In this work the structural transitions of the rare earth oxides praseodymia and ceria grown on Si(111) are investigated. It is demonstrated that several of the rare earth intermediate phases can be stabilizied by post deposition annealing in ultra high vacuum. However, in most cases no single phased but coexisting species are observed. In addition, the surface structure and morphology of hex-Pr2O3(0001) as well as reduced ceria films are investigated.

The details of the interplay between the electronic structure and the magnetic properties of matter represent a state of the art challenge. In the present work spectroscopic investigations on the electronic structure of some interesting materials are presented. The achieved information has been used in order to answer specific questions related to the magnetic behavior of the investigated materials. For the transition metal dicyanamide compounds it is shown that the electronic states arising from carbon and nitrogen remain roughly unchanged for all compositions. A model for the magnetic superexchange interaction was proposed. In this model the geometry of the crystallographic structure accounts for a particular interaction pattern while the occupancy of the 3d transition metal band is the factor which triggers the changeover from antiferromagnetic to ferromagnetic interaction. In the case of six-membered ferric-wheel molecules the comparison between experimental and theoretical data issued estimations for the magnitude of magnetic exchange interactions. The information on the electronic structure of the LaNi5-xMex (Me=Cu, Al) allowed a better understanding of their magnetic behavior. The changes induced in the electronic structure of the parent compound by partial substitutions of nickel by copper or aluminum are discussed.

