Le développement considérable des signaux dans les bandes hyperfréquences (communications sans fil, radars, etc.) rend leur traitement extrêmement difficile. Il est en effet nécessaire d'analyser de larges bandes spectrales avec une grande résolution, ce que ne permettent pas les dispositifs purement électroniques. Une solution très prometteuse consiste à transposer les signaux hyperfréquences (hf) sur un laser puis à utiliser un cristal dopé par des ions de terres rares comme processeur. Cette technique tire parti des propriétés optiques exceptionnelles des ces matériaux, qui présentent des transitions à la fois extrêmement fines pour un centre unique (largeur homogène donnant la résolution spectrale) et larges pour un ensemble d'ions (largeur inhomogène donnant la bande d'analyse). Grâce à des techniques de pompage optique, ceci permet d'obtenir des bandes d'analyse de 20 GHz avec une résolution de 100 kHz, soit un rapport de 105. Ces résultats, obtenus dans des cristaux de Tm3+:Y3Al5O12, font l'objet d'études industrielles avancées, notamment en France par Thales, et auxquelles participe l'IRCP-MPOE. Le but du projet est de développer des cristaux dopés Er3+ pour une nouvelle génération d'analyseurs fonctionnant à 1.5 μm dans le but d'exploiter pleinement les technologies télécom en terme de transmission longue distance et de composants opto-électroniques. Les meilleurs cristaux actuels ne possèdent pas une largeur inhomogène suffisante pour l'application visée. Notre approche consistera à introduire un désordre chimique contrôlé, par exemple en utilisant un co-dopant, dans des monocristaux dopés Er3+ de haute qualité. Ceci permettra d'augmenter la largeur inhomogène optique mais pourrait aussi influencer la largeur homogène, point crucial pour l'analyse des signaux hf. Il s'agira de déterminer la nature et le niveau de désordre optimaux permettant d'obtenir les meilleures performances. D'une façon plus générale, ce travail permettra une compréhension approfondie des phénomènes dynamiques contrôlant la largeur homogène optique et de leur relation avec les structures cristallines. Outre l'analyse de signaux hyperfréquences, ceci pourra déboucher sur des avancées dans le traitement quantique de l'information dans le domaine télécom. / Significant progresses have been made recently on radar communications. However it is still difficult to analyse radar communications both efficiently and on a large frequency span. This is due to the fact that pure electronic processors are not able to process rapidly signals with high bandwidths. A very promising solution consists in transposing radar signals on a optical carrier (laser) and process the signals via rare-earths-doped single-crystals, which are able to interact efficiently with light. Rare-earth-ion doped crystals can have very narrow optical transitions at liquid helium temperature, making them attractive for applications in quantum information processing and advanced RF signal processing. One key property of these materials is the potential for a high ratio between the optical inhomogeneous and homogeneous linewidths. This allows signals with high bandwidth to be stored in quantum memories for a long time, or alternatively, the high resolution spectral analysis of RF signals. Er3+ is particularly interesting because it has a transition at 1.5 mm that is directly compatible with telecommunication components in existing optical fiber networks. The aim of the project is to enhance the bandwidths of those atomic processors by introducing a chemical disorder in the single crystals doped with Er3+. This will lead to an inhomogeneous broadening of the optical transitions and could also reduce the optical homogeneous linewidths, and so, increase the processing bandwidth for radar signals. For that, a better understanding of the nature of the dynamical processes acting on the optical homogeneous linewidth is needed.

No description available.

