Cs-corrected Transmission Electron Microscopy Research Articles

Influence of V/B ratio and rolling treatment on the grain refinement efficacy and anti-fading ability of Al-V-B master alloys

As a promising anti Si-poisoning grain refiner, Al-V-B master alloys have gained increasing attention in the past several years. However, the effect of V:B ratio on the grain refinement effect, as well as the refining mechanism has not been clearly investigated. This work investigates the impact of V:B ratio on microstructures of Al-V-B master alloys and grain refinement efficacy for α-Al grains in AlSi alloys. In addition, the effect of rolling treatment on the agglomeration and sedimentation of Al-V-B master alloys has been investigated. It demonstrates that the V:B ratio significantly affects primary secondary phases within Al-V-B master alloys, with an optimal ratio of 2.33 (Al-4.2 V-1.8B) leading to the finest average α-Al grain size of ~98 μm in the Al8Si alloy. Moreover, rolling treatment further enhances the grain refinement efficacy of Al-4.2 V-1.8B master alloy by dispersing VB2 agglomerates/particles and reducing agglomeration and sedimentation during smelting. Accordingly, the rolled Al-4.2 V-1.8B master alloy reduces the average α-Al grain size to ~75 μm at only 50 % amount of the as-cast Al-4.2 V-1.8B master alloy, improving the grain refinement efficacy by 25 % (75 μm v.s. 98 μm). At the same time, the rolled Al-4.2 V-1.8B master alloy exhibits exceptional anti-fading ability, maintaining a stable grain refinement efficacy for up to 120 min. Cs-corrected transmission electron microscopy (TEM) reveals that VB2 particles act as primary nucleation sites for α-Al grains, with a specific crystallographic orientation relationship: [1 1 2¯ 0] (0001) VB2 || [1¯ 1 0] (111) α-Al. This study provides valuable insights into the design of Al-V-B master alloys with superior grain refinement efficacy and anti-fading ability for AlSi alloys.

Combining Machine Learning Algorithms with Traditional Methods for Resolving the Atomic-Scale Dynamic Structure Reconstruction of Monolayer MoS2 in High-Resolution Transmission Electron Microscopy

Abstract High-resolution transmission electron microscopy promises rapid atomic-scale dynamic structure imaging. Yet, the precision limitations of aberration parameters and the challenge of eliminating aberrations in Cs-corrected transmission electron microscopy constrain resolution. A machine learning algorithm is developed to determine the aberration parameters with higher precision from small, lattice-periodic crystal images. The proposed algorithm is then validated with simulated HRTEM images of graphene and applied to the experimental images of a molybdenum disulfide (MoS2) monolayer with 25 variables (14 aberrations) resolved in wide ranges. Using these measured parameters, the phases of the exit-wave functions are reconstructed for each image in a focal series of MoS2 monolayers. The images were acquired due to the unexpected movement of the specimen holder. Four-dimensional data extraction reveals time-varying atomic structures and ripple. In particular, the atomic evolution of the sulfur-vacancy point and line defects, as well as the edge structure near the amorphous, are visualized as the resolution has been improved from about 1.75 Å to 0.90 Å. This method can help salvage important transmission electron microscope images and is beneficial for the images obtained from electron microscopes with average stability.

Modification of RuO2 nanosheet microstructure via hydrogen plasma treatment for transparent electrode applications

Although monolayer RuO2 nanosheets exhibit semiconducting characteristics, their conductivity must be improved to enable application in transparent electrodes. In this study, RuO2 nanosheet microstructures are modified through H2 plasma treatment to reduce sheet resistance without deteriorating the optical characteristics necessary for flexible transparent electrode applications. Microstructural and chemical changes in the RuO2 nanosheets are observed using monochromator Cs-corrected high-resolution transmission electron microscopy combined with electron energy loss spectroscopy. Single-crystalline RuO2 nanosheets are transformed into nanometer-sized polycrystalline nanosheets. The phase-transformed polycrystalline RuO2 nanosheets contain newly formed defects such as grain boundaries and O vacancies, which result in lower sheet resistances because these defects provide leakage-current paths in the semiconducting RuO2 nanosheets. These defects are located parallel to the direction of light transmission. Consequently, transparency and haze do not significantly deteriorate after H2 plasma treatment. These results indicate that the H2 plasma creates a more stable O-vacancy row which alters the microstructure of the RuO2 nanosheets. Our results demonstrate that H2 plasma treatment can be widely used to fabricate two-dimensional material-based transparent electrodes.

