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Hybridized Guided-Mode Resonances via Colloidal Plasmonic Self-Assembled Grating
For many photonic applications, it is important to confine light of a specific wavelength at a certain volume of interest at low losses. So far, it is only possible to use the polarized light perpendicular to the solid grid lines to excite waveguide-plasmon polaritons in a waveguide-supported hybrid structure. In our work, we use a plasmonic grating fabricated by colloidal self-assembly and an ultrathin injection layer to guide the resonant modes selectively. We use gold nanoparticles self-assembled in a linear template on a titanium dioxide (TiO 2 ) layer to study the dispersion relation with conventional ultraviolet-visible-near-infrared spectroscopic methods. Supported with finite-difference in time-domain simulations, we identify the optical band gaps as hybridized modes: plasmonic and photonic resonances. Compared to metallic grids, the observation range of hybridized guided modes can now be extended to modes along the nanoparticle chain lines. With future applications in energy conversion and optical filters employing these cost-efficient and upscalable directed self-assembly methods, we discuss also the application in refractive index sensing of the particle-based hybridized guided modes. Copyright © 2019 American Chemical Society.
Self‐Patterning of Multifunctional Heusler Membranes by Dewetting
Ni-Mn-based Heusler alloys are an emerging class of materials which enable actuation by (magnetic) shape memory effects, magnetocaloric cooling, and thermomagnetic energy harvesting. Multifunctional materials have a particular advantage for miniaturization since their functionality is already built within the material. However, often complex microtechnological processing is required to bring these materials into shape. Here, self-organized formation of single crystalline membranes having arrays of rectangular holes with high aspect ratio is demonstrated. Dewetting avoids the need for complicated processing and allows to prepare freestanding Ni–Mn–Ga–Co membranes. These membranes are martensitic and magnetic, and their functional properties are not disturbed by self-patterning. Feature sizes of these membranes can be tailored by film thickness and heat treatment, and the tendencies can be explained with dewetting. As an outlook, the advantages of these multifunctional membranes for magnetocaloric and thermomagnetic microsystems are sketched. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
Silk Hydrogel Substrate Stress Relaxation Primes Mesenchymal Stem Cell Behavior in 2D
Tissue-mimetic silk hydrogels are being explored for diverse healthcare applications, including stem cell delivery. However, the impact of stress relaxation of silk hydrogels on human mesenchymal stem cell (MSC) biology is poorly defined. The aim of this study was to fabricate silk hydrogels with tuned mechanical properties that allowed the regulation of MSC biology in two dimensions. The silk content and stiffness of both elastic and viscoelastic silk hydrogels were kept constant to permit direct comparisons. Gene expression of IL-1β, IL-6, LIF, BMP-6, BMP-7, and protein tyrosine phosphatase receptor type C were substantially higher in MSCs cultured on elastic hydrogels than those on viscoelastic hydrogels, whereas this pattern was reversed for insulin, HNF-1A, and SOX-2. Protein expression was also mechanosensitive and the elastic cultures showed strong activation of IL-1β signaling in response to hydrogel mechanics. An elastic substrate also induced higher consumption of glucose and aspartate, coupled with a higher secretion of lactate, than was observed in MSCs grown on viscoelastic substrate. However, both silk hydrogels changed the magnitude of consumption of glucose, pyruvate, glutamine, and aspartate, and also metabolite secretion, resulting in an overall lower metabolic activity than that found in control cells. Together, these findings describe how stress relaxation impacts the overall biology of MSCs cultured on silk hydrogels. ©
Transparent Power-Generating Windows Based on Solar-Thermal-Electric Conversion
Integrating transparent solar-harvesting systems into windows can provide renewable on-site energy supply without altering building aesthetics or imposing further design constraints. Transparent photovoltaics have shown great potential, but the increased transparency comes at the expense of reduced power-conversion efficiency. Here, a new technology that overcomes this limitation by combining solar-thermal-electric conversion with a material's wavelength-selective absorption is presented. A wavelength-selective film consisting of Cs0.33WO3 and resin facilitates high visible-light transmittance (up to 88%) and outstanding ultraviolet and infrared absorbance, thereby converting absorbed light into heat without sacrificing transparency. A prototype that couples the film with thermoelectric power generation produces an extraordinary output voltage of ≈4 V within an area of 0.01 m2 exposed to sunshine. Further optimization design and experimental verification demonstrate high conversion efficiency comparable to state-of-the-art transparent photovoltaics, enriching the library of on-site energy-saving and transparent power generation.
