Universität Stuttgart
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Item Open Access Growth and characterization of transition metal oxide heterostructures with a tailored work function(2018) Rastegar Alam, Iman; Mannhart, Jochen (Prof. Dr.)Item Open Access Spectroscopic investigations of the magnetic anisotropy of lanthanide- and cobalt-based molecular nanomagnets(2016) Rechkemmer, Yvonne; Slageren, Joris van (Prof. Dr.)Single-molecule magnets are metal complexes exhibiting an energy barrier for spin reversal, leading to magnetic bistability and slow relaxation of the magnetization. Their potential for practical applications such as high-density magnetic data storage was recognized early on and with the goal of achieving high energy barriers, different kinds of single-molecule magnets have been synthesized. The quadratic dependence of the barrier height on the spin motivated chemists to synthesize metal complexes with very high total spins; however, with limited success. It was shown that high spins come along with low anisotropies and increased interest thus focused on the synthesis and investigation of (mononuclear) complexes of highly anisotropic metal centers, e.g. lanthanide or cobalt complexes. Although rather high energy barriers can be achieved in such systems, practical application remains problematic and has not been realized yet. Reasons are for example the lack of rational design criteria and the complex interplay of different magnetic relaxation pathways. The aim of this work was therefore the comprehensive magnetic and spectroscopic investigation of selected molecular lanthanide and cobalt compounds in order to obtain a deeper insight into the correlation of molecular and electronic structures as well as the corresponding magnetic properties. The applied spectroscopic methods included electron paramagnetic resonance spectroscopy, far-infrared spectroscopy and optical methods. Special emphasis was placed on magnetic circular dichroism (MCD) spectroscopy, which served as a main tool for electronic structure determination. However, since the MCD-spectrometer was not part of the available experimental equipment at the University of Stuttgart, its design, setup and characterization were the first part of this work. In the further course of this work MCD-spectroscopy was employed for the electronic structure determination of selected lanthanide and cobalt compounds. The studied lanthanide compounds were literature-known molecular tetra-carbonates of erbium (1-Er) and dysprosium (1-Dy). Detailed magnetometric studies showed that both 1-Er and 1-Dy are field-induced single-molecule magnets; however, 1-Er and 1-Dy show significant differences in their magnetic relaxation behavior. The magnetic studies were complemented by detailed spectroscopic investigations.The combination of far-infrared-, luminescence- and MCD-spectroscopy allowed for the experimental determination of 48 energy levels for 1-Er and 55 levels for 1-Dy, which built the foundation for the subsequent crystal field analysis and electronic structure determination. In addition, the results of EPR-spectroscopic studies were used for fine-tuning and verifying the respectively determined crystal field parameters. Calculating the magnetic dipole strengths for transitions between the relevant states led to a quantitative understanding of the magnetic relaxation pathways. Besides the investigation of lanthanide compounds, this thesis deals with two classes of cobalt complexes. The first class comprises mononuclear complexes in which one Co(II) ion is ligated by the nitrogen donors of two doubly deprotonated 1,2-bis(methanesulfonamido)-benzene-ligands. Rather acute N-Co-N bite angles indicate strong deviations from ideal tetrahedral symmetry. The static magnetic properties hint at very high energy barriers for spin reversal and with the help of far-infrared spectroscopy, largely negative axial zero-field splitting parameters were determined. The corresponding energy barriers belong to the highest ever reported for 3d-transition metal complexes and investigating the dynamic magnetic properties confirmed single-molecule magnet behavior. The unique magnetic properties were fully explained by analyzing spectroscopic results. The MCD-spectra showed intense signals that were assigned to spin-allowed d-d-transitions. Subsequent crystal field analysis revealed that the strong axial crystal field generated by the ligands leads to a large splitting of the electronic terms and thus in turn to a relatively small energy gap between the electronic ground state and the first excited state. The resulting increase in second-order spin-orbit coupling explains the high energy barriers observed in the studied complexes. The second class of cobalt compounds studied in this