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- ItemAdvanced fiber in-coupling through nanoprinted axially symmetric structures(New York, NY : AIP, 2023) Yermakov, Oleh; Zeisberger, Matthias; Schneidewind, Henrik; Kim, Jisoo; Bogdanov, Andrey; Kivshar, Yuri; Schmidt, Markus A.Here, we introduce and demonstrate nanoprinted all-dielectric nanostructures located on fiber end faces as a novel concept for the efficient coupling of light into optical fibers, especially at multiple incidence angles and across large angular intervals. Taking advantage of the unique properties of the nanoprinting technology, such as flexibly varying the width, height, and gap distance of each individual element, we realize different polymeric axial-symmetric structures, such as double-pitch gratings and aperiodic arrays, placed on the facet of commercial step-index fibers. Of particular note is the aperiodic geometry, enabling an unprecedentedly high average coupling efficiency across the entire angular range up to 80°, outperforming regular gratings and especially bare fibers by orders of magnitude. The excellent agreement between simulation and experiment clearly demonstrates the quality of the fabricated structures and the high accuracy of the nanoprinting process. Our approach enables realizing highly integrated and ready-to-use fiber devices, defining a new class of compact, flexible, and practically relevant all-fiber devices beyond the state-of-art. Applications can be found in a variety of cutting-edge fields that require highly efficient light collection over selected angular intervals, such as endoscopy or quantum technologies. Furthermore, fiber functionalization through nanoprinting represents a promising approach for interfacing highly complex functional photonic structures with optical fibers.
- ItemSolvent-antisolvent interactions in metal halide perovskites(Cambridge : Soc., 2023) Bautista-Quijano, Jose Roberto; Telschow, Oscar; Paulus, Fabian; Vaynzof, YanaThe fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method.
- ItemPerspectives on weak interactions in complex materials at different length scales(Cambridge : RSC Publ., 2022) Fiedler, J.; Berland, K.; Borchert, J.W.; Corkery, R. W.; Eisfeld, A.; Gelbwaser-Klimovsky, D.; Greve, M.M.; Holst, B.; Jacobs, K.; Krüger, M.; Parsons, D. F.; Persson, C.; Presselt, M.; Reisinger, T.; Scheel, S.; Stienkemeier, F.; Tømterud, M.; Walter, M.; Weitz, R.T.; Zalieckas, J.Nanocomposite materials consist of nanometer-sized quantum objects such as atoms, molecules, voids or nanoparticles embedded in a host material. These quantum objects can be exploited as a super-structure, which can be designed to create material properties targeted for specific applications. For electromagnetism, such targeted properties include field enhancements around the bandgap of a semiconductor used for solar cells, directional decay in topological insulators, high kinetic inductance in superconducting circuits, and many more. Despite very different application areas, all of these properties are united by the common aim of exploiting collective interaction effects between quantum objects. The literature on the topic spreads over very many different disciplines and scientific communities. In this review, we present a cross-disciplinary overview of different approaches for the creation, analysis and theoretical description of nanocomposites with applications related to electromagnetic properties.
- ItemCurled cation structures accelerate the dynamics of ionic liquids(Cambridge : RSC Publ., 2021) Rauber, Daniel; Philippi, Frederik; Kuttich, Björn; Becker, Julian; Kraus, Tobias; Hunt, Patricia; Welton, Tom; Hempelmann, Rolf; Kay, Christopher W.M.Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids – slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation–anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure–property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure–property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.
- ItemSuppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films(College Park, MD : APS, 2023) Lou, Rui; Suvorov, Oleksandr; Grafe, Hans-Joachim; Kuibarov, Andrii; Krivenkov, Maxim; Rader, Oliver; Büchner, Bernd; Borisenko, Sergey; Fedorov, AlexanderThe nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity.
