<p>Mechanical property and thermal stability of Zr-Si-N films of varying silicon contents deposited on Al₂O₃ (0001) substrates are characterized. All films provided for characterization were deposited by reactive DC magnetron sputter deposition technique from elemental Zr and Si targets in a N₂/Ar plasma at 800 ^oC. The hardness and microstructures of the as deposited films and post-annealed films up to 1100 ^oC are evaluated by means of nanoindentation, X-ray diffractometry and transmission electron microscopy. The Zr-Si-N films with 9.4 at.% Si exhibit hardness as high as 34 GPa and a strong (002) texture within which vertically elongated ZrN crystallites are embedded in a Si₃N₄ matrix. The hardness of these two dimensional nanocomposite films remains stable up to 1000 ^oC annealing temperatures which is in contrast to ZrN films where hardness degradation occurs already above 800 ^oC. The enhanced thermal stability is attributed to the presence of Si₃N₄ grain boundaries which act as efficient barriers to hinder the oxygen diffusion. X-ray amorphous or nanocrystalline structures are observed in Zr-Si-N films with silicon contents > 13.4 at.%. After the annealing treatments, crystalline phases such as ZrSi₂, ZrO₂ and Zr₂O are formed above 1000 ^oC in the Si-containing films while only zirconia crystallites are observed at 800 ^oC in pure ZrN films because oxygen acts as artifacts in the vacuum furnace. The structural, compositional and hardness comparison of as-deposited and annealed films reveal that the addition of silicon enhances the thermal stability compared to pure ZrN films and the hardness degradation stems from the formation of oxides at elevated temperatures.</p>

Mechanical property and thermal stability of Zr-Si-N films of varying silicon contents deposited on Al2O3 (0001) substrates are characterized. All films provided for characterization were deposited by reactive DC magnetron sputter deposition technique from elemental Zr and Si targets in a N2/Ar plasma at 800 oC. The hardness and microstructures of the as deposited films and post-annealed films up to 1100 oC are evaluated by means of nanoindentation, X-ray diffractometry and transmission electron microscopy. The Zr-Si-N films with 9.4 at.% Si exhibit hardness as high as 34 GPa and a strong (002) texture within which vertically elongated ZrN crystallites are embedded in a Si3N4 matrix. The hardness of these two dimensional nanocomposite films remains stable up to 1000 oC annealing temperatures which is in contrast to ZrN films where hardness degradation occurs already above 800 oC. The enhanced thermal stability is attributed to the presence of Si3N4 grain boundaries which act as efficient barriers to hinder the oxygen diffusion. X-ray amorphous or nanocrystalline structures are observed in Zr-Si-N films with silicon contents &gt; 13.4 at.%. After the annealing treatments, crystalline phases such as ZrSi2, ZrO2 and Zr2O are formed above 1000 oC in the Si-containing films while only zirconia crystallites are observed at 800 oC in pure ZrN films because oxygen acts as artifacts in the vacuum furnace. The structural, compositional and hardness comparison of as-deposited and annealed films reveal that the addition of silicon enhances the thermal stability compared to pure ZrN films and the hardness degradation stems from the formation of oxides at elevated temperatures.

Thin film nanostructures may be defined as assemblies, arrays, or randomly distributed nanoparticles, nanowires, or nanotubes, which together form a layer of materials supported on a substrate surface. Because such nanostructures are supported on a substrate surface, their potential applications cover a wide area in optical, magnetic, electrochemical, electromagnetic, and optoelectronic devices. The focus of the present thesis is the development of methodologies to grow certain thin film nanostructures of some transition metal oxides (TMOs), including copper oxides and LixCoO2, through CVD, sol-gel, and electromagnetic radiation-mediated approaches. The work towards this objective can be divided into three parts: first, the design, synthesis, and systematic identification of novel metalorganic precursors of copper (monometallic) and Li and Co (bimetallic); second, the growth of nanostructured oxides thin films using these precursors; and third, the application of electromagnetic radiation to control or tailor the growth of as grown nanostructures. The underlying growth mechanisms substantiated by appropriate evidence have been put forward, wherever found relevant and intriguing. It may be added that the principal objective of the work reported here has been to explore the several ideas noted above and examine possibilities, rather than to study any specific one of them in significant detail. It is hoped earnestly that this has been accomplished to a reasonable extent. Chapter 1 reviews briefly the reports available in the literature on three specific methods of growing thin films nanostructures, namely chemical vapour deposition, sol-gel processing and light-induced approach. The objective of this chapter has been to provide the background of the work done in the thesis, and is substantiated with a number of illustrative examples. Some of the fundamental concepts involved, viz., plasmons and excitons, have been defined with illustration wherever found relevant in the context of the work. Chapter 2 describes the various techniques used for synthesis and characterisation of the metalorganic