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Abstract Space charge layers (SCLs) formed at grain boundaries (GBs) are considered to critically influence the properties of polycrystalline materials such as ion conductivities. Despite the extensive researches on this issue, the presence of GB SCLs and their relationship with GB orientations, atomic-scale structures and impurity/solute segregation behaviors remain controversial, primarily due to the difficulties in directly observing charge distribution at GBs. In this study, we directly observe electric field distribution across the well-defined yttria-stabilized zirconia (YSZ) GBs by tilt-scan averaged differential phase contrast scanning transmission electron microscopy. Our observation clearly reveals the existence of SCLs across the YSZ GBs with nanometer precision, which are significantly varied depending on the GB orientations and the resultant core atomic structures. Moreover, the magnitude of SCLs show a strong correlation with yttrium segregation amounts. This study provides critical insights into the complex interplay between SCLs, orientations, atomic structures and segregation of GBs in ionic crystals.

Abstract 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.

Abstract 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.

Controlling nanoscale heat generation, dissipation, and transport is crucial for miniaturizing electronic devices and for designing highly efficient thermoelectric materials. However, it has been challenging to directly measure thermal properties at individual atom level. Herein, direct atomic‐resolution column‐by‐column imaging of the rattling motion of Ba atoms in a clathrate compound Ba8Ga16Ge30 using atomic‐resolution scanning transmission electron microscopy with a segmented detector is shown. The directional anisotropy of the rattling motion is clearly visualized in real space and its amplitude and anisotropy are quantitatively evaluated by Bayesian analysis of the thermal diffuse scattering distribution. These results open a new possibility for directly characterizing nanoscale thermal properties in materials and devices, even those containing heavy elements such as thermoelectric materials.

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.

Grain boundaries can improve the radiation resistance of a material by annihilating point defects formed under irradiation, however the atomistic mechanism is still unclear. Here the authors demonstrate grain boundaries absorb point defects through the climb motion of disconnections.

Abstract Grain boundary (GB) migration is one of the most important phenomena in materials science, which plays a key role in modifying the microstructures and properties of polycrystalline ceramic materials. Understanding how GB migrates is thus a fundamental and critical issue for future ceramic material design. While the understanding of GB atomic structures has evolved significantly over the past several decades due to the progress of atomic‐resolution electron microscopy and atomistic simulation, the understanding of the atomistic grain boundary migration is still lacking. The present article briefly reviews our recent progress on the direct observation of atomistic GB migration in ceramic material by atomic‐resolution scanning transmission electron microscopy (STEM). Using Al2O3 as a model material, we found that the atomistic GB migration proceeds with atom shuffling accompanied by GB structural change and/or nucleation of disconnection, which is highly dependent on the GB atomic structures.

Rare-earth iron garnets are ferrimagnetic insulators commercially known for their large magneto-optical effect. Here, strain-gradient engineering is used to induce flexoelectricity in Sm3Fe5O12 films, achieving simultaneous remanent dielectric polarization and magnetization at room temperature.

The delithiation process in lithium excess cathode material of Li2MnO3 has remained unclear. Here, the authors report that delithiation is governed by the climb motion of dislocations with the aid of oxygen-release, determined directly by atomic-resolution STEM imaging and spectroscopy.

Abstract 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 (=S2σ) is significantly enhanced to 300 µW m−1K−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.

Cracks propagate preferentially along grain boundaries, but what happens exactly at the atomic scale remains unclear. Here, the authors combine atomic-scale observations and first-principles calculations to show local coordination chemistry at a dopant-segregated alumina grain boundary core dictates the atomic bond-breaking path.

Imaging chemical bonding states in defective graphene is important to determine its functional properties. Here, the authors report triangular and rectangular atomic electric fields in monolayer graphene induced by silicon as imaged by differential phase contrast STEM.

Improving the performance of Li-ion batteries relies on understanding charging/discharging mechanisms. Here the authors visualize the interfacial structure and composition of a partially delithiated lithium iron phosphate single crystal as a function of time, revealing a mechanism of relaxation.

The accelerated growth of thermoelectric technology that efficiently converts waste heat to electricity necessitates the development of high-performance materials. Here, the authors experimentally demonstrate a 2D electron system with enhanced two-dimensionality and thermoelectric power factor.

Liquid crystals comprising inorganic species are far less common than their organic-based counterparts. Here, the authors report a magneto-optically responsive liquid crystal based on nanorods of the biomineral hydroxyapatite, representing a new type of biologically-based, dynamic inorganic material.

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.

Understanding ferroelectricity at reduced dimensions will be important for future sub-nanoscale devices based on ferroelectrics. Using high resolution electron microscopy; Gaoet al., observe the existence of a measurable polarization at a thickness of just 1.5-unit cells

The ability of scanning transmission electron microscopy (STEM) to image single atoms is becoming increasingly sophisticated. Here, the authors use differential phase contrast STEM to map the atomic electric fields within single Au atoms and SrTiO3crystals, a step toward visualizing such intra- and interatomic electronic structure as chemical bonds.

