CONTROLLED FABRICATIONS OF 3D CARBON NANOTUBES ARCHITECTURES WITH IMPROVED FUNCTIONALITIES

Master'sMASTER OF SCIENC

A study of hybrid carbon nanotubes - polymers microstructures

Master'sMASTER OF SCIENC

Carbon nanotube based electromechanical probes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (p. 131-137).Electromechanical probing applications continuously require smaller pitches, faster manufacturing and lower electrical resistance. Conventional techniques, such as MEMS based cantilever probes have their shortcomings in terms of the lowest pitch that can be achieved, cost and yield. Given their promising mechanical and electrical properties, carbon nanotubes (CNTs) are strong candidates for future probing applications. A new class of metal-CNT hybrid electromechanical probes is presented where vertically aligned carbon nanotube structures, grown with a chemical vapor deposition (CVD) technique, act as elastic springs, and a metal coating on the probes is used for increased electrical conduction. This design and architecture presents a scalable approach where thousands of probes can be fabricated in very short production times. 1.5 Ohm resistance and reliable performance for 6000 cycles at 50 [mu]m over-travel was achieved for a column of 200 [mu]m x 200[mu]m cross-section and 1plm of Au deposition. In-situ scanning electron microscope mechanical compression tests revealed a unique deformation mechanism of the CNT structures where continued compression results in successive buckle formation which later can serve as micro-bellows and elastic springs.(cont.) A novel stiffness tuning method is presented to control the elastic properties of a given CNT probe by controlling the initial compressing amount. Further stiffness tuning is achieved by changing gas composition during CVD growth where CNT diameter and density is modified. Lateral compression and densification tests show that these CNT structures are highly anisotropic and have very different deformation mechanisms in vertical and lateral directions. Mechanical properties resulting from two main CVD growth techniques, namely fixed catalyst where a thin film of catalyst layer is deposited onto the growth substrate, and floating catalyst where the catalyst particles are introduced in the gas phase, are compared. It is found that floating catalyst CVD growth yields much stiffer structures due to the relatively larger CNT diameters. As the adhesion of CNT structures to the growth substrate is very weak and the support layer is typically an insulator, a versatile transfer printing technique is developed which enables simultaneous placement and reinforcement of the probes on a wide range of substrates, including metals and printed circuit boards. Electromechanical performance and failure mechanisms of fully functional metal-CNT hybrid probes are presented.by Onnik Yaglioglu.Ph.D

Studies on Electrocatalysts for Oxygen Electrochemistry, Hydrogen Evolution, and Carbon Dioxide Conversion and Their Applications

