Zeitschriftenaufsätze und Buchbeiträge

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Pikushina, Alena; Centeno, Luis Fernando; Stehr, Uwe; Jacobs, Heiko O.; Hein, Matthias
Electrical lengths and phase constants of stretchable coplanar transmission lines at GHz frequencies. - In: Flexible and printed electronics, ISSN 2058-8585, Bd. 9 (2024), 1, 015005, S. 1-12

Elastic, bendable and stretchable electronics establish a new and promising area of multi-physics engineering for a variety of applications, e.g. on wearables or in complex-shaped machine parts. While the area of metamorphic electronics has been investigated comprehensively, the behavior at radio frequencies (RFs), especially in the GHz range, is much less well studied. The mechanical deformation of the soft substrates, for instance, due to stretching, changes the geometrical dimensions and the electrical properties of RF transmission lines. This effect could be desirable in some cases, e.g. for smart devices with shape-dependent transmission or radiation characteristics, or undesirable in other cases, e.g. in feed and distribution networks due to the variable electrical lengths and thus phase variations. This contribution describes the results of a systematic study of the broadband RF properties of coplanar transmission lines on Ecoflex® substrates, based on numerical simulations and experimental data. Two types of stretchable transmission line structures were studied: Meander- and circular ring-segmented lines. Modeling and simulation were performed combining a 2D circuit simulation software with electromagnetic full-wave simulations. The experimental part of the work included the fabrication of metamorphic substrates metallized with thin copper layers and systematic measurements of the electrical lengths and phase constants of coplanar waveguides in the frequency range from 1 to 5 GHz based on vector network analysis for different stretching levels. With the given substrate technology, we succeeded in demonstrating stretchability up to a level of 21%, while the theoretical limit is expected at 57%. The meander- and circular-shaped line structures revealed markedly different sensitivities to the stretching level, which was lower for circular structures compared to the meander structures by approximately a factor of three.



https://doi.org/10.1088/2058-8585/ad1efd
Günther-Müller, Sarah; Azizy, Raschid; Strehle, Steffen
Droplet motion driven by liquid dielectrophoresis in the low-frequency range. - In: Micromachines, ISSN 2072-666X, Bd. 15 (2024), 1, 151, S. 1-16

Electrohydrodynamic wetting manipulation plays a major role in modern microfluidic technologies such as lab-on-a-chip applications and digital microfluidics. Liquid dielectrophoresis (LDEP) is a common driving mechanism, which induces hydrodynamic motion in liquids by the application of nonhomogeneous electrical fields. Among strategies to analyze droplet movement, systematic research on the influence of different frequencies under AC voltage is missing. In this paper, we therefore present a first study covering the motion characteristics of LDEP-driven droplets of the dielectric liquids ethylene glycol and glycerol carbonate in the driving voltage frequency range from 50 Hz to 1600 Hz. A correlation between the switching speed of LDEP-actuated droplets in a planar electrode configuration and the frequency of the applied voltage is shown. Hereby, motion times of different-sized droplets could be reduced by up to a factor of 5.3. A possible excitation of the droplets within their range of eigenfrequencies is investigated using numerical calculations. The featured fluidic device is designed using larger-sized electrodes rather than typical finger or strip electrodes, which are commonly employed in LDEP devices. The influence of the electrode shape is considered simulatively by studying the electric field gradients.



https://doi.org/10.3390/mi15010151
Schaaf, Peter; Constantinescu, Catalin; Matei, Andreea
Laser material processing: from fundamental interactions to innovative applications (E-MRS). - In: Applied surface science advances, ISSN 2666-5239, Bd. 21 (2024), 100592, insges. 1 S.

https://doi.org/10.1016/j.apsadv.2024.100592
Zhang, Yuanpeng; Cheng, Pengfei; Wang, Dong; Wang, Hui; Tang, Yongliang; Wang, Wei; Li, Yuhang; Sun, Zeqi; Lv, Wenmei; Liu, Qingxiang
Evaluating the field emission properties of N-type black silicon cold cathodes based on a three-dimensional model. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 16 (2024), 2, S. 2932-2939