complexes as well as of the thin films. This chapters covers mostly experimental details, with brief descriptions of the working principles of the analytical procedures adopted, namely, infrared spectroscopy, mass spectroscopy, elemental analysis, and thermal analysis for characterisation of the metalorganic complexes. This is followed by a similarly brief account of techniques employed to characterize the thin films prepared in this work, viz., glancing incidence X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), electrostatic force microscopy (EFM), transmission electron microscopy (TEM), glancing incidence infra-red spectroscopy (GIIR) and, UV-visible spectroscopy. The metalorganic chemical vapour deposition (MOCVD) systems built in house and used for growth of films are described in detail. The topics in the different sections of the chapter are accompanied by pertinent diagrams. Chapter 3 deals with the design, synthesis and characterisation of novel polynuclear complexes of copper and cobalt. Keeping in mind the various advantages such as low toxicity, ease of synthesis, non-pyrophoricity, and low temperature volatility, of environmentally benign complexes based on biologically compatible such as triethanolamine, diethanolamine, the objective has been to synthesize complexes containing triethanolamine and diethanolamine of transition metals such as cobalt and copper, and to investigate their applicability in MOCVD processes as a novel class of precursors. With the notion of ‘better’ and efficient design of precursors, an attempt has been made, through a semi-empirical modeling, to understand the correlation between volatility and various intrinsic molecular parameters such as lattice energy, vibrational-rotational energy, and internal symmetry. Chapter 4 discusses the growth of nanoporous Cu4O3-C composite films through the MOCVD process employing Cu4(deaH)(dea)(oAc)5.(CH3)2CO as the precursor. The various characteristic aspects of as-grown films, such as their crystallinity, morphology, and composition have been covered elaborately in various sections of this chapter. The chapter describes the efficient guiding and confining of light exploiting the photonic band gap of these nanoporous films, which indicates the potential usefulness of these and similar films as optical waveguides. A model described in the literature on absorbing photonic crystals, wherein a periodically modulated absorption entails an inevitable spatial modulation of dispersion, i.e., of the index contrast to open a photonic band gap, has been used to calculate the indices of refraction of one of these nanoporous films. The chapter also reports briefly the preliminary electrochemical investigations carried out on a typical film, examining the notion of its application as the anode in a Li-ion rechargeable battery. Chapter 5 describes the synthesis of nanocrystalline LixCoO2 films by the sol-gel method. Reports available in literature indicate that the various phases of LixCoO2 are extremely sensitive to processing temperature, making it difficult to control dimensionality of a given phase using temperature as one of process parameters. We have investigated the possibility of using incoherent light to tailor the particle size/shape of this material. The as-grown and irradiated films were characterised by X-ray diffraction, and by microscopic and spectroscopic techniques.Optical spectroscopy was carried out in order to gain insight into the physico-chemical mechanism involved in such structural and morphological transformation. Chapter 6 deals with the synthesis of self-assembled nanostructures from the pre-synthesized nanocrystals building blocks, through optical means of exciton formation and dissociation. It has been demonstrated that, upon prolonged exposure to (incoherent) ultraviolet-visible radiation, LixCoO2 nanocrystals self-assemble into acicular architectures, through intermediate excitation of excitons. Furthermore, it has been shown that such self-assembly occurs in nanocrystals, which are initially anchored to the substrate surface such as that of fused quartz. This new type of process for the self-assembly of nanocrystals, which is driven by light has been investigated by available microscopic and spectroscopic techniques. Chapter 7 describes the stabilisation of chemically reactive metallic lithium in a carbonaceous nanostructure, viz., a carbon nanotube, achieved through the MOCVD process involving a lithium-alkyl moiety. This moiety is formed in situ during deposition through partial decomposition of a metalorganic precursor synthesized in house, which contains both lithium and cobalt. It is surmised that the stabilization of metallic Li in the nanostructure in situ occurs through the partial decomposition of the metalorganic precursor. Quantitative X-ray photoelectron spectroscopy carried out on such a film reveals that as much as 33.4% metallic lithium is trapped in carbon. Lastly, Chapter 8 briefly highlights the outlook for further investigations suggested by the work undertaken for this thesis. Novel precursors derived from biologically compatible ligands can open up possibility of growing new type of micro/nano-structures, and of unusual phases in the CVD grown films. Furthermore, it is proposed that the novel method of growth and alignment of nanocrystals through irradiation with incoherent light, employed for the specific material LixCoO2, may be employed for various other metallic and semiconducting materials.

