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Ultra-high temperature performances are achieved by introducing and engineering homogeneous/heterogeneous interfaces within the capacitors. The ultra-wide working temperature range from −100 to 400 °C is suitable for extreme environments.

Transmission electron microscopy is an indispensable tool in modern materials science. It enables the structure of materials to be studied with high spatial resolution, and thus makes a decisive contribution to the fact that it is now possible to understand the microstructure-related physical and chemical characteristics and to correlate these with the macroscopic materials properties. It was tantamount to a paradigm shift when electron microscopy reached atomic resolution in the late 1990s due to the invention of aberration-corrected electron optics. It is now generally accepted practice to perform picometer-scale measurements and chemical analyses with reference to single atomic units. This review has three objectives. Microscopy in atomic dimensions is applied quantum physics. The consequences of this for practical work and for the understanding and application of the results shall be worked out. Typical applications in materials science will be used to show what can be done with this kind of microscopy and where its limitations lie. In the absence of relevant monographs, the aim is to provide an introduction to this new type of electron microscopy and to enable the reader to access the literature in which special issues are addressed. The paper begins with a brief presentation of the principles of optical aberration correction. It then discusses the fundamentals of atomic imaging and covers typical examples of practical applications to problems in modern materials science. It is emphasized that in atomic-resolution electron microscopy the quantitative interpretation of the images must always be based on the solution of the quantum physical and optical problem on a computer.

Nanoprecipitation is a routine method to decrease the thermal conductivity for advancing thermoelectric performance. However, the coarsening/Ostwald ripening of precipitates under temperature gradients in long-duration service deteriorates the efficacy of this strategy. Enlightened by the Gibbs adsorption-induced suppression of Ostwald ripening, we designed a highly stable thermoelectric system SnAg0.05Te-x%CdSe with embedded novel core/shell nanoprecipitates. Scanning transmission electron microscopy and atom probe tomography reveal that the precipitate core is CdTe and the shell is formed by Ag segregation with structures and compositions different from its abutting phases, termed interfacial complexions. This complexions layer suppresses the coarsening of precipitates by decreasing the interfacial Gibbs free energy, resulting in a high number of nanoscale precipitates. The enlarged phonon scattering cross-section of nanoprecipitates enabled by Gibbs adsorption along with interfacial complexions restrains the lattice thermal conductivity to its minimum while maintaining high carrier mobility. This eventually leads to an outstanding figure-of-merit (zT) of 1.5 at 873 K in thermally aged SnAg0.05Te-6%CdSe, assisted with valence band flattening and convergence. This work explores the mechanisms underpinning precipitate stabilization and provides a new paradigm to design nanostructured thermoelectrics.

Silicon integrated lead-free oxide thin film capacitors with high energy storage density (Wre), high efficiency (η) and good thermal stability have great application potential in modern communication fields. Here, 1 mol% SiO2-doped Ba(Zr0.35Ti0.65)O3 (BZTS) thin film capacitors are integrated on Si and HfO2 buffered Si substrates by using a radio-frequency magnetron sputtering system. It is found that the energy storage performances are significantly improved by inserting an HfO2 buffer layer (about 13.5 nm) between the BZTS layer and the Si substrate. The improved Wre of the BZTS/HfO2 thin film capacitors can be up to 93.48 J cm−3 at room temperature, which is about 65% higher than that of the film without the HfO2 buffer layer, and the η is ∼ 71.44%. Moreover, the introduction of the HfO2 buffer layer leads to a superior thermal stability in a wide temperature range from −100 °C to 200 °C with a very small change rate of ∼3.39%. Under different irradiations with doses of He+ from 1 × 1012 ions per cm2 to 7 × 1015 ions per cm2 and neutrons from 5 × 1012 ions per cm2 to 1 × 1014 ions per cm2, the BZTS thin film capacitors with the HfO2 buffer layer show ultra-stable energy storage performance. Our research provides an effective strategy for the integration of high performance BZTS thin film capacitors on Si substrates, and our results demonstrate potential of the Si integrated thin film capacitors for the application in nuclear technology, space stations, satellites, radiation centers and other harsh environments.

