Transmission Electron Microscopy Experiments Research Articles

Thermodynamic Understanding of Impurity Phase Segregation in a PdCrO2/CuCrO2 Heterostructure

AbstractPdCrO2 films are synthesized on CuCrO2 buffer layers on Al2O3 substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO2 films. Potential causes ofth impurity phase segregation are studied by using a combination of experiments and ab initio calculations. X‐ray diffraction and scanning transmission electron microscopy experiments reveal impurity phases of CuxPd1 − x alloy and chromium oxides, Cr2O3 and Cr3O4, in PdCrO2. Calculations determine that oxygen deficiency can cause the impurity phase segregation. Therefore, preventing oxygen release from delafossites can suppress the impurity phase segregation. The amounts of Cr2O3 and Cr3O4 depend on temperature and oxygen partial pressure. A reasonable theory‐based explanation for this experimental observation is provided.

Direct observation of oxygen vacancy formation and migration over ceria surface by in situ environmental transmission electron microscopy

The extremely high structural tolerance of ceria to oxygen vacancies (Ov) has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants. It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property. However, the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown. Herein, the structural and valence evolution of CeO2 (111) surfaces in reductive, oxidative and vacuum environments from room temperature up to 700 °C was studied with in situ aberration-corrected environmental transmission electron microscopy (ETEM) experiments. Ov is found to form under a high vacuum at elevated temperatures; however, the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment. Furthermore, in hydrogen environment, the step-CeO2 (111) surface is not stable at elevated temperatures; thus, the steps tend to be eliminated with increasing temperature. Combined with first-principles density function calculations (DFT), it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface. Furthermore, in a reductive environment, H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope. These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas, thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO2 during catalytic processes.

The improvement of endurance characteristics in a superlattice-like material-based phase change device

Improvement of endurance characteristics has been a hot area of phase-change memoryresearch. The properties of a phase-change material are believed to play an important role in device endurance. Repeated SET–RESET operation always leads to material failure problems, such as composition deviation and phase separation. Moreover, the quality of the electrode and the electrode contact also determine the endurance characteristics. In this study, C nanolayers were periodically inserted into the phase-change material Ge2Sb2Te5 (GST) to fabricate a superlattice-like (SLL) structure. Although some of C bonded with some of the Ge, Sb and Te atoms, more C atoms prefer nanometer-scale clusters at the grain boundary in the SLL film. The typical local configuration of GST was unchanged when artificial C nanolayers were inserted. Transmission electron microscopy experiments revealed that the bonded C atoms and nanometer-scale C clusters may occupy the spontaneously created holes and defects, preventing composition deviation of the phase-change material and prolonging the electrode service life. The contact surface between the phase-change material and the electrode is also improved. As a result, we found that the endurance cycle could be improved by up to 106 for a GST/C SLL film-based device.

Sunlight-mediated CaO2 inactivation of pathogen indicator organisms in surface water system: Roles of reactive species, characterization of pathogen inactivation

In the era of the current epidemic, it is urgent to control pathogens in sewage, eliminate the source of infection, and optimize the technology for killing pathogens. Combining calcium peroxide (CaO2) with sunlight is considered a potentially efficient, economical, and eco-friendly method for pathogen-contaminated water remediation. This paper evaluated the solar activating properties of CaO2 for inactivating pathogenic indicators and explored the roles of reactive species contributing to pathogen inactivation. Moreover, these reactive species’ average steady-state concentrations and second-order reaction rate were tentatively explored, and mechanistic model for photoinactivation were establishment. Pathogen's inactivation was mainly attributed to direct photoinactivation (13∼50%) and exogenous indirect mechanisms with corresponding contributions of reactive species, i.e., OH− (14∼23%), 1O2 (12∼28%), •OH (20∼32%), O2•– (12∼16%), and H2O2 (6∼11%). Furthermore, cell membrane rupture and DNA damage were observed by transmission electron microscopy (TEM) and agarose gel electrophoresis (AGE) experiments. Among experiments on common aqueous constituents influencing photoinactivation, copper and iron ions were found to promote a pathogen-inactivating ability of the system, while fulvic acids (FA) and humic acid (HA) had the opposite effect. This study revealed the potential of CaO2/sunlight to inactivate pathogens and laid a foundation for its application in inactivating pathogens in surface water.