The group of materials with ABO3 type perovskite structure are very important due to their attractive electrical and magnetic properties for technological applications and have been studied in the form of single crystals, bulk polycrystalline materials and thin films. Recently, efforts have been made to synthesize and understand the growth of ABO3 type perovskite nanostructures because of their distinctive physical properties and potential applications in the nanodevices. The primary aim of the present thesis is to synthesize the perovskites at nano-scale, with zero-dimension (0D), and one-dimension (1D) configurations. Basic work was carried in terms of synthesis – structure – composition correlation. Due to the small nature of the synthesized materials, few attempts were done to examine the physical properties, but to a limited extant. Efforts were also done to emphasize the structural behavior of nano perovskite in comparison with their bulk counterparts. Chapter 1 provides a brief introduction to perovskite materials and nanostructures, their technological applications and the fundamental physics involved. A brief review of the perovskite nanostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the experimental studies carried out in this thesis. It describes the methods used for the synthesis, experimental set up and the basic operation principles of various structural and physical characterizations such as X-ray diffraction (XRD), thermal analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), compositional analysis (EDX), focused ion beam (FIB), electrical and magnetic studies of the materials prepared. Chapter 3 describes the fabrication of porous anodic aluminum oxide (AAO) templates with different pore size, basic steps for synthesis of nanotubes and the possible growth mechanism of nanotubes in the AAO template. In chapter 4, we report the synthesis of ferroelectric Ba1-xSrxTiO3 (x = 0.0, 0.3) nanoparticles (diameter range: 20-40nm) and Ba1-xSrxTiO3 (x = 0.0, 0.4) nanotubes with diameter about 200nm by the sol-gel method. The Ba1-xSrxTiO3 nanostructures so obtained were characterized by number of techniques, including FE-SEM, XRD, DTA/TGA, FTIR spectroscopy, TEM, HRTEM as well as EDX and SAED. Formation of Y-junctions and multi-branches in Ba1-xSrxTiO3 nanotubes were also observed. The wall of the nanotubes were found to be made of randomly oriented nanoparticles which were confirmed from the HRTEM image. The average thickness of the wall of the nanotubes was found around 15(±5) nm and nanoparticles consisting the wall were found to be in the range of 5-10nm. Diffused phase transition (cubic to tetragonal), shifted to lower temperature side and leaky ferroelectric P–E loops were observed in Ba1-xSrxTiO3 (x = 0.0) ceramic prepared from nanoparticles. Curie temperature was observed at 120oC in the BT nanotube array as confirmed by the dielectric study. The P–E loops of as-prepared Ba1-xSrxTiO3 (x = 0.0) nanotube array were also measured and the hysteresis clearly demonstrates the room temperature ferroelectricity in the as prepared nanotubes, indicating these nanotube array is potential media as ferroelectric information storage. In chapter 5, we report the synthesis of single crystalline nanoparticles and polycrystalline nanotubes of Pb0.76Ca0.24TiO3 (PCT24) by sol-gel processing and characterized by various techniques. The crystallinity and phase purity of the PCT24 nanoparticles and nanotubes were confirmed by the XRD and SAED pattern. Compositional homogeneity and their crystalline structure confirms the formation of the tetragonal perovskite phase. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotubes was found around 20nm and nanoparticles consisting the wall were found to be in the range of 5-8nm. Formation of some single crystalline PCT24 nanorods was also observed as confirmed by SAED and HRTEM analysis. Formations of Y-junctions and multi-branches in this complex functional oxide were observed. Dielectric measurements shows the diffuse phase transition and frequency dependence of Tm (temperature at which real part of dielectric constant shows maxima) suggesting the relaxor type behavior in the PCT24 ceramic prepared from nanoparticles. Polarization study was carried out on PCT24 nanotube array, which shows the ferroelectric nature at room temperature. Chapter 6 reports the synthesis and studies of PbZrO3 (PZ) nanoparticles and PbZr1-xTixO3 for x = 0.0, 0.48 and 1.0 nanotubes. PZ nanoparticles were prepared by a novel sol-gel method based on diol-based solution. Initially, PZ was crystallized with some intermediate m-Z and t-Z phases at 400-550oC and start transforming to orthorhombic at around 600oC and then finally transformed into pure orthorhombic PZ phase at about 700oC. XRD