The huge axial ratio of supertetragonal perovskites can induce large polarization due to the coupling between the unit cell and ion displacements, which often results in higher Curie temperature or better ferroelectric properties. With the progress of vertical strain engineering, experimentally available supertetragonal PbTiO3 raises much interest in the relation between its intrinsic polarization and structural characteristics, such as the axial ratio and ion displacements that can be obtained with a high-resolution (scanning) transmission electron microscope nowadays. However, the interpretation and use of such obtained structural information are not without peril. Here, employing first-principles calculations, we report on the relationship between the polarization and the structural information for supertetragonal PbTiO3. Our investigation shows that, unlike normal PbTiO3, the strong sublinear correlation raises the difficulty to quantitatively estimate the spontaneous polarization for the axial ratio larger than 1.15. In addition, the relative displacement between cations (i.e., Pb and Ti) proves to be an unsuitable structural parameter to determine the spontaneous polarization, emphasizing the importance of obtaining the positions of anions, albeit presumably more difficult, along with cations.

Owing to their excellent mixed-ionic and electronic conductivity, fast oxygen kinetics, and cost efficiency, layered oxygen-deficient perovskite oxides hold great potential as highly efficient cathodes for solid oxide fuel cells and anodes for water oxidation. Under working conditions, cation ordering is believed to substantially enhance oxygen diffusion while maintaining structural stability owing to the formation of double perovskite (DP), thus attracting extensive research attention. In contrast, the incorporation of oxygen vacancies and the associated vacancy ordering have rarely been studied at the atomic scale, despite their decisive roles in regulating the electronic and spin structures as well as in differentiating the crystal structure from DP. Here, atomic-resolution transmission electron microscopy is used to directly image oxygen vacancies and measure their concentration in (Pr,Ba)CoO3-δ films grown on SrTiO3 substrates. We find that accompanied by the presence of oxygen vacancy ordering at Co–O planes, the A–O (A = Pr/Ba) planes also exhibit a breathing-like lattice modulation. Specifically, as confirmed by first-principle calculations, the AO–AO interplanar spacings are found to be linearly correlated with the vacancy concentration in the enclosing Co–O planes. On this basis, potential consequences of oxygen occupancy for the catalytic properties of structurally pure PBCO phases are discussed. Through establishing a simple correlation of oxygen concentration with the easily achievable lattice measurement, our results pave a way for better understanding the structure–performance relationship of oxygen-deficient complex cobaltites used for electrocatalysis.

As one of the most fundamental order parameters, the coordinate order of atoms is a cornerstone in determining the physical and chemical properties of materials. Off-centering shifts of atoms from their centrosymmetric positions, a phenomenon that is closely related to symmetry breaking and structural transitions in crystals, is of key importance in modifying a broad range of fundamental properties, such as ferroelectricity, antiferroelectricity, and piezoelectricity. In thermoelectric materials, which can be used to directly interconvert heat to electricity, off-centering has been proposed in, e.g., lead tellurides, halide perovskites, type I and II clathrates, and filled skutterudites, to be the physical origin of the experimentally measured exceptionally low thermal conductivity, but only indirectly supported from structural refinements and experimental/theoretical vibration mode analyses. In this work, Yb partially filled and Ce fully filled skutterudite is taken as a model system for directly measuring the off-centering shifts of filler atoms, i.e., rattlers, with picometer precision in real space, by means of atomic-resolution negative spherical aberration imaging transmission electron microscopy. This finding allows an in-depth understanding of the relationship between the off-centering and the thermoelectric performance, and the presented methodology here is also applicable to investigate the off-centering phenomena in other functional materials, e.g., ferroelectrics and solar cells.

