Diffraction contrast of ferroelectric domains in DPC STEM images

Differential phase contrast scanning transmission electron microscopy (DPC STEM) is a powerful technique for directly visualizing electromagnetic fields inside materials at high spatial resolution. Electric field observation within ferroelectric materials is potentially possible by DPC STEM, but concomitant diffraction contrast hinders the quantitative electric field evaluation. Diffraction contrast is basically caused by the diffraction-condition variation inside a field-of-view, but in the case of ferroelectric materials, the diffraction conditions can also change with respect to the polarization orientations. To quantitatively observe electric field distribution inside ferroelectric domains, the formation mechanism of diffraction contrast should be clarified in detail. In this study, we systematically simulated diffraction contrast of ferroelectric domains in DPC STEM images based on the dynamical diffraction theory, and clarify the issues for quantitatively observing electric fields inside ferroelectric domains. Furthermore, we conducted experimental DPC STEM observations for a ferroelectric material to confirm the influence of diffraction contrast predicted by the simulations.

Applications of electron microscopic observations to electrochemistry in liquid electrolytes for batteries

Herein, we review notable points from observations of electrochemical reactions in a liquid electrolyte by liquid-phase electron microscopy. In situ microscopic observations of electrochemical reactions are urgently required, particularly to solve various battery issues. Battery performance is evaluated by various electrochemical measurements of bulk samples. However, it is necessary to understand the physical/chemical phenomena occurring in batteries to elucidate the reaction mechanisms. Thus, in situ microscopic observation is effective for understanding the reactions that occur in batteries. Herein, we focus on two methods, of the liquid phase (scanning) transmission electron microscopy and liquid phase scanning electron microscopy, and summarize the advantages and disadvantages of both methods.

Nanoscale Localized Phonons at Al2O3 Grain Boundaries

Nanoscale defects like grain boundaries (GBs) would introduce local phonon modes and affect the bulk materials' thermal, electrical, optical, and mechanical properties. It is highly desirable to correlate the phonon modes and atomic arrangements for individual defects to precisely understand the structure-property relation. Here we investigated the localized phonon modes of Al2O3 GBs by combination of the vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscope and density functional perturbation theory (DFPT). The differences between GB and bulk obtained from the vibrational EELS show that the GB exhibited more active vibration at the energy range of <50 meV and >80 meV, and further DFPT results proved the wide distribution of bond lengths at GB are the main factor for the emergence of local phonon modes. This research provides insights into the phonon-defect relation and would be of importance in the design and application of polycrystalline materials.

Atomistic Investigation of Grain Boundary Fracture in Alumina

Grain boundary (GB) fracture is a major mechanism of material failure in polycrystalline ceramics. However, the intricate atomic arrangements of GBs have impeded our understanding of the atomistic mechanisms of these processes. In this study, we investigated the atomic-scale crack propagation behavior of an α-Al2O3 ∑13 grain boundary, using a combination of in situ transmission electron microscopy (TEM) and scanning TEM. The atomic-scale fracture path along the GB core was directly determined by the observation of the atomic structures of the fractured surfaces, which is consistent with density functional theory calculations. We found that the GB fracture can be attributed to the weaker local bonds and a smaller number of bonds along the fracture path. Our findings provide atomistic insights into the mechanisms of crack propagation along GBs, offering significant implications for GB engineering and the toughening of ceramics.

Real-time tracking of three-dimensional atomic dynamics of Pt trimer on TiO2 (110)

Single and multi-atoms supported on oxide substrates ultimately increase the efficiency of noble metal atom use, and moreover, catalytic activity and selectivity are also improved substantially. However, single and multi-atoms are unstable under catalytic conditions, and these metal atoms spontaneously aggregate and grow into nanoparticles. Catalytic performance is strongly related to local atomic configurations, and hence, it is essential to determine the three-dimensional (3D) atomic structures of multi-atoms on the substrate and their structural dynamics. Here, we show the real-time tracking of the 3D structural evolution of a Pt trimer on TiO2 (110) substrate at a high temperature, using high-spatiotemporal-resolution scanning transmission electron microscopy, where sub-angstrom spatial resolution is maintained, while the temporal resolution reaches 40 milliseconds. With the aid of prior structural knowledge of a Pt trimer for 3D reconstruction, the present method could open the way to characterize in situ atomic-scale structural dynamics, especially meta-stable structural transition.

Ultra-stable and highly reactive colloidal gold nanoparticle catalysts protected using multi-dentate metal oxide nanoclusters

Owing to their remarkable properties, gold nanoparticles are applied in diverse fields, including catalysis, electronics, energy conversion and sensors. However, for catalytic applications of colloidal gold nanoparticles, the trade-off between their reactivity and stability is a significant concern. Here we report a universal approach for preparing stable and reactive colloidal small (~3 nm) gold nanoparticles by using multi-dentate polyoxometalates as protecting agents in non-polar solvents. These nanoparticles exhibit exceptional stability even under conditions of high concentration, long-term storage, heating and addition of bases. Moreover, they display excellent catalytic performance in various oxidation reactions of organic substrates using molecular oxygen as the sole oxidant. Our findings highlight the ability of inorganic multi-dentate ligands with structural stability and robust steric and electronic effects to confer stability and reactivity upon gold nanoparticles. This approach can be extended to prepare metal nanoparticles other than gold, enabling the design of novel nanomaterials with promising applications.

