arXiv subjects

Andreas Beyer

Publications and source records attributed to Andreas Beyer.

16 recordsLinked to original sources

arXiv2026

Orientation Mapping via Dictionary Indexing of AC-STEM Kikuchi patterns using Open-source Software

The properties of polycrystalline materials are strongly influenced by the spatial arrangement and orientations of individual grains within the microstructure, making nanoscale characterization of grain orientation essential. This is also often the case for small grains in the nm regime explored using scanning transmission electron microscopy (STEM). Automated crystal orientation mapping (ACOM) is traditionally performed using spot-like diffraction patterns. In contrast, orientation mapping based on transmission Kikuchi diffraction (TKD) using an aberration-corrected (AC) convergent STEM probe remains relatively underexplored, despite its superior orientation sensitivity and higher spatial resolution. In this work, we present an open-source software-based template-matching approach for orientation mapping using AC-STEM TKD. A master pattern (a simulated angular distribution of Kikuchi band intensities on the unit sphere) is first generated through a dynamical simulation implemented in open-source software. This resulting pattern is subsequently imported into another open-source package for geometric simulations and orientation indexing. We demonstrate the capability of the proposed method by applying it to orientation mapping in BaZr0.4Ce0.4Y0.1Yb0.1O3-{\delta} (BZCYYb4411) fuel-cell material and LiNiO2 (LNO) lithium-ion battery cathode material. The best-matched simulated patterns exhibit strong agreement with experimental data, even under the challenging conditions with limited diffraction space available for matching.

cond-mat.mtrl-sci↗

arXiv2025

Robust coherent phonon mode at GaP/Si(001) heterointerface

Lattice-matched GaP layers without extended defects can be grown on Si(001) substrate via a two-step growth procedure, consisting of low-temperature nucleation followed by high-temperature overgrowth. A transient reflectivity experiment on a thin, low-temperature nucleation layer discovered a previously unknown phonon mode at 2 THz upon below-bandgap optical excitation (Adv. Mater. Interfaces 2025, 2400573). Here we examine the influence of the two-step growth process on the ultrafast carrier and phonon dynamics of the GaP/Si interface. We find that the discrete electronic state, which governed the interfacial carrier dynamics of the thin nucleation layer, becomes suppressed when a thicker layer is formed by high-temperature overgrowth. The coherent 2-THz oscillation is observed also in the high-temperature overgrown structures, at the constant frequency regardless of the GaP layer thickness. Its resonance behavior closely follows that of the carrier dynamics at the respective growth stage. This supports its assignment to a phonon mode generated at the heterointerface and strongly coupled to the interfacial carriers. The phonon amplitude exhibits a non-monotonic dependence on the GaP layer thickness, and its optical polarization-dependence is qualitatively altered by the high-temperature overgrowth, neither of which is accounted for by the carrier-phonon coupling alone. Our results demonstrate that the 2-THz interfacial phonon mode is robust against high-temperature overgrowth, while its amplitude is determined by both coupling to interfacial electronic transitions and atomic-scale structural reorganization at the interface.

cond-mat.mtrl-sci↗

arXiv2025

Low Temperature MOCVD Synthesis of high-mobility 2D InSe

Two-dimensional (2D) indium selenide (InSe) is a layered semiconductor with high electron mobility and a tunable band gap ranging from 1.25 eV in the bulk to 2.8 eV in the monolayer limit. These properties make these materials strong candidates for future logic and optoelectronic devices. However, growing phase-pure InSe remains challenging due to the complex indium-selenium (In-Se) phase diagram. This complexity and the sensitivity of chemical precursors to growth conditions make it difficult to control which In-Se phase forms during synthesis during, e.g., metal-organic chemical vapor deposition (MOCVD). Despite the challenges, MOCVD is considered the most promising approach for growing InSe, as it enables wafer-scale, uniform, and controllable deposition-key requirements for device integration. In this study, we present a systematic investigation of InSe synthesis via MOCVD on c-plane sapphire substrates at low temperatures, which are highly relevant for various integration schemes. By varying the Se/In precursor ratio and the growth temperature, we create a phase diagram that covers the In-rich, equal stoichiometric, and Se-rich InxSey phases. Raman spectroscopy and atomic force microscopy, supported by energy dispersive X-ray spectroscopy and scanning transmission electron microscopy, confirm conditions, under which the formation of 2D InSe is observed. Atomically-resolved cross-sectional scanning transmission electron microscopy also reveals an epitaxial alignment of the InSe with the sapphire substrate mediated by a specific interface reconstruction. The epitaxial alignment is verified by in-plane X-ray diffraction across large length scales. Samples grown under optimized conditions exhibit a strong optical absorption in the visible range and especially a comparably high electron mobility underlining the potential of the MOCVD-grown material for future applications.