Department of Energy Engineering (Energy Engineering)Global energy demand and consumption of humankind have increased significantly over the industrial period and continue to increase steadily. A stable retention of energy resources has become a critical issue in terms of economic, industrial, technical, military and national competitiveness. Fossil fuels have been used extensively throughout the world owing to the miniaturization of combustion energy converters together with their high technological maturity. Accordingly, atmospheric carbon dioxide (CO2) has increased from 278 to 412 parts per million (ppm) over a century and has critically impacted on environmental issues and climate changes. Human beings are now faced irreversible energy requirements and environmental issues, and thus finding new energy sources and energy conversion devices for clean and efficient energy storage and generation is becoming an important challenge. Metal-air batteries have been regarded as prime alternatives for energy storage and conversion devices from their far higher specific energy than that of lithium-ion batteries. However, many complications related to metal anodes (e.g., self-corrosion, rechargeability, etc.), electrolytes (e.g., solution resistance, short-circuit, sluggish ion transfer of separators, etc.), and catalysts (e.g., activities, asymmetrical redox trends, corrosions, durability, high-cost, etc.) should be addressed for the development and implementation of metal-air batteries. Hydrogen (H2), a clean energy source, is widely viewed as a promising alternative energy source to finite fossil fuels, but it has been pointed out that H2 is mainly produced by a hydrocarbon thermolysis (e.g., steam methane reforming) releasing a significant amount of CO2. Alkaline water electrolysis has been known as the green H2 production technology from its nature of no CO2 emissions. However, its energy-intensive electrolysis processes require the development of efficient hydrogen and oxygen evolution catalysts. To reduce carbon footprint, considerable research has been focused on a carbon capture, utilization and storage/sequestration (CCUS) technology to recycle CO2 as a resource to produce high value-added carbon compounds, such as methanol, organic materials, and plastics. So far, however, the economic feasibility of the existing conversion technologies is still inadequate due to sluggish CO2 conversion. Thus, the development of efficient CO2 utilizing electrochemical cells and active electrocatalysts is required.?????? This dissertation focuses on the studies of electrocatalysts for oxygen electrochemistry, hydrogen evolution, and carbon dioxide conversion applicable to metal-air batteries, alkaline water electrolysis, and metal-CO2 cells. This dissertation discovers the active electrocatalysts and the promising electrochemical cells with the detailed discussions organized by material characterizations, electrochemical analyses, mechanism analyses, thermodynamic studies, computational studies, and efficiency calculations. This dissertation starts with brief backgrounds of electrocatalysis, metal-air batteries, alkaline water electrolysis, and CO2 conversion technology in Chapter 1. And the detailed experimental techniques for electrochemical analyses for half-cell and full-cell configurations will be covered in Chapter 2. The rest of chapters will cover the studies of electrocatalysts for oxygen electrochemistry, hydrogen evolution, and carbon dioxide conversion and their applications. The chapters are categorized as follows: Chapter 3 discovers new composite catalysts consisting of nanorod type perovskites and edge-iodinated graphene nanoplatelets for efficient bifunctional catalysts toward oxygen reduction reaction and oxygen evolution reaction, with the application on hybrid Li-air batteries. Chapter 4 discovers new binder-free electrodes prepared by structuring polypyrrole-assisted cobalt oxide anchored carbon fiber for efficient bifunctional catalysts toward oxygen reduction reaction and oxygen evolution reaction, with the application on seawater batteries. Chapter 5 discovers new heterostructure electrocatalysts consisting of perovskite oxides and transition metal dichalcogenides for efficient overall water electrolysis. During overall water splitting operation, the catalysts performed excellent performance and durable stability for a long-term. Chapter 6 discovers new hybrid Na-CO2 electrochemical cells that producing electric energy and hydrogen by efficiently consuming CO2 from the nature of spontaneous CO2 dissolution in an aqueous solution with a long-term stable operation. Chapter 7 discovers new aqueous Zn/Al-CO2 electrochemical cells that utilize CO2 as a useful resource to produce electricity and hydrogen gas using Zn and Al metals, which are abundant, low-cost, and environmentally friendly. The proposed systems presented the best performance among metal-CO2 systems reported so far.clos

Atomic layer deposition functionalization and modification of three dimensional nanostructures for energy storage and conversion