Industry has been seeking a thin-film capacitor that can work at high temperature in a harsh environment, where cooling systems are not desired. Up to now, the working temperature of the thin-film capacitor is still limited up to 200 °C. Herein, we design a multilayer structure with layers of paraferroelectric (Ba0.3Sr0.7TiO3, BST) and relaxor ferroelectric (0.85BaTiO3–0.15Bi(Mg0.5Zr0.5)O3, BT–BMZ) to realize optimum properties with a flat platform of dielectric constant and high breakdown strength for excellent energy storage performance at high temperature. Through optimizing the multilayer structure, a highly stable relaxor ferroelectric state is obtained for the BST/BT–BMZ multilayer thin-film capacitor with a total thickness of 230 nm, a period number N = 8, and a layer thickness ratio of BST/BT–BMZ = 3/7. The optimized multilayer film shows significantly improved energy storage density (up to 30.64 J/cm3) and energy storage efficiency (over 70.93%) in an ultrawide temperature range from room temperature to 250 °C. Moreover, the multilayer system also exhibits excellent thermal stability in such an ultrawide temperature range with a change of 5.15 and 12.75% for the recoverable energy density and energy storage efficiency, respectively. Our results demonstrate that the designed thin-film capacitor is promising for the application in a harsh environment and open a way to tailor a thin-film capacitor toward higher working temperature with enhanced energy storage performance.

Integration of high-quality multifunctional oxide thin films in silicon technology promises a wide range of potential applications in low loss spintronic devices such as sensors, detectors, data storage media, and so on. However, the heteroepitaxial growth of functional complex oxides on silicon substrates has been proved challenging, which limits the development of semiconductor-based spintronic devices. In this work, epitaxial single-crystalline La0.67Sr0.33MnO3 (LSMO) thin films have been integrated on silicon substrates by epitaxy and transfer carried out at room temperature. The microwave magnetisms of the LSMO thin films transferred on silicon substrates have been investigated under multiple directions of magnetic field by ferromagnetic resonance. The transferred LSMO thin films on silicon substrates preserve the microwave magnetic characteristics of the primary as-grown LSMO thin films. Our results demonstrate that the epitaxy and transfer method has enormous potential in future spintronic applications of functional oxide devices compatible with semiconductor technology without thermal damage.

Getting more bits out of PCRAM Phase-change random access memory (PCRAM) has the ability to both store and process information. It also suffers from noise and electrical drift due to damage that accumulates during the cycling process. Ding et al. developed a phase-change heterostructure where a phase-change material is separated by a confinement material, creating an alternating stack (see the Perspective by Gholipour). This architecture results in ultralow noise, lower drift, and stable multilevel storage capacity, which are potentially useful for new forms of computing. Science , this issue p. 210 ; see also p. 186

High-performance dielectric capacitors are in high demand for advanced electronics and electric power systems. They possess high power density (on the order of Megawatt) and exhibit ultrafast charge/discharge capability (on a microsecond scale) and long-term storage lifetime1-5, and thus they are particularly demanded in pulse power systems such as high-power microwaves, hybrid electric vehicles, and high-frequency inverters. However, their relatively low operating temperature limits their widespread applications6-9. Here, guided by phase-field simulations, we synthesized capacitors with an energy storage density of 55.4 joules per cubic centimeter, energy efficiency of over 82%, and superior thermal stability and fatigue properties at record high operating temperature of 400°C. These ultrahigh-temperature performances are achieved through a relatively simple method of introduction and engineering of interfaces within the capacitors, which greatly improve their high-temperature stability, relaxation behavior, and breakdown strength. Our work not only successfully fabricated capacitors with potential applications in high-temperature electric power systems and electronic technologies but also opens up a promising and general route for designing high-performance electrostatic capacitors through interface engineering.

