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- ItemRevealing all states of dewetting of a thin gold layer on a silicon surface by nanosecond laser conditioning(Amsterdam : Elsevier, 2021) Ernst, Owen C.; Uebel, David; Kayser, Stefan; Lange, Felix; Teubner, Thomas; Boeck, TorstenDewetting is a ubiquitous phenomenon which can be applied to the laser synthesis of nanoparticles. A classical spinodal dewetting process takes place in four successive states, which differ from each other in their morphology. In this study all states are revealed by interaction of pulsed nanosecond UV laser light with thin gold layers with thicknesses between 1 nm and 10 nm on (100) silicon wafers. The specific morphologies of the dewetting states are discussed with particular emphasis on the state boundaries. The main parameter determining which state is formed is not the duration for which the gold remains liquid, but rather the input energy provided by the laser. This shows that each state transition has a separate measurable activation energy. The temperature during the nanosecond pulses and the duration during which the gold remains liquid was determined by simulation using the COMSOL Multiphysics® software package. Using these calculations, an accurate local temperature profile and its development over time was simulated. An analytical study of the morphologies and formed structures was performed using Minkowski measures. With aid of this tool, the laser induced structures were compared with thermally annealed samples, with perfectly ordered structures and with perfectly random structures. The results show that both, structures of the laser induced and the annealed samples, strongly resemble the perfectly ordered structures. This reveals a close relationship between these structures and suggests that the phenomenon under investigation is indeed a spinodal dewetting generated by an internal material wave function. The purposeful generation of these structures and the elucidation of the underlying mechanism of dewetting by short pulse lasers may assist the realisation of various technical elements such as nanowires in science and industry. © 2020
- ItemMultistatic Specular Meteor Radar Network in Peru: System Description and Initial Results(Malden, Mass. : American Geophysical Union, 2021) Chau, J.L.; Urco, J.M.; Vierinen, J.; Harding, B.J.; Clahsen, M.; Pfeffer, N.; Kuyeng, K.M.; Milla, M.A.; Erickson, P.J.The mesosphere and lower thermosphere (MLT) region is dominated globally by dynamics at various scales: planetary waves, tides, gravity waves, and stratified turbulence. The latter two can coexist and be significant at horizontal scales less than 500 km, scales that are difficult to measure. This study presents a recently deployed multistatic specular meteor radar system, SIMONe Peru, which can be used to observe these scales. The radars are positioned at and around the Jicamarca Radio Observatory, which is located at the magnetic equator. Besides presenting preliminary results of typically reported large-scale features, like the dominant diurnal tide at low latitudes, we show results on selected days of spatially and temporally resolved winds obtained with two methods based on: (a) estimation of mean wind and their gradients (gradient method), and (b) an inverse theory with Tikhonov regularization (regularized wind field inversion method). The gradient method allows improved MLT vertical velocities and, for the first time, low-latitude wind field parameters such as horizontal divergence and relative vorticity. The regularized wind field inversion method allows the estimation of spatial structure within the observed area and has the potential to outperform the gradient method, in particular when more detections are available or when fine adaptive tuning of the regularization factor is done. SIMONe Peru adds important information at low latitudes to currently scarce MLT continuous observing capabilities. Results contribute to studies of the MLT dynamics at different scales inherently connected to lower atmospheric forcing and E-region dynamo related ionospheric variability.
- ItemBio-inspired deposition of electrochemically exfoliated graphene layers for electrical resistance heating applications(Bristol : IOP Publishing, 2020-12-4) Utech, Toni; Pötschke, Petra; Simon, Frank; Janke, Andreas; Kettner, Hannes; Paiva, Maria; Zimmerer, CordeliaElectrochemically exfoliated graphene (eeG) layers possess a variety of potential applications, e.g. as susceptor material for contactless induction heating in dynamic electro-magnetic fields, and as flexible and transparent electrode or resistivity heating elements. Spray coating of eeG dispersions was investigated in detail as a simple and fast method to deposit both, thin conducting layers and ring structures on polycarbonate substrates. The spray coating process was examined by systematic variation of dispersion concentration and volume applied to heated substrates. Properties of the obtained layers were characterized by UV-VIS spectroscopy, SEM and Confocal Scanning Microscopy. Electrical conductivity of eeG ring structures was measured using micro-four-point measurements. Modification of eeG with poly(dopamine) and post-thermal treatment yields in the reduction of the oxidized graphene proportion, an increase in electrical conductivity, and mechanical stabilization of the deposited thin layers. The chemical composition of modified eeG layer was analyzed via x-ray photoelectron spectroscopy pointing to the reductive behavior of poly(dopamine). Application oriented experiments demonstrate the direct electric current heating (Joule-Heating) effect of spray-coated eeG layers.
