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Item type:Publication, Access status: with files , Water in mesoporous materials: investigating capillary flow, elastocapillarity, and phase transitions with molecular dynamics simulations(2026); Water-driven mesoporous materials offer great potential for sustainable applications. This work elucidates the analytical modeling of capillary flows of water in ordered, cylindrical silica mesopores and the resulting imbibition-induced elastocapillarity. The results could support the development of novel sensors, and actuators. In addition, a method for enabling liquid-to-solid phase transitions in molecular dynamics simulations is presented. In the future this method could be used to investigate liquid-to-solid phase transitions in mesoporous materials.Publicationtype: Doctoral Thesis42 55 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electrochemical actuation of porous silicon in aqueous electrolytesPorous silicon is highly versatile functional material. In this thesis two different electrochemistry-inspired approaches are developed and investigated to integrate an actuator functionality into porous silicon. The first approach aims to utilise electrochemo-mechanical coupling at the interface of a porous silicon electrode immersed in an aqueous electrolyte solution. The second approach comprises the integration of the electroactive, electrically conductive polymer polypyrrole into the porous silicon pore structure to create a hybrid material. Both approaches achieve a robust and highly reproducible actuation that follows the applied potential linear and whose work density compares well to classical piezo actuators.Publicationtype: Doctoral Thesis197 2910 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Understanding interfaces in metal-oxide/organic-acid hybrid materials from first-principles calculationsMetal-oxide/organic-acid interfaces play an important role in various applications ranging from hybrid materials to catalysis. The present work studies the structure and energetics of these interfaces on the atomic scale. Employing first-principles calculations and a novel structure search tool, the most stable interface structures are determined, finding a restructuring of magnetite (Fe3O4) surfaces, and identifying shape-influencing factors for titania (TiO2) nanoparticles. Finally, the facet dependence of mechanical interface properties is examined, yielding deeper insights in the origin of failure in hybrid materials.Publicationtype: Doctoral Thesis315 296 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Self-assembly of confined liquid crystals: from nanoscale physics to designing photonic metamaterials(2020); Liquid crystals in combination with nanoporous media, exhibiting structures significantly smaller than the wavelengths of visible light, are promising candidates to design new soft-hard photonic metamaterials. The liquid crystal provides functionality, resulting from temperature tunable anisotropic properties, i.\,e. optical birefringence, while the nanoporous solid provides a mechanical robust scaffold structure. Then the optical functionality of prospective metamaterials is not determined by the properties of base materials, but from newly designed multiscale structures often absent in the base materials. However, the extreme spatial and topological constraints can result in inhomogeneous molecular arrangements or even the complete suppression of liquid crystallinity in nanoconfinement hampering the optical functionality. Depending on the type of confining space (meso- and macroporous silica and anodized aluminium oxide) and as a function of temperature, pore size and pore surface chemistry, the molecular self-assembling process and the resulting metamaterial potential of a discotic liquid crystal (HAT6) in nanoconfinement is analyzed. In order to correlate orientational and translational ordering, synchrotron-based 2D X-ray diffraction in combination with high-resolution optical polarimetry measurements are performed. Resulting from a quantized phase transition a rarely measured material specific parameter giving a measure of the tendency of the liquid crystal to bend along the preferred alignment direction is found. Additionally, the experiments and complementary Monte Carlo simulations show a remarkably rich self-assembly behavior resulting in structures absent in the bulk state. These diverse configurations on the nanoscale result in integrated material systems exhibiting a temperature- and liquid-solid interaction-dependent positive and negative optical birefringence. Hence, the self-organized nanoporous solids in combination with liquid-crystalline self-assembly allow the fabrication of newly designed photonic metamaterials with tailorable optical anisotropy.Publicationtype: Doctoral Thesis461 496 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Surface properties of silver-gold alloys – a quantum mechanics-based approach combining theory and experiment(2020); A prominent tool in computational materials modeling is density functional theory (DFT), which allows one to calculate macroscopic material properties from the atomic scale and based solely upon physical principles. The cluster expansion (CE), a statistical physics-based method, makes it possible to match those properties to atomic arrangements, and thus identify the most favorable structures within the whole configuration space. Local properties at the material’s surface are also influenced by the atomic arrangement within a small number of surface layers. A typical example is surface segregation, where the atomic order at the surface differs from that of the bulk because of a difference in chemical potential. In this thesis, CE fits were performed to predict the most favorable surface configurations and analyze the segregation behavior at flat and stepped Ag-Au surfaces. The Ag-Au system was of particular interest here, since small amounts of silver that remain after the fabrication process may explain the origin of the high catalytic reactivity of the sponge-like nanoporous gold. Interestingly, gold segregation to the topmost layer of the adsorbate-free Ag-Au surfaces was obtained in this work, whereas numerous experimental and theoretical studies from the past report silver surface segregation. It is shown here by means of an analysis of Bader charges and the partial density of states that gold is stabilized in the topmost layer by a charge transfer from silver to the more electronegative gold. In a next step, it is revealed that for oxygen-covered Ag-Au surfaces, silver impurities are drawn to the surface layer. The special case of an infinite oxide chain on the stepped Au(321) surface with Ag impurities is characterized by an analysis of the bonding characters and the electronic surface structure. Furthermore, the electromechanical coupling behavior at the Ag-Au (111) surface is studied. This can be evaluated by calculating the response of the electronic work function to in-plane strain. The resulting coupling parameter was then expanded in a CE fit to examine the influence of the silver surface concentration and the atomic arrangement at the surface. In summary, a strong influence of the surface layer composition on the coupling parameter is found for the Ag-Au alloy surface. Finally, the atomic structure composition of the adsorbate-free Ag-Au (111) surface is characterized experimentally via low-energy electron diffraction (LEED). The LEED structure analysis indicates good agreement with the calculated segregation behavior, namely slight gold enrichment in the surface layer and silver enrichment in the subsurface layer. The obtained silver concentrations in the first layers match those of a ground state obtained in the surface CE. The first-principles results from this thesis combined with the cluster expansion technique help to shed new light on surface phenomena in the Ag-Au alloy. Such data are very difficult to acquire experimentally, as they take into consideration hundreds of thousands of atomic configurations. Here, we verify our calculations by performing a LEED structure analysis, which yields fairly good agreement with the first principles data.Publicationtype: Doctoral Thesis292 1635 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Herstellung und chemo-elektromechanische Charakterisierung von Aktoren aus nanoporösem Gold und Polypyrrol(2020); In this thesis, millimeter-sized actuators made from nanoporous gold and polypyrrole were manufactured and investigated with respect to their chemo-electromechanical coupling. The electrochemically controlled residual stress within the polypyrrole was quantified and related to the macroscopic deformation of the actuators. It was shown that the deformation of the polypyrrole layer between adjacent structural units of the nanoporous substrate has a significant impact on the actuation.Publicationtype: Doctoral Thesis494 700