Publications at the Department of Mathematics and Natural Sciences from 2019

Results: 907
Created on: Sun, 26 May 2024 16:55:10 +0200 in 0.0641 sec

Rothe, Karl; Néel, Nicolas; Kröger, Jörg
Unveiling the nature of atomic defects in graphene on a metal surface. - In: Beilstein journal of nanotechnology, ISSN 2190-4286, Bd. 15 (2024), S. 416-425

Low-energy argon ion bombardment of graphene on Ir(111) induces atomic-scale defects at the surface. Using a scanning tunneling microscope, the two smallest defects appear as a depression without discernible interior structure suggesting the presence of vacancy sites in the graphene lattice. With an atomic force microscope, however, only one kind can be identified as a vacancy defect with four missing carbon atoms, while the other kind reveals an intact graphene sheet. Spatially resolved spectroscopy of the differential conductance and the measurement of total-force variations as a function of the lateral and vertical probe–defect distance corroborate the different character of the defects. The tendency of the vacancy defect to form a chemical bond with the microscope probe is reflected by the strongest attraction at the vacancy center as well as by hysteresis effects in force traces recorded for tip approach to and retraction from the Pauli repulsion range of vertical distances.
Eingartner, Anna; Naumann, Steffi; Schmitz, Philipp; Worthmann, Karl
Adjustable robust energy operation planning under uncertain renewable energy production. - In: Energies, ISSN 1996-1073, Bd. 17 (2024), 8, 1917, S. 1-14

In this paper, the application of the method of affinely adjustable robust optimization to a planning model of an energy system under uncertain parameters is presented, and the total scheduling costs in comparison with the deterministic model are evaluated. First, the basics of optimization under uncertain data are recapped, and it is described how these methods can be used in different applications for energy systems. This is followed by the methodology of adjustable robust optimization by defining the affinely adjustable robust counterpart. Finally, a numerical case study is conducted to compare the adjustable robust method with a rolling deterministic scheduling method. Both are implemented on a model of an energy system and compared with each other by simulation using real-world data. By calculating the total operating costs for both methods, it can be concluded that the adjustable robust optimization provides a significantly more cost-effective solution to the scheduling problem.
Bohm, Sebastian; Phi, Hai Binh; Dittrich, Lars; Runge, Erich
Chip-integrated non-mechanical microfluidic pump driven by electrowetting on dielectrics. - In: Lab on a chip, ISSN 1473-0189, Bd. 0 (2024), 0, insges. 13 S.

A microfluidic pump is presented that generates its pumping action via the EWOD (electrowetting-on-dielectric) effect. The flow is generated by the periodic movement of liquid-vapor interfaces in a large number (≈10^6) of microcavities resulting in a volume change of approx. 0.5 pl per cavity per pump stroke. The total flow resulting from all microcavities adds up to a few hundred nanolitres per cycle. Passive, topologically optimized, non-mechanical Tesla valves are used to rectify the flow. As a result, the micropump operates without any moving components. The dimensioning, fabrication, and characterization process of the micropump are described. Device fabrication is done using conventional manufacturing processes from microsystems technology, enabling cost-effective mass production on wafer-level without additional assembly steps like piezo chip-level bonding, etc. This allows for direct integration into wafer-based microfluidic or lab-on-a-chip applications. Furthermore, first measurement results obtained with prototypes of the micropump are presented. The voltage- and frequency-dependent pump performance is determined. The measurements show that a continuous flow rate larger than 0.2 ml min^−1 can be achieved at a maximum pump pressure larger than 12 mbar.
Harrison, T. G.; Böhme, Thomas; Kubek, Mario; Unger, Herwig
GraphLearner: an approach to sequence recognition and generation. - In: IEEE Xplore digital library, ISSN 2473-2001, (2024), S. 445-451

This paper presents GraphLearner a neuromorphic sequence generator with similarities to Markov Chain Models. GraphLearner is proposed as an alternative to ‘black box’ deep neural network models which lack explainability and adaptability. Bloom Filters are used to simplify otherwise computationally expensive Markov chain probability calculations. It is demonstrated with Natural Language Processing tasks, generating sentences of remarkable quality.
Noßmann, Bastian; Yu, Zuodong; Das, Ankur; Schulte, Stefan; Néel, Nicolas; Wu, Chien-Te; Kirchner, Stefan; Kröger, Jörg
Yu-Shiba-Rusinov states induced by single Fe atoms on reconstructed compound superconductor V3Si. - In: Surface science, ISSN 1879-2758, Bd. 746 (2024), 122504, S. 1-10

Reconstructed surfaces of the A15-compound superconductor V3Si(100) that are possibly induced by the segregation of bulk impurities serve as platforms to study the dependence of Yu-Shiba-Rusinov states induced by a single Fe atom on the adsorption site. Their number, energy and electron-hole asymmetry vary strongly with the atomic environment of the Fe atom. These variations are indicative of different Fe d-orbitals being active in the site-dependent exchange coupling with the substrate Cooper pairs. Spatially resolved spectroscopy gives rise to a short decay length of the Yu-Shiba-Rusinov states and thereby suggests the three-dimensional character of the scattering process underlying the bound states.
Christensen, Ole; Hasannasab, Marzieh; Philipp, Friedrich; Stoeva, Diana
The mystery of Carleson frames. - In: Applied and computational harmonic analysis, ISSN 1096-603X, Bd. 72 (2024), 101659, S. 1-5