- ItemDesign of high-performance antimony/MXene hybrid electrodes for sodium-ion batteries(London [u.a.] : RSC, 2022) Arnold, Stefanie; Gentile, Antonio; Li, Yunjie; Wang, Qingsong; Marchionna, Stefano; Ruffo, Riccardo; Presser, VolkerDue to their versatile properties and excellent electrical conductivity, MXenes have become attractive materials for alkali metal-ion batteries. However, as the capacity is limited to lower values due to the intercalation mechanism, these materials can hardly keep up in the ever-fast-growing community of battery research. Antimony has a promisingly high theoretical sodiation capacity characterized by an alloying reaction. The main drawback of this type of battery material is related to the high volume changes during cycling, often leading to electrode cracking and pulverization, resulting in poor electrochemical performance. A synergistic effect of combing antimony and MXene can be expected to obtain an optimized electrochemical system to overcome capacity fading of antimony while taking advantage of MXene charge storage ability. In this work, variation of the synthesis parameters and material design strategy have been dedicated to achieving the optimized antimony/MXene hybrid electrodes for high-performance sodium-ion batteries. The optimized performance does not align with the highest amount of antimony, the smallest nanoparticles, or the largest interlayer distance of MXene but with the most homogeneous distribution of antimony and MXene while both components remain electrochemically addressable. As a result, the electrode with 40 mass% MXene, not previously expanded, etched with 5 mass% HF and 60% antimony synthesized on the surfaces of MXene emerged as the best electrode. We obtained a high reversible capacity of 450 mA h g−1 at 0.1 A g−1 with a capacity retention of around 96% after 100 cycles with this hybrid material. Besides the successful cycling stability, this material also exhibits high rate capability with a capacity of 365 mA h g−1 at 4 A g−1. In situ XRD measurements and post mortem analysis were used to investigate the reaction mechanism.
- ItemInvestigating the electrochemical stability of Li7La3Zr2O12 solid electrolytes using field stress experiments(London [u.a.] : RSC, 2021) Smetaczek, Stefan; Pycha, Eva; Ring, Joseph; Siebenhofer, Matthäus; Ganschow, Steffen; Berendts, Stefan; Nenning, Andreas; Kubicek, Markus; Rettenwander, Daniel; Limbeck, Andreas; Fleig, JürgenCubic Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for solid-state batteries with the potential to exceed conventional battery concepts in terms of energy density and safety. The electrochemical stability of LLZO is crucial for its application, however, controversial reports in the literature show that it is still an unsettled matter. Here, we investigate the electrochemical stability of LLZO single crystals by applying electric field stress via macro- and microscopic ionically blocking Au electrodes in ambient air. Induced material changes are subsequently probed using various locally resolved analysis techniques, including microelectrode electrochemical impedance spectroscopy (EIS), laser induced breakdown spectroscopy (LIBS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and microfocus X-ray diffraction (XRD). Our experiments indicate that LLZO decomposes at 4.1–4.3 V vs. Li+/Li, leading to the formation of Li-poor phases like La2Zr2O7 beneath the positively polarized electrode. The reaction is still on-going even after several days of polarization, indicating that no blocking interfacial layer is formed. The decomposition can be observed at elevated as well as room temperature and suggests that LLZO is truly not compatible with high voltage cathode materials.
- ItemPhase equilibria in the Gd–Cr–Ge system at 1070 K(Ivano-Frankivsʹk : Fizyko-chimičnyj instytut DVNZ "Prykarpatsʹkyj nacionalʹnyj universytet imeni Vasylja Stefanyka", 2021) Konyk, M.; Romaka, L.; Stadnyk, Yu.; Romaka, V.V.; Pashkevych, V.The isothermal section of the phase diagram of the Gd–Cr–Ge ternary system was constructed at 1070 K over the whole concentration range using X-ray diffractometry, metallography and electron microprobe (EPM) analysis. Three ternary compounds are realized in the Gd–Cr–Ge system at the temperature of annealing: Gd117Cr52Ge112 (Tb117Fe52Ge112 structure type, space group Fm-3m, Pearson symbol cF1124, a = 2.8971(6) nm), GdCr6Ge6 (SmMn6Sn6 structure type, space group P6/mmm, Pearson symbol hP16, a = 0.51797(2), c = 0.82901(4) nm) and GdCr1-хGe2 (CeNiSi2 structure type, space group Cmcm, Pearson symbol oS16, a = 0.41569(1)-0.41593(8), b = 1.60895(6)-1.60738(3), c = 0.40318(1)-0.40305(8) nm). For the GdCr1-xGe2 compound the homogeneity range was determined (x=0.73 – 0,69).