Incommensurate grain-boundary atomic structure

Grain-boundary atomic structures of crystalline materials have long been believed to be commensurate with the crystal periodicity of the adjacent crystals. In the present study, we experimentally observed a Σ9 grain-boundary atomic structure of a bcc crystal (Fe-3%Si). It is found that the Σ9 grain-boundary structure is largely reconstructed and forms a dense packing of icosahedral clusters in its core. Combining with the detailed theoretical calculations, the Σ9 grain-boundary atomic structure is discovered to be incommensurate with the adjacent crystal structures. The present findings shed new light on the study of stable grain-boundary atomic structures in crystalline materials.

Direct imaging of local atomic structures in zeolite using optimum bright-field scanning transmission electron microscopy

Zeolites are used in industries as catalysts, ion exchangers, and molecular sieves because of their unique porous atomic structures. However, direct observation of zeolitic local atomic structures via electron microscopy is difficult owing to low electron irradiation resistance. Subsequently, their fundamental structure-property relationships remain unclear. A low-electron-dose imaging technique, optimum bright-field scanning transmission electron microscopy (OBF STEM), has recently been developed. It reconstructs images with a high signal-to-noise ratio and a dose efficiency approximately two orders of magnitude higher than that of conventional methods. Here, we performed low-dose atomic-resolution OBF STEM observations of two types of zeolite, effectively visualizing all atomic sites in their frameworks. In addition, we visualized the complex local atomic structure of the twin boundaries in a faujasite (FAU)-type zeolite and Na+ ions with low occupancy in eight-membered rings in a Na-Linde Type A (LTA) zeolite. The results of this study facilitate the characterization of local atomic structures in many electron beam-sensitive materials.

Ultrahigh toughness zirconia ceramics

The attainment of both high strength and toughness is the ultimate goal for most structural materials. Although ceramic material has been considered for use as a structural material due to its high strength and good chemical stability, it suffers from the limitation of low toughness. For instance, although Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZPs) exhibit remarkable toughness among ceramics due to their phase transformation toughening mechanism, this toughness is still much weaker than that of metals. Here, we report Y-TZP-based ceramic materials with toughnesses exceeding 20 MPa m^1/2, which is comparable to those of metals, while maintaining strengths over 1,200 MPa. The superior mechanical properties are realized by reducing the phase stability of tetragonal zirconia by tailoring the microstructure and chemistry of the Y-TZP. The proposed ceramic materials can further advance the design and application of ceramic-based structural materials.

Real-space observation of a two-dimensional electron gas at semiconductor heterointerfaces

Mobile charge carriers are essential components in high-performance, nano-engineered semiconductor devices. Employing charge carriers confined to heterointerfaces, the so-called two-dimensional electron gas, is essential for improving device performance. The real-space visualization of a two-dimensional electron gas at the nanometre scale is desirable. However, it is challenging to accomplish by means of electron microscopy due to an unavoidable strong diffraction contrast formation at the heterointerfaces. We performed direct, nanoscale electric field imaging across a GaN-based semiconductor heterointerface using differential phase contrast scanning transmission electron microscopy by suppressing diffraction contrasts. For both nearly the lattice-matched GaN/Al_0.81In_0.19N interface and pseudomorphic GaN/Al_0.88In_0.12N interface, the extracted quantitative electric field profiles show excellent agreement with profiles predicted using Poisson simulation. Furthermore, we used the electric field profiles to quantify the density and distribution of the two-dimensional electron gas across the heterointerfaces with nanometre precision. This study is expected to guide the real-space characterization of local charge carrier density and distribution in semiconductor devices.

Grain boundary structural transformation induced by co-segregation of aliovalent dopants

Impurity doping is a conventional but one of the most effective ways to control the functional properties of materials. In insulating materials, the dopant solubility limit is considerably low in general, and the dopants often segregate to grain boundaries (GBs) in polycrystals, which significantly alter their entire properties. However, detailed mechanisms on how dopant atoms form structures at GBs and change their properties remain a matter of conjecture. Here, we show GB structural transformation in α-Al₂O₃ induced by co-segregation of Ca and Si aliovalent dopants using atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations. To accommodate large-sized Ca ions at the GB core, the pristine GB atomic structure is transformed into a new GB structure with larger free volumes. Moreover, the Si and Ca dopants form a chemically ordered structure, and the charge compensation is achieved within the narrow GB core region rather than forming broader space charge layers. Our findings give an insight into GB engineering by utilizing aliovalent co-segregation.

The effect of aliovalent doping on grain boundary is not yet fully understood at the atomic level. Here, the authors report grain boundary structural transformation in α-Al₂O₃ is induced by co-segregation of multiple dopants using atomic-resolution electron microscopy and theoretical calculations.

Spatial and phase resolution in electron microscopy

With the invention of the aberration corrector in electron optics, the spatial resolution in electron microscopy has progressively improved and has now reached the sub-50 picometre regime, and atomic-resolution electron microscopy has become a versatile tool for investigating the atomic structures in materials and devices. Furthermore, the phase resolution in electron microscopy also exhibits outstanding progress, and it has become possible to visualise electromagnetic fields at atomic dimensions, which strongly contributes to understanding the physical and chemical properties of materials. The electron microscopy society has grown with the improvements in spatial and phase resolutions, and hence we must continuously develop new hardware, software and methodologies to boost these resolutions. Here, we review the historical progress of spatial and phase resolutions in electron microscopy, where we clarify the definition of these resolutions. We also discuss the future targets in electron microscopy.

