IUCrJ 13 (2026): 282–290 

https://doi.org/10.1107/S2052252526002782

Understanding structure at the atomic scale is fundamental for understanding the functioning and the development of materials with improved properties. Compared with other probes providing atomic resolution, electrons offer the strongest interaction in combination with minimal radiation damage, which makes them an ideal tool for investigating very small and radiation-sensitive samples [Henderson (1995), Q. Rev. Biophys. 28, 171–193]. However, these benefits are often offset by the laborious preparation of nanometre-sized samples that are not visible using a light microscope, and the fact that experiments are largely restricted to ultra-high vacuum [Duyvesteyn et al. (2018), Proc. Natl Acad. Sci. USA 115, 9569–9573; Gruene et al. (2021), Nat. Rev. Chem. 5, 660–668]. Here, we report the successful implementation of MeV electron diffraction for ab initio 3D structure determination of the quasi-2D material muscovite and the quantum material 1T-TaS2 at atomic resolution. By employing ultrashort electron pulses from the REGAE (Relativistic electron gun for atomic exploration) accelerator, we obtained high-quality diffraction datasets suitable for structural refinements based on dynamical scattering theory, enabling precise localization of even hydrogen atoms. The increased penetration depth of MeV electrons significantly expands the applicable thickness range of samples, overcoming previous restrictions associated with traditional electron diffraction. These findings establish MeVelectron diffraction as a viable approach for investigating a broad range of materials, including nanostructures and radiation-sensitive compounds, and open up new opportunities for in situ and time-resolved experiments [Chao et al. (2023), Chem. Rev. 123, 8347–8394; Filippetto et al. (2022), Rev. Mod. Phys. 94, 045004].

Journal of Solid State Chemistry 359 (2026): 126008

https://doi.org/10.1016/j.jssc.2026.126008

Each member of the mullite-type Pb₂(Pb_1-xSn_x)O₄ solid-solution can be described with the schafarzikite structure type. The endmember Pb₂PbO₄ (x = 0) crystallizes in space group P4₂/mbc at ambient conditions, whereas the orthorhombic subgroup Pbam is needed for the structural description of the phases for x ≥ 0.3. On cooling, Pb₂PbO₄ undergoes a P4₂/mbc → Pbam phase transition between 171(1) K and 174(4) K confirmed by FTIR and Raman data analysis, respectively. On heating, the other endmember Pb₂SnO₄ (x = 1) shows the reverse Pbam → P4₂/mbc phase transition at 1221(19) K, confirmed by a Landau-type fit of the lattice parameter difference ∆_ab . During thermogravimetric analysis Pb₂PbO₄ decomposes into PbO and oxygen. However, samples with 0.3 < x < 0.8 undergo an exsolution of PbO along with an enrichment of the Sn-content in the respective mullite-type phases. Due to slow decomposition kinetics several exsolution steps are visible in the temperature-dependent unit-cell volumes for different stochiometric x-values. For all phases, the lattice thermal expansion was modeled using the Debye-Einstein-Anharmonicity-Gliding (DEAG) approach. A single Debye term well describes the internal energy contribution followed by different phase-change properties. The initial phase-change temperatures can be derived from the respective fits of the gliding components. The stoichiometries of the intermediate phases are estimated from the calculated 0K unit-cell volume of the DEAG model. Based on the structure analysis a polyhedral distortion sum was calculated from the weighted octahedral distortion of Pb⁴⁺O₆ and the Wang-Liebau-eccentricity parameter of Pb²⁺O₄E nido-pyramids. Whereas the latter one does not play an important role for the temperature-dependent behavior, the structural distortion originating from the size difference of the cations on the octahedra position is found to be the main reason for the miscibility-gap of the solid solution for 0.0 < x < 0.3, as can be derived from the sum distortion parameters relative to their structural average.

Surface Science767 (2026): 122927

https://doi.org/10.1016/j.susc.2025.122927

Here we report an investigation of ultrathin tin oxide films on Pt₃Sn(111) using low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (μ-LEED), scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and high-resolution X-ray photoelectron spectroscopy (XPS). Oxidation at ~390–410 °C produces triangular, two-dimensional oxide islands that nucleate rapidly and exhibit self-limited lateral growth, attributed to limited Sn diffusion from the subsurface of the crystal. μ-LEED shows that the initially formed (4 X 4) Sn oxide is subsequently converted to a more oxygen-rich (2 X 2n) “stripe” phase. At 630 °C , enhanced Sn mobility enables a closed (4 X 4) film. The (2 X 2n) phase is shown to consist of a (2 X 2) Sn lattice modulated by 1D stripe defects with spacings of n= 4–6 atomic rows; LEED and SXRD measurements show diffraction features corresponding to this striped superstructure. The two oxides can be distinguished in XPS by their O 1s lineshapes: the (4 X 4) phase shows a clear doublet attributable to distinct O species, whereas the (2 X 2n) phase exhibits a broader envelope consistent with a distribution of O coordination environments. The Sn 3d⁵ᐟ₂ spectra are similar for both phases, reflecting closely related Sn bonding motifs. The spectra are consistent with those of previous near-ambient-pressure XPS measurements, suggesting that the surface oxides forming under CO oxidation conditions are similar to those studied here.

Geochimica et Cosmochimica Acta (2026): S0016703726001365

https://doi.org/10.1016/j.gca.2026.02.043

Sedimentary basins are at the interface between deep (lithosphere) and superficial (hydrosphere, atmosphere and biosphere) reservoirs, with the capacity to trap large quantities of volatile elements (e.g. S, C, H, Cl, F) over time through sedimentation and diagenesis. These volatiles can be released abruptly due to contact metamorphism associated to the emplacement of magmatic intrusions in the sediments, further disturbing the geochemical cycles in the atmosphere and biosphere. The interactions between magma and sediments during sill emplacement are complex and include contact metamorphism and contamination processes, but also the assimilation of sediments into the sill. In this study, we examine the impact of magma-sediment interactions on the sulphur cycle. The Ringvent sill in the Guaymas Basin drilled during Expedition IODP 385, is a funnel-shaped sill that emplaced in soft sediments at the subseafloor, representing an exceptional in-situ natural laboratory for studying magma-sediment interactions. The petrological and geochemical study of the sediments, magmatic rocks and magma-sediment mixtures reveals a high concentration of pyrite at the magma-sediment interfaces. Multiple types of pyrite indicate different sulphur-trapping processes occurring during the emplacement of the sill. Trace elements and isotopic (δ³⁴S) analyses were used to identify the different sources of sulphur and to better constrain the processes. We demonstrate that the assimilation of sediment and the contamination of porewater in the Ringvent sill enables the formation of these different pyrite types from the time of sill emplacement until to late hydrothermal circulations, which remain still active. Such processes enable the storage of a minimum of 700,000 tonnes of sulphur within the sill, while there is minimal evidence of the release of volatiles in the surrounding area, indicating that, under certain conditions, these systems can function as a significant sulphur sink.

Ultramicroscopy 278 (2025): 114243

https://doi.org/10.1016/j.ultramic.2025.114243

The preparation of ultra-thin PtSn-alloyed layers by molecular beam epitaxy was studied using low-energy electron microscopy (LEEM) and micro-diffraction (μ-LEED). Deposition at a sample temperature of 435 °C initially results in the formation of a Pt₃Sn/Pt(111) layer showing a (2 × 2) reconstruction. With continued Sn deposition, a Pt₂Sn/Pt(111) layer develops, showing a (√3 × √3)R30° reconstruction. An ultra-thin tin oxide was formed from the (2 × 2) surface by exposure to molecular oxygen at temperatures of 500 °C and 590 °C, respectively. LEED shows the evolution of a new surface structure, which could be identified as an incommensurate rectangular (2.3 0 1.8​ 3.6​) reconstruction with lattice parameters of a = (6.4 ± 0.1) Å  and b = (8.6 ± 0.1) Å  present in three domains rotated by 120° with respect to each other. This structure can be related to the zigzag reconstructions found for similar ultra-thin oxide systems. Contrarily, the (√3 × √3)R30° structure showed no oxide formation even after extensive exposure to molecular oxygen. The usage of atomic oxygen, however, allows for oxidation of this surface and the growth of thicker oxides on both types of overlayers. At 500 °C this process is accompanied by substantial roughening of the surface.

Applied Physics Reviews 12 (2025): 041413

https://doi.org/10.1063/5.0293660

As an exemplary member of the MXene family belonging to the class of two-dimensional materials, titanium carbide (Ti₃C₂T_x) MXene stands bright and is explored owing to its exceptional tunable properties. The full ambient oxidation of MXene in a spectrum of varying elevated temperatures toward the application of memristor devices is reported for the first time in this work. A Ti₃C₂T_x MXene free-standing film is oxidized in air from the temperature of 100 to 700 °C upon which the MXene completely transforms into the TiO₂ film yet retaining its free-standing nature in the form of MXene-derived TiO₂ films. Extensive surface, morphological, and bulk characterizations, such as x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray diffraction, confirmed the increasing Ti–O and decreasing Ti–C bond strength amid increasing oxidation. Furthermore, exceptional resistance switching properties are unveiled employing these heated MXene devices in tri-layer memristors utilizing flexible reduced graphene oxide as electrodes. The memristor device utilizing Ti₃C₂T_x MXene heated at 700 °C exhibited outstanding performance compared to the other series of devices with low switching voltage, a high OFF/ON ratio of >10², cycle-to-cycle repeatability, and exceptional endurance of over 6000 cycles. This work on MXene-derived TiO₂ free-standing films will lay open ways to obtain oxide based flexible electronic devices through easy fabrication methods along with the possible capability to mimic unmatched synaptic features.

Journal of Materials Science: Materials in Engineering20 (2025): 140 

https://doi.org/10.1186/s40712-025-00340-6

Hydrogen offers a high-energy, carbon-free fuel alternative; however, conventional flame-based hydrogen combustion poses challenges, including NO_x emissions and the risk of flame flashback. Catalytic combustion provides a safer, low-temperature approach to hydrogen utilization, but realizing it within compact, integrated systems have remained a significant challenge. This study introduces an innovative approach to hydrogen catalytic combustion by directly integrating noble metal single quantum-crystallites of Pt and Ru within a porous silica aerogel matrix embedded in a silicon chip. This configuration enables deep nanoparticle (np) penetration throughout the aerogel network, maximizing the catalytic surface area and providing efficient on-chip hydrogen combustion. The np@aerogel systems are systematically synthesized and incorporated within silicon chips equipped with a polyimide membrane and Pt thermal structures. This unique setup allows for direct, real-time characterization of hydrogen catalytic combustion by measuring resistance changes in an embedded thermistor. The Pt@SiO₂ system demonstrates a rapid and substantial temperature increase of up to 40 K upon hydrogen exposure, independent of both preheating and Pt concentration, underscoring its robustness and adaptability for micro-scale hydrogen combustion. This on-chip integration of np@aerogel catalysts marks a significant advancement for hydrogen-based energy applications, offering a compact, scalable platform for efficient catalytic combustion. This approach opens pathways for applications in thermoelectric generators and other micro-reactor technologies where controlled, localized energy generation is critical.

Powder Technology 465 (2025): 121318

https://doi.org/10.1016/j.powtec.2025.121318

The synthesis of Cu₁.₈S, ZnS, and Cu₁.₈S-ZnS composite nanoparticles is achieved via reactive spray combustion, wherein rapid vaporization of thiophene initiates micro-explosions that promote high-temperature vapor-phase reactions under reducing conditions. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) analyses reveal that the synthesized nanoparticles consist of agglomerated spherical primary crystallites, with average sizes of 12.2 nm for Cu₁.₈S, 10 nm for ZnS, and 10.8 nm for the Cu₁.₈S-ZnS composite. Elemental analysis via energy-dispersive X-ray spectroscopy (EDX) coupled with scanning transmission electron microscopy (STEM) confirms homogeneous spatial distribution of Cu and S in Cu₁.₈S, elevated surface oxygen content in ZnS attributed to physisorption, and substantial Cu incorporation into the ZnS lattice within the Cu₁.₈S-ZnS composite system. Structural analysis indicates that the contrast features observed in Cu₁.₈S, ZnS, and Cu-Zn mixed sulfides are consistent with their respective crystallographic symmetries, where sulfur atoms adopt well-ordered lattice positions, while copper exhibits partial site occupancy and electron density disorder attributed to the comparable ionic radii of Cu²⁺ and Zn²⁺ ions. This study underscores the efficacy of oxygen-deficient, reducing flame environments in facilitating the synthesis of binary and mixed-metal sulfide nanomaterials, enabling the formation of metastable phases providing a scalable, cost-effective route for producing advanced functional materials with broad application potential.

Cement and Concrete Research 197 (2025): 107965 

https://doi.org/10.1016/j.cemconres.2025.107965

Autogenous self-healing of cracked concrete remains a highly uncertain phenomenon in building practice, with the influence of the binder composition and curing time not yet fully understood. This study investigates self-healing of cracked concrete produced with CEM I, CEM II/A-LL and CEM III/A after 7 or 28 days of curing, aiming to model reactive transport and validate previous research. Healing efficiency generally decreased as crack width increased because flow rate scales with the third power of the crack width, while crack closure through CaCO₃ precipitation increases only linearly. Rather than portlandite content of the binders, the results suggest that CaCO₃ precipitation and thus healing efficiency is governed by portlandite availability and potential microstructural changes due to carbonation. Notably, young limestone and slag cement samples showed a significantly lower capability to self-heal than comparable Portland cement reference samples, highlighting the need to reassess autogenous self-healing in practical applications.

Small 21 (2025): 2409993

https://doi.org/10.1002/smll.202409993

The objective of this study is to investigate the influence of various process parameters, such as the fuel-to-oxygen ratio, precursor flow rate, co-flow rate, and different metal-to-sulfur ratios on the properties of metal sulfide particles synthesized via flame spray pyrolysis (FSP). The particle size increases with increasing dispersion oxygen flow and copper sulfide is obtained only when the fuel-to-oxygen ratio is equal to or higher than 1.5. The temperature of the flame rises with an increasing precursor flow rate and copper sulfide is formed at a precursor flow rate of 5 mL min⁻¹ or lower, while contamination occurs above 5 mL min⁻¹. A Co-flow rate above 100 L min⁻¹ is required to cool the aerosol stream before deposition on the filter. A pure copper sulfide phase is produced when sulfur is more than 5 times in molar ratio compared to Cu in the liquid solution and particle size decreases with increasing sulfur concentration. This research will contribute to a better understanding of the fundamental formation process of metal sulfides under oxygen-lean gas-phase conditions and serve as a milestone in optimizing synthesis parameters for various applications.

JGR Solid Earth 130 (2025): e2024JB030354

https://doi.org/10.1029/2024JB030354

Magma emplacement in the top unconsolidated sediments of rift basins is poorly understood. We compare two shallow sills from the Guaymas Basin (Gulf of California) using core data and analyses from IODP Expedition 385, and high-resolution 2-D seismic data. We show that magma stalling in the top uncemented sediment layer is controlled by the transition from siliceous claystone to uncemented silica-rich sediment, favoring flat sill formation. Space is created through a combination of viscous indentation, magma-sediment mingling and fluidization processes. We show that sills emplace above the opal-A/CT diagenetic barrier. Our model suggests that in low magma input regions sills emplace at constant depth from the seafloor, while high magma input leads to upward stacking of sills, culminating in a funnel-shaped intrusions. Our petrophysical, petrographic, and textural analyses show that magma-sediment mingling creates significant porosity (up to 20%) through thermal cracking of the assimilated sediment. Stable isotope data suggest carbonate formation at 70–90°C, consistent with background geothermal gradient at 250–325 m depth. The unconsolidated, water-rich host sediments produce little thermogenic gas through contact metamorphism, but deep diagenetically formed gas bypasses the low-permeability top sediments via hydrothermal fluids flowing through the magma plumbing system. This hydrothermal system provides a steady supply of hydrocarbons at temperatures amendable for microbial life, serving as an incubator that may be abundant in magma-rich young rift basins and play a key role in sustaining subseafloor ecosystems.

J. Phys. Energy7 (2025): 025012

https://doi.org/10.1088/2515-7655/adbad9

The work's objective is to enhance the generation of H₂ via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce_1−xU_xO₂. While CeO₂ is the most active and stable known reducible oxide for the TCWS reaction, it is below par to make it practical. This has motivated many works to enhance its reduction capacity and therefore increase its activity. In this work the presence of both metal cations (Ce⁴⁺ and U⁴⁺) has allowed for the charge transfer reaction to occur (Ce⁴⁺ + U⁴⁺ ➔ Ce³⁺ + U⁵⁺) and therefore increased its capacity to generate oxygen vacancies, V_O (2 Ce³⁺ + V_O), needed for the TCWS reaction. Test reactions on the polycrystalline mixed oxides indicated that small atomic percentages of U (<10%) were found to be optimal for H₂ production (ca. 7 µmol g⁻¹) due to a considerable increase of Ce³⁺ states. Further studies of the Ce–U interaction were performed on thin epitaxial Ce_1−xU_xO₂ (111) films of about 6 nm. In situ x-ray photoelectron spectroscopy showed clear evidences of charge transfer at low U content (ca. 50% of surface/near surface Ce⁴⁺ cations were reduced in the case of Ce_0.95U_0.05O_2−δ). Moreover, it was found that while increasing the content of U decreased the charge transfer efficiency, it protected reduced Ce³⁺ from being oxidized. Our computational results using the DFT + U method gave evidence of charge transfer at 3.5 and 6.2 at.% of U. In agreement with experiments, theoretical calculations also showed that the charge transfer is sensitive to the distribution of U⁴⁺ around the Ce⁴⁺ cations, which in turn affected the creation of V_O needed for water splitting. Our results point out to the important yet often neglected effect of statistical entropy (cations distribution in the lattice), in addition to composition, in increasing the density of reduced states and consequently enhancing H₂ production from water.