cond-mat.mtrl-sci↗

arXiv2025

Microstructure of Silicon Anodes in Solid-State Batteries -- From Crystalline to Amorphous

Silicon offers great promise as a potential anode active material and the optimum alternative to lithium metal in all-solid-state lithium-ion batteries. However, its practical application is limited by severe volume expansion (~300%) during lithiation, leading to cracking upon delithiation. In this study, we investigated the microstructural evolution of microcrystalline silicon electrodes in a solid-electrolyte-free environment using cryogenic scanning transmission electron microscopy (STEM) during electrochemical cycling. A controlled workflow prevents ambient exposure, and cryo-TEM ensures structural integrity. After the first lithiation, the electrode shows a heterogeneous mix of crystalline Li15Si4, various amorphous LixSi phases, and residual crystalline silicon. After delithiation, the structure becomes predominantly amorphous with thread-like features and minimal remaining crystallinity. By the 10th delithiation, the microstructure is more uniform, with thread-like regions mainly at grain boundaries. Our results reveal that starting from a crystalline phase, a stationary microstructure emerges in bulk silicon only after several cycles. Thus, to have a more controlled behavior of the electrode and minimize cracking, the starting material should be carefully chosen along with an optimized electrode architecture to help stabilize the microstructure throughout cycling.

cond-mat.mtrl-sci↗

arXiv2025

Determining the grain orientations of battery materials from electron diffraction patterns using convolutional neural networks

Polycrystalline materials have numerous applications due to their unique properties, which are often determined by the grain boundaries. Hence, quantitative characterization of grain as well as interface orientation is essential to optimize these materials, particularly energy materials. Using scanning transmission electron microscopy, matter can be analysed in an extremely fine grid of scan points via electron diffraction patterns at each scan point. By matching the diffraction patterns to a simulated database, the crystal orientation of the material as well as the orientation of the grain boundaries at each scan point can be determined. This pattern matching approach is highly time intensive. Artificial intelligence promises to be a very powerful tool for pattern recognition. In this work, we train convolutional neural networks (CNNs) on dynamically simulated diffraction patterns of LiNiO2, an important cathode-active material for Lithium-ion batteries, to predict the orientation of grains in terms of three Euler angles for the complete fundamental orientation region. Results demonstrate that these networks outperform the conventional pattern matching algorithm with increased accuracy and efficiency. The increased accuracy of the CNN models can be attributed to the fact that these models are trained by data incorporating dynamical effects. This work is the first attempt to apply deep learning for analysis of electron diffraction data and enlightens the great potential of ML to accelerate the analysis of electron microscopy data, toward high-throughput characterization technique.