Thin film dielectrics are the most selected materials for many power electronics owing to their inherent advantages, such as high power density, fast charging-discharging, and long lifetime. Nowadays, additional demands for the film dielectrics are the high performances under harsh operating conditions, e.g. at high temperatures, which is highly favourable to significantly reduce the size and cost of energy devices. Here, we demonstrated that through design and optimization of the film systems with 1 mol% SiO2-doped BaZr0.35Ti0.65O3 layer sandwiched between two undoped BaZr0.35Ti0.65O3 layers, it is capable to concomitantly enhance breakdown strength and electrical polarization of the systems. The optimized sandwich-structure films yield a greatly improved discharged energy densities of ~130.1 J/cm3 with a high charge-discharge efficiency of ~73.8% at room temperature, as well as retain an ultrahigh discharged energy densities of ~77.8 J/cm3 in the ultra-wide temperature range from −100 to 200 °C. The presented combination of property modulation with structure engineering paves an effective way to meet the increasingly technological challenges and the requirements of modern electrical energy storage applications.

High-quality epitaxial (111) BaTiO3 thin film was successfully fabricated on flexible fluorophlogopite mica substrate by radio frequency magnetron sputtering. The flexible BaTiO3 thin-film element shows observably stable ferroelectric properties with the bending radius of 4 mm and well-mechanical fatigue resisting performance with 104 bending cycles at the bending radius of 6mm. Furthermore, it exhibits excellent ferroelectric fatigue resistance after suffering 106 bipolar switching cycles. Most importantly, the element shows good thermal stability in temperature range from 25°C to 100°C. All these stable ferroelectric properties indicate their enormous potential in flexible non-volatile memory devices under harsh working environment.

Antiphase boundaries in spinel ferrites have been considered as the functional elements with great potential for applications in spintronic devices. Engineering the antiphase boundaries is a big challenge in experiments. Here, we report on that a high density of antiphase boundaries in CuFe2O4 films have been prepared on rough surface of MgAl2O4 (001) substrates. By means of advanced electron microscopy these antiphase boundaries are found to be of {011} 1 4 〈011〉-type and bound by interfacial dislocations with Burgers vector of a/4〈011〉 that contribute to relaxation of in-plane and out-of-plane lattice strain in the heterostructure. Our results provide evidence that the rough substrate surface can be applied for the formation of antiphase boundaries in the CuFe2O4/MgAl2O4 system to tune the magnetic and electrical transport properties of epitaxial spinel ferrite films.

Here, large-scaled flexible Ba(Zr0.35Ti0.65)O3 ferroelectric film capacitors not only exhibit ultrahigh energy storage performances but also have excellent mechanical flexibility and ferroelectric fatigue endurance. In ultra-wide temperature range from −100 °C to 200 °C, the flexible Ba(Zr0.35Ti0.65)O3 film capacitors show excellent thermal stability with recoverable energy density (Wrec) of over 65.1 J/cm3 and efficiency (η) of over 72.9%, which presents the highest Wrec obtained up to now not only in flexible polymer film dielectric capacitors but also in the ceramic film capacitors including lead-free and lead-based film capacitors on rigid substrates in this measuring temperature range. All these results demonstrate that the flexible Ba(Zr0.35Ti0.65)O3 film capacitors have huge potential in the applications of flexible electronics operated in cold polar regions, extreme altitudes and outer space, as well as high temperature regions.

Controlling the Dirac point voltage of graphene is essential for realizing various practical applications of graphene. Here, control of the doping state is achieved in flexible graphene field effect transistors (GFETs) by applying mechanical bending stress. By gradually increasing the bending strain (the decrease of upward/downward bending radius), the Dirac point (VDirac) linearly shifts to left/right, which is induced by the flexoelectric effect of the ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) gate. In addition, a superior mechanical antifatigue character is obtained in the flexible GFETs, and the doping effect is recoverable. The sensitivity to strain and high bending stability not only offer an easy, controllable and nonintrusive method to obtain a specific doping level in graphene for flexible electric devices, but also highlight the enormous potential of the flexible ferroelectric PLZT-gated GFETs as wearable sensors.