Cette thèse s’intéresse à l’analyse de cellules silicium cristallin à l’échelle nanométrique, à l’aide de techniques de microscopie à sonde locale (SPM). En particulier, nous avons choisi d’analyser les propriétés électriques à l’échelle locale, grâce à deux techniques SPM : la microcopie à sonde de Kelvin (KPFM) et la microscopie à force atomique à sonde conductrice (CP-AFM).Tout d’abord, nous présentons les forces et faiblesses de ces deux techniques, comparées à la microscopie électronique, qui permet également d’analyser les propriétés électrique à l’échelle nanométrique. Cette comparaison approfondie nous permet d’identifier des mesures où le KPFM et le CP-AFM sont particulièrement adéquat et peuvent apporter de la valeur. Ces mesures sont divisées en deux catégories : les analyses matériaux et les analyses dispositifs.Ensuite, nous nous focalisons sur les analyses matériaux à l’échelle nanométrique. Nous présentons d’abord des mesures de dopage à l’échelle nanométrique, à l’aide d’une technique avancée de CP-AFM, appelée Resiscope. Nous montrons que cette technique peut détecter des changements de dopage dans la gamme 1015 à 1020 atomes.cm-3, avec une résolution nanométrique et un bon ratio signal/bruit. Puis, nous présentons des mesures de durée de décroissance sur des wafers silicium cristallin passivés. Les mesures sont réalisées sur la tranche non-passivée des échantillons. Nous montrons que, même si la tranche n’est pas passivée, les durées de décroissance obtenue par KPFM ont une bonne corrélation avec les temps de vie des wafers mesurées par décroissance de la photoconductivité détectée par micro-ondes.Par la suite, nous nous concentrons sur les analyses dispositif. A l’aide du KPFM, nous analysons deux types de cellules solaires silicium cristallin : les cellules solaires silicium épitaxié (epi-Si) et les cellules solaires hétérojonctions à contact arrière (IBC). En particulier, nous nous focalisons sur l’analyse de dispositifs en condition d’opération. Nous étudions d’abord l’influence de la tension électrique appliquée et nous montrons que les effets de résistance et de diode peuvent être détectés à l’échelle nanométrique. Les mesures de KPFM sont comparées aux mesures de microscopie électronique à balayage (SEM) dans les mêmes conditions, puisque le SEM est aussi sensible au potentiel de surface. Nous montrons que les mesures KPFM sur la tranche de cellules solaires epi-Si peuvent permettre d’étudier les changements de champ électrique avec la tension électrique appliquée. De plus, si la tension électrique est modulée en fréquence, nous montrons que des mesures de temps de vie peuvent être effectuées à l’échelle locale sur la tranche de cellules solaires epi-Si, ce qui peut permettre de détecter les interfaces limitantes. Puis, nous étudions l’influence de l’illumination sur les mesures KPFM et CP-AFM. Nous effectuons des mesures sur la tranche de cellules epi-Si sous différentes valeurs d’intensité et longueurs d’onde d’illumination. Nous montrons une bonne sensibilité des mesures KPFM à l’illumination. Cependant, nous montrons que pour différentes longueurs d’onde, à tension de circuit ouvert fixé, nos mesures ne sont pas corrélées avec les mesures de rendement quantique interne, comme nous le pensions.Enfin, nous résumons notre travail dans un tableau qui représente les forces et faiblesses des techniques pour les différentes mesures d’intérêt exposées précédemment. A partir de ce tableau, nous imaginons un setup de microscopie « idéal » qui permette d’analyser les cellules solaires de manière fiable, versatile et précise. Pour finir, nous proposons des mesures d’intérêt qui pourraient être réalisées avec ce setup « idéal ». / This thesis focuses on the investigation of crystalline silicon solar cells at the nano-scale using scanning probe microscopy (SPM) techniques. In particular, we chose to investigate electrical properties at the nano-scale using two SPM techniques: Kelvin Probe Force Microscopy (KPFM) and Conducting Probe Atomic Force Microscopy (CP-AFM).First, we highlight the strengths and weaknesses of both these techniques compared to electron microscopy techniques, which can also help investigate electrical properties at the nano-scale. This comprehensive comparison enables to identify measurements where KPFM and CP-AFM are particularly adequate. These measurements are divided in two categories: material investigation and devices investigation.Then, we focus on materials investigation at the nano-scale using SPM techniques. We first present doping measurements at the nano-scale using an advanced CP-AFM technique called Resiscope. We prove that this technique could detect doping changes in the range 1015 and 1020 atoms.cm-3 with a nano-scale resolution and a high signal/noise ratio. Then, we highlight decay time measurements on passivated crystalline silicon wafers using KPFM. Measurements are performed on the unpassivated cross-section. We show that, even though the cross-section is not passivated, decay times measurements obtained with KPFM are in good agreement with lifetimes measured by microwave photoconductivity decay.Subsequently, we focus on device measurements. Using KPFM, we investigate two different crystalline silicon solar cell architectures: epitaxial silicon (epi-Si) solar cells and interdigitated back contact (IBC) heterojunction solar cells. In particular, we focus on measurements on devices under operating conditions. We first study the influence of the applied electrical bias. We study the sensitivity of surface potential to electrical bias and we show that diode and resistance effects can be detected at the nano-scale. KPFM measurements are compared to scanning electron microscopy (SEM) measurements in the same conditions since SEM is also sensitive to surface potential. We show that KPFM measurements on the cross-section of epi-Si solar cells can help detect electric field changes with electrical bias. Besides, if the electrical bias is frequency modulated, we show that lifetime measurements can be performed on the cross-section of epi-Si solar cells and can help detect limiting interfaces and layers. Then, we study the influence of illumination on KPFM and CP-AFM measurements. We perform photovoltage and photocurrent measurements on the cross-section of epi-Si solar under different values of illumination intensity and illumination wavelength. We show a good sensitivity of KPFM measurements to illumination. However, we show that measurements for different wavelengths at a given open circuit voltage, are not correlated with the internal quantum efficiency, as we could have expected.Finally, we summarize our work in a table showing the impact of strengths and weaknesses of the techniques for the different measurements highlighted. From this table, we imagine an “ideal” microscopy setup to investigate crystalline silicon solar cells in a reliable, versatile and accurate way. We propose investigations of interest that could be carried out using this “ideal” setup.