- ItemPolysulfide driven degradation in lithium–sulfur batteries during cycling – quantitative and high time-resolution operando X-ray absorption study for dissolved polysulfides probed at both electrode sides(London [u.a.] : RSC, 2021) Zech, Claudia; Hönicke, Philipp; Kayser, Yves; Risse, Sebastian; Grätz, Olga; Stamm, Manfred; Beckhoff, BurkhardThe development of operando characterization techniques on realistic batteries is essential for an advanced mechanistic understanding in battery chemistry and, therefore, contributes to their further performance improvement. This manuscript presents operando Near-Edge X-ray Absorption Spectroscopy (NEXAFS) traceable to the SI units (SI is the abbreviation for the International System of Units) during multiple charge–discharge cycles on both electrodes of lithium–sulfur (Li/S) coin cells which enables an absolute quantification of dissolved polysulfides with no need for calibration samples or reference material. We could reveal that during the charging process, polysulfide (PS) movement from the negative to the positive electrode is inhibited. This leads to a steady increase of dissolved polysulfides at the anode side and, therefore, is one of the key points for capacity fading. We quantitatively track the polysulfides dissolved in the electrolyte and correlate for the first time their evolution with the capacity fading of the cell. We analyze the appearance of PS during cell operation at the cathode and anode side to characterize the transport mechanisms of the polysulfide shuttle phenomena and to reveal quantitative information about their evolution at different states of charge and states of health. Our cell design suppresses the contribution of cathodic sulfur, which is mandatory for reference-sample-free quantification in X-ray spectrometry and allows us to use only slightly modified standard coin cell batteries.
- ItemRhodamine 6G and 800 intermolecular heteroaggregates embedded in PMMA for near-infrared wavelength shifting(London [u.a.] : RSC, 2022) Castillo-Seoane, Javier; Gonzalez-Garcia, Lola; Obrero-Perez, José M.; Aparicio, Francisco J.; Borrás, Ana; González-Elipe, Agustín R.; Barranco, Ángel; Sanchez-Valencia, Juan R.The opto-electronic properties of small-molecules and functional dyes usually differ when incorporated into solid matrices with respect to their isolated form due to an aggregation phenomenon that alters their optical and fluorescent properties. These spectroscopic modifications are studied in the framework of the exciton theory of aggregates, which has been extensively applied in the literature for the study of molecular aggregates of the same type of molecules (homoaggregation). Despite the demonstrated potential of the control of the heteroaggregation process (aggregation of different types of molecules), most of the reported works are devoted to intramolecular aggregates, complex molecules formed by several chromophores attached by organic linkers. The intramolecular aggregates are specifically designed to hold a certain molecular structure that, on the basis of the exciton theory, modifies their optical and fluorescent properties with respect to the isolated chromophores that form the molecule. The present article describes in detail the incorporation of Rhodamine 6G (Rh6G) and 800 (Rh800) into polymeric matrices of poly-(methyl methacrylate), PMMA. The simultaneous incorporation of both dyes results in an enhanced fluorescent emission in the near-infrared (NIR), originating from the formation of ground-state Rh6G–Rh800 intermolecular heteroaggregates. The systematic control of the concentration of both rhodamines provides a model system for the elucidation of the heteroaggregate formation. The efficient energy transfer between Rh6G and Rh800 molecules can be used as wavelength shifters to convert effectively the light from visible to NIR, a very convenient wavelength range for many practical applications which make use of inexpensive commercial detectors and systems.
- ItemOn the viscous dissipation caused by randomly rough indenters in smooth sliding motion(Amsterdam : Elsevier, 2021) Sukhomlinov, Sergey; Müser, Martin H.The viscous dissipation between rigid, randomly rough indenters and linearly elastic counter bodies sliding past them is investigated using Green’s function molecular dynamics. The study encompasses a variety of models differing in the height spectra properties of the rigid indenter, in the viscoelasticity of the elastomer, and in their interaction. All systems reveal the expected damping linear in sliding velocity at small and a pronounced maximum at intermediate . Persson’s theory of rubber friction, which is adopted to the studied model systems, reflects all observed trends. However, close quantitative agreement is only found up to intermediate sliding velocities. Relative errors in the friction force become significant once the contact area is substantially reduced by sliding.
- ItemDry etching of monocrystalline silicon using a laser-induced reactive micro plasma(Amsterdam : Elsevier, 2021) Heinke, Robert; Ehrhardt, Martin; Lorenz, Pierre; Zimmer, KlausDry etching is a prevalent technique for pattern transfer and material removal in microelectronics, optics and photonics due to its high precision material removal with low surface and subsurface damage. These processes, including reactive ion etching (RIE) and plasma etching (PE), are performed at vacuum conditions and provide high selectivity and vertical side wall etched patterns but create high costs and efforts in maintenance due to the required machinery. In contrast to electrically generated plasmas, laser-induced micro plasmas are controllable sources of reactive species in gases at atmospheric pressure that can be used for dry etching of materials. In the present study, we have demonstrated the laser-induced plasma etching of monocrystalline silicon. A Ti:Sapphire laser has been used for igniting an optically pumped plasma in a CF4/O2 gas mixture near atmospheric pressure. The influence of process parameters, like substrate temperature, O2 concentration, plasma-surface distance, etching duration, pulse energy and crystal orientation on etching rate and surface morphology has been investigated. Typical etching rates of 2–12 µm x min−1 can be achieved by varying mentioned parameters with a decreasing etching rate during the process. Different morphologies can be observed due to the parameters set, smooth as well as rough surfaces or even inverted pyramids. The presented etching method provides an approach for precise machining of silicon surfaces with good surface qualities near atmospheric pressure and sufficiently high material removal rates for ultraprecise surface machining. © 2021 The Author(s)