The anisotropic sublimation induces rhombohedral to rock-salt phase transformation of chalcogenide SnSb2Te4 two-dimensional nanomaterial

We report the sublimation and phase transformation mechanisms of a two-dimensional (2D) chalcogenide SnSb2Te4 using a lab-developed in-situ electro-thermal testing system on a Cs-corrected transmission electron microscope. It reveals that the sublimation process is primarily governed by the surface energy mechanism, following an anisotropic route. The van der Waals (vdW) layers ending with Te atoms lead to the preferential sublimation on (003) plane, with the (1 1‾ 4) and (117) planes accommodating the sublimation. It is uncovered for the first time, that the preferential and accommodated sublimation of Te and Sb atoms along the vdW planes results in short-range diffusion inducing a phase transition from rhombohedral SnSb2Te4 to rock-salt (Sn1-xSbx)Te. These results highlight complex physics processes of 2D materials under service conditions and the route of atomic-resolved electro-thermal research platforms for investigations of complex atomistic mechanisms of 2D materials by thermal-electric external fields.

Engineering Phase Transition from 2H to 1T in MoSe2 by W Cluster Doping toward Lithium-Ion Battery.

Phase engineering synthesis strategy is extremely challenging to achieve stable metallic phase molybdenum diselenide for a better physicochemical property than the thermodynamically stable semiconducting phase. Herein, we introduce tungsten atom clusters into the MoSe2 layered structure, realizing the phase transition from the 2H semiconductor to 1T metallic phase at a high temperature. The combination of synchrotron radiation X-ray absorption spectroscopy, Cs-corrected transmission electron microscopy, and theoretical calculation demonstrates that the aggregation doping of W atoms is the factor of MoSe2 structure transformation. When utilizing this distinct structure as an anode component, it demonstrates outstanding rate capability and durability. After 500 cycles, this results in a specific capacity of 1007.4 mAh g-1 at 500 mA g-1. These discoveries could open the door for the future development of high-performance anodes for ion battery applications.

The γ” Phase in Mg-RE-TM Alloys: A Review on the Structure and Stability of the γ” Phase and Its Effect on Mechanical Properties

In magnesium–rare earth–transition metal (Mg-RE-TM) alloys, the γ” phase (with a hexagonal structure with the space group P6¯2m) is a critical strengthening phase that can significantly improve their mechanical properties. However, compared to other phases in Mg-RE-TM alloys, research on the γ” phase is less documented, and an understanding of the γ” phase is not well established. As a result, different models of the structure of the γ” phase have been proposed. In this review, we summarize these structural models and find that the γ” phase is different from the Guinier–Preston (G.P.) zone, as revealed via Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculations. In addition, we briefly summarize the stability of the γ” phase and its effect on the mechanical properties of Mg-RE-TM alloys.

Nucleation and evolution mechanism of precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy

The evolution process and nucleation mechanism of the precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy were studied using the combination of the Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and first-principles calculations. A new nucleation mode of η' phase with the thickness of 7 at. layers was found, which is parallel to {111}Al planes and consists of 5-layer transition structures and 2 transition layers at the interface. The precipitate is the initial aged η' phase with the simplest structure. Its interfaces are coherent with the Al matrix and meet the stacking periodicity, which is more stable than ordinary η' phases. Besides, the interfaces have a tendency to segregate heavy elements. The presence of segregating atoms enhances the interface interaction and makes it more stable. These two aspects improve the stability of the nano-scale 7-layer particle and maintain the aging strengthening effect of Al alloy.