Acid/Base characterizations of metal oxide surfaces are often used to explain their catalytic behavior. However, the vast majority of these studies have been performed on powders or supported oxides, and there is very little information available in the literature on the interaction of acid/base probe molecules with well-defined oxide surfaces of known coordination geometry and oxidation state. The well-defined, single crystal surfaces of Cu₂O (111), SnO₂ (110), and Cr₂O₃ (101̲2) were investigated for their acid/base properties by the interactions between the probe molecules and the well-defined surface features. The adsorption of NH₃ at cation sites was used to characterize the Lewis acidity of SnO₂ (110) and Cu₂O (111) surfaces. The adsorption of CO₂, a standard acidic probe molecule, was used to characterize the Lewis basicity of the oxygen anions on SnO₂ (110), Cu₂O (111) , and Cr₂O₃ (101̲2) surfaces. BF₃, while not a standard probe molecule, has been tested as a probe of the Lewis basicity of the oxygen anions on SnO₂ (110) and Cr₂O₃ (101̲2). By studying probe molecules on well-defined metal oxide surfaces with known coordination geometry and oxidation state, an overall evaluation of NH₃, CO₂, and BF₃ as probe molecules can be made using the surfaces studied. NH₃ probed differences in Lewis acidity of Sn cations on SnO₂ (110), which had differences in coordination environments and oxidation states. But, NH₃ adsorption failed to provide any direct information on differences in Lewis acidity of Cu cations in different local coordination geometries on Cu₂O (111). CO₂ is a poor probe of the Lewis basicity of oxygen anions on the metal oxide surfaces studied here. CO₂ does not strongly adsorb to either SnO₂ (110) or Cu₂O (111). On Cr₂O₃ (101̲2), CO₂ does interact with oxygen sites but in two different coordinations, which vary with surface condition, making a comparison of basicity difficult. In the cases studied here, CO₂ either does not adsorb, or it does not provide a clear set of results that can be related simply to Lewis basicity. BF₃ seems to be a much better probe of the Lewis basicity than CO₂ for the well-defined metal oxide surfaces studied here. On SnO₂ (110) and Cr₂O₃ (101̲2), the boron atom of BF₃ directly interacts with oxygen sites by accepting their electrons. BF₃ thermal desorption seems to provide a direct measure of the Lewis basicity of different surface oxygen species as long as they are thermally-stable in vacuum. / Ph. D.

The growing concern about climate change is pushing the global community towards greener solutions to cut down the greenhouse gases emissions. As such, producing energy from sustainable sources becomes mandatory to achieve the net zero emissions goal by 2050, as set by the United Nations. Solar panels offer the possibility to generate power from light harvesting, but it’s the use of organic materials that offers great advantages in terms of functionality and life-cycle. In particular, organic semiconductors properties such as their tunable colours, lightweight, flexibility, and semi-transparency enable the use of Organic Photovoltaics (OPV) in building façades and contribute to the realisation of Net Zero Energy Buildings (NZEB). However, the OPV scalability to terawatts of installed capacity is still non competitive with respect to its cost when compared to the conventional inorganic silicon-based technologies. One of the reasons is the lower performance achieved by the state-of-the-art OPV devices, whose active layer (the film where the light is absorbed and converted into free charges, electrons and holes, i.e. electricity) is typically composed of a blend made of an electron donor material (conjugated polymer) and a smaller compound as electron acceptor (Non-Fullerene Acceptor, NFA). A crucial factor determining the low performance of OPVs made with NFAs is related to their poor charge transport properties (e.g. low electron mobility and high recombination), which are intimately related to how these molecules are arranged in the solid film, i.e. their molecular organisation. Great progress was made in the field of organic electronics to obtain higher mobility by understanding the crystalline behaviour of organic molecules from their single crystals, and using these knowledge in the design of new compounds with the desired properties. At the beginning of this thesis project, little was known about the solid-state organisation of NFAs as very few single crystal structures were disclosed. For these reasons, we were first dedicated to the study of the intrinsic propensity of NFAs to crystallise by growing single crystals. At this fundamental level, we found that the NFA packing geometry is strongly affecting the isotropy of the charge transport, and potentially the electron mobility. On a following step, we developed a methodology to track the NFA packing geometry as we move from ideal systems (single crystals) to the most complex scenario of the solar cell active layer films, which include a donor and an acceptor (NFA) component. We discovered that NFAs generally tracks their packing motif from single crystals to blend films, and we quantified the benefit of using crystalline compounds with specific packing geometry in terms of electron mobility. Interestingly, we also found that these motifs are not necessary to obtain high performance in organic solar cells as the efficiency is mostly driven by charge recombination and domain purity, rather than electron mobility.