Resolution of MoS2 Nanosheets-Induced Pulmonary Inflammation Driven by Nanoscale Intracellular Transformation and Extracellular-Vesicle Shuttles.

Pulmonary exposure to some engineered nanomaterials can cause chronic lesions as a result of unresolved inflammation. Among 2D nanomaterials and graphene, MoS2 has received tremendous attention in optoelectronics and nanomedicine. Here an integrated approach is proposed to follow up the transformation of MoS2 nanosheets at the nanoscale and assessstheir impact on lung inflammation status over 1 month after a single inhalation in mice. Analysis of immune cells, alveolar macrophages, extracellular vesicles, and cytokine profiling in bronchoalveolar lavage fluid (BALF) shows that MoS2 nanosheets induced initiation of lung inflammation. However, the inflammation is rapidly resolved despite the persistence of various biotransformed molybdenum-based nanostructures in the alveolar macrophages and the extracellular vesicles forup to 1 month. Using in situ liquid phase transmission electron microscopy experiments, the dynamics of MoS2 nanosheets transformation triggered by reactive oxygen species could be evidenced. Three main transformation mechanisms are observed directly at the nanoscale level: 1) scrolling of the dispersed sheets leading to the formation of nanoscrolls and folded patches, 2) etching releasing soluble MoO4 - , and 3) oxidation generating oxidized sheet fragments. Extracellular vesicles released in BALF are also identified as a potential shuttle of MoS2 nanostructures and their degradation products and more importantly as mediators of inflammation resolution.

Phenomenological understanding of kappa-carbides on strengthening mechanism of low-density steel

We elucidate here the phenomenological understanding of deformation mechanisms over a wide range of strain rates (5.5 × 10–4s–1 to 5.5 × 10–1s–1) based on the evolution of microstructure. Atom probe tomography (APT), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) experiments were conducted to reveal the spatial distribution of precipitates and microstructure evolution. The ordered kappa-carbides were characterised by a layered-type structure, which made it difficult for dislocations to by-pass kappa-carbides in the same layer individually and the corresponding strengthening mechanism was cutting mechanism. Dislocations and microbands evolution analysis shows that compared to dislocations, slip band width was the key factor that governed the strength of the experimental steel at different strain rates.

Development of novel HBV capsid assembly modifiers through molecular dynamics, docking, and experiments.

Hepatitis B Virus (HBV) is the leading cause of liver complications, including cirrhosis, hepatocellular carcinoma, and liver failure. Interfering with the self-assembly process of virus nucleocapsid is a promising approach for the development of the novel antiviral agents. Starting from microsecond-scale molecular dynamic simulations of a tetramer of the HBV capsid protein Cp149, we identified three novel conformations for docking based on their pocket volume and principal component analysis of inter-dimer motions. Compounds were selected from the DivV set of NCI and ZINC0.9 databases for experimental testing according to their docking scores for the selected pocket. Combining molecular dynamics simulations and in vitro testing, we discovered several novel compounds with moderate activity. Additionally, transmission electron microscopy and capsid assembly experiments suggest that our compounds possess a novel mechanism for assembly modulation, potentially opening new avenues for HBV inhibition.

Preparation of High-Quality Samples for MEMS-Based In-Situ (S)TEM Experiments.