work included dimers of distorted octahedrally coordinated Co(II) ions bridged by symmetrical or asymmetrical quinone based bridging ligands. The main focus of investigation lay on the impact of the bridging ligand on the magnetic coupling between the cobalt centers. Thus, the magnetic properties of the complexes were studied with the help of static susceptibility and magnetization measurements and analyzed by means of different models. Depending on the bridging ligand, different signs for the exchange coupling constants were found. The varying signs can be explained by different relative contributions of possible exchange paths, influenced by the different substituents at the bridging ligands or slight geometry differences. The observations indicate that electron withdrawing substituents favor ferromagnetic couplings, which are preferred in the context of molecular magnetism. All in all, it can be concluded that this work provides a contribution to the deeper understanding of the features relevant for single-molecule magnets. The electronic structure determination for selected lanthanide and cobalt complexes applying advanced magnetometric and spectroscopic techniques not only led to an understanding of the static and dynamic magnetic properties but also allowed for the development of design criteria and new approaches for improved single-molecule magnets in the future.Item Open Access Modeling and simulation of closed low-pressure adsorbers for thermal energy storage(2019) Schäfer, Micha; Thess, André (Prof. Dr. rer. nat.)Closed low-pressure adsorption systems can be applied for thermal energy storage. Their performance is determined by the mass and heat transport processes in the adsorber. Therefore, thorough knowledge of these transport processes is required for further storage development. The present thesis contributes to this by providing detailed models of closed low-pressure adsorbers and by conducting simulations over a broad range of parameters and configurations. The focus is on adsorbers of larger scale (length L = 0.1 . . . 1 m) and on the discharging process. As the adsorption pair, binderless zeolite 13X with water is examined. The models are developed in a stepwise manner from pore to storage scale. The Finite-Difference-Method is implemented to numerically solve the models. Simulations are conducted for defined reference cases as well as over a broad range of geometric and process parameters. The reference cases are analyzed in detail to gain a better understanding of the transport processes. Furthermore, the results are analyzed with respect to two particular modeling aspects: equilibrium assumptions and rarefaction effects (e. g. slip effect). With respect to the application, the discharging performance is analyzed in terms of thermal power and a defined discharging degree. Both the adsorber and the adsorbent configurations are varied. In addition, the effect of the discharging conditions is evaluated. Finally, one exemplary charging process is examined. The detailed analysis of the reference cases reveals that the mass and heat transport and the adsorption processes are strongly coupled and can only be understood in their interaction. For onedimensional adsorber configurations, that is the mass and heat transport are in the same direction, the discharging process is generally limited by the heat transport. This leads to insufficient thermal power and unsuitable discharging durations of up to one year. In contrast, for two-dimensional adsorber configurations, that is the mass and heat transport are in perpendicular directions, the discharging process can be limited either by the mass or heat transport or by the adsorption. The limitation depends on the configuration of the adsorber and adsorbent. Moreover, the twodimensional adsorber configurations can provide sufficient thermal power. With respect to the modeling, it is found that the assumption of a uniform pressure distribution is applicable for one-dimensional adsorber configurations. In contrast, for two-dimensional configurations, no equilibrium assumptions can be applied in general. However, for powder adsorbent it is always valid to assume local adsorption equilibrium. Regarding the rarefaction effects in twodimensional adsorber configurations with honeycombs and granules, the slip effect is relevant for small channel and particle diameters (d = 1 mm). For adsorbers with powder adsorbent, the reduction of the effective heat conductivity due to the rarefaction effect becomes relevant. With respect to the application, the variation of the adsorber configuration shows that the volumetric thermal power generally decreases with increasing adsorber length. Furthermore, the power decreases with increasing width between the parallel heat exchanger