Lithium Lanthanum Titanate Single Crystals: Dependence of Lithium-Ion Conductivity on Crystal Domain Orientation

Lithium lanthanum titanate La_2/3-xLi_3xTiO₃ (LLTO) has the potential to exhibit the highest Li-ion conductivity among oxide-based electrolytes because of the fast Li-ion diffusion derived from its crystal structure. Herein, bulk Li-ion conductivity of up to σ_bulk = 4.0 × 10^-3 S/cm at 300 K, which is approximately three to four times higher than that of LLTO polycrystals, was demonstrated using LLTO single crystals, and their dependence on crystal domain orientation was examined. A change in the activation energy, which was previously obscured because of random crystal orientation, was observed at approximately 260 K. Furthermore, electron microscopy analysis indicated that the ionic conductivity of LLTOs remained higher because the region with the highest ionic conductivity was tilted away from the ideal conduction orientation. The results reported herein provide the highest conductivity in LLTO and important insights into their crystal structures, enabling higher conductivity in novel oxide-based electrolyte design.

Atomic-Resolution STEM Image Denoising by Total Variation Regularization

Atomic-resolution electron microscopy imaging of solid state material is a powerful method for structural analysis. Scanning transmission electron microscopy (STEM) is one of the actively used techniques to directly observe atoms in materials. However, some materials are easily damaged by the electron beam irradiation, and only noisy images are available when we decrease the electron dose to avoid beam damages. Therefore, a denoising process is necessary for precise structural analysis in low-dose STEM. In this study, we propose total variation (TV) denoising algorithm to remove quantum noise in a STEM image. We defined an entropy of STEM image that corresponds to the image contrast to determine a hyperparameter and we found that there is a hyperparameter that maximize the entropy. We acquired atomic resolution STEM image of CaF2 viewed along the [001] direction, and executed TV denoising. The atomic columns of Ca and F are clearly visualized by the TV denoising, and atomic position of Ca and F are determined with the error of ± 1 pm and ± 4 pm, respectively.

Reliable Electrochemical Setup for in situ Observations with an Atmospheric SEM

A novel setup for the in situ observation of electrochemical reactions in liquids through atmospheric scanning electron microscopy is presented. The proposed liquid-phase electrochemical SEM system consists of a working electrode (WE) on an electrochemical chip (e-chip) and other two electrodes inserted into a liquid electrolyte; electrochemical reactions occurring at the WE are controlled precisely with an external potentiostat/galvanostat connected to the three electrodes. Copper deposition from a CuSO4 aqueous solution was conducted onto the WE, and simultaneous acquisition of nanoscale images and reliable electrochemical data was achieved with the proposed setup.

Direct Observation of Atomistic Reaction Process between Pt Nanoparticles and TiO2 (110)

The catalytic activity and selectivity of heterogeneous catalysts are governed by atomic and electronic structures at the heterointerface between noble metal nanoparticles (NPs) and oxide substrates. In specific chemical reactions, it is well-known that the catalytic activity is strongly suppressed by annealing in a reducing atmosphere, so-called strong metal-support interaction (SMSI). However, it is still unclear the formation process and atomistic origin of the SMSI. By preparing well-defined platinum (Pt) NPs supported on atomically flat TiO₂ (110) substrate, we directly show the formation of chemically ordered Pt-Ti intermetallic NPs and impregnation of NPs into TiO₂ substrate at high temperatures by using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Furthermore, we observed negative charge transfer from the Pt-Ti intermetallic NPs to the TiO₂ surface, which would strongly affect the catalytic activities.

Design and Fabrication of an Electrochemical Chip for Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) can be used with an electrochemical chip (e-chip) to observe electrochemical reactions in a liquid in situ. The design of electrodes on an e-chip fabricated using microelectromechanical system (MEMS) technology cannot be easily changed. Here, we report a newly designed e-chip and its fabrication process. Electrodes with a desired shape were fabricated with various metals via an additional step of vacuum deposition onto our e-chip with a shadow mask. For precise control of the electrochemical reactions in LP-TEM, optimization of the electrode shape and material is critical.

Direct imaging of the disconnection climb mediated point defects absorption by a grain boundary

Grain boundaries (GBs) are considered as the effective sinks for point defects, which improve the radiation resistance of materials. However, the fundamental mechanisms of how the GBs absorb and annihilate point defects under irradiation are still not well understood at atomic scale. With the aid of the atomic resolution scanning transmission electron microscope, we experimentally investigate the atomistic mechanism of point defects absorption by a ∑31 GB in α-Al₂O₃ under high energy electron beam irradiation. It is shown that a disconnection pair is formed, during which all the Al atomic columns are tracked. We demonstrate that the formation of the disconnection pair is proceeded with disappearing of atomic columns in the GB core, which suggests that the GB absorbs vacancies. Such point defect absorption is attributed to the nucleation and climb motion of disconnections. These experimental results provide an atomistic understanding of how GBs improve the radiation resistance of materials.

