Prof. Dr. Knut Müller-Caspary
Academic Research Focus
- Multidimensional electron microscopy
- Structure & chemistry in solid-state nanostructures & 2D materials
- Electrical characterization
- High-contrast low-dose imaging of soft and biological matter
- Development and application of new imaging techniques
- Simulation of electron scattering, Density functional theory, Artificial Intelligence
- Structure & chemistry in solid-state nanostructures & 2D materials
- Electrical characterization
- High-contrast low-dose imaging of soft and biological matter
- Development and application of new imaging techniques
- Simulation of electron scattering, Density functional theory, Artificial Intelligence
Fields of Application
Data Science / Machine Learning, Nanomaterials, Semiconductor Materials
Publications
25.
M Kost, J F Dushimineza, K Müller-Caspary, T Bein
Optimized Oxidation Temperature Enhances OER Performance of IrO2-Loaded SnO2 Nanofibers - Role of Charge Carrier Percolation Pathways Journal Article
In: Advanced Materials Interfaces, vol. 12, no. 14, 2025, ISSN: 2196-7350.
@article{nokey,
title = {Optimized Oxidation Temperature Enhances OER Performance of IrO2-Loaded SnO2 Nanofibers - Role of Charge Carrier Percolation Pathways},
author = {M Kost and J F Dushimineza and K M\"{u}ller-Caspary and T Bein},
url = {\<Go to ISI\>://WOS:001527460600001},
doi = {10.1002/admi.202400997},
issn = {2196-7350},
year = {2025},
date = {2025-07-14},
journal = {Advanced Materials Interfaces},
volume = {12},
number = {14},
abstract = {The potential for reducing iridium content in large-scale proton-exchange membrane (PEM) electrolysis is examined using a fibrous support morphology to enhance electron percolation. Focusing on high activity, stability, and conductivity, ultra-small, interconnected IrOx/IrO2 nanoparticles anchored to electrospun SnO2 nanofibers (IrOx/IrO2@SnO2) are investigated, with particular attention to the crystallinity of the iridium phase. Scanning transmission electron microscopy (STEM), conducted both before and after use as an electrocatalyst for the oxygen evolution reaction (OER), reveals how the oxidation temperature impacts the crystallinity and stability of the iridium oxide phase. The results suggest that further reductions in iridium content may be achieved by optimizing synthesis parameters. Here, the highest iridium utilization is achieved at an oxidation temperature of 375 degrees C, with improved conductivity and electrochemical activity. Transmission electron microscopy (TEM) indicates that higher oxidation temperatures result in fragmentation of conduction pathways, negatively affecting catalyst performance. Furthermore, TEM reveals the onset of IrO2 crystallization between 365 and 375 degrees C, with cyclic voltammetry (CVA) emphasizing the critical role of conductivity in ensuring efficient charge carrier transport to active sites. This study not only deepens the understanding of iridium-based catalysts but also identifies practical strategies to enhance cost-effectiveness and efficiency in PEM electrolysis technologies.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The potential for reducing iridium content in large-scale proton-exchange membrane (PEM) electrolysis is examined using a fibrous support morphology to enhance electron percolation. Focusing on high activity, stability, and conductivity, ultra-small, interconnected IrOx/IrO2 nanoparticles anchored to electrospun SnO2 nanofibers (IrOx/IrO2@SnO2) are investigated, with particular attention to the crystallinity of the iridium phase. Scanning transmission electron microscopy (STEM), conducted both before and after use as an electrocatalyst for the oxygen evolution reaction (OER), reveals how the oxidation temperature impacts the crystallinity and stability of the iridium oxide phase. The results suggest that further reductions in iridium content may be achieved by optimizing synthesis parameters. Here, the highest iridium utilization is achieved at an oxidation temperature of 375 degrees C, with improved conductivity and electrochemical activity. Transmission electron microscopy (TEM) indicates that higher oxidation temperatures result in fragmentation of conduction pathways, negatively affecting catalyst performance. Furthermore, TEM reveals the onset of IrO2 crystallization between 365 and 375 degrees C, with cyclic voltammetry (CVA) emphasizing the critical role of conductivity in ensuring efficient charge carrier transport to active sites. This study not only deepens the understanding of iridium-based catalysts but also identifies practical strategies to enhance cost-effectiveness and efficiency in PEM electrolysis technologies.
24.
R Holfeuer, C Maheu, H Illner, R Hoojier, H Balakrishnan, B März, S Lotfi, H Sezen, K Müller-Caspary, T Bein, J P Hofmann, T Ameri, A Hartschuh, A Yousefiamin
Printed CsMg–ZnO ETLs achieve over 9 % efficiency in PbS quantum dot solar cells Journal Article
In: Materials Today Energy, vol. 48, pp. 101813, 2025, ISSN: 2468-6069.
@article{nokey,
title = {Printed CsMg\textendashZnO ETLs achieve over 9 % efficiency in PbS quantum dot solar cells},
author = {R Holfeuer and C Maheu and H Illner and R Hoojier and H Balakrishnan and B M\"{a}rz and S Lotfi and H Sezen and K M\"{u}ller-Caspary and T Bein and J P Hofmann and T Ameri and A Hartschuh and A Yousefiamin},
url = {https://www.sciencedirect.com/science/article/pii/S2468606925000218},
doi = {https://doi.org/10.1016/j.mtener.2025.101813},
issn = {2468-6069},
year = {2025},
date = {2025-03-01},
journal = {Materials Today Energy},
volume = {48},
pages = {101813},
abstract = {Zinc oxide (ZnO) is a key electron transport layer (ETL) material in next-generation lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) due to its high transparency, strong exciton binding energy, and good electron mobility. Here, we demonstrate a scalable doctor-blading printing protocol for ZnO ETLs that integrates dual defect passivation with magnesium (Mg2⁺) and caesium (Cs⁺) and employs solvent engineering to achieve uniform, defect-minimized films. Using a ternary solvent blend (methanol, chloroform, and 2-methoxyethanol) optimizes the ink's viscosity and boiling point, preventing particle migration and ensuring full substrate coverage. Our modified ZnO ink leads to improved crystallinity, smoother surfaces, and reduced trap states, boosting the fill factor (FF) and short-circuit current (Jsc). Consequently, we achieve a power conversion efficiency increase from 5.98 % to 9.53 % using a printed CsMg-ZnO film. Notably, 80 % of dual-doped devices exceeded 7.5 % efficiency, demonstrating high reproducibility and reliability. This performance enhancement underscores the effectiveness of dual metal ion treatment and solvent engineering strategies in overcoming printability challenges. Moreover, the compatibility of our approach with low-temperature processing and established coating techniques paves the way for seamless integration into large-scale manufacturing, bringing PbS CQDSCs closer to commercial viability.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Zinc oxide (ZnO) is a key electron transport layer (ETL) material in next-generation lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) due to its high transparency, strong exciton binding energy, and good electron mobility. Here, we demonstrate a scalable doctor-blading printing protocol for ZnO ETLs that integrates dual defect passivation with magnesium (Mg2⁺) and caesium (Cs⁺) and employs solvent engineering to achieve uniform, defect-minimized films. Using a ternary solvent blend (methanol, chloroform, and 2-methoxyethanol) optimizes the ink's viscosity and boiling point, preventing particle migration and ensuring full substrate coverage. Our modified ZnO ink leads to improved crystallinity, smoother surfaces, and reduced trap states, boosting the fill factor (FF) and short-circuit current (Jsc). Consequently, we achieve a power conversion efficiency increase from 5.98 % to 9.53 % using a printed CsMg-ZnO film. Notably, 80 % of dual-doped devices exceeded 7.5 % efficiency, demonstrating high reproducibility and reliability. This performance enhancement underscores the effectiveness of dual metal ion treatment and solvent engineering strategies in overcoming printability challenges. Moreover, the compatibility of our approach with low-temperature processing and established coating techniques paves the way for seamless integration into large-scale manufacturing, bringing PbS CQDSCs closer to commercial viability.