cond-mat.mtrl-sci↗

arXiv2025

Sequential tilting 4D-STEM for improved momentum-resolved STEM field mapping

Momentum-resolved scanning transmission electron microscopy (MRSTEM) is a powerful phase-contrast technique that can map lateral magnetic and electric fields ranging from the micrometer to the subatomic scale. Resolving fields ranging from a few nanometers to a few hundred nanometers, as well as across material junctions, is particularly important since these fields often determine the functional properties of devices. However, it is also challenging since they are orders of magnitude smaller than atomic electric fields. Thus, subtle changes in diffraction conditions lead to significant changes in the measured MRSTEM signal. One established approach to partially overcome this problem is precession electron diffraction, in which the incident electron beam is continuously precessed while precession-averaged diffraction patterns are acquired. Here, we present an alternative approach in which we sequentially tilt the incident electron beam and record a full diffraction pattern for each tilt and spatial position. This approach requires no hardware modification of the instrument and enables the use of arbitrary beam tilt patterns that can be optimized for specific applications. Furthermore, recording diffraction patterns for every beam tilt allows access to additional information. In this work, we use this information to create virtual large-angle convergent beam electron diffraction (vLACBED) patterns to assess MRSTEM data quality and improve field measurements by applying different data analysis methods beyond simple averaging. The presented data acquisition concept can readily be applied to other 4D-STEM applications.

cond-mat.mtrl-sci↗

arXiv2024

Excitons in epitaxially grown WS2 on Graphene: a nanometer-resolved EELS and DFT study

In this study, we investigate excitonic properties of epitaxially grown WS2, which is of particular interest for various applications due to its potential for upscaling to wafer sized structures. Understanding the effect of the dielectric environment due to changing layer numbers and multi-material heterostructures on the optical properties is crucial for tailoring device properties. Monochromated electron energy loss spectroscopy in a scanning transmission electron microscope is employed to characterize the excitonic spectrum of WS2 on graphene grown by metal organic chemical vapor deposition. This technique provides the required spatial resolution at the nanometer scale in combination with high quality spectra. To complement the experimental results, theoretical investigations using density functional theory and applying the Bethe-Salpeter equations are conducted. We find that by transitioning from mono- to bi- to multilayers of WS2 the spectra show redshifts for both, the K-valley excitons at about 2.0 and 2.4 eV as well as excitonic features of higher energies. The latter features originate from so called band nesting of transitions between the Gamma- and K-point. In summary, this study provides valuable insights into the excitonic properties of WS2 in different layer configurations and environments, which are realistically needed for future device fabrication and property tuning. Finally, we can show that nanometer scale electron spectroscopy supported by careful theoretical modelling can successfully link atomic structure and optical properties, such as exciton shifts, in non-idealized complex material systems like multilayer 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

A Small Step for Epitaxy, a Large Step Towards Twist Angle Control in 2D Heterostructures

Two-dimensional (2D) materials have received a lot of interest over the past decade. Especially van der Waals (vdW) 2D materials, such as transition metal dichalcogenides (TMDCs), and their heterostructures exhibit semiconducting properties that make them highly suitable for novel device applications. Controllable mixing and matching of the 2D materials with different properties and a precise control of the in-plane twist angle in these heterostructures are essential to achieve superior properties and need to be established through large-scale device fabrication. To gain fundamental insight into the control of these twist angles, 2D heterostructures of tungsten disulfide (WS2) and graphene grown by bottom-up synthesis via metal-organic chemical vapor deposition (MOCVD) are investigated using a scanning transmission electron microscope (STEM). Specifically, the combination of conventional high-resolution imaging with scanning nano beam diffraction (SNBD) using advanced 4D STEM techniques is used to analyze moir\'e structures. The latter technique is used to reveal the epitaxial alignment within the WS2/Gr heterostructure, showing a direct influence of the underlying graphene layers on the moir\'e formation in the subsequent WS2 layers. In particular, the importance of grain boundaries within the underlying WS2 and Gr layers for the formation of moir\'e patterns with rotation angles below 2{\deg} is discussed.

cond-mat.mtrl-sci↗

arXiv2023

Measuring spatially-resolved potential drops at semiconductor hetero-interfaces using 4D-STEM

