Mario Fratschko
Metal-organic frameworks (MOFs) are porous materials with metal ions or clusters connected by organic ligands.1 Due to their high surface areas, tunable pores, and high stability, they are highly versatile. While traditionally synthesized as polycrystalline bulk materials through methods like solvothermal synthesis, thin films are needed for electrical devices. Thin films are produced by specific techniques, e.g. layer-by-layer growth. This research investigates the crystallographic properties of MOFs in thin films by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Two systems namely Cu2(bdc) and Cu2(bdc)2(dabco) (bdc = benzene-1,4-dicarboxylic acid; dabco = 1,4-diazabicyclo[2.2.2]octane) have been investigated for their potential as a fluorescence sensor6 and catalyst, respectively. Therefore, thin films were prepared by solvothermal processing and a layer-by-layer approach. To solve the crystal structure of the MOFs a combined experimental and theoretical approach is used, as the limited diffraction peaks in thin films made this task particularly challenging. For Cu₂(bdc), the findings suggest a two-dimensional structure with Cu²⁺ ions coordinated to bdc ligands. Cu₂(bdc)₂(dabco) is composed of two-dimensional layers of Cu paddlewheel complexes with bdc linkers and dabco pillars, forming a three-dimensional structure. The growth mechanism of Cu₂(bdc)₂(dabco) was also examined due to its catalytic performance dependence on film thickness.7 Nine thin films with different deposition cycles (0.5, 1, 2, 3, 4, 5, 10, 20, 50) were prepared using the layer-by-layer method. AFM revealed non-linear Volmer-Weber growth with full coverage of the sample at 10 deposition cycles. The crystal structure of Cu₂(bdc)₂(dabco) is confirmed across all deposition cycles, while an additional Cu₂(bdc) phase emerges at 20 cycles. Quantitative analysis indicated increasing Cu₂(bdc) volume, reaching 12% and 22% for 20 and 50 cycles, respectively. Pole figures show that Cu₂(bdc)₂(dabco) forms a uniplanar texture, with 75% exhibiting uniplanar texture and 25% exhibiting random texture. Cu₂(bdc), on the other hand, displays only random texture. GIXD provides a detailed characterization of thin film formation, which allows the optimization of the thin film formation process.
Lukas Legenstein
Lattice vibrations (phonons) affect transport properties in crystalline organic semiconductors either by scattering with charge carriers, or as the main carriers of thermal energy. Modelling these phonons and their transport gives a distinct advantage of providing direct insight into the relevant processes at an atomistic level compared to experiments. Of particular relevance for such simulations are machine-learned potentials, which often achieve accuracies comparable to the ab initio methods they are trained on, albeit at hugely reduced computational costs. We show that the applied Moment Tensor Potentials excellently reproduce the phonon bands calculated previously using dispersion-corrected density-functional theory. Further, we determine the lattice thermal conductivity by solving the Wigner transport equation. With this methodology the conduction mechanisms arising from inter-band tunneling are accounted, which turns out to be crucial for matching the temperature-dependent experimental values. Importantly, the presented approach provides direct insight into the (anisotropic) contributions of individual modes to the thermal conductivities.
Josef Simbrunner
Grazing Incidence X-ray Diffraction (GIXD) has been established as a powerful tool for the structural characterization of thin films. However, indexing of the experimentally observed diffraction peaks without prior knowledge of the involved crystal lattices has turned out as a challenging task. During the last years, we published a series of works which introduce indexing methods for different methods of GIXD experiments. Static GIXD measurements are performed at fixed sample positions for thin films with preferred orientation of the crystallites relative to the substrate surface but without any in-plane order. Rotating GIXD measurements use rotation of the thin film sample about the substrate normal and collect for each rotation angle a single detector image. This method is used for crystals with azimuthal alignments within the thin film. A comprehensive mathematical framework has been developed which provides the assignment of Laue indices to the individual diffraction peaks. The algorithms are even reduced from the three-dimensional case to two-dimensional representation of the experimental results. Despite the fact that GIXD experiments provide only a limited number of diffraction peaks, indexing became possible even for thin film crystals with low symmetry, different preferred orientations and multiple azimuthal alignments.