Miniature of electronic devices is attractive yet challenging due to structural variation at nanoscale. Here, Gao et al. report atomic imaging of reconstruction and unusual domain walls on Pb(Zr0.2Ti0.8)O3surfaces, providing possibilities to engineer nanoscale structural change.

Solute segregation is challenging to experimentally identify at the atomic scale. Here, the authors report the direct observation of atomic site-dependent solute segregation behaviour in an yttria-stabilized zirconia grain boundary by atomic-resolution energy-dispersive X-ray spectroscopy.

Studies on dislocations in prototypic binary and ternary oxides (here TiO2 and SrTiO3) using modern TEM and scanning probe microscopy (SPM) techniques, combined with classical etch pits methods, are reviewed. Our review focuses on the important role of dislocations in the insulator-to-metal transition and for redox processes, which can be preferentially induced along dislocations using chemical and electrical gradients. It is surprising that, independently of the growth techniques, the density of dislocations in the surface layers of both prototypical oxides is high (109/cm2 for epipolished surfaces and up to 1012/cm2 for the rough surface). The TEM and locally-conducting atomic force microscopy (LCAFM) measurements show that the dislocations create a network with the character of a hierarchical tree. The distribution of the dislocations in the plane of the surface is, in principle, inhomogeneous, namely a strong tendency for the bundling and creation of arrays or bands in the crystallographic and directions can be observed. The analysis of the core of dislocations using scanning transmission electron microscopy (STEM) techniques (such as EDX with atomic resolution, electron-energy loss spectroscopy (EELS)) shows unequivocally that the core of dislocations possesses a different crystallographic structure, electronic structure and chemical composition relative to the matrix. Because the Burgers vector of dislocations is per se invariant, the network of dislocations (with additional d1 electrons) causes an electrical short-circuit of the matrix. This behavior is confirmed by LCAFM measurements for the stoichiometric crystals, moreover a similar dominant role of dislocations in channeling of the current after thermal reduction of the crystals or during resistive switching can be observed. In our opinion, the easy transformation of the chemical composition of the surface layers of both model oxides should be associated with the high concentration of extended defects in this region. Another important insight for the analysis of the physical properties in real oxide crystals (matrix + dislocations) comes from the studies of the nucleation of dislocations via in situ STEM indentation, namely that the dislocations can be simply nucleated under mechanical stimulus and can be easily moved at room temperature.

Alumina (α-Al2O3) is one of the representative high-temperature structural materials. Dislocations in alumina play an important role in its plastic deformation, and they have attracted much attention for many years. However, little is known about their core atomic structures, with a few exceptions, because of lack of experimental observations at the atomic level. Low-angle grain boundaries are known to consist of an array of dislocations, and they are useful to compose dislocation structures. So far, we have systematically fabricated several types of alumina bicrystals with a low-angle grain boundary and characterized the dislocation structures by transmission electron microscopy (TEM). Here, we review the dislocation structures in { 11 2 ¯ 0 } / [ 0001 ] , { 11 2 ¯ 0 } / 〈 1 1 ¯ 00 〉 , { 1 1 ¯ 00 } / 〈 11 2 ¯ 0 〉 , ( 0001 ) / 〈 1 1 ¯ 00 〉 , { 1 ¯ 104 } / 〈 11 2 ¯ 0 〉 , and ( 0001 ) / [ 0001 ] low-angle grain boundaries of alumina. Our observations revealed the core atomic structures of b = 1 / 3 〈 11 2 ¯ 0 〉 edge and screw dislocations, 〈 1 1 ¯ 00 〉 edge dislocation, and 1 / 3 〈 1 ¯ 101 〉 edge and mixed dislocations. Moreover, the stacking faults on { 11 2 ¯ 0 } , { 1 1 ¯ 00 } , and ( 0001 ) planes formed due to the dissociation reaction of the dislocations are discussed, focusing on their atomic structure and formation energy.

Basal dislocations having a Burgers vector of 1/3 in zinc oxide (ZnO) with the wurtzite structure are known to strongly affect physical properties in bulk. However, the core structure of the basal dislocation remains unclear. In the present study, ZnO bicrystals with a {2 1 ¯ 1 ¯ 0}/ 2° low-angle tilt grain boundary were fabricated by diffusion bonding. The resultant dislocation core structure was observed by using scanning transmission electron microscopy (STEM) at an atomic resolution. It was found that a basal edge dislocation in α-type is dissociated into two partial dislocations on the (0001) plane with a separation distance of 1.5 nm, indicating the glide dissociation. The Burgers vectors of the two partial dislocations were 1/3 and 1/3, and the stacking fault between the two partials on the (0001) plane has a formation energy of 0.14 J/m2. Although the bicrystals have a boundary plane of {2 1 ¯ 1 ¯ 0}, the boundary basal dislocations do not exhibit dissociation along the boundary plane, but along the (0001) plane perpendicular to the boundary plane. From DFT calculations, the stacking fault on the (0001) plane was found to be much more stable than that on {2 1 ¯ 1 ¯ 0}. Such an extremely low energy of the (0001) stacking fault can realize transverse dissociation of the basal dislocation of ZnO.

