux-ci a été observée pour certaines conditions. La meilleure réplication du motif a été obtenue pour le TPU. Les surfaces ainsi obtenues pourront être utilisées et optimisées lors de l'étude ultérieure de l'adhésion du biofilm. / Ureteral stents and nephrostomy catheters are made of silicone or thermoplastic polyurethane (TPU). A topographical modification creating an ordered porous surface could limit the infectious risks during their implantation, by reducing bacterial adhesion and creating a loading platform from which anti-infectious compounds could be released.In this work, a two-steps replication method was used to create ordered porous polymer surfaces (silicone or TPU) using porous anodic aluminium oxide (PAAO) as master template. The PAAO surfaces were prepared by double mild or double hard/mild anodization. Three intermediate mould materials were tested (acrylonitrile butadiene styrene, polystyrene, polyacrylate resin). The polymer material (silicone or TPU) was then moulded onto the intermediate mould surfaces that possessed freestanding pillar arrays, to imprint pores. The obtained surfaces were characterized by microscopic and spectroscopic methods. The initial PAAO surfaces prepared by double mild anodization possessed pores of about 50 nm diameter and 100 nm depth, whereas those prepared after development of the double hard/mild anodization method were bigger, of about 125 nm diameter and several hundred nanometers deep. The intermediate mould structure possessed freestanding arrays, but instabilities (lateral adhesion) were noted for certain conditions. The best pattern replication was observed for TPU. In conclusion, these novel porous polymeric surfaces could be optimized and tested for an anti-biofilm effect.