Zeitschriftenaufsätze

Monolithic integrated photovoltaic-driven electrochemical (PV-EC) artificial photosynthesis is reported for unassisted CO2 reduction. The PV-EC structures employ triple junction photoelectrodes with a front mounted semitransparent catalyst layer as a photocathode. The catalyst layer is comprised of an array of microscale triangular metallic prisms that redirect incoming light toward open areas of the photoelectrode to reduce shadow losses. Full wave electromagnetic simulations of the prism array (PA) structure guide optimization of geometries and length scales. An integrated device is constructed with Ag catalyst prisms covering 35% of the surface area. The experimental device has close to 80% of the transmittance with a catalytic surface area equivalent 144% of the glass substrate area. Experimentally this photocathode demonstrates a direct solar-to-CO conversion efficiency of 5.9% with 50 h stability. Selective electrodeposition of Cu catalysts onto the surface of the Ag triangular prisms allows CO2 conversion to higher value products enabling demonstration of a solar-to-C2+ product efficiency of 3.1%. This design featuring structures that have a semitransparent catalyst layer on a PV-EC cell is a general solution to light loss by shadowing for front surface mounted metal catalysts, and opens a route for the development of artificial photosynthesis based on this scalable design approach.

Owing to high power density and long cycle life, micro-supercapacitors (MSCs) are regarded as a prevalent energy storage unit for miniaturized electronics in modern life. A major bottleneck is achieving enhanced energy density without sacrificing both power density and cycle life. To this end, designing electrodes in a “smart” way has emerged as an effective strategy to achieve a trade-off between the energy and power densities of MSCs. In the past few years, considerable research efforts have been devoted to exploring new electrode materials for high capacitance, but designing clever configurations for electrodes has rarely been investigated from a structural point of view, which is also important for MSCs within a limited footprint area, in particular. This review article categorizes and arranges these “smart” design strategies of electrodes into three design concepts: layer-by-layer, scaffold-assisted and rolling origami. The corresponding strengths and challenges are comprehensively summarized, and the potential solutions to resolve these challenges are pointed out. Finally, the smart design principle of the electrodes of MSCs and key perspectives for future research in this field are outlined.

V-groove nanopatterning of Si substrates has recently demonstrated promise for achieving high-quality III-V-on-Si epitaxy while providing a lower-cost processing route than chemo-mechanical polishing to produce epi-ready planar wafers. A key factor in determining the crystalline quality of III-V buffer layers is the Si surface structure and its chemical composition. Unlike planar Si surfaces, the surfaces of V-grooves prior to growth have not been studied in detail. Here, we study the surface of V-groove Si prepared for GaP nucleation via X-ray photoelectron spectroscopy and low-energy electron diffraction. We identify several pretreatments, using both 830˚C and 1000˚C annealing under an As background pressure, as being suitable for deoxidizing and cleaning the V-groove Si surface. The V-groove Si was found to behave similarly to reference Si(0 0 1) and Si(1 1 1) planar samples, demonstrating that in situ techniques such as reflection anisotropy spectroscopy can be used on reference samples to infer the state of the V-groove surface, and indicating that the extensive research on planar Si surfaces can be directly applied to V-grooves.

A multistage optimization method is developed yielding Tesla valves that are efficient even at low flow rates, characteristic, e.g., for almost all microfluidic systems, where passive valves have intrinsic advantages over active ones. We report on optimized structures that show a diodicity of up to 1.8 already at flow rates of 20 μl s^-1 corresponding to a Reynolds number of 36. Centerpiece of the design is a topological optimization based on the finite element method. It is set-up to yield easy-to-fabricate valve structures with a small footprint that can be directly used in microfluidic systems. Our numerical two-dimensional optimization takes into account the finite height of the channel approximately by means of a so-called shallow-channel approximation. Based on the three-dimensionally extruded optimized designs, various test structures were fabricated using standard, widely available microsystem manufacturing techniques. The manufacturing process is described in detail since it can be used for the production of similar cost-effective microfluidic systems. For the experimentally fabricated chips, the efficiency of the different valve designs, i.e., the diodicity defined as the ratio of the measured pressure drops in backward and forward flow directions, respectively, is measured and compared to theoretical predictions obtained from full 3D calculations of the Tesla valves. Good agreement is found. In addition to the direct measurement of the diodicities, the flow profiles in the fabricated test structures are determined using a two-dimensional microscopic particle image velocimetry (μPIV) method. Again, a reasonable good agreement of the measured flow profiles with simulated predictions is observed.

Low-gain avalanche detectors (LGAD) suffer from an acceptor removal phenomenon due to irradiation. This acceptor removal phenomenon is investigated in boron, gallium, and indium implanted samples by 4-point-probe (4pp) measurements, low-temperature photoluminescence spectroscopy (LTPL), and secondary ion mass spectrometry (SIMS) before and after irradiation with electrons and protons. Different co-implantation species are evaluated with respect to their ability to reduce the acceptor removal phenomenon. In case of boron, the beneficial effect is found to be most pronounced for the low-dose fluorine and high-dose nitrogen co-implantation. In case of gallium, the low-dose implantations of carbon and oxygen are found to be beneficial. For indium, the different co-implantation species have no beneficial effect. SIMS boron concentration depth profiles measured before and after irradiation show no indication of a fast movement of boron at room temperature. Hence, the discussed BSi-Sii-defect explanation approach of the acceptor removal phenomenon seems to be more likely than the other discussed Bi-Oi-defect explanation approach.