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.

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.

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.

Micromechanism for Copper-Deficiency Enhanced Stability: A Quasi In Situ TEM Study of Electric-Induced Structural Evolution in Cu2-x(S, Se) Liquid-Like Thermoelectric Materials.

Cu-based liquid-like thermoelectric materials have garnered tremendous attention due to their inherent ultralow lattice thermal conductivity. However, their practical application is hampered by stability issues under a large current or temperature gradient. It has been reported that introduction of copper vacancies can enhance the chemical stability, whereas the micromechanism behind this macroscopic improvement still remains unknown. Here, we have established a quasi in situ TEM method to examine and compare the structural evolution of Cu2-xS0.2Se0.8 (x = 0, 0.05) under external electric fields. It is then found that the preset Cu vacancies could favor the electric-induced formation of a more stable intermediate phase, i.e., the hexagonal CuSe-type structure in the form of either lamellar defects (majorly) or long-range order (minorly), in which ordering of S and Se also occurred. Thereby, copper and chalcogen atoms could largely be solidified into the matrix, and the elemental deposition and evaporation process is mitigated under an electric field.

Fracture Resistant CrSi2-Doped Silicon Nanoparticle Anodes for Fast-Charge Lithium-Ion Batteries.

Lithium-ion batteries (LIBs) has been developed over the last three decades. Increased amount of silicon (Si) is added into graphite anode to increase the energy density of LIBs. However, the amount of Si is limited, due to its structural instability and poor electronic conductivity so a novel approach is needed to overcome these issues. In this work, the synthesized chromium silicide (CrSi2) doped Si nanoparticle anode material achieves an initial capacity of 1729.3 mAh g-1 at 0.2C and retains 1085 mAh g-1 after 500 cycles. The new anode also shows fast charge capability due to the enhanced electronic conductivity provided by CrSi2 dopant, delivering a capacity of 815.9 mAh g-1 at 1C after 1000 cycles with a capacity degradation rate of <0.05% per cycle. An in situ transmission electron microscopy is used to study the structural stability of the CrSi2-doped Si, indicating that the high control of CrSi2 dopant prevents the fracture of Si during lithiation and results in long cycle life. Molecular dynamics simulation shows that CrSi2 doping optimizes the crack propagation path and dissipates the fracture energy. In this work a comprehensive information is provided to study the function of metal ion doping in electrode materials.

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.

Revealing the Dynamic Lithiation Process of Copper Disulfide by in Situ TEM.

Transition metal oxides, fluorides, and sulfides are extensively studied as candidate electrode materials for lithium-ion batteries driven by the urgency of developing next-generation higher energy density lithium batteries. These conversion-type electrode materials often require nanosized active materials to enable a "smooth" lithiation and de-lithiation process during charge/discharge cycles, determined by their size, structure, and phase. Herein, the structural and chemical changes of Copper Disulfide (CuS2 ) hollow nanoparticles during the lithiation process through an in situ transmission electron microscopy (TEM) method are investigated. The study finds the hollow structure of CuS2 facilitates the quick formation of fluidic Li2 S "drops," accompanied by a de-sulfurization to the Cu7 S4 phase. Meanwhile, the metallic Cu phase emerges as fine nanoparticles and grows into nano-strips, which are embedded in the Li2 S/Cu7 S4 matrix. These complex nanostructured phases and their spatial distribution can lead to a low de-lithiation barrier, enabling fast reaction kinetics.

An In Situ TEM Study of the Influence of Water Vapor on Reduction of Nickel Phyllosilicate - Retarded Growth of Metal Nanoparticles at Higher Rates.

Unavoidable water formation during the reduction of solid catalyst precursors has long been known to influence the nanoparticle size and dispersion in the active catalyst. This in situ transmission electron microscopy study provides insight into the influence of water vapor at the nanoscale on the nucleation and growth of the nanoparticles (2-16 nm) during the reduction of a nickel phyllosilicate catalyst precursor under H2/Ar gas at 700 °C. Water suppresses and delays nucleation, but counterintuitively increases the rate of particle growth. After full reduction is achieved, water vapor significantly enhances Ostwald ripening which in turn increases the likelihood of particle coalescence. This study proposes that water leads to formation of mobile nickel hydroxide species, leading to faster rates of particle growth during and after reduction.

Three-step nucleation of metal-organic framework nanocrystals.

Metal-organic frameworks (MOFs) are crystalline nanoporous materials with great potential for a wide range of industrial applications. Understanding the nucleation and early growth stages of these materials from a solution is critical for their design and synthesis. Despite their importance, the pathways through which MOFs nucleate are largely unknown. Using a combination of in situ liquid-phase and cryogenic transmission electron microscopy, we show that zeolitic imidazolate framework-8 MOF nanocrystals nucleate from precursor solution via three distinct steps: 1) liquid-liquid phase separation into solute-rich and solute-poor regions, followed by 2) direct condensation of the solute-rich region into an amorphous aggregate and 3) crystallization of the aggregate into a MOF. The three-step pathway for MOF nucleation shown here cannot be accounted for by conventional nucleation models and provides direct evidence for the nonclassical nucleation pathways in open-framework materials, suggesting that a solute-rich phase is a common precursor for crystallization from a solution.