A novel focused ion beam (FIB)-based methodology for the preparation of clean and artifact-free specimens on micro-electro-mechanical-system (MEMS)-based chips for in-situ electrical and electro-thermal experiments in a (scanning) transmission electron microscope ((S)TEM) is introduced. Owing to an alternative geometry, the lamellae are attached to a MEMS-based chip directly after the lift-out procedure and afterward further treated or thinned to electron transparency. The quality of produced lamellae on a chip resembles the quality of a classical FIB-prepared sample that is here demonstrated by high-resolution STEM imaging and analytical techniques. Various sample preparation parameters and the performance of in-situ prepared samples have been evaluated through electrical-biasing experiments.

Screening of Coatings for an All-Solid-State Battery using In Situ Transmission Electron Microscopy.

With the ever-increasing use of Li-ion batteries, especially due to their adoption in electric vehicles, their safety is in prime focus. Thus, the all-solid-state batteries (ASSBs) that use solid electrolytes instead of liquid electrolytes, which reduce the risk of flammability, have been the center stage of battery research for the last few years. However, in the ASSB, the ion transportation through the solid-solid electrolyte-electrode interface poses a challenge due to contact and chemical/electrochemical stability issues. Applying a suitable coating around the electrode and/or electrolyte particles offers a convenient solution, leading to better performance. For this, researchers are screening potential electronic/ionic conductive and nonconductive coatings to find the best coatings with suitable thickness for long-term chemical, electrochemical, and mechanical stability. Operando transmission electron microscopy (TEM) couples high spatial resolution with high temporal resolution to allow visualization of dynamic processes, and thus is an ideal tool to evaluate electrode/electrolyte coatings via studying (de)lithiation at a single particle level in real-time. However, the accumulated electron dose during a typical high-resolution in situ work may affect the electrochemical pathways, evaluation of which can be time-consuming. The current protocol presents an alternative procedure in which the potential coatings are applied on Si nanoparticles and are subjected to (de)lithiation during operando TEM experiments. The high volume changes of Si nanoparticles during (de)lithiation allow monitoring of the coating behavior at a relatively low magnification. Thus, the whole process is very electron-dose efficient and offers quick screening of potential coatings.

Pseudopyrolysis of Metal-Organic Frameworks: A Synchronous Nucleation Mechanism to Synthesize Ultrafine Metal Compound Nanoparticles.

Metal-Organic frameworks (MOFs) are increasingly being investigated for the synthesis of carbon-supported metal-based ultrafine nanoparticles (UNPs). However, the collapse of the carbon framework and aggregation of metal particles in the pyrolysis process have severely hindered their stability and applications. Here, we report the synchronous nucleation pseudopyrolysis of MOFs to confine Fe/FeOx UNPs in intact porous carbon nanorods (IPCNs), revealed by in situ transmission electron microscopy experiments and ex situ structure analysis. The pseudopyrolysis mechanism enables strong physical and chemical confinement effects between UNPs and carbon by moderate thermal kinetics and abundant oxygen defects. Further, this strong confinement is greatly beneficial for subsequent chemical transformations to obtain different Fe-based UNPs and excellent electrochemical performance. As a proof of concept, the as-prepared FeSe UNPs in IPCNs show superior lithium storage performance with an ultrahigh and stable capacity of 815.1 mAh g-1 at 0.1 A g -1 and 379.7 mAh g-1 at 5 A g-1 for 1000 cycles.

Effects of Preformed Martensite on Microstructure and Mechanical Properties of Nanobainite Bearing Steel

Previous studies have indicated that the preformed martensite has a significant effect on the subsequent bainite transformation of nanobainite bearing steel, but its effects on the microstructure and mechanical properties are still unclear. In this paper, the effects of preformed martensite and its volume fraction on microstructure and mechanical properties of nanobainite bearing steel were investigated by using scanning electron microscope, transmission electron microscope, X-ray diffractometer and wear resistance experiments. It was demonstrated that the introduction of preformed martensite can refine bainitic microstructure and increase the volume fraction of retained austenite, but reduces the mechanical stability of retained austenite. When the volume fraction of preformed martensite is less than 20%, the hardness and wear resistance of nanobainite bearing steel gradually increases with the increase in the volume fraction of preformed martensite, but the toughness of steel decreases gradually. When the volume fraction of preformed martensite reaches 25%, the properties of nanobainite bearing steel, especially the wear resistance, obviously deteriorate. The optimized volume fraction of preformed martensite is ~10%–15% by comprehensive comparison the mechanical properties of nanobainite bearing steel.

