Atomic-scale Characterization Research Articles

Exploring multi-scale damage mechanisms in 8Cr4Mo4V alloy by molecular dynamics simulations and experiments

8Cr4Mo4V, as an ideal choice for aeroengines bearing materials, involves multi-scale damage mechanisms during the process of material failure. Herein, we propose a multi-scale analysis method to explore the crack initiation and propagation damage mechanisms of 8Cr4Mo4V alloy. We find that different crystal orientations produce distinctly different damage mechanisms. The second phase doping also has different strengthening effects on the matrix conformed to cut-off and Orowan mechanism. Meanwhile, the study of the polycrystalline model verified inverse Hall-Petch relation at the micro-scale, where material strength decreases with grain refinement. Phase transitions caused by dislocations lead to stress concentration at specific grain boundaries, becoming the main inducer for the nucleation of initial voids. To address micro-to-macroscale-crossing issues, we constructed a large-scale mesoscopic molecular dynamics simulation and proposed an analytical formula to predict the relationship between average grain size and yield stress. Finally, we combine macroscopic experiments, crystallographic analyses, and atomic-scale characterization, verifying the combined action of dislocations and stacking faults leads to the failure of the material.

Real-Space Tilting Method for Atomic Resolution STEM Imaging of Nanocrystalline Materials.

Atomic-resolution scanning transmission electron microscopy (STEM) characterization requires precise tilting of the specimen to a high symmetric zone axis, which is usually processed in reciprocal space by following the diffraction patterns. However, for small-sized nanocrystalline materials, their diffraction patterns are often too faint to guide the tilting process. Here, a simple and effective tilting method is developed based on the diffraction contrast change of the shadow image in the Ronchigram. The misorientation angle of the specimen can be calculated and tilted to the zone axis based on the position of the shadow image with lowest intensity. This method requires no prior knowledge of the sample and the maximum misorientation angle that can be corrected is >±6.9° with sub-mrad accuracy. It operates in real space, without recording the diffraction patterns of the specimens, making it particularly effective for nanocrystalline materials. Combined with the scripting to control the microscope, the sample can be automatically tilted to the zone axis under low dose conditions (<0.17 e-Å- 2s-1), facilitating the imaging of beam sensitive materials such as zeolites or metal-organic frameworks. This automated tilting method can significantly contribute to the atomic-scale characterization of the nanocrystalline materials by STEM imaging.

Atomic-Scale Characterization of Dilute Dopants in Topological Insulators via STEM-EDS Using Registration and Cell Averaging Techniques.

Magnetic dopants in three-dimensional topological insulators (TIs) offer a promising avenue for realizing the quantum anomalous Hall effect (QAHE) without the necessity for an external magnetic field. Understanding the relationship between site occupancy of magnetic dopant elements and their effect on macroscopic property is crucial for controlling the QAHE. By combining atomic-scale energy-dispersive X-ray spectroscopy (EDS) maps obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM) and novel data processing methodologies, including semi-automatic lattice averaging and frame registration, we have determined the substitutional sites of Mn atoms within the 1.2% Mn-doped Sb2Te3 crystal. More importantly, the methodology developed in this study extends beyond Mn-doped Sb2Te3 to other quantum materials, traditional semiconductors, and even electron irradiation sensitive materials.

Low-voltage single-atom electron microscopy with carbon-based nanomaterials

The properties of materials are strongly correlated with their atomic scale structures. Achieving a comprehensive understanding of the atomic-scale structure-property relationship requires advancements of imaging and spectroscopy techniques. Aberration-corrected scanning transmission electron microscopy (STEM) has seen rapid development over the past decades and is now routinely employed for atomic-scale characterization. However, quantitative STEM imaging and spectroscopy analysis at the single-atom level is challenging due to the extremely weak signals generated from individual atom, thus imposing stringent requirements for analysis sensitivity. This review discusses the development and application of low-voltage STEM techniques with single-atom sensitivity, primarily based on recent research presented on an invited talk at the 5th 2D23 SALVE Symposium, including annular dark-field (ADF) imaging, functional imaging and electron energy-loss spectroscopy (EELS) analysis. Carbon-based nanomaterials were chosen as model systems for demonstrating the capabilities of single-atom STEM imaging and EELS analysis, due to their structural stability under low accelerating voltages and their rich physical and chemical properties. Moreover, this review summarizes recent advancements and applications of low-voltage single-atom STEM imaging and spectroscopy in the study of functional materials and discusses prospects for future developments.