23.
T Lorenzen, B Diederichs, C Otieno Ogolla, B Butz, K Müller-Caspary
Aberration measurement by electron ptychography and consistency among different algorithms Journal Article
In: Applied Physics Letters, vol. 126, no. 8, pp. 081602, 2025, ISSN: 0003-6951.
@article{nokey,
title = {Aberration measurement by electron ptychography and consistency among different algorithms},
author = {T Lorenzen and B Diederichs and C Otieno Ogolla and B Butz and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1063/5.0238580},
doi = {10.1063/5.0238580},
issn = {0003-6951},
year = {2025},
date = {2025-02-24},
journal = {Applied Physics Letters},
volume = {126},
number = {8},
pages = {081602},
abstract = {Control over and knowledge of the electron probe is important in all scanning transmission electron microscopy (STEM) techniques. This is emphasized especially in electron ptychography, where the accurate probe wave function is required to deconvolve illumination from the specimen. The majority of ptychographic algorithms, such as the extended ptychographic iterative engine, reconstruct the electron probe on a pixelated grid numerically self-consistently. Solutions are thus not necessarily bound to wave functions physically realizable by the optical system. A method is presented to characterize reconstructed probes by conventional lens aberrations. The fitted aberrations are then used to investigate the quality of the retrieved probes, and their consistency is examined in a systematic study using a 4D-STEM focal series recorded for a thin SnS2 2D flake. Additionally, the influences of partial coherence and limited electron dose on the retrieved probes are analyzed, and the usefulness of the retrieved probes for different ptychographic methods, such as single sideband, Wigner distribution deconvolution ptychography, and gradient descent-based schemes, is elucidated. Finally, applications for ptychography-driven alignments of aberration-correcting electron optics are outlined.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Control over and knowledge of the electron probe is important in all scanning transmission electron microscopy (STEM) techniques. This is emphasized especially in electron ptychography, where the accurate probe wave function is required to deconvolve illumination from the specimen. The majority of ptychographic algorithms, such as the extended ptychographic iterative engine, reconstruct the electron probe on a pixelated grid numerically self-consistently. Solutions are thus not necessarily bound to wave functions physically realizable by the optical system. A method is presented to characterize reconstructed probes by conventional lens aberrations. The fitted aberrations are then used to investigate the quality of the retrieved probes, and their consistency is examined in a systematic study using a 4D-STEM focal series recorded for a thin SnS2 2D flake. Additionally, the influences of partial coherence and limited electron dose on the retrieved probes are analyzed, and the usefulness of the retrieved probes for different ptychographic methods, such as single sideband, Wigner distribution deconvolution ptychography, and gradient descent-based schemes, is elucidated. Finally, applications for ptychography-driven alignments of aberration-correcting electron optics are outlined.
22.
M Cattaneo, K Müller-Caspary, J Barthel, K E Macarthur, N Gauquelin, M Lipinska-Chwalek, J Verbeeck, L J Allen, R E Dunin-Borkowski
Automated detection and mapping of crystal tilt using thermal diffuse scattering in transmission electron microscopy Journal Article
In: Ultramicroscopy, vol. 267, pp. 114050, 2024, ISSN: 0304-3991.
@article{nokey,
title = {Automated detection and mapping of crystal tilt using thermal diffuse scattering in transmission electron microscopy},
author = {M Cattaneo and K M\"{u}ller-Caspary and J Barthel and K E Macarthur and N Gauquelin and M Lipinska-Chwalek and J Verbeeck and L J Allen and R E Dunin-Borkowski},
url = {https://www.sciencedirect.com/science/article/pii/S0304399124001293},
doi = {https://doi.org/10.1016/j.ultramic.2024.114050},
issn = {0304-3991},
year = {2024},
date = {2024-12-01},
journal = {Ultramicroscopy},
volume = {267},
pages = {114050},
abstract = {Quantitative interpretation of transmission electron microscopy (TEM) data of crystalline specimens often requires the accurate knowledge of the local crystal orientation. A method is presented which exploits momentum-resolved scanning TEM (STEM) data to determine the local mistilt from a major zone axis. It is based on a geometric analysis of Kikuchi bands within a single diffraction pattern, yielding the center of the Laue circle. Whereas the approach is not limited to convergent illumination, it is here developed using unit-cell averaged diffraction patterns corresponding to high-resolution STEM settings. In simulation studies, an accuracy of approximately 0.1 mrad is found. The method is implemented in automated software and applied to crystallographic tilt and in-plane rotation mapping in two experimental cases. In particular, orientation maps of high-Mn steel and an epitaxially grown La0.7Sr0.3MnO3-SrTiO3 interface are presented. The results confirm the estimates of the simulation study and indicate that tilt mapping can be performed consistently over a wide field of view with diameters well above 100 nm at unit cell real space sampling.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Quantitative interpretation of transmission electron microscopy (TEM) data of crystalline specimens often requires the accurate knowledge of the local crystal orientation. A method is presented which exploits momentum-resolved scanning TEM (STEM) data to determine the local mistilt from a major zone axis. It is based on a geometric analysis of Kikuchi bands within a single diffraction pattern, yielding the center of the Laue circle. Whereas the approach is not limited to convergent illumination, it is here developed using unit-cell averaged diffraction patterns corresponding to high-resolution STEM settings. In simulation studies, an accuracy of approximately 0.1 mrad is found. The method is implemented in automated software and applied to crystallographic tilt and in-plane rotation mapping in two experimental cases. In particular, orientation maps of high-Mn steel and an epitaxially grown La0.7Sr0.3MnO3-SrTiO3 interface are presented. The results confirm the estimates of the simulation study and indicate that tilt mapping can be performed consistently over a wide field of view with diameters well above 100 nm at unit cell real space sampling.
21.
E Sirotti, L I Wagner, C-M Jiang, J Eichhorn, F Munnik, V Streibel, M J Schilcher, B März, F S Hegner, M Kuhl, T Höldrich, K Müller-Caspary, D A Egger, I D Sharp
Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3 Journal Article
In: Advanced Energy Materials, vol. 14, no. 42, pp. 2402540, 2024, ISSN: 1614-6832.
@article{nokey,
title = {Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3},
author = {E Sirotti and L I Wagner and C-M Jiang and J Eichhorn and F Munnik and V Streibel and M J Schilcher and B M\"{a}rz and F S Hegner and M Kuhl and T H\"{o}ldrich and K M\"{u}ller-Caspary and D A Egger and I D Sharp},
url = {https://doi.org/10.1002/aenm.202402540},
doi = {https://doi.org/10.1002/aenm.202402540},
issn = {1614-6832},
year = {2024},
date = {2024-11-01},
journal = {Advanced Energy Materials},
volume = {14},
number = {42},
pages = {2402540},
abstract = {Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.
20.
K Arslanova, P Ganswindt, T Lorenzen, E Kostyurina, K Karaghiosoff, B Nickel, K Müller-Caspary, A S Urban
Synthesis of Cs3Cu2I5 Nanocrystals in a Continuous Flow System Journal Article
In: Small, vol. 20, no. 44, pp. 2403572, 2024, ISSN: 1613-6810.