and TEM confirmed the nanocrystalline nature of PZ particles. Curie temperature in the PZ ceramic prepared from PZ nanoparticles was observed around at 205oC, which is lower as compared to the bulk (233oC). P–E hysteresis loops of PZ ceramic prepared from nanoparticles were measured at different applied voltages and single ferroelectric loops of leaky nature were observed rather than antiferroelectrics. The lead zirconate nanoparticles produced may have potential applications as materials used in microelectronics and microelectromechanical systems. PbZr1-xTixO3 for x = 0.0 (PZ), 0.48 (PZT48) and 1 (PT) nanotubes were fabricated by sol-gel method within the closely packed porous alumina templates and characterized by various techniques. The crystallinity of the PZ, PZT48 and PT nanotubes were confirmed via XRD and SAED studies. EDX analysis demonstrated that stoichiometry was formed. Formation of Y-junctions in this complex functional oxide was also observed. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles, which were confirmed by the HRTEM studies and also by a typical SEM image. The average thickness of the wall of the nanotubes was found to be around 10-20nm and nanoparticles consisting the wall was found to be in the range of 3 – 8nm. The Curie temperature was observed at 220oC in the PZ nanotube array. For the first time, PLD has been employed for the synthesis of lead zirconate nanotubes using AAO template. Well-registered arrays of these nanotubes could function as three dimensional (3D) device elements in miniaturized ferroelectric random access memory (FRAM). In chapter 7, we report the synthesis of single crystalline 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) nanoparticles. PMN-PT nanoparticles were developed by a novel sol-gel method based on diol route. After partial calcination at 450oC/1h, PMN-PT powder morphology started transforming from pyrochlore to perovskite phase. It is interesting to note that this partially crystallized PMN-PT powder was unstable under electron beam and generated freestanding lead nanoparticles after absorbing energy from a focused electron beam. PMN-PT powder annealed at 700°C was fully transformed to perovskite phase and was stable under electron beam. XRD calculations and TEM imaging confirmed the nanocrystalline nature of PMN-PT particles. Magnetic measurements on PMN-PT nanoparticles prepared at 650 and 750oC show room temperature ferromagnetic hysteresis, whereas the bulk or the agglomerated particles show diamagnetic behavior. With an increase of annealing temperature or the particle size the magnetic moment decreases. PMN-PT nanotubes with diameter about 200nm were fabricated successfully by the sol-gel method based on diol route within the closely packed porous nanochannel alumina templates. Phase purity and crystalline perovskite phase formation of PMN-PT nanotubes were confirmed by the XRD and SAED pattern. EDX analysis demonstrated that stoichiometry was formed within accepted limit. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotube was found around 20 nm and nanoparticles consisting the wall were found to be in the range of 10-20 nm. Since electroceramic materials are following a similar trend to miniaturization as conventional semiconductors, the synthesis of nanosized oxidic building blocks is moving into the focus of scientific and technological interest. Ferroelectrics are promising class of materials for the fabrication of electronic devices, as they are already an integral part of modern nanotechnological operations. Chapter 8 deals with the synthesis and properties of BiFeO3 (BFO) nanoparticles and nanotubes. Single crystalline BFO nanoparticles of different size and polycrystalline BFO nanotubes were prepared by sol-gel method. As prepared nanostructures were characterized by various techniques such as XRD, TGA-DTA, FTIR, scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM and energy-dispersive X-ray spectroscopy (EDX). The crystallinity and phase purity of the BFO nanoparticles and nanotubes were confirmed by the XRD, SAED pattern and HRTEM analysis. Compositional homogeneity and their crystalline structure confirms the formation of the rhombohedrally distorted perovskite phase. EDX analysis demonstrated that stoichiometric BiFeO3 was formed within accepted limit. The HRTEM analysis confirmed that wall of the BFO nanotubes was made of nanoparticles, which were randomly oriented in the wall. The average thickness of the wall of the nanotubes was found to be around 15 nm and nanoparticles consisting the wall were found to be in the range of 3-6nm. Formation of Y-junctions in this complex functional oxide was observed. Magnetic measurements show clearly the enhancement of ferromagnetism in BFO nanotubes and