Characterizing long-range electric fields and built-in potentials in functional materials at nano- to micrometer scales is of supreme importance for optimizing devices. E.g., the functionality of semiconductor heterostructures or battery materials is determined by the electric fields established at interfaces which can also vary spatially. In this study, we propose momentum-resolved four-dimensional scanning transmission electron microscopy (4D-STEM) for the quantification of these potentials and show the optimization steps required to reach quantitative agreement with simulations for the GaAs / AlAs hetero-junction model system. Using STEM the differences in the mean inner potentials (DELTA MIP) of two materials forming an interface and resulting dynamic diffraction effects have to be considered. We show that the measurement quality is significantly improved by precession, energy filtering and a non-zone-axis alignment of the specimen. Complementary simulations yielding a DELTA MIP of 1.3 V confirm that the potential drop due to charge transfer at the intrinsic interface is about 0.1 V, in agreement with experimental and theoretical values found in literture. These results show the feasibility of accurately measuring built-in potentials across hetero-interfaces of real device structures and its promising application for more complex interfaces of other polycrystalline materials on the nanometer scale.

cond-mat.mtrl-sci↗

arXiv2022

Adamantanes as white-light emitters: Controlling arrangement and functionality by external Coulomb forces

Functionalized adamantane molecular cluster materials show highly transient nonlinear optical properties of currently unclear structural origin. Several interaction mechanisms in compounds comprising molecular clusters, their inter- and intramolecular interactions as well as the interplay of their electronic systems and vibrations of their backbone are viable concepts to explain these nonlinear optical properties. We show that transient Coulomb forces also have to be considered as they can lead to intramolecular structure transformations and intermolecular rearrangements in the crystal. Both strongly influence the nonlinear optical properties. Moreover, selective bromine functionalization can trigger a photochemical rearrangement of the molecules. The structure and chemical bonding within the compounds are investigated in dependence on the laser irradiation at different stages of their nonlinear emission by electron diffraction and electron energy loss spectroscopy. The transient structural and chemical states observed are benchmarked by similar observations during electron irradiation, which makes quantification of structural changes possible and allows the correlation with first principles calculations. The functionalization and its subsequent usage to exploit photochemical effects can either enhance two-photon absorption or facilitate white-light emission rather than second-harmonic generation.

cond-mat.mtrl-sci↗

arXiv2022

White-light generating molecular materials: correlation between the amorphous/crystalline structure and nonlinear optical properties

Amorphous materials are integral part of todays technology, they commonly are performant and versatile in integration. Consequently, future applications increasingly aim to harvest the potential of the amorphous state. Establishing its structure-property relationship, however, is inherently challenging using diffraction-based techniques yet is extremely desirable for developing advanced functionalities. In this article, we introduce a set of transmission electron microscopy-based techniques to locally quantify the structure of a material. This unique approach allows to clearly identify the spatial distribution of amorphous and crystalline regions and to quantify atomic arrangements of amorphous regions of a representative model system. We study an ensemble of well-defined, functionalized adamantane-type cluster molecules exhibiting exceptionally promising nonlinear optical properties of unclear origin. The nanoscopic structure for three model compounds ([(PhSn)4S6], [(NpSn)4S6], [(CpSn)4S6]) correlates with their characteristic optical responses. These results highlight the advantageous properties of amorphous molecular materials when understanding the microscopic origin.

cond-mat.mtrl-sci↗

arXiv2018

Coherent optical and acoustic phonons generated at lattice-matched GaP/Si(001) heterointerfaces

Thin GaP films can be grown on an exact Si(001) substrate with nearly perfect lattice match. We perform all-optical pump-probe measurements to investigate the ultrafast electron-phonon coupling at the buried interface of GaP/Si. Above-bandgap excitation with a femtosecond laser pulse can induce coherent longitudinal optical (LO) phonons both in the GaP opverlayer and in the Si substrate. The coupling of the GaP LO phonons with photoexcited plasma is reduced significantly with decreasing the GaP layer thickness from 56 to 16 nm due to the quasi-two-dimensional confinement of the plasma. The same laser pulse can also generate coherent longitudinal acoustic (LA) phonons in the form of a strain pulse. The strain induces not only a periodic modulation in the optical reflectivity as they propagate in the semiconductors, but also a sharp spike when it arrives at the GaP layer boundaries. The acoustic pulse induced at the GaP/Si interface is remarkably stronger than that at the Si surface, suggesting a possible application of the GaP/Si heterostructure as an opto-acoustic transducer. The amplitude and the phase of the reflectivity modulation varies with the GaP layer thickness, which can be understood in terms of the interference caused by the multiple acoustic pulses generated at the top surface and at the buried interface.