Excellent thermal stability with high energy storage density in ultra-wide range of temperatures is the extremely important property of capacitors for applications in cold polar regions, extreme altitudes and high temperature regions. Here, we report on designing and preparing the BaZr0.15Ti0.85O3/BaZr0.35Ti0.65O3 (BZT15/BZT35) multilayer thin film capacitors. Under a given total thickness, the energy storage performances of the multilayer films can be optimized by controlling the number of interfaces. For the capacitor with an optimum period number N = 6, the markedly enhanced breakdown strength and large dielectric constant are achieved, which leads to a giant energy storage density (Wre) of ~83.9 J/cm3 with the efficiency (η) of ~78.4% and a superior power density of 1.47 MW/cm3 at room temperature. Moreover, the N = 6 multilayer capacitor also exhibits ultra-stable Wre of 69.1 J/cm3 (efficiency: 84.9%) to 63.2 J/cm3 (efficiency: 66.9%) from − 100 °C to 200 °C and a good reliability in Wre and η even after 106 cycles at 200 °C. The excellent performances demonstrate that the multilayer films are a promising material system to meet the wide requirements of future applications, ranging from portable electronics to hybrid electric vehicles and aerospace power electronics.

Pyrochlores characterized by the chemical formula A2B2O7 form an extended class of materials with interesting physical and chemical properties. The compound Bi1.5ZnNb1.5O7 is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. Bi1.5ZnNb1.5O7 shows an intriguing frequency-dispersive dielectric relaxation at 50 K ≤ T ≤ 250 K, which has been studied intensively but is still not fully understood. In this first study on a pyrochlore by atomic-resolution transmission electron microscopy we observe the Bi atoms on A sites since, due to their low nuclear charge, the contribution of Zn atoms to the contrast of these sites is negligible. We find in our [ 1 ¯ 00 ] and [112] oriented images that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split A-type sites. The Bi atoms are sessile, only occasionally we observe in time sequences of images jumps between individual split-site positions. The apertaining jump rate of the order of 0.1–1 Hz is by ten orders of magnitude lower than the values derived in the literature from Arrhenius plots of the low-temperature dielectric relaxation data. It is argued that these jumps are radiation induced. Therefore our observations are ruling out a contribution of Bi-atom jumps to low-temperature dielectric A sites-related relaxation. It is suggested that this relaxation is mediated by jumps of Zn atoms.

Flexible epitaxial La0.67Ca0.33MnO3 (LCMO) thin films are fabricated on an SrTiO3 buffered (001)-oriented fluorophlogopite substrate. The metal-to-insulator transition tends toward lower temperature when subjected to mechanical bending. Moreover, the transport behavior of the bent LCMO films in the insulating region follows the variable range hopping model and the resistivity increases with the reduction in the bending curvature radii because the applied strain aggravates the distortion of the LCMO crystal structure, decreases the hopping distance, and, hence, impedes the transport of charge carriers. The resistivity change induced by the mechanical bending can go up to 104% at 100 K and 105% at 10 K. Such a large resistivity change makes the flexible LCMO thin film promising as a mechanical-bending switch device at low temperature.

Lattice defects are typically used to tailor the thermoelectric properties of materials. It is desirable that such defects improve the electrical conductivity, while, at the same time, reduce the thermal conductivity, for an overall improvement on the thermoelectric properties of materials. Here, we report on an extended defect in Sb2Te3 consisting of swapped bilayers with chemical intermixing of Sb and Te atoms, which can be generated and effectively manipulated in polycrystalline samples through synthetic methods and thermal treatments. The swapped bilayers bridge the spatial gaps between the Sb2Te3 quintuple-layer blocks, enhancing the charge carrier mobility and thus the electrical conductivity. These defects also result in a reduced lattice thermal conductivity through suppressing phonon transport. These synergistic effects contribute together to an improved thermoelectric quality factor and an enhanced figure of merit (zT) value in Sb2Te3.