Recent findings of multiferroicity and magnetoelectric effects in rare earth manganites have fuelled research in this class of materials. These multiferroics can be structurally divided into two classes – orthorhombic and hexagonal. Especially attractive are TbMnO3, HoMnO3 and DyMnO3. Since the ionic radius of Dy is at the boundary that separates the orthorhombic and hexagonal RMnO3, DyMnO3 can be synthesized in both the structures using different synthesis conditions. In this thesis, DyMnO3 single crystals (both hexagonal and orthorhombic) prepared using optical floating zone furnace are studied through structural, magnetic and thermal properties. The influence of rare earth ion on the magnetic phase transitions is revealed in magnetisation, ac susceptibility and specific heat studies. Moreover, doping RMnO3 (small R) with alkaline earth ions creates an arena to test the interesting physics of spin-glass-like phenomena in manganites that arises due to quenched disorder. In this regard, 50% strontium diluted DyMnO3 could be an ideal system to study the effects of quenched disorder and structural/magnetic inhomogeneities that govern the magnetic phases in manganites. Structural phase-coexistence and ensuing anomalous magnetism in Pr–based manganite Pr0.6Sr0.4MnO3 are also presented in this thesis. Details of how the thesis is organized into eight chapters and a brief summary of each chapter follows: Chapter 1 is an introduction to the physics of manganites which progresses into multiferroics and eventually discusses the spin-glass-like effects arising due to size mismatch. A discussion on the phase-coexistence and its effect on physical properties are also presented. Eventually, the scope of the thesis is outlined in the last section. Chapter 2 outlines the basic experimental methods employed in this thesis work. Chapter 3 describes the details of crystal growth by optical floating zone method. DyMnO3 crystals in both hexagonal and orthorhombic structures are grown by employing the ambience of argon and air respectively. The crystals in the two crystallographic variants are characterized by X ray diffraction, Energy dispersive X ray analysis and Inductively coupled plasma atomic emission spectroscopy. The crystal structures are refined using Rietveld method with FULLPROF code and found to be P63cm for hexagonal and Pnma for orthorhombic DyMnO3. Details of crystal growth of Dy1−xSrxMnO3 are also presented. The change in ambience has no effect in the crystal structure of this doped manganite. A comparison of the growth of undoped and doped systems is given. In a later section, the crystal growth and structure refinement of Pr0.6Sr0.4MnO3 are discussed and the optimized growth parameters are tabulated for various manganite systems grown in the present work. Chapter 4 deals with the magnetic and thermal characterization of hexagonal and orthorhombic DyMnO3 single crystals. Magnetic measurements reveal the importance of rare earth magnetism in these compounds. The antiferromagnetic transition to a stacked triangular antiferromagnet is discernible from the specific heat studies of hexagonal DyMnO3, which is masked in the bulk magnetisation measurements. Various magnetic transitions pertaining to the antiferromagnetic sinusoidal – spiral – incommensurate magnet, are evident in the magnetisation and specific heat of orthorhombic DyMnO3 which belongs to the class of non-collinear magnets. Chapter 5 deals with basic investigations on the spin-glass-like state in Dy0.5Sr0.5MnO3. Preliminary dc magnetisation shows indication of spin-glass state as a split in field-cooled and zero-field-cooled magnetisation cycles. Further, the failure of scaling of M(T) with H/T indicates the absence of superparamagnetism in Dy0.5Sr0.5MnO3. The dynamic susceptibility and its analysis using the theory of critical slowing down yield exponents pertaining to the spin-glasses. However, a four-order magnitude change is observed in the characteristic spin-flip time. This leads to the assumption that in Dy0.5Sr0.5MnO3 the spin entities are not atomic spins as in canonical spin-glasses but clusters of spins. The specific heat is analysed for signatures of spin-glass state and is found that a linear term in temperature is essential in fitting the observed data. The crystalline electric fields of Dy ion is also analysed attempting multiple Schottky-levels instead of two. Chapter 6 concerns with the aging experiments performed in the spin-glass-like state in Dy0.5Sr0.5MnO3. Striking aging and chaos effects are observed through these measurements. However, owing to the clusters of spins present, deviations from the typical time-dependent behavior seen in canonical spin-glass materials are anticipated in Dy0.5Sr0.5MnO3. In fact, the relaxation measurements indicate that the glassy magnetic properties are due to a cooperative and frustrated dynamics in a heterogeneous or clustered magnetic state. In particular, the microscopic spin flip time obtained from dynamical scaling near the spin-glass transition temperature is four orders of magnitude larger than microscopic times found in atomic spin-glasses. Magnetic viscosity, deduced from the waiting time dependence of the zero field cooled magnetisation, exhibits a peak at a temperature T<Tsg. Waiting time experiments prove that the dynamics is collective and that the observed memory effects are not due to superparamagnetism of separate magnetic entities. Chapter 7 discusses the Electron paramagnetic resonance (EPR) studies on single crystals of DyMnO3 in hexagonal as well as orthorhombic structures. The interesting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is also probed using EPR. The lineshapes are fitted to broad Lorentzian in the case of pure DyMnO3 and to modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity and geff derived from the signals are analysed as a function of temperature. The EPR results corroborate well with the magnetisation measurements. The study clearly reveals the signature of frustrated magnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systems persist even above the transition. Moreover, a spinglass-like behaviour in Dy0.5Sr0.5MnO3 is indicated by a step-like feature in the EPR signals at low fields. Chapter 8 deals with the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This crystal undergoes two prominent phase transitions – a paramagnetic to ferromagnetic at Tc~300 K and a structural transition at Tstr ~ 64 K. These phase transitions are evident in the static magnetisation as well as in frequency-dependent susceptibility. In these measurements, the structural transition is associated with a sizeable hysteresis typical of a first-order transition. The M–H curves below Tc show clear indication of anomalous magnetism at low temperatures: the virgin curve lies outside the subsequent magnetisation loops. These observations are explained by assuming structural coexistence of a high–temperature orthorhombic and a low–temperature monoclinic ferromagnetic phases. The nature of static magnetisation data is analysed in the critical region. Modified Arrott’s plots yielded perfect straight lines with the isotherm at ~ 300 K passing through the origin. The exponent values thus should be very close to those expected for the universality class of Heisenberg ferromagnets. The temperature dependence of resistivity also shows critical nature with an exponent belonging to the Heisenberg class. The thesis concludes with a chapter on General conclusions and future scope on these systems.