A novel approach for the atomic scale characterization of Li-ion battery components probed by positron annihilation lifetime spectroscopy

Recent research has focused on solid polymer electrolytes (SPEs) as the best liquid electrolyte replacements. Poly(ethylene oxide) (PEO) is the most frequent one because of its fast segmental movements in the free volume which controls lithium-ion transport between electrodes. Therefore, PEO-based copolymer electrolytes that can be tailored for mechanical strength and ionic conductivity are often studied. The most popular material is PEO chains containing PMMA because PEO transports ions, and PMMA has good mechanical properties. Further investigations on PEO-based solid polymer electrolytes (SPEs) have shown that chain designs with more nonlinear branches prevent crystallinity and maintain fast segmental dynamics. For this purpose, we worked on PEO-grafted-PMMA copolymers in which the free volume probed by positron annihilation lifetime spectroscopy considerably affects the structure-ionic conductivity relationship. Finally, we investigated the free volume theory of ionic condcutivity using Yahsi-Ulutas-Tav (YUT) theory.

Structure–Activity–Stability Relationship of Ru-Ir-Ti Electrocatalysts for the Oxygen Evolution Reaction

Environmentally friendly strategies for energy conversion and storage are urgently needed to account for natural intensity variations of power generation by renewables and to ensure a stable electrical grid. Green hydrogen, produced by water electrolysis, can be considered as one of the most promising energy carriers since it can balance the energy demand in multiple energy sectors. Proton exchange membrane water electrolyzers (PEMWE) are state of the art devices for on-site hydrogen production. Still, their upscaling is challenged by the corrosive acidic environment and sluggish anodic oxygen evolution reaction (OER), which demand using noble metal based catalysts [1, 2]. Currently, Ir-based oxides are employed as OER catalysts in commercial PEMWE as they provide the best balance between activity and stability. Yet, the scarcity and high market price of Ir lead to poor cost-benefit factors. In the last years, Ir-Ru mixed oxides have attracted significant attention of the community due to their improved electrocatalytic activity [2, 3]. However, Ru degradation during the OER becomes challenging in conditions of long-term operation of the electrolyzer. Therefore, balancing stability and activity towards the OER at reduced Ir loading remains a significant challenge in PEMWE.Here, we examine the effect of low additions of Ti (1 to 6 at. %) on the catalytic activity and stability of Ru-Ir alloys towards the OER [4]. The catalysts are synthesized as ternary Ru-Ir-Ti thin film material libraries with Ir contents below 50 at. %. After the high-throughput screening of these libraries for desired electrocatalytic properties by online inductively coupled plasma mass spectrometry (ICP-MS), the most promising alloy compositions are selected and tested in conditions of long-term electrolysis (Figure 1). The observed activity-stability trends are correlated with the electronic structure of the Ru-Ir-Ti catalysts derived from X-ray photoelectron (XPS) and X-ray absorption spectroscopies (XAS). Furthermore, the structural changes in the near-surface regions of these catalysts induced by the OER, are investigated with the aid of atom probe tomography (APT). Our data reveal formation of Ir-Ru hydrous oxide layers in the course of the OER, providing high catalytic activity, while Ti additions promote stabilization of these reactive oxides in the near surface region. Our multidisciplinary approach of combining advanced electrochemical, spectroscopic and atomic-scale characterization methods provides insights on structure-function relationships in electrocatalysis, guiding the rational design of active and stable electrocatalysts.[1.] Carmo, M., et al., A comprehensive review on PEM water electrolysis. International journal of hydrogen energy, 2013. 38(12): p. 4901-4934.[2.] Kasian, O., et al., On the Origin of the Improved Ruthenium Stability in RuO2-IrO2 Mixed Oxides. Journal of the Electrochemical Society, 2016. 163(11): p. F3099-F3104.[3.] Saveleva, V.A., et al., Uncovering the stabilization mechanism in bimetallic ruthenium–iridium anodes for proton exchange membrane electrolyzers. The journal of physical chemistry letters, 2016. 7(16): p. 3240-3245.[4.] Lahn, L., et al. Low Ti Additions to Stabilize Ru‐Ir Electrocatalysts for the Oxygen Evolution Reaction. ChemElectroChem, 2023, e202300399. DOI: https://doi.org/10.1002/celc.202300399. Figure 1

Determining the density and spatial descriptors of atomic scale defects of 2H–WSe2 with ensemble deep learning

We have demonstrated atomic-scale defect characterization in scanning tunneling microscopy images of single crystal tungsten diselenide using an ensemble of U-Net-like convolutional neural networks. Coordinates, counts, densities, and spatial extents were determined from almost 16 000 defect detections, leading to the rapid identification of defect types and their densities. Our results show that analysis aided by machine learning can be used to rapidly determine the quality of transition metal dichalcogenides and provide much needed quantitative input, which may improve the synthesis process.