Materials Today Advances25 (2025): 100566

https://doi.org/10.1016/j.mtadv.2025.100566

Manufacturing pure metals on Mars is challenging due to limited energy resources and unavoidable contamination of raw materials and production equipment with the Martian dust (regolith) resulting in impure materials. Understanding the effect of contamination on material properties is crucial for establishing materials manufacturing on Mars. This study investigates the influence of regolith contamination on its processability, and properties of the Fe-based material manufactured via laser powder bed fusion (L-PBF) for potential extraterrestrial applications. To simulate a contamination, water-atomized iron powder was mixed with 1 wt% Martian regolith simulant and processed by L-PBF. It was found the regolith is uniformly distributed within the iron matrix transforming from contaminant to reinforcement material. The crack-free interface between iron and regolith systematically studied using STEM reveals segregation of some elements but absence of notable reaction between matrix and particles. The Fe-regolith composite demonstrate moderate strength and large plastic deformability. The results suggest that unavoidable regolith contamination during production on Mars can be rethink as in-situ resource utilization for manufacturing of regolith reinforced iron matrix composites.

J. Phys. Chem. C129 (2025): 3583–3594

https://doi.org/10.1021/acs.jpcc.4c08072

Inverse oxide–metal model catalysts can show superior activity and selectivity compared with the traditional supported metal–oxide architecture, commonly attributed to the synergistic overlayer–support interaction. We have investigated the growth and redox properties of ceria nanoislands grown on Au(111) between 700 and 890 ˚C, which yields the CeO₂–Au(111) model catalyst system. We have observed a distinct correlation between deposition temperature, structural order, and oxide composition through low-energy electron microscopy, low-energy electron diffraction, intensity–voltage curves, and X-ray absorption spectroscopy. Improved structural order and thermal stability of the oxide have been achieved by increasing the oxygen chemical potential at the substrate surface using reactive oxygen (O/O₂) instead of molecular O₂ during growth. In situ characterization under reducing (H₂) and oxidizing atmospheres (O₂, CO₂) indicates an irreversible loss of structural order and redox activity at high reduction temperatures, while moderate temperatures result in partial decomposition of the ceria nanoislands (Ce³⁺/Ce⁴⁺) to metallic cerium (Ce⁰). The weak interaction between Au(111) and CeO_x would facilitate its reduction to the Ce⁰ metallic state, especially considering the comparatively strong interaction between Ce⁰ and Au⁰. Besides, the higher reactivity of atomic oxygen promotes a stronger interaction between the gold and oxide islands during the nucleation process, explaining the improved stability. Thus, we propose that by driving the nucleation and growth of the ceria/Au system in a highly oxidizing regime, novel chemical properties can be obtained.

Acta Materialia 283 (2025): 120488

https://doi.org/10.1016/j.actamat.2024.120488

Additively manufactured components are generally heat treated to remove the undesired microstructure formed during the repeated heating-cooling cycles inherent to the process, known as intrinsic heat treatment (IHT). Recently, the IHT has been explored as a driving force for precipitation hardening in steels which can potentially shorten the manufacturing chain of AM components. However, the mechanisms behind the formation of secondary phase precipitates during the complex thermal history remains unclear. In this work, a combination of in situ high energy X-ray diffraction, atom probe tomography, scanning and transmission electron microscopy were used to reveal the precipitation sequence in an X40CrMoV5–1 tool steel during laser-directed energy deposition (L-DED). V-rich MCN and V₈CN₇ carbonitrides, as well as, Fe-Cr-rich M₃C and M₇C₃ carbides were formed at different stages of the L-DED. Their evolution and resulting chemical stoichiometry was correlated to the exact phase transformation occurring in the microstructure during the IHT over different regions along the built direction. Finally, the combined results from the in situ and ex situ experiments enabled us to retrace the history of the full microstructure during the L-DED process. The findings lead to the conclusion that secondary hardening effect in tool steel is, as expected, sensitive to the severity of the IHT, and if limited, can result in a tempered microstructure comparable to the ones conventionally obtained after tempering heat treatments.

Composite Interfaces (2025): 1-21

https://doi.org/10.1080/09276440.2024.2448883

This research aims to enhance fibre-matrix adhesion in bio-based fibre-reinforced polyolefins without using adhesion promoters. The primary focus is to establish a cross-linking mechanism between cellulose fibres and polyethylene by applying UV irradiation to a UV-transparent matrix and UV-absorbing fibres. The influence of UV treatment on the composite properties is evaluated by tensile, interfacial and interlaminar shear strength tests. The UV irradiation decreases the critical fragment length in single fibre fragmentation tests, indicating an improved fibre-matrix adhesion. The UV-irradiated composites’ tensile strength and Young’s modulus are found to be ~10% (for 3- and 8-minute irradiation) and ~50% (for 8-minute irradiation), respectively, higher than those of the untreated samples. Furthermore, the UV irradiation leads to an improvement in the interlaminar shear strength by 25%. The variation of the UV-irradiation time (3 min and 8 min) and the comparison of the properties of semi-finished composite sheets and composites also reveal chemical and physical changes in the regenerated cellulose fibres due to heat adsorption. The proposed mechanism of interfacial crosslinking is confirmed by FTIR spectroscopy. The results suggest an approach to overcome poor compatibility between hydrophobic polyolefin matrix and hydrophilic cellulose-based fibres, resulting in adhesive-free bio-based composites.

Carbon 231 (2025): 119600

https://doi.org/10.1016/j.carbon.2024.119600

Ruthenium is emerging as a promising candidate to replace copper in highly integrated electronics by enabling barrierless metallization in ultrathin interconnects. From this perspective, the study of graphene growth on such surface templates is of paramount importance as a platform for graphene integration in electronic devices. In particular, graphene growth on the Ru(101‾0) surface allows selective growth of different graphene orientations, one-dimensional structures, and reduced substrate interaction compared to the well-established hexagonal Ru(0001) substrate. Real-time growth observations using low-energy electron microscopy and micro-diffraction highlight the influence of substrate symmetry on graphene growth, leading to the formation of rectangular islands with distinct zigzag- or armchair-terminated edges. Bilayer formation on Ru(101‾0) occurs by nucleation of graphene nanoribbons under the monolayer. Micro-spot angle-resolved photoemission spectroscopy shows significantly less charge-transfer doping in these freestanding, zigzag-terminated bilayer graphene nanoribbons, indicating reduced graphene-substrate interaction and hence more effective decoupling as compared to graphene/Ru(0001). Our results show that the growth of graphene on non-hexagonal substrates opens new pathways for tailoring the graphene-substrate interaction at the interface, and thus the properties of graphene beyond the limits imposed by hexagonal substrates.

Zeitschrift Für Kristallographie - Crystalline Materials (2024)

https://doi.org/10.1515/zkri-2024-0117

Plagioclase feldspars draw intensive research attention in planetary sciences because of their abundance in the Martian regolith. Crystal chemical studies on plagioclase feldspars would be of crucial importance for possible in situ resource utilization for future human settlement on Mars. This study focuses on the synthesis of representative plagioclase feldspars followed by simulation of mechanical weathering using ball milling. A series of (Ca_1-xNa _x )(Al_2-xSi_2+x)O₈ plagioclase feldspars is synthesized perfoming the solid-state method, where the endmembers are the anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈). The bulk chemical composition, particularly the Al/Si ratio, of each member is determined from energy-dispersive X-ray spectroscopy, which is supported by X-ray powder diffraction data Rietveld refinements. Selective plagioclase members (x = 0.0, 0.4 and 1.0) are mechanically weathered using high-energy ball milling, leading to significant changes of microstructural features such as average crystallite size and micro-strain. Total scattering data are collected using in-house X-ray facilities and analyzed by pair distribution function refinements. The vibrational modes of the samples are evaluated by Raman spectroscopy, complementing the local structural description.

Ultramicroscopy 265 (2024): 114020

https://doi.org/10.1016/j.ultramic.2024.114020

Structural and chemical characterization of nanomaterials provides important information for understanding their functional properties. Nanomaterials with characteristic structure sizes in the nanometer range can be characterized by scanning transmission electron microscopy (STEM). In conventional STEM, two-dimensional (2D) projection images of the samples are acquired, information about the third dimension is lost. This drawback can be overcome by STEM tomography, where the three-dimensional (3D) structure is reconstructed from a series of projection images acquired using various projection directions. However, 3D measurements are expensive with respect to acquisition and evaluation time. Furthermore, the method is hardly applicable to beam-sensitive materials, i.e. samples that degrade under the electron beam. For this reason, it is desirable to know whether sufficient information on structural and chemical information can be extracted from 2D-projection measurements. In the present work, a comparison between 3D-reconstruction and 2D-projection characterization of structure and mixing in nanoparticle hetero-aggregates is provided. To this end, convolutional neural networks are trained in 2D and 3D to extract particle positions and material types from the simulated or experimental measurement. Results are used to evaluate structure, particle size distributions, hetero-aggregate compositions and mixing of particles quantitatively and to find an answer to the question, whether an expensive 3D characterization is required for this material system for future characterizations.

IUCrJ 11 (2024): 977-990

https://doi.org/10.1107/S2052252524009722

Regolith draws intensive research attention because of its importance as the basis for fabricating materials for future human space exploration. Martian regolith is predicted to consist of defect-rich crystal structures due to long-term space weathering. The present report focuses on the structural differences between defect-rich and defect-poor forsterite (Mg₂SiO₄) – one of the major phases in Martian regolith. In this work, forsterites were synthesized using reverse strike co-precipitation and high-energy ball milling (BM). Subsequent post-processing was also carried out using BM to enhance the defects. The crystal structures of the samples were characterized by X-ray powder diffraction and total scattering using Cu and synchrotron radiation followed by Rietveld refinement and pair distribution function (PDF) analysis, respectively. The structural models were deduced by density functional theory assisted PDF refinements, describing both long-range and short-range order caused by defects. The Raman spectral features of the synthetic forsterites complement the ab initio simulation for an in-depth understanding of the associated structural defects.

Advanced Functional Materials (2024): 2411521

https://doi.org/10.1002/adfm.202411521

The innovative development of reactive-spray systems for gas-phase production of metal sulfides are potential materials for next-generation technologies. These flame-synthesized sulfides (doped, functionalized, and heterogeneously mixed derivatives) hold significant potential as photocatalysts for water splitting. The knowledge acquired from nonaqueous precursor-solvent and high-temperature aerosol chemistries, optimal process parameters are established to generate In_2-(4/3)xSn_xS₃, solid-solutions. The thermally driven reducing gas-phase reactions are controlled through fuel/oxygen ratio. Particles characterizations (X-ray diffraction, transmission electron microscopy (TEM) and imaging) revealed structural stability and crystallinity. The In_2-(4/3)xSn_xS₃, at higher Sn doping had enhanced photoexcitation. Donor-acceptor levels within the material facilitated electron-hole pair trapping, crucial for redox reactions. With suitable band gap energies for water oxidation (1.9-1.1 eV) closely matched flat band potentials (4.38-4.67 eV) for redox reactions. The powder characterization showed 8% In₂O₃ in InSn_0.75S₃ after photocatalysis due to S-degradation in the initial light “on/off cycles”. The pioneering process of employing oxygen-deficient reducing flame enabled a series of photo-catalytically active metal sulfide nanoparticles with work function energies in the range of 5.19-5.37 eV. This synthesis strategy holds the potential for impactful advancements in both industry and R&D, addressing the urgent need for new materials capable of inducing water oxidation under visible light.

APL Materials 12 (2024): 091104

https://doi.org/10.1063/5.0226857

We investigate the composition of α-phase intermediate layers at epitaxial Ga₂O₃/Al₂O₃ interfaces using high angle annular dark field scanning transmission electron microscopy. Their presence is considered a general phenomenon as they are observed independent of the growth technique [Schewski et al., Appl. Phys. Exp. 8, 011101]. Samples were grown by plasma assisted molecular beam epitaxy using different growth conditions. Almost independent of these, the quantitative evaluation of the measured intensities gave Ga concentrations of ∼25%. We show that the previously published model, based on a pure α-Ga₂O₃ interlayer, fails if it is adapted to the measured composition. Density functional theory (DFT) computations were used to overcome the approximations made in this model and suggest that a stabilization of the layer is possible due to the low Ga concentration (⁠≤35%) at which the α-phase is the most stable. Our surface model computations suggest an exchange of Ga atoms at the surface with Al atoms from the underlying substrate as a possible formation mechanism.

Zeitschrift für Kristallographie - Crystalline Materials (2024)

https://doi.org/10.1515/zkri-2024-0078

To fabricate metals from the base materials for future Mars exploration, synthesis of representative olivine phases and their structural and spectroscopic characterizations are of crucial importance. Using mechanochemical technique that mimics the mechanical weathering, a complete solid solution of (Mg_1−xFe _x )₂SiO₄ has been synthesized to investigate the associated crystal chemical properties. X-ray powder diffraction data Rietveld analysis confirms that each polycrystalline sample crystallizes in space group Pbnm. The average crystallite size ranges between 80(1) nm and 223(4) nm. Each lattice parameter increases with increasing Fe-content due to the larger Fe²⁺ radius than that of Mg²⁺, following Vegard’s rule. For a given nominal chemical composition, substitution of Mg with Fe at the M1-site (4a: 0,0,0) is preferred to the M2-site (4c: x,y,¼). As a consequence, the average Fe-content lies slightly below the equivalence line for x = 0.2–0.8, indicating that the Fe/Mg ratio in the amorphous scattering content is most likely greater than unity. Characteristic Raman spectral features of the olivines have been explained in terms of the chemical composition (x). Simple regression models are demonstrated based on both X-ray diffraction and Raman spectroscopic data for the calculation of Mg/Fe in olivines. Diffuse reflectance UV/Vis spectra RATD analysis shows each olivine phase possesses direct band-gap between 3.38(3) eV and 4.90(3) eV. This study could keep valuable information to relevant databases for future human missions on Mars, in particular, for precise estimation of the representative olivines from the remote X-ray diffraction and spectroscopic data.

ChemCatChem (2024): e202400280

https://doi.org/10.1002/cctc.202400280

Nanoporous gold (npAu) attracted increasing attention over the last 20 years as a highly active and selective oxidation catalyst in particular at low temperatures. Previous research mainly focused on npAu that was fabricated by corrosive dealloying of AuAg parent alloys. Yet, the use of other binary alloys, such as AuCu, promises interesting variations of the catalytic properties, when considering that residual amounts of the less noble metal were shown to be co-catalytically involved. Aiming at providing a platform for systematic studies in this direction for Cu, we not only dealt with strategies for a reliable and reproducible preparation of npAu(Cu) catalysts from AuCu, but also with their potential for CO oxidation in comparison to npAu(Ag). We were able to develop an approach based on thermally quenched Au_0.3Cu_0.7 alloys, providing distinct synthetic advantages as a starting material for the catalyst fabrication versus the thermodynamically more stable AuCu₃ intermetallic compound. Using PCD (potentiostatically controlled dealloying), well-defined pore structures with ligament diameters of ∼40 nm and variable residual Cu concentrations in the range between ∼0.6 at % and ∼1.2 at % could be straightforwardly obtained. After activating such catalysts at 150 °C, they reproducibly showed catalytic activity for aerobic CO oxidation in a broad temperature window between 40 °C and 250 °C. As opposed to npAu(Ag), the activity increased with decreasing residual Cu content, outperforming the former at temperatures above ∼60 °C not only with respect to CO₂ formation rates but also with respect to thermal stability. Based on X-ray photoelectron spectroscopic and transmission electron microscopic results, it was possible to conclude that Cu segregates to the surface and, with rising Cu bulk content, increasingly occurs in form of Cu²⁺ species at the surface. While the latter are expected to be catalytically inactive, Cu and Cu⁺ species are likely candidates for the activation of oxygen being not possible on pure Au.

Nanoscale Advances 6(15) 2024: 3895-3903

https://doi.org/10.1039/D4NA00276H

Spark ablation was used to continuously synthesize bimetallic L1₀ Pt/Fe nanoparticles in an aerosol process involving a furnace and hydrogen as a reducing process gas. For the formation of Pt/Fe in the favorable L1₀ crystal configuration, which is a promising electrocatalyst, the Pt–Fe ratio plays a crucial role. State-of-the-art analytics for such multi-element nanoparticles include, among others, electron microscopy (EM) with an element mapping function, such as scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDXS). Morphological characteristics, local compositions, and element distributions within single particles can be easily derived from EM for a small number of particles. However, a statistical evaluation aiming at the composition of hundreds of single Pt/Fe particles can barely be addressed with such analytics. Driven by the lack of analytical setups aiming at the recording of composition and size distribution of nanoparticles by online diagnostics, this work focuses on a single-particle inductively coupled plasma mass spectrometry (spICP-MS) setup able to resolve this issue. The combination of nanoparticle dilution and classification with spICP-MS allows for the analysis of thousands of multi-element aerosol nanoparticles within minutes. Hence, this article elaborates on the synergy of conducting STEM-EDXS and spICP-MS measurements in parallel, giving the opportunity to multi-dimensionally characterize nanoparticles consisting of more than one element. Beyond metallic particles, the presented setup even allows for the analysis of hetero-aggregated oxidic particles, such as Pt/Fe₂O₃. Including further offline analytics like X-ray diffraction (XRD), the formation of L1₀ Pt/Fe was found to be process gas-dependent and to set in at 400 °C, yielding particles with 56% L1₀ content at 1000 °C under a reducing atmosphere.