Black silicon (BS), a nanostructured silicon surface containing highly roughened surface morphology, has recently emerged as a promising candidate for field emission (FE) cathodes in novel electron sources due to its huge number of sharp tips with ease of large-scale fabrication and controllable geometrical shapes. However, evaluating the FE performance of BS-based nanostructures with high accuracy is still a challenge due to the increasing complexity in the surface morphology. Here, we demonstrate a 3D modeling methodology to fully characterize highly disordered BS-based field emitters randomly distributed on a roughened nonflat surface. We fabricated BS cathode samples with different morphological features to demonstrate the validity of this method. We utilize parametrized scanning electron microscopy images that provide high-precision morphology details, successfully describing the electric field distribution in field emitters and linking the theoretical analysis with the measured FE property of the complex nanostructures with high precision. The 3D model developed here reveals a relationship between the field emission performance and the density of the cones, successfully reproducing the classical relationship between current density J and electric field E (J-E curve). The proposed modeling approach is expected to offer a powerful tool to accurately describe the field emission properties of large-scale, disordered nano cold cathodes, thus serving as a guide for the design and application of BS as a field electron emission material.



https://doi.org/10.1021/acsami.3c15402
Schaaf, Peter; Zyabkin, Dmitry
Mössbauer spectroscopy. - In: Encyclopedia of condensed matter physics, (2024), S. 15-28

The current chapter provides the reader with a general introduction of Mössbauer effect following by its unique utilization, which became known as Mössbauer spectroscopy. Mössbauer spectroscopy is based on the recoilless emission and following resonant absorption of gamma radiation by atomic nuclei and has been at the scientific forefront of physics, chemistry, biology, mineralogy for more than 60 years. Soon after the discovery of the Mössbauer effect, it became obvious that this effect can be used to study various properties of materials on a microscopic scale via hyperfine interactions with an unprecedented resolution. This was the beginning of a new analytical tool - Mössbauer spectroscopy. Today, it has developed into a standard analytical technique used in many laboratories and big research facilities. The current chapter provides the reader with a general introduction, explains the underlying hyperfine interactions and gives examples of the possible application of the method.



Koch, Juliane; Liborius, Lisa; Kleinschmidt, Peter; Prost, Werner; Weimann, Nils; Hannappel, Thomas
Impact of the tip-to-semiconductor contact in the electrical characterization of nanowires. - In: ACS omega, ISSN 2470-1343, Bd. 9 (2024), 5, S. 5788-5797

Well-defined semiconductor heterostructures are a basic requirement for the development of high-performance optoelectronic devices. In order to achieve the desired properties, a thorough study of the electrical behavior with a suitable spatial resolution is essential. For this, various sophisticated tip-based methods can be employed, such as conductive atomic force microscopy or multitip scanning tunneling microscopy (MT-STM). We demonstrate that in any tip-based measurement method, the tip-to-semiconductor contact is decisive for reliable and precise measurements and in interpreting the properties of the sample. For that, we used our ultrahigh-vacuum-based MT-STM coupled in vacuo to a reactor for the preparation of nanowires (NWs) with metal organic vapor phase epitaxy, and operated our MT-STM as a four-point nanoprober on III-V semiconductor NW heterostructures. We investigated a variety of upright, free-standing NWs with axial as well as coaxial heterostructures on the growth substrates. Our investigation reveals charging currents at the interface between the measuring tip and the semiconductor via native insulating oxide layers, which act as a metal-insulator-semiconductor capacitor with charging and discharging conditions in the operating voltage range. We analyze in detail the observed I-V characteristics and propose a strategy to achieve an optimized tip-to-semiconductor junction, which includes the influence of the native oxide layer on the overall electrical measurements. Our advanced experimental procedure enables a direct relation between the tip-to-NW junction and the electronic properties of as-grown (co)axial NWs providing precise guidance for all future tip-based investigations.



https://doi.org/10.1021/acsomega.3c08729
Liu, Fengli; Yan, Yong; Chen, Ge; Wang, Dong
Recent advances in ambient electrochemical methane conversion to oxygenates using metal oxide electrocatalysts. - In: Green chemistry, ISSN 1463-9270, Bd. 26 (2024), 2, S. 655-677

To reach a decarbonized future, the conversion of greenhouse gases into green fuels and valuable chemicals is of crucial importance. Methane emissions are the second most significant contributor to global warming. Recent advances in electrocatalytic partial oxidation of methane to high-value fuels at ambient temperatures promise to sidestep the requirement of high temperature in conventional thermal catalysis and provide a revolutionary, sustainable, and decentralized alternative to flaring. Electrocatalysts that can selectively produce valuable compounds from methane under mild conditions are essential for commercialization. This review covers current developments in the electrochemical partial oxidation of methane to oxygenates, with an emphasis on metal oxide electrocatalysts. The regularly deployed strategies, including doping and interface engineering, are systematically reviewed in detail. In addition, the design of the electrolytic cell, the electrolyte, time, potential, and temperature are examined thoroughly and discussed.