In order to fulfill the increasing demands for various sustainable and renewable energy sources in future, many efforts have been paid to construct high-efficient energy storage and conversion devices for the corresponding energy sources. The boom of nanomaterials provides new opportunities for the development of high efficient energy related devices. Meanwhile, the fabrication of three-dimensional architecture nanomaterials to replace their planer counterparts for devices fabrication has been regarded as one of the promising strategy to improve the efficiency of devices. In this thesis, through effectively combining three-dimensional micro/nanoarchitecture with atomic layer deposition, we carried out a series of systematic research works on controllable fabrication, assembly and functionalization of three-dimensional micro/nanoarchitecture for high-efficient energy storage and conversion devices. The main achievements are outlined as following: 1. A low-cost and controlled assembly route was employed to construct three-dimensional aluminum doped zinc oxide transparent electrode using atomic layer deposition on varies micro/nanostructures, including three-dimensional nanopore array and three-dimensional porous nanostructure and so on. The two main properties of transparent electrode, electroconductivity and transparence of the synthesized three-dimensional aluminum doped zinc were systematically investigated by the adjusting of doping and growth conditions by atomic layer deposition. The constructed three-dimensional aluminum doped zinc oxide could serve as a good transparent electrode to be used in the new generations of photovoltaic and optoelectronic devices. 2. Core/shell nanostructures with optimal structure and composition could maximize the solar light utilization. A feasible route was performed toward scalable fabrication of well-modulated core/shell nanostructures and can be easily applied to other metal/semiconductor composites for high-performance photoelectrochemical electrodes. An aluminum nanocone array as a substrate, well-defined regular array of aluminum doped zinc oxide/titanium dioxide core/shell nanocones with uniformly dispersed gold nanoparticles was successfully realized through three sequential steps of atomic layer deposition, physical vapor deposition and annealing processes. By tuning the structural and compositional parameters, the advantages of light trapping and short carrier diffusion from the core/shell nanocone array, as well as the surface plasmon resonance and catalytic effects from the gold nanoparticles can be maximally utilized. Accordingly, a remarkable photoelectrochemical performance could be acquired. 3. A cost-effective atomic layer deposition process was introduced to realize well-defined three-dimensional platinum nanotube array based on alumina nano-porous template. Through the special introduction of a low-nitrogen-filling step and the control of atomic layer deposition conditions, continuous and smooth surface of platinum nanotube array could be obtained. And to achieve those platinum nanotube arrays, half numbers of the atomic layer deposition cycles and 10% platinum precursor pulsing time are only needed, compared to conventional atomic layer deposition process. The achieved platinum nanotube array was explored as a current collector to construct three-dimensional core/shell platinum/manganese dioxide nanotube array for supercapacitors. The constructed three-dimensional core/shell nanostructure electrode exhibited a high specific capacitance, an excellent rate capability and a negligible capacitance loss after long-term charging-discharging cycling. 