Pyrrhotites, characterized by the chemical formula Fe1–δS (0 < δ ≤ 1/8), represent an extended group of minerals that are derived from the NiAs-type FeS aristotype. They contain layered arrangements of ordered Fe vacancies, which are at the origin of the various magnetic signals registered from certain natural rocks and can act as efficient electrocatalysts in oxygen evolution reactions in ultrathin form. Despite extensive studies over the past century, the local structural details of pyrrhotite superstructures formed by different arrangements of Fe vacancies remain unclear, in particular at the atomic scale. Here, atomic-resolution high-angle annular dark-field imaging and nanobeam electron diffraction in the scanning transmission electron microscope are used to study natural pyrrhotite samples that contain commensurate 4C and incommensurate 4.91 ± 0.02C constituents. Local measurements of both the intensities and the picometer-scale shifts of individual Fe atomic columns are shown to be consistent with a model for the structure of 4C pyrrhotite, which was derived using X-ray diffraction by Tokonami et al. (1972). In 4.91 ± 0.02C pyrrhotite, 5C-like unequally sized nano-regions are found to join at anti-phase-like boundaries, leading to the incommensurability observed in the present pyrrhotite sample. This conclusion is supported by computer simulations. The local magnetic properties of each phase are inferred from the measurements. A discussion of perspectives for the quantitative counting of Fe vacancies at the atomic scale is presented.

Phase transition is established to govern electrostatic energy storage for antiferroelectric (AFE)‐type dielectric capacitors. However, the source of inducing the phase transition and the pathway of storing the energy remains elusive so far given the ultrafast charging/discharging process under normal working conditions. Here, by slowing down the phase‐transition speed using electron‐beam irradiation as an external stimulus, the in situ dynamic energy‐storage process in AFE PbZrO3 is captured by using atomic‐resolution transmission electron microscopy. Specifically, it is found that oxygen‐lead‐vacancy‐induced defect core acts as a seed to initiate the antiferrodistortive‐to‐ferrodistortive transition in antiparallel‐Pb‐based structural frames. Associated with polarity evolution of the compressively strained defect core, the ferroelectric (FE)–ferrodistortive state expands bilaterally along the b‐axis direction and then develops into charged domain configurations during the energy‐storage process, which is further evidenced by observations at the ordinary FE states. With filling the gap of perception, the findings here provide a straightforward approach of unveiling the unit‐cell‐wise energy storage pathway in chemical defect‐engineered dielectric ceramics.

With the development of miniaturization, lightweight and integration of electronic devices, the demand for high-temperature dielectric capacitors is becoming urgent. Nevertheless, the breakdown strength and polarization are deteriorated at high temperatures due to the thermal energy assisting the electron transport and impeding the dipole alignment. Here, a structure of capacitor with double gradients of dielectric constant gradient and strain gradient is designed to achieve high breakdown strength, high working temperature, and high energy storage density simultaneously. It is found that the designed structure of BaHf

Industry has been seeking a thin-film capacitor that can work at high temperature in a harsh environment, where cooling systems are not desired. Up to now, the working temperature of the thin-film capacitor is still limited up to 200 °C. Herein, we design a multilayer structure with layers of paraferroelectric (Ba

Materials with layered crystal structures and high in-plane anisotropy, such as black phosphorus, present unique properties and thus promise for applications in electronic and photonic devices. Recently, the layered structures of GeS

Domains and domain walls are among the key factors that determine the performance of ferroelectric materials. In recent years, a unique type of domain walls, i.e., the sawtooth-shaped domain walls, has been observed in ${\mathrm{BiFeO}}_{3}$ and ${\mathrm{PbTiO}}_{3}$. Here, we build a minimal model to reveal the origin of these sawtooth-shaped domain walls. Incorporating this model into Monte Carlo simulations shows that (i) the competition between the long-range Coulomb interaction (due to bound charges) and short-range interaction (due to opposite dipoles) is responsible for the formation of these peculiar domain walls and (ii) their relative strength is critical in determining the periodicity of these sawtooth-shaped domain walls. Necessary conditions to form such domain walls are also discussed.