Capturing and observing 1–6-nm-sized nanocarbon particles by transmission electron microscopy during the diamond hot filament chemical vapor deposition process

Nanocarbon particles ~1 nm in size were first suggested to exist in the gas phase in 1996, and to be the building block of diamond growth during the hot filament chemical vapor deposition process. However, only nanocarbon particles larger than ~2 nm have been captured and observed in recent research. We used single-layer graphene to capture ~1-nm-sized nanocarbon particles at 300 °C and observed them by Cs-corrected monochromated transmission electron microscopy (Cs-corrected TEM). Nanocarbon particles 1–6 nm in size were captured for 1 and 10 s with a filament temperature of 2100 °C, reactor pressure of 20 Torr, and gas mixture composition of 1 % CH4–99 % H2. According to our Cs-corrected TEM observations, most nanocarbon particles larger than 3.5 nm were crystalline, whereas most nanocarbon particles smaller than 2.0 nm were amorphous. Analysis of high-resolution TEM images and their fast Fourier transformations confirmed that most of the crystalline nanocarbon particles were i‑carbon, in contrast with a previous study reporting that they consisted of various allotropes.

Nanoindentation investigation of incoherent twin boundary migration in Au nanocrystalline films

Incoherent twin boundary (ITB) evolution and migration induced by nanoindentation in Au nanocrystalline films were systematically investigated by Cs-corrected transmission electron microscopy (TEM). With a gradient stress distribution at various indentation depths, 9R/3R phase nucleation is greatly facilitated at the ITBs. The 9R/3R phase formation can be achieved by collective glide of triple partial dislocations with different Burgers vectors at two phase boundaries. With 9R/3R phase at ITBs, atoms on consecutive {111} planes shear in a helical manner, generating zero macroscopic strain during ITB migration. Dislocation interactions with ITBs were further analyzed by combining atomic-scale high-angle annular dark-field scanning TEM (HAADF-STEM) with geometric phase analysis method. ITBs can both absorb partial dislocations and emit partial dislocations from the junctions between coherent twin boundaries and ITBs, providing complementary mechanisms for ITB migration in Au nanocrystalline films under nanoindentation. These results suggest the critical role of 9R/3R phase nucleation in ITB migration and provide a deeper understanding of nanoindentation-induced deformation twinning in nanocrystalline films.

Revealing Surface Restraint-Induced Hexagonal Pd Nanocrystals via In Situ Transmission Electron Microscopy.

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

A new insight into heterogeneous nucleation mechanism of Al by non-stoichiometric TiCx

Al–Ti–C master alloy has been known as an efficient grain refiner for a series of Al alloys since 1950s. However, the grain refinement mechanism is still not fully elucidated up to now. Herein, we systematically investigate this mechanism by combining Cs-corrected transmission electron microscopy and theoretical calculations. First, the Al/TiCx nucleation interface is determined to be a “Al ∼ Interfacial transition layer ∼ TiCx” sandwich-like structure, with the orientation relationship of [011]Al//[011]TiCx and (11¯1)[011]Al plane inclined 21° to (11¯1)[011]TiCx plane that is beneficial to eliminate the lattice constant misfit between Al and TiCx. The interfacial transition layer, consisting of Al and Ti atoms, play a crucial role in reducing atomic arrangement mismatch between (13¯3)[011]Al and (11¯1)[011]TiCx, thus promoting Al nucleation. Second, rather than directly reacting with Al melt as conventionally reckoned, TiCx with x < 0.92 actually releases Ti atoms to Al melt first after inoculation (720 °C), contributing to the formation of the Ti-containing interfacial transition layer. On the other hand, after Ti releasing, C/Ti ratio in TiCx is increased, which inversely decreases the Ti-releasing capability and leads to grain refinement fading gradually. This study sheds new light on the grain refinement mechanism in Al–Ti–C-refined Al and should build up the foundation for developing high-efficiency TiCx-containing grain refiners.