The solid-state dye sensitized solar cell (SDSC) is an emerging photovoltaic technology which promises inexpensive materials, roll-to-roll processing, and a stable architecture. In this thesis, I seek to enhance electronic transport in order to enable thicker devices and yield higher power conversion efficiencies. I adopt a multipronged approach to advance three aims, employing analytical, computational, and experimental methodologies. First, I generalize existing models of the dye sensitized solar cell (DSSC) to allow simple parameter fitting of real devices and to account for previously ignored electronic processes. In Chapter 3 and Chapter 4 I present a nondimensional model capable of fitting real devices and simulating transient behavior without extensive material knowledge. Subsequently in Chapter 5, I introduce a novel three-dimensional model which incorporates electronic drift. Second, in Chapter 4 I critically assess a widespread method of measuring the charge collection efficiency, the summary metric that describes the efficacy of charge transport in the SDSC. I discover that the conventional method is inaccurate for values of the collection efficiency below 90% because of large experimental error and an intrinsic inaccuracy in applying a transient method to measure a steady-state parameter. Third, I aim to increase the rate of charge transport by employing new materials and nanostructures in the place of conventional nanocrystalline TiO2. In Chapter 5, I present evidence of faster transport and enhanced efficiency in flexible SnO2 nanowire SDSCs, ZnO nanowire SDSCs, and the first viable SnO2/P3HT SDSC, where photoanode and hole transporter have been replaced with higher mobility materials. Finally, in Chapter 6, I investigate use of TiO2 mesoporous single crystals (MSCs) with high surface area and extended crystallinity. After demonstrating the viability of MSCs in SDSCs, I examine enhanced transport caused by the background doping effect of thermal treatment. Together, the progress achieved toward diverse and ambitious goals advances the field and delineates routes to future progress for SDSC development.

Studies on ac losses in superconductors have been a subject of great interest for a long time not only as an important topic in fundamental science, but also as a basic requirement for the application of superconductors. A proper understanding of the mechanisms of ac losses and their quantitative knowledge is an essential requirement for any application. Such studies not only yield information on the material parameters crucial for applications but can also provide a test for any possible microscopic theory of superconductivity. The main focus of the current thesis is to understand the mechanisms of ac losses in superconductors and to gain more knowledge on the ac dissipative behavior of type II superconductors. In this thesis we report our investigations on the ac losses in certain type II superconductors at different ranges of frequency through different experimental techniques. We have investigated the ac losses that arise in high Tc superconducting single crystals at rf frequency (8 MHz) using a simple LC oscillator technique. The result shows a surprising ac dissipation behavior in which the loss in the superconducting state is more than the normal state loss. Even though the superconducting state is defined as the zero resistive state, this is true only for dc transport. The ac resistivity studies have been made also on high Tc polycrystalline samples using the standard four-probe technique using a lock-in amplifier (100 kHz). The result shows different ac resistive behavior for samples with different microstructures. Non-resonant microwave absorption (NRMA) studies in MgB2 thin films is reported for the first time. The experiment has been performed using a continuous wave X band EPR spectrometer. The recorded signals give information on the ac losses that occurs at microwave frequency (9.43 GHz). The effects of ac magnetic field on the superconductors have been investigated through a dc four-probe resistivity measurements in the presence of an ac field of different magnitudes applied at different frequencies. Also a simple experimental technique based on the concept of kinetic inductance designed to study the ac losses that arise due to vortex motion is reported. In the following a chapter-wise summary of the thesis is presented.m Chapter 1 surveys the related literature on experimental and theoretical reports on ac losses in superconductors. In this chapter a brief introduction to superconductors is given with an emphasis on the high Tc superconductors. The superconducting materials studied in the thesis are described in detail along with their superconducting parameters and the form of the specimen. The origins of ac losses are discussed with various models proposed so far to explain the ac losses in superconductors. Since most of the ac losses reported in this thesis arise due to the Josephson junctions and vortex motion, they are discussed in detail. The occurrence of Josephson junctions and the various models used to describe the junctions’ characteristics are discussed. The formation of vortices their various forms in layered superconductors and the mechanisms of flux flow and flux creep are discussed. Chapter 2 describes the studies on ac losses in superconducting Bi2Sr2CaCu2O8 single crystals [1,2]. Generally in the superconducting state the dissipation is expected to be less compared with that in the normal state. However, we observe that the ac losses in the superconducting state are larger than the normal state losses. In this chapter we report on the ac losses in superconducting Bi2Sr2CaCu2O8 single crystals at radio frequencies determined from direct measurement of the absorbed power using an rf oscillator [3]. The ac response of Bi2Sr2CaCu2O8 single crystals is investigated as a function of temperature from the measured shift in current and the frequency of the oscillator. The studies are carried