ferroelectric loops were also observed in these nanotubes, that indicates the multiferroic nature of these nanotubes. BFO nanostructures at a large scale might be important for many applications such as memory elements in nanoscale devices in future. Chapter 9 reports the synthesis of a series of crystalline La1-xCaxMnO3 (x = 0, 0.3, 0.5, 0.7) nanoparticles with average diameter about 20 nm by an improved sol-gel method. The crystallinity and phase formation of as prepared nanoparticles was confirmed via XRD, SAED and HRTEM studies. EDX analysis demonstrated that desired stoichiometric was formed. Magnetic characterization reveals that the PM-FM transitions (Tc) occurs around at 205, 235, 235 and 230 K for x = 0, 0.3, 0.5, 0.7, respectively. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves, a cusplike peak in ZFC curve and unusual shape of M versus H loop at T = 5 K gives strong support for surface spin glass behavior. The highly stable charge ordering state in bulk manganites is suppressed, while the ferromagnetism is enhanced in these nanoparticles (x = 0.5 and 0.7). La0.7Ca0.3MnO3 were fabricated by sol-gel method within the closely packed porous alumina templates. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles (8-12nm) as confirmed by HRTEM studies. The strong irreversibility between ZFC and FC magnetization curves as well as a cusplike peak in ZFC curve gives strong support for surface spin glass behavior. Magnetization value as obtained from M-H loop was about 28.5% of expected value, suggesting the existence of a magnetic dead layer, which avoids the propagation of exchange interaction between magnetic grains. The PM-FM transition was observed at 235 K. Chapter 10 gives the summary and conclusions of the present study and also discusses the possible future work that could after more insights into the understanding of the perovskite nanostructures. Highlight of the present work (i) Successful growth of nanostructures in both particles and tube forms, and study of their structure – composition correlations. (ii) Present work could optimize the necessary chemistry to successfully grow nanoparticles and nanotubes of various perovskite compositions. (iii) Successful studies of physical properties of nanoparticles and nanotubes, ofcourse, to a limited extent. However the properties observed in the present nanostructures have a strong indication of nonlinear phenomena similar to their bulk counterparts. (iv) It was reported in the literature, the observation of ferromagnetic behavior in several nonmagnetic compositions at nano-scale. Surprisingly, similar ferroelectric behavior was noticed even in our perovskite complex oxides such as relaxors (PMN-PT). A clear interaction of magnetic spin and an electric dipole was evident in these oxides such as relaxors and also multiferroics at nano-scale (~10-20 nm). (v) In ferromagnetic compositions such as LCMO, a very interesting spin-glass type behavior was observed.

With the advancements in the field of wireless sensor networks (WSNs), more and more applications require the sensor nodes to have long lifetime. Energy harvesting sources, e.g. thermoelectric generators (TEGs), can be used to increase the lifetime and capability of the WSNs. Integration of energy harvesters into sensor nodes of WSNs can realize self powered systems, providing the possibility for maintenance free WSNs. TEGs can convert the existing temperature differences into electricity. The efficiency of TEGs is directly related to the dimensionless figure of merit (ZT) of materials, which is given as ZT=σS^2 T/k, where σ is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, T is the temperature and σS^2 is the power factor. Traditional thermoelectric (TE) materials are based on inorganic materials, of which the thermal conductivity is high. Over the past decade, the use of nanostructuring technology, e.g. superlattice, could decrease the thermal conductivity in order to enhance the efficiency of TE materials. However, the high cost and the rigidity of inorganic TE materials are limiting factors. As alternatives, polymer based materials have become the research focus due to their intrinsic low thermal conductivity, high flexibility and high electrical conductivity. Moreover, polymer based materials could be fabricated in solution form, giving the possibility for employing printing techniques hence a decrease in the production cost. Unlike the typical approach, in which secondary dopants are added into PEDOT:PSS solutions to modify the power factor of polymer films, this thesis is focused on a more efficient method to improve TE properties. This thesis demonstrates for the first time that post treatment of PEDOT:PSS films with the secondary dopant DMSO as the medium results in a much