Jaytee Kanwar
The James Webb Space Telescope (JWST) revolutionizes our understanding of planet-forming disks, offering unprecedented insights into their physical and chemical structures. Among these, the very low-mass stars are known to have the highest occurrence rate of the terrestrial planets around them. In the MIRI Mid-Infrared Disk Survey (MINDS), we observe 10 such disks around very low-mass stars.
Here, we present the JWST MIRI/MRS spectrum of a disk around a very low-mass star, revealing dust features and, a rich array of large molecules such as C6H6, C4H2, C3H4, C2H6, HC3N, C2H2, CO2 etc. and isotopologues such as 13CCH2 and 13CO2. By leveraging the recently developed extended hydrocarbon chemical network that can form simple aromatics such as C6H6 and using the thermo-chemical disk models, we check if these molecular detections are consistent with our astro-chemical understanding in the high-density inner regions of the disks. As we predominantly observe hydrocarbons in the disk, varying the carbon-to-oxygen (C/O) ratio leads to the formation of these detected species in the surface layers of the disk. We still have unidentified spectral features in the spectrum that led us to employ these models to predict additional detectable species. We need the spectrum for these predicted species to confirm or rule out their presence.
We then use thermo-chemical disk models to place the slab model results into a larger context and identify the 2D geometry conducive to those conditions. Our study paves the way for a deeper understanding of the spectra. It provides new constraints for planet formation in disks around VLMS and highlights the instrumental role of JWST in providing insights into the origins of planetary systems.
Patrick Lainer
The class of plasma instabilities known as edge-localized modes (ELMs) is of special concern in tokamaks operating in high-confinement mode, such as ASDEX Upgrade and ITER. One strategy for ELM mitigation is the application of resonant magnetic perturbations (RMPs) via external coils. Kinetic modeling accurately describes the plasma response to these RMPs from first principles, particularly the parallel shielding currents at resonant surfaces. Away from resonant surfaces, ideal magnetohydrodynamics (iMHD) is expected to yield sufficiently accurate results, providing a computationally less expensive option that could complement kinetic modeling.
The code MEPHIT has been developed to solve the linearized iMHD equations in a way that is compatible with iterative kinetic modeling approaches. We consider an axisymmetric iMHD equilibrium in realistic tokamak geometry under the influence of a quasi-static non-axisymmetric external perturbation from ELM mitigation coils. The plasma responds to this external magnetic perturbation with a current perturbation, which in turn produces a magnetic field perturbation. The resulting fixed-point equation can be solved in a self-consistent manner by preconditioned iterations in which Ampère's equation and the magnetic differential equations for pressure and current are solved in alternation until convergence is reached. After expansion in toroidal Fourier harmonics, these equations are solved on a triangular mesh in the poloidal plane using finite elements. These results are then benchmarked against established codes.
Hana Hampel
Extreme ultraviolet light (XUV) offers considerable potential in microscopy to improve resolution but implementations are hampered by the difficulty to construct optical elements for this wavelength range. Particular interest lies in the XUV range (10 – 124 nm) where – next to improving the spatial resolution according to the Abbe-limit - synthesis of the microscopy light through high-harmonic generation (HHG) would add unrivaled temporal resolution. Such a tool could provide unprecedented insights into the interaction between light and nanostructured matter (in particular: electronic circuitry), raising hope to pave the way towards novel photonic devices.
This study set out to validate vacuum-guiding, a new design concept for transmissive optics in the XUV spectral region suitable to establish attosecond microscopy with sub-optical-wavelength and sub-optical-period spatio-temporal resolution. We experimentally demonstrate the focusing power of a XUV metalens that uses vacuum-guiding through a lithographically nano-perforated silicon membrane to confine attosecond light into a sub-micrometer beam waist.