Coaxially grafting conjugated microporous polymers containing single-atom cobalt catalysts to carbon nanotubes enhances sulfur cathode reaction kinetics

Conjugated microporous polymers (CMPs) hold great potential for use in energy related applications due to their extended π-conjugated structures, tunable pore sizes, and modular molecular functionalities. Herein, we report a novel composite material (labeled as Co-CMP-MWNTs) that consists of a CMP containing Co single-atom catalysts (Co SACs) and being coaxially grafted to multi-walled carbon nanotubes (MWNTs), and show that the material synergistically promotes the cathode reaction kinetics in lithium−sulfur (Li−S) batteries. The Co-CMP-MWNTs are synthesized by coupling 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine to a dibromobipyridine-Co complex in the presence of bromopyrimidinyl-functionalized MWNTs. The composite features a conductive MWNT-based core and a CMP-based shell that contains nitrogen as well as Co. Cs-corrected high-resolution transmission electron microscopy and X-ray absorption near-edge structure (XANES) spectroscopy reveal that the Co species exist as single atoms. Additional XANES data coupled with density functional theory calculations elucidate the adsorption interactions formed between the Co SACs and various sulfur species as well as their electrocatalytic effects. Li−S cells prepared using Co-CMP-MWNTs as a cathode host material exhibit excellent performance in terms of specific capacity (1485 mA h g−1 at 0.1 C), rate capability (602 mA h g−1 at 3 C), and cycling stability (510 mA h g−1 at 0.5 C after 1000 cycles, which corresponds to a capacity decay of 0.050% per cycle). Collectively, the results demonstrate that SACs can be prepared under benign conditions and used to enhance sulfur cathode reaction kinetics. The methodology described may be extended to enable the use of SACs in other contemporary energy conversion technologies.

Atomic-Scale Observation of Unusual Dislocations in GaAs-GaAsSb Heterostructured Nanowires.

Cognizing the structural characteristics of a heterointerface is significant to understand the growth mechanism of heterostructured nanowires. Here, the structural characteristics of a heterointerface in GaAs-GaAsSb heterostructured nanowires were investigated by employing spherical aberration (CS)-corrected transmission electron microscopy (TEM). It is found that some unusual dislocations are formed at the heterointerface, leading to the bending of nanowires. Further, the atomically inhomogeneous distribution of Sb content near the heterointerface is revealed, which is responsible for the formation of dislocations. By applying a thermal electric system equipped in the Cs-corrected TEM, a direct observation of structural evolution at the heterointerface was enabled and the stability of GaAs-GaAsSb heterostructured nanowires was evaluated. In situ high-resolution TEM imaging indicates that the destabilization of the heterointerface occurs during nanowire annealing. This study builds a direct correlation between the nanowire heterointerfacial structure with nanowire growth behavior and its stability, which is of importance for heterostructured nanowire design for practical use.

Improvement of Measurement Method of Precipitate Number Densities in 9Cr Steel by Cs-corrected Transmission Electron Microscopy

Improvement of Measurement Method of Precipitate Number Densities in 9Cr Steel by Cs-corrected Transmission Electron Microscopy

The intrinsic Josephson effect of Bi-2212 superconducting thin films prepared by sol-gel method

In this paper, Bi2Sr2Ca1Cu2O8+σ (Bi-2212) superconducting thin films was prepared by using a sol-gel method on a LaAlO3 (LAO) single crystal substrate, with acetates of bismuth, strontium, calcium, and copper as the starting raw materials. A step structure was specifically designed to measure the intrinsic Josephson effect. The microstructure of the Bi-2212 thin films and their epitaxial growth on the LAO substrate were analyzed by a Cs-corrected transmission electron microscope (Cs-TEM) with an atomic-level resolution and the electrical properties were measured. The results showed that the prepared Bi-2212 thin films had good epitaxial growth characteristics and good in-plane and out-plane growth texture and that its critical transition temperature, Tc, was 87 K. The interface between the substrate and the film was well-defined; the films grow epitaxially on the substrate from the CuO layer, with a highly ordered atom arrangement. The I–V characteristic curve of the prepared step micro-bridging sample showed zero-voltage overcurrent phenomenon, and the critical current was 2.3 mA. In addition, multiple current-voltage jumps were observed on the curve, indicating that the prepared biaxial texture thin films showed the intrinsic Josephson effect.