4. An ultra-low loading amount of ultrasmall platinum nanoparticles on three-dimensional bacterial cellulose derived carbon nanofiber was achieved by using a convenient modified atomic layer deposition process. The ultrasmall platinum nanoparticles surface-modified three-dimensional carbon nanofiber exhibited good electrocatalytic activity and stability towards hydrogen evolution reaction. The synthesis process provides a general strategy for minimizing the demand of precious metal catalysts while maintaining their high catalytic efficiency. The achieved results within this dissertation on three-dimensional nanostructures fabrication and functionalization, and the integration in energy storage and conversion device should provide a strong insight and guidance on the design and structure of the high efficient energy storage and conversion devices.Um der steigende Nachfrage nach nachhaltigen und erneuerbaren Energiequellen in der Zukunft gerecht zu werden, wurden viele Anstrengen unternommen um hoch effiziente Energiespeicher und Bauelemente zur Energieumwandlung zu entwickeln. Insbesondere bieten Nanomaterialien neue Möglichkeiten um energiebezogene Bauelemente noch effizienter zu machen. Hier verspricht man sich von der Herstellung von dreidimensionalen Nanostrukturen weitere Effizienzsteigerungen im Vergleich zu planaren Strukturen. In dieser Arbeit werden hocheffiziente Energiespeicher und -umwandler durch die effektive Kombination von dreidimensionaler Mikro- und Nanotechnologie mit Atomlagenabscheidung hergestellt und systematisch charakterisiert. Im Folgenden die wichtigsten Ergebnisse: 1. Dreidimensionale nanoporöse Aluminiumdotierte Zinkoxid Elektroden wurden kostengünstig und kontrolliert mit Hilfe von Atomlagenabscheidung hergestellt. Die wichtigsten Parameter, Transparenz und elektrische Leitfähigkeit der Elektrode, wurde systematisch charakterisiert und der Einfluss der Dopingkonzentration und der Wachstumsbedingungen wurde analysiert. Es hat sich herausgestellt, dass die dreidimensionalen nanoporöse Aluminiumdotierte Zinkoxid Elektroden sich insbesondere als gute transparente Elektroden in der Photovoltaik und in optoelektronischen Bauelementen eignen. 2. Kern/Mantelnanostrukturen mit optimierter Struktur und Zusammensetzung können die ausbaute von Sonnenlicht deutlich erhöhen. Eine vielversprechende Route mit starkem Fokus auf die skalierbare Herstellung von gut modulierten Kern/Mantel-Nanostrukturen wurde entwickelt, welche leicht an andere Metall und Halbleiter für photoelektrochemische Elektroden angepasst werden kann. Als Substrat dient ein regelmäßig angeordnetes Aluminium nano-Kegel-Array, welches mit einer Aluminium-dotiertem Zinkoxid / Titandioxid Kern/Mantel Struktur und regelmäßig verteilten Goldnanopartikeln überzogen ist. Die Herstellung wurde Hilfe von Atomlagenabscheidung, physikalischer Dampfabscheidung und einem Glühprozess realisiert. Durch gezielte Abstimmung der Struktur und Zusammensetzung konnte der Lichteinfang verbessert und die Ladungsträgerdiffusion optimiert werden. Plasmonenresonanz und katalytische Effekte konnten durch Goldnanopartikel kontrolliert werden. Dementsprechend konnte eine bemerkenswerte photoelektrochemische Leistungsfähigkeit erzielt werden. 3. Ein kostengünstiger Prozess für die Synthese von dreidimensionalen Platin Nanoröhren-Arrays, basierend auf der Atomlagenabscheidung und nanoporösen Templaten, wurde entwickelt. Dies gelang durch die Einführung eines low-nitrogen-filling Schritts. Kontinuierliche Platin Nanoröhren mit glatter Oberfläche wurden erzielt. Dabei wurde die Anzahl der Zyklen halbiert und die Pulszeit des Platinprecursors um 10 % reduziert im Vergleich zu herkömmlichen Verfahren. Die hergestellten Platinnanoröhren-Arrays wurden als Stromkollektoren für dreidimensionale Pt/MnO2 Kern/Mantel Strukturen in Superkondensatoren eingesetzt. Die synthetisierte Struktur zeigte eine hohe spezifische Kapazität, gute Performance unter schneller Entladung und eine gute Zyklenbeständigkeit. 4. Eine ultra-niedrige Lademenge von sehr kleinen Platin Nanopartikeln auf Kohlenstoffnanofasern, welche mittels bakterieller Zellulose hergestellt wurde, wurde mit Hilfe der Atomlagenabscheidung erzielt. Die mit Platinpartikeln oberflächenmodifizierte Kohlenstoffnanofasern zeigten gute elektrokatalytische Aktivität und Stabilität gegenüber der Wasserstoffentwicklungsreaktion. Das Syntheseverfahren stellt eine allgemeine Strategie dar, um den Einsatz von Edelmetallkatalysatoren unter Beibehaltung ihrer hohen katalytischen Effizienz zu minimieren. Die im Rahmen dieser Arbeit erzielten Ergebnisse in Bezug auf die Herstellung von dreidimensionalen Nanostrukturen, ihre Funktionalisierung und die Implementierung in Bauelemente zur Energiespeicherung und -umwandlung, sollte eine starke Basis für zukünftige Bauelemente mit verbesserter Leistung liefern