Factors limiting quantitative phase retrieval in atomic-resolution differential phase contrast scanning transmission electron microscopy using a segmented detector

Quantitative differential phase contrast imaging of materials in atomic-resolution scanning transmission electron microscopy using segmented detectors is limited by various factors, including coherent and incoherent aberrations, detector positioning and uniformity, and scan-distortion. By comparing experimental case studies of monolayer and few-layer graphene with image simulations, we explore which parameters require the most precise characterisation for reliable and quantitative interpretation of the reconstructed phases. Coherent and incoherent lens aberrations are found to have the most significant impact. For images over a large field of view, the impact of noise and non-periodic boundary conditions are appreciable, but in this case study have less of an impact than artefacts introduced by beam deflections coupling to beam scanning (imperfect tilt-shift purity).

Real-space visualization of intrinsic magnetic fields of an antiferromagnet

Characterizing magnetic structures down to atomic dimensions is central to the design and control of nanoscale magnetism in materials and devices. However, real-space visualization of magnetic fields at such dimensions has been extremely challenging. In recent years, atomic-resolution differential phase contrast scanning transmission electron microscopy (DPC STEM)¹ has enabled direct imaging of electric field distribution even inside single atoms². Here we show real-space visualization of magnetic field distribution inside antiferromagnetic haematite (α-Fe₂O₃) using atomic-resolution DPC STEM in a magnetic-field-free environment³. After removing the phase-shift component due to atomic electric fields and improving the signal-to-noise ratio by unit-cell averaging, real-space visualization of the intrinsic magnetic fields in α-Fe₂O₃ is realized. These results open a new possibility for real-space characterization of many magnetic structures.

Atomic-Level Changes during Electrochemical Cycling of Oriented LiMn2O4 Cathodic Thin Films

Spinel LiMn₂O₄ is an attractive lithium-ion battery cathode material that undergoes a complex series of structural changes during electrochemical cycling that lead to rapid capacity fading, compromising its long-term performance. To gain insights into this behavior, in this report we analyze changes in epitaxial LiMn₂O₄ thin films during the first few charge-discharge cycles with atomic resolution and correlate them with changes in the electrochemical properties. Impedance spectroscopy and scanning transmission electron microscopy are used to show that defect-rich LiMn₂O₄ surfaces contribute greatly to the increased resistivity of the battery after only a single charge. Sequences of {111} stacking faults within the films were also observed upon charging, increasing in number with further cycling. The atomic structures of these stacking faults are reported for the first time, showing that Li deintercalation is accompanied by local oxygen loss and relaxation of Mn atoms onto previously unoccupied sites. The stacking faults have a more compressed structure than the spinel matrix and impede Li-ion migration, which explains the observed increase in thin-film resistivity as the number of cycles increases. These results are used to identify key factors contributing to conductivity degradation and capacity fading in LiMn₂O₄ cathodes, highlighting the need to develop techniques that minimize defect formation in spinel cathodes to improve cycle performance.

Spin Polarization-Assisted Dopant Segregation at a Coherent Phase Boundary

Coherent phase boundaries are widely expected as segregation-free boundaries due to their low interfacial energies and lack of trapping sites for impurities. Here, we report an equilibrium segregation of W atoms at fully coherent terraces of a Fe₃O₄ (111)/Fe₂O₃ (0001) phase boundary that was never expected previously. Through comparison of pristine and W-doped Fe₃O₄/Fe₂O₃ phase boundaries, it is revealed that the spin polarization of O atoms at the interface plays an important role in the periodic segregation of W atoms. Unusual spin-polarized O atoms with large magnetic moments are periodically arranged in the interfacial O plane of the pristine phase boundary. After doping of W at this boundary, W atoms will selectively substitute the Fe atoms of Fe₂O₃ that directly bond with three spin-polarized O atoms, thereby resulting in the complete neutralization of the magnetic moments of the spin-polarized O atoms. These findings reveal that coherent phase boundaries are able to trap impurities and local spin polarization is one of the driving forces for dopant segregation, suggesting that elemental doping is an efficient way for tailoring the physical properties of boundaries in magnetic materials and devices.

Breaking of Thermopower-Conductivity Trade-Off in LaTiO3 Film around Mott Insulator to Metal Transition

Introducing artificial strain in epitaxial thin films is an effective strategy to alter electronic structures of transition metal oxides (TMOs) and to induce novel phenomena and functionalities not realized in bulk crystals. This study reports a breaking of the conventional trade-off relation in thermopower (S)-conductivity (σ) and demonstrates a 2 orders of magnitude enhancement of power factor (PF) in compressively strained LaTiO3 (LTO) films. By varying substrates and reducing film thickness down to 4 nm, the out-of-plane to the in-plane lattice parameter ratio is controlled from 0.992 (tensile strain) to 1.034 (compressive strain). This tuning induces the electronic structure change from a Mott insulator to a metal and leads to a 103 -fold increase in σ up to 2920 S cm-1 . Concomitantly, the sign of S inverts from positive to negative, and both σ and S increase and break the trade-off relation between them in the n-type region. As a result, the PF (=S2 σ) is significantly enhanced to 300 µW m- 1 K-2 , which is 102 times larger than that of bulk LTO. Present results propose epitaxial strain as a means to finely tune strongly correlated TMOs close to their Mott transition, and thus to harness the hidden large thermoelectric PF.