Science 385(6706) (2024): 318-321

DOI: 10.1126/science.adp4963

Nitrenes are a highly reactive, yet fundamental, compound class. They possess a monovalent nitrogen atom and usually a short life span, typically in the nanosecond range. Here, we report on the synthesis of a stable nitrene by photolysis of the arylazide M^SFluindN₃ (1), which gave rise to the quantitative formation of the arylnitrene M^SFluindN (2) (M^SFluind is dispiro[fluorene-9,3′-(1′,1′,7′,7′-tetramethyl-s-hydrindacen-4′-yl)-5′,9′′-fluorene]) that remains unchanged for at least 3 days when stored under argon atmosphere at room temperature. The extraordinary life span permitted the full characterization of 2 by single-crystal x-ray crystallography, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry, which supported a triplet ground state. Theoretical simulations suggest that in addition to the kinetic stabilization conferred by the bulky M^SFluind aryl substituent, electron delocalization across the central aromatic ring contributes to the electron stabilization of 2.

ChemPhysChem 25 (16) (2024): e202400186

https://doi.org/10.1002/cphc.202400186

Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VO_x/Rh(110) surface have been investigated in the 10⁻⁴ mbar range with low-energy electron microscopy (LEEM) and micro spot low-energy electron diffraction (micro-LEED) as in situ methods. V coverages of θ_V=0.2 and 0.4 MLE (monolayer equivalents) were studied. Pulses display a c(2×2) pattern in the reduced part and (1×2) and c(2×8) structures in the oxidized part of the surface. At θ_V=0.4 MLE (1×2)/(1×4) patterns with streaks along the [001]-direction at the 1/8 positions are present on the oxidized part of the surface. This phase can be assigned to V-oxide. On a tentative basis, an excitation mechanism for pulses is presented, Annealing the surface to 990 K under reaction conditions results in a macroscopic hole pattern in which holes of low VO_x coverage are surrounded by a V-oxide layer. Chemical waves propagate inside the holes as well as on the VO_x covered parts of the surface. The results demonstrate for the first time that also in supported oxidic overlayers selforganization processes can take place leading to chemical waves and a large scale redistribution of the oxide.

Applied Surface Science 655 (2024): 159634

https://doi.org/10.1016/j.apsusc.2024.159634

Alkaline solid wastes containing olivine minerals can be suitable for CO₂ mineralization due to their relatively higher reactivity and intrinsic alkalinity. However, an activation process is required to improve cation dissolution rates. Four pristine facets in an olivine single crystal were used to study their dissolution kinetics at far from thermodynamic equilibrium conditions. Vertical scanning interferometry was employed to analyze the distribution of surface dissolution rate and the evolution of etch pits. The dissolution rate exhibited an anisotropic distribution, by up to two orders of magnitude among facets. The rate law differs for each plane, with (0 1 0) face following zero-order, (1 2 0) plane exhibiting first-order, and (1 1 0) and (1 1 1) planes showing second-order reactions. Etch pits also displayed significant anisotropy, with spindle-shaped on (0 1 0) plane, droplet-shaped on (1 1 0) plane, and funnel-shaped on (1 1 1) plane. Strip-like etch pits only formed along certain scratches on (1 2 0) plane. Initially, the etch pits deepened, but after 200 h of reaction time, they grew in size. Raman and TEM analysis confirmed atomic-scale rearrangement of the dissolved surface, providing insights into etch pit morphology evolution. This work unveiled the anisotropic dissolution mechanism of olivine surfaces, enabling predictions of long-term reaction rates under various geochemical conditions.

Nanoscale 16(19) (2024): 9603-9616

https://doi.org/10.1039/D4NR00046C

Dealloying of Ag–Au alloy nanoparticles (NPs) strongly differs from the corresponding bulk alloy materials. Here, we have investigated the effects of potentiodynamic and potentiostatic dealloying on structure and distribution of residual Ag atoms for Au rich NPs. Two different sizes of Ag rich alloy NPs, 77 ± 26 nm Ag₇₇Au₂₃ and 12 ± 5 nm Ag₈₆Au₁₄, were prepared. 77 nm Ag₇₇Au₂₃ NPs form a homogeneous alloy, while 12 nm Ag₈₆Au₁₄ NPs show an Ag rich shell–Au rich core arrangement. The two groups of as-prepared NPs were dealloyed either under potentiodynamic (0.2–1.3 V_RHE) or potentiostatic (0.9, 1.2, and 1.6 V_RHE) conditions in 0.1 M HClO₄. For the initial 77 nm Ag₇₇Au₂₃ NPs, both dealloying protocols lead to pore evolution. Interestingly, instead of homogenous Ag distribution, numerous Ag rich regions form and locate near the pores and particle edges. The critical dealloying potential also differs by ∼500 mV depending on the dealloying method. The initial 12 nm Ag₈₆Au₁₄ NPs remain dense and solid, but Ag distribution and thickness of the Au passivation layer vary between both dealloying protocols. When the Au passivation layer is very thin, the residual Ag atoms tend to segregate to the particle surface after dealloying. Due to the size effect, small NPs are less electrochemically stable and show a lower critical dealloying potential. In this systematic study, we demonstrate that the mobility of Au surface atoms and dealloying conditions control the structure and residual Ag distribution within dealloyed NPs.

2D Materials 11(2) (2024): 025031

DOI: 10.1088/2053-1583/ad3134

MoS₂ and WS₂ mono- and multilayers were grown on SiO₂/Si substrates. Growth by atomic layer deposition (ALD) at fast growth rates is compared to sub-ALD, which is a slow growth rate process with only partial precursor surface coverage per cycle. A Raman spectroscopic analysis of the intensity and frequency difference of the modes reveals different stages of growth from partial to full surface layer coverage followed by layer-by-layer formation. The initial layer thickness and structural quality strongly depend on the growth rate and monolayers only form using sub-ALD. Optical activity is demonstrated by photoluminescence (PL) characterization which shows typical excitonic emission from MoS₂ and WS₂ monolayers. A chemical analysis confirming the stoichiometry of MoS₂ is performed by x-ray photoelectron spectroscopy. The surface morphology of layers grown with different growth rates is studied by atomic force microscopy. Plan-view transmission electron microscopy analysis of MoS₂ directly grown on freestanding graphene reveals the local crystalline quality of the layers, in agreement with Raman and PL results.

Machine Learning: Science and Technology 5 (2024): 025007

https://doi.org/10.1088/2632-2153/ad38fd

The 3D nano/microstructure of materials can significantly influence their macroscopic properties. In order to enable a better understanding of such structure-property relationships, 3D microscopy techniques can be deployed, which are however often expensive in both time and costs. Often 2D imaging techniques are more accessible, yet they have the disadvantage that the 3D nano/microstructure of materials cannot be directly retrieved from such measurements. The motivation of this work is to overcome the issues of characterizing 3D structures from 2D measurements for hetero-aggregate materials. For this purpose, a method is presented that relies on machine learning combined with methods of spatial stochastic modeling for characterizing the 3D nano/microstructure of materials from 2D data. More precisely, a stochastic model is used for the generation of synthetic training data. This kind of training data has the advantage that time-consuming experiments for the synthesis of differently structured materials followed by their 3D imaging can be avoided. More precisely, a parametric stochastic 3D model is presented, from which a wide spectrum of virtual hetero-aggregates can be generated. Additionally, the virtual structures are passed to a physics-based simulation tool in order to generate virtual scanning transmission electron microscopy (STEM) images. The preset parameters of the 3D model together with the simulated STEM images serve as a database for the training of convolutional neural networks, which can be used to determine the parameters of the underlying 3D model and, consequently, to predict 3D structures of hetero-aggregates from 2DSTEM images. Furthermore, an error analysis is performed with respect to structural descriptors, eg the hetero-coordination number. The proposed method is applied to image data of TiO ₂ -WO ₃  hetero-aggregates, which are highly relevant in photocatalysis processes. However, the proposed method can be transferred to other types of aggregates and to different 2D microscopy techniques. Consequently, the method is relevant for industrial or laboratory setups in which product quality is to be quantified by means of inexpensive 2D image acquisition.

Journal of Crystallography - Crystalline Materials 239 (2024): 3-4

https://doi.org/10.1515/zkri-2023-0056

The presence of  ns ²  stereo-chemical active lone electron pairs (LEPs) causes asymmetric atomic environments around a given  p -block cation, leading to change the crystal chemistry of a respective system. Here we report a series of mullite-type compounds to understand at what extend Sr ²⁺  replaces the stereochemical active Pb ²⁺  cation in (Pb _{1− x} Sr _x )MnBO ₄ . Each member of the solid solution has been synthesized by conventional solid-state method. The polycrystalline samples are characterized using X-ray powder diffraction followed by Rietveld refinement. Substitution of Pb ²⁺  with Sr ²⁺  leads to contraction of the  a  lattice parameter with slight elongation in the  b  and  c  direction. For a difference of 1 pm of the ionic radius between Sr ²⁺  and Pb ²⁺ , the cell volume contracts about 4% between the end members as the spatial requirement of the LEP activity in the MBO ₄ ²⁻  channels significantly decreases. Within the solid solution, two distinct Pb/Sr–O ₂  bond distances significantly differ, which gradually decreases with increasing strontium content leading to a more symmetric coordination around strontium. The calculated BVS of Pb ²⁺ /Sr ²⁺  exhibits a linear correlation with the Wang–Liebau eccentricity parameter, indicating to an increased bonding ability cation. The vibrational properties are characterized by both Raman and FTIR spectroscopy, complementing the XPRD results. Electronic band gaps of selected (Pb _{1− x} Sr _x )MnBO ₄  samples were obtained from diffuse reflectance spectroscopy data. Additionally, the Sr containing samples show higher thermal stability than the Pb containing counterparts.    

Journal of Microscopy 295 (2024): 140-146

https://doi.org/10.1111/jmi.13276

Atomic electric fields in a thin GaN sample are measured with the centre-of-mass approach in 4D-scanning transmission electron microscopy (4D-STEM) using a 12-segmented STEM detector in a Spectra 300 microscope. The electric fields, charge density and potential are compared to simulations and an experimental measurement using a pixelated 4D-STEM detector. The segmented detector benefits from a high recording speed, which enables measurements at low radiation doses. However, there is measurement uncertainty due to the limited number of segments analysed in this study.

Ultramicroscopy 256 (2024): 113867

https://doi.org/10.1016/j.ultramic.2023.113867

If quantitative scanning transmission electron microscopy is used for very precise thickness measurements with atomic resolution, it is commonly referred to as »atom counting«. Due to scattering and the finite probe extent the signal recorded in one atomic column is dependent not only on its own height but also on the height of its neighbours. Especially for thicker specimens this crosstalk effect can have significant impact on the measured intensity. If it is not appropriately accounted for in the evaluation, it can result in a deterioration of accuracy that impedes the possibility of actual atom counting. However, as the number of possible neighbour configurations can be excessively large, a comprehensive consideration of all in the evaluation reference is neigh impossible. This work proposes a method that allows for the a-posteriori reduction of crosstalk during the evaluation by algebraic means. Based on a parametric model, which is described in detail in the article, the crosstalk is expressed by an invertible matrix. Applying the inverted matrix to the measurement yields crosstalk corrected intensity values with very little computational effort. These can subsequently be evaluated by direct comparison to simple reference data. The working principle of the method is presented on the example of crystalline gold. The crosstalk parametrisation is found by fitting a model to sets of specifically created multislice simulations. The parameters are given for both aberration corrected and uncorrected STEM. Subsequently the abilities and potential of the technique are assessed in simulative studies on multiple model systems including gold nanoparticles. Overall a significant and robust improvement of the attainable precision can be demonstrated making the proposed method a promising tool for reference-based atom counting.

APL Materials 12 (2024): 011120

https://doi.org/10.1063/5.0180041

The growth of α-Ga₂O₃ and α-(In_xGa_1−x)₂O₃ on m-plane α-Al₂O₃(⁠101̄0⁠) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga₂O₃(⁠101̄0⁠) and α-(In_xGa_1−x)₂O₃(⁠101̄0⁠) thin films are realized. The presence of In on the α-Ga₂O₃ growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (R_O), In incorporates into α-(In_xGa_1−x)₂O₃ up to x ≤ 0.08. Upon a critical thickness, β-(In_xGa_1−x)₂O₃ nucleates and, subsequently, heteroepitaxially grows on top of α-(In_xGa_1−x)₂O₃ facets. Metal-rich MOCATAXY growth conditions, where α-Ga₂O₃ would not conventionally stabilize, lead to single-crystalline α-Ga₂O₃ with negligible In incorporation and improved surface morphology. Higher T_TC further results in single-crystalline α-Ga₂O₃ with well-defined terraces and step edges at their surfaces. For R_O ≤ 0.53, In acts as a surfactant on the α-Ga₂O₃ growth surface by favoring step edges, while for R_O ≥ 0.8, In incorporates and leads to a-plane α-(In_xGa_1−x)₂O₃ faceting and the subsequent (⁠2̄01) β-(In_xGa_1−x)₂O₃ growth on top. Thin film analysis by scanning transmission electron microscopy reveals highly crystalline α-Ga₂O₃ layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga₂O₃ on α-Al₂O₃(⁠101̄0⁠).

Nano Select 5 (2024): 2300128

https://doi.org/10.1002/nano.202300128

Formation of hetero-contacts between particles of different materials in nanoparticle hetero-aggregates can lead to new functional properties. Improvement of the functional behavior requires a detailed characterization of mixing between the two types of particles, in order to correlate different mixing with the performance of the material. Scanning transmission electron microscopy (STEM) is an option for this task. To obtain statistically relevant results, STEM-images of many hetero-aggregates have to be acquired and evaluated. This can be time-consuming if it is done manually. In the present work, the applicability of convolutional neural networks for the automated analysis of STEM-images acquired from TiO₂–WO₃ nanoparticle hetero-aggregates is investigated. Hetero-aggregates are obtained in a double flame spray pyrolysis (DFSP) setup, in which a variation of setup parameters is expected to affect the mixing of TiO₂ and WO₃. Mixing is investigated by a measurement of cluster sizes (the number of connected particles of the same material within an aggregate) and coordination numbers (the number of particle contacts with particles of the same or the different material). Results show that the distribution of measured values is wide for both quantities, rendering it challenging to correlate mixing with parameters varied in the DFSP setup.

IEEE Xplore   (2023) 

Doi: 10.1109/NMDC57951.2023.10343972

Gallium oxide (β−Ga2O3) has been considered as a promising candidate for non-volatile resistive switching devices. However, it has been challenging for various deposition techniques to grow crystalline gallium oxide on metal substrates acting as bottom electrode in vertically stacked devices. We studied the synthesis of crystalline β−Ga2O3 on Ru/Al2O3 substrates using radio-frequency (RF) magnetron sputtering and characterized the electrical properties of Al/Ga2O3/Ru devices with respect to the effect of deposition time and temperature. The obtained devices showed more than 70 consistent IV cycles and ON-OFF ratios of up to 10 ³ . With increasing temperature and thickness, enhanced stability was observed in the endurance and retention cycles. The resistive switching (RS) behavior in these devices seems to be related to the formation and rupture of oxygen vacancy filaments.

Applied Physics Letters 123 (2023): 213504

https://doi.org/10.1063/5.0170354

In the last few years, there has been significant interest in gallium oxide devices for resistive switching technologies due to its remarkable sensitivity to oxygen. In this study, we present the growth and resistive switching of a (2¯01) oriented (75 ± 3) nm β-Ga₂O₃ thin film on a  Ru/Al₂O₃ substrate using magnetron radio frequency sputtering. The observed resistive switching was attributed to the formation and rupture of conductive filaments constituted by oxygen vacancies in the  β-Ga₂O₃  film as confirmed by x-ray photoelectron spectroscopy and energydispersive x-ray spectroscopy. The electrical conduction was found to be of Ohmic nature in the low-resistance ON state, while the highresistance OFF state was governed by the Poole–Frenkel transport mechanism. Exhibiting stable endurance cycles, long retention times, and ON/OFF ratios of up to 10⁴ , the devices can be considered as promising prototypes for future nonvolatile resistive switching random access memory with respect to both switching performance and device stability.

Advanced Engineering Materials 25 (2023): 2300942

https://doi.org/10.1002/adem.202300942

Herein, feasibility of plasmonically enhanced molecule detection via surface-enhanced Raman scattering for ceramics that are commonly used as bone ortooth replacement materials is evaluated. Open cell foams of Bioglass 45S5, thecommercial hydroxyapatite-based product Bio-Oss, and bioinert zirconia-toughened-alumina (ZTA) are coated with Au nanoparticles via colloidal depo-sition to introduce plasmonic effects. Depending on the pore size, gold-func-tionalized plasmonic porous Bioglass shows effective Raman enhancementfactor (eEF) up to 5.4
·10⁴, while depositing gold nanoparticles on Bio-Oss andporous ZTA resulted in eEF of 1.1
·10⁴ and 2.4·
10⁵ respectively. The per-formance of the plasmonic porous bioceramics under simulated biologicalconditions is examined in situ in the biological medium fetal bovine serum (FBS)and during extended incubation in mineralizing simulated bodyfluid (SBF). Mostnotably, the plasmonic porous Bioglass still delivered an eEF around 7.2
·10³ after 28 days of incubation in SBF, indicating promising stability in simulatedbiological conditions without significant difference in SBF bioactivity before andafter Au deposition. Accordingly, the plasmonically enhanced porous bioceramicsoffer the possibility for real-time and sensitive molecule detection at SBF and FBSconditions and can be further developed for sensing of specific biomarkers, forexample, in the context of osseointegration of bone replacement materials.

Geology 51 (2023): 875

https://doi.org/10.1130/G51238.1

Carbonate formation during the alteration of oceanic crust is a global CO₂ sink. Its timing and controls are not well understood, particularly in volcanic seamounts, which react with seawater over tens of millions of years. We report in situ U-Pb age dates of carbonate vein and void fill in 50–74 Ma basaltic basement of the Louisville Seamount Chain. More than 90% of the carbonate formed <20 m.y. after seamount emplacement. Vesicle carbonate precipitated within 8 m.y. (median = 2.9 m.y.) whereas vein carbonate grew over longer time spans (median = 8.1 m.y.). The duration of carbonation was hence limited despite the basement’s long-term exposure to seawater. The age dates imply a rapid infill of vesicles by alteration of confined domains around vesicles. Carbonate formation in veins extended for longer periods of time, likely due to the late opening of fractures, which exposed fresh reactive rock surface to circulating seawater long after the formation of the basement.