@article{nokey,
title = {Synthesis of Cs3Cu2I5 Nanocrystals in a Continuous Flow System},
author = {K Arslanova and P Ganswindt and T Lorenzen and E Kostyurina and K Karaghiosoff and B Nickel and K M\"{u}ller-Caspary and A S Urban},
url = {https://doi.org/10.1002/smll.202403572},
doi = {https://doi.org/10.1002/smll.202403572},
issn = {1613-6810},
year = {2024},
date = {2024-11-01},
journal = {Small},
volume = {20},
number = {44},
pages = {2403572},
abstract = {Abstract Achieving the goal of generating all of the world's energy via renewable sources and significantly reducing the energy usage will require the development of novel, abundant, nontoxic energy conversion materials. Here, a cost-efficient and scalable continuous flow synthesis of Cs3Cu2I5 nanocrystals is developed as a basis for the rapid advancement of novel nanomaterials. Ideal precursor solutions are obtained through a novel batch synthesis, whose product served as a benchmark for the subsequent flow synthesis. Realizing this setup enabled a reproducible fabrication of Cs3Cu2I5 nanocrystals. The effect of volumetric flow rate and temperature on the final product's morphology and optical properties are determined, obtaining 21% quantum yield with the optimal configuration. Consequently, the size and morphology of the nanocrystals can be tuned with far more precision and in a much broader range than previously achievable. The flow setup is readily applicable to other relevant nanomaterials. It should enable a rapid determination of a material's potential and subsequently optimize its desired properties for renewable energy generation or efficient optoelectronics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Achieving the goal of generating all of the world's energy via renewable sources and significantly reducing the energy usage will require the development of novel, abundant, nontoxic energy conversion materials. Here, a cost-efficient and scalable continuous flow synthesis of Cs3Cu2I5 nanocrystals is developed as a basis for the rapid advancement of novel nanomaterials. Ideal precursor solutions are obtained through a novel batch synthesis, whose product served as a benchmark for the subsequent flow synthesis. Realizing this setup enabled a reproducible fabrication of Cs3Cu2I5 nanocrystals. The effect of volumetric flow rate and temperature on the final product's morphology and optical properties are determined, obtaining 21% quantum yield with the optimal configuration. Consequently, the size and morphology of the nanocrystals can be tuned with far more precision and in a much broader range than previously achievable. The flow setup is readily applicable to other relevant nanomaterials. It should enable a rapid determination of a material's potential and subsequently optimize its desired properties for renewable energy generation or efficient optoelectronics.
19.
T Lorenzen, B Diederichs, C Ogolla, B Butz, K Müller-Caspary
Consistency and reliability of ptychographic deconvolution approaches Journal Article
In: BIO Web Conf., vol. 129, pp. 04016, 2024.
@article{nokey,
title = {Consistency and reliability of ptychographic deconvolution approaches},
author = {T Lorenzen and B Diederichs and C Ogolla and B Butz and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1051/bioconf/202412904016},
year = {2024},
date = {2024-10-17},
journal = {BIO Web Conf.},
volume = {129},
pages = {04016},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
18.
K Frank, N A Henke, C Lampe, T Lorenzen, B März, X Sun, S Haas, O Gutowski, A-C Dippel, V Mayer, K Müller-Caspary, A S Urban, B Nickel
Antisolvent controls the shape and size of anisotropic lead halide perovskite nanocrystals Journal Article
In: Nature Communications, vol. 15, no. 1, pp. 8952, 2024, ISSN: 2041-1723.
@article{nokey,
title = {Antisolvent controls the shape and size of anisotropic lead halide perovskite nanocrystals},
author = {K Frank and N A Henke and C Lampe and T Lorenzen and B M\"{a}rz and X Sun and S Haas and O Gutowski and A-C Dippel and V Mayer and K M\"{u}ller-Caspary and A S Urban and B Nickel},
url = {https://doi.org/10.1038/s41467-024-53221-5},
doi = {10.1038/s41467-024-53221-5},
issn = {2041-1723},
year = {2024},
date = {2024-10-17},
journal = {Nature Communications},
volume = {15},
number = {1},
pages = {8952},
abstract = {Colloidal lead halide perovskite nanocrystals have potential for lighting applications due to their optical properties. Precise control of the nanocrystal dimensions and composition is a prerequisite for establishing practical applications. However, the rapid nature of their synthesis precludes a detailed understanding of the synthetic pathways, thereby limiting the optimisation. Here, we deduce the formation mechanisms of anisotropic lead halide perovskite nanocrystals, 1D nanorods and 2D nanoplatelets, by combining in situ X-ray scattering and photoluminescence spectroscopy. In both cases, emissive prolate nanoclusters form when the two precursor solutions are mixed. The ensuing antisolvent addition induces the divergent anisotropy: The intermediate nanoclusters are driven into a dense hexagonal mesophase, fusing to form nanorods. Contrastingly, nanoplatelets grow freely dispersed from dissolving nanoclusters, stacking subsequently in lamellar superstructures. Shape and size control of the nanocrystals are determined primarily by the antisolvent’s dipole moment and Hansen hydrogen bonding parameter. Exploiting the interplay of antisolvent and organic ligands could enable more complex nanocrystal geometries in the future.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Colloidal lead halide perovskite nanocrystals have potential for lighting applications due to their optical properties. Precise control of the nanocrystal dimensions and composition is a prerequisite for establishing practical applications. However, the rapid nature of their synthesis precludes a detailed understanding of the synthetic pathways, thereby limiting the optimisation. Here, we deduce the formation mechanisms of anisotropic lead halide perovskite nanocrystals, 1D nanorods and 2D nanoplatelets, by combining in situ X-ray scattering and photoluminescence spectroscopy. In both cases, emissive prolate nanoclusters form when the two precursor solutions are mixed. The ensuing antisolvent addition induces the divergent anisotropy: The intermediate nanoclusters are driven into a dense hexagonal mesophase, fusing to form nanorods. Contrastingly, nanoplatelets grow freely dispersed from dissolving nanoclusters, stacking subsequently in lamellar superstructures. Shape and size control of the nanocrystals are determined primarily by the antisolvent’s dipole moment and Hansen hydrogen bonding parameter. Exploiting the interplay of antisolvent and organic ligands could enable more complex nanocrystal geometries in the future.
17.
T Lorenzen, B Diederichs, C Ogolla, B Butz, K Müller-Caspary
Consistency and reliability of ptychographic deconvolution approaches Journal Article
In: BIO Web Conf., vol. 129, pp. 04016, 2024.
@article{nokey,
title = {Consistency and reliability of ptychographic deconvolution approaches},
author = {T Lorenzen and B Diederichs and C Ogolla and B Butz and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1051/bioconf/202412904016},
year = {2024},
date = {2024-10-17},
journal = {BIO Web Conf.},
volume = {129},
pages = {04016},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
16.
K Müller-Caspary, B Diederichs, Z Herdegen, A Strauch
Structure retrieval by parameterised inverse multislice accounting for partial coherence and thermal effects Journal Article
In: BIO Web Conf., vol. 129, pp. 07009, 2024.
@article{nokey,
title = {Structure retrieval by parameterised inverse multislice accounting for partial coherence and thermal effects},
author = {K M\"{u}ller-Caspary and B Diederichs and Z Herdegen and A Strauch},
url = {https://doi.org/10.1051/bioconf/202412907009},
year = {2024},
date = {2024-10-17},
journal = {BIO Web Conf.},
volume = {129},
pages = {07009},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
15.
Z Herdegen, B Diederichs, K Müller-Caspary
Thermal vibrations in the inversion of dynamical electron scattering Journal Article
In: Physical Review B, vol. 110, no. 6, pp. 064102, 2024.
@article{nokey,
title = {Thermal vibrations in the inversion of dynamical electron scattering},
author = {Z Herdegen and B Diederichs and K M\"{u}ller-Caspary},
url = {https://link.aps.org/doi/10.1103/PhysRevB.110.064102},
doi = {10.1103/PhysRevB.110.064102},
year = {2024},
date = {2024-08-21},
journal = {Physical Review B},
volume = {110},
number = {6},
pages = {064102},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
14.