Antiferroelectric‐based dielectric capacitors are receiving tremendous attention for their outstanding energy‐storage performance and extraordinary flexibility in collecting pulsed powers. Nevertheless, the in situ atomic‐scale structural‐evolution pathway, inherently coupling to the energy storage process, has not been elucidated for the ultimate mechanistic understanding so far. Here, time‐ and atomic‐resolution structural phase evolution in antiferroelectric PbZrO3 during storage of energy from the electron‐beam illumination is reported. By employing state‐of‐the‐art negative‐spherical‐aberration imaging technique, the quantitative transmission electron microscopy study presented herein clarifies that the hierarchical evolution of polar oxygen octahedra associated with the unit‐cell volume change and polarization rotation accounts for the stepwise antiferroelectric‐to‐ferroelectric phase transition. In particular, an unconventional ferroelectric category—the ferrodistortive phase characteristic of a unique cycloidal polarization order—is established during the dynamic structure investigation. Through clarifying the atomic‐scale phase transformation pathway, findings of this work unveil a new territory to explore novel ferrodistortive phases in energy‐storage materials with the nonpolar‐to‐polar phase transitions.

A disorder-driven metal insulator transition has been reported in crystalline Ge-Sb-Te phase-change materials (PCMs). The high concentration and statistical distribution of atomic vacancies were identified as the key factors in shaping the localization properties of electrons and, thus, the electrical transport. Vacancy ordering has been consistently observed in crystalline Ge-Sb-Te thin films upon thermal annealing, triggering a structural transition from a cubic rocksalt structure to a layered hexagonal structure and an insulator to metal transition. In this work, we demonstrate an opposite vacancy disordering process upon extensive electron beam irradiation, which is accompanied by the reverse transition from the stable hexagonal phase to the metastable cubic phase. The combined in situ transmission electron microscopy experiments and density functional theory nudged elastic band calculations reveal three transition stages, including (I) the vacancy diffusion in the hexagonal phase, (II) the change in atomic stacking, and (III) the disappearance of vacancy-rich planes. The mechanism of vacancy disordering is attributed to kinetic knock-on collision effects of the high-energy electron beam, which prevail over the heating effects.

Summary Spontaneous polarization is essential for ferroelectric functionality in non-centrosymmetric crystals. High-integration-density ferroelectric devices require the stabilization of polarization in small volumes. Here, atomic-resolution transmission electron microscopy imaging reveals that twinning gives rise to multiple polarization orders without symmetry breaking in colloidal nanocrystals of HfO2. The polarization orders are associated with sub-nanometer ferroelectric and antiferroelectric phases. The minimum size limit of the ferroelectric phase is found to be ∼4 nm3. Density functional theory calculations indicate that transformations between the ferroelectric and antiferroelectric phases are energetically possible. This work provides a route toward applications of HfO2 nanocrystals in information storage at densities that are more than an order of magnitude higher than the scaling limit defined by the nanocrystal size. Our results on the formation of twinning-induced polarization orders without symmetry breaking may provide general guidance for the discovery of new ferroelectric phases in ionic compounds that are not restricted to oxides.

Elemental phosphorus nanostructures are notorious for a large number of allotropes, which limits their usefulness as semiconductors. To limit this structural diversity, we synthesize selectively quasi-1D phosphorus nanostructures inside carbon nanotubes (CNTs) that act both as stable templates and nanoreactors. Whereas zigzag phosphorus nanoribbons form preferably in CNTs with an inner diameter exceeding 1.4 nm, a previously unknown square columnar structure of phosphorus is observed to form inside narrower nanotubes. Our findings are supported by electron microscopy and Raman spectroscopy observations as well as ab initio density functional theory calculations. Our computational results suggest that square columnar structures form preferably in CNTs with an inner diameter around 1.0 nm, whereas black phosphorus nanoribbons form preferably inside CNTs with a 4.1 nm inner diameter, with zigzag nanoribbons energetically favored over armchair nanoribbons. Our theoretical predictions agree with the experimental findings.