out at different rf amplitudes by varying the supply voltage to the oscillator circuit. To understand the magnetic field dependent behavior of ac losses, studies have been performed in the presence of magnetic field of various magnitudes applied parallel to the c-axis of the crystal. In the presence of the magnetic field two peaks are observed in ac losses in the superconducting state as a function of temperature. The presence of the peaks and their behavior are studied in detail by varying the orientation of the applied field with respect to the c-axis of the crystal. The results are discussed in terms of a new model proposed recently by us [4], which explains ac losses as a consequence of cumulative effect of the energy spent in repetitive decoupling of the Josephson junctions and in terms of Lorentz force driven motion of vortices. In Chapter 3, we discuss the ac resistivity behavior of the polycrystalline superconducting samples with different microstructures. Measurement of resistivity is the basic characterization method not only for superconductors but for any material. The superconducting state is defined as the zero resistive state; but this statement is true only for dc and not for ac. The presence of ac resistance in superconductors leads to losses. In the present work we report on the behavior of ac resistance in the superconductors. The application of a magnetic field and the variation of temperature alter the AC penetration depth of the superconducting sample, which in turn changes the AC impedance associated with it. In this chapter we report the results on the complex AC conductivity that has been measured in two types of polycrystalline YBa2Cu3O7 samples at frequencies starting from 100 Hz to 100 kHz and at temperatures from 10 K to 300 K. In the first pellet which is sintered, the possibility of presence of extrinsic Josephson junctions is less, but a large number of Josephson junctions is present in the second non-sintered pellet. In general it is expected that the AC or the DC resistivity in superconductors should decrease below Tc. In the case of DC resistivity the value of resistance goes exactly to zero and in the case of AC resistivity it keeps on decreasing towards zero with decreasing temperature. But surprisingly we find that in superconducting samples with Josephson junctions, the AC resistivity drops very close to zero at the critical temperature and instead of decreasing it increases slowly with decreasing temperature below the critical temperature. This property is also strongly dependent on the applied AC frequency. Investigation of the above phenomenon gives information regarding the contribution of JJ decoupling towards the AC resistivity of superconducting samples. The observed ac resistive behavior is well fitted with the Ambegaokar-Baratoff model for temperature dependence of critical current in the Josephson junction. In Chapter 4, the possibility of the presence of weak links in the intermetallic superconductor MgB2 is reported. The role of weak links in superconductors has been studied for a long time. Understanding the behavior of weak links has great importance for the applications of superconductors. Presence of weak links in high Tc materials due to its insulating grain boundaries limits the application potential of those materials. These weak links lead to lower critical current density and lower critical field of superconductors and lead to losses. The discovery of superconductivity in the simple intermetallic compound MgB2 has created a lot of interest from both application aspects and of fundamental science. MgB2 differs from high Tc materials and is considered as a potential candidate for applications, because of its high critical current density which arises due to the absence of weak links in MgB2. Absence of weak links is reported in most of the MgB2 literature and only in a very few studies possibility of the presence of weak links is reported. Here, our NRMA studies on the MgB2 thin films show the presence of weak links [5]. NRMA is a highly sensitive, non-invasive technique, which has proven to be a valuable tool for detecting weak links in superconductors and their characterization [6]. In this technique the sample is studied using a continuous wave electron paramagnetic resonance (EPR) spectrometer, by recording the magnetic field dependence of the power absorption. The NRMA studies on the MgB2 thin film shows the presence of weak links and hysteresis in the signal. The origin of weak links is discussed as being due to the presence of oxygen in the grain boundaries. The hysteresis appears because of remnant magnetization and due to the pinning of flux lines when there is a change in the sweeping field direction. The NRMA studies are carried out as a function of temperature, modulation field, microwave power and the scan range and the results are reported in this Chapter. In chapter 5 we report on the resistive behavior of superconducting MgB2 film in the presence of an ac field using a novel technique. In this simple technique the resistive measurements are done using the general four-probe method, but a coil is wound over the sample and connected to an ac source to generate the ac field. The resistivity measurements are carried out as a function of temperature, amplitude and the frequency of ac field. The ac field shifts the Tc towards lower temperature and increases the broadening in transition from normal to superconducting state. In the absence of Lorentz force due to the parallel orientation of ac field with the transport current, we find that Josephson junction decoupling is one of the main origins of resistivity. The results are compared with the resistive behavior of YBCO film. The epitaxial YBCO film which is free from weak links shows a different frequency dependent resistive behavior, which is explained in terms of flux-creep. In the MgB2 film the studies are carried out in the presence of a dc field that is applied perpendicular to direction of transport