larger power factor than the traditional addition method. The post treatment method also avoids the usually required mixing step involved in the addition method. Different solvents were selected to discuss the impact factors in the modification of the power factor by this post treatment approach. The post treatment of PEDOT:PSS films was then extended to utilize a green solvent EMIMBF_4 (an ionic liquid) as the medium. EMIMBF_4 is found to exchange ions with PEDOT:PSS films. As a result, the EMIM^+ cations remain in the films and reduce the oxidation level of PEDOT chains, which affects the Seebeck coefficient and the electrical conductivity. Furthermore, TE materials based on hybrid composites with polymer as the matrix and Te nanostructures as the nanoinclusions were investigated. This thesis successfully developed a green synthesis method to obtain Te nanostructures, in which a non toxic reductant and a non toxic Te sources were used. Well controlled Te nanostructures including nanorods, nanowires and nanotubes were synthesized by wet chemical and hydrothermal synthesis. Those as synthesized Te nanowires were then integrated into PEDOT:PSS solution for composite films fabrication. A high Seebeck coefficient up to 200 μV/K was observed in the composite film. / Mit den Weiterentwicklungen der Drahtlosen Sensornetzwerke (engl. WSN, wireless sensor networks) stellen immer mehr Anwendungen die Forderung einer langen Lebensdauer der Sensorknoten. Energiegewinnungssysteme (engl. Energy Harvesters) wie z.B. thermoelektrische Generatoren (TEGs) können genutzt werden, um die Lebensdauer und Leistungsfähigkeit der WSN zu steigern. Mit der Integration von Energy Harvesters können WSN ohne äußere Stromversorgung realisiert und somit die Möglichkeit zur Wartungsfreiheit geschaffen werden. TEGs liefern Energie durch die Umwandlung einer Temperaturdifferenz in Elektrizität. Die Effektivität der TEG ist direkt verbunden mit der Material-Kennzahl ZT und ist gegeben durch ZT=σS^2 T/k, wobei σ die elektrische Leitfähigkeit ist, S der Seebeck Koeffizient, k die thermische Leifähigkeit, T die Temperatur und σS^2 der Leistungsfaktor. Herkömmliche thermoelektrische (TE) Materialien basieren auf anorganischen Materialien, von denen die thermische Leitfähigkeit hoch ist. Im Laufe des letzten Jahrzehnts konnte durch den Einsatz der Nanostrukturierung die thermische Leitfähigkeit verringern werden um damit die Effizienz von TE-Materialien zu steigern. Die Steifigkeit dieser Materialien ist ein anderer Aspekt. Als Alternative für anorganische TE Materialien sind Polymer basierte TE Materialien zum Fokus der Forschung geworden aufgrund einer intrinsisch niedrigen thermischen Leitfähigkeit, hohen Flexibilität und hohen elektrischen Leitfähigkeit. Des Weiteren können diese Polymere in gelöster Form verarbeitet werden, was die Möglichkeit für den Einsatz von Drucktechnologien und damit geringeren Produktionskosten gibt. Anders als der herkömmliche Ansatz den Leistungsfaktor der Polymerfilme durch die Ergänzung von sekundären Dotanten in PEDOT:PSS Lösungen zu verändern, wurde in dieser Arbeit eine effizientere Methode zur Verbesserung der TE Eigenschaften gesucht. In dieser Arbeit wird zum ersten Mal gezeigt, dass die Nachbehandlung von PEDOT:PSS Schichten mit sekundären Dotanten Dimethylsulfoxid (DMSO) als Medium der Nachbehandlung zu einem viel höheren Leistungsfaktor führt als bei der Zugabemethode und außerdem die sonst erforderliche Mischprocedur vermeidet. Es wurden verschiedene Lösungsmittel ausgewählt um die Einflussfaktoren bei der Modifikation des Leistungsfaktors durch die Nachbehandlung von Polymerschichten zu diskutieren. Die Nachbehandlung von PEDOT:PSS Schichten wurde nachfolgend erweitert um das umweltfreundliche Lösungsmittel EMIMBF4 (eine ionische Flüssigkeit) als das Medium einzusetzen. EMIMBF4 ist bekannt für den Austausch von Ionen mit PEDOT:PSS Schichten, so dass EMIM Kationen in der Schicht verbleiben, die Oxidationsstufe der PEDOT-Ketten senken und damit den Seebeck-Koeffizient und die elektrische Leitfähigkeit beeinflussen. Des Weiteren konzentriert sich diese Arbeit auf TE Materialien basierend auf Kompositen aus Polymeren mit Nanoeinlagerungen. Erfolgreiche Syntheseansätze wurden für Tellur-Nanostrukturen entwickelt, bei denen keine giftigen Reduktionsmittel und keine giftigen Tellur-Quellen zur Verwendung kamen. Es erfolgte die Erzeugung von kontrollierten Tellur-Nanostrukturen, einschließlich Nanostäben, Nanodrähten und Nanoröhren, mit nass-chemischer und hydrothermaler Synthese. Die so hergestellten Nanodrähte wurden dann in PEDOT:PSS Lösungen integriert für die Herstellung von Komposite-Schichten. Dabei konnte ein hoher Seebeck-Koeffizienten, bis zu 200 μV/K, festgestellt werden.

No description available.