Solid-State Electrochemical Switch of Superconductor-Metal-Insulators

Controlling the oxygen content can manipulate the electrical conductivity of transition metal oxides (TMOs). Although the superconductor-metal-insulator transition is useful for functional devices, an electrical path must be developed to manipulate the oxygen deficiency (δ) while maintaining the solid state. YBa₂Cu₃O_7-δ (YBCO, 0 ≤ δ ≤ 1) is a high transition temperature (T_c) TMO that can be modulated from a superconductor (T_c ≈ 92 K when δ = 0) to an insulator (δ ≈ 1). Here, we show a simple and efficient way to manipulate δ in YBCO films using a solid-state electrochemical redox treatment. Applying a negative voltage injects oxide ions to the YBCO films, increasing T_c. Employing a positive voltage suppresses the superconducting transition and modulates the electrical conductivity. The present results demonstrate that the superconductor-metal-insulator transition of YBCO is modulated electrochemically in the solid state, opening possibilities of superconducting oxide-based device applications.

Unveiling the Electronic Structure of Grain Boundaries in Anatase with Electron Microscopy and First-Principles Modeling

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in high-efficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.

Reprint of: Automated geometric aberration correction for large-angle illumination STEM

Depth resolution in scanning transmission electron microscopy (STEM) is physically limited by the illumination angle. In recent notable progress on aberration correction technology, the illumination angle is significantly improved to be larger than 60 milliradians, which is 2 or 3 times larger than those in the previous generation. However, for three-dimensional depth sectioning with the large illumination angles, it is prerequisite to ultimately minimize lower orders of aberrations such as 2- and 3-fold astigmatisms and axial coma. Here, we demonstrate a live aberration correction using atomic-resolution STEM images rather than Ronchigram images. The present method could save the required time for aberration correction, and moreover, it is possible to build up a fully automated program. We demonstrate the method should be useful not only for axial depth sectioning but also phase imaging in STEM including differential phase-contrast imaging.

An elastic metal-organic crystal with a densely catenated backbone

What particular mechanical properties can be expected for materials composed of interlocked backbones has been a long-standing issue in materials science since the first reports on polycatenane and polyrotaxane in the 1970s^1-3. Here we report a three-dimensional porous metal-organic crystal, which is exceptional in that its warps and wefts are connected only by catenation. This porous crystal is composed of a tetragonal lattice and dynamically changes its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range. We indented⁴ the crystal along its a/b axes and obtained the Young's moduli of 1.77 ± 0.16 GPa in N,N-dimethylformamide and 1.63 ± 0.13 GPa in tetrahydrofuran, which are the lowest among those reported so far for porous metal-organic crystals⁵. To our surprise, hydrostatic compression showed that this elastic porous crystal was the most deformable along its c axis, where 5% contraction occurred without structural deterioration upon compression up to 0.88 GPa. The crystal structure obtained at 0.46 GPa showed that the catenated macrocycles move translationally upon contraction. We anticipate our mechanically interlocked molecule-based design to be a starting point for the development of porous materials with exotic mechanical properties. For example, squeezable porous crystals that may address an essential difficulty in realizing both high abilities of guest uptake and release are on the horizon.

Atomistic Origin of Li-Ion Conductivity Reduction at (Li3xLa2/3-x)TiO3 Grain Boundary

Lithium lanthanum titanate (LLTO) is one of the excellent candidates for an electrolyte in the all-solid-state Li-ion battery, owing to the high Li-ion conductivity in the bulk. However, the Li-ion conductivity at the grain boundary (GB) is largely reduced, and it is therefore important to reveal the origin of Li-ion conductivity reduction at the GB. Here, by using atomic-resolution scanning transmission electron microscopy combined with atomic force microscopy, we investigate the charge states, Li-ion conductivities, atomic and electronic structures at the LLTO Σ5 and Σ13 GBs. Although the Σ5 GB has no significant influence on Li-ion conductivity, the Σ13 GB shows the evident reduction of Li-ion conductivity. We further elucidate that the Σ13 GB is positively charged by the formation of oxygen vacancies at the GB. Such a positive charge would form the Li-ion depletion layers adjacent to the GB, which causes the significant reduction of Li-ion conductivity.

Single-Dislocation Schottky Diodes

Dislocations often exhibit unique physical properties distinct from those of the bulk material. However, functional applications of dislocations are challenging due to difficulties in the construction of high-performance devices of dislocations. Here we demonstrate unidirectional single-dislocation Schottky diode arrays in a Fe₂O₃ thin film on Nb-doped SrTiO₃ substrates. Conductivity measurements using conductive atomic force microscopy indicate that a net current will flow through individual dislocation Schottky diodes under forward bias and disappear under reverse bias. Under cyclic bias voltages, the single-dislocation Schottky diodes exhibit a distinct resistive switching behavior containing low-resistance and high-resistance states with a high resistance ratio of ∼10³. A combined study of transmission electron microscopy and first-principles calculations reveals that the Fe₂O₃ dislocations comprise mixed Fe²⁺ and Fe³⁺ ions due to O deficiency and exhibit a one-dimensional electrical conductivity. The single-dislocation Schottky diodes may find applications for developing ultrahigh-density electronic and memory devices.

