Cs-corrected Transmission Electron Microscopy Research Articles

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.

Formation Mechanism of GaN Nanowires with Various Shapes on Si(111)

We discuss the formation mechanism of GaN nanowires (NWs) with various shapes grown on Si(111) by using a plasma-assisted molecular-beam epitaxy. The GaN NWs have not only symmetrical structures but also various features such as reverse-mesa and reverse-funnel shapes. To manipulate the shape of GaN NWs, we controlled the growth kinetics of gallium (Ga) atoms by varying V/III ratio, defined as the ratio of the nitrogen (N) flux to the Ga flux, and the growth time. Narrow linewidth broadenings of the x-ray diffraction rocking curves and the coherent crystal structure, which are confirmed by using Cs-corrected transmission electron microscope images, indicate the formation of highly crystalline GaN NWs. The formation of GaN NWs, particularly reverse-funnel shaped NWs (RFS-NWs), can be explained by using the so-called wedding-cake growth mechanism, which is related to a reduction in the number of Ga atoms reaching the top surfaces of the GaN NWs from the surface of SiNx/Si(111) as the height of the GaN NWs increases. Photoluminescence spectra from the GaN RFS-NWs show a double-peak feature at wavelengths of 361.79 and 373.58 nm, which is attributed to two different widths of the GaN NWs in the vertical direction.

The Effects of Trace Sb and Zn Additions on Cu6Sn5 Lithium-Ion Battery Anodes.

Sn-based compounds are promising candidates for application as anodes in lithium-ion batteries (LIBs) due to the favourable storage capacity of Sn at 993 mAh g-1 compared to carbon at 372 mAh g-1. The use of Sn-based anodes also avoids some of the safety concerns associated with carbon anodes. However, the large volume changes during lithiation and delithiation of pure Sn anodes often results in poor cyclic performance. Alloying Sn with Cu, an element inactive with respect to Li, buffers the expansion stresses and can improve cycling performance. Cu6Sn5 is therefore a promising candidate anode material. In this work, the effects of Sb and Zn additions on the morphology, crystal structure, atomic arrangements and the electrochemical performance of the anodes were evaluated. Characterisation with synchrotron X-ray powder diffraction and Cs-corrected transmission electron microscopy revealed the larger lattice parameters, higher symmetry crystal structures and well-ordered atomic arrangements in the Sb and Zn modified electrodes, which resulted in a more than 50% increase in cycling capacity from 490 mAh g-1 to 760 mAh g-1.

Local structure tuning in Fe-N-C catalysts through support effect for boosting CO2 electroreduction

Metal-N-C materials have attracted considerable attention for electrocatalytic CO2 reduction. Local electronic and geometric structure of active sites in the metal-N-C catalysts significantly determine catalytic reactivity. In this work, atomically dispersed Fe species with different coordination environment are anchored into carbon supports through introducing aminated carbon nanotubes, as effectively revealed by the Cs-corrected transmission electron microscope and X-ray absorption near edge structure spectra. Electrochemical tests show that the catalyst with the aminated carbon nanotubes has a low overpotential (0.39 V) and high selectivity of CO up to 95.47 % at -0.6 V vs. RHE. The density functional theory calculation results indicate that the additional axial Fe-N coordination compared with the planar Fe-N coordination can reduce the free energy required for the desorption of CO from the adsorption site and inhibit the occurrence of hydrogen evolution reaction, thus leading to improve the selectivity of CO.