We suggest that carbonate growth ceased after the volcanic rocks were too altered to liberate sufficient Ca²⁺ and generate the alkalinity required for carbonate precipitation. A critical extent of rock reactivity is required to sustain carbonate formation. Carbonate precipitation likely ends after much of the exposed basaltic substrate has been altered and the rock reactivity drops below this critical threshold. These findings help to explain the generally short duration of carbonation in the flanks of mid-ocean ridges.

Terra Nova 35 (2023): 396–403

https://doi.org/10.1111/ter.12663

Pleistocene basanitic rocks of Vesteris Seamount in the Greenland Sea had been found to exhibit an endolithic habitat largely consisting of marine fungi that dwell within abundant vesicles, therefore representing cryptoendoliths. For the first time, we demonstrate that 3D X-ray microscopy can unravel how microorganisms access and migrate through vesicular rock. The fossil assemblages occur within a set of vesicles connected by microcracks. Such microcracks, which are ubiquitous features in submarine volcanic rocks, enable the dispersal of marine microorganisms in the rock. This study suggests that this pathway for the colonization of marine volcanic rocks forms in consequence of early tensional stress due to variable rates of cooling of the lava flow. Subsequently, the interconnected vesicles get populated by rock-dwelling microorganisms. This cryptoendolithic habitat exists at least since the Paleoproterozoic.

DiscoverNano 18 (2023): 27

https://doi.org/10.1186/s11671-023-03808-6

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 In_xGa_1-xN regions with a period of 2–3 nm that are covered by a GaN shell and that transition to a more homogenous In_xGa_1-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.

Journal of Microscopy  289 (2023): 91-106

https://doi.org/10.1111/jmi.13155

Low-energy electron microscopy (LEEM) taken as intensity–voltage (I–V) curves provides hyperspectral images of surfaces, which can be used to identify the surface type, but are difficult to analyse. Here, we demonstrate the use of an algorithm for factorizing the data into spectra and concentrations of characteristic components (FSC³) for identifying distinct physical surface phases. Importantly, FSC³ is an unsupervised and fast algorithm. As example data we use experiments on the growth of praseodymium oxide or ruthenium oxide on ruthenium single crystal substrates, both featuring a complex distribution of coexisting surface components, varying in both chemical composition and crystallographic structure. With the factorization result a sparse sampling method is demonstrated, reducing the measurement time by 1–2 orders of magnitude, relevant for dynamic surface studies. The FSC³ concentrations are providing the features for a support vector machine-based supervised classification of the surface types. Here, specific surface regions which have been identified structurally, via their diffraction pattern, as well as chemically by complementary spectro-microscopic techniques, are used as training sets. A reliable classification is demonstrated on both example LEEM I–V data sets.

The Journal of Physical Chemistry C. 127(6) (2023): 2988-2994

https://doi.org/10.1021/acs.jpcc.2c09041

The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt₃Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn²⁺. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below.

Zeitschrift für Kristallographie - Crystalline Materials 238 (2023): 27 - 38

https://doi.org/10.1515/zkri-2022-0037

Sodalites of the general type |Na₈X₂|[T¹T²O₄]₆ with X = Cl⁻, Br⁻, I⁻ have been synthesized for Al–Si, Ga–Si, Al–Ge and Ga–Ge as T¹–T² frameworks. The structures were examined using in-house and synchrotron X-ray diffraction, Raman spectroscopy, force-field structure optimizations and DFT based ab-initio molecular dynamics (MD) computations. Calculated phonon density of states (PDOS) of the 12 compounds show only minor differences within a framework composition with a lowering of certain phonon energies with increasing anion size. Earlier published Debye and Einstein temperatures obtained with a Debye-Einstein-anharmonicity (DEA) model approach are confirmed using the determined low-temperature lattice parameters (18 K–293 K) and show no correlation with the respective PDOS. Small-box refinements against radial pair distribution functions (PDF) allowed the determination of anisotropic displacement ellipsoids (ADP) for Na⁺ and O²⁻, indicating a strong dependency of the ADP of Na⁺ on the chemical composition. Significantly lower thermal displacements from MD calculations suggested an influence of structural displacements. For compounds with an aspherical ADP for sodium, structural models could be refined in which the sodium is located on two 8e or one 24i site (both partially occupied), and also temperature-dependent (100 K–300 K) for the compounds with Ga–Ge framework. 3D-plots of the bond-valence sums of Na⁺ further validate the structural differences. These results imply that the local structure of halide-sodalites in many cases is not best described by the known average structure and may even not be cubic.

Materials 16 (2023): 435

https://doi.org/10.3390/ma16010435

Duplex stainless steel powders for laser additive manufacturing have not been developed extensively. In this study, the melts of a super duplex stainless steel X2CrNiMoCuWN25-7-4 (AISI F55, 1.4501) were atomized with different process gases (Ar or N₂) at different atomization gas temperatures. The process gas N₂ in the melting chamber leads to a higher nitrogen dissolution in the steel and a higher nitrogen content of the atomized powders. The argon-atomized powders have more gas porosity inside the particles than the nitrogen-atomized powders. In addition, the higher the atomization gas temperature, the finer the powder particles. The duplex stainless steel powders showed good processability during PBF-LB/M (Laser powder bed fusion). The gas entrapment in the powder particles, regardless of the gas chemistry and the gas content, appears to have a negligible effect on the porosity of the as-built parts.

Advanced Optical Materials 11 (2022): 2201809

https://doi.org/10.1002/adom.202201809

Photonic crystals can be used to achieve high-performance surface-emitting lasers and enable novel photonic topological insulator devices. In this work, a GaAs/InGaAs heterojunction nanowire platform by selective area metalorganic vapor phase epitaxy for such applications is demonstrated. The nanowires are arranged into deformed honeycomb lattices on silicon-on-insulator substrate to exploit the quadrupolar photonic band-edge mode. Core–shell and axial heterostructures are formed with their crystalline properties studied by scanning transmission electron microscopy. Room-temperature, single mode lasing from both stretched and compressed honeycomb lattices within the telecom-O band, with lasing threshold as low as 1.25 µJ cm⁻² is demonstrated. The potential of using InGaAs nanowire-based honeycomb lattices for small-divergence surface-emitting lasers and topological edge mode lasers is investigated. Finite-difference time-domain far field simulations suggest a sub-10° beam divergence can be achieved thanks to the out-of-plane diffraction.

ACS Appl. Nano Mater. 5 (2022): 17702–17710

https://doi.org/10.1021/acsanm.2c03584

The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness.

Catal Lett 153 (2023): 3405–3422

https://doi.org/10.1007/s10562-022-04218-6

In this article we shed light on newly emerging perspectives to characterize and understand the interplay of diffusive mass transport and surface catalytic processes in pores of gas phase metal catalysts. As a case study, nanoporous gold, as an interesting example exhibiting a well-defined pore structure and a high activity for total and partial oxidation reactions is considered. PFG NMR (pulsed field gradient nuclear magnetic resonance) measurements allowed here for a quantitative evaluation of gas diffusivities within the material. STEM (scanning transmission electron microscopy) tomography furthermore provided additional insight into the structural details of the pore system, helping to judge which of its features are most decisive for slowing down mass transport. Based on the quantitative knowledge about the diffusion coefficients inside a porous catalyst, it becomes possible to disentangle mass transport contributions form the measured reaction kinetics and to determine the kinetic rate constant of the underlying catalytic surface reaction. In addition, predictions can be made for an improved effectiveness of the catalyst, i.e., optimized conversion rates. This approach will be discussed at the example of low-temperature CO oxidation, efficiently catalysed by npAu at 30 °C. The case study shall reveal that novel porous materials exhibiting well-defined micro- and mesoscopic features and sufficient catalytic activity, in combination with modern techniques to evaluate diffusive transport, offer interesting new opportunities for an integral understanding of catalytic processes.

Ultramicroscopy 245 (2023): 113661

https://doi.org/10.1016/j.ultramic.2022.113661

The ISTEM mode for TEM has been demonstrated to have several advantages in regard to resolution and precision. While previous works primarily focussed on the advantages due to the reduced spatial coherence, the actual image contrast, i.e. how bright or dark certain atom columns are imaged, has mostly been of secondary concern. The present work sets out to achieve the contrast of annular bright field STEM in ISTEM, producing the high contrast of light elements, for which this method is popular. It is shown from theoretical considerations that using an annular condenser aperture this aim can be realised. The optimal size of this aperture is found by simulative studies. It is then manufactured from platinum foil and installed in an image-aberration corrected microscope. ABF-like ISTEM images of strontium titanate [100] in projection are acquired. The pure oxygen columns are clearly resolved with significant contrast. The image pattern is indeed identical to what is achieved by ABF STEM. A close look at the image formation also shows that the dose needed for a given signal-to-noise ratio is at least a quarter smaller for ABF-like ISTEM compared to ABF STEM, assuming detectors of similar detective quantum efficiency.

Z. Anorg. Allg. Chem. 648 (2022): e20220031

https://doi.org/10.1002/zaac.202200311

Polycrystalline red tin(II)oxide (RSO) has been synthesized by a fast reflux method. X-ray powder diffraction (XRPD) data Rietveld refinement confirms the orthorhombic space group Cmc2₁ (Z=8). The presence of 5 s² lone electron pair (LEP) of the Sn²⁺ cation results in layers of highly distorted SnO₄ tetrahedra with an averaged Wang-Liebau eccentricity (WLE) parameter of 4.3(1) x10⁻⁵ as a measure of the stereochemical activity. The significantly different Sn−O bond distances demonstrate the under-bonding nature of both the tin and the oxygen atoms in the SnO₄ coordination. The band gap of red tin(II)oxide is found to be 1.75(1) eV which is compared with those of blue-black tin(II) (BSO) and white tin(IV) oxide (WSO) based on the UV-Vis diffuse reflectance spectral data. Temperature-dependent XRPD reveals the phase transitions, which is complemented by thermogravimetric and differential scanning calorimetry (TG/DSC) investigations. Moreover, in-situ Raman spectroscopy additionally hints to an intermediate phase either of Sn₂O₃ or Sn₃O₄ appears within a short temperature range before the RSO to BSO transition occurs. Due to axial negative thermal expansion for the b-lattice parameter RSO exhibits nearly a zero thermal expansion coefficient for a given temperature range above room temperature.

J. Phys. Chem. C. 126 (2022): 19101–19112

https://doi.org/10.1021/acs.jpcc.2c04174

Submonolayer coverages of V-oxide on Rh(111) condense during catalytic methanol oxidation into a pattern of macroscopic stripes or islands. Under reaction conditions, a phase separation occurs within the VO_x islands that has been studied in a pressure range of 10^–6–10^–4 mbar with photoemission electron microscopy (PEEM), low-energy electron microscopy (LEEM), microspot-low-energy electron diffraction (μLEED), and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized outer ring with a (√7 × √7)R19.1° structure coexists with an inner (12 × 12) Moiré-type boundary layer and a reduced core exhibiting a (√3 × √3)R30° Moiré type pattern. The dependence of the substructure on the reaction conditions, on V coverage, and on island size was investigated. With μXPS, the V coverages of the different phases in the VO_x islands were determined.

Dalton Transactions 45 (2022): 4086

https://doi.org/10.1039/D2DT02447K

The double-perovskite series, Sr₂(Fe_1−xNi_x)TeO₆ (x = 0, 0.25, 0.50, 0.75, and 1) has been synthesized in polycrystalline form by solid-state reaction at 1300 K in air. Their crystal structures were probed by powder X-ray diffraction at room temperature. Rietveld analysis revealed that all samples crystallize in the monoclinic space group I2/m. The double-perovskite structures ideally contain two alternating types of octahedra (Fe/Ni)^2dO₆ and (Te)^2aO₆, tilted in the system (a⁻a⁻c⁰). However, the refinements have shown a complex distribution of all three cations over the two available octahedral sites; 2d (½, ½, 0) and 2a (0, 0, 0). Raman spectroscopy further complements the obtained results, by revealing a tiny increase of the wavenumber of some Raman modes when Fe is substituted by Ni. The optical characteristics of the series were determined by fitting diffuse reflectance UV/Vis spectra enabling the optical band gaps to be derived from Tauc method and derivation of absorption spectra fitting (DASF) techniques. Analyses of the obtained ⁵⁷Fe Mössbauer hyperfine parameters at room temperature of samples with compositions x = 0, 0.25, 0.50 and 0.75 reveal the presence of Fe³⁺ in high-spin state with an anti-site disorder of Fe–Ni–Te cations in distorted octahedral environments (site 2d and 2a). The results show that significant correlations exist between the crystal structures and physical properties of double perovskites containing B site transition elements of different charge and size. Temperature-dependent magnetic susceptibility data show magnetic transitions below 40(1) K (38(1) K, 31(1) K, 25(1) K, 20(1) K, and 35(1) K for x = 0, 0.25, 0.50, 0.75, and 1, respectively. A divergence between FC and ZFC curves for all compositions has been observed. The results show that the ground states of the doped materials might be spin glasses or magnetically ordered.

Journal of Materials Science 57 (2022): 19280-19299

https://doi.org/10.1007/s10853-022-07854-w

A new precursor for the formation of mullite-type visible-light active photocatalyst Bi₂Al₄O₉ has been identified. The crystal structure of the organic–inorganic hybrid perovskite can be described using the hexagonal setting of the rhombohedral unit cell with lattice parameters a = 1.1342(2) nm, c = 2.746(1) nm, and V = 3.059(2) nm³. The presence of di-nitro-glycerin as organic component, which is centered together with two bismuth atoms at the A-sites of the ABX₃-type perovskite, suggests for doubling of the a- and c-lattice parameters compared to isostructural BiAlO₃ perovskite. The nano-crystalline precursor with the chemical composition [Bi₂(C₃H₅N₂O₇)]Al₄[O₉(□_1-x(H₂O)_x)₃] (□: vacancies) decomposes at 540(10) K to a quantum-crystalline phase with an average crystallite size of 1.4(1) nm, refined from X-ray powder data Bragg reflections and confirmed by atomic pair distribution function data analysis. Further heating enables a controlled formation of quantum- or nano-crystalline mullite-type phases, depending on temperature and time. The same precursor structure could also be obtained as iron-containing phase and for Al/Fe solid-solution samples. UV/Vis diffuse reflectance spectroscopy suggests an indirect band-gap transition energy of 3.50(3) eV calculated by the Reflectance-Absorption-Tauc-DASF (RATD) method. Temperature-dependent UV/Vis allows to follow the change of band-gap energy across all associated phase transformations. The long- and short-range appearance of each phase has been presented using X-ray Bragg scattering and total scattering data analyses. This is supported by Raman and infrared spectroscopic investigations complemented by density functional theory (DFT) calculations. Moreover, the theoretical calculation confirms the incorporated di-nitro-glycerin. Thermal stabilities of the phases are investigated by using thermal analysis and temperature-dependent X-ray diffraction.

Materials Research Letters 10 (2022): 824-831

https://doi.org/10.1080/21663831.2022.2109941

Nanoporous gold as obtained by corrosion of alloys of gold and a less noble metal is a system with manifold applications. The process of dealloying, however, can result in partially porous samples, when dealloying conditions are not optimized with respect to composition and pre-treatment of the master alloy. In the present work, we investigate the dealloying interface between porous material and non-porous alloy in two samples, in which dealloying either stopped automatically or was interrupted manually. Reasons for unintended termination of dealloying in the former case are suggested.

American Mineralogist 107 (2022): 1668–1680

https://doi.org/10.2138/am-2022-8057

In basaltic volcanic ash recovered from a seamount at 3000 m water depth, we discovered marcasite and pyrite precipitation within cavities that formed by partial to complete dissolution of olivine. In places, these cavities are reminiscent of negative crystal shapes; elsewhere they apparently continue along cracks. In strong contrast, adjacent volcanic glass shows little, if any, evidence for dissolution. The FeS2 precipitates were commonly found to be conjoined and planar aggregates, occurring in the center of the voids. Their maximum volume fraction in relation to the void space as determined by 2D and 3D imaging techniques corresponds to the amount of iron released by olivine dissolution. Almost all occurrences of FeS2 precipitation are related to Cr-spinel inclusions in the former olivine. We propose that rapid olivine dissolution was initiated by reduced H2S-bearing fluids at olivine grain boundaries or surfaces exposed by cracks. Many of these cracks are connected to spinel grains, where the iron liberated from olivine is mineralized as FeS2, initially facilitated by heterogeneous nucleation. Subsequent pyrite and/or marcasite precipitation occurred as overgrowths on existing FeS2 aggregates. The particular chemical environment of low-pH, hydrogen sulfide-bearing fluids may have enhanced olivine dissolution by (1) keeping Fe in solution and (2) sequestering important quantities of Fe as FeS2. The in situ oxidation of ferrous Fe and precipitation of ferric hydroxides at the olivine surface commonly observed in oxic environments were obviously impeded. It would have slowed down olivine dissolution to rates more similar to the dissolution of basaltic glass. We have no direct indication that the process of rapid olivine dissolution was aided by subseafloor life. However, the presence of fibrous structures with small sulfide particles could indicate late colonization of sulfate-reducing bacteria that may add an additional path of iron fixation.

Ultramicroscopy 238 (2022): 113503

https://doi.org/10.1016/j.ultramic.2022.113535

In this paper we perform angular resolved annular-dark field (ADF) scanning-transmission electron microscopy (STEM) to study the scattered intensity in an InGaN layer buried in GaN as a function of the scattering angle. We achieved angular resolution with a motorized iris aperture in front of the ADF detector. Using this setup, we investigated how the intensities measured in various angular ranges agree with multislice simulations in the frozen-lattice approximation. We observed a strong influence of relaxation induced surface-strain fields on the ADF intensity, measured its angular characteristics and compared the result with simulations.