Atomic diffusion behavior in electromagnetic pulse welding

• Establish molecular dynamics model to investigate the interface diffusion behavior. • A narrow diffusion zone of amorphous layer was found at the bonding interface. • Improve the theoretical basis on the bonding mechanism at the atomic level. This paper investigates the atomic diffusion behavior and bonding mechanism in a Cu-Al joint fabricated by electromagnetic pulse welding. The molecular dynamics simulation is proposed to investigate the atomic diffusion behavior. The equation of diffusion layer thickness is deduced to calculate the diffusion layer thickness according to the simulation results. The welding and transmission electron microscope experiments are carried out, and the simulation results are in good agreement with the experiments. There is a diffusion zone of amorphous layer at the bonding interface, but the diffusion thickness is narrow. So this work reveals that the inter-atomic bonding is not a major factor in electromagnetic pulse welding.

Phosphates form spectroscopically dark state assemblies in common aqueous solutions

Phosphates and polyphosphates play ubiquitous roles in biology as integral structural components of cell membranes and bone, or as vehicles of energy storage via adenosine triphosphate and phosphocreatine. The solution phase space of phosphate species appears more complex than previously known. We present nuclear magnetic resonance (NMR) and cryogenic transmission electron microscopy (cryo-TEM) experiments that suggest phosphate species including orthophosphates, pyrophosphates, and adenosine phosphates associate into dynamic assemblies in dilute solutions that are spectroscopically "dark." Cryo-TEM provides visual evidence of the formation of spherical assemblies tens of nanometers in size, while NMR indicates that a majority population of phosphates remain as unassociated ions in exchange with spectroscopically invisible assemblies. The formation of these assemblies is reversibly and entropically driven by the partial dehydration of phosphate groups, as verified by diffusion-ordered spectroscopy (DOSY), indicating a thermodynamic state of assembly held together by multivalent interactions between the phosphates. Molecular dynamics simulations further corroborate that orthophosphates readily cluster in aqueous solutions. This study presents the surprising discovery that phosphate-containing molecules, ubiquitously present in the biological milieu, can readily form dynamic assemblies under a wide range of commonly used solution conditions, highlighting a hitherto unreported property of phosphate's native state in biological solutions.

FFT pattern recognition of crystal HRTEM image with deep learning

Rapid analysis and processing of large quantities of data obtained from in-situ transmission electron microscope (TEM) experiments can save researchers from the burdensome manual analysis work. The method mentioned in this paper combines deep learning and computer vision technology to realize the rapid automatic processing of end-to-end crystal high-resolution transmission electron microscope (HRTEM) images, which has great potential in assisting TEM image analysis. For the fine-grained result, the HRTEM image is divided into multiple patches by sliding window, and 2D fast Fourier transform (FFT) is performed, and then all FFT images are inputted into the designed LCA-Unet to extract bright spots. LCA-Unet combines local contrast and attention mechanism on the basis of U-net. Even if the bright spots in FFT images are weak, the proposed neural network can extract bright spots effectively. Using computer vision and the information of bright spots above mentioned, the automatic FFT pattern recognition is completed by three steps. First step is to calculate the precise coordinates of the bright spots, the lattice spacings and the inter-plane angles in each patch. Second step is to match the lattice spacing and the angles with the powder diffraction file (PDF) to determine the material phase of each patch. Third step is to merge the patches with same phase. Taking the HRTEM image of zirconium and its oxide nanoparticles as an example, the results obtained by the proposed method are basically consistent with manual identification. Thus the approach could be used to automatically and effectively find the phase region of interest. It takes about 3 s to process a 4 K × 4 K HRTEM image on a modern desktop computer with NVIDIA GPU.