Direct imaging of atomistic grain boundary migration

Grain boundary (GB) migration plays an important role in modifying the microstructures and the related properties of polycrystalline materials, and is governed by the atomistic mechanism by which the atoms are displaced from one grain to another. Although such an atomistic mechanism has been intensively investigated, it is still experimentally unclear as to how the GB migration proceeds at the atomic scale. With the aid of high-energy electron-beam irradiation in atomic-resolution scanning transmission electron microscopy, we controllably triggered the GB migration in α-Al₂O₃ and directly visualized the atomistic GB migration as a stop motion movie. It was revealed that the GB migration proceeds by the cooperative shuffling of atoms on GB ledges along specific routes, passing through several different stable and metastable GB structures with low energies. We demonstrated that GB migration could be facilitated by the GB structural transformations between these low-energy structures.

Surfactant-mediated morphology evolution and self-assembly of cerium oxide nanocrystals for catalytic and supercapacitor applications

Surfactant plays a remarkable role in determining the growth process (facet exposition) of colloidal nanocrystals (NCs) and the formation of self-assembled NC superstructures, the underlying mechanism of which, however, still requires elucidation. In this work, the mechanism of surfactant-mediated morphology evolution and self-assembly of CeO2 nanocrystals is elucidated by exploring the effect that surfactant modification has on the shape, size, exposed facets, and arrangement of the CeO2 NCs. It is directly proved that surfactant molecules determine the morphologies of the CeO2 NCs by preferentially bonding onto Ce-terminated {100} facets, changing from large truncated octahedra (mostly {111} and {100} exposed), to cubes (mostly {100} exposed) and small cuboctahedra (mostly {100} and {111} exposed) by increasing the amount of surfactant. The exposure degree of the {100} facets largely affects the concentration of Ce3+ in the CeO2 NCs, thus the cubic CeO2 NCs exhibit superior oxygen storage capacity and excellent supercapacitor performance due to a high fraction of exposed active {100} facets with great superstructure stability.

Defect Engineering and Anisotropic Modulation of Ionic Transport in Perovskite Solid Electrolyte LixLa(1-x)/3NbO3

Solid electrolytes, such as perovskite Li3xLa2/1-xTiO3, LixLa(1-x)/3NbO3 and garnet Li7La3Zr2O12 ceramic oxides, have attracted extensive attention in lithium-ion battery research due to their good chemical stability and the improvability of their ionic conductivity with great potential in solid electrolyte battery applications. These solid oxides eliminate safety issues and cycling instability, which are common challenges in the current commercial lithium-ion batteries based on organic liquid electrolytes. However, in practical applications, structural disorders such as point defects and grain boundaries play a dominating role in the ionic transport of these solid electrolytes, where defect engineering to tailor or improve the ionic conductive property is still seldom reported. Here, we demonstrate a defect engineering approach to alter the ionic conductive channels in LixLa(1-x)/3NbO3 (x = 0.1~0.13) electrolytes based on the rearrangements of La sites through a quenching process. The changes in the occupancy and interstitial defects of La ions lead to anisotropic modulation of ionic conductivity with the increase in quenching temperatures. Our trial in this work on the defect engineering of quenched electrolytes will offer opportunities to optimize ionic conductivity and benefit the solid electrolyte battery applications.

Two-Dimensional Room-Temperature Giant Antiferrodistortive SrTiO_{3} at a Grain Boundary

The broken symmetry at structural defects such as grain boundaries (GBs) discontinues chemical bonds, leading to the emergence of new properties that are absent in the bulk owing to the couplings between the lattice and other parameters. Here, we create a two-dimensional antiferrodistortive (AFD) strontium titanate (SrTiO_{3}) phase at a Σ13(510)/[001] SrTiO_{3} tilt GB at room temperature. We find that such an anomalous room-temperature AFD phase with the thickness of approximate six unit cells is stabilized by the charge doping from oxygen vacancies. The localized AFD originated from the strong lattice-charge couplings at a SrTiO_{3} GB is expected to play important roles in the electrical and optical activity of GBs and can explain past experiments such as the transport properties of electroceramic SrTiO_{3}. Our study also provides new strategies to create low-dimensional anomalous elements for future nanoelectronics via grain boundary engineering.

Atomic-Resolution Topographic Imaging of Crystal Surfaces

The surface of metal oxides is of technological importance and is extensively used as a substrate for various electronic and chemical applications. A real surface, however, is not a perfectly well-defined and clean surface, but rather contains a diverse class of atomistic defects. Here, we show the direct determination of the 3D surface atomic structure of SrTiO₃ (001) including termination layers and atomistic defects such as vacancies, adatoms, ledges, kinks, and their complex combinations, by using depth sectioning of atomic-resolution annular dark-field scanning transmission electron microscopy (ADF STEM). To overcome the poor depth resolution in STEM, we statistically analyze the column by column depth profiles of ADF STEM images with a Bayesian framework fitting algorithm, and we achieve depth resolution at the entrance surface of ±0.9 Å for 1518 individual atomic columns. The present atomic-resolution 3D electron microscopy at the surface will provide fertile ground especially in surface science.