To assess the agreement of the measured intensity with simulations, we evaluated the specimen thickness in GaN and the indium concentration in InGaN for each angular interval by comparing the measured intensities with simulations. The thickness was strongly overestimated for scattering angles below 40 mrad and also the evaluated indium concentration varies with the considered angular range. Using simulations, we investigated which angular ranges show a high sensitivity to variations of the thickness and which intervals strongly depend on the indium concentration. By combining two angular intervals, the indium concentration and the specimen thickness were determined simultaneously, which has potential advantages over the usual quantification method. It is shown that inelastic scattering, surface contamination and mistilt can have an influence on the measured intensity, especially at lower scattering angles below 30–50 mrad, which might explain the observed difference between the frozen lattice simulation and the experiment.

Ultramicroscopy 236 (2022): 113503

https://doi.org/10.1016/j.ultramic.2022.113503

The measurement of electric fields in scanning transmission electron microscopy (STEM) is a highly investigated field of research. The constant improvement of spatial resolution in STEM and the development of new hardware for the fast acquisition of diffraction patterns even paved the way for the measurement of atomic electric fields. Although the basic principle that an electric field leads to a tilt of the focussed electron probe that can be detected as a shift of the diffraction pattern in the back focal plane of the objective lens seems quite simple, many challenges arose in the measurement of fields in a quantitative way. In the present study we investigate whether a shift of the diffraction pattern that occurs at an interface between two materials can be related to the electric field which is caused by the difference of the mean inner potentials of the two materials. To this end, experiments and simulations are compared. It is demonstrated that the difference in mean inner potential has an influence on the observed effect, but a quantitative interpretation is difficult. The influence of image recording effects such as shot noise and the modulation transfer function are investigated as well as further effects such as e.g. sample tilt. In addition, the influence of the observed effect on a strain measurement is shown.

ACS Catal. 12 (2022): 4415–4429

https://doi.org/10.1021/acscatal.1c05160

Nanoporous gold (NPG) obtained by dealloying Ag75Au25 with an overall residual Ag content of less than 1% was investigated as an electrocatalyst for the oxidation of methanol, formaldehyde, and formate in aqueous 0.1 M NaOH solution. The NPG was used to fill cavity microelectrodes, which allowed the recording of well-resolved voltammetry from the porous material. NPG differs from polycrystalline Au (Au(poly)) by its microstructure and its residual Ag content and also behaves distinctly different than Au(poly). The residual Ag content is higher at the surface of the ligaments than in the bulk. By cycling the NPG electrodes in 0.1 M H2SO4, the surface concentration of Ag could be decreased. It could then be set to a defined value by underpotential deposition (UPD) of Ag. The surface structure, and specifically its evolution upon the removal of Ag from the surface, was analyzed by the characteristic voltammetric features of Pb UPD. The effect of Ag on the electrocatalytic methanol oxidation reaction (MOR) is different in different potential regions. Ag coverage shifts the onset of the methanol oxidation current to less positive potentials. In the range of the peak current density, only a defined low Ag concentration enhanced the MOR current density compared to the Ag-free NPG. The {1 0 0} and {1 1 1} facets contributed the largest current, as concluded from selective poisoning experiments. At a potential of 1.63 V vs RHE, Ag2O at the surface is oxidized to AgO. Those layers can oxidize methanol and formate to CO2. The oxidation of formaldehyde proceeds at a much higher reaction rate than the MOR and formate oxidation; the reaction leads to CO and CO2 depending on the applied potential. Given the high oxidation rate of formaldehyde, it would be immediately further oxidized should it be formed as an intermediate of MOR. This is an important difference to the methanol oxidation at Pt. The water oxidation that occurs at the same potential range in the methanol-free solution was suppressed during CO2 formation.

Crystal Growth and Design 22 (2022): 2557–2568

https://doi.org/10.1021/acs.cgd.2c00030

We demonstrate the growth of large (Mg,Zr):SrGa₁₂O₁₉ (SGMZ) single crystals and use a combination of X-ray imaging techniques to analyze their structural and chemical homogeneity. Single-crystal cylinders with lengths and diameters up to about 2.5 cm are achieved. Our characterization of polished sections reveals a localized (0001) facet that is typically formed at the center of the growth interface. Such facets are seen as the key factor limiting the growth of large-area crystals with excellent structural quality due to local deviations in the segregation behavior of the dopants. We developed a lab-based X-ray diffraction imaging technique with high sensitivity that exposes subtle variations in lattice parameters and lattice tilts, which are attributed to changes in the chemical composition and the resulting elastic deformation. The relationship between unit-cell dimensions and composition is verified by micro X-ray fluorescence mapping. In this way, we find a Ga-rich center region with a reduced unit-cell volume that is surrounded by a ring of increased tilt and elastic strain. Furthermore, we observe a 6-fold in-plane anisotropy of dopant incorporation and tree-ring-shaped structures caused by macrosteps. With rocking curve widths below 23 arcsec in ∼90% of the crystal, SGMZ crystals are largely homogeneous and hence suitable for the preparation of high-quality substrates. For most applications, the substantially enhanced crystal size enabled by very high Mg and Zr codoping levels outweighs the issues related to concentration variations arising from their addition.

Open Ceramics 9 (2022): 100228

https://doi.org/10.1016/j.oceram.2022.100228

We demonstrate the feasibility of plasmonic porous ceramics which combine the optical properties of plasmonic nanoparticles with the advantages of open porous ceramics. To this end, we prepared open porous structures for surface-enhanced Raman spectroscopy (SERS) based on zirconia-toughened alumina on which we deposited silver nanoparticles. The Raman enhancement of the plasmonic structures was analyzed as a function of the amount of deposited silver nanoparticles, pore diameter and strut diameter of the ceramic structure using the probe molecule pyridine. Flat substrates of the same chemical composition and non-porous fragments of the porous structure were used for comparison. The Raman signal is found to be significantly augmented by the porous structure compared to that collected on flat substrates with similar composition. Accordingly, we propose that the plasmonic porous ceramics are well suited as 3D SERS substrates, allowing real-time Raman sensing of trace amounts of molecules.

Zeitschrift für Kristallographie - Crystalline Materials 237 (2022): 1-3

https://doi.org/10.1515/zkri-2022-0004

Twelve cubic sodalites |Na₈X₂|[T¹T²O₄]₆ (T¹ = Al³⁺, Ga³⁺; T² = Si⁴⁺, Ge⁴⁺; X = Cl⁻, Br⁻, I⁻) were examined using high-temperature (HT) X-ray diffraction experiments and TGA-DSC measurements. Temperature-dependent structure data was obtained by Rietveld refinements. Decomposition temperatures were determined using TGA-DSC data for all compounds. The temperature-dependent volume expansion was used to determine Debye and Einstein temperatures using DEA fits. Distinct relations between thermal expansion, bond lengths and the decomposition temperature could not be found. Determination of Lindemann constants of all compounds enables a classification of the sodalites in three groups.

Journal of the American Ceramic Society (2021), 105, 2702-2712

https://doi.org/10.1111/jace.18261

Mullite-type RMn₂O₅ (R = Y, rare-earth element) ceramics are of ongoing research attention because of their interesting crystal-chemical and magnetic properties. We report nuclear and magnetic structures of NdMnTiO₅ together with its spectroscopic, thermogravimetric, and magnetic properties. The polycrystalline sample is prepared by solid-state synthesis and characterized from neutron and X-ray powder diffraction data Rietveld refinements. NdMnTiO₅ crystallizes in the orthorhombic space group Pbam with metric parameter a  =  755.20(1) pm, b  =  869.91(1) pm, c  =  582.42(1) pm, and V  =  382.62(1) 10⁶ pm³. The Mn³⁺ and Ti⁴⁺ cations are observed to be located in the octahedral and pyramidal sites, respectively. The vibrational features in these polyhedral sites are characterized by Raman and Fourier transform infrared spectroscopes. The higher decomposition temperature of NdMnTiO₅, compared to other RMn₂O₅ phases, is explained in terms of the higher bond strength of Ti-O bonds than those of Mn-O bonds. Temperature-dependent DC magnetic susceptibility suggests a paramagnetic to antiferromagnetic phase transition at 43(1) K. Inverse susceptibility in the paramagnetic region above 120 K follows the Curie-Weiss law, resulting in a magnetic moment of 6.33(1) μ_B per formula unit. Neutron diffraction data collected at 7.5 K reveal that the magnetic moments of Nd³⁺ and Mn³⁺ in NdMnTiO₅ are incommensurately ordered with a propagation vector k = (0, 0.238, 0.117).

Zeitschrift für Kristallographie - Crystalline Materials 236 (2021): 283-292

https://doi.org/10.1515/zkri-2021-2046

Pb₂Sc₂Si₂O₉ and Pb₂In₂Si₂O₉, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna2₁ space group and undergo phase transitions to the centrosymmetric Pbcn space group at elevated temperatures. The Pbcn into Pna2₁ phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.

Journal of Physical Chemistry C 125 (2021): 22539–22546

https://doi.org/10.1021/acs.jpcc.1c06106

With LEEM (low-energy electron microscopy) and micro-LEED as in situ techniques we have studied the structural transitions in the excitation of traveling interface pulses (TIPs) in bistable methanol oxidation on a bare Rh(110) and on a Rh(110) surface covered with a 0.1 monolayer of V oxide in the 10^–4 mbar range. Close to equistability, a (1×1) structure coexists with O-induced reconstructions of the “missing row” type at the interface. An oxidation pulse traveling along the interface exhibits a substructure consisting of various reconstructions of the “missing row” type; on the reduced surface, the slow development of a c(2×2) structure is accompanied by a strong loss of the (0,0)-beam intensity. The addition of 0.1 monolayer of V oxide increases structural disorder but causes no qualitative changes in the structural transitions.

Geochimica et Cosmochimica Acta 309 (2021): 329-351

https://doi.org/10.1016/j.gca.2021.06.032

Intrinsic heterogeneities of crystals almost always impose challenges on the investigation of dissolution kinetics. In the case of the incomplete solid solution series of alkali feldspars, additional complexity to the wide range in chemical composition and crystallographic characteristics is introduced in the form of chemical differentiation at low cooling temperatures. In this paper, we use direct measurements of intact crystal surfaces to quantify the dissolution rate of an alkali feldspar, providing details unavailable to powder dissolution experiments alone. In particular, we show how complex intergrowth textures characteristic of feldspars may play an important role in terms of how these minerals dissolve.

By means of a comprehensive experimental and analytical framework using Raman coupled vertical scanning interferometry (Rc-VSI), atomic force microscopy (AFM), light microscopy (LM), electron probe microanalysis (EPMA) and scanning electron microscopy (SEM), detailed information on heterogeneities in surface reactivity, morphology, chemistry, structure and texture of the dissolving surface are provided. The combination of spatially resolved qualitative and quantitative data revealed significant differences both in absolute rates and rate distributions associated with the potassium- and sodium-rich compositional compounds of which the latter dissolved slower with less dispersion in rates than the former. Whereas the spatial dissolution rate variances could not be explained by compositional differences between the two distinct feldspar phases alone, surface reactivity was successfully shown to vary with unique properties of the surface texture as well. A close relationship between enhanced dissolution and defect site distribution was observed, which critically depends on intergrowth geometry and phase internal twinning patterns. These observations are of great importance to any kind of experimental study and modeling approach which aims to improve the understanding of the dissolution kinetics of chemically and crystallographically complex materials.

© The Authors, https://doi.org/10.1016/j.gca.2021.06.032, licensed under CC BY-NC-ND 4.0

Metals 11 (2021): 1173

https://doi.org/10.3390/met11081173

Recently, laser additive manufacturing (AM) techniques have emerged as a promising alternative for the synthesis of bulk metallic glasses (BMGs) with massively increased freedom in part size and geometry, thus extending their economic applicability of this material class. Nevertheless, porosity, compositional inhomogeneity, and crystallization display themselves to be the emerging challenges for this processing route. The impact of these “defects” on the surface reactivity and susceptibility to corrosion was seldom investigated but is critical for the further development of 3D-printed BMGs. This work compares the surface reactivity of cast and additively manufactured (via laser powder bed fusion—LPBF) Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ metallic glass after 21 days of immersion in a corrosive HCl solution. The cast material presents lower oxygen content, homogeneous chemical distribution of the main elements, and the surface remains unaffected after the corrosion experimentation based on vertical scanning interferometry (VSI) investigation. On the contrary, the LPBF material presents a considerably higher reactivity seen through crack propagations on the surface. It exhibits higher oxygen content, heterogeneous chemical distribution, and presence of defects (porosity and cracks) generated during the manufacturing process.

Front. Phys. 9 (2021): 654845

https://doi.org/10.3389/fphy.2021.654845

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS₂ bilayer forms at the boundaries of single-layer single-domain MoS₂ islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.

Microscopy and Microanalysis 27 (2021): 678 - 686

https://doi.org/10.1017/S1431927621000519

Quantitative structural characterization of nanomaterials is important to tailor their functional properties. Corrosion of AgAu-alloy nanoparticles (NPs) results in porous structures, making them interesting for applications especially in the fields of catalysis and surface-enhanced Raman spectroscopy. For the present report, structures of dealloyed NPs were reconstructed three-dimensionally using scanning transmission electron microscopy tomography. These reconstructions were evaluated quantitatively, revealing structural information such as pore size, porosity, specific surface area, and tortuosity. Results show significant differences compared to the structure of dealloyed bulk samples and can be used as input for simulations of diffusion or mass transport processes, for example, in catalytic applications.

Applied Surface Science 549 (2021): 149317

https://doi.org/10.1016/j.apsusc.2021.149317

The surface reaction of olivine materials is critical for their extensive applications in the global cycle of elements, future CO₂ sequestration and fuel production. Here, we investigate a pristine (0 1 0) surface dissolved in flow-through cells with acidic solutions at surface control regime. The multiscale surface topography changes during dissolution are firstly measured by a combination of ex-situ vertical scanning interferometry and in-situ atomic force microscopy (AFM). The deduced dissolution rate maps and rate spectra from surface topography vary temporally and spatially, wherein the average rate is higher at initial stages and reaches to a constant value, about 6.6 ± 1 × 10⁻⁸ mol m⁻² s⁻¹, after 100 h reaction accompanied with the formation of crystallographically controlled pits. Furthermore, in-situ AFM shows that the average height of steps on the pit walls is equal to 1/2b and Raman spectra prevails that the (0 1 0) surface is favorably dissolved layer by layer at Mg2 glide plane with SiO₄⁴⁻ distortion. This work reveals that at far from equilibrium conditions with constant Gibbs free energy, surface reactivity is better defined by rate ranges rather than a single mean rate. This superior method is powerful to quantify surface reaction variabilities in multiscale range and understand the geochemical cycles.

Ultramicroscopy 223 (2021): 113221

https://doi.org/10.1016/j.ultramic.2021.113221

Modern quantitative TEM methods such as the -factor technique require precise knowledge of the electron beam current. To this end, a macroscopic Faraday cup was designed and constructed. It can replace the viewing screen in the projection chamber of a TEM and guarantees highly accurate measurement of the electron beam with precision only limited by the used amperemeter. The easy to install, affordable device is shown to be highly apt for precision measurement of currents > 5pA. The Faraday cup results are used for an assessment and a comparison of various other beam current measurement methods. It is found that the built-in screen amperemeter of the used TEM is quite inaccurate and that measurements using the screen in general tend to underestimate the current. If present, the drift tube of a spectrometer can also be used as a Faraday cup, but certain described peculiarities have to be taken into account. Direct ultrafast electron detection cameras allow precise measurement at very small currents. For the electron counting technique, which exploits single electron detection capabilities of STEM detectors, a systematic current underestimation was observed and investigated. This results in a reformulated routine for the method and with these improvements it is demonstrated to be capable of accurate high-precision measurements for currents < 5pA.

Ultramicroscopy 221 (2021): 113175

https://doi.org/10.1016/j.ultramic.2020.113175

The angle-resolved electron scattering is investigated in scanning-transmission electron microscopy (STEM) using a motorised iris aperture placed above a conventional annular detector. The electron intensity scattered into various angle ranges is compared with simulations that were carried out in the frozen-lattice approximation. As figure of merit for the agreement of experiment and simulation we evaluate the specimen thickness which is compared with the thickness obtained from position-averaged convergent beam electron diffraction (PACBED). We find deviations whose strengths depend on the angular range of the detected electrons. As possible sources of error we investigate, for example, the influences of amorphous surface layers, inelastic scattering (plasmon excitation), phonon-correlation within the frozen-lattice approach, and distortions in the diffraction plane of the microscope. The evaluation is performed for four experimental thicknesses and two angle-resolved STEM series under different camera lengths. The results clearly show that especially for scattering angles below 50 mrad, it is mandatory that the simulations take scattering effects into account which are usually neglected for simulating high-angle scattering. Most influences predominantly affect the low-angle range, but also high scattering angles can be affected (e.g. by amorphous surface covering).

Lithos 382-383 (2021): 105949

https://doi.org/10.1016/j.lithos.2020.105949

Crystal morphologies are essential for deciphering the reaction history of igneous and metamorphic rocks because they often record the interplay between nucleation and growth rates controlled by the departure from equilibrium. Here, we report an exceptional record of the morphological transition of olivine formed during subduction metamorphism and high-pressure dehydration of antigorite-serpentinite to prograde chlorite-harzburgite in the Almirez ultramafic massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain). In this massif, rare varied-textured chlorite-harzburgite (olivine+enstantite+chlorite+oxides) —formed after high–P dehydration of antigorite-serpentinite— exhibits large olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains, similar to single tabular olivines occurring in the matrix. The correlative X-ray μ-CT and EBSD study of two varied-textured chlorite-harzburgite samples show that tabular olivine in coronas is tabular on (100)_Ol with c > b >> a, and grew in nearly the same crystallographic orientation as the rounded olivine cores of the porphyroblast.