P Ding, J Kühne, S Santra, R Zell, P Zellner, T Rieth, J Gao, J Chen, G Zhou, J Dittloff, K Müller-Caspary, I D Sharp
Tailoring Microenvironments and In Situ Transformations of Cu Catalysts for Selective and Stable Electrosynthesis of Multicarbon Products Journal Article
In: Advanced Energy Materials, vol. 14, no. 20, pp. 2303936, 2024, ISSN: 1614-6832.
@article{nokey,
title = {Tailoring Microenvironments and In Situ Transformations of Cu Catalysts for Selective and Stable Electrosynthesis of Multicarbon Products},
author = {P Ding and J K\"{u}hne and S Santra and R Zell and P Zellner and T Rieth and J Gao and J Chen and G Zhou and J Dittloff and K M\"{u}ller-Caspary and I D Sharp},
url = {https://doi.org/10.1002/aenm.202303936},
doi = {https://doi.org/10.1002/aenm.202303936},
issn = {1614-6832},
year = {2024},
date = {2024-04-09},
journal = {Advanced Energy Materials},
volume = {14},
number = {20},
pages = {2303936},
abstract = {Abstract Electrochemical CO2 reduction is of tremendous interest for storing chemical energy from renewable sources while reducing CO2 emissions. While copper is one of the most effective catalysts, it suffers from low selectivity and limited long-term durability. Here, these limitations are overcome by engineering Nafion coatings on CuO nanoparticle-based catalysts supported on glassy carbon. By tuning the Nafion thickness and internal structure, it is shown that both the selectivity to multicarbon (C2+) products and long-term stability can be dramatically enhanced. Optimized catalyst layers reach Faradaic efficiencies for C2+ products of 86% during long-term testing for 200 h, with no evidence for performance degradation. Indeed, the C2+ Faradaic efficiency increases during testing, which is attributed to favorable in situ electrochemical fragmentation of catalytic nanoparticles. Finally, the optimized Nafion/Cu catalytic coatings are utilized to create scalable membrane electrode assemblies for CO2 electrolysis, yielding significantly enhanced C2H4 selectivity (≈58%) and activity at technologically-relevant currents of 1?2 A. These results highlight the potential for creating multi-functional Nafion coatings on CO2 reduction catalysts to favorably tune the reaction environment, while also promoting in situ transformations to active and selective nanoscale structures and morphologies, not just on model surfaces but also in state-of-the-art gas diffusion electrodes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Electrochemical CO2 reduction is of tremendous interest for storing chemical energy from renewable sources while reducing CO2 emissions. While copper is one of the most effective catalysts, it suffers from low selectivity and limited long-term durability. Here, these limitations are overcome by engineering Nafion coatings on CuO nanoparticle-based catalysts supported on glassy carbon. By tuning the Nafion thickness and internal structure, it is shown that both the selectivity to multicarbon (C2+) products and long-term stability can be dramatically enhanced. Optimized catalyst layers reach Faradaic efficiencies for C2+ products of 86% during long-term testing for 200 h, with no evidence for performance degradation. Indeed, the C2+ Faradaic efficiency increases during testing, which is attributed to favorable in situ electrochemical fragmentation of catalytic nanoparticles. Finally, the optimized Nafion/Cu catalytic coatings are utilized to create scalable membrane electrode assemblies for CO2 electrolysis, yielding significantly enhanced C2H4 selectivity (≈58%) and activity at technologically-relevant currents of 1?2 A. These results highlight the potential for creating multi-functional Nafion coatings on CO2 reduction catalysts to favorably tune the reaction environment, while also promoting in situ transformations to active and selective nanoscale structures and morphologies, not just on model surfaces but also in state-of-the-art gas diffusion electrodes.
13.
H W Jeong, A Ajay, M Döblinger, S Sturm, M Gómez Ruiz, R Zell, N Mukhundhan, D Stelzner, J Lähnemann, K Müller-Caspary, J J Finley, G Koblmüller
Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices Journal Article
In: ACS Applied Nano Materials, vol. 7, no. 3, pp. 3032-3041, 2024.
@article{nokey,
title = {Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices},
author = {H W Jeong and A Ajay and M D\"{o}blinger and S Sturm and M G\'{o}mez Ruiz and R Zell and N Mukhundhan and D Stelzner and J L\"{a}hnemann and K M\"{u}ller-Caspary and J J Finley and G Koblm\"{u}ller},
url = {https://doi.org/10.1021/acsanm.3c05392},
doi = {10.1021/acsanm.3c05392},
year = {2024},
date = {2024-01-24},
journal = {ACS Applied Nano Materials},
volume = {7},
number = {3},
pages = {3032-3041},
abstract = {III\textendashV semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array\textendashgeometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
III–V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array–geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.
12.
T Lorenzen, B März, T Xue, A Beyer, K Volz, T Bein, K Müller-Caspary
Imaging built-in electric fields and light matter by Fourier-precession TEM Journal Article
In: Scientific Reports, vol. 14, no. 1, pp. 1320, 2024, ISSN: 2045-2322.
@article{nokey,
title = {Imaging built-in electric fields and light matter by Fourier-precession TEM},
author = {T Lorenzen and B M\"{a}rz and T Xue and A Beyer and K Volz and T Bein and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1038/s41598-024-51423-x},
doi = {10.1038/s41598-024-51423-x},
issn = {2045-2322},
year = {2024},
date = {2024-01-15},
journal = {Scientific Reports},
volume = {14},
number = {1},
pages = {1320},
abstract = {We report the precise measurement of electric fields in nanostructures, and high-contrast imaging of soft matter at ultralow electron doses by transmission electron microscopy (TEM). In particular, a versatile method based on the theorem of reciprocity is introduced to enable differential phase contrast imaging and ptychography in conventional, plane-wave illumination TEM. This is realised by a series of TEM images acquired under different tilts, thereby introducing the sampling rate in reciprocal space as a tuneable parameter, in contrast to momentum-resolved scanning techniques. First, the electric field of a p\textendashn junction in GaAs is imaged. Second, low-dose, in-focus ptychographic and DPC characterisation of Kagome pores in weakly scattering covalent organic frameworks is demonstrated by using a precessing electron beam in combination with a direct electron detector. The approach offers utmost flexibility to record relevant spatial frequencies selectively, while acquisition times and dose requirements are significantly reduced compared to the 4D-STEM counterpart.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We report the precise measurement of electric fields in nanostructures, and high-contrast imaging of soft matter at ultralow electron doses by transmission electron microscopy (TEM). In particular, a versatile method based on the theorem of reciprocity is introduced to enable differential phase contrast imaging and ptychography in conventional, plane-wave illumination TEM. This is realised by a series of TEM images acquired under different tilts, thereby introducing the sampling rate in reciprocal space as a tuneable parameter, in contrast to momentum-resolved scanning techniques. First, the electric field of a p–n junction in GaAs is imaged. Second, low-dose, in-focus ptychographic and DPC characterisation of Kagome pores in weakly scattering covalent organic frameworks is demonstrated by using a precessing electron beam in combination with a direct electron detector. The approach offers utmost flexibility to record relevant spatial frequencies selectively, while acquisition times and dose requirements are significantly reduced compared to the 4D-STEM counterpart.
11.
B Diederichs, Z Herdegen, A Strauch, F Filbir, K Müller-Caspary
Exact inversion of partially coherent dynamical electron scattering for picometric structure retrieval Journal Article
In: Nature Communications, vol. 15, no. 1, pp. 101, 2024, ISSN: 2041-1723.