Anataselike Grain Boundary Structure in Rutile Titanium Dioxide

The formation of nanoscale phases at grain boundaries in polycrystalline materials has attracted much attention, since it offers a route toward targeted and controlled design of interface properties. However, understanding structure-property relationships at these complex interfacial defects is hampered by the great challenge of accurately determining their atomic structure. Here, we combine advanced electron microscopy together with ab initio random structure searching to determine the atomic structure of an experimentally fabricated Σ13 (221) [11̅0] grain boundary in rutile TiO2. Through careful analysis of the atomic structure and complementary electron energy-loss spectroscopy analysis we identify the existence of a unique nanoscale phase at the grain boundary with striking similarities to the bulk anatase crystal structure. Our results show a path to embed nanoscale anatase into rutile TiO2 and showcase how the atomic structure of even complex internal interfaces can be accurately determined using a combined theoretical and experimental approach.

Direct visualization of anionic electrons in an electride reveals inhomogeneities

Electrides are an unusual family of materials that feature loosely bonded electrons that occupy special interstitial sites and serve as anions. They are attracting increasing attention because of their wide range of exotic physical and chemical properties. Despite the critical role of the anionic electrons in inducing these properties, their presence has not been directly observed experimentally. Here, we visualize the columnar anionic electron density within the prototype electride Y₅Si₃ with sub-angstrom spatial resolution using differential phase-contrast imaging in a scanning transmission electron microscope. The data further reveal an unexpected charge variation at different anionic sites. Density functional theory simulations show that the presence of trace H impurities is the cause of this inhomogeneity. The visualization and quantification of charge inhomogeneities in crystals will serve as valuable input in future theoretical predictions and experimental analysis of exotic properties in electrides and materials beyond.

Improving the depth resolution of STEM-ADF sectioning by 3D deconvolution

Although the possibility of locating single atom in three dimensions using the scanning transmission electron microscope (STEM) has been discussed with the advent of aberration correction technology, it is still a big challenge. In this report we have developed deconvolution routines based on maximum entropy method (MEM) and Richardson-Lucy algorithm (RLA), which are applicable to the STEM-annular dark-field (ADF) though-focus images to improve the depth resolution. The new three-dimensional (3D) deconvolution routines require a limited defocus-range of STEM-ADF images that covers a whole sample and some vacuum regions. Since the STEM-ADF probe is infinitely elongated along the optical axis, a 3D convolution is performed with a two-dimensional (2D) convolution over xy-plane using the 2D fast Fourier transform in reciprocal space, and a one-dimensional convolution along the z-direction in real space. Using our new deconvolution routines, we have processed simulated focal series of STEM-ADF images for single Ce dopants embedded in wurtzite-type AlN. Applying the MEM, the Ce peaks are clearly localized along the depth, and the peak width is reduced down to almost one half. We also applied the new deconvolution routines to experimental focal series of STEM-ADF images of a monolayer graphene. The RLA gives smooth and high-P/B ratio scattering distribution, and the graphene layer can be easily detected. Using our deconvolution algorithms, we can determine the depth locations of the heavy dopants and the graphene layer within the precision of 0.1 and 0.2 nm, respectively. Thus, the deconvolution must be extremely useful for the optical sectioning with 3D STEM-ADF images.

Photoindentation: A New Route to Understanding Dislocation Behavior in Light

It was recently found that extremely large plasticity is exhibited in bulk compression of single-crystal ZnS in complete darkness. Such effects are believed to be caused by the interactions between dislocations and photoexcited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here, we propose the "photoindentation" technique to solve this issue by combining nanoscale indentation tests with a fully controlled lighting system. The quantitative data analyses based on this photoindentation approach successfully demonstrate that the first pop-in stress indicating dislocation nucleation near the surface of ZnS clearly increases by light irradiation. Additionally, the room-temperature indentation creep tests show a drastic reduction of the dislocation mobility under light. Our approach demonstrates great potential in understanding the light effects on dislocation nucleation and mobility at the nanoscale, as most advanced technology-related semiconductors are limited in dimensions.

Anisotropic Electrical Conductivity of Oxygen-Deficient Tungsten Oxide Films with Epitaxially Stabilized 1D Atomic Defect Tunnels

Materials having an anisotropic crystal structure often exhibit anisotropy in the electrical conductivity. Compared to complex transition-metal oxides (TMOs), simple TMOs rarely show large anisotropic electrical conductivity due to their simple crystal structure. Here, we focus on the anisotropy in the electrical conductivity of a simple TMO, oxygen-deficient tungsten oxide (WO_x) with an anisotropic crystal structure. We fabricated several WO_x films by the pulsed laser deposition technique on the lattice-matched (110)-oriented LaAlO₃ substrate under a controlled oxygen atmosphere. The crystallographic analyses of the WO_x films revealed that highly dense atomic defect tunnels were aligned one-dimensionally (1D) along [001] LaAlO₃. The electrical conductivity along the 1D atomic defect tunnels was ∼5 times larger than that across the tunnels. The present approach, introduction of 1D atomic defect tunnels, might be useful to design simple TMOs exhibiting anisotropic electrical conductivity.

Automated geometric aberration correction for large-angle illumination STEM

Depth resolution in scanning transmission electron microscopy (STEM) is physically limited by the illumination angle. In recent notable progress on aberration correction technology, the illumination angle is significantly improved to be larger than 60 milliradians, which is 2 or 3 times larger than those in the previous generation. However, for three-dimensional depth sectioning with the large illumination angles, it is prerequisite to ultimately minimize lower orders of aberrations such as 2- and 3-fold astigmatisms and axial coma. Here, we demonstrate a live aberration correction using atomic-resolution STEM images rather than Ronchigram images. The present method could save the required time for aberration correction, and moreover, it is possible to build up a fully automated program. We demonstrate the method should be useful not only for axial depth sectioning but also phase imaging in STEM including differential phase-contrast imaging.