current in the film along with the presence and the absence of the ac field. The studies show that in superconductors the presence of ac field leads to more loss than that of dc field. Chapter 6 describes a simple experimental technique using the property of kinetic inductance to measure the vortex resistivity arising from the ac current. Since the discovery of the superconductors much attention has been given to the dynamics of the vortices because of their importance from both scientific and application point of view. When a magnetic field of amplitude more than Hc1 is applied the type II superconductors enter in to the ‘mixed state’ due to the presence of vortices. In the presence of a current, the vortices experience Lorentz force of magnitude F = J x B normal to the current and the field. The vortices move under the influence of the Lorentz force along its direction which leads to resistivity. The electric field generated by the vortex movement has two components, one acting along the current direction and the other normal to the current direction. But most of the vortex resistivity measurements are carried out either in the presence of high magnetic field or at temperatures closer to Tc due to the limitation of experimental techniques. In this chapter we are reporting a simple experimental technique to measure vortex resistivity with very high resolution even at low temperatures and fields based on the concept of kinetic inductance. Kinetic inductance is the property which arises mostly in superconductors due to the inertial mass of the charge carriers. In our measurement kinetic inductance is measured through a simple four-probe ac impedance technique, which is more commonly used for measuring resistivity. The penetration depth due to vortices is related to their resistivity and from the relation between the measured kinetic inductance and penetration depth vortex resistivity is calculated. In this report we discuss the experimental setup, principle of the method and present the results of our measurements carried out on YBa2Cu3O7 thin films.

Surfaces and interfaces are of fundamental importance from the nucleation to growth of crystals formed under different conditions such as vapor phase, liquid phase including biomineralisation conditions. Recently there is lot of interest in controlling the shape of nanoparticles during the synthesis due to their excellent shape dependent properties. Understanding the role of surfaces and interfaces is vital for such shapecontrolled synthesis of nanomaterials. On the surface, coordination number, structure, density and composition are different from that of bulk and hence the properties are completely different in the surfaces and interfaces of any crystalline material. Especially when the length scale become nanoscale, the surface and interface play a dominant important role and leads to several new and interesting phenomena. In this dissertation, the role of surfaces and interfaces on the synthesis and the properties of inorganic functional nanostructures have been studied. The work primarily relies on basic chemistry to synthesize nanostructures that brings the importance of surfaces/interfaces into the picture. Though several basic characterization techniques have been used, electron microscopy has been the emphasis and has been used extensively through the work to probe and explore the materials for characterizing the structures over a variety of length scales. The entire thesis based on the results and findings obtained from the present investigation are organized as follows: Chapter1 gives a general introduction to the surfaces and interfaces to create a background for the investigation. This emphasizes the role of surfaces and interfaces in several aspects starting from nucleation, growth to the properties of inorganic crystals. It gives some exposure in to the different type of surface phenomenon which is common in nanoscale materials. Chapter 2 deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter 3 deals with predicting the morphology of 2D nanostructures by combining the crystal growth theory into chemical thermodynamics. Morphology diagrams have been developed for Au, Ag, Pt and Pd to predict conditions under which two-dimensional nanostructures form as a result of a chemical reaction. In addition, it provides the general understanding of shape control in 2D nanostructures with atomistic mechanism. The validity of the morphology diagram has been tested for various noble metals by carrying out critical experiments. As a result, 2D nanostructures of metals projecting the lowest energy facet resulted in a complete novel way in the absence of any capping/reducing agents. Chapter 4 deals with predicting the formation of 2D nanostructures of inorganic crystals formed as a result of precipitation reaction. Morphology diagram has been developed for the case of hydroxyapatite, an inorganic part of the human bone. This answers some of the long standing question related to the shape of the HA crystals formed in the bone by biomineralisation. The generality of the method has been tested to few other inorganic crystals such as CaCO3, ZnO and CuO formed through precipitation reaction. The key finding of the above two chapter is that the low driving force of the chemical reactions results in two dimensional nanostructures. On contrary, high chemical driving force combined with the optimum zeta potential results in porous aggregate of nanoparticles. Chapter 5 discusses the formation of porous clusters of metals and ceramics at specific conditions. The mechanism behind the formation of monodisperse aggregates are investigated based on the interaction energies of nanoparticles in aqueous medium. This chapter reveals the role of surface charge and the surface energy in controlling the stability of nanoparticles in aqueous medium. In addition, it provides the simple methodology to produce well controlled porous clusters by exploiting the competition between surface charge and surface