@article{nokey,
title = {Exact inversion of partially coherent dynamical electron scattering for picometric structure retrieval},
author = {B Diederichs and Z Herdegen and A Strauch and F Filbir and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1038/s41467-023-44268-x},
doi = {10.1038/s41467-023-44268-x},
issn = {2041-1723},
year = {2024},
date = {2024-01-02},
journal = {Nature Communications},
volume = {15},
number = {1},
pages = {101},
abstract = {The greatly nonlinear diffraction of high-energy electron probes focused to subatomic diameters frustrates the direct inversion of ptychographic data sets to decipher the atomic structure. Several iterative algorithms have been proposed to yield atomically-resolved phase distributions within slices of a 3D specimen, corresponding to the scattering centers of the electron wave. By pixelwise phase retrieval, current approaches do not only involve orders of magnitude more free parameters than necessary, but also neglect essential details of scattering physics such as the atomistic nature of the specimen and thermal effects. Here, we introduce a parametrized, fully differentiable scheme employing neural network concepts which allows the inversion of ptychographic data by means of entirely physical quantities. Omnipresent thermal diffuse scattering in thick specimens is treated accurately using frozen phonons, and atom types, positions and partial coherence are accounted for in the inverse model as relativistic scattering theory demands. Our approach exploits 4D experimental data collected in an aberration-corrected momentum-resolved scanning transmission electron microscopy setup. Atom positions in a 20 nm thick PbZr0.2Ti0.8O3 ferroelectric are measured with picometer precision, including the discrimination of different atom types and positions in mixed columns.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The greatly nonlinear diffraction of high-energy electron probes focused to subatomic diameters frustrates the direct inversion of ptychographic data sets to decipher the atomic structure. Several iterative algorithms have been proposed to yield atomically-resolved phase distributions within slices of a 3D specimen, corresponding to the scattering centers of the electron wave. By pixelwise phase retrieval, current approaches do not only involve orders of magnitude more free parameters than necessary, but also neglect essential details of scattering physics such as the atomistic nature of the specimen and thermal effects. Here, we introduce a parametrized, fully differentiable scheme employing neural network concepts which allows the inversion of ptychographic data by means of entirely physical quantities. Omnipresent thermal diffuse scattering in thick specimens is treated accurately using frozen phonons, and atom types, positions and partial coherence are accounted for in the inverse model as relativistic scattering theory demands. Our approach exploits 4D experimental data collected in an aberration-corrected momentum-resolved scanning transmission electron microscopy setup. Atom positions in a 20 nm thick PbZr0.2Ti0.8O3 ferroelectric are measured with picometer precision, including the discrimination of different atom types and positions in mixed columns.
10.
J F Dushimineza, J Jo, R E Dunin-Borkowski, K Müller-Caspary
Quantitative electric field mapping between electrically biased needles by scanning transmission electron microscopy and electron holography Journal Article
In: Ultramicroscopy, vol. 253, pp. 113808, 2023, ISSN: 0304-3991.
@article{nokey,
title = {Quantitative electric field mapping between electrically biased needles by scanning transmission electron microscopy and electron holography},
author = {J F Dushimineza and J Jo and R E Dunin-Borkowski and K M\"{u}ller-Caspary},
url = {https://www.sciencedirect.com/science/article/pii/S0304399123001250},
doi = {https://doi.org/10.1016/j.ultramic.2023.113808},
issn = {0304-3991},
year = {2023},
date = {2023-07-04},
journal = {Ultramicroscopy},
volume = {253},
pages = {113808},
abstract = {Stray electric fields in free space generated by two biased gold needles have been quantified in comprehensive finite-element (FE) simulations, accompanied by first moment (FM) scanning TEM (STEM) and electron holography (EH) experiments. The projected electrostatic potential and electric field have been derived numerically under geometrical variations of the needle setup. In contrast to the FE simulation, application of an analytical model based on line charges yields a qualitative understanding. By experimentally probing the electric field employing FM STEM and EH under alike conditions, a discrepancy of about 60% became apparent initially. However, the EH setup suggests the reconstructed phase to be significantly affected by the perturbed reference wave effect, opposite to STEM where the field-free reference was recorded subsequently with unbiased needles in which possibly remaining electrostatic influences are regarded as being minor. In that respect, the observed discrepancy between FM imaging and EH is resolved after including the long-range potential landscape from FE simulations into the phase of the reference wave in EH.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Stray electric fields in free space generated by two biased gold needles have been quantified in comprehensive finite-element (FE) simulations, accompanied by first moment (FM) scanning TEM (STEM) and electron holography (EH) experiments. The projected electrostatic potential and electric field have been derived numerically under geometrical variations of the needle setup. In contrast to the FE simulation, application of an analytical model based on line charges yields a qualitative understanding. By experimentally probing the electric field employing FM STEM and EH under alike conditions, a discrepancy of about 60% became apparent initially. However, the EH setup suggests the reconstructed phase to be significantly affected by the perturbed reference wave effect, opposite to STEM where the field-free reference was recorded subsequently with unbiased needles in which possibly remaining electrostatic influences are regarded as being minor. In that respect, the observed discrepancy between FM imaging and EH is resolved after including the long-range potential landscape from FE simulations into the phase of the reference wave in EH.
9.
M L Leidl, C Sachse, K Muller-Caspary
Dynamical scattering in ice-embedded proteins in conventional and scanning transmission electron microscopy Journal Article
In: IUCrJ, vol. 10, no. 4, pp. 475-486, 2023, ISSN: 2052-2525.
@article{nokey,
title = {Dynamical scattering in ice-embedded proteins in conventional and scanning transmission electron microscopy},
author = {M L Leidl and C Sachse and K Muller-Caspary},
url = {https://doi.org/10.1107/S2052252523004505},
doi = {doi:10.1107/S2052252523004505},
issn = {2052-2525},
year = {2023},
date = {2023-07-01},
journal = {IUCrJ},
volume = {10},
number = {4},
pages = {475-486},
abstract = {Structure determination of biological macromolecules using cryogenic electron microscopy is based on applying the phase object (PO) assumption and the weak phase object (WPO) approximation to reconstruct the 3D potential density of the molecule. To enhance the understanding of image formation of protein complexes embedded in glass-like ice in a transmission electron microscope, this study addresses multiple scattering in tobacco mosaic virus (TMV) specimens. This includes the propagation inside the molecule while also accounting for the effect of structural noise. The atoms in biological macromolecules are light but are distributed over several nanometres. Commonly, PO and WPO approximations are used in most simulations and reconstruction models. Therefore, dynamical multislice simulations of TMV specimens embedded in glass-like ice were performed based on fully atomistic molecular-dynamics simulations. In the first part, the impact of multiple scattering is studied using different numbers of slices. In the second part, different sample thicknesses of the ice-embedded TMV are considered in terms of additional ice layers. It is found that single-slice models yield full frequency transfer up to a resolution of 2.5 A, followed by attenuation up to 1.4 A. Three slices are sufficient to reach an information transfer up to 1.0 A. In the third part, ptychographic reconstructions based on scanning transmission electron microscopy (STEM) and single-slice models are compared with conventional TEM simulations. The ptychographic reconstructions do not need the deliberate introduction of aberrations, are capable of post-acquisition aberration correction and promise benefits for information transfer, especially at resolutions beyond 1.8 A.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Structure determination of biological macromolecules using cryogenic electron microscopy is based on applying the phase object (PO) assumption and the weak phase object (WPO) approximation to reconstruct the 3D potential density of the molecule. To enhance the understanding of image formation of protein complexes embedded in glass-like ice in a transmission electron microscope, this study addresses multiple scattering in tobacco mosaic virus (TMV) specimens. This includes the propagation inside the molecule while also accounting for the effect of structural noise. The atoms in biological macromolecules are light but are distributed over several nanometres. Commonly, PO and WPO approximations are used in most simulations and reconstruction models. Therefore, dynamical multislice simulations of TMV specimens embedded in glass-like ice were performed based on fully atomistic molecular-dynamics simulations. In the first part, the impact of multiple scattering is studied using different numbers of slices. In the second part, different sample thicknesses of the ice-embedded TMV are considered in terms of additional ice layers. It is found that single-slice models yield full frequency transfer up to a resolution of 2.5 A, followed by attenuation up to 1.4 A. Three slices are sufficient to reach an information transfer up to 1.0 A. In the third part, ptychographic reconstructions based on scanning transmission electron microscopy (STEM) and single-slice models are compared with conventional TEM simulations. The ptychographic reconstructions do not need the deliberate introduction of aberrations, are capable of post-acquisition aberration correction and promise benefits for information transfer, especially at resolutions beyond 1.8 A.
