Defect-Mediated Formation of Oriented Phase Domains in a Lithium-Ion Insertion Electrode.

The performance and robustness of electrodes are closely related to transformation-induced nanoscale structural heterogeneity during (de)lithiation. As a result, it is critical to understand at atomic scale the origin of such structural heterogeneity and ultimately control the transformation microstructure, which remains a formidable task. Here, by performing in situ studies on a model intercalation electrode material, anatase TiO2, we reveal that defects─both preexisting and as-formed during lithiation─can mediate the local anisotropic volume expansion direction, resulting in the formation of multiple differently oriented phase domains and eventually a network structure within the lithiated matrix. Our results indicate that such a mechanism operated by defects, if properly harnessed, could not only improve lithium transport kinetics but also facilitate strain accommodation and mitigate chemomechanical degradation. These findings provide insights into the connection of defects to the robustness and rate performance of electrodes, which help guide the development of advanced lithium-ion batteries via defect engineering.

Estimation of local variation in Young's modulus over a gold nanocontact using microscopic nanomechanical measurement method.

Nanoscale materials tend to have a single crystal domain, leading to not only size dependence but also orientation dependence of their mechanical properties. Recently, we developed a microscopic nanomechanical measurement method (MNMM), which enabled us to obtain equivalent spring constants (force gradients) of nanocontacts while observing their atomic structures by transmission electron microscopy (TEM). Therein, we evaluated Young's modulus based on a model that a newly introduced layer at the thinnest section of a nanocontact determined the change in the measured equivalent spring constant, and discussed their size dependence. However, this model is not general for other nanomaterials that do not exhibit the introduction of a new atomic layer while stretching. In this study, using MNMM, we propose a new analytical method to directly retrieve the local Young's modulus of nanomaterials by measuring initial lattice spacing and its displacement of a local region in the TEM image during the stretching of the nanocontact. This reveals the size dependence of local Young's modulus at various positions of the nanocontact at once. As a result, our estimated Young's modulus for a gold [111] nanocontact showed a size dependence similar to the one previously reported. This indicates that this analytical method benefits in revealing the mechanical properties of not only nanomaterials but also structurally heterogeneous materials such as high-entropy alloys. .

In Situ TEM Imaging Reveals the Dynamic Interplay Between Attraction, Repulsion and Sequential Attraction-Repulsion in Gold Nanoparticles.

Recent efforts on manipulating metal nanoparticles (NPs) using an electron beam have offered new insights into nanoparticle behavior, structural transition, and the emergence of new properties. Despite an increasing understanding of the dynamics of electron beam-induced coalescence of NPs, several phenomena are yet to be investigated. Here, we show that repulsion between two NPs is as favorable as coalescence under electron beam irradiation at room temperature. Using small-sized (D ≈ 5.9 nm) and large-sized (D ≈ 11.0 nm) gold (Au) NPs, and different electron dose rates, a unique sequential attraction-repulsion between NPs is disclosed. The real-time in situ transmission electron microscopy imaging suggest that at a low dose rate, two small-sized AuNPs with 1.0 nm particle-particle distance undergo repulsion to 18 nm with a diffusion rate of 0.4 nm min-1. For large-sized AuNPs, the repulsion rate is 0.08 nm min-1 at a low dose rate and is comparable to that of small-sized AuNPs at a high dose rate. Surprisingly, large-sized AuNPs at a high electron dose rate displayed attraction in the first 15 min, followed by rapid repulsion. This unique sequential attraction-repulsion behavior of NPs offers possibilities to manipulate interparticle distance and properties without inducing dimensional changes for advanced photonic and plasmonic nanodevices.

Revealing Asymmetric Phase Transformation of the BiVO4 Anodes for Potassium-Ion Batteries by In Situ Transmission Electron Microscopy.

Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), thanks to the cost-effectiveness of potassium resources and a favorable redox potential of approximately -2.936 V. The monoclinic BiVO4, known for its layered structure, shows noteworthy electrochemical properties when utilized as an anode material for both LIBs and sodium-ion batteries. However, the fundamental electrochemical reaction mechanisms of the BiVO4 anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO4 anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO4 during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO4 nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO4 material is transformed into an amorphous KxBiVO4 phase after the first cycle. More interestingly, the anode materials near and far from the potassium source exhibit opposite volume-changing trends under the same voltage potential. Also, this phenomenon should be attributed to the mass flow of the unstable K-Bi alloy under the electric field. Our findings provide significant insights into the electrochemical mechanism of BiVO4 nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.

Strain Field Around Individual Dislocations Controls Failure.

Understanding material failure on a fundamental level is a key aspect in the design of robust structural materials, especially for metals and alloys capable to undergo plastic deformation. In the last decade, significant progress is made in quantifying the stresses associated with failure in both experiments and simulations. Nonetheless, the processes occurring on the most essential level of individual dislocations that govern semi-brittle and ductile fracture are still experimentally not accessible, limiting the failure prediction capabilities. Therefore, in the present work, a one-of-a-kind nanoscale fracture experiment is conducted on a single crystalline Cr bending beam in situ in the transmission electron microscope and for the first time quantify the transient strains around individual dislocations, as well as of the whole dislocation network during crack opening. The results reveal the importance of both pre-existing and newly emitted dislocations for crack-tip shielding via their intrinsic strain field and provide guidelines to design more damage tolerant materials.

Unraveling the Dynamic Properties of New-Age Energy Materials Chemistry Using Advanced In Situ Transmission Electron Microscopy.

The field of energy storage and conversion materials has witnessed transformative advancements owing to the integration of advanced in situ characterization techniques. Among them, numerous real-time characterization techniques, especially in situ transmission electron microscopy (TEM)/scanning TEM (STEM) have tremendously increased the atomic-level understanding of the minute transition states in energy materials during electrochemical processes. Advanced forms of in situ/operando TEM and STEM microscopic techniques also provide incredible insights into material phenomena at the finest scale and aid to monitor phase transformations and degradation mechanisms in lithium-ion batteries. Notably, the solid-electrolyte interface (SEI) is one the most significant factors that associated with the performance of rechargeable batteries. The SEI critically controls the electrochemical reactions occur at the electrode-electrolyte interface. Intricate chemical reactions in energy materials interfaces can be effectively monitored using temperature-sensitive in situ STEM techniques, deciphering the reaction mechanisms prevailing in the degradation pathways of energy materials with nano- to micrometer-scale spatial resolution. Further, the advent of cryogenic (Cryo)-TEM has enhanced these studies by preserving the native state of sensitive materials. Cryo-TEM also allows the observation of metastable phases and reaction intermediates that are otherwise challenging to capture. Along with these sophisticated techniques, Focused ion beam (FIB) induction has also been instrumental in preparing site-specific cross-sectional samples, facilitating the high-resolution analysis of interfaces and layers within energy devices. The holistic integration of these advanced characterization techniques provides a comprehensive understanding of the dynamic changes in energy materials. This review highlights the recent progress in employing state-of-the-art characterization techniques such as in situ TEM, STEM, Cryo-TEM, and FIB for detailed investigation into the structural and chemical dynamics of energy storage and conversion materials.

Growth Mechanism of Carbon Nanotubes Revealed by in situ Transmission Electron Microscopy.