- ItemPorous spherical gold nanoparticles via a laser induced process(Cambridge : Royal Society of Chemistry, 2022) Schmidl, Gabriele; Raugust, Marc; Jia, Guobin; Dellith, Andrea; Dellith, Jan; Schmidl, Frank; Plentz, JonathanNanoparticles consisting of a mixture of several metals and also porous nanoparticles due to their special structure exhibit properties that find applications in spectroscopic detection or catalysis. Different approaches of top down or bottom up technologies exist for the fabrication of such particles. We present a novel combined approach for the fabrication of spherical porous gold nanoparticles on low-cost glass substrates under ambient conditions using a UV-laser induced particle preparation process with subsequent wet chemical selective etching. In this preparation route, nanometer-sized branched structures are formed in spherical particles. The laser process, which is applied to a silver/gold bilayer system with different individual layer thicknesses, generates spherical mixed particles in a nanosecond range and influences the properties of the fabricated nanoparticles, such as the size and the mixture and thus the spectral response. The subsequent etching process is performed by selective wet chemical removal of silver from the nanoparticles with diluted nitric acid. The gold to silver ratio was investigated by energy-dispersive X-ray spectroscopy. The porosity depends on laser parameters and film thickness as well as on etching parameters such as time. After etching, the surface area of the remaining Au nanoparticles increases which makes these particles interesting for catalysis and also as carrier particles for substances. Such substances can be positioned at defined locations or be released in appropriate environments. Absorbance spectra are also analyzed to show how the altered fractured shape of the particles changes localized plasmon resonances of the resultingt particles.
- ItemMagnetization-driven Lifshitz transition and charge-spin coupling in the kagome metal YMn6Sn6(London : Springer Nature, 2022) Siegfried, Peter E.; Bhandari, Hari; Jones, David C.; Ghimire, Madhav P.; Dally, Rebecca L.; Poudel, Lekh; Bleuel, Markus; Lynn, Jeffrey W.; Mazin, Igor I.; Ghimire, Nirmal J.The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics.
- ItemChemical Bonded PA66-PTFE-Oil Composites as Novel Tribologically Effective Materials: Part 2(London [u.a.] : Institute of Physics, 2021) Nguyen, Thanh-Duong; Kamga, Lionel Simo; Gedan-Smolka, Michaela; Sauer, Bernd; Emrich, Stefan; Kopnarski, Michael; Voit, Brigitte; Karjust, Krist; Otto, Tauno; Kübarsepp, Jakob; Hussainova, IrinaPolytetrafluoroethylene (PTFE) exhibits excellent non-stick properties and a very low coefficient of friction under tribological stress, but it is incompatible with almost all other polymers. In the first part of this study we presented the generation of the novel tribological material based on unsaturated oil, radiation-modified PTFE (MP1100) and Polyamide 66 (PA66). To get a better understanding of the chemical properties and chemical composition of the compounds, the PA66-MP1100-oil-cb (chemical bonded) compounds were examined by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). In this part, the mechanical properties of the compounds are compared with plain PA66 and PA66-MP1100-cb. The tribological investigation was carried out using the Block-on-Ring tribometer. It was found that the mechanical properties of PA66-MP1100-oil-cb with 20 wt.% MP1100-oil-cb only show slight differences compared to PA66, but the tribological properties of the compounds have been significantly improved through chemical coupling between the three components. Finally, the amount of the compound that was deposited on the surface of the steel disc counterpart was analyzed after the tribological testing.
- ItemEvidence for “dark charge” from photoluminescence measurements in wide InGaN quantum wells(Washington, DC : Optical Society of America, OSA, 2023) Bercha, A.; Trzeciakowski, W.; Muziol, G.; Tomm, J. W.; Suski, T.Wide (15-25 nm) InGaN/GaN quantum wells in LED structures were studied by time-resolved photoluminescence (PL) spectroscopy and compared with narrow (2.6 nm) wells in similar LED structures. Using below-barrier pulsed excitation in the microsecond range, we measured increase and decay of PL pulses. These pulses in wide wells at low-intensity excitation show very slow increase and fast decay. Moreover, the shape of the pulses changes when we vary the separation between them. None of these effects occurs for samples with narrow wells. The unusual properties of wide wells are attributed to the presence of “dark charge” i.e., electrons and holes in the ground states. Their wave functions are spatially separated and due to negligible overlap they do not contribute to emission. However, they screen the built-in field in the well very effectively so that excited states appear with significant overlap and give rise to PL. A simple model of recombination kinetics including “dark charge” explains the observations qualitatively.