energy during the aggregation. The application of the porous clusters of Pt has been tested for methanol oxidation which is essential for fuel cell applications. Chapter 6 deals with the development of porous biphasic scaffolds through the morphology transition of nanorods. Rod shape is not stable when subjected to high temperature due to instability and spherodisation takes place to minimize the surface energy. Here in this chapter, by exploiting spherodisation along with the phase transition, highly interconnected porous structure of hydroxyapatite and tricalcium phosphate is achieved. Combined with the morphology transition, by adding naphthalene as a template, the possibility of achieving hierarchical porous structure also presented. The mechanical strength of the biphasic porous scaffold has been tested by microindentation. Mechanical properties of apatite are generally poor and there are lots of efforts to improve the mechanical properties apatite by the composite approach. Chapter 7 deals with the HA-Alumina and HA-TCP composites. In spite of much attention given to the mechanical properties of the composites, the interfacial phenomenon that takes place between the components of the nanocomposite has not been studied in detail. In the present study, interfacial reactions in hydroxyapatite-alumina nanocomposites have been investigated and new reaction mechanism also proposed. The degradation of densification process has been observed for the HATCP composites due to the creation of porous interface between HA crystals and TCP matrix. Mechanical properties of these two composites have been studied using microindentation. The mechanical properties of HA and TCP single crystals are important for developing the biphasic composites with reliable mechanical properties. Chapter8deals with the mechanical behavior of hydroxyapatite and tricalcium phosphate single crystals. The mechanical properties of HA and TCP have been studied by performing nanoand microindentation on specific crystallographic facets. In case of hydroxyapatite, the anisotropy in mechanical properties has been explored by performing indentation on its prism and basal planes. Nanoscale plasticity is observed in both HA and TCP crystals which arise due to the easy movement of surface atoms with lesser coordination compared to the bulk. Nanoindentation has been performed in the calciumdeficient HA platelets provides important clues about the role of calcium deficiency on the mechanical behavior of bone and has implications for the properties of osteoporotic bones.

Das Ziel dieser Arbeit ist die optische Untersuchung von ferroelektrischen Domänen und Domänenwänden auf lokaler Skala. Dafür wurden neuartige nichtinvasive Ansätze entwickelt, die auf der Anwendung optischer Rastersondenmikroskopie basieren. Die untersuchten Schlüsseleigenschaften umfassen den elektrooptischen Effekt für verschiedene Domänenorientierungen und die Brechungindexänderungen an Domänenwänden an Bariumtitanat-Einkristallen. Die lokale Messung der elektrooptischen Eigenschaften wurde mit räumlich stark begrenzten elektrischen Feldern durchgeführt, die mittels elektrisch leitfähigen Spitzen angelegt wurden. Dieser experimentelle Ansatz erlaubt nicht nur die Messung verschiedener elektrooptischer Koeffzienten, sondern auch die Unterscheidung von allen auftretenden, auch antiparallelen, Domänenausrichtungen. Durch Anlegen eines zusätzlichen elektrischen Feldes mittels der gleichen Spitze konnte auch das ferroelektrische Schalten mit dieser optischen Methode untersucht werden. Die Experimente wurden durch eine numerische Modellierung der elektrischen Feldverteilung und der resultierenden elektrooptischen Antwort begleitet. Die Ergebnisse der Modellierung sind dabei in sehr guter Übereinstimmung mit den experimentellen Ergebnissen. Dies erlaubt auch die Trennung von Beiträgen verschiedener elektrooptischer Koeffzienten und den entsprechenden Feldkomponenten. ür die experimentelle Untersuchung von den theoretisch vorhergesagten Brechungsindexprofilen einzelner Domänenwände, wurde die Sensitivität der optischen Sonde auf lokale Änderungen des Brechungsindex mittels Polarisations- und Positionsmodulation erhöht. Obwohl die Abbildung einer einzelnen Domänenwand nicht gelang, konnte damit zumindest eine obere Grenze für den optischen Effekt einer Domänenwand experimentell gewonnen werden, welche verträglich mit den theoretischen Vorhersagen ist. / The goal of this thesis is the optical investigation of ferroelectric domains and domain walls at the very local scale. For that, novel noninvasive approaches based on optical scanning probe microscopy are developed. The key properties investigated are the electrooptic effect for different domain orientations and refractive-index changes at single domain walls of barium titanate single crystals. The local probing of the electro-optic response is performed with strongly confined electric fields, applied via a conductive tip. With this approach we can not only probe different electro-optic coeffcients, but also identify all occurring domain orientations, even antiparallel ones. The application of additional bias fields by the same tip is used to investigate ferroelectric switching and domain growth by optical means. The experiments are supported by numerical modelling of the electric-field distribution and the resulting electro-optic response. The modelling shows excellent agreement with the measurements, and allows us to separate the contributions of different electro-optic coeffcients and their associated electric-field components. For the experimental observation of the theoretically predicted refractive-index profiles at single ferroelectric domain walls, polarization and position modulation of the optical probe is used to obtain high sensitivity to local modifications of the refractive index. An upper limit to the optical effect to the optical effect of a single domain wall is deduced from the experiment, which is compatible with the effect predicted by theory.

