Journal articles and book contributions

Understanding the connection of molecular structure and optical properties of rare earth doped luminescent materials is essential for fabrication of state-of-the-art active laser media. On the other hand, rare earth ions can be used as a probe ion for the molecular structure of the host material if the structure-property correlations are known. Therefore, this work combines molecular dynamics simulations, Judd-Ofelt theory and UV-vis-NIR absorption spectroscopy including the behavior of the structure-sensitive hypersensitive absorption transitions of Er3+ to expand the knowledge on the local molecular structure in the immediate vicinity of the doped rare earth ions in dependence of glass composition. For this purpose, glasses of the compositions (35-x) BaO &hahog; x MgO &hahog; 10 Al2O3 &hahog; 55 SiO2 (mol%) (x = 0, 7.5, 15, 25, 35) and (20-x) BaO &hahog; x MgO &hahog; 20 Al2O3 &hahog; 60 SiO2 (mol%) (x = 0, 10, 20), doped with 2 × 10^20 ions/cm^3 Er3+ were prepared and analyzed. Clear differences in the absorption spectra between glasses of different BaO/MgO ratios, i.e. different network modifier field strengths, and different network modifier oxide to Al2O3 ratios are found and discussed in detail. Glasses with high BaO concentrations and high network modifier oxide to Al2O3 ratios provide lower rare earth coordination numbers with oxygen in general but higher coordination probabilities with non-bridging oxygen, which results in notably increased splitting of the optical transitions of the doped rare earth ions and higher hypersensitivity / lower local site symmetry for the doped rare earth ions in the investigated compositions. Based on our results and results from other publications the local rare earth site symmetry in glasses can in general be correlated with the rare earth coordination number.

Miniaturized on-chip micropumps with no moving parts are intriguing components for advanced lab-on-chip systems. Magnetohydrodynamic pumping is one possibility but requires further research with respect to microsystems design and fabrication. In this paper, the design and fabrication of a magnetohydrodynamic micropump is discussed using a composite of patterned glass and stacked dry film photoresist as demonstrator platform. The magnetohydrodynamic pumping effect is achieved by the superposition of an electric ion current generated by integrated electrodes and an external magnetic field provided by a permanent magnet. As test electrolytes, potassium chloride with potassium hexacyanoferrate (III) and potassium hexacyane iron (II) were used. Seamless fluid channel sidewalls were achieved from stacked dry film resists, which appear to be cast from a single mold. A liquid-tight sealing of the microchannels was realized by covering them with a thermally bonded laser-structured glass lid. Although, a complete characterization of the pump performance was not yet realized, the micropump in its current state serves as a technology demonstrator for further research of microfluidic on-chip micropumps that utilize the magnetohydrodynamic effect and also for other microfluidic systems.

Reactive multilayer systems are nanostructures of great interest for various technological applications because of their high energy release rate during the self-propagating reaction of their components. Therefore, many efforts are aimed at controlling the propagation velocity of these reactions. Herein, reactive multilayer systems of Al/Ni in the shape of free-standing foils with a wavelike surface morphology prepared by using sacrificial substrates with well-aligned waves are presented and the propagation of the reaction along different directions of the reproduced waves is analyzed. During the ignition test, the propagation front is recorded with a high-speed camera, and the maximum temperature is measured using a pyrometer. The propagation of the reaction is favored in the direction of the waves, which points out the influence of the anisotropy generated by this morphology and how it affects the propagation dynamics and the resulting microstructure. Furthermore, compared to their counterparts fabricated on flat substrates, these reactive multilayers with wavelike morphology exhibit a remarkable reduction in the propagation velocity of the reaction of about 50%, without significantly affecting the maximum temperature registered during the reaction.

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO2 ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh3) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh3 as catalysts for CO2 ECR. Unlike the layered CoPS3 and NiPS3 nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS3 nanosheets. Correspondingly, the nonlayered SnPS3 nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with -7.51 mA cm^-2 at -0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS3 surface. These results provide a new avenue to understand the novel CO2 ECR mechanism of Sn-based and MPCh3-based catalysts.

Interfaces are key elements that define electronic properties of the final device. Inevitably, most of the active interfaces of III-V semiconductor devices are buried and it is therefore not straightforward to characterize them. The Tapered Cross Section Photoelectron Spectroscopy (TCS-PES) approach is promising to address such a challenge. That the TCS-PES can be used to study the relevant heterojunction in epitaxial III-V architectures prepared by metalorganic chemical vapor deposition is demonstrated here. A MULTIPREP polishing system that enables controlling the angle between the sample holder and the polishing plate has been employed to improve the reproducibility of the polishing procedure. With this procedure, that preparing the TCS of III-V semiconductor devices with tapering angles lower than 0.02˚ is possible is demonstrated. The PES provides then information about the buried interfaces of Ge|GaInP and GaAs|GaInP layer stacks. Both, chemical and electronic properties have been measured by PES. It evidences that the preparation of the TCSs under an uncontrolled atmosphere modifies the pristine properties of the critical buried heterointerfaces. Surface states and reaction layers are created on the TCS surface, which restrict unambiguous conclusions on buried interface energetics.