In 2016 Aldroubi et al. constructed the first class of frames having the form {Tkφ}k=0∞ for a bounded linear operator on the underlying Hilbert space. In this paper we show that a subclass of these frames has a number of additional remarkable features that have not been identified for any other frames in the literature. Most importantly, the subfamily obtained by selecting each Nth element from the frame is itself a frame, regardless of the choice of N∈N. Furthermore, the frame property is kept upon removal of an arbitrarily finite number of elements.
Phani Kumar, Bandaru V. N.; Lobo, Nitin P.; Mattea, Carlos; Stapf, Siegfried
Exploring crystal-phase molecular dynamics of the low-viscous mesogen 6CHBT: a combined FFC and high-field NMR relaxometry investigation. - In: The journal of physical chemistry, ISSN 1520-5207, Bd. 128 (2024), 16, S. 3997-4007

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.
Saupe, Mario; Wiedemeier, Stefan; Gastrock, Gunter; Römer, Robert; Lemke, Karen
Flexible toolbox of high-precision microfluidic modules for versatile droplet-based applications. - In: Micromachines, ISSN 2072-666X, Bd. 15 (2024), 2, 250, S. 1-19

Although the enormous potential of droplet-based microfluidics has been successfully demonstrated in the past two decades for medical, pharmaceutical, and academic applications, its inherent potential has not been fully exploited until now. Nevertheless, the cultivation of biological cells and 3D cell structures like spheroids and organoids, located in serially arranged droplets in micro-channels, has a range of benefits compared to established cultivation techniques based on, e.g., microplates and microchips. To exploit the enormous potential of the droplet-based cell cultivation technique, a number of basic functions have to be fulfilled. In this paper, we describe microfluidic modules to realize the following basic functions with high precision: (i) droplet generation, (ii) mixing of cell suspensions and cell culture media in the droplets, (iii) droplet content detection, and (iv) active fluid injection into serially arranged droplets. The robustness of the functionality of the Two-Fluid Probe is further investigated regarding its droplet generation using different flow rates. Advantages and disadvantages in comparison to chip-based solutions are discussed. New chip-based modules like the gradient, the piezo valve-based conditioning, the analysis, and the microscopy module are characterized in detail and their high-precision functionalities are demonstrated. These microfluidic modules are micro-machined, and as the surfaces of their micro-channels are plasma-treated, we are able to perform cell cultivation experiments using any kind of cell culture media, but without needing to use surfactants. This is even more considerable when droplets are used to investigate cell cultures like stem cells or cancer cells as cell suspensions, as 3D cell structures, or as tissue fragments over days or even weeks for versatile applications.
Zhao, Lanqing; Hou, Minjie; Ren, Kun; Yang, Dongrong; Li, Fupeng; Yang, Xiecheng; Zhou, Yingjie; Zhang, Da; Liu, Shan; Lei, Yong; Liang, Feng
Hot-pressing enhances mechanical strength of PEO solid polymer electrolyte for all-solid-state sodium metal batteries. - In: Small Methods, ISSN 2366-9608, Bd. 0 (2024), 0, 2301579, S. 1-9

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are widely utilized in all-solid-state sodium metal batteries (ASSSMBs) due to their excellent flexibility and safety. However, poor ionic conductivity and mechanical strength limit its development. In this work, an emerging solvent-free hot-pressing method is used to prepare mechanically robust PEO-based SPE, while sodium superionic conductors Na3Zr2Si2PO12 (NZSP) and NaClO4 are introduced to improve ionic conductivity. The as-prepared electrolyte exhibits a high ionic conductivity of 4.42 × 10−4 S cm−1 and a suitable electrochemical stability window (4.5 V vs Na/Na+). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na3V2(PO4)3C ASSSMB delivers a high-capacity retention of 97.1% after 100 cycles at 0.5 C and 60 ˚C, and exhibits excellent Coulombic efficiency (CE) (close to 100%). The ASSSMB with the 20 µm thick electrolyte also demonstrates excellent cyclic stability. This study provides a promising strategy for designing stable polymer-ceramic composite electrolyte membranes through hot-pressing to realize high-energy-density sodium metal batteries.
Yang, Xiecheng; Zhang, Dantong; Zhao, Lanqing; Peng, Chao; Ren, Kun; Xu, Changfan; Liu, Pan; Zhou, Yingjie; Lei, Yong; Yang, Bin; Xue, Dongfeng; Liang, Feng
Upgrading cycling stability and capability of hybrid Na-CO2 batteries via tailoring reaction environment for efficient conversion CO2 to HCOOH. - In: Advanced energy materials, ISSN 1614-6840, Bd. 14 (2024), 16, 2304365, S. 1-12

Rechargeable Na-CO2 batteries are considered to be an effective way to address the energy crisis and greenhouse effect due to their dual functions of CO2 fixation/utilization and energy storage. However, the insolubility and irreversibility of solid discharge products lead to poor discharge capacity and poor cycle performance. Herein, a novel strategy is proposed to enhance the electrochemical performance of hybrid Na-CO2 batteries, using water-in-salt electrolyte (WiSE) to establish an optimal reaction environment, regulate the CO2 reduction pathway, and ultimately convert the discharge product of the battery from Na2CO3 to formic acid (HCOOH). This strategy effectively resolves the issue of poor reversibility, allowing the battery to exhibit excellent cycle performance (over 1200 cycles at 30 ˚C), especially under low-temperature conditions (2534 cycles at −20 ˚C). Furthermore, density functional theory (DFT) calculations and experiments indicate that by adjusting the relative concentration of H/O atoms at the electrolyte/catalyst interface, the CO2 reduction pathway in the battery can be regulated, thus effectively enhancing CO2 capture capability and consequently achieving an ultra-high discharge specific capacity of 148.1 mAh cm−2. This work effectively promotes the practical application of hybrid Na-CO2 batteries and shall provide a guidance for converting CO2 into products with high-value-added chemicals.