THz vs NIR laser-assisted atom probe tomography of LaB6 samples

Terahertz (THz) radiation with low-energy photons (meV) is used in a wide range of applications, such as microscopy, sensing, and spectroscopy. However, recently, high amplitude THz pulses of MV/cm have been generated and used for electron emission and ion evaporation from field emitters, opening up the possibility of using high amplitude THz pulses for material imaging by THz-assisted atom probe tomography (APT). In this work, we compare the APT analyses of lanthanum hexaboride (LaB6) samples using a femtosecond near-infrared laser with those obtained using high-amplitude single-cycle THz pulses. The atomic-scale characterization of stoichiometric LaB6 is challenging in laser-assisted APT due to the detection losses of boron ions. Here, we show that the THz radiation reduces the emission of molecular ions and multiple detection events, and it increases the charge state of the emitted ions. All these effects result in an improvement in boron detection. Furthermore, the emission dynamics of boron and lanthanum ions differ in their evaporation times when using THz radiation. This work emphasizes the ability of high-amplitude, single-cycle THz pulses to well control material analysis in APT, leading to better results on chemical composition. It also paves the way for the use of this radiation for material manipulation.

Cooperative dislocations for pressure-dependent sequential deformation of multi-principal element alloys under shock loading

Multi-principal element alloys (MPEAs) are promising materials for structural applications under extreme conditions. Their outstanding mechanical properties are closely related to the activation of multiple deformation modes of dislocation gliding, twinning, and phase transformation that appear in sequence during deformation at low temperatures, high pressures, or high strain rates. However, the inherent correlations among these deformation modes and, thus, underlying deformation mechanisms of MPEAs remain largely unknown. We report soft-recovery plate impact experiments of face-centered-cubic (FCC) CrCoNi MPEAs, demonstrating pressure-dependent deformation modes from low-pressure stacking faults to medium-pressure twinning and high-pressure FCC to hexagonal-close-packed (HCP) phase transformation. Atomic-scale characterizations unveil that the sequential deformation is manipulated by the cooperation of 90° and 30° Shockley partial dislocations at deformation fronts, which is facilitated by low stacking fault energy and pressure-dependent phase stability of the MPEAs. Moreover, the cooperative dislocation behavior can also be observed at twin fronts of shock-loaded CrMnFeCoNi MPEA, validating the universality of the cooperative deformation mode in FCC alloys with a low stacking fault energy. Theoretical analyses suggest that the distinctive cooperative dislocation behavior results in the self-compensation of dislocation strain fields and the minimization of interfacial elastic energy at incoherent twin and FCC/HCP interfaces.

In situ investigation of hydrogen embrittlement induced by δ phase in selective laser-melted GH4169 superalloy

Direct evidence of hydrogen-assisted crack nucleation and propagation associated with the δ phase in the selective laser melted GH4169 superalloy was obtained. The analysis of hydrogen trapping sites using thermal desorption spectroscopy revealed that the δ phase exhibits strong hydrogen capture capability, with a hydrogen desorption activation energy of 35.45 ± 2.51 kJ/mol. In addition, spatially resolved hydrogen mapping conducted by scanning Kelvin probe force microscopy and hydrogen microprint technique provided further evidence for the δ phase as a deep hydrogen trapping site. The atomic-scale characterization sufficiently reveals the deformation mechanism of the δ phase induced by dislocation accumulation. Hydrogen-promoted dislocation slip localization facilitates the formation of microvoid defects in the δ phase, which is the main reason for the δ phase fracture, and induces intergranular and transgranular cracks.

Nanoscale Analysis of Frozen Water by Atom Probe Tomography Using Graphene Encapsulation and Cryo-Workflows.

There has been an increasing interest in atom probe tomography (APT) to characterize hydrated and biological materials. A major benefit of APT compared to microscopy techniques more commonly used in biology is its combination of outstanding three-dimensional (3D) spatial resolution and mass sensitivity. APT has already been successfully used to characterize biominerals, revealing key structural information at the atomic scale, however there are many challenges inherent to the analysis of soft hydrated materials. New preparation protocols, often involving specimen preparation and transfer at cryogenic temperature, enable APT analysis of hydrated materials and have the potential to enable 3D atomic scale characterization of biological materials in the near-native hydrated state. In this study, samples of pure water at the tips of tungsten needle specimens were prepared at room temperature by graphene encapsulation. A comparative study was conducted where specimens were transferred at either room temperature or cryo-temperature and analyzed by APT by varying the flight path and pulsing mode. The differences between the analysis workflows are presented along with recommendations for future studies, and the compatibility between graphene coating and cryogenic workflows is demonstrated.