Conversion of nitrogen to ammonia using a Cu/C electrocatalyst in a polymeric electrolyte reactor

The electrochemical conversion of N2 to NH3 using a polymeric electrolyte reactor is a promising method to accelerate the green production of hydrogen carriers. On this basis, we report the efficiency of ammonia production by the nitrogen reduction reaction using a Cu/C catalyst in a polymeric electrolyte membrane reactor. The Cu/C catalyst was prepared by the NaBH4 reduction method and characterized by X-ray diffraction, transmission electron microscopy, cyclic voltammetry, and conversion experiments performed in a polymer electrolyte membrane fuel cell type reactor. The X-ray diffraction results showed the presence of CuO2 and carbon phases, while the TEM images showed a high agglomeration of copper nanoparticles on carbon. The onset potential of nitrogen reduction was near to the Cu (I) to Cu0 reduction peak. Mass spectroscopy was used to observe the production of N2H2 and NH3 and the consumption of N2. Maximum ammonia production was detected at 0.0 V with a NH3 yield rate of 38.4 µg h−1 cm−2 and a faradaic efficiency of 42.57 %.

Operando Liquid-Phase TEM Experiments for the Investigation of Dissolution Kinetics: Application to Li-Ion Battery Materials

Abstract We present here a novel and universal sample preparation method for liquid-phase transmission electron microscopy (TEM) experiments involving powder materials. Thanks to this new protocol, we demonstrate the ability to study the dissolution of cathode materials by sulfuric acid using operando TEM. Based on ultramicrotomy, this method has been developed to avoid compatibility issues between sample size and liquid cell geometry. The observation parameters have been optimized to get the best contrast in scanning TEM (STEM) images. The reaction kinetics have been investigated by image analysis and compared to those obtained ex situ. By taking into account the sample geometry and the reacting surfaces in both experiments, we show that the reaction kinetics are equivalent and could be overlaid. This proves that the protocol developed enables real operando experiments, as it provides quantitative data at the nanoscale that are representative of the bulk specimen. The main challenges of such a protocol for operando liquid-phase TEM, as well as the accuracy of the image analysis method used, are also discussed.

Double Diffraction or ω-Fe Diffraction in Body-Centered Cubic {112}<111>-Type Twinned Martensite

The martensitic substructure in steels plays a major role in understanding the nature of martensite transformation and improving mechanical properties. However, the substructure in the quenched martensite of carbon steel, particularly in high carbon martensite, has not been clarified yet. Local crystal structure in martensite has been characterized by means of transmission electron microscope (TEM). A specimen tilting experiment in TEM is a practical method to simply confirm that the electron diffraction spots are caused by either double diffraction effect or a second crystalline phase. By tilting the TEM specimen containing Fe–C twinned martensite structure, the so-called double diffraction spots of {112}<111>-type twin are still visible even after the diffraction spots from the twinned crystals become invisible. This experimental result is the direct evidence that the so-called twinning double diffraction spots come from a second crystalline phase (ω-phase), which is distributed in the body-centered cubic (BCC){112}<111>-type twinning boundary region. An ideal BCC {112}<111>-type twin is just a model, which has not been reported yet in any experiment.