8.
Z Li, F Tabataba-Vakili, S Zhao, A Rupp, I Bilgin, Z Herdegen, B März, K Watanabe, T Taniguchi, G R Schleder, A S Baimuratov, E Kaxiras, K Müller-Caspary, A Högele
Lattice Reconstruction in MoSe2–WSe2 Heterobilayers Synthesized by Chemical Vapor Deposition Journal Article
In: Nano Letters, vol. 23, no. 10, pp. 4160-4166, 2023, ISSN: 1530-6984.
@article{nokey,
title = {Lattice Reconstruction in MoSe2\textendashWSe2 Heterobilayers Synthesized by Chemical Vapor Deposition},
author = {Z Li and F Tabataba-Vakili and S Zhao and A Rupp and I Bilgin and Z Herdegen and B M\"{a}rz and K Watanabe and T Taniguchi and G R Schleder and A S Baimuratov and E Kaxiras and K M\"{u}ller-Caspary and A H\"{o}gele},
url = {https://doi.org/10.1021/acs.nanolett.2c05094},
doi = {10.1021/acs.nanolett.2c05094},
issn = {1530-6984},
year = {2023},
date = {2023-05-24},
journal = {Nano Letters},
volume = {23},
number = {10},
pages = {4160-4166},
abstract = {Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moir\'{e} systems with rich correlated electron phases and moir\'{e} exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe2\textendashWSe2, however, lattice reconstruction eliminates the canonical moir\'{e} pattern and instead gives rise to arrays of periodically reconstructed nanoscale domains and mesoscopically extended areas of one atomic registry. Here, we elucidate the role of atomic reconstruction in MoSe2\textendashWSe2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moir\'{e}-type cores and extended moir\'{e}-free regions in heterostacks with parallel and antiparallel alignment. Our work highlights the potential of chemical vapor deposition for applications requiring laterally extended heterosystems of one atomic registry or exciton-confining heterostack arrays.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moiré systems with rich correlated electron phases and moiré exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe2–WSe2, however, lattice reconstruction eliminates the canonical moiré pattern and instead gives rise to arrays of periodically reconstructed nanoscale domains and mesoscopically extended areas of one atomic registry. Here, we elucidate the role of atomic reconstruction in MoSe2–WSe2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moiré-type cores and extended moiré-free regions in heterostacks with parallel and antiparallel alignment. Our work highlights the potential of chemical vapor deposition for applications requiring laterally extended heterosystems of one atomic registry or exciton-confining heterostack arrays.
7.
J Zweck, F Schwarzhuber, S Pöllath, K Müller-Caspary
Advanced processing of differential phase contrast data: Distinction between different causes of electron phase shifts Journal Article
In: Ultramicroscopy, vol. 250, pp. 113752, 2023, ISSN: 0304-3991.
@article{nokey,
title = {Advanced processing of differential phase contrast data: Distinction between different causes of electron phase shifts},
author = {J Zweck and F Schwarzhuber and S P\"{o}llath and K M\"{u}ller-Caspary},
url = {https://www.sciencedirect.com/science/article/pii/S0304399123000694},
doi = {https://doi.org/10.1016/j.ultramic.2023.113752},
issn = {0304-3991},
year = {2023},
date = {2023-05-12},
journal = {Ultramicroscopy},
volume = {250},
pages = {113752},
abstract = {Differential phase contrast, in its high resolution modification also known as first moment microscopy or momentum resolved STEM [1], [2], [3], [4], [5], [6], [7] , basically measures the lateral momentum transfer to the electron probe due to the beam interaction with either electrostatic and/or magnetic fields, when the probe transmits the specimen. In other words, the result of the measurement is a vector field p→(x,y) which describes the lateral momentum transfer to the probe electrons. In the case of electric fields, this momentum transfer is easily converted to the electric field E→(x,y) causing the deflection, and from ϱ=ɛ0∇⋅E→ the local charge density can be calculated from the divergence of the electric field. However, from experimental data it is known that also the calculation of the vector field’s curl ∇→×p→ in general yields non-zero results. In this paper, we use the Helmholtz decomposition (Wikipedia contributors, 2022), also known as the fundamental theorem of vector calculus, to split the measured vector fields into their curl-free and divergence-free components and to interpret the physical meaning of these components in detail. It will be shown, that non-zero curl components may be used to measure geometric phases occurring from irregularities in crystal structure such as a screw dislocation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Differential phase contrast, in its high resolution modification also known as first moment microscopy or momentum resolved STEM [1], [2], [3], [4], [5], [6], [7] , basically measures the lateral momentum transfer to the electron probe due to the beam interaction with either electrostatic and/or magnetic fields, when the probe transmits the specimen. In other words, the result of the measurement is a vector field p→(x,y) which describes the lateral momentum transfer to the probe electrons. In the case of electric fields, this momentum transfer is easily converted to the electric field E→(x,y) causing the deflection, and from ϱ=ɛ0∇⋅E→ the local charge density can be calculated from the divergence of the electric field. However, from experimental data it is known that also the calculation of the vector field’s curl ∇→×p→ in general yields non-zero results. In this paper, we use the Helmholtz decomposition (Wikipedia contributors, 2022), also known as the fundamental theorem of vector calculus, to split the measured vector fields into their curl-free and divergence-free components and to interpret the physical meaning of these components in detail. It will be shown, that non-zero curl components may be used to measure geometric phases occurring from irregularities in crystal structure such as a screw dislocation.
6.
N Aspiotis, K Morgan, B März, K Müller-Caspary, M Ebert, E Weatherby, M E Light, C-C Huang, D W Hewak, S Majumdar, I Zeimpekis
Large-area synthesis of high electrical performance MoS2 by a commercially scalable atomic layer deposition process Journal Article
In: npj 2D Materials and Applications, vol. 7, no. 1, pp. 18, 2023, ISSN: 2397-7132.