Ultramicroscopy 221 (2021): 113196

https://doi.org/10.1016/j.ultramic.2020.113196

Strain analysis by nano-beam electron diffraction allows for measurements of strain with nanometre resolution in a large field of view. This is done by evaluating distances between diffraction discs in diffraction patterns acquired while a focussed electron beam is scanned across the sample in a transmission electron microscope. The bottleneck of this method is a precise determination of diffraction disc positions, which suffers from the inner structure of the discs caused by dynamical diffraction. Electron beam precession is a tool that solves this problem but it is not commonly available in every microscope. Without precession significant progress has been reported recently by using patterned condenser apertures. The pattern of the aperture is reproduced in patterns of the diffraction discs allowing for a more precise position determination. In this report the accuracy of measured strain profiles using patterned apertures is investigated by evaluation of realistic simulations. This is done especially at interfaces between regions with different lattice plane spacing. It is found by evaluation of the simulations that measured strain profiles are more blurred and hence the accuracy at the interface is worse the more patterns are imprinted to the condenser aperture. An explanation of this effect is given and as a proof of principle a solution to this problem is provided applying geometric phase analysis ptychography.

Nature Scientific Reports 10 (2020): 22374

https://doi.org/10.1038/s41598-020-78584-9

Vanadium dioxide (VO₂) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.

Inorg. Chem 59 (2020): 18214–18224

https://doi.org/10.1021/acs.inorgchem.0c02684

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi₂RE(BO₃)₂ (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi₂RE(BO₃)₂ members are isotypic and crystallize in the space group P2₁/n. The novel structure type is comprised of [RE₂(BO₃)₄O₄]^14– anionic clusters where the edge-sharing REO₇ pentagonal bipyramids are connected by BO₃ groups and both K⁺ and Li⁺ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi₂RE(BO₃)₂ and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV–vis spectra display the characteristic absorption behaviors of the RE³⁺ cations and the fundamental absorption edge. Both the Tauc’s and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi₂RE(BO₃)₂. The incongruent melting behavior and the thermal stability of each member of the KLi₂RE(BO₃)₂ series were also studied by thermogravimetric analyses.

Zeitschrift für Naturforschung B 75 (2020): 9-10

https://doi.org/10.1515/znb-2020-0159

Synthesis, crystal structure and temperature-dependent behavior of Na₂H₄Ga₂GeO₈ are reported. This novel gallogermanate crystallizes in space group I4₁/acd with room-temperature powder diffraction lattice parameters of a = 1298.05(1) pm and c = 870.66(1) pm. The structure consists of MO₄ (M = Ga, Ge) tetrahedra in four-ring chains, which are connected by two different (left- and right-handed) helical chains of NaO₆ octahedra. Protons coordinating the oxygen atoms of the GaO₄ tetrahedra not linked to germanium atoms ensure the charge balance. Structure solution and refinement are based on single crystal X-ray diffraction measurements. Proton positions are estimated using a combined approach of DFT calculations and NMR, FTIR and Raman spectroscopic techniques. The thermal expansion was examined in the range between T = 20(2) K and the compound’s decomposition temperature at 568(5) K, in which no phase transition could be observed, and Debye temperatures of 266(11) and 1566(65) K were determined for the volume expansion.

Nature Scientific Reports 10 (2020): 17890

https://doi.org/10.1038/s41598-020-74434-w

Scanning transmission electron microscopy (STEM) allows to gain quantitative information on the atomic-scale structure and composition of materials, satisfying one of todays major needs in the development of novel nanoscale devices. The aim of this study is to quantify the impact of inelastic, i.e. plasmon excitations (PE), on the angular dependence of STEM intensities and answer the question whether these excitations are responsible for a drastic mismatch between experiments and contemporary image simulations observed at scattering angles below ∼ 40 mrad. For the two materials silicon and platinum, the angular dependencies of elastic and inelastic scattering are investigated. We utilize energy filtering in two complementary microscopes, which are representative for the systems used for quantitative STEM, to form position-averaged diffraction patterns as well as atomically resolved 4D STEM data sets for different energy ranges. The resulting five-dimensional data are used to elucidate the distinct features in real and momentum space for different energy losses. We find different angular distributions for the elastic and inelastic scattering, resulting in an increased low-angle intensity (∼10–40 mrad). The ratio of inelastic/elastic scattering increases with rising sample thickness, while the general shape of the angular dependency is maintained. Moreover, the ratio increases with the distance to an atomic column in the low-angle regime. Since PE are usually neglected in image simulations, consequently the experimental intensity is underestimated at these angles, which especially affects bright field or low-angle annular dark field imaging. The high-angle regime, however, is unaffected. In addition, we find negligible impact of inelastic scattering on first-moment imaging in momentum-resolved STEM, which is important for STEM techniques to measure internal electric fields in functional nanostructures. To resolve the discrepancies between experiment and simulation, we present an adopted simulation scheme including PE. This study highlights the necessity to take into account PE to achieve quantitative agreement between simulation and experiment. Besides solving the fundamental question of missing physics in established simulations, this finally allows for the quantitative evaluation of low-angle scattering, which contains valuable information about the material investigated.

Zeitschrift für Kristallographie - Crystalline Materials 235 (2020): 6-7

https://doi.org/10.1515/zkri-2020-0027

The temperature-dependent structure-property relationships of the aluminosilicate perrhenate sodalite |Na₈(ReO₄)₂|[AlSiO₄]₆ (ReO₄-SOD) were analysed via powder X-ray diffraction (PXRD), Raman spectroscopy and heat capacity measurements. ReO₄-SOD shows two phase transitions in the investigated temperature range (13 K < T < 1480 K). The first one at 218.6(1) K is correlated to the transition of dynamically ordered P¯43n (> 218.6(1 K) to a statically disordered (<218.6(1) K) SOD template in P¯43n. The loss of the dynamics of the template anion during cooling causes an increase of disorder, indicated by an unusual intensity decrease of the 011-reflection and an increase of the Re-O₂ bond length with decreasing temperature. Additionally, Raman spectroscopy shows a distortion of the ReO₄ anion. Upon heating the thermal expansion of the sodalite cage originated in the tilt-mechanism causes the second phase transition at 442(1) K resulting in a symmetry-increase from P¯43n to Pm¯3n, the structure with the sodalites full framework expansion. Noteworthy is the high decomposition temperature of 1320(10) K.

Advanced Engineering Materials 22 (2020): 2000352

https://doi.org/10.1002/adem.202000352

To translate the exceptional properties of colloidal nanoparticles (NPs) to macroscale geometries, assembly techniques must bridge a 10⁶‐fold range of length. Moreover, for successfully attaining a final mechanically robust nanocomposite macroscale material, some of the intrinsic NPs’ properties have to be maintained while minimizing the density of strength‐limiting defects. However, the assembly of nanoscale building blocks into macroscopic dimensions, and their effective macroscale properties, are inherently affected by the precision of the conditions required for assembly and emergent flaws including point defects, dislocations, grain boundaries, and cracks. Herein, a direct‐write self‐assembly technique is used to construct free‐standing, millimeter‐scale columns comprising spherical iron oxide NPs (15 nm diameter) surface functionalized with oleic acid (OA), which self‐assemble into face‐centered cubic (FCC) supercrystals in minutes during the direct‐writing process. The subsequent crosslinking of OA molecules results in nanocomposites with a maximum strength of 110 MPa and elastic modulus up to 58 GPa. These mechanical properties are interpreted according to the flaw size distribution and are as high as newly engineered platelet‐based nanocomposites. The findings indicate a broad potential to create mechanically robust, multifunctional 3D structures by combining additive manufacturing with colloidal assembly.

Surface Science 694 (2020): 121561

https://doi.org/10.1016/j.susc.2019.121561

The adsorption of S on Si(111)- 7  ×  7 has been investigated for different preparation schemes and parameters. S was supplied from an electrochemical Ag₂S cell. For room temperature adsorption and subsequent annealing, no ordered S induced reconstruction can be observed with spot profile analysis low-energy electron diffraction (SPALEED). S deposition at temperatures above about 400 ^∘C, however, leads to a well-ordered reconstruction. Judging from the LEED pattern, the same reconstruction was already observed by Metzner et al. [Surf. Sci. 377–379 (1997) 71–74] who identified it as 4  ×  4 reconstruction. The upper temperature limit for the preparation of this superstructure depends on S flux, which is needed to compensate for desorption. Prolonged S exposure leads to surface roughening, as observed with SPALEED and scanning tunneling microscopy (STM), pointing to surface etching by S. From our SPALEED data, we can conclude that the observed reconstruction is not a 4  ×  4 reconstruction, but a superstructure with a rectangular unit cell that exists in three rotational domains, as confirmed by STM. Different structural trial models have been assessed with density functional theory. Among these model structures, a configuration with dimers adsorbed on bridging sites, with a S coverage of 1 monolayer, is most likely, since it is energetically favorable and is in agreement with all experimental results.

American Journal of Science 320 (2020): 27-52

doi: 10.2475/01.2020.03 

Dissolution rates of porous crystalline materials reflect the superposition of transport and surface control, mainly via the parameters saturation of the ambient fluid and distribution of surface energy. As a result, reacting surfaces evolve over time showing a heterogeneous distribution of surface rates. The spatiotemporal heterogeneity of surface reaction rates is analyzed using the rate map and rate spectra concept. Here, we quantify the dissolution rate variability covering the nm- to mm-scale of dissolving single-crystal and polycrystalline calcite samples, using a combined approach of X-ray micro-computed tomography (μ-CT) and vertical scanning interferometry (VSI). The dissolution experiments cover reaction periods from 15 minutes up to 54 days. The observed rate ranges are remarkably consistent over the entire reaction period but include a variability of about two orders of magnitude (10⁻⁹ − 3 × 10⁻⁷ mol m⁻² s⁻¹). The rate map data underscore the concurrent and superimposing impact of surface- vs. fluid flow controlled rate portions. The impact of fluid flow on reactivity at the mm-scale in the transport-controlled system is confirmed by 2-D reactive transport modeling. The sub-mm spatial heterogeneity of low vs. high reactivity surface portions of polycrystalline calcite is clearly below the mean crystal size. This suggests the dominant impact of highly reactive surface portions irrespective of the orientation of larger crystals on the overall surface reactivity. Correspondingly, the overall range of intrinsic reactivity heterogeneity as observed using singly crystal material is not further expanded for polycrystalline material. As a general conclusion, numerical reactive transport concepts would benefit from the implementation of a reactivity term resembling the experimentally observed existence of multiple rate components.

Nano Letters (2019) 19, 6554–6563

https://doi.org/10.1021/acs.nanolett.9b02798

As a key player in blood coagulation and tissue repair, fibrinogen has gained increasing attention to develop nanofibrous biomaterial scaffolds for wound healing. Current techniques to prepare protein nanofibers, like electrospinning or extrusion, are known to induce lasting changes in the protein conformation. Often, such secondary changes are associated with amyloid transitions, which can evoke unwanted disease mechanisms. Starting from our recently introduced technique to self-assemble fibrinogen scaffolds in physiological salt buffers, we here investigated the morphology and secondary structure of our novel fibrinogen nanofibers. Aiming at optimum self-assembly conditions for wound healing scaffolds, we studied the influence of fibrinogen concentration and pH on the protein conformation. Using circular dichroism and Fourier-transform infrared spectroscopy, we observed partial transitions from α-helical structures to β-strands upon fiber formation. Interestingly, a staining with thioflavin T revealed that this conformational transition was not associated with any amyloid formation. Toward novel scaffolds for wound healing, which are stable in aqueous environment, we also introduced cross-linking of fibrinogen scaffolds in formaldehyde vapor. This treatment allowed us to maintain the nanofibrous morphology while the conformation of fibrinogen nanofibers was redeveloped toward a more native state after rehydration. Altogether, self-assembled fibrinogen scaffolds are excellent candidates for novel wound healing systems since their multiscale structures can be well controlled without inducing any pathogenic amyloid transitions.

Science of The Total Environment (2019) 673, 13-621

https://doi.org/10.1016/j.scitotenv.2019.03.493

Fate, bioavailability and toxicity of silver nanoparticles (AgNP) are largely affected by soil properties. Here we focused on how these processes are connected in simulated soil pore water. OECD soil components (sand, kaolin clay, peat) were covered with NM-300K-, AgNO₃- and NM-300K dispersant-contaminated water, and Folsomia candida were exposed on the water surface. After 14 days the majority of AgNP was in nano form in sand pore water where also silver uptake was highest. Multilayered cross sections from X-ray micrographs of Collembola exposed to AgNP showed that silver was located in animal areas of direct contact to the contaminated pore water and was ingested. In contrast, in simulated peat pore water only a small fraction of silver was bioavailable. AgNO₃ was only bioavailable at the start of the test and not anymore at test end. AgNP and AgNO₃ caused immobilization in sand and kaolin pore water while no toxicity was found with peat and OECD soil. A strong correlation (correlation coefficient = 0.901) existed between the concentration of nano silver and immobilization; for ionic silver this was not the case. The dispersant of AgNP was toxic on its own in sand and kaolin pore water. As there are analytical limitations of quantifying AgNP in complex matrices this test system enables a mechanistic view of exposure and uptake of AgNP (and other substances) by F. candida from soil pore water.

Journal of Anatomy (2019) 234, 656-667

https://doi.org/10.1111/joa.12964

Starfish (order: Asteroidea) possess a complex endoskeleton composed of thousands of calcareous ossicles. These ossicles are embedded in a body wall mostly consisting of a complex collagen fiber array. The combination of soft and hard tissue provides a challenge for detailed morphological and histological studies. As a consequence, very little is known about the general biomechanics of echinoderm endoskeletons and the possible role of ossicle shape in enabling or limiting skeletal movements. In this study, we used high‐resolution X‐ray microscopy to investigate individual ossicle shape in unprecedented detail. Our results show the variation of ossicle shape within ossicles of marginal, reticular and carinal type. Based on these results we propose an additional classification to categorize ossicles not only by shape but also by function into ‘connecting’ and ‘node’ ossicles. We also used soft tissue staining with phosphotungstic acid successfully and were able to visualize the ossicle ultrastructure at 2‐μm resolution. We also identified two new joint types in the aboral skeleton (groove‐on‐groove joint) and between adambulacral ossicles (ball‐and‐socket joint). To demonstrate the possibilities of micro‐computed tomographic methods in analyzing the biomechanics of echinoderm skeletons we exemplarily quantified changes in ossicle orientation for a bent ray for ambulacral ossicles. This study provides a first step for future biomechanical studies focusing on the interaction of ossicles and soft tissues during ray movements.

Physical Review Letters (2019) 122, 106102

https://doi.org/10.1103/PhysRevLett.122.106102

We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of 5.3±1.5  MV/cm is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.

Chemical Geology (2019) 504, 216-235

https://doi.org/10.1016/j.chemgeo.2018.11.016

In a unique “perspectives” format that examines both past and future, we appraise the field of crystal dissolution kinetics, showing how the last century's strong progress in experimental discovery has both driven, and been driven by, the tandem evolution of basic theory. To provide context for examining the current state-of-the-art in this critical field, we highlight the key milestones that have punctuated our progress in understanding the dynamics of crystalline surfaces. For crystal growth, these are the energy relations between kinks on stepped surfaces, and the phenomena of screw dislocations sustaining steady state growth at low thermodynamic overstep. For crystal dissolution, the corresponding recognition is the tie between defects, hollow cores, and macroscopic etch pits. These latter relationships have been more recently formalized in the stepwave model, incorporating etch pit nucleation, step generation, and global retreat of the crystal surface: the total dissolution rate. All these conceptual advances contain an assertion of a link, fundamental but often implicit, between mass action and kinetics, where chemical potential is the primary driver of rates of physical process. This link is inherent in many “classical” rate equations, whose parameterization is often the endgame of laboratory observations.

Today, this extant framework serves as the conceptual basis for organizing the data available from a sophisticated suite of analytical and experimental instrumentation. These resources permit ever-increasing resolution of reacting surfaces in breathtaking detail, often under in situ conditions. These direct observations are now further enhanced by powerful computer-driven simulation and numerical modelling, allowing the virtual exploration of complex reaction systems, ranging from isolated single crystals to porous, multiphase networks. Despite the exhilarating breadth and detail of these accomplishments, it is also becoming increasingly apparent that we are moving further, not closer, from the goal of predictive understanding, a goal that is an increasingly vital social responsibility of our science. A major source of this divergence reflects the fact that at key intersecting points of study, our prowess in technical observation has effectively outpaced our theoretical understanding. In confronting the daunting complexity of these systems, we must be careful to first identify major vacancies in theory. Until we resolve these deficits, more observations may be of only limited utility.

In assessing this problem, a major uncertainty is how to properly reconcile thermodynamics, by its very nature a macroscopic formalism, with our current focus on atomic scales of reaction. This may be a problem unique to crystalline materials and their interactions with phases whose components are otherwise mobile. Detailed balancing and related microscopic reversibility, the implicit link referred to above, is often used to form a mechanistic bridge between the macroscopic distribution of energy and microscopic heterogeneity of events in crystal surfaces, but its employment creates two problems: spatial and temporal. First, reaction mechanism is truly atomic in dimension, involving actual, nondegenerate collisions at crystal surface sites, whereas ∆G_r or ∆μ is macroscopic. Second, the rate at which a crystal surface dissolves reflects both the chemical composition of the ambient fluid and the distribution of surface energy. Reaction towards “equilibrium”, involving the typically slow redistribution of surface energy, may thus inherit topography inconsistent with the computed “driving force”. This reactivity mismatch yields surfaces that evolve over time, producing a heterogeneous distribution of rates. This distribution can be efficiently characterized by rate spectra: the span of non-steady-state rates reflecting diversity of reactive sites established under previous ∆G_r regimes. We use these spectra as a basic compact variable: a signal that encodes the complex link between site-specific surficial energy distributions, solution and surface chemistry, and the cumulative rate that results. Because this encoding is efficiently captured by numerous surface analytical microscopies (VSI, AFM), this approach permits the testing of hypotheses regarding the probabilistic nature of rate distributions, a process we hope the community will embrace, serving ultimately as a key step forward in establishing useful predictive approaches. We illustrate this potential with a series of case studies that target a range of composition, space, and time scales.