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.

Novel Method for the Preparation of Lamellas From Porous and Brittle Materials for In Situ TEM Heating/Biasing.

A novel method for the preparation of lamellas made from porous and brittle compressed green powder using a focused ion beam (FIB) is described. One of the main purposes for the development of this methodology is to use this type of samples in micro-electromechanical systems (MEMS) chips for in situ transmission electron microscopy heating/biasing experiments, concomitant with maintaining the mechanical integrity and the absence of contamination of samples. This is accomplished through a modification of the standard FIB procedure for the preparation of lamellas, the adaptation of conventional chips, as well as the specific transfer of the lamella onto the chips. This method is versatile enough to be implemented in most commercially available FIB systems and MEMS chips.

Graphene Microheater Chips for In Situ TEM.

Low-dimensional materials are bringing significant innovations to in situ TEM characterization. Here a new graphene microheater chip for TEM was developed by stacking graphene on a suspended SiNx membrane as the Joule heating element. It could be heated up to 800 °C within 26.31 ms with a low power consumption of 0.025 mW/1000 µm2. The bulging was only ∼50 nm at 650 °C, which is 2 orders of magnitude smaller than those of conventional MEMS heaters at similar temperatures. The performances benefit from the employment of graphene, since its monolayer structure greatly reduces the heat capacity, and the vdW contact significantly reduces the interfacial interaction. The TEM observation on the Sn melting process verifies its great potential in resolving thermodynamic processes. Moreover, more multifunctional in situ chips could be developed by integrating other stimuli to such chips. This work opens a new frontier for both graphene and in situ characterization techniques.

In Situ Transmission Electron Microscopy Investigation on Oriented Attachment of Nanodiamonds.

Oriented attachment (OA) plays an important role in the assembly of nanoparticles and the regulation of their size and morphology, which is expected to be an effective means to modulate the properties of nanodiamonds (NDs). However, there remains a dearth of comprehensive investigation into the OA mechanism of NDs. Using in situ transmission electron microscopy, we conducted atomic-resolution investigation on the OA events of ND pairs under electron beam irradiation. The occurrence of an OA event is contingent upon the alignment between two ND surfaces, and the coalesced particles undergo recrystallization to form spherical shapes. Both experimental observations and molecular dynamics (MD) simulations reveal that ND pairs exhibit a preference for coalescing along the {111} surfaces. Additionally, MD simulations indicate that kinetic factors, such as contact surface area and contact angle, also influence the coalescence process.

Directional Migration and Rapid Coalescence of Au Nanoparticles on Anisotropic ReS2.

Interfacial atomic configuration and its evolution play critical roles in the structural stability and functionality of mixed zero-dimensional (0D) metal nanoparticles (NPs) and two-dimensional (2D) semiconductors. In situ observation of the interface evolution at atomic resolution is a vital method. Herein, the directional migration and structural evolution of Au NPs on anisotropic ReS2 were investigated in situ by aberration-corrected transmission electron microscopy. Statistically, the migration of Au NPs with diameters below 3 nm on ReS2 takes priority with greater probability along the b-axis direction. Density functional theory calculations suggest that the lower diffusion energy barrier enables the directional migration. The coalescence kinetics of Au NPs is quantitatively described by the relation of neck radius (r) and time (t), expressed as r2=Kt. Our work provides an atomic-resolved dynamic analysis method to study the interfacial structural evolution of metal/2D materials, which is essential to the study of the stability of nanodevices based on mixed-dimensional nanomaterials.

Revealing Temperature-Dependent Oxidation Dynamics of Ni Nanoparticles via Ambient Pressure Transmission Electron Microscopy.

Understanding the oxidation mechanism of metal nanoparticles under ambient pressure is extremely important to make the best use of them in a variety of applications. Through ambient pressure transmission electron microscopy, we in situ investigated the dynamic oxidation processes of Ni nanoparticles at different temperatures under atmospheric pressure, and a temperature-dependent oxidation behavior was revealed. At a relatively low temperature (e.g., 600 °C), the oxidation of Ni nanoparticles underwent a classic Kirkendall process, accompanied by the formation of oxide shells. In contrast, at a higher temperature (e.g., 800 °C), the oxidation began with a single crystal nucleus at the metal surface and then proceeded along the metal/oxide interface without voids formed during the whole process. Through our experiments and density functional theory calculations, a temperature-dependent oxidation mechanism based on Ni nanoparticles was proposed, which was derived from the discrepancy of gas adsorption and diffusion rates under different temperatures.