Controlling the morphology of nanocrystals provides provides a possible pathway to tune properties and hence has been explored in depth. However, to obtain a wider spectrum of properties or for multi-functionality. Other strategies need to be devised. Combining different functional nanostructures to obtain a functional hybrid is one such strategy that holds promise for a wide range of applications. While this is simple in principle, there are no simple and general protocols for synthesis of such functional heterostructure. The challenge lies in producing a hybrid with good control over the structure and chemistry of the interfaces in the system. The use of molecular linkers or physical forces to form the hybrid has several drawbacks in terms of interface quality and stability. In this dissertation, a rational basis is developed for the evolution of symmetry forbidden FCC nanocrystals via wet chemical route which relies on appropriate choice of reagents and the reaction conditions for nucleation and growth. The concept is extended to devise general synthetic strategies for functional nanoheterostrcutres in solution via economic, facile and environment friendly routes. Electron microscopy and X-ray photoelectron spectroscopy has been used as the major tools for structural characterization of the materials and to investigates the reaction/formation mechanism. The properties of the synthesized materials are investigated primarily targeting the nanoelectronic and catalytic applications. The entire study reported in the thesis is organized as follow: chapter I leads to a general introduction of nanocrystals and role in different fields of application. It describes the motivation behind controlling the shape of nanocrystals and combining two or more nanostructures to obtain a functional heterostructure. The existing methodologies to achieve shape control and nanoscale hybrid/heterostructure with active interfaces are elaborated while indicating the role of morphology, interfaces and composition for enhanced activity/performance. The information on the chemical used for synthesis, routers adopted for synthesizing and the basic techniques utilized to characterize the materials in study are detailed in the respective chapters. Chapter 2 provides a study by which one can easily select an appropriate reductant for a metal couple to achieve the desired morphology. Moreover, the role of kinetics and the factors driving the kinetics in obtaining the symmetry breaking shapes like 2-D and I-D for Ag and Au nanocrystals is discussed in detail and validated by experiments. Chapter 3 describes the methodology to attach ultrafine Au nanowires to different nanosubstrates ranging from oxides to carbon (CNT/graphene) where the key step is heteronucleation of the Au (I) precursor on the substrate. Chapter 4 deals with the growth of ultrafine Au nanowires on various substrates and between pre-defined contacts to fabricate nanodevices. The mechanistic investigation directs to the controlled heterogeneous nucleation of the building units (Au nanoparticles) on substrate as the key step followed by its subsequent growth into wires in presence of Au nanoparticles in the medium. Kinetic control of the nucleation and growth step enabled precise control over the population and length of the wires. This is of immense importance for application like catalysis, sensors and nanoelectronics. Moreover, the method enabled the first time electrical transport studies on these wires which revealed an insulating behavior in such metallic wires on progressive lowering of temperature down to few kelvins. The concept of heterogeneous nucleation is extended to design nanoscale heterogeneous in the following three chapters where primarily a precursor coating is formed on a nanosubstrate, viz. ZnO nanorods and graphene, and converted to the phase of interest in a controlled manner to obtain the desired morphology. In each of the chapters the mechanisms of formation of the heterostructure are discussed in detail. Chapter 5 deals with formation of semiconductor based heterostructure like ZnO/CdS in solution by aqueous route. The material has been demonstrated as a potential visible light catalyst for dye degradation with enhanced activity. The interfacial chemistry could be tuned appropriately to achieve high activity in the catalyst by simple wet chemical route. In chapter 6, an ultrafast, facile, green route to obtain oxide supported metal catalyst has been demonstrated. ZnO/Au heterostructures were designed with well defined morphology and studied for low temperature CO oxidation reaction. Detail investigation reveals the surface doping of ZnO with Au the nucleation process leading to active ionic sites for CO oxidation. Chapter 7 demonstrate a rapid and economically viable route to graphene based pt catalysts where a synergistic co-reduction mechanism operates between the metal precursor and the graphic oxide to from the heterostructure. The obtained G-Pt heterostructure exhibits high catalytic activity for methanol oxidation reaction and hydrogen convention at ambient conditions. Finally a conclusion is drawn, highlighting the possibilities and prospects that the study leads to.

Le graphène est un plan unique d'atomes de carbone formant une structure en nid d'abeilles. Dans le cas idéal, le graphène possède des propriétés physiques étonnantes, comme une structure électronique en " cône de Dirac ". Depuis 2004, il est connu qu'on peut obtenir ce matériau bidimensionnel à partir de la graphitisation du carbure de silicium (SiC). Sur la base de calculs ab initio et d'expériences de microscopie à effet tunnel (STM), nous avons entrepris de sonder les propriétés électroniques et structurales du graphène sur SiC et de déterminer en quoi elles sont similaires ou au contraire différentes du graphène idéal. Ce manuscrit commence par une introduction générale sur la thématique du graphène et se poursuit par une description des deux méthodes utilisées durant ce travail. Il vient ensuite l'exposé de nos résultats obtenus pour le graphène sur la face terminée Si et celle terminée C des polytypes hexagonaux du SiC. Nous avons montré notamment que le premier plan de carbone généré sur la face terminée Si se comporte comme un plan tampon, lequel permet aux autres plans qui le recouvrent d'avoir une structure électronique de type monoplan/multiplan de graphène. D'autres aspects liés à la nature complexe de l'interface comme la présence d'états localisés ou l'existence d'une forte structuration du plan tampon sont également discutés. Pour une surface terminée C suffisamment graphitisée, nos travaux révèlent l'existence d'un désordre rotationnel entre les plans de graphène successifs qui se manifeste sous forme de Moiré sur les images STM. Nous montrons par des calculs ab initio qu'une simple rotation permet de découpler électroniquement les plans de graphène.