Chemical, mechanical, and thermal control of substrate-bound carbon nanotube growth

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 323-357).Carbon nanotubes (CNTs) are long molecules having exceptional properties, including several times the strength of steel piano wire at one fourth the density, at least five times the thermal conductivity of pure copper, and high electrical conductivity and current-carrying capacity. This thesis presents methods of CNT synthesis by atmospheric-pressure thermal chemical vapor deposition (CVD), where effective choice of the catalyst composition and processing conditions enables growth of tangled single-wall CNTs or structures of aligned multi-wall CNTs, on bare silicon, microstructured silicon, and ceramic fibers. Applying mechanical pressure during growth controls the structure of a CNT film while causing significant defects in the CNTs. This mechanochemisty approach is used to "grow-mold" CNTs into 3D-shaped microforms. A new reactor apparatus featuring a resistively-heated suspended platform enables rapid ( 100 °C/s) temperature control and versatile in situ characterization, including laser measurement of CNT film growth kinetics, and imaging of stress-induced film cracking. By thermally pre-treating the reactant mixture before it reaches the substrate platform, aligned CNTs are grown to 3 mm length in just 15 minutes.(cont.) A microchannel array is created for combinatorial flow studies of nanomaterials growth, having velocity range and resolution far exceeding those of conventional furnaces. A detailed design methodology considers compressible slip flows within the microchannels and flow leaks across the array, and the devices are fabricated by KOH etching of silicon. Initial experiments with this system demonstrate chemically-driven transitions in CNT yield and morphology along the microchannels, and flow-directed alignment of isolated CNTs and CNT strands. Applications of aligned CNTs in reinforced composites and electromechanical probes are enabled by the CNT synthesis technologies presented here, and show significant initial promise through collaborative research projects. Overall, controlling the packing density and matrix reinforcement of aligned CNTs gives material attributes spanning from those of energy-absorbing foams to stiff solids; however, significant increases in CNT length, growth rate, and packing density must be achieved to realize macroscopic fibers and films having the properties of individual CNTs. New machines can be created for studying the limiting aspects of growth reactions, for exploring new reaction regimes, and for producing exceptionally long nanostructures, looking ahead to fabrication of CNT-based materials in a continuous and industrially-scalable fashion.by Anastasios John Hart.Ph.D

Transmission Electron Microscopy studies of the nanostructural characteristics of the Inductively Coupled Plasma synthesized Silicon Nanowires.