Lattice parameter fluctuations of relaxor ferroelectrics determined by X‐ray diffraction method

AbstractRelaxor ferroelectrics possess prominent dielectric and piezoelectric properties thus have been utilized in many advanced electromechanical devices. However, the atomic‐scale mechanism of their excellent electromechanical properties remains vague, which hinders the development of high‐performance ferroelectrics. In this work, we investigated the lattice parameter fluctuations for Pb(Mg1/3Nb2/3)O3–PbTiO3 relaxor ferroelectric ceramics, with a comparison of non‐relaxor ferroelectric ceramics, including BaTiO3, SrTiO3, and Pb(Zr,Ti)O3, at their respective paraelectric phase by X‐ray diffraction. We found that the fluctuations of lattice parameter were much larger in relaxor ferroelectrics than that in conventional ferroelectrics in the paraelectric phase, revealing a significant distinction between relaxor ferroelectrics and conventional ferroelectrics from the respect of atomic arrangement. Transmission electron microscopy experiments and X‐ray scattering‐intensity simulations indicated that the large fluctuations of lattice parameter in relaxor ferroelectrics can be attributed to the ordered–disordered arrangement of B‐site cations at the nanoscale. This work offers a new method to study the chemical arrangement difference between relaxor ferroelectrics and conventional ferroelectrics and may help us to explore the atomic‐scale origin of ultrahigh piezoelectric properties in relaxor ferroelectrics.

Using θ′ interfaces as templates for planar L12 precipitation in AlCuMnZr alloys

• Al 3 Zr L1 2 / θ' co-precipitates form faster in additively manufactured Al-Cu-Mn-Zr (ACMZ) alloys • Rapid solidification allows for higher matrix solute contents • Interfacial Zr segregation rate is dependent on the Zr matrix content • Theta prime precipitates are perfect templates for planar Al 3 Zr L1 2 formation Controlled Mn and Zr additions to Al-Cu alloys have allowed for the improved retention of mechanical properties after extended 350°C exposures by stabilizing the main strengthening θ' (Al 2 Cu) phase. Ultimately, θ' /L1 2 (Al 3 Zr) co-precipitate formation stabilizes θ' most effectively; however, Zr diffuses sluggishly and has low solubility in aluminum castings. Increasing the Zr segregation rate would allow for faster and more effective θ' /L1 2 co-precipitation. It is demonstrated that the Zr segregation rate is faster when the Zr matrix content is higher. A much higher Zr matrix content facilitated by rapid cooling during additive manufacturing (AM) was achieved that produces faster θ' /L1 2 co-precipitation, which is shown by scanning transmission electron microscopy and atom probe tomography experiments. It was also found that Zr continuously segregates to θ' interfaces up to the most aggressive heat treatment studied such that planar L1 2 precipitates remain after the metastable θ' dissolves. In this manner, we demonstrate that θ' coherent interfaces serve as perfect templates to form stable planar L1 2 precipitates that can provide strength at higher temperatures than traditional θ' strengthened AlCu alloys. This work introduces an alloy design strategy that uses metastable precipitates to quickly nucleate and grow co-precipitates with a desired geometry that contain slow diffusing elements. These ideas can be applied to engineer more heat resistant alloys by taking advantage of high solute matrix contents enabled by rapid cooling during additive manufacturing.

Bias‐Free Graphene Solid Cell to Realize the Real‐Time Observation on Dynamical Changes of Electrodes

AbstractTo realize the development of high‐performance electrodes materials, real‐time observation tool that can visualize dynamical changes of electrode materials upon lithiation and sodiation is highly critical. Recently, in situ transmission electron microscopy (TEM) has gained much attraction as it provides rich information on both phase and morphological transitions of electrode materials upon lithiation and sodiation in an atomic scale. Nevertheless, two significant limitations are present for current in situ TEM study: i) most of the in situ TEM experiments use a specially designed TEM holder, which is costly and suitable for use in only selective types of TEM; and ii) recently developed in situ TEM techniques with no applied bias have inherent limitations in the size of the sample (difficult for a size above 50 nm). Overcoming these two aforementioned issues, bias‐free graphene solid cells (GSCs) are now introduced, which can conduct real‐time observation of electrode materials in any size dimensions and morphologies without using bias. In this work, dynamical changes of hollow SnO2 nanotubes (NTs) are visualized upon lithiation and sodiation in real time using GSCs, where rich information is obtained on the morphology‐dependent, ion‐dependent characteristics of dynamical changes.