https://doi.org/10.1039/D3GC03513A
Li, Feitao; Tan, Xinu; Flock, Dominik; Oliva Ramírez, Manuel; Wang, Dong; Qiu, Risheng; Schaaf, Peter
Structure-dependent oxidation behavior of Au-Cu nanoparticles. - In: Journal of alloys and compounds, ISSN 1873-4669, Bd. 976 (2024), 173179, S. 1-8

Thermal oxidation is an easily controlled method to change the physical and chemical properties of nanoparticles, thus optimizing and expanding their applications. Unfortunately, less attention has been paid to the role of the crystal structure whose atomic arrangements can be critical for oxidation. Au-Cu nanoparticles showing a fast order-disorder transformation are oxidized at two temperatures of ordered (L10) and disordered (A1) phase regions. The oxidation rates between the two phases are compared by the Arrhenius equation, and a lower oxidation rate is determined in the L10 lattice than in the A1 lattice based on the time required for the complete oxidation. One possible reason is attributed to the longer diffusion length in the L10 lattice compared to the A1 lattice due to the anisotropic diffusion path of the former while isotropic diffusion of the latter, resulting in longer oxidation time and then slower oxidation for the ordered sample. The crystalline phase of Au-Cu nanoparticles can be straightforwardly tuned and the resulting atomic disposition is a powerful tool to control oxidation evolution.



https://doi.org/10.1016/j.jallcom.2023.173179
Grad, Marius; Honig, Hauke; Diemar, Andreas; Flock, Dominik; Spieß, Lothar
Complex material analysis of a TiC coating produced by hot pressing with optical light microscopy, EDS, XRD, GDOES and EBSD. - In: Surface and coatings technology, ISSN 1879-3347, Bd. 476 (2024), 130265, S. 1-11

The present study investigates the interface between carbon steel and titanium samples annealed at different temperatures (ϑ1 = Image 1 and ϑ2 = Image 2). In both cases, an observable layer forms at the interface, with its thickness increasing from tϑ1 = 2.75 ± Image 3 at Image 1 to tϑ2 = 8.86 ± Image 4 at Image 2. The layer's composition and thickness evolve with temperature. Analysis reveals approximately 40 at.-% carbon concentration in the exterior region, indicating likely titanium carbide creation. X-ray diffraction identifies titanium carbide peaks, while microscopy and elemental mapping confirm compositional gradients at the interface. Electron Backscatter Diffraction (EBSD) shows a gradient in grain size near the TiC surface, reflecting TiC nucleation rates. XRD data detect both titanium carbide and titanium phases, with TiC becoming more prominent at Image 2. Rietveld analysis further confirms TiC formation. Notably, distinct diffraction patterns on the contact and rear sides suggest a Ti(C, O, N) presence. Depth profiles exhibit varying surface and depth carbon concentrations, attributed to temperature effects. The study successfully demonstrates TiC coating fabrication through hot pressing, wherein Ti(C, O, N) coatings arise from titanium's affinity for reacting with oxygen and nitrogen. This research contributes to the understanding of phase transformations and interfacial properties in titanium-carbon steel systems.



https://doi.org/10.1016/j.surfcoat.2023.130265
Li, Zhiyong; Chen, Guangshen; Cheng, Pengfei; Zhang, Zhang; Liu, Junming
Phototactic photocatalysis enabled by functionalizing active microorganisms with photocatalyst. - In: Advanced sustainable systems, ISSN 2366-7486, Bd. 8 (2024), 2, 2300302, S. 1-10

Positive phototropism enables plants to take advantage of sunlight more efficiently. However, positive phototropism of plant-like photocatalyst has not been reported yet, which cause people's limited understanding on it. Therefore, developing new photocatalysts that can move toward the light source and thus speed up the photocatalytic process, is a great challenge. Herein, a biologically active photocatalyst (graphitic carbon nitride combined with algae microorganisms, g-C3N4/alga) is reported first that can behave like green plants and move toward light source, leading to a great enhancement in photocatalysis. The photocatalytic degradation efficiency of the phototactic g-C3N4/alga is improved up to 570% than that of pure g-C3N4. The phototactic g-C3N4/alga photocatalyst can effectively utilize the synergy of phototaxis of microalgae and photocatalytic activity of g-C3N4 to promote the pollutant decomposition using sunlight. Imparting photocatalyst with positive phototropism will open a new door in photocatalysis field for clean energy production, pollutant treatment, and biomass conversion.



https://doi.org/10.1002/adsu.202300302