Ultramicroscopy (2019) 196, 74-82

Images acquired in transmission electron microscopes can be distorted for various reasons such as e.g. aberrations of the lenses of the imaging system or inaccuracies of the image recording system. This results in inaccuracies of measures obtained from the distorted images. Here we report on measurement and correction of elliptical distortions of diffraction patterns. The effect of this correction on the measurement of crystal lattice strain is investigated. We show that the effect of the distortions is smaller than the precision of the measurement in cases where the strain is obtained from shifts of diffracted discs with respect to their positions in images acquired in an unstrained reference area of the sample. This can be explained by the fact that diffraction patterns acquired in the strain free reference area of the sample are distorted in the same manner as the diffraction patterns acquired in the strained region of interest. In contrast, for samples without a strain free reference region such as nanoparticles or nanoporous structures, where we evaluate ratios of lattice plane distances along different directions, the distortions are usually not negligible. Furthermore, two techniques for the detection of diffraction disc positions are compared showing that for samples in which the crystal orientation changes over the investigated area it is more precise to detect the positions of many diffraction discs simultaneously instead of detecting each disc position independently.

npj Materials Degradation (2018) 2, 40

https://doi.org/10.1038/s41529-018-0061-2

The corrosion and degradation of materials, such as pipeline steel, have a strong effect on both the environment and the economy. The quantification of these processes can therefore provide important information needed to manage their impact. In this study, a concept for the characterization and quantification of corrosion is demonstrated on API X70 steel immersed in 3.5 wt.% NaCl solution. Due to the difficulty of quantifying corrosion rates, e.g., through single mean values, a unique system is applied that directly couples Raman spectroscopy with vertical scanning interferometry to assess the physical and chemical aspects of steel corrosion kinetics. Vertical scanning interferometry allows the quantification of the topographical evolution of corrosion product formation and material dissolution in combination with the direct measurements of the respective rates. The Raman spectroscopy provides additional information about the (mineral) phases. Rate variations ranging from uniform corrosion to areas of high pit densities are quantified and analyzed in rate maps and subsequently visualized in rate spectra. The rate distribution is classified into different domains and pitting rates. Thus, a comprehensive quantitative assessment of the characteristic corrosion behavior is discussed.

Zeitschrift für Kristallographie - Crystalline Materials (2018) 234, Heft 4

https://doi.org/10.1515/zkri-2018-2122

The aluminosilicate iodide sodalite |Na₈I₂|[AlSiO₄]₆ was examined by temperature-dependent neutron time-of-flight powder diffraction from 5 K to 290 K and X-ray diffraction from 298 K to 1200 K. The temperature-dependent properties of the mean structure in space group P4̅3n were obtained by Rietveld analysis. A negative slope for the thermal expansion coefficient below 50 K could be observed, and the displacement parameters of the iodide ions indicate anharmonic effects. Local structure models (8×8×8 super cells) were refined against pair-distribution functions calculated from total scattering data collected at 5 K, 165 K and 240 K. The results indicate isotropic displacements for all atoms except for I-atoms, showing the effects of an anharmonic potential around this anion at very low temperatures.

Lithos (2018) 320-321, 470-489

https://doi.org/10.1016/j.lithos.2018.09.033

The Almirez ultramafic massif (SE Spain) preserves the transformation of high-P antigorite (Atg-) serpentinite to chlorite (Chl-) harzburgite (1.6–1.9 GPa; 680–710 °C), a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs. We present a detailed μ-CT and EBSD study of oriented samples across the Atg-serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs.

Above the Atg-out isograd, Atg-serpentinite shows a prograde mylonitic foliation and a weak Shape Preferred Orientation (SPO) of oxide aggregates defining a N—S stretching lineation. The antigorite Crystal Preferred Orientation (CPO) is characterized by [001]_Atg perpendicular to the foliation, and the poles to (100)_Atg and (010)_Atg distributed in a girdle-like symmetry with [100]_Atg nearly parallel to the stretching lineation. The antigorite microstructure and CPO are consistent with deformation by dislocation creep, twinning, and dissolution-precipitation creep. These textures record the long-term shear deformation near the slab interface where the main compressive stress, σ₁, was at an acute angle to the foliation.

Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grainedChl-harzburgite with granofels or spinifex textures distributed in alternating decameter-sized lenses. Crystallization of granofels and spinifex Chl-harzburgite records, respectively, a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the Atg-serpentinite. Both textural types exhibit coarse-grained textures with systematic mineral CPOs and SPOs and microstructures without evidence of major ductile deformation. The texture of the granofels Chl-harzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The olivines of granofels Chl-harzburgite are rounded and display a weak CPO that can be accounted for by the topotactic reaction 〈100〉_Atg||〈100〉_Ol and (001)_Atg||(010)_Ol after Atg-serpentinite. Similarly, orthopyroxene shows a marked CPO consistent with the topotactic reaction (100)_Opx||(001)_Atg and [001]_Opx||[100]_Atg.

Spinifex Chl-harzburgite displays systematic mineral SPO and CPO. Spinifex olivines are tabular on (100)_Ol and elongated along [001]_Ol (c > b >> a), and define a ESE–WNW platelet lineation. The average texture is characterized by [001]_Ol,Opx subparallel to a strong ESE–WNW oxide aggregate lineation, and [100]_Ol,Opx and [001]_Ol,Opx within a plane of similar orientation to the Atg-serpentinite foliation. The SPOs and CPOs of spinifex Chl-harzburgites are composed of up to four orientation populations of tabular olivines, where one population is volumetrically dominant in all samples. Relative to the main orientation population, the CPO of olivine shows clustered distribution of [100]_Ol and [010]_Ol maxima rotated at systematic angles around [001]_Ol. These clustered CPOs and SPOs show a remarkable correlation with the orientation of the principal paleostresses. The preponderant orientation population of tabular olivines lies on the plane of maximum compression (σ₂–σ₃ plane) and has [010]_Ol and its (100)_Ol tabular faces nearly perpendicular to the Atg-serpentinite foliation. This population can be accounted for by oriented growth of platy crystals perpendicular to σ₁. The other populations lie in the plane perpendicular to the least compressive stress (σ₃) and the planes of maximum shear. The driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain, but their correlation with paleostresses suggests a cause-effect relationship.

Spinifex and granofels Chl-harzburgites show a marked ESE–WNW oxide aggregate lineation that differs in orientation from that of Atg-serpentinite and is approximately parallel to the intermediate compressive stress (σ₂). These lineations and the platelet lineation of spinifex Chl-harzburgite may be due to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. Our study shows that the kinematics of the slab, paleostresses, and fluid flow exert a dynamic control on the textures of Atg-serpentinite dehydrating to peridotite in subducting slabs.

The Journal of Physical Chemistry C (2018) 122 (49), 28280–28291

https://doi.org/10.1021/acs.jpcc.8b05740

We report on temperature-dependent structural and spectroscopic features of (Bi_1–xFe_x)FeO₃ perovskite for x = 0.15 and 0.25. Samples were synthesized by heating quantum crystalline precursors obtained by the polyol method. Crystal structures of each composition were obtained from in-house X-ray, synchrotron X-ray, and time-of-flight neutron powder diffraction data Rietveld refinements. Partial replacement of the Bi site by the Fe³⁺ cation significantly changes the crystal physicochemical properties, such as thermal expansion, polyhedral distortion, Debye temperature, and vibrational and magnetic properties. Whereas BiFeO₃ is multiferroic, both Bi_0.85Fe_0.15FeO₃ and Bi_0.75Fe_0.25FeO₃ are found to be superparamagnetic, as observed by temperature-dependent Mössbauer and SQUID measurements. Lattice thermal expansion was modeled using the Debye–Einstein-anharmonicity approach. Debye temperatures obtained from the mean-squared atomic displacement parameter and lattice thermal expansion are compared. Temperature dependence of selective Raman modes is also analyzed.

Archaeometry (2019) 61

https://doi.org/10.1111/arcm.12410

Finishing techniques are significant markers of the technological ‘know‐how’ involved in the production of the traditional, clay‐based ceramic ware. In order to provide a reliable tool to discriminate among two main surface processing techniques—that is, smoothing and burnishing—vertical scanning interferometry (VSI), a recently developed non‐destructive technique for analysing the surface roughness and topography, is applied. The smoothed areas have an obvious roughness expressed by linear structures. The latter are made of parallel ridges and trenches with an average depth of 15–20 μm. Burnishing leads to a lower topography and a lower roughness compared to the smoothed surface section. VSI quantifies the spatial distribution of the surface building blocks, which consist of phyllosilicate aggregates of variable size. The statistical treatment of the roughness data obtained by VSI shows that the surface topography provides significant information on the pottery processing and a clear qualitative and quantitative discrimination between different surfaces. VSI supports the reconstitution of the chaîne opératoire for traditional ceramic pottery and the recognition of the surface finishing techniques.

Computers & Geosciences (2018) 115, 75-87

https://doi.org/10.1016/j.cageo.2018.03.007

During the last decades, X-ray (micro-)computed tomography has gained increasing attention for the description of porous skeletal and shell structures of various organism groups. However, their quantitative analysis is often hampered by the difficulty to discriminate cavities and pores within the object from the surrounding region.

Herein, we test the ambient occlusion (AO) algorithm and newly implemented optimisations for the segmentation of cavities (implemented in the software Amira). The segmentation accuracy is evaluated as a function of (i) changes in the ray length input variable, and (ii) the usage of AO (scalar) field and other AO-derived (scalar) fields. The results clearly indicate that the AO field itself outperforms all other AO-derived fields in terms of segmentation accuracy and robustness against variations in the ray length input variable. The newly implemented optimisations improved the AO field-based segmentation only slightly, while the segmentations based on the AO-derived fields improved considerably.

Additionally, we evaluated the potential of the AO field and AO-derived fields for the separation and classification of cavities as well as skeletal structures by comparing them with commonly used distance-map-based segmentations. For this, we tested the zooid separation within a bryozoan colony, the stereom classification of an ophiuroid tooth, the separation of bioerosion traces within a marble block and the calice (central cavity)-pore separation within a dendrophyllid coral. The obtained results clearly indicate that the ideal input field depends on the three-dimensional morphology of the object of interest. The segmentations based on the AO-derived fields often provided cavity separations and skeleton classifications that were superior to or impossible to obtain with commonly used distance-map-based segmentations. The combined usage of various AO-derived fields by supervised or unsupervised segmentation algorithms might provide a promising target for future research to further improve the results for this kind of high-end data segmentation and classification. Furthermore, the application of the developed segmentation algorithm is not restricted to X-ray (micro-)computed tomographic data but may potentially be useful for the segmentation of 3D volume data from other sources.

Ultramicroscopy (2018) 184, 29-36

https://doi.org/10.1016/j.ultramic.2017.09.012

The chemical composition of four Si_1-xGe_x layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of Si_1-xGe_x in STEM.

Journal of Solid State Chemistry (2017) 254, 82-89 

https://doi.org/10.1016/j.jssc.2017.07.013

The structural and spectroscopic features of the visible light photocatalyst Bi₂WO₆ have been studied. Polycrystalline (PC), nanocrystalline (NC) and quantum dot (QD) sized samples were produced using solid state reaction, hydrothermal and flame spray pyrolysis methods, respectively. While the crystal structures of PC and NC Bi₂WO₆ are well characterized using X-ray powder diffraction data Rietveld refinements, the structural information of the QD are obtained from the complementary pair distribution function analysis and high-resolution transmission electron microscopy. The Raman spectra of the samples are compared with the phonon density of states calculated by DFT. A continuous phenomenological model describes selective optical phonon confinement into the QDs. The type of the electronic bandgaps obtained from the UV-VIS absorbance-spectra have been analyzed using two different methods, and compared with those calculated from the electronic band structures.

Journal of Catalysis (2017) 352, 52-58

https://doi.org/10.1016/j.jcat.2017.05.002

Large efforts have been made trying to understand the origin of the high catalytic activity of dealloyed nanoporous gold as a green catalyst for the selective promotion of chemical reactions at low temperatures. Residual silver, left in the sample after dealloying of a gold-silver alloy, has been shown to have a strong influence on the activity of the catalyst. But the question of how the silver is distributed within the porous structure has not finally been answered yet. We show by quantitative energy dispersive X-ray tomography measurements that silver forms clusters that are distributed irregularly, both on the surface and inside the ligaments building up the porous structure. Furthermore, we find that the role of the residual silver is ambiguous. Whereas CO oxidation is supported by more residual silver, methanol oxidation to methyl formate is hindered. Structural characterisation reveals larger ligaments and pores for decreasing residual silver concentration.

Fossil Record (2017) 20, 173–199

doi:10.5194/fr-20-173-2017

The ongoing technical revolution in non-destructive 3-D visualisation via micro-computed tomography (micro-CT) finds a valuable application in the studies of bioerosion trace fossils, since their three-dimensional architecture is hidden within hard substrates. This technique, in concert with advanced segmentation algorithms, allows a detailed visualisation and targeted morphometric analyses even of those bioerosion traces that are otherwise inaccessible to the widely applied cast-embedding technique, because they either are filled with lithified sediment or cement or are preserved in inherently insoluble or silicified host substrates, or because they are established type material and should not be altered.

In the present contribution selected examples of such cases are illustrated by reference to bioerosion trace fossils preserved in Late Cretaceous belemnite guards from the European Chalk Province. These case studies comprise an analysis of a diverse ichno-assemblage found associated with the lectotype of the microboring Dendrina dendrina (Morris, 1851) in a belemnite from the upper Campanian to lower Maastrichtian chalk of Norfolk, England, and the description of two new bioerosion trace fossils with type specimens found in belemnite guards from the lower Campanian limestones of Höver, Germany. The latter are Lapispecus hastatus isp. n., a tubular and occasionally branched macroboring for which a sipunculan or a phoronid trace maker are discussed, and Entobia colaria isp. n., a camerate network formed by an excavating sponge that eroded diagnostic grated apertures at the locations of the presumed inhalant papillae or exhaling pores, adding to or replacing filtering devices that are otherwise made of tissue and spicules.

As an added value to the non-destructive visualisation procedure, the processed X-ray micro-CT scans of the studied type material provide 3-D models that may now serve as digitypes that can be studied as digital facsimile without the necessity of consulting the actual type specimens.

Journal of microscopy (2017) 268, 193-207

https://doi.org/10.1111/jmi.12598

We reconstruct the 3‐D microstructure of centimetre‐sized olivine crystals in rocks from the Almirez ultramafic massif (SE Spain) using combined X‐ray micro computed tomography (‐CT) and electron backscatter diffraction (EBSD). The semidestructive sample treatment involves geographically oriented drill pressing of rocks and preparation of oriented thin sections for EBSD from the ‐CT scanned cores. The ‐CT results show that the mean intercept length (MIL) analyses provide reliable information on the shape preferred orientation (SPO) of texturally different olivine groups. We show that statistical interpretation of crystal preferred orientation (CPO) and SPO of olivine becomes feasible because the highest densities of the distribution of main olivine crystal axes from EBSD are aligned with the three axes of the 3‐D ellipsoid calculated from the MIL analyses from ‐CT. From EBSD data we distinguish multiple CPO groups and by locating the thin sections within the ‐CT volume, we assign SPO to the corresponding olivine crystal aggregates, which confirm the results of statistical comparison. We demonstrate that the limitations of both methods (i.e. no crystal orientation data in ‐CT and no spatial information in EBSD) can be overcome, and the 3‐D orientation of the crystallographic axes of olivines from different orientation groups can be successfully correlated with the crystal shapes of representative olivine grains. Through this approach one can establish the link among geological structures, macrostructure, fabric and 3‐D SPO‐CPO relationship at the hand specimen scale even in complex, coarse‐grained geomaterials.

Lithos (2016) 268-271, 274-284.

https://doi.org/10.1016/j.lithos.2016.11.014

In a drillcore sample of serpentinized harzburgite from the uppermost oceanic crust (Mid-Atlantic Ridge, ODP Leg 209, Site 1270), we demonstrate using high-resolution 3D-microtomography that micron-sized open cavities are present. The development of porosity is interpreted to result from dissolution of brucite and/or olivine. Petrographic observations indicate that voids are integrated in a network of carbonate veins, the formation of which is linked to changing alkalinity in conjunction with dissolution reactions. Partial carbonate filling of pore spaces indicates that under static conditions low-temperature carbonation leads to clogging of fluid pathways and thus to a reduction in permeability. Electron microprobe analyses show that the inner walls of open voids are lined with Fe-rich precipitates. We propose that the iron in those phases was released by brucite or olivine dissolution and was subsequently oxidized and precipitated as ferric hydroxide. Thermodynamic computations show that this process may be a potential source of catabolic energy for microorganisms inhabiting serpentinites. The proposed carbonation mechanism implies that carbonate precipitation may start soon after exposure of the abyssal peridotites, when dissolution of brucite and weathering of olivine begin, and continue until the phases become inaccessible to seawater. Predicting carbonation rates of abyssal peridotites will hence require understanding of permeability reactions.

Physical Chemistry Chemical Physics (2017) 19, 3480-3485

https://doi.org/10.1039/C6CP06853G

The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 °C, the oxide is shown to form hexagonally close-packed (A-type) Pr₂O₃(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.