Atomic resolution in situ observation on photon-induced reshaping and phase transitions of CsPbBr3 nanocube and quantum dot

Controlling the photon-induced reshaping and phase transitions of low-dimensional halide perovskites is a significant and challenging task. Using in situ Cs-corrected transmission electron microscopy with a nanosecond pulsed laser source, we traced CsPbBr3 nanocubes and quantum dots under different laser irradiation conditions, through a low-dose observation mode. We found that a high laser fluence with a short irradiation time (e.g., 300 mJ/cm2 in the range of seconds) triggers defect growth and a cubic-to-orthorhombic phase transition in perovskite nanocubes; however, a low laser fluence with a long irradiation time (e.g., 30 mJ/cm2 in the range of minutes) is actually like an annealing process that removes defects and gradually reshapes the perovskite samples into a round morphology, with the cubic phase well stabilized by the strong surface tension. Based on the in situ studies, we present a feasible and effective laser engineering approach for low-dimensional halide perovskite materials.

Atomic-scale observation on the precipitates in various aging stages of Mg–Gd–Y–Cu alloy

This paper reports a comprehensive investigation of long-period stacking ordered (LPSO) structures and precipitates in various stages of aging treatment in Mg96Gd2Y1Cu1(at%) alloy, using atomic-scaled Cs-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). 14H- and 18R-LPSO structures coexisted in as-cast alloy, dissolved in solid solution state and reappeared after aging treatment. The aging peak hardness reached 147 HV at 200 °C for 19 h. β' and LPSO structures interconnected with each other via two spatial interaction types during aging treatment, contacting with each other directly or being separated by α-Mg layers. Novel rectangular and rhombic precipitates forming during aging treatment are identified as Gd3Y compound, which are observed for the first time by HAADF-STEM. Composite strengthening of LPSO, β' and Gd3Y precipitates contributes to a significant strengthening effect on the alloy.

Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines

Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines

Unraveling a Biomass-Derived Multiphase Catalyst for the Dehydrogenative Coupling of Silanes with Alcohols under Aerobic Conditions

Herein, a novel silver and chromium nanostructured N-doped carbonaceous material has been synthesized by a biomass-annealing approach using readily available chitosan as a raw material. The resulting catalyst AgCr@CN-800 has been applied for the dehydrogenative coupling reaction of various silanes with different alcohols to obtain the corresponding silyl ethers under aerobic and mild conditions. Besides excellent activity and selectivity, the as-prepared catalyst exhibits good stability and reusability. Characterization by X-ray diffraction, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and high-resolution transmission electron microscopy (TEM) in combination with careful examination of the structure with Cs-corrected high-angle annular dark-field scanning TEM revealed that the catalyst AgCr@CN-800 comprises Ag- and CrN-aggregated particles, as well as highly dispersed Ag–Nₓ and Cr–Nₓ sites embedded in N-doped graphitic structures. A comparative catalytic study using structure-related catalysts in combination with acid-leaching treatments has shown that the most active species are the Ag particles and that their activity is boosted by the presence of Cr-derived species. By in situ Raman spectroscopy experiments, it has been found that the dehydrogenative coupling of silanes with alcohols in the presence of catalyst AgCr@CN-800 takes place through an oxygen-assisted mechanism.

Macro-Micro Relationship of Dimensional Changing Behavior of Aluminum Matrix Composite

Accuracy and stability have always been the eternal pursuit of inertial navigation. Among them, the dimensional stability of inertial instrument materials is a key factor in determining the accuracy level of equipment. The dimensional changing(DC) behavior is a material problem extracted from the common accuracy drift problem in engineering technology. The DC behavior of 45vol% SiC P /2024Al composite under no load at constant temperature was systematically studied in-depth using in-situ heating XRD and double Cs-corrected transmission electron microscopy. The trend and magnitude of lattice constant and strain mapping of matrix near SiC/Al interface were both tracked and examined. A high-resolution thermal dilatometer was used to measure the macroscopic DC. Thermal mismatch residual stress has a decisive effect on the DC of SiC P /2024Al composite. The effect of exsolution on the DC of SiC P /2024Al composite was also discussed. Different dominant mechanism of DC between aluminum alloy and composites leads to the difference in the magnitude, rate, and stabilization of the dimensional change. In order to verify the validity of the mechanism, the DC results of different matrix and different heat treatment states were compared and explained. The estimation method of DC based on the volume change of unit cell could accurately predict the trend of DC and the magnitude of SiC P /2024Al composite. This provides instructive guidance for deeply understanding the DC behavior of aluminum matrix composites, and enriches the research work of micro-plastic deformation.