In a crystalline ordered magnet, coherent excitations called spin waves, or magnons, propagate in the material forming band structures in an analogous way to electrons. Spin waves can possess non trivial topology associated with novel response functions of fundamental and potential technological interest. In particular, topologically protected surface states of magnons offer a new path towards coherent spin transport for spintronics applications. One of the central issues in this area is to establish the conditions under which band topology can arise in magnons and explore its variety. In this work we harness the full power of symmetry as applied to magnetism, to facilitate the discovery of new topological magnon models and materials. We show how to efficiently identify such systems by adapting the electronic topological quantum chemistry scheme to magnons, using constraints imposed by time reversal and crystalline symmetries to determine possible gapped and nodal topology in magnon models. Further, we explore enhanced symmetries beyond this paradigm, which are nevertheless natural for magnons: the spin-space groups. Exploring spin-space symmetry, which has wholly or partially decoupled magnetic and lattice degrees of freedom, reveals a proliferation of nodal points, lines, and planes beyond the standard crystalline symmetries. Linear spin wave theory is one of the most valuable techniques to study magnons, however, it can fail in different scenarios. Because of its importance to the community, we explore cases where it contains spurious symmetries which can potentially hide important physics in the spectra, like topology. We provide therefore a simple way to identify and resolve such cases within the linear theory. Finally, a pressing issue in magnons is the experimental detection and manipulation of topological surface states. Even more, the characterisation of generic 2D magnetism is an open problem. We contribute to this by devising an experimental setup based on quasi-particle interference which potentially could solve this long-lasting challenge. / Kohärente Anregungen, wie Spinwellen, auch Magnonen genannt, formen Bandstrukturen in kristallin geordneten Materialien. Diese Magnonen können eine nicht triviale Topologie aufwei- sen, welche neuartige Antwortfunktionen erzeugen können. Sie sind daher von technologischem Interesse. Insbesondere die topologisch geschützten Oberflächenzustände der Magnonen ermöglichen eine Realisierung von kohärentem Spin Transport und erlauben eine potentielle Anwendung in der Spintronik. Zentraler Punkt der aktuellen Forschung sind Bedingungen, unter denen eine nicht triviale Magnon-Bandtopologie entstehen kann und welche Möglichkeiten diese eröffnen. In dieser Arbeit untersuchen wir diese neuartigen topologischen Phasen für verschiedene Mo- delle unter Nutzung verschiedener Symmetrien. Die Erweiterung des elektronischen “topological quantum chemistry” Ansatzes für Magnonen erlaubt eine effiziente Identifikation dieser topologischen Eigenschaften. Der Ansatz basiert auf verschiedenen Einschränkungen, welche von der Zeitumkehr und kristallinen Symmetrien induziert werden. Darüber hinaus untersuchen wir die Anwendung von weiteren Symmetrien, welche relevant für Magnonen sind: die Spin-Raumgruppen. Die Erforschung der Spin-Raum-Symmetrie, welche magnetische Freiheitsgrade und Gittersyme- trien ganz oder teilweise entkoppelt, führt zur Ausbreitung von Knotenpunkten, Linien und Ebenen jenseits der standardmäßigen Kristallsymmetrien. Die lineare Spinwellentheorie ist eine der erfolgreichsten Methoden zur Untersuchung von Magnonen, kann jedoch unter verschiedenen Umständen versagen, da künstliche Symmetrien essenzielle Physik, wie beispielsweise topologische Eigenschaften, verbergen. Ansätze, die im Rahmen dieser Dissertation erarbeitet worden sind, helfen dabei, solche Fälle zu identifizieren und zu verstehen. Aktuelle Experimente zur Manipulation topologischer Oberflächenzustände von Magnonen, sowie die allgemeine Untersuchung von Magnetismus in zwei Dimensionen, fehlen. Daher präsentieren wir einen möglichen experimentellen Aufbau, basierend auf Quasi-Teilchen-Interferenz, welcher einen möglichen Ausweg aufzeigt.

Many transition metal oxides exhibit a Metal-Insulator Transition (MIT). This is an interesting phenomenon where the material undergoes a structural phase transition, and the electronic charge is redistributed in the crystal lattice. The crystallographic, optical, electrical, and magnetic properties are drastically changed when a material undergoes a MIT. Vanadium oxides are thermochromic materials, which means that the optical properties change with temperature, closely connected to the MIT. Vanadium sesquioxide (V2O3) experiences a MIT at the transition temperature (Tt) 160-170 K. Below the transition temperature, it is a monoclinic ferromagnetic insulator, and above the transition temperature it becomes a paramagnetic rhombohedral corundum-type metal. In the insulating phase, infrared (IR) light is transmitted, whereas in the metallic phase, IR light is reflected. The aim of this Master thesis is to improve the methodology to produce V2O3 thin films, characterize them, and study how different parameters affect the structural and optical properties of the film. V2O3 thin films were synthesized by sol-gel and deposited by spin-coating on sapphire substrates. Thereafter, the resulting films underwent an annealing process in a reducing atmosphere to achieve crystalline V2O3. The obtained crystal phase was examined by X-ray diffraction. The surface structure and morphology were studied with an optical microscope and scanning electron microscope. The optical transmittance in the IR region was examined by Fourier transform infrared spectroscopy. At last, a laser test was performed on one of the samples. In conclusion, a majority of the thin films consisted of polycrystalline V2O3. The MIT was studied with the optical hysteresis loop, which showed great difference among the different samples studied. The thin film that exhibited best optical properties showed a maximum transmission of 66 % below the Tt, and a minimum transmission of 6 % above the Tt. In comparison to this one, the other samples exhibited lower transmission and a decrease in Tt. This difference was attributed to the different morphologies and crystal orientations of the samples.