Récemment, la recherche sur les nanomatériaux a atteint sa maturité puisque plusieurs applications de la nanotechnologie ont déjà rejoint le marché. À juste titre, plusieurs nanomatériaux se retrouvent déjà dans les produits de la vie quotidienne. Dans ce contexte, l'étude des matériaux à base de silicium (Si), pierre angulaire du développement de la microélectronique pendant les dernières décennies, conservera sans doute un rôle important dans le panorama scientifique et technologique. En particulier, les nanostructures de Si, grâce aux effets de confinement quantique (QC), peuvent avoir des propriétés améliorées et de nouvelles fonctionnalités par rapport au Si massif. Parmi les nanostructures de Si, les nanofils de Si, structure unidimensionnelle, soulèvent un grand intérêt de la part des chercheurs au cours des dernières années. En effet, les nanofils de Si ont été intégrés avec succès dans des dispositifs de détection chimique et biologique avec haute performance, dans des anodes de batterie au lithium de haute qualité et dans les dispositifs thermoélectriques. Les effets de QC dans les nanostructures de Si ont conduit à une émission optique remarquablement efficace, ce qui ne serait pas le cas pour le Si massif puisque il s’agit d’un matériau à bande interdite indirecte. Ceci ouvre la perspective d’aller vers des dispositifs nanophotoniques et photovoltaïques de nouvelle génération. De plus, l'émission optique des nanostructures de silicium peut être ajustée dans le visible en contrôlant la largeur de leur bande interdite, qui dépend directement de la taille de nanocristaux de silicium. Dans cette perspective, ce projet de thèse porte sur l'étude des nanofils de Si synthétisés au moyen d'une nouvelle approche basée sur l’utilisation d’une torche de plasma inductif (connu sous le terme anglosaxon par Inductively coupled Plasma; ICP). La technique ICP est utilisée, depuis quelques années, pour la production industrielle de microsphères de Si au moyen d'un procédé de sphéroïdisation. Au cours de ce procédé de sphéroïdisation, il y a formation concomitante de nanostructures de silicium avec une large proportion de nanofils de Si, qui feront l’objet d’étude de cette thèse de doctorat. Nous nous sommes plus concentrés sur l’étude et la compréhension des mécanismes de croissance de ces nanofils de Si produits dans le système ICP, qui ont, entre autres, la particularité d’être le plus fins dans tous les nanofils rapportés dans la littérature à ce jour. Pour ce faire, nous avons adopté une approche basée sur les caractérisations par une foule de techniques de microscopie électronique à transmission (MET) pour étudier en profondeur les caractéristiques nanostructurales morphologiques et chimiques des nanofils de Si. Ainsi, des techniques liées à la microscopie électronique à transmission ont été utilisées, tels que l’énergie filtrée, le MET à haute résolution, la tomographie électronique, la spectroscopie par rayons X, la cathodoluminescènce, et la microscopie électronique à perte d'énergie. Nous avons ainsi pu identifier trois familles de nanofils de Si, à savoir, (i) les nanofils de Si avec un coeur cylindrique continu de Si (ayant des diamètres allant de 2 à 15 nm) entouré d’une couche concentrique de SiO₂ (ayant une épaisseur de 4 à 15 nm); (ii) les nanofils de Si dont le coeur en Si est formé d’un chapelet de nanocristaux sous forme d’amande connectés par un nanofil de Si très fin, et le tout enveloppé dans une couche de silice; et (iii) les nanofils de Si dont le coeur en Si est formé par une chaîne de nanocristaux de Si déconnectés les uns des autres et le tout est dans la croute nanocylindrique de silice (de 4 à 5 nm d’épaisseur). En étudiant ces différentes nanostructures de Si, nous avons pu mettre en évidence deux mécanismes de croissance compétitifs qui mènent à la croissance unidimensionnelle de ces trois classes de nanofils de Si via la technique ICP, à savoir la croissance assistée par l’oxyde et, dans une moindre mesure, la croissance catalysée par des nanoparticules de Fe issues des impuretés résiduelles de la poudre de silicium injectée initialement dans le réacteur ICP. Nous avons ensuite, mis l'accent sur la corrélation entre la caractérisation structurale de ces nanostructures de Si et leurs propriétés optoélectronqiues par de mesures de photoluminescence (PL), l’objectif étant de confirmer l’émission dans le visible de ces nanostructures de Si, ce qui constitue une signature de la présence d'effets de confinement quantique dans nos nanofils produits par ICP. De plus, la mise au point d'un procédé de purification approprié a été développée par une méthode basée sur la centrifugation afin d’extraire les nanofils de Si du reste du la poudre de nano-silicium issue du procédé de sphéroïdisation par ICP. Nous avons, enfin, étudié l'effet des traitements thermiques sur les changements nanostructuraux des nanofils-ICP, via des analyses MET in-situ et ex-situ. De cette manière, nous avons pu atteindre les conditions d'instabilité de Rayleigh et transformer structurellement les nanofils de Si en chaînes de nanocristaux de Si à une température de 1200°C. La formation « contrôlée » d'un tel nanocomposite a contribué à la compréhension de l'apparition de structures similaires dans les nanofils de Si tel que produits par ICP (nonrecuits). Les propriétés optoélectroniques des nanofils de Si recuits ont été caractérisés par PL et les valeurs de leur émissions optiques ont été déterminés et corrélés avec les tailles des nanocristaux de Si (mesurées directement par MET). Puisque le procédé ICP constitue déjà une technique bien développée industriellement, ces études peuvent ouvrir la voie à l'optimisation du système ICP pour la synthèse intentionnelle de nanofils de Si en