Greta Cappello
Parker Solar Probe (PSP) and Solar Orbiter (SolO) observe the Sun from unprecedented close-in orbits out of the Sun-Earth line. They both provide high-resolution observations of the heliosphere through their white light heliospheric imagers, respectively WISPR and SolO-HI. In combination with EUV imagery (e.g., EUVI/SolO and AIA/SDO) and coronographic data (e.g., COR1-COR2/STEREO and C2-C3/SOHO), these unique data from different vantage points will give us new insights into the early evolution of coronal mass ejections (CMEs) in the low corona and inner heliosphere. This allows us to investigate the 3D location, morphology, and evolution of the internal magnetic fine structures in the interiors of CMEs. We track localized density enhancements, reflecting small-scale magnetic structures belonging to a filament-related coronal mass ejection (CME). Specifically, we ask about their relationship with the filament/source region and the flux rope. Using triangulation techniques, we derive the three-dimensional information of selected CME features. We group small-scale structures with roughly similar speeds, longitude, and latitude, into distinct morphological groups. For the December 8 2022 event, we find twisted magnetic field patterns close to the eastern leg of the CME that may be related to 'horns' outlining the edges of the flux-rope cavity. Aligned thread-like bundles are identified close to the western leg. They may be related to confined density enhancements evolving during the filament eruption. High-density blob-like features (magnetic islands) are widely spread in longitude (∼40°) close to the flanks and rear part of the CME. We demonstrate that CME flux ropes may comprise different morphological groups with a clustering behavior, apart from the blobs that span a wide range of longitudes instead. This may hint either to the three-dimensionality of the post-CME current sheet (CS) or to the influence of the ambient corona in the evolutionary behavior of the CS. Importantly, we show that the global appearance of the CME can be very different in WISPR (0.11--0.16~AU) and instruments near 1~AU because of the shorter line-of-sight integration of WISPR.
Richard Berger
When simulating molecular adlayers on inorganic substrates, most often only homogeneous adlayers made of one single molecular species are taken into account. While this avoids numerous computational problems, it often does not represent the physically correct situation where the adlayer consists of multiple building blocks, rather forming heterogeneous adlayers adsorbed on the substrate.
Here we computationally studied two different cases of heterogeneous adlayers. The first consists of both organic molecules and adatoms extracted from the metallic bulk phase. The second is formed from two different molecular species also adsorbed on a metal substrate.
As the first system, we investigate the adsorption of F4TCNQ on Au (111), which is a prototypical system for the adsorption of an acceptor type molecule on a metallic substrate. The presence of the adatom in the adlayer causes significant changes in the electronic structure of the interface affecting properties such as the adsorption geometry, the bonding type, and the work function. Using density functional theory, we show that incorporating adatoms significantly changes the interface charge transfer and modifies the Fermi-level pinning mechanism for the adsorbed species. Furthermore, we find that the 5d orbitals of the Au adatom hybridize with the F4TCNQ molecular orbitals, introducing covalent coupling within the adlayer. The combination of this effect explains why the incorporation of adatoms, despite the high cost of extracting them from the bulk, is energetically favorable.
As the second system, we investigated the exemplary donor-acceptor system of F4TCNQ and 1H,1’H-[4,4’]bipyridinylidene on the Cu (111) surface. We predict the thermodynamically stable adlayer structure for different mixing stoichiometries. This allows us to forecast the tunability of the on-surface stoichiometry and structure via the gas phase concentration ratio during the deposition process. Using these predictions, we explore how the complex electrostatic potential of the donor/acceptor mixture affects the work function modification at the interface.
Eva Kogler: Pathway to High Temperature Superconductors: Theoretical Insights into BaSiH
Nina Kainbacher: Investigation of the interplay between the polymorphism of organic thin films and their optical properties
David Loibner: Light-field driven molecular wave packet dynamics
Robbin Steentjes: Disordered stacking models and stacking-dependent property determination of 2D MOFs and COFs
Elena Unterleutner: Atom by Atom analysis of defect structures in doped STO
Lorenz Huber: Investigation of Particle-Plasmon Fields with a Tip and Structured Light
Stefan Eber: Magneto-ionic effects in nanoporous Pd-Co alloys
Relindis Rott: Fast Physical Environment Simulation Models for Virtual Testing of Autonomous Systems