Elucidating the growth mechanism of carbon nanotubes (CNTs) is critical to obtaining CNTs with desired structures and tailored properties for their practical applications. With atomic resolution imaging, in situ transmission electron microscopy (TEM) has been a key technique to reveal the microstructure and dynamics of CNTs in real time. In this review, recent advances in the development of in situ TEM with different types of environmental reactors will be introduced. The catalytic growth mechanisms of CNTs revealed by in situ TEM under realistic conditions are discussed from fundamental thermodynamics and kinetics to the detailed nucleation, growth, and termination mechanisms, including the state and phase of active catalysts, interfacial connections between catalyst and growing CNTs, and catalyst-related growth kinetics of CNTs. Great progresses have been made on how a CNT nucleates, grows and terminates, focusing on the interface dynamics and kinetic fluctuations. Finally, challenges and future directions for understanding the atomic dynamics under the real growth conditions are proposed. It is expected that breakthroughs in the fundamental growth mechanisms will pave the way to the ultimate goal of designing and controlling the atomic structures of CNTs for their applications in various devices.

3D Shape Reconstruction of Ge Nanowires during Vapor-Liquid-Solid Growth under Modulating Electric Field.

Bottom-up growth offers precise control over the structure and geometry of semiconductor nanowires (NWs), enabling a wide range of possible shapes and seamless heterostructures for applications in nanophotonics and electronics. The most common vapor-liquid-solid (VLS) growth method features a complex interaction between the liquid metal catalyst droplet and the anisotropic structure of the crystalline NW, and the growth is mainly orchestrated by the triple-phase line (TPL). Despite the intrinsic mismatch between the droplet and the NW symmetries, its discussion has been largely avoided because of its complexity, which has led to the situation when multiple observed phenomena such as NW axial asymmetry or the oscillating truncation at the TPL still lack detailed explanation. The introduction of an electric field control of the droplet has opened even more questions, which cannot be answered without properly addressing three-dimensional (3D) structure and morphology of the NW and the droplet. This work describes the details of electric-field-controlled VLS growth of germanium (Ge) NWs using environmental transmission electron microscopy (ETEM). We perform TEM tomography of the droplet-NW system during an unperturbed growth, then track its evolution while modulating the bias potential. Using 3D finite element method (FEM) modeling and crystallographic considerations, we provide a detailed and consistent mechanism for VLS growth, which naturally explains the observed asymmetries and features of a growing NW based on its crystal structure. Our findings provide a solid framework for the fabrication of complex 3D semiconductor nanostructures with ultimate control over their morphology.

Oxidation-Induced Oxide Shell Rupture and Phase Separation in Eutectic Gallium-Indium Nanoparticles.

Eutectic gallium-indium (EGaIn), a room-temperature liquid metal, has garnered significant attention for its applications in soft electronics, multifunctional materials, energy engineering and drug delivery. A key factor influencing these diverse applications is the spontaneous formation of a native passivating oxide shell that not only encapsulates the liquid metal but also alters the properties from the bulk counterpart. Using environmental scanning transmission electron microscopy, we report in situ observations of the oxidation of EGaIn nanoparticles by ambient air under high-energy electron beam irradiation. Our findings demonstrate that uneven oxide shell growth, driven by inward diffusion of adsorbed O species, creates unbalanced stresses. This compels the liquid metal to deform toward regions with slower oxide growth, resulting in shell rupture and allowing the liquid metal core to flow out. This process initiates top-down self-similar replication of the core-shell liquid metal nanoparticles, causing larger particles to break down into smaller particles. Additionally, internal oxidation triggers phase separation within the liquid core, ultimately leading to the pulverization of the liquid metal into finer solid particles rich in indium. These mechanistic insights into the oxidation behavior of the liquid metal hold practical implications for leveraging this process to reconfigure EGaIn nanoparticles for various applications.

Surficial Reconstruction of Pt-Ni/NiS and Its Effect on Electrocatalytic Hydrogen Evolution in Alkaline Medium.