grand volume, répondant ainsi aux besoins des applications à grande échelle. Les propriétés optoélectroniques de ces nanofils-ICP (émission PL intense dans le visible et potentiellement contrôlable par une sélection fine de leurs diamètres) ouvre définitivement la voie à leur utilisation dans les futurs dispositifs nanophotoniques et photovoltaïques. Recently, nanomaterial research has reached its maturity as long as several nanotechnology applications have reached the market. Deservedly, several nanomaterials can be found in every-day life items. In this regard, the study of Silicon (Si) based materials, which represented the cornerstone for the development of microelectronics in the past few decades, will most likely preserve an important role in the scientific and technological panorama. In particular, Si nanostructures have demonstrated to possess enhanced properties and even new functionalities with respect to their bulk counterpart via Quantum Confinement (QC) effects. Among Si nanostructures, Si nanowires (SiNWs), i.e. one dimensional Si nanostructures, have grasped a lot of interest from the researchers in the past few years. In fact, SiNWs have been successfully integrated into high-performance chemical sensing and bio-sensing devices, high-quality lithium battery anodes or thermoelectric devices. QC effects in Si nanostructures have been demonstrated to lead to an efficient optical emission, which is not possible in the bulk Si, as indirect band-gap material. This opens the possibility to prospect new generation nanophotonic and photovoltaic devices. Moreover, the optical emission of Si nanostructures can be tuned in the visible range by controlling the band-gap amplitude, which depends on the Si nanocrystal size. In this perspective, this PhD project has been devoted to the study of SiNWs synthetized by means of a novel approach based on the exploitation of an inductively coupled plasma torch (indicated as Inductively Coupled Plasma; ICP). So far ICP technique was exploited for the industrial production of Si microspheres by means of a spheroidization process. During this process, the concomitant formation of Si nanostructures occurs, mostly constituted of SiNWs, which have been the subject of this PhD thesis. Here, it will be focused both the study and the understanding of the growth mechanism of these ICP-SiNWs, which have the characteristic of being the thinnest SiNWs reported in literature so far. To this aim, an approach based on Transmission Electron Microscopy (TEM) characterizations has been applied to thoroughly investigate the ICP-SiNW nanostructural morphological and chemical characteristics of the ICP-SiNWs. Hence, TEM related techniques have been used, i.e. Energy Filtered TEM (EFTEM), High Resolution TEM (HRTEM), electron tomography, Scanning TEM Energy Dispersive X-ray spectroscopy (STEM-EDX), cathodoluminescence (CL) and Electron Energy Loss Spectroscopy (EELS). In this way, it has been possible to identify three families of ICP-SiNWs, namely (i) cylindrical SiNWs having a continuous cylindrical Si core (with diameter between 2-15 nm) surrounded by a concentric SiO₂ shell (having thickness from 4 to 15 nm), (ii) SiNWs of which the Si core is represented by a chapelet-like structure, constituted of almond-shaped Si nanocrystals (SiNCs) connected via a very-thin SiNW, all surrounded by a silica shell, (iii) SiNWs of which the Si core is represented by a chain of SiNCs separated from each other, located along an otherwise silica nanocylinder (with thickness of 4-5 nm). By studying these different Si nanostructures, it has been possible to infer two competitive growth mechanisms which lead to the one dimensional growth of these three classes of SiNWs via the ICP technique, namely the oxide assisted growth (OAG) mechanism and, to a lesser extent, the catalyzed growth by means of iron (Fe) nanoparticles present as impurities in the Si powder initially injected in the ICP reactor as a feedstock. In addition, focus has been put on the correlation between the structural characterization of these ICP-Si nanostructures and their optical properties, probed via Photoluminescence (PL) spectroscopy, in order to confirm the visible emission of these Si nanostructures as a signal of the occurrence of QC effects in the ICP-SiNWs. Moreover, the design of a suitable purification process was developed via a centrifugation based method in order to separate the SiNWs from the residuals of the Si nanopowders produced by the main ICP spheroidization process. As a further step of the project, the effect of thermal treatments on the nanostructural changes of ICP- SiNWs via both in-situ and ex-situ TEM analyses was studied. In this way, we were able to reach the Rayleigh instability conditions and structurally transform the SiNWs into SiNC chains as long as the temperature was set at 1200°C. The controlled formation of such nanocomposite contributed to the understanding of the occurrence of similar structures in the as-produced ICP-SiNWs (not annealed). The optoelectronic properties of the annealed SiNWs have been probed by means of PL spectroscopy and their optical emissions have been correlated with the SiNC sizes determined by the TEM characterizations. Since the ICP process already constitutes a welldeveloped technique for nanostructure synthesis at industrial scale, these studies may open the route for the optimization of the ICP system for the intentional synthesis of large volumes of SiNWs, matching the requirements of large-scale applications. The optical properties of ICP-SiNWs (intense PL emission in the visible range and potentially controllable by a precise selection of their diameter) also foster their application in future nanophotonic and photovoltaic devices