In der vorliegenden Arbeit werden Ergebnisse von mechanischen, elektronenmikroskopischen und rasterkraft-mikroskopischen Untersuchungen an mittelorientierten Nickeleinkristallen dargestellt, die bei 77 K bis in den Bereich der Stabilisierung der mechanischen Eigenschaften zyklisch verformt worden sind. Im Zentrum des Interesses steht neben der Analyse der stabilisierten Versetzungsstruktur auf makroskopischer, mesoskopischer und mikroskopischer Maßstabsskala das Studium der Gleitlokalisationsphänomene bei tiefen Temperaturen. Nach Verformung bei 77 K ergibt sich sowohl hinsichtlich des mechanischen Verhaltens und der Oberflächengleiterscheinungen als auch bezüglich der Versetzungsanordnungen auf mesoskopischem Strukturniveau (Strukturlängen in der Größenordnung 1 µm) prinzipiell ein ähnliches Erscheinungsbild wie bei höheren Verformungstemperaturen: Die zyklische Spannungs-Dehnungs-Kurve ist in drei Bereiche gegliedert, sie weist ein Plateaugebiet auf; es treten Grobgleitspuren auf der Probenoberfläche auf, die eine Lokalisation der Gleitung anzeigen; es kommt zur Ausbildung einer mesoskopisch heterogenen Versetzungsstruktur. Die Tendenz, daß mit sinkender Verformungstemperatur die Strukturlängen auf den unterschiedlichen Maßstabsniveaus kleiner werden, setzt sich bei der Wechselplastizierung bei 77 K fort. Die detaillierten quantitativen Untersuchungen zur Gleitlokalisation bei 77 K haben zu neuen Erkenntnissen geführt, deren Konsequenzen nicht nur die zyklische Plastizität bei tiefen Temperaturen betreffen. Die Ergebnisse lassen sich in folgenden Punkten zusammenfassen: Nach Halbzyklusverformung existiert ein weites Spektrum von lokalen Abgleitamplituden, das von 0,5 % bis 30 % reicht; die mittlere plastische Dehnung der Gleitbänder ist intrinsisch; das aktive Gleitvolumen im Halbzyklus beträgt nur 30 % des Volumens, in dem Gleitlokalisation nach einer hinreichend großen Zyklenzahl beobachtet wird; die Abgleitung in den aktiven Gleitbändern liegt um ungefähr einen Faktor drei über der oberen Plateaugrenzdehnung der zyklischen Spannungs-Dehnungs-Kurve. Die Meßergebnisse zum aktiven Gleitvolumen führen zu dem Schluß, daß in den Gleitbändern zeitlich gestaffelt Phasen hoher und niedriger Gleitaktivität auftreten. Die Diskussion der experimentellen Befunde zeigt, daß die Kerngedanken des WINTER-Modells bezüglich der Eigenschaften der Gleitbänder gültig bleiben, wenn man die Aussagen des Modells auf das mittlere räumliche und mittlere zeitliche Verhalten der Gleitbänder bezieht. Anhaltspunkte für die physikalischen Ursachen der Existenz eines Abgleitungsspektrums ergeben sich zum einen aus einer Modifikation des phänomenologischen WINTER-Modells nach ZAISER und zum anderen durch die Annahme von Spektren des Verfestigungskoeffizienten und/oder der effektiven Spannung.