Cyclic voltammetry pretreatment of Pt-based electrocatalysts has been proven to be a normal activation process on achieving the optimal alkaline hydrogen evolution performance. Until now, the congruent relationship between the microstructural evolution and performance improvement during this process has rarely been reported. Herein, when the in situ transmission electron microscopy and in situ Raman analyses are employed, a self-reconstruction process from crystalline NiS into amorphous nickel hydroxide hydrate [Ni(OH)2-x·H2O, where x ≈ 0.3] on the surface of platinum-nickel nanowires has first been captured, which is the critical water dissociation active site to offer a sufficient proton supply. Furthermore, such a surficial reconstruction triggers an increase in the current density from -2.3 to -38.8 mA/cm2 (at -70 mV), which is nearly 17 times. These observations point to the fact that it is essential to consider the fundamental mechanisms of hydrogen evolution on the active sites when the process is scaled up.

Using In situ Transmission Electron Microscopy to Study Strong Metal-Support Interactions in Heterogeneous Catalysis.

Precisely controlling the microstructure of supported metal catalysts and regulating metal-support interactions at the atomic level are essential for achieving highly efficient heterogeneous catalysts. Strong metal-support interaction (SMSI) not only stabilizes metal nanoparticles and improves their resistance to sintering but also modulates the electrical interaction between metal species and the support, optimizing the catalytic activity and selectivity. Therefore, understating the formation mechanism of SMSI and its dynamic evolution during the chemical reaction at the atomic scale is crucial for guiding the structural design and performance optimization of supported metal catalysts. Recent advancements in in situ transmission electron microscopy (TEM) have shed new light on these complex phenomena, providing deeper insights into the SMSI dynamics. Here, the research progress of in situ TEM investigation on SMSI in heterogeneous catalysis is systematically reviewed, focusing on the formation dynamics, structural evolution during the catalytic reactions, and regulation methods of SMSI. The significant advantages of in situ TEM technologies for SMSI research are also highlighted. Moreover, the challenges and probable development paths of in situ TEM studies on the SMSI are also provided.

The Investigation of Neuromimetic Dynamics in Ferroelectrics via In Situ TEM.

The core task of neuromorphic devices is to effectively simulate the behavior of neurons and synapses. Based on the functionality of ferroelectric domains with the advantages of low power consumption and high-speed response, great progress has been made in realizing neuromimetic behaviors such as ferroelectric synaptic devices. However, the correlation between the ferroelectric domain dynamics and neuromimetic behavior remains unclear. Here, we reveal the correlation between domain/domain wall dynamics and neuromimetic behaviors from a microscopic perspective in real-time by using high temporal and spatial resolution in situ transmission electron microscopy. Furthermore, we propose utilizing ferroelectric microstructures for the simultaneous simulation of neuronal and synaptic plasticity, which is expected to improve the integration and performance of ferroelectric neuromorphic devices. We believe that this work to study neuromimetic behavior from the perspective of domain dynamics is instructive for the development of ferroelectric neuromorphic devices.

Freeze Metal Halide Perovskite for Dramatic Laser Tuning: Direct Observation via In Situ Cryo-Electron Microscope.

A frozen-temperature (below -28 °C) laser tuning way is developed to optimize metal halide perovskite (MHP)'s stability and opto-electronic properties, for emitter, photovoltaic and detector applications. Here freezing can adjust the competitive laser irradiation effects between damaging and annealing/repairing. And the ligand shells on MHP surface, which are widely present for many MHP materials, can be frozen and act as transparent solid templates for MHP's re-crystallization/re-growth during the laser tuning. With model samples of different types of CsPbBr3 nanocube arrays,an attempt is made to turn the dominant exposure facet from low-energy [100] facet to high-energy [111], [-211], [113] and [210] ones respectively; selectively removing the surface impurities and defects of CsPbBr3 nanocubes to enhance the irradiation durability by 101 times; and quickly (tens of seconds) modifying a Ruddlesden-Popper (RP) boundary into another type of boundary like twinning, and so on. The laser tuning mechanism is revealed by an innovative in situ cryo-transmission electron microscope (cryo-TEM) exploration at atomic resolution.

Measuring Electrical Resistivity at the Nanoscale in Phase-Change Materials.