cond-mat.mtrl-sci↗

arXiv2017

Sub-picosecond acoustic pulses at buried GaP/Si interfaces

We report on the optical generation and detection of ultrashort acoustic pulses that propagate in three-dimensional semiconductor crystals. Photoexcitaiton of lattice-matched GaP layers grown on Si(001) gives rise to a sharp spike in transient reflectivity due to the acoustic pulse generated at the GaP/Si interface and detected at the GaP surface and vice versa. The extremely short width of the reflectivity spike, 0.5 ps, would translate to a spatial extent of 3 nm or 10 atomic bilayers, which is comparable with the width of the intermixing layer at the GaP/Si interface. The reflectivity signals are also modified by quasi-periodic Brillouin oscillations of GaP and Si arising from the acoustic pulses during the propagation in the crystals. The present results demonstrate the potential application of the simple optical pump-probe scheme in the nondestructive evaluation of the buried semiconductor interface quality.

cond-mat.mtrl-sci↗

arXiv2016

Direct Characterization of Band Bending in GaP/Si(001) Heterostructures with Hard X-ray Photoelectron Spectroscopy

We apply hard X-ray photoelectron spectroscopy (HAXPES) to investigate the electronic structures in ~50-nm thick epitaxial GaP layers grown on Si(001) under different conditions. Depth profiles of the local binding energies for the core levels are obtained by measuring the photoemission spectra at different incident photon energies between 3 and 7 keV and analyzing them with simple numerical models. The obtained depth profiles are in quantitative agreement with the band bending determinations for the same samples in a previous coherent phonon spectroscopic study. Our results demonstrate the applicability of the HAXPES with varying incident photon energy to characterize the electric potential profiles at buried semiconductor heterointerfaces.

cond-mat.mes-hall↗

arXiv2014

ILATO Project: Fusion of Optical Surface Models and Volumetric CT Data

Project ILATO focuses on Improving Limited Angle computed Tomography by Optical data integration in order to enhance image quality and shorten acquisition times in X-ray based industrial quality inspection. Limited angle computed tomography is indicated whenever specimen dimensions exceed cone beam limits or the object is impenetrable from certain angles. Thus, acquiring only a subset of a full circle CT scan poses problems for reconstruction algorithms due to incomplete data which introduces blurred edges and other artifacts. To support volumetric data reconstruction algorithm a surface mesh of the object obtained via structured light optical scan acts as a mask defining boundaries of the reconstructed image. The registration of optically acquired surfaces with data acquired from computed tomography is our current challenge. This article presents our setup, the methods applied and discusses the problems arising from registration of data sets created with considerably different imaging techniques.

cs.CG↗

arXiv2013

Stress induces remodelling of yeast interaction and co-expression networks

Network analysis provides a powerful framework for the interpretation of genome-wide data. While static network approaches have proved fruitful, there is increasing interest in the insights gained from the analysis of cellular networks under different conditions. In this work, we study the effect of stress on cellular networks in fission yeast. Stress elicits a sophisticated and large scale cellular response, involving a shift of resources from cell growth and metabolism towards protection and maintenance. Previous work has suggested that these changes can be appreciated at the network level. In this paper, we study two types of cellular networks: gene co-regulation networks and weighted protein interaction networks. We show that in response to oxidative stress, the co-regulation networks re-organize towards a more modularised structure: while sets of genes become more tightly co-regulated, co-regulation between these modules is decreased. This shift translates into longer average shortest path length, increased transitivity, and decreased modular overlap in these networks. We also find a similar change in structure in the weighted protein interaction network in response to both oxidative stress and nitrogen starvation, confirming and extending previous findings. These changes in network structure could represent an increase in network robustness and/or the emergence of more specialised functional modules. Additionally, we find stress induces tighter co-regulation of non-coding RNAs, decreased functional importance of splicing factors, as well as changes in the centrality of genes involved in chromatin organization, cytoskeleton organization, cell division, and protein turnover.

q-bio.MN↗