Flexible Materials for High-Resolution 3D Printing of Microfluidic Devices with Integrated Droplet Size Regulation
We develop resins for high-resolution additive manufacturing of flexible micromaterials via projection microstereolithography (PμSL) screening formulations made from monomer 2-phenoxyethyl acrylate, the cross-linkers Ebecryl 8413, tri(propyleneglycol) diacrylate or 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, the photoabsorber Sudan 1, and the photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. PμSL-printed polymer micromaterials made from this resin library are characterized regarding achievable layer thickness depending on UV exposure energy, and for mechanical as well as optical properties. The best-candidate resin from this screening approach allows for 3D-printing transparent microchannels with a minimum cross section of approximately 35 × 46 μm2, which exhibit proper solvent resistance against water, isopropanol, ethanol, n-hexane, and HFE-7500. The mechanical properties are predestined for 3D-printing microfluidic devices with integrated functional units that require high material flexibility. Exemplarily, we design flexible microchannels for on-demand regulation of microdroplet sizes in microemulsion formation. Our two outlines of integrated droplet regulators operate by injecting defined volumes of air, which deform the droplet-forming microchannel cross-junction, and change the droplet size therein. With this study, we expand the library of functional resins for PμSL printing toward flexible materials with micrometer resolution and provide the basis for further exploration of these materials, e.g., as microstructured cell-culturing substrates with defined mechanics. © 2021 American Chemical Society. All rights reserved.
Increasing Antibacterial Efficiency of Cu Surfaces by targeted Surface Functionalization via Ultrashort Pulsed Direct Laser Interference Patterning
Copper (Cu) exhibits great potential for application in the design of antimicrobial contact surfaces aiming to reduce pathogenic contamination in public areas as well as clinically critical environments. However, current application perspectives rely purely on the toxic effect of emitted Cu ions, without considering influences on the interaction of pathogenic microorganisms with the surface to enhance antimicrobial efficiency. In this study, it is investigated on how antibacterial properties of Cu surfaces against Escherichia coli can be increased by tailored functionalization of the substrate surface by means of ultrashort pulsed direct laser interference patterning (USP-DLIP). Surface patterns in the scale range of single bacteria cells are fabricated to purposefully increase bacteria/surface contact area, while parallel modification of the surface chemistry allows to involve the aspect of surface wettability into bacterial attachment and the resulting antibacterial effectivity. The results exhibit a delicate interplay between bacterial adhesion and the expression of antibacterial properties, where a reduction of bacterial cell viability of up to 15-fold can be achieved for E. coli on USP-DLIP surfaces in comparison to smooth Cu surfaces. Thereby, it can be shown how the antimicrobial properties of copper surfaces can be additionally enhanced by targeted surface functionalization. © 2020 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
Nanometer-thick lateral polyelectrolyte micropatterns induce macrosopic electro-osmotic chaotic fluid instabilities
Electro-convective vortices in ion concentration polarization under shear flow have been of practical relevance for desalination processes using electrodialysis. The phenomenon has been scientifically disregarded for decades, but is recently embraced by a growing fluid dynamics community due its complex superposition of multi-scale gradients in electrochemical potential and space charge interacting with emerging complex fluid momentum gradients. While the visualization, quantification and fundamental understanding of the often-chaotic fluid dynamics is evolving rapidly due to sophisticated simulations and experimentation, little is known whether these instabilities can be induced and affected by chemical topological heterogeneity in surface properties. In this letter, we report that polyelectrolyte layers applied as micropatterns on ion exchange membranes induce and facilitate the electro-osmotic fluid instabilities. The findings stimulate a variety of fundamental questions comparable to the complexity of today's turbulence research.
Robust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet
The ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time- and angle-resolved photoemission spectroscopy and time-resolved resonant magnetic soft X-ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two-step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence-dependent increased Néel temperature. The results indicate that effects beyond a mean-field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.
Layered Nano‐Mosaic of Niobium Disulfide Heterostructures by Direct Sulfidation of Niobium Carbide MXenes for Hydrogen Evolution
MXene-transition metal dichalcogenide (TMD) heterostructures are synthesized through a one-step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre-treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar-like structures on the parent MXene, Nb4C3 produces nano-mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano-mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3-derived hybrids show better performance than Nb2C derivatives. The Nb4C3-derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte.
Microscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite
The microstructural changes caused by the addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL-containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross-linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re-established electrical connections after stress release but reduces the absolute conductivity.