Electrical resistivity is the key parameter in the active regions of many current nanoscale devices, from memristors to resistive random-access memory and phase-change memories. The local resistivity of the materials is engineered on the nanoscale to fit the performance requirements. Phase-change memories, for example, rely on materials whose electrical resistance increases dramatically with a change from a crystalline to an amorphous phase. Electrical characterization methods have been developed to measure the response of individual devices, but they cannot map the local resistance across the active area. Here, we propose a method based on operando electron holography to determine the local resistance within working devices. Upon switching the device, we show that electrical resistance is inhomogeneous on the scale of only a few nanometers.

Visualizing the Structure-Property Nexus of Wide-Bandgap Perovskite Solar Cells under Thermal Stress.

Wide-bandgap perovskite solar cells (PSCs) toward tandem photovoltaic applications are confronted with the challenge of device thermal stability, which motivates to figure out a thorough cognition of wide-bandgap PSCs under thermal stress, using in situ atomic-resolved transmission electron microscopy (TEM) tools combing with photovoltaic performance characterizations of these devices. The in situ dynamic process of morphology-dependent defects formation at initial thermal stage and their proliferations in perovskites as the temperature increased are captured. Meanwhile, considerable iodine enables to diffuse into the hole-transport-layer along the damaged perovskite surface, which significantly degrade device performance and stability. With more intense thermal treatment, atomistic phase transition reveals the perovskite transform to PbI2 along the topo-coherent interface of PbI2/perovskite. In conjunction with density functional theory calculations, a mutual inducement mechanism of perovskite surface damage and iodide diffusion is proposed to account for the structure-property nexus of wide-bandgap PSCs under thermal stress. The entire interpretation also guided to develop a thermal-stable monolithic perovskite/silicon tandem solar cell.

Microheater Chips with Carbon Nanotube Resistors.

The specific and excellent properties of the low-dimensional nanomaterials have made them promising building blocks to be integrated into microelectromechanical systems with high performances. Here, we present a new microheater chip for in situ TEM, in which a cross-stacked superaligned carbon nanotube (CNT) film resistor is located on a suspended SiNx membrane via van der Waals (vdW) interactions. The CNT microheater has a fast high-temperature response and low power consumption, thanks to the micro/nanostructure of the CNT materials. Moreover, the membrane bulging amplitude is significantly reduced to only ∼100 nm at 800 °C for the vdW interaction between the CNTs and the SiNx membrane. An in situ observation of the Sn melting process is successfully conducted with the assistance of a customized flexible temperature control system. The uniform wafer-scaled CNT films enable a high level of consistency and cost-effective mass production of such chips. The as-developed in situ chips, as well as the related techniques, hold great promise in nanoscience, materials science, and electrochemistry.

Structural Reversibility of Nanoscaled Sn Anodes.

Alloying-type anode materials provide high capacity for lithium-ion batteries; however, they suffer pulverization problems resulting from the volume change during cycling. Realizing the cycling reversibility of these anodes is therefore critical for sustaining their electrochemical performance. Here, we investigate the structural reversibility of Sn NPs during cycling at atomic-level resolution utilizing in situ high-resolution TEM. We observed a surprisingly near-perfect structural reversibility after a complete cycle. A three-step phase transition happens during lithiation, accompanied by the generation of a significant number of defects, grain boundaries, and up to 202% volume expansion. In subsequent delithiation, the volume, morphology, and crystallinity of the Sn NPs were restored to their initial state. Theoretical calculations show that compressive stress drives the removal of vacancies generated within the NPs during delithiation, therefore maintaining their intact morphology. This work demonstrates that removing vacancies during cycling can efficiently improve the structural reversibility of high-capacity anode materials.

Thermal Stability and Crystallization Processes of Pd78Au4Si18 Thin Films Visualized via In Situ TEM.