We report the microstructure and gas-sensing properties of a nonequilibrium TiO2-SnO2 solid solution prepared by the sol-gel method. In particular, we focus on the effect of Cd doping on the sensing behavior of the TiO2-SnO2 sensor. Of all volatile organic compound gases examined, the sensor with Cd doping exhibits exclusive selectivity as well as high sensitivity to formaldehyde, a main harmful indoor gas. The key gas-sensing quantities, maximum sensitivity, optimal working temperature, and response and recovery time, are found to meet the basic industrial needs. This makes the Cd-doped TiO2-SnO2 composite a promising sensor material for detecting the formaldehyde gas.

High structural susceptibility of multiferroic BiFeO3 (BFO) makes it a potential replacement of current Pb-based piezoelectrics. In this study, a tetragonal phase is identified based on a combination of x-ray diffraction, scanning transmission electronic microscopy, x-ray absorption spectroscopy, and Raman spectroscopy when BFO is grown on yttria-stabilized zirconia (YSZ) substrates. To distinguish the discrepancy between this tetragonal phase and common cases of monoclinic BFO, piezoelectric force microscopy images and optical property are also performed. It shows a lower electrostatic energy of ferroelectric domains and a large reduction of band gap for BFO grown on YSZ substrate comparing to the well-known one grown on LaAlO3 substrate. Our findings in this work can provide more insights to understand the structural diversity of multiferroic BFO system for further applications.

The addition of glass phase can control the grain boundary structure and hence the mechanical properties of tetragonal zirconia polycrystals (TZP). To reveal the effect of the glass dopant on the high-temperature deformation behavior of TZP, SiO2-doped TZP, (SiO2–Al2O3)-doped TZP, (SiO2–MgO)-doped TZP and undoped TZP were prepared and their grain boundary structure, chemical composition and chemical bonding state were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a field-emission-type transmission electron microscope (FE-TEM). It was found that no amorphous film was formed along the grain boundaries in any of the specimens examined, but amorphous pockets formed at multiple grain boundary junctions in three kinds of glass-doped specimens. In the glass-doped specimens, the segregation of yttrium, silicon and the added metal ions (Al3+ or Mg2+) was observed over a width of several nm across the grain boundaries. The addition of pure SiO2 much enhanced the ductility in TZP, although further addition of a small amount of Al2O3 or MgO to SiO2 phase resulted in a marked reduction in the tensile ductility of SiO2-doped TZP. EELS measurements and molecular orbital (MO) calculations using a cluster model revealed that the ductility of TZP was related to the bond overlap population (BOP) at the grain boundaries, which was influenced by the kinds of segregated dopants. That is, the presence of Si4+ increases the BOP, strengthening the grain boundary bonding strength and thus preventing cavity formation, but Al3+ and Mg2+ decrease the BOP, enhancing the grain boundary cavitation and thus reducing the ductility. Furthermore, the dynamic behavior of SiO2 in TZP was observed using a TEM in situ heating technique, and the results supported the fact that that Si segregates along the grain boundaries.

Metamaterial has been captivated a popular notion, offering photonic functionalities beyond the capabilities of natural materials. Its desirable functionality primarily relies on well-controlled conditions such as structural resonance, dispersion, geometry, filling fraction, external actuation, etc. However, its fundamental building blocks--meta-atoms--still rely on naturally occurring substances. Here, we propose and validate the concept of gradient and reversible atomic-engineered metamaterials (GRAM), which represents a platform for continuously tunable solid metaphotonics by atomic manipulation. GRAM consists of an atomic heterogenous interface of amorphous host and noble metals at the bottom, and the top interface was designed to facilitate the reversible movement of foreign atoms. Continuous and reversible changes in GRAM's refractive index and atomic structures are observed in the presence of a thermal field. We achieve multiple optical states of GRAM at varying temperature and time and demonstrate GRAM-based tunable nanophotonic devices in the visible spectrum. Further, high-efficiency and programmable laser raster-scanning patterns can be locally controlled by adjusting power and speed, without any mask-assisted or complex nanofabrication. Our approach casts a distinct, multilevel, and reversible postfabrication recipe to modify a solid material's properties at the atomic scale, opening avenues for optical materials engineering, information storage, display, and encryption, as well as advanced thermal optics and photonics. [ABSTRACT FROM AUTHOR]