@article{nokey,
title = {Large-area synthesis of high electrical performance MoS2 by a commercially scalable atomic layer deposition process},
author = {N Aspiotis and K Morgan and B M\"{a}rz and K M\"{u}ller-Caspary and M Ebert and E Weatherby and M E Light and C-C Huang and D W Hewak and S Majumdar and I Zeimpekis},
url = {https://doi.org/10.1038/s41699-023-00379-z},
doi = {10.1038/s41699-023-00379-z},
issn = {2397-7132},
year = {2023},
date = {2023-03-27},
journal = {npj 2D Materials and Applications},
volume = {7},
number = {1},
pages = {18},
abstract = {This work demonstrates a large area process for atomically thin 2D semiconductors to unlock the technological upscale required for their commercial uptake. The new atomic layer deposition (ALD) and conversion technique yields large area performance uniformity and tunability. Like graphene, 2D Transition Metal Dichalcogenides (TMDCs) are prone to upscaling challenges limiting their commercial uptake. They are challenging to grow uniformly on large substrates and to transfer on alternative substrates while they often lack in large area electrical performance uniformity. The scalable ALD process of this work enables uniform growth of 2D TMDCs on large area with independent control of layer thickness, stoichiometry and crystallinity while allowing chemical free transfers to application substrates. Field effect transistors (FETs) fabricated on flexible substrates using the process present a field effect mobility of up to 55 cm2/Vs, subthreshold slope down to 80 mV/dec and on/off ratios of 107. In addition, non-volatile memory transistors using ferroelectric FETs (FeFETs) operating at ±5 V with on/off ratio of 107 and a memory window of 3.25 V are demonstrated. These FeFETs demonstrate state-of-the-art performance with multiple state switching, suitable for one-transistor non-volatile memory and for synaptic transistors revealing the applicability of the process to flexible neuromorphic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This work demonstrates a large area process for atomically thin 2D semiconductors to unlock the technological upscale required for their commercial uptake. The new atomic layer deposition (ALD) and conversion technique yields large area performance uniformity and tunability. Like graphene, 2D Transition Metal Dichalcogenides (TMDCs) are prone to upscaling challenges limiting their commercial uptake. They are challenging to grow uniformly on large substrates and to transfer on alternative substrates while they often lack in large area electrical performance uniformity. The scalable ALD process of this work enables uniform growth of 2D TMDCs on large area with independent control of layer thickness, stoichiometry and crystallinity while allowing chemical free transfers to application substrates. Field effect transistors (FETs) fabricated on flexible substrates using the process present a field effect mobility of up to 55 cm2/Vs, subthreshold slope down to 80 mV/dec and on/off ratios of 107. In addition, non-volatile memory transistors using ferroelectric FETs (FeFETs) operating at ±5 V with on/off ratio of 107 and a memory window of 3.25 V are demonstrated. These FeFETs demonstrate state-of-the-art performance with multiple state switching, suitable for one-transistor non-volatile memory and for synaptic transistors revealing the applicability of the process to flexible neuromorphic applications.
5.
M Alonso-Orts, R Hötzel, T Grieb, M Auf Der Maur, M Ries, F Nippert, B März, K Müller-Caspary, M R Wagner, A Rosenauer, M Eickhoff
Correlative analysis on InGaN/GaN nanowires: structural and optical properties of self-assembled short-period superlattices Journal Article
In: Discover Nano, vol. 18, no. 1, pp. 27, 2023, ISSN: 2731-9229.
@article{nokey,
title = {Correlative analysis on InGaN/GaN nanowires: structural and optical properties of self-assembled short-period superlattices},
author = {M Alonso-Orts and R H\"{o}tzel and T Grieb and M Auf Der Maur and M Ries and F Nippert and B M\"{a}rz and K M\"{u}ller-Caspary and M R Wagner and A Rosenauer and M Eickhoff},
url = {https://doi.org/10.1186/s11671-023-03808-6},
doi = {10.1186/s11671-023-03808-6},
issn = {2731-9229},
year = {2023},
date = {2023-03-01},
journal = {Discover Nano},
volume = {18},
number = {1},
pages = {27},
abstract = {The influence of self-assembled short-period superlattices (SPSLs) on the structural and optical properties of InGaN/GaN nanowires (NWs) grown by PAMBE on Si (111) was investigated by STEM, EDXS, µ-PL analysis and k·p simulations. STEM analysis on single NWs indicates that in most of the studied nanostructures, SPSLs self-assemble during growth. The SPSLs display short-range ordering of In-rich and In-poor InxGa1-xN regions with a period of 2\textendash3 nm that are covered by a GaN shell and that transition to a more homogenous InxGa1-xN core. Polarization- and temperature-resolved PL analysis performed on the same NWs shows that they exhibit a strong parallel polarized red-yellow emission and a predominantly perpendicular polarized blue emission, which are ascribed to different In-rich regions in the nanostructures. The correlation between STEM, µ-PL and k·p simulations provides better understanding of the rich optical emission of complex III-N nanostructures and how they are impacted by structural properties, yielding the significant impact of strain on self-assembly and spectral emission.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The influence of self-assembled short-period superlattices (SPSLs) on the structural and optical properties of InGaN/GaN nanowires (NWs) grown by PAMBE on Si (111) was investigated by STEM, EDXS, µ-PL analysis and k·p simulations. STEM analysis on single NWs indicates that in most of the studied nanostructures, SPSLs self-assemble during growth. The SPSLs display short-range ordering of In-rich and In-poor InxGa1-xN regions with a period of 2–3 nm that are covered by a GaN shell and that transition to a more homogenous InxGa1-xN core. Polarization- and temperature-resolved PL analysis performed on the same NWs shows that they exhibit a strong parallel polarized red-yellow emission and a predominantly perpendicular polarized blue emission, which are ascribed to different In-rich regions in the nanostructures. The correlation between STEM, µ-PL and k·p simulations provides better understanding of the rich optical emission of complex III-N nanostructures and how they are impacted by structural properties, yielding the significant impact of strain on self-assembly and spectral emission.
4.
A Strauch, B März, T Denneulin, M Cattaneo, A Rosenauer, K Müller-Caspary
Systematic Errors of Electric Field Measurements in Ferroelectrics by Unit Cell Averaged Momentum Transfers in STEM Journal Article
In: Microscopy and Microanalysis, vol. 29, no. 2, pp. 499-511, 2023, ISSN: 1431-9276.
@article{nokey,
title = {Systematic Errors of Electric Field Measurements in Ferroelectrics by Unit Cell Averaged Momentum Transfers in STEM},
author = {A Strauch and B M\"{a}rz and T Denneulin and M Cattaneo and A Rosenauer and K M\"{u}ller-Caspary},
url = {https://doi.org/10.1093/micmic/ozad016},
doi = {10.1093/micmic/ozad016},
issn = {1431-9276},
year = {2023},
date = {2023-02-23},
journal = {Microscopy and Microanalysis},
volume = {29},
number = {2},
pages = {499-511},
abstract = {When using the unit cell average of first moment data from four-dimensional scanning transmission electron microscopy (4D-STEM) to characterize ferroelectric materials, a variety of sources of systematic errors needs to be taken into account. In particular, these are the magnitude of the acceleration voltage, STEM probe semi-convergence angle, sample thickness, and sample tilt out of zone axis. Simulations show that a systematic error of calculated electric fields using the unit cell averaged momentum transfer originates from violation of point symmetry within the unit cells. Thus, values can easily exceed those of potential polarization-induced electric fields in ferroelectrics. Importantly, this systematic error produces deflection gradients between different domains seemingly representing measured fields. However, it could be shown that for PbZr0.2Ti0.8O3, many adjacent domains exhibit a relative crystallographic mistilt and in-plane rotation. The experimental results show that the method gives qualitative domain contrast. Comparison of the calculated electric field with the systematic error showed that the domain contrast of the unit cell averaged electric fields is mainly caused by dynamical scattering effects and the electric field plays only a minor role, if present at all.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
When using the unit cell average of first moment data from four-dimensional scanning transmission electron microscopy (4D-STEM) to characterize ferroelectric materials, a variety of sources of systematic errors needs to be taken into account. In particular, these are the magnitude of the acceleration voltage, STEM probe semi-convergence angle, sample thickness, and sample tilt out of zone axis. Simulations show that a systematic error of calculated electric fields using the unit cell averaged momentum transfer originates from violation of point symmetry within the unit cells. Thus, values can easily exceed those of potential polarization-induced electric fields in ferroelectrics. Importantly, this systematic error produces deflection gradients between different domains seemingly representing measured fields. However, it could be shown that for PbZr0.2Ti0.8O3, many adjacent domains exhibit a relative crystallographic mistilt and in-plane rotation. The experimental results show that the method gives qualitative domain contrast. Comparison of the calculated electric field with the systematic error showed that the domain contrast of the unit cell averaged electric fields is mainly caused by dynamical scattering effects and the electric field plays only a minor role, if present at all.