Amorphous alloys or metallic glasses (MGs) thin films have attracted extensive attention in various fields due to their unique functional properties. Here, we use in situ heating transmission electron microscopy (TEM) to investigate the thermal stability and crystallization behavior of Pd-Au-Si thin films prepared by a pulsed laser deposition (PLD) method. Upon heating treatment inside a TEM, we trace the structural changes in the Pd-Au-Si thin films through directly recording high-resolution images and diffraction patterns at different temperatures. TEM observations reveal that the Pd-Au-Si thin films started to nucleate with small crystalline embryos uniformly distributed in the glassy matrix upon approaching the glass transition temperature Tg=625K, and subsequently, the growth of crystalline nuclei into sub-10 nm Pd-Si nanocrystals commenced. Upon further increasing the temperature to 673K, the thin films transformed to micro-sized patches of stacking-faulty lamellae that further crystallized into Pd9Si2 and Pd3Si intermetallic compounds. Interestingly, with prolonged thermal heating at elevated temperatures, the Pd9Si2 transformed to Pd3Si. Simultaneously, the solute Au atoms initially dissolved in glassy alloys and eventually precipitated out of the Pd9Si2 and Pd3Si intermetallics, forming nearly spherical Au nanocrystals. Our TEM results reveal the unique thermal stability and crystallization processes of the PLD-prepared Pd-Au-Si thin films as well as demonstrate a possibility of producing a large quantity of pure nanocrystals out of amorphous solids for various applications.

Ultrauniform Plating of Lithium on 10-nm-Scale Ordered Carbon Grids for Long Lifespan Lithium Metal Batteries.

Tailorable lithium (Li) nucleation and uniform early-stage plating is essential for long-lifespan Li metal batteries. Among factors influencing the early plating of Li anode, the substrate is critical, but a fine control of the substrate structure on a scale of ≈10 nm has been rarely achieved. Herein, a carbon consisting of ordered grids is prepared, as a model to investigate the effect of substrate structure on the Li nucleation. In contrast to the individual spherical Li nuclei formed on the flat graphene, an ultrauniform and nuclei-free Li plating is obtained on the ordered carbon with a grid size smaller than the thermodynamical critical radius of Li nucleation (≈26 nm). Simultaneously, an inorganic-rich solid-electrolyte-interphase is promoted by the cross-sectional carbon layers of such ordered grids which are exposed to the electrolyte. Consequently, the carbon grids with a grid size of ≈10 nm show a favorable cycling stability for more than 1100 cycles measured at 2 mA cm-2 in a half cell. With LiNi0.8Co0.1Mn0.1O2 as cathode, the assembled full cell with a cathode capacity of 3 mAh cm-2 and a negative/positive ratio of 1.67 demonstrates a stable cycling for over 130 cycles with a capacity retention of 88%.

Multi-Stimuli Operando Transmission Electron Microscopy for Two-Terminal Oxide-Based Devices.

The integration of microelectromechanical systems (MEMS)-based chips for in situ transmission electron microscopy (TEM) has emerged as a highly promising technique in the study of nanoelectronic devices within their operational parameters. This innovative approach facilitates the comprehensive exploration of electrical properties resulting from the simultaneous exposure of these devices to a diverse range of stimuli. However, the control of each individual stimulus within the confined environment of an electron microscope is challenging. In this study, we present novel findings on the effect of a multi-stimuli application on the electrical performance of TEM lamella devices. To approximate the leakage current measurements of macroscale electronic devices in TEM lamellae, we have developed a postfocused ion beam (FIB) healing technique. This technique combines dedicated MEMS-based chips and in situ TEM gas cells, enabling biasing experiments under environmental conditions. Notably, our observations reveal a reoxidation process that leads to a decrease in leakage current for SrTiO3-based memristors and BaSrTiO3-based tunable capacitor devices following ion and electron bombardment in oxygen-rich environments. These findings represent a significant step toward the realization of multi-stimuli TEM experiments on metal-insulator-metal devices, offering the potential for further exploration and a deeper understanding of their intricate behavior.