Toward quantitative electromagnetic field imaging by differential-phase-contrast scanning transmission electron microscopy

Differential-phase-contrast scanning transmission electron microscopy (DPC STEM) is a technique to directly visualize local electromagnetic field distribution inside materials and devices at very high spatial resolution. Owing to the recent progress in the development of high-speed segmented and pixelated detectors, DPC STEM now constitutes one of the major imaging modes in modern aberration-corrected STEM. While qualitative imaging of electromagnetic fields by DPC STEM is readily possible, quantitative imaging by DPC STEM is still under development because of the several fundamental issues inherent in the technique. In this report, we review the current status and future prospects of DPC STEM for quantitative electromagnetic field imaging from atomic scale to mesoscopic scale.

Magnetic-structure imaging in polycrystalline materials by specimen-tilt series averaged DPC STEM

Differential phase contrast (DPC) imaging in scanning transmission electron microscopy is a technique to visualize electromagnetic field distribution inside specimens at high spatial resolution. However, diffraction contrast strongly hampers electromagnetic contrast in DPC images especially in polycrystalline samples. In this paper, we develop an imaging technique to effectively suppress diffraction contrast in DPC images. It is shown that a magnetic structure in a Nd-Fe-B permanent magnet was clearly visualized by averaging 64 DPC images with various specimen-tilt conditions. This is because the diffraction contrast in DPC images sensitively and randomly varies with crystal orientation and thus almost vanishes by averaging specimen-tilt image series. We further investigated two types of residual diffraction contrast in the tilt-series averaged DPC images: weak contrast inside grains and strong contrast at grain boundaries. We found that the former can be suppressed by averaging more DPC images, whereas the latter can be suppressed by the tilt-series averaging with wider range of specimen tilt. The tilt-series averaging method enables DPC to visualize electromagnetic structures even inside polycrystalline materials.

Magnetic-structure imaging in polycrystalline materials by specimen-tilt series averaged DPC STEM

Differential phase contrast (DPC) imaging in scanning transmission electron microscopy is a technique to visualize electromagnetic field distribution inside specimens at high spatial resolution. However, diffraction contrast strongly hampers electromagnetic contrast in DPC images especially in polycrystalline samples. In this paper, we develop an imaging technique to effectively suppress diffraction contrast in DPC images. It is shown that a magnetic structure in a Nd-Fe-B permanent magnet was clearly visualized by averaging 64 DPC images with various specimen-tilt conditions. This is because the diffraction contrast in DPC images sensitively and randomly varies with crystal orientation and thus almost vanishes by averaging specimen-tilt image series. We further investigated two types of residual diffraction contrast in the tilt-series averaged DPC images: weak contrast inside grains and strong contrast at grain boundaries. We found that the former can be suppressed by averaging more DPC images, whereas the latter can be suppressed by the tilt-series averaging with wider range of specimen tilt. The tilt-series averaging method enables DPC to visualize electromagnetic structures even inside polycrystalline materials.

Dislocation and oxygen-release driven delithiation in Li2MnO3

Lithium-excess layered cathode materials such as Li2MnO3 have attracted much attention owing to their high energy densities. It has been proposed that oxygen-release and cation-mixing might be induced by delithiation. However, it is still unclear as to how the delithiated-region grows. Here, by using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy, we directly observe the atomic structures at the interface between pristine and delithiated regions in the partially delithiated Li2MnO3 single crystal. We elucidate that the delithiated regions have extensive amounts of irreversible defects such as oxygen-release and Mn/Li cation-mixing. At the interface, a partially cation disordered structure is formed, where Mn migration occurred only in the specific Mn/Li layers. Besides, a number of dislocations are formed at the interface to compensate the lattice mismatch between the pristine and delithiated regions. The observed oxygen-release and dislocations could govern the growth of delithiated-regions and performance degradation in Li2MnO3.

Ultrafast Encapsulation of Metal Nanoclusters into MFI Zeolite in the Course of Its Crystallization: Catalytic Application for Propane Dehydrogenation

Encapsulating metal nanoclusters into zeolites combines the superior catalytic activity of the nanoclusters with high stability and unique shape selectivity of the crystalline microporous materials. The preparation of such bifunctional catalysts, however, is often restricted by the mismatching in time scale between the fast formation of nanoclusters and the slow crystallization of zeolites. We herein demonstrate a novel strategy to overcome the mismatching issue, in which the crystallization of zeolites is expedited so as to synchronize it with the rapid formation of nanoclusters. The concept was demonstrated by confining Pt and Sn nanoclusters into a ZSM-5 (MFI) zeolite in the course of its crystallization, leading to an ultrafast, in situ encapsulation within just 5 min. The Pt/Sn-ZSM-5 exhibited exceptional activity and selectivity with stability in the dehydrogenation of propane to propene. This method of ultrafast encapsulation opens up a new avenue for designing and synthesizing composite zeolitic materials with structural and compositional complexity.