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

FeSiAl soft magnetic composite with double Al2O3 insulation layers for simultaneous high mechanical and magnetic properties

FeSiAl-based soft magnetic composites (SMCs), prepared from insulated FeSiAl powders, are widely applied in electronic devices. However, it is still challenging to achieve high magnetic and mechanical properties simultaneously due to the undesirable insulation layer. Here, double Al2O3 insulation layers are prepared for FeSiAl SMC. Atomic-scale characterizations reveal an in-situ epitaxial Al2O3 layer at FeSiAl surface under the catalysis of NaAlO2, and an outer amorphous Al2O3 layer by subsequent NaAlO2 hydrolysis. The above structure ensures effective insulation of FeSiAl powders and excellent magnetic properties of the FeSiAl/NaAlO2 SMC, with permeability of 101 and power loss of 128 mW/cm3 (50 mT, 100 kHz) respectively. Moreover, in-situ Al2O3/amorphous Al2O3 on FeSiAl matrix also leads to distinguished crush strength of 36.5 MPa for the core sample, which is ascribed to the enhanced adhesion at different interfaces as evidenced by similar local oxygen coordination and low strain distribution. This work provides a novel method to fabricate high-performance FeSiAl SMCs.

Regulation of antiphase boundary density in Fe3O4 thin films and its effect on the electrical and magnetic properties

Antiphase boundaries (APBs) widely exist in Fe3O4 thin films and affect significantly their electrical and magnetic properties. Here, effects of APBs on the electrical and magnetic properties of Fe3O4 thin films have been quantitatively investigated by a combined study of pulsed laser deposition, transmission electron microscopy (TEM) and first-principles calculations. It was revealed that the substrates affected greatly the density of APBs in Fe3O4 thin films but did not change the atomic structures of APBs. The APB density in the Fe3O4 (111) thin film grown on the MgO (111) substrate was one order of magnitude larger than that on the Al2O3 (0001) substrate. Atomic-scale TEM characterizations revealed that the APBs in the Fe3O4 thin films on both substrates had three types of atomic structures. Due to the higher APB density in the Fe3O4/MgO thin film, the electrical resistivity at 300 K was two orders of magnitude larger than that of the Fe3O4/Al2O3 thin film. Meanwhile, the saturation magnetization and the coercivity of the Fe3O4/MgO thin film were reduced by 86 % and 35 %, respectively. First-principles calculations suggested that the antiferromagnetic coupling was easily formed at all the three types of APBs, which led to the decrease of spin polarization and the increase of electrical resistance of the Fe3O4 thin films.

Atomic-Scale Characterization of Microscale Battery Particles Enabled by a High-Throughput Focused Ion Beam Milling Technique.

The cathode materials in lithium-ion batteries (LIBs) require improvements to address issues such as surface degradation, short-circuiting, and the formation of dendrites. One such method for addressing these issues is using surface coatings. Coatings can be sought to improve the durability of cathode materials, but the characterization of the uniformity and stability of the coating is important to assess the performance and lifetime of these materials. For microscale particles, there are, however, challenges associated with characterizing their surface modifications by transmission electron microscopy (TEM) techniques due to the size of these particles. Often, techniques such as focused ion beam (FIB)-assisted lift-out can be used to prepare thin cross sections to enable TEM analysis, but these techniques are very time-consuming and have a relatively low throughput. The work outlined herein demonstrates a FIB technique with direct support of microscale cathode materials on a TEM grid that increases sample throughput and reduces the processing time by 60-80% (i.e., from >5 to ∼1.5 h). The demonstrated workflow incorporates an air-liquid particle assembly followed by direct particle transfer to a TEM grid, FIB milling, and subsequent TEM analysis, which was illustrated with lithium nickel cobalt aluminum oxide particles and lithium manganese nickel oxide particles. These TEM analyses included mapping the elemental composition of cross sections of the microscale particles using energy-dispersive X-ray spectroscopy. The methods developed in this study can be extended to high-throughput characterization of additional LIB cathode materials (e.g., new compositions, coating, end-of-life studies), as well as to other microparticles and their coatings as prepared for a variety of applications.