Limitations and Advancements in Soft x-ray Spectroscopy

Soft x-ray absorption spectroscopy (XAS) is a widely used method for probing the electronic structure of materials, yet it suffers from many complications related to the reliable measurement of absorption spectra and the proper theoretical modelling of spectral features. Problems due to different experimental aspects, such as sample charging, surface contamination and saturation effects, often introduce artifacts or distortions into a measured absorption spectrum. Even when measured accurately, the interpretation of absorption spectra is complicated by the absence of rigorous theoretical methods that properly account for the effect of the core hole on the remaining electronic structure, leading to inaccurate spectral simulations. The limits of the one electron model of core excitation were explored through a high resolution XAS study of the linear polyacenes. When measured at high resolution, the linear polyacenes exhibited a fine structure that could be assigned to excitation of non-equivalent carbon atoms in the molecules. The energies and intensities of these transitions were extracted and compared to transition intensities calculated from density functional theory, where the multi-electronic effects were approximated using a full core hole and half core hole model. The full core hole model was found to better approximate the trends in the spectra for the smaller molecules, but neither model was able to properly calculate the changes in the absorption spectra of the larger molecules. These results demonstrated the deficiencies of the one electron picture of core hole excitation and highlighted the need for incorporating multi-electronic effects into XAS simulations. To extend the capabilities of XAS, the use of partial fluorescence yields (PFY) was explored. One of the primary experimental limitation of XAS is that absorption spectra are measured by monitoring the total yield of electrons or photons from the sample. In both of these methods, a direct relationship between the measured spectra and the linear attenuation coefficient of the sample is not guaranteed. To overcome this drawback, the partial fluorescence yield measurement technique, employing a silicon drift detector, was used. By measuring partial fluorescence yields (PFY), as opposed to total yields, the effects of background fluorescence could be avoided. The inverse of the partial fluorescence yield was also demonstrated to be an effective way of avoiding saturation effects in some samples. The utility of the inverse partial fluorescence yield (IPFY) method was demonstrated in a study of several iron oxide minerals that were not possible to measure using conventional total yield methods. IPFY was also demonstrated for liquid samples where differences between the Fe PFY and IPFY were noted and attributed to resonant scattering effects in the PFY. In both the iron oxide study and the polyacene study, the experimental limitations related to the measurement of XAS were evaluated by comparison to x-ray Raman spectroscopy (XRS). XRS is a hard x-ray based method that probes core excitation in low-Z elements by measuring the energy loss of the scattered photons. Both XRS and XAS involve the same electronic transitions allowing for a direct comparison of spectra measured using the two methods, but the limitations of XAS related to the short penetration depth of soft x-rays are not encountered in XRS. While XRS measurements do not suffer from saturation or surface effects, the experimental resolution and count rates of this technique are limited. This dissertation demonstrated that, while XAS is an established method, there are many aspects of the technique that require additional development. Emerging calculation methods that can incorporate the interaction between the electron and core hole will improve data interpretation capabilities. New detector systems that can measure partial yields with high resolution and can be placed in specific geometries will continue to improve measurement quality and allow for the development of novel techniques, like IPFY. A growing use of XAS and XRS in concert will also improve the general applicability of core level excitation spectroscopies and help to advance our understanding of the materials around us

Modeling and Optimization of Renewable Energy Systems

This book includes solar energy, wind energy, hybrid systems, biofuels, energy management and efficiency, optimization of renewable energy systems and much more. Subsequently, the book presents the physical and technical principles of promising ways of utilizing renewable energies. The authors provide the important data and parameter sets for the major possibilities of renewable energies utilization which allow an economic and environmental assessment. Such an assessment enables us to judge the chances and limits of the multiple options utilizing renewable energy sources. It will provide useful insights in the modeling and optimization of different renewable systems. The primary target audience for the book includes students, researchers, and people working on renewable energy systems