plates in the adsorber. Regarding the adsorbent configuration in two-dimensional adsorber configurations, it is found that the volumetric thermal power can be optimized by variation of the channel or particle diameter. Interestingly, the optima for peak and mean power do not coincide. In addition, the discharging degree is found to strongly depend on the discharging conditions in terms of discharging temperature and volume flow of the heat transfer fluid extracting the heat from the adsorber. In general, the discharging degree decreases with increasing discharging temperature. Similarly, the discharging degree decreases with increasing volume flow of the heat transfer fluid. Finally, the analysis of an exemplary charging process revealed that the pressure in the adsorber can increase significantly (> 50%) due to the desorption.Item Open Access Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)Item Open Access Präzise Fahrzeugpositionierung durch Entzerrung der gepulsten magnetischen Flussdichteverteilung einer Ladespule(2017) Martinovic, Dean; Reuss, Hans-Christian (Prof. Dr.-Ing.)Elektrofahrzeuge werden in Zukunft nicht mehr per Kabel, sondern mittels induktiver Ladesysteme mit Strom versorgt. Um eine hohe Ladeleistung sicher übertragen zu können, müssen die Spulen hinreichend genau übereinander positioniert werden, was für den Fahrer eine kaum lösbare Aufgabe darstellt. Das allgemeine Ziel der vorliegenden Arbeit ist es daher, eine neue Methode zu untersuchen, die ein gepulstes Magnetfeld der Ladespule zu dessen Ortung nutzt. Hierbei wird das magnetische Pulssignal durch den ferromagnetischen Unterboden des Elektrofahrzeugs verzerrt. Dieser verändert die Pulsamplitude entsprechend einer unbekannten Abbildung, ohne deren Kenntnis eine präzise und eindeutige Positionierung nicht möglich ist. Die Herausforderung der vorliegenden Arbeit ist daher die Bestimmung dieser Abbildung samt ihrer Eigenschaften und Abhängigkeiten. Theoretische Untersuchungen zeigen, dass die Abbildung allgemein vom nicht-deterministischen magnetischen Zustand des Unterbodenmaterials abhängt und dessen messtechnische Erfassung kaum möglich ist. Im weiteren Verlauf der Untersuchungen wird jedoch hergeleitet, dass die Ladespule, das Elektrofahrzeug und die umgebende Atmosphäre zusammen einen magnetischen Kreis bilden, der aufgrund der sehr hohen Reluktanz der Atmosphäre linear ist. Änderungen des magnetischen Zustands haben folglich keinen Einfluss auf die Abbildung. Diese ist somit reproduzierbar und kann messtechnisch einfach erfasst werden. Die These wird für unterschiedliche magnetische Zustände experimentell nachgewiesen. Basierend auf den Forschungsergebnissen wird ein vollständiger Prototyp entwickelt und in ein Versuchsfahrzeug integriert. Das Gesamtsystem wird anschließend erfolgreich getestet. Die gefundenen Ergebnisse zeigen, dass mittels gepulster magnetischer Felder eine universelle, kostengünstige, sichere und präzise Positionierung von Elektrofahrzeugen möglich ist. Dies unterstreicht das Potential des neuen, komfortablen Positionierungsverfahrens eine Schlüsseltechnologie für die Elektromobilität zu werden.Item Open Access Chiral metamaterials(2016) Eslami, Sahand; Fischer, Peer (Prof. Dr.)Item Open Access Mid-infrared resonant nanostructures for in-vitro monitoring of polypeptides(2019) Semenyshyn, Rostyslav; Giessen, Harald (Prof. Dr.)Infrared vibrational spectroscopy is a technique based on the molecular vibrations, that is, the oscillation of individual atoms with respect to each other. Each of these vibrations has a characteristic resonance frequency which leads to the distinct vibrational fingerprint of a molecule and thus enables a label-free, non-destructive, and chemically specific detection of molecular species. The infrared absorption cross-sections, which characterize the optical interaction strength, are relatively small. This is of minor importance for conventional spectroscopy, where large ensembles of molecules can be measured and thus contribute to the overall signal. However, the small infrared absorption hampers detection of molecules at low concentrations, which is of large importance for medical diagnostics, for instance, where the determination of the secondary structures of proteins is crucial due to their role in many incurable diseases. A key to overcome this limitation is to utilize plasmonic nanostructures, which confine the electromagnetic radiation on the nanometer scale and allow higher overall absorption. In this dissertation, it is demonstrated that even a monolayer of proteins can be detected using mid-infrared resonant gold nanostructures. We use polypeptides as a model system and were able to investigate the secondary structure of molecular monolayers in-vitro. Applying different external stimuli, we