Cooperative Characterization of In Situ TEM and Cantilever-TGA to Optimize Calcination Conditions of MnO2 Nanowire Precursors.

Calcination plays a vital role during material preparation. However, the calcination conditions have often been determined empirically or have been based on trial and error. Herein we present a cooperative characterization approach to optimize calcination conditions by gas-cell in situ TEM in collaboration with microcantilever-based thermogravimetric analysis (cantilever-TGA) techniques. The morphological evolution of precursors under atmospheric conditions is observed with in situ TEM, and the right calcination temperature is provided by cantilever-TGA. The proposed approach successfully optimizes the calcination conditions of fragile MnO2 nanowire precursors with multiple valence products. The cantilever-TGA shows that a calcination temperature above 560 °C is required to transform the MnO2 precursor to Mn3O4 under an N2 atmosphere, but the in situ TEM indicates that the nanowire structure is destroyed within only 30 min under calcination conditions. Our method further suggests that heating the precursor at 400 °C using an H2-containing atmosphere can produce Mn3O4 nanowires with good electrical properties.

Direct Observation of Hollow Bimetallic Nanoparticle Formation through Galvanic Replacement and Etching Reactions.

Hollow bimetallic nanoparticles (NPs) formed from metal oxide NP templates are widely used catalysts for hydrogen evolution and CO2 reduction reactions. Despite their importance in catalysis, the details of how these NPs form on the NP templates remain unclear. Here, using in situ liquid-phase transmission electron microscopy (TEM) imaging, we describe the conversion of Cu2O template NPs to hollow PdCu NPs. Our observations show that a polycrystalline PdCu shell forms on the surface of the template via a galvanic replacement reaction while the template undergoes anisotropic etching. This study provides important insights into the synthesis of hollow metallic nanostructures from metal oxide templates.

Particle Attachment Growth of Au@Ag Core-Shell Nanocuboids.

In the templated synthesis of colloidal core-shell nanoparticles, the monomer attachment growth mechanism has been widely accepted to describe the growth process of shells. In this work, by using advanced transmission electron microscope techniques, we directly observe two alternative particle attachment growth pathways that dominate the growth of Au@Ag core-shell nanocuboids. One pathway involves the in situ reduction of AgCl nanoparticles attached to Au nanorods and the subsequent epitaxial growth of the Ag shell. The other pathway involves the adherence of Ag-AgCl Janus nanoparticles to Au nanorods with random orientations, followed by nanoparticle redispersion and the resulting formation of epitaxial Ag shells on the Au nanorods. The particle-mediated growth of Ag shells is accompanied by the redispersion of surface atoms, tending to form a uniform structure. The validation of the particle attachment growth processes at the atomic scale provides a new mechanistic understanding of core-shell nanostructure synthesis.

In Situ Visualization and Mechanistic Understandings on Facet-Dependent Atomic Redispersion of Platinum on CeO2.

Redispersion is an effective method for regeneration of sintered metal-supported catalysts. However, the ambiguous mechanistic understanding hinders the delicate controlling of active metals at the atomic level. Herein, the redispersion mechanism of atomically dispersed Pt on CeO2 is revealed and manipulated by in situ techniques combining well-designed model catalysts. Pt nanoparticles (NPs) sintered on CeO2 nano-octahedra under reduction and oxidation conditions, while redispersed on CeO2 nanocubes above ∼500 °C in an oxidizing atmosphere. The dynamic shrinkage and disappearance of Pt NPs on CeO2 (100) facets was directly visualized by in situ TEM. The generated atomically dispersed Pt with the square-planar [PtO4]2+ structure on CeO2 (100) facets was also confirmed by combining Cs-corrected STEM and spectroscopy techniques. The redispersion and atomic control were ascribed to the high mobility of PtO2 at high temperatures and its strong binding with square-planar O4 sites over CeO2 (100). These understandings are important for the regulation of atomically dispersed platinum catalysts.

Direct Observation of Grain-Boundary-Migration-Assisted Radiation Damage Healing in Ultrafine Grained Gold under Mechanical Stress.

Nanostructured metals are a promising class of radiation-tolerant materials. A large volume fraction of grain boundaries (GBs) can provide plenty of sinks for radiation damage, and understanding the underlying healing mechanisms is key to developing more effective radiation tolerant materials. Here, we observe radiation damage absorption by stress-assisted GB migration in ultrafine-grained Au thin films using a quantitative in situ transmission electron microscopy nanomechanical testing technique. We show that the GB migration rate is significantly higher in the unirradiated specimens. This behavior is attributed to the presence of smaller grains in the unirradiated specimens that are nearly absent in the irradiated specimens. Our experimental results also suggest that the GB mobility is decreased as a result of irradiation. This work implies that the deleterious effects of irradiation can be reduced by an evolving network of migrating GBs under stress.