Atomic-Scale Multimodal Characterization of Self-Assembled InAs/InGaAlAs Quantum Dots.

Self-assembled quantum dots (QDs) are potential candidates for photoelectric and photovoltaic devices, because of their discrete energy levels. The characterization of QDs at the atomic level using a multimodal approach is crucial to improving device performance because QDs are nanostructures with highly correlated structural parameters. In this study, scanning transmission electron microscopy, geometric phase analysis, and atom probe tomography were employed to characterize structural parameters such as the shape, strain, and composition of self-assembled InAs-QDs with InGaAlAs spacer layers. The measurements revealed characteristic AlAs-rich regions above the QDs and InAs-rich regions surrounding the QD columns, which can be explained by the relationship between the effect of strain and surface curvature around the QD. The methodology described in this study accelerates the development of future QD devices because its multiple perspectives reveal phenomena such as atomic-scale segregations and allow for more detailed discussions of the mechanisms of these phenomena.

“Invisible” radioactive cesium atoms revealed: Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant

Understanding radioactive Cs contamination has been a central issue at Fukushima Daiichi and other nuclear legacy sites; however, atomic-scale characterization of radioactive Cs in environmental samples has never been achieved. Here we report, for the first time, the direct imaging of radioactive Cs atoms using high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In Cs-rich microparticles collected from Japan, we document inclusions that contain 27 – 36 wt% of Cs (reported as Cs2O) in a zeolite: pollucite. The compositions of three pollucite inclusions are (Cs1.86K0.11Rb0.19Ba0.22)2.4(Fe0.85Zn0.84X0.31)2.0Si4.1O12, (Cs1.19K0.05Rb0.19Ba0.22)1.7(Fe0.66Zn0.32X0.41)1.4Si4.6O12, and (Cs1.27K0.21Rb0.29Ba0.15)1.9(Fe0.60Zn0.32X0.69)1.6Si4.4O12 (X includes other cations). HAADF-STEM imaging of pollucite, viewed along the [111] zone axis, revealed an array of Cs atoms, which is consistent with a simulated image using the multi-slice method. The occurrence of pollucite indicates that locally enriched Cs reacted with siliceous substances during the Fukushima meltdowns, presumably through volatilization and condensation. Beta radiation doses from the incorporated Cs are estimated to reach 106 – 107 Gy, which is more than three orders of magnitude less than typical amorphization dose of zeolite. The atomic-resolution imaging of radioactive Cs is an important advance for better understanding the fate of radioactive Cs inside and outside of nuclear reactors damaged by meltdown events.

Atomic-scale observation and characterization of deformation twins in uniaxial tensile-deformed 120Mn13 steel

High-resolution transmission electron microscope was used to observe and characterize the different formation stages of zero-strain twins in uniaxial tensile-deformed 120Mn13 steel at the atomic scale, which revealed the growth process of zero-strain twins in coarse-grained austenitic manganese steels. Here, plenty of zero-strain twins are observed in deformed 120Mn13 steel and they can be formed by three types of precursors. Their processes of expansion and connection, thickening and thinning can be carried out through the transformation of 9R phase with the matrix and the twin. A new 9R phase with different stacking sequences is firstly discovered and defined as 9R-Ⅱ to distinguish it from the classical 9R phase (9R-Ⅰ). 9R-Ⅰ and 9R-Ⅱ have the same lattice structure but different internal atomic arrangements and satisfy a twin-like relationship. In addition, a new formation mechanism of zero-strain twins based on the 9R phase transformation is proposed.

Atomic-scale understanding of twin intersection rotation and ε-martensite transformation in a high Mn twinning-induced plasticity steel

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions.

Atomic-scale characterization of V-shaped interface structure of η1 precipitates in Al–Zn–Mg alloy

Al–Zn–Mg alloys have attracted significant interest in the automotive industry owing to their high strength and light weight. Precipitation hardening is the primary mechanism by which these alloys are strengthened, meaning the analysis of the shape, size, and fraction of the precipitates is crucial. In this study, the interfacial structure of precipitates, which influences the mechanical properties of alloys, was investigated. Aberration-corrected scanning transmission electron microscopy studies revealed the atomic structure of the unique V-shaped interface structure of the η1 precipitates, which are the most prevalent among the η precipitates produced in this alloy. The structure was investigated from an energetic perspective using first-principles calculations, which revealed that the formation of the V-shaped interface structure increased the stability through strain relaxation in both the aluminum matrix and η1. The results provide valuable insights into the formation and growth mechanisms of precipitates, paving the way for further advancements in this field.