are able to induce structural changes of polypeptides in aqueous environments. In addition to a mid-infrared resonant nanoantenna, nanoslits (or inverse antennas) can also enhance the optical response of polypeptides, which allowed us to detect the secondary structure of minicollagen monolayers. Both nanostructure designs provided the possibility even to monitor reversible conformational transitions of molecular monolayers. Scaling this approach down to a single nanostructure allows to detect only a few thousands of polypeptides in liquid environments. The demonstrated concept could lead to integrated chip-level technology for biological and even medical applications, where biosamples with minute concentrations are investigated. With further advances, it could be possible to scale the process to a few or single proteins and observe the structural changes of individual entities.Item Open Access Ion beam lithographic and multilayer fresnel zone plates for soft and hard X-rays: nanofabrication and characterization(2015) Keskinbora, Kahraman; Schütz, Gisela (Prof. Dr.)X-ray microscopy has become an important analytical characterization method for a plethora of applications in materials science, physics, chemistry and biology, thanks to the emergence of modern synchrotron radiation facilities. These facilities enable high brilliance, energy tunable, variable polarization X-rays which gives access to mass density, elemental, chemical, electronic and magnetic properties of materials. In the soft X-ray energies nearly all elements can be probed by spectromicroscopic methods. Another important property of synchrotron radiation is the time structure in the ns to ps range, which can be utilized for sophisticated time resolution studies. These opportunities can be combined with high spatial resolution which is determined by the focusing method and the optic. Focusing of X-rays has historically been a difficult task due to strong absorption and weak phase shift of X-rays within matter. The required phase shift of X-rays, which depends on the real part of the complex refractive index, differs from 1 (the vacuum refractive index) only on the order of 10^-2 to 10^-6 and conventional lenses do not work. One very successful X-ray optic is the Fresnel Zone Plate (FZP), a diffractive optic that act as a lens under certain conditions and can focus X-rays to nanometer sized spots. The resolution of the FZP depends on the width of the outermost zone and is highly correlated with the smallest feature that can be fabricated. Conventionally, the e-beam lithography (EBL) is used for production FZPs which could resolve up to 10 nm structures with serious limitations. One difficulty of EBL is its ever increasing complexity for many-step fabrication of smaller features or intricate geometries. Therefore, EBL is mostly constrained to planar, binary geometries with moderate efficiencies strongly decreasing with energy and not effective for hard X-rays. Special 3D geometries in the form of kinoform lenses can theoretically have 100 % focusing efficiencies. Attempts to approximate these geometries via EBL increased the number of process steps even further. The smallest FZP feature size even for low aspect ratios achievable via EBL is fundamentally limited due to the proximity effect which is the interaction and spread of electrons within the resist material. We addressed these issues by focusing our research on alternative FZP fabrication techniques as high-speed ion beam lithography (IBL), and gray scale ion lithography to realize efficient kinoforms. Another approach towards full-material multilayer FZPs with infinite aspect ratio was based on atomic layer deposition (ALD) with subsequent ion beam slicing. Each of these three methods targets specific challenges faced by the e-beam lithography based FZP fabrication techniques. All the fabricated FZPs were tested for their resolution and efficiency performances at a state of the art scanning transmission X-ray microscope at BESSY for soft X-rays and/or at optical test stations at ESRF and PETRA III for hard X-rays. Using IBL the rapid preparation of a 110 nm thick Au FZP with 50 µm diameter and 50 nm ∆r in less than 13 minutes is demonstrated. Employed for X-ray microscopy, the FZP clearly resolved 28.5 nm features with a cut-off of 24.3 nm at ~1120 eV. Additional process improvements were made towards smaller zones with higher zone quality. They allowed the preparation of a FZP with 30 nm outermost half-period remarkably, in about 8 min. This FZP was shown to clearly resolve 21 nm features on a multilayer test object with large room for improvement. This high through-put FZP production route is of special interest not only concerning the low cost and easy availability. A large array of these optical