Journal of Physics: Condensed Matter (2016) 28, 475003

https://doi.org/10.1088/0953-8984/28/47/475003

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Solid Earth (2016) 7, 651-658

doi:10.5194/se-7-651-2016

A new flow-through reaction cell consisting of an X-ray-transparent semicrystalline thermoplastic has been developed for percolation experiments. Core holder, tubing and all confining parts are constructed using PEEK (polyetheretherketone) to allow concomitant surveillance of the reaction progress by X-ray microtomography (μ-CT). With this cell setup, corrosive or oversaturated fluids can be forced through rock cores (up to ∅ 19 mm) or powders at pressures up to 100 bar and temperatures up to 200 °C. The reaction progress of the experiment can be monitored without dismantling the sample from the core holder.

The combination of this flow-through reaction cell setup with a laboratory X-ray μ-CT system facilitates on-demand monitoring of the reaction progress of (long-term) hydrothermal experiments in the own laboratory, keeping interruption times as short as possible. To demonstrate both the suitability of the cell construction material for X-ray imaging purposes and the experimental performance of the flow-through system, we report the virtually non-existent bias of the PEEK cell setup with distinctive X-ray observations (e.g., differing states of pore fillings: air vs. fluid; detection of delicate fabric elements: filigree zeolite crystals overgrowing weathered muscovite), and the monitoring of the gypsum/anhydrite transition as a case study of a 4-D fabric evolution.

Nature Communications (2014) 5, 5653

doi: 10.1038/ncomms6653

By focusing electrons on probes with a diameter of 50 pm, aberration-corrected scanning transmission electron microscopy (STEM) is currently crossing the border to probing subatomic details. A major challenge is the measurement of atomic electric fields using differential phase contrast (DPC) microscopy, traditionally exploiting the concept of a field-induced shift of diffraction patterns. Here we present a simplified quantum theoretical interpretation of DPC. This enables us to calculate the momentum transferred to the STEM probe from diffracted intensities recorded on a pixel array instead of conventional segmented bright-field detectors. The methodical development yielding atomic electric field, charge and electron density is performed using simulations for binary GaN as an ideal model system. We then present a detailed experimental study of SrTiO₃ yielding atomic electric fields, validated by comprehensive simulations. With this interpretation and upgraded instrumentation, STEM is capable of quantifying atomic electric fields and high-contrast imaging of light atoms.

Physical Review Letters (2014) 113, 096101

doi: 10.1103/PhysRevLett.113.096101

There are mainly two complementary imaging modes in transmission electron microscopy (TEM): Conventional TEM (CTEM) and scanning TEM (STEM). In the CTEM mode the specimen is illuminated with a plane electron wave, and the direct image formed by the objective lens is recorded in the image plane. STEM is based on scanning the specimen surface with a focused electron beam and collecting scattered electrons with an extended disk or ring-shaped detector. Here we show that combination of CTEM imaging with STEM illumination generally allows extending the point resolution of CTEM imaging beyond the diffraction limit. This new imaging mode improves imaging characteristics, is more robust against chromatic aberration, exhibits direct structural imaging with superior precision, visualizes light elements with excellent contrast, and even allows us to overcome the conventional information limit of a microscope.

An autocorrelator records an approximation of the, so-called, g⁽²⁾-function. This function includes valuable information about the photon statistics of any light sources, enabling, e.g., the distinction of coherent, thermal, and single photon sources.

The upgrade includes: The autocorrelator is based on a low-noise, continuous-wave UV laser, a time-resolved photon counting unit with two UV-VIS (visible) light detectors, and a piezo mirror system for precise laser beam positioning.

Key benefits: From a worldwide perspective, the system is one of the very few detection systems for quantum light in the UV spectral range. Exactly this UV spectral range is of particular interest for secured inter-satellite data transmission in space. The system has a high quantum efficiency, which reduces data acquisition times.

Features: This UV autocorrelator is sensitive from around 230 – 700 nm, while its optimal optical detection window stretches from 230 – 550 nm. The applied detectors yield a biphoton time resolution of ≤ 220 ps. The low noise UV laser source in use has a wavelength of 266 nm, which enables the characterization of most UV single photon sources (e.g., based on quantum dots, point defects). Due to the acquired piezo mirror, this UV autocorrelator can automatically readjust itself, which is a key feature for longer acquisition times needed for pioneering materials. In addition, this feature enhances the usability of this setup for external users. Furthermore, conventional time-resolved photoluminescence studies (TRPL) can be performed on the basis of this UV autocorrelator.

Applied by:

Prof. Dr. Gordon Callsen

(Solid State Spectroscopy)

Atomic layer deposition (ALD) is a thin film deposition technique based on sequential, self-limiting chemical reactions of precursors on surfaces. Plasma-enhanced ALD (PE-ALD) increases surface reactivity, enabling conformal coatings on complex structures at moderate temperatures. Applications include semiconductor device fabrication, high-aspect-ratio nanostructure coatings, 2D materials synthesis, and Bragg reflector optical coatings.

The upgrade includes:

• Replacement of the exhaust absorber for safe neutralization of process gases.

• Repair of the ACP 28 roots pump to restore vacuum integrity.

Key benefits:

• Ensures safe and uninterrupted operation of the ALD system.
• Maintains high-quality deposition for collaborative MAPEX research projects.
• Supports advanced applications including photocatalysis, optical coatings and 2D materials.

Features:

• PE-ALD capability for a wide range of oxide, nitride and sulphide materials.
• Dry scrubbing system for chemical safety.
• High vacuum connectivity to MBE and XPS systems for integrated processing and analysis.

Applied by:

Dr. Manuel Alonso-Orts (AG Eickhoff, IFP)

Raman and photoluminescence spectroscopy is an established technique for spectroscopy of all kind of materials (solid state materials, liquids, gases, inorganic and organic) using laser excitation in the UV-VIS-NIR spectral range.

The upgrade includes:  Installation of a new InGaAs array detector, a 1064 nm laser diode and a long-pass edge optical filter for Raman and photoluminescence spectroscopy in the infrared spectral range up to 1600 nm.

Key benefits:   So far, the PL spectral analysis and imaging in the Horiba LabRAM Evolution system was limited to wavelengths up to 1000 nm. However, many materials are optically active in the short-wavelength infrared (SWIR) spectral range above 1000 nm such as Si, InN, InGaN (with a high In content), InGaAs and several 2D materials (e.g. MoTe₂, WTe₂).  To get more detailed spectral and spatio-spatial information of such materials, PL spectroscopy and imaging beyond 1000 nm is needed. 

Raman spectroscopy and imaging in the SWIR spectral range is also of interest. Some materials show bright PL or fluoresence in the UV-VIS-NIR spectral range which outshines the Raman signal or is degraded by the irradiation with shorter wavelength lasers, so that Raman spectroscopy and imaging is only possible using a SWIR laser and thus detection in the SWIR spectral range. 

Features: The Horiba LabRAM Evolution system is used for Raman, photoluminescence and time-resolved spectroscopy of all kind of materials. The detection range is 200 to 1600 nm (UV-VIS-NIR) using a combination of a CCD and an InGaAs array detector. A Princeton Instruments Spectrometer and a Hamamatsu Streak-Scope enable time-resolved measurements with a detection range up to 1000 nm. Laser excitation for Raman and PL spectroscopy is possible with 325 nm, 442 nm, 633 nm, 785 nm and 1064 nm. For PL spectroscopy only, additional lasers are available at 260 nm, 406 nm and 520 nm. The laser wavelengths at 260 and 520 nm are pulsed laser sources suitable for time-resolved spectroscopy. The system is equipped with a high spatial resolution sample stage covering a range of 7 cm x 7 cm for Raman and photoluminescence mapping. A cryostat is available for low temperature measurements down to 4 K. A mass flow controlled gas flow chamber allows measurements of samples in non-toxic gas environments. 

Applied by: 
Dr. Christian Tessarek
(Solid State Materials – Institute of Solid State Physics)

The use of graphics processing units (GPUs) in classic simulations greatly accelerates computing times. They also form the basis for applications in the field of machine learning. 

The compute-server includes: 2 AMD EPYC 9374F central processing units (CPUs) each with 32 cores; 8 NVIDIA RTX 6000 Ada GPUs each with 48 GB GDDR6 GPU memory, 18,176 NVIDIA CUDA Cores, 568 NVIDIA Tensor Cores and 142 NVIDIA RT cores; 1,152 GB main memory.

Key benefits: The compute server allows simulations with GPU acceleration and machine learning applications to be performed on modern state-of-the-art hardware, with full access to the hardware and software.

Features: First and foremost, the compute server allows all types of simulations for any application. However, its special structure makes it particularly well suited for the use of GPUs. Accordingly, the focus is on simulations that use GPUs to accelerate calculations. The main use case, however, is machine learning methods. Here, the GPUs drastically reduce the computing times during the training of artificial neural networks and subsequent evaluation. This compute server provides local, low-threshold access to the software, enabling the installation of special software packages or similar.

Applied by: 

Prof. Dr. rer. nat. Andreas Rademacher 

(Center for Industrial Mathematics)

With the rise of high-resolution X-ray imaging, datasets often exceed 1 TB per experiment, far beyond standard computing capabilities. TOMCAT addresses this critical gap by offering centralized, high-performance computing for efficient processing and visualization of large volumetric and time-resolved (4D) data. Unlike isolated workstation setups, TOMCAT is a shared, remotely accessible server infrastructure designed for cross-faculty digital collaboration on imaging data across materials science, geoscience, engineering, and life sciences.

The system includes: 

A Supermicro A+ Server 4125GS-TNRT equipped with 4× NVIDIA Ada Lovelace L40S GPUs (48 GB GDDR6 ECC VRAM each, 192 GB total), dual AMD EPYC 9754 processors (128 cores / 256 threads each, 256C/512T total), 1 TB DDR5-6400 RAM, and 15.36 TB high-speed NVMe SSD storage. The server runs Dragonfly 3D World for AI-driven 3D/4D image visualization, segmentation, and quantification, alongside open-source tools such as TomoPy, ParaView, Tomviz, and ImageJ/Fiji, as well as a pre-configured Python environment with scientific and ML libraries.

Key benefits: 

TOMCAT is the first instrument at MAPEX explicitly designed for cross-faculty digital collaboration on imaging data. It enables processing of terabyte-scale datasets that exceed the memory and compute capacity of standard workstations. 

Authorized users access the server remotely via web-based tools, including remote desktops for GUI applications with GPU-accelerated 3D rendering. The system supports both commercial (Dragonfly) and open-source frameworks, ensuring broad usability across disciplines.

Features: 

4× NVIDIA L40S GPUs with 192 GB total VRAM and 18,176 CUDA cores each for GPU-accelerated volume rendering, deep learning model training, and CT reconstruction. 1 TB DDR5 system memory for handling large datasets in-memory. 15.36 TB NVMe SSD storage for fast data access. Dragonfly 3D World software supporting AI-powered segmentation, quantification, 3D surface reconstruction, porosity analysis, and publication-ready visualization from multi-modal imaging data including X-ray micro-CT, synchrotron tomography, FIB-SEM, and correlative microscopy. Open data formats (OME-TIFF, Zarr) to ensure FAIR, scalable access.

Applied by: 

Prof. Dr. Oliver Plümper 

(Mineralogy, Faculty of Geosciences/ MARUM)

The Bruker D8 Venture four-circle single-crystal diffractometer in GEO 2380 is currently fitted with a hot air gas blower from XDS-Oxford. This allows for the advanced study of phase transitions and diffuse scattering phenomena of single crystals as a function of temperature. Temperature control with a precision of ±1°C is possible between room temperature and ~1000°C. 

The upgrade includes: A hot air gas blower and a pre-calibrated controller with a computer software interface. The functionality has currently been successfully tested ex-situ, and the next step is to adapt the stand to the diffractometer housing for convenient operation. The system will then be calibrated to minimize the offset between factory-calibrated and actual temperatures. 

Key benefits: The temperature-dependent behaviour of crystal structures can be studied with exceptional temperature stability and accuracy in situ. 

Usage: To use the system, the hot air blower is aligned to the center of the single crystal with a precision of less than 0.1 mm, using a well- centered micro-thermocouple in the position of the single crystal. The micro-thermocouple will then be replaced by a 50–200 µm single crystal, fixed to the sample holder with high-temperature ceramic glue. 

Features: The system enables detailed analysis of changes to and transitions in crystal structures based on high-quality data collected at user-selected temperatures.

Applied by: 

Prof. Dr. Ella Mara Schmidt 

Electron microscopy is a standard technique in modern nano science due to its spatial resolution up to the atomic scale. However, ultimate resolution requires versatile specimen preparation in order to achieve optimized contrasts at samples surfaces, to prevent specimen from charging or oxidation, stabilization of nanoparticle aggregates etc. Such requirements are often solved by deposition of a conductive layer or a low Z-material onto the specimen. 

Key Feature:  The Leica sputter coater ACE600 allows to deposit ultrathin chromium (or any other available sputter target material) or carbon layers with thicknesses down to about 0.5 nm onto specimens with lateral size of several centimeters.

Functionalities: 

• Pulsed carbon-wire deposition: with about 0.5nm film deposition per pulse
• Deposition of two materials without break of vacuum
• Bayonet lock for quick exchange of sputtering targets
• Rotating specimen holder for homogenous coating of rough and porous samples
• Front loading of samples enabling the coating of large samples (4-inch wafers)
• Thickness determination monitor using an oscillating quartz
• High vacuum coater (<2*10^-6 mbar)

Scientific uses:

• Use for preventing charging of SEM/FIB samples.
• Use for TEM grid preparation with ultrathin carbon films.
• Protection of specimen surface against focused ion beam damage for samples with surface near features or objects of interest on the surface.
• Use as a low intensity/contrast material for imaging of surface near regions.
• Stabilization of nanoparticle aggregates for investigation in TEM.

Applied by:

 Dr. Marco Schowalter

(Institute of Solid State Physics)

Transmission electron microscopy is an established technique for imaging of nanoscale materials using a beam of electrons that is transmitted through the specimen.

The upgrade includes:  Complete replacement of the previous control electronics and vacuum control. The TEM is now able to perform selected area electron diffraction (SAED). SAED provides diffractograms of specimen, which, depending on the focus, can comprise a single nanoparticle or individual grains in a polycrystalline material.

Key benefits:   It's ease of operation, allowing even inexperienced researchers to become proficient within two hours, and its rapid sample change, taking under five minutes—or less than one minute for experienced operators.

Features: The EM900 provides accelerating voltages of 50 and 80 kV, an energy range leading to a stronger interaction with the sample especially suitable for nanoparticles and low -Z materials. The microscope delivers high-quality images with a resolution of approximately 5 nm. As with most TEMs, samples are deposited on a carbon-coated copper grid, and analysis takes place at high vacuum conditions.

Applied by:
PD Dr. rer. nat. Michael Maas
(Advanced Ceramics)

Feature: This ellipsometer is a highly automated, spectroscopic ellipsometer for the wavelength range of 190 - 2100 nm.

Measured quantity: The ellipsometer is a micro-spot ellipsometer, which can achieve very small spot sizes (eg 35 µm x 85 µm), making it possible to examine even smaller structures. In addition to the high-precision measurement of layer thicknesses even below a monolayer, this ellipsometer enables comprehensive experimental access to a large number of optical constants.

In addition to adjustable spot sizes, a wide-angle range of 35° - 90° can also be measured with this ellipsometer. In addition, the system is equipped with a “vision” system, which not only allows the micro-spot to be positioned precisely on the sample, but also enables autofocus, among other things.

Principle: The ellipsometer is based on the “photo-elastic modulator (PEM)” measuring principle, which allows an extremely low-noise measuring signal to be recorded.

Evaluation software: The measurement data is evaluated using the DeltaPsi2 software from Horiba

Applied by: 

Prof. Gordon Callsen

(Semiconductor Optics, Faculty 1)

The integrating sphere is shown in the photo and can be inserted directly by the user in the form of an insert into the Flurolog 3 spectrometer from AG Callsen, which considerably simplifies operation.

Measured quantity: The user is then able to measure quantum efficiencies of a variety of samples (layer samples, powder samples, etc.) using this integrating sphere.The supported integrating sphere is the “K SPHERE-Petite” system from Horiba, which has a diameter of 81 mm (compact design) and enables the automated measurement of quantum efficiencies from 320 - 950 nm in the Fluorolog 3 spectrometer.

Feature: A special feature of the entire system is the resulting possibility of measuring quantum efficiencies as a function of the excitation wavelength, which is particularly important for semiconductor heterostructures.

Applied by: Prof. Gordon Callsen  (Semiconductor Optics - Faculty 1) 

Acoustic emission (AE) monitoring is a powerful technique for structural health monitoring of components and in-situ microstructural analysis of complex materials.

Measured quantity: The newly acquired AE measuring system AMSY-6 (Vallen Systeme GmbH) can be used for constant measurement of acoustic waves or hit-based signal measurement with measuring frequency of up to 40 MS/s for different types of samples. The system is equipped with four high-frequency piezo electric sensors, as well as four parametric channels to record external sensors for simultaneous measurement of mechanical load, deformation, temperature, etc. This allows us to detect, localize and identify microstructural changes and related them to external conditions, such as the applied stress (eg, mechanical testing), temperature and atmosphere (eg, processing and in-field application).

Future possibilities: There is also the possibility to combine AE monitoring with other in-situ techniques for a robust microstructural characterization.

Thus, the new device will be vital for the flowing planned projects:

- Combination of AE monitoring, computer tomography and machine learning for in-situ microstructural analysis of multi-phase materials

- Crack detection in fiber-reinforced composites at extreme temperatures from cryogenic to up to 1200°C

- Evaluation of chemical reactions and mechanical deformation during fluid-rock interactions.

Applied by: Dr. Renato Almeida (Advanced Ceramics group)