Dans cette thèse nous considérons un semi-conducteur de GaAs dopé, où nous étudions la relaxation du spin du côté métallique de la transition metal-isolant. Nous considérons deux types différents d'interaction de spin-orbite. Le premier d'entre eux est associé aux impuretés et l'autre est de type Dresselhaus. La dynamique du spin est traitée à travers une formulation analytique basée sur la diffusion du spin de l'électron, et un calcul numérique de la durée de vie du spin.Ensuite, nous considérons une boîte quantique hébergée dans un nanofil de matériau InAs (avec une structure cristalline de type wurtzite), afin d'étudier l'effet de l'interaction spin-orbite sur les états propres du système. Nous développons ici une solution analytique pour la boîte quantique en incluant l'interaction spin-orbite (de type Dresselhaus propre à la structure wurtzite). Nous avons calculé le facteur g effectif, ainsi que la relaxation du spin dûe aux phonons acoustiques, en utilisant les potentiels d'interaction electron-phonon propres à la structure wurtzite.

Les cellules solaires nanostructurées ont été développées pour réduire le coût du photovoltaïque et le rendre compétitif aux autres sources d’énergies. Dans ce but deux cellules solaires ont été étudié durant la thèse: la cellule « eta » (Extremely Thin Absorber) et la cellule hybride à polymères. Dans un premier temps, des couches 2D et nanofils de ZnO ont été réalisés par voie électrochimique sur des substrats verre/TCO (oxyde transparent et conducteur). Il est montré que la température du bain, la densité de charge et la concentration de l’électrolyte support (KCl) infleuncent la morphologie, composition, cristallisation et propriétés optiques des couches. Les films déposés à 0,1 M KCl et à T ≥ 50°C, présente de bonnes propriétés physico-chimiques. La couche 2D est ensuite utilisée pour la croissance des nanofils de ZnO et leurs dimensions sont ajustées avec la moprhologie et l’épaisseur de cette couche. L’électrolyte support et la densité de charge permettent également de contrôler les dimensions des nanofils. Dans un deuxième temps, les nanofils de ZnO ont été photo-sensibilisés par deux types d’absorbeurs : CuInS2 (CIS) et Cu2ZnSnS4 (CZTS). Ils ont été réalisés par différentes méthodes : SILAR (Successive Ion Layer Adsorption and Reaction), électrodépôt et dépôt de nanoparticules pré-synthétisées (pour CIS). Les films préparés par voie SILAR sont très uniformes autour des nanofils. Tandis que ceux réalisés par électrodépôt sont moins homogènes mais de très bonnes qualités cristallines. Grâce à la fonctionnalisation des nanofils, une couche de nanoparticules de CuInS2 très uniforme est déposée. Les cellules « eta » réalisées avec ces structures cœur/coquille montrent un effet photovoltaïque. Les films de ZnO électrodéposés ont été intégrés dans des cellules solaires hybrides à polymères sur substrats verres et plastiques. Ces cellules ont montré de bons rendements et une haute stabilité. / Nanostructured solar cells have been proposed as a solution for photovoltaic cost reduction and to rival the cost of grid-powered electricity. Regarding this challenge, two kinds of solar cells have been studied within the PhD thesis: the Extremely Thin Absorber Solar cells (eta) and the polymer hybrid solar cell. First, we are reporting on the electrochemical deposition of ZnO 2D layers and nanowires on glass substrates covered with TCO (Transparent Conducting Oxide). It is shown that the bath temperature and the supporting electrolyte concentration (KCl) play an important role on the ZnO layer morphology, composition, crystallization and optical properties. The film deposited from 0.1 M KCl and T ≥ 50°C exhibit very good optical and structural properties. These 2D layers are used for consequent ZnO nanowires electrodeposition and their dimensions could be tailored by the seed layer morphology and thickness. The supporting electrolyte concentration and the passed charge density could be additionally used to control their dimensions. Then, the ZnO nanowires have been photosensitized with two absorbers: CuInS2 (CIS) and Cu2ZnSnS4 (CZTS). These materials are prepared by: Successive Ion Layer Adsorption and Reaction (SILAR), electrodeposition and nanoparticules deposition (for CIS). The SILAR films are very uniform around the nanowires. The layers prepared by electrodeposition are less uniform but exhibit very good structural properties. Uniform thin film of CuInS2 nanoparticules are deposited onto functionalized ZnO nanowires. The eta solar cells fabricated with these core/shell nanostructures have shown a photovoltaic effect. The ZnO thin films have been integrated in hybrid solar cells on flexible and rigid substrates. These cells show good power conversion efficiency and a high stability.