components is attractive, for experiments such as one-shot ultra-high brilliance FEL investigations due to the radiation damage or for instance for coded-aperture arrays for high-angle resolving X-ray astronomy. Towards fabrication of kinoforms for high efficiency X-ray focusing, we have performed various materials optimization studies in order to achieve a high surface quality optic. After various trials the materials were finally optimized and the fabricated lenses achieved more than 14 % absolute diffraction efficiency that is almost 90 % compared to the theoretical prediction. This confirms how closely we were able to replicate the ideal three dimensional surface relief structure for the first time. It was possible to carry out imaging with these lenses with half-pitch resolutions down to 60 nm. The kinoform lenses were tested at the soft X-ray range where a significant absorption is present in materials. These results also potentially pave the way for very high efficiency hard X-ray focusing which can in principle be utilized in laboratory based X-ray sources, X-ray astronomy and the new rising field of X-ray ptychography. To fabricate high resolution ML-FZPs, Al2O3/Ta2O5multilayers, deposited on a smooth glass optical fiber via atomic layer deposition using non-dedicated instruments were carefully cut-out, sliced and polished to a high quality surface finish using focused ion beams. Following the transfer of the slice to a TEM grid as holder the slices were polished to a high surface finish quality, also via a focused ion beam. Fabricated ML-FZPs were synchrotron tested using an in-house constructed 2-axis tilt stage specially designed for aligning ML-FZP with respect to the X-ray optical axis. The results showed that it was possible to resolve 21 nm features in direct imaging at 1200 eV and sub-30 nm focusing at 8 keV. This is the highest demonstrated resolving power for a multilayer type FZP, to date to the best of our knowledge. Results exhibit the potential for high-resolution hard X-ray focusing where this type of optics are especially efficient. For ultra-high resolution hard and soft X-ray imaging, with potentially achievable ∆r of a few nm is well below what can be achieved through any lithography method available today.Item Open Access Diagnostik und Modellierung eines Mikrowellen-Plasmabrenners bei Atmosphärendruck(2017) Gaiser, Sandra; Hirth, Thomas (Prof. Dr.)Mikrowellen-Plasmaprozesse bei Atmosphärendruck bieten eine Vielzahl von Anwendungsmöglichkeiten. Dazu gehören das Plasmaspritzen zur Beschichtung, die Behandlung von Oberflächen für die Reinigung oder Aktivierung sowie der Abbau schädlicher Abgase. Für die Entwicklung und Optimierung dieser Verfahren sind sowohl experimentelle Untersuchungen als auch eine theoretische Betrachtung von Bedeutung. Diese Arbeit beschäftigt sich deshalb neben der Diagnostik vor allem mit der Modellierung und numerischen Simulation eines bei Atmosphärendruck betriebenen Mikrowellen-Plasmabrenners. Dazu wird die Simulationssoftware Comsol Multiphysics verwendet. Das Ziel ist es, mittels einzelner Modelle die unterschiedlichen physikalischen Vorgänge zu beschreiben und das Brennersystem zu optimieren. Die Simulationen werden schließlich schrittweise miteinander verknüpft, um so ein möglichst selbstkonsistentes Modell der Plasmaquelle zu erhalten. Die Simulationsergebnisse werden zudem mit experimentellen Daten verglichen. Zunächst werden die Verteilung des Mikrowellenfeldes im Plasmabrenner sowie die Resonanzfrequenzen der Resonatoranordnung berechnet, was die Grundlage für eine zuverlässige Zündung und den Betrieb des Plasmas bildet. Anschließend wird ein Modell der kalten Gasströmung erstellt. In dieses wird schließlich eine Wärmequelle implementiert, um den Einfluss des heißen Plasmas auf die Strömung zu untersuchen. Die Gasströmung soll dahingehend optimiert werden, dass sie das Plasma einschließt, um so eine Beschädigung des Gas führenden Quarzrohres zu vermeiden. In einer weiteren Simulation wird das Plasma mit Hilfe des Drude-Modells beschrieben. Hierbei werden dem Plasma eine Permittivität und eine Leitfähigkeit zugewiesen. Eine Erweiterung erfolgt durch das Fluid-Modell, das Bilanzgleichungen für die Elektronendichte sowie Reaktionsmechanismen für ein Argon-Plasma enthält. Die Simulationsergebnisse werden durch den Vergleich mit experimentellen Ergebnissen verifiziert. Dazu wird zum einen die räumliche Lage des Plasmas mit Hilfe von Kameraaufnahmen qualitativ untersucht. Zum anderen stehen Messwerte aus der optischen Emissionsspektroskopie zur Verfügung.Item Open Access Rydberg polaritons and Rydberg superatoms - novel tools for quantum nonlinear optics(2017) Tresp, Christoph; Hofferberth, Sebastian (Dr.)
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