3.
H L Robert, B Diederichs, K Müller-Caspary
Contribution of multiple plasmon scattering in low-angle electron diffraction investigated by energy-filtered atomically resolved 4D-STEM Journal Article
In: Applied Physics Letters, vol. 121, no. 21, pp. 213502, 2022.
@article{nokey,
title = {Contribution of multiple plasmon scattering in low-angle electron diffraction investigated by energy-filtered atomically resolved 4D-STEM},
author = {H L Robert and B Diederichs and K M\"{u}ller-Caspary},
url = {https://aip.scitation.org/doi/abs/10.1063/5.0129692},
doi = {10.1063/5.0129692},
year = {2022},
date = {2022-10-06},
journal = {Applied Physics Letters},
volume = {121},
number = {21},
pages = {213502},
abstract = {We report the influence of multiple plasmon losses on the dynamical diffraction of high-energy electrons, in a scanning transmission electron microscopy (STEM) study. Using an experimental setup enabling energy-filtered momentum-resolved STEM, it is shown that the successive excitation of up to five plasmons within the imaged material results in a subsequent and significant redistribution of low-angle intensity in diffraction space. An empirical approach, based on the convolution with a Lorentzian kernel, is shown to reliably model this redistribution in dependence of the energy-loss. Our study demonstrates that both the significant impact of inelastic scattering in low-angle diffraction at elevated specimen thickness and a rather straightforward model can be applied to mimic multiple plasmon scattering, which otherwise is currently not within reach for multislice simulations due to computational complexity.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We report the influence of multiple plasmon losses on the dynamical diffraction of high-energy electrons, in a scanning transmission electron microscopy (STEM) study. Using an experimental setup enabling energy-filtered momentum-resolved STEM, it is shown that the successive excitation of up to five plasmons within the imaged material results in a subsequent and significant redistribution of low-angle intensity in diffraction space. An empirical approach, based on the convolution with a Lorentzian kernel, is shown to reliably model this redistribution in dependence of the energy-loss. Our study demonstrates that both the significant impact of inelastic scattering in low-angle diffraction at elevated specimen thickness and a rather straightforward model can be applied to mimic multiple plasmon scattering, which otherwise is currently not within reach for multislice simulations due to computational complexity.
2.
A Bangun, O Melnyk, B März, B Diederichs, A Clausen, D Weber, F Filbir, K Müller-Caspary
Inverse Multislice Ptychography by Layer-wise Optimisation and Sparse Matrix Decomposition Journal Article
In: arXiv preprint arXiv:2205.03902, 2022.
@article{nokey,
title = {Inverse Multislice Ptychography by Layer-wise Optimisation and Sparse Matrix Decomposition},
author = {A Bangun and O Melnyk and B M\"{a}rz and B Diederichs and A Clausen and D Weber and F Filbir and K M\"{u}ller-Caspary},
url = {https://arxiv.org/abs/2205.03902},
doi = {https://doi.org/10.48550/arXiv.2205.03902},
year = {2022},
date = {2022-05-08},
journal = {arXiv preprint arXiv:2205.03902},
abstract = {We propose algorithms based on an optimisation method for inverse multislice ptychography in, e.g. electron microscopy. The multislice method is widely used to model the interaction between relativistic electrons and thick specimens. Since only the intensity of diffraction patterns can be recorded, the challenge in applying inverse multislice ptychography is to uniquely reconstruct the electrostatic potential in each slice up to some ambiguities. In this conceptual study, we show that a unique separation of atomic layers for simulated data is possible when considering a low acceleration voltage. We also introduce an adaptation for estimating the illuminating probe. For the sake of practical application, we finally present slice reconstructions using experimental 4D scanning transmission electron microscopy (STEM) data.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We propose algorithms based on an optimisation method for inverse multislice ptychography in, e.g. electron microscopy. The multislice method is widely used to model the interaction between relativistic electrons and thick specimens. Since only the intensity of diffraction patterns can be recorded, the challenge in applying inverse multislice ptychography is to uniquely reconstruct the electrostatic potential in each slice up to some ambiguities. In this conceptual study, we show that a unique separation of atomic layers for simulated data is possible when considering a low acceleration voltage. We also introduce an adaptation for estimating the illuminating probe. For the sake of practical application, we finally present slice reconstructions using experimental 4D scanning transmission electron microscopy (STEM) data.
1.
N Aspiotis, K Morgan, B März, K Müller-Caspary, M Ebert, C-C Huang, D W Hewak, S Majumdar, I Zeimpekis
Scalable, Highly Crystalline, 2D Semiconductor Atomic Layer Deposition Process for High Performance Electronic Applications Journal Article
In: arXiv preprint arXiv:2203.10309, 2022.
@article{nokey,
title = {Scalable, Highly Crystalline, 2D Semiconductor Atomic Layer Deposition Process for High Performance Electronic Applications},
author = {N Aspiotis and K Morgan and B M\"{a}rz and K M\"{u}ller-Caspary and M Ebert and C-C Huang and D W Hewak and S Majumdar and I Zeimpekis},
url = {https://arxiv.org/abs/2203.10309},
doi = {https://doi.org/10.48550/arXiv.2203.10309},
year = {2022},
date = {2022-03-19},
journal = {arXiv preprint arXiv:2203.10309},
abstract = {This work demonstrates a large area process for atomically thin 2D semiconductors to unlock the technological upscale required for their commercial uptake. The new atomic layer deposition (ALD) and conversion technique yields large area performance uniformity and tunability. Like graphene, 2D Transition Metal Dichalcogenides (TMDCs) are prone to upscaling challenges limiting their commercial uptake. They are challenging to grow uniformly on large substrates and to transfer on alternative substrates while they often lack in large area electrical performance uniformity. The scalable ALD process of this work enables uniform growth of 2D TMDCs on large area with independent control of layer thickness, stoichiometry and crystallinity while allowing chemical free transfers to application substrates. Field effect transistors (FETs) fabricated on flexible substrates using the process present a field effect mobility of up to 55 cm^2/Vs, subthreshold slope down to 80 mV/dec and on/off ratios of 10^7. Additionally, non-volatile memory transistors using ferroelectric FETs (FeFETs) operating at +-5 V with on/off ratio of 107 and a memory window of 3.25 V are demonstrated. These FeFETs demonstrate state-of-the-art performance with multiple state switching, suitable for one-transistor non-volatile memory and for synaptic transistors revealing the applicability of the process to flexible neuromorphic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This work demonstrates a large area process for atomically thin 2D semiconductors to unlock the technological upscale required for their commercial uptake. The new atomic layer deposition (ALD) and conversion technique yields large area performance uniformity and tunability. Like graphene, 2D Transition Metal Dichalcogenides (TMDCs) are prone to upscaling challenges limiting their commercial uptake. They are challenging to grow uniformly on large substrates and to transfer on alternative substrates while they often lack in large area electrical performance uniformity. The scalable ALD process of this work enables uniform growth of 2D TMDCs on large area with independent control of layer thickness, stoichiometry and crystallinity while allowing chemical free transfers to application substrates. Field effect transistors (FETs) fabricated on flexible substrates using the process present a field effect mobility of up to 55 cm^2/Vs, subthreshold slope down to 80 mV/dec and on/off ratios of 10^7. Additionally, non-volatile memory transistors using ferroelectric FETs (FeFETs) operating at +-5 V with on/off ratio of 107 and a memory window of 3.25 V are demonstrated. These FeFETs demonstrate state-of-the-art performance with multiple state switching, suitable for one-transistor non-volatile memory and for synaptic transistors revealing the applicability of the process to flexible neuromorphic applications.