Due to the study basic structural features of ultrafine ceramics, using the scanning electron microscopy method, were identified. Dependency of particle sizes on thermal treatment modes was determined, and average size of the sample grains was determined due to statistical processing of the SEM images, using hardware and software image analysis package SIAMS 700. It has been established that photoluminescent properties of single-crystalline samples under the study correspond to the publicly available literature data. Photoluminescence spectra were measured after sample finishing and dependence of optical absorption on exposure dose was identified. The ultrafine ceramic FL was examined and comparison study of the resulting parameters with a single-crystalline sample was carried out. Excitation and emission spectra under various modes of ceramic synthesis were also produced. This study makes it possible to consider ultrafine ceramics as a non-conventional material for high-dose measurements. / В результате исследования были выявлены основные структурные особенности ультрадисперсной керамики методом растровой электронной микроскопии. Определена зависимость размеров частиц от режимов термической обработки, а так же найден средний размер зерен образцов за счет проведения статистической обработки изображений РЭМ на программно-аппаратном комплексе анализа изображений SIAMS 700. Установлено, что фотолюминесцентные свойства исследуемых монокристаллических образцов соответствуют общеизвестным литературными данными. Измерены спектры фотолюминесценции после полировки образцов и найдена зависимость оптического поглощения от дозы облучения. Рассмотрена ФЛ ультрадисперсной керамики и проведен сравнительный анализ полученных параметров с монокристаллическим образцом. Так же получены спектры возбуждения и излучения при различных режимах синтеза керамики. Данные исследования позволяют рассматривать ультрадисперсную керамику как перспективный материал для высокодозных измерений.

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No description available.

Spontaneous Raman spectroscopy (SR) is a versatile method for analysis and visualization of ferroelectric crystal structures, including domain walls. Nevertheless, the necessary acquisition time makes SR impractical for in situ analysis and large scale imaging. In this work, we introduce broadband coherent anti-Stokes Raman spectroscopy (B-CARS) as a high-speed alternative to conventional Raman techniques and demonstrate its benefits for ferroelectric domain wall analysis. Using the example of poled lithium niobate, we compare the spectral output of both techniques in terms of domain wall signatures and imaging capabilities. We extract the Raman-like resonant part of the coherent anti-Stokes signal via a Kramers–Kronigbased phase retrieval algorithm and compare the raw and phase-retrieved signals to SR characteristics. Finally, we propose a mechanism for the observed domain wall signal strength that resembles a Cerenkov-like behavior, in close analogy to domain wall signatures obtained by secondharmonic generation imaging.We, thus, lay here the foundations for future investigations on other poled ferroelectric crystals using B-CARS.

Two-dimensional (2D) membranes are emerging candidates for osmotic energy conversion. However, the trade-off between ion selectivity and conductivity remains the key bottleneck. Here we demonstrate a fully crystalline imine-based 2D polymer (2DPI) membrane capable of combining excellent ionic conductivity and high selectivity for osmotic energy conversion. The 2DPI can preferentially transport cations with Na+ selectivity coefficient of 0.98 (Na+/Cl− selectivity ratio ~84) and K+ selectivity coefficient of 0.93 (K+/Cl− ratio ~29). Moreover, the nanometer-scale thickness (~70 nm) generates a substantially high ionic flux, contributing to a record power density of up to ~53 W m−2, which is superior to most of nanoporous 2D membranes (0.8~35 W m−2). Density functional theory unveils that the oxygen and imine nitrogen can both function as the active sites depending on the ionization state of hydroxyl groups, and the enhanced interaction of Na+ versus K+ with 2DPI plays a significant role in directing the ion selectivity.