Microstructural properties of spinel LiFe5O8 (LFO) films grown on (001)-oriented SrTiO3 (STO) substrates have been investigated at the atomic-scale by advanced electron microscopy techniques. Two types of orientation relationship (OR) between the LFO films and the STO substrates have been determined, cube-on-cube and (111)[ 1 ¯ 10]LFO//(111)[1 1 ¯ 0]STO. Antiphase boundaries (APBs) and three types of twin boundaries (TBs) form within the LFO films, and the propagation of TBs and APBs results in their complex interactions. In most cases, interactions between TBs and APBs change the type of TBs and terminate the propagation of APBs since the APBs introduce a displacement vector of (a/4)〈110〉 into the TBs. In addition, the interactions between two coherent TBs are observed to generate the incoherent TB. The epitaxial strain of the LFO/STO (001) heterosystem can be released by the formation of TBs and APBs in the films and misfit dislocations at the interface. Considering that the magnetic coupling across the APBs and TBs can lead to novel physical properties, the appearance of APBs and TBs with a high density in the LFO films would affect the magnetic properties of the films.

Materials with layered crystal structures and high in-plane anisotropy, such as black phosphorus, present unique properties and thus promise for applications in electronic and photonic devices. Recently, the layered structures of GeS2 and GeSe2 were utilized for high-performance polarization-sensitive photodetection in the short wavelength region due to their high in-plane optical anisotropy and wide band gap. The highly complex, low-symmetric (monoclinic) crystal structures are at the origin of the high in-plane optical anisotropy, but the structural nature of the corresponding nanostructures remains to be fully understood. Here, we present an atomic-scale characterization of monoclinic GeS2 nanostructures and quantify the in-plane structural anisotropy at the sub-angstrom level in real space by Cs-corrected scanning transmission electron microscopy. We elucidate the origin of this high in-plane anisotropy in terms of ordered and disordered arrangement of [GeS4] tetrahedra in GeS2 monolayers, through density functional theory (DFT) calculations and orbital-based bonding analyses. We also demonstrate high in-plane mechanical, electronic, and optical anisotropies in monolayer GeS2 and envision phase transitions under uniaxial strain that could potentially be exploited for nonvolatile memory applications.

The excellent functionalities of perovskite oxides and the growing demands for flexible devices lead to great interests on epitaxial growth of functional oxide films on flexible mica substrates. Understanding the film epitaxy on the substrate with a very different crystal structure is a key issue for the optimization of the film growth and hence properties. Such understanding largely depends on knowing the atomic structure of the interfaces between the films and the substrates. Here, the interface between the epitaxial films of SrTiO3 on the fluorophlogopite mica substrate is studied in detail. Two types of interfaces, clean or with secondary phase, exist in this system, leading to two types of crystallographic orientation relationships. Atomic‐resolution scanning transmission electron microscopy images reveal that at the clean interface the (111) Sr–O3 atomic plane of SrTiO3 interacts with the (001) (SiAl)2–O3 plane of mica. This interface structure and thus the epitaxy of the film are understood in light of the strong similarity of the oxygen sublattices in these two atomic planes. First‐principles calculations demonstrate strong bonding of the atoms at the interface, which is also corroborated by the observation of misfit dislocations at the interfaces.

Antiferroelectric-based dielectric capacitors are receiving tremendous attention for their outstanding energy-storage performance and extraordinary flexibility in collecting pulsed powers. Nevertheless, the in situ atomic-scale structural-evolution pathway, inherently coupling to the energy storage process, has not been elucidated for the ultimate mechanistic understanding so far. Here, time- and atomic-resolution structural phase evolution in antiferroelectric PbZrO

Disorder plays an essential role in shaping the transport properties of GeSbTe phase-change materials (PCMs) to enable nonvolatile memory technology. Recently, increasing efforts have been undertaken to investigate disorder in the stable hexagonal phase of GeSbTe compounds, focusing on a special type of swapping bilayer defects. This configuration has been claimed to be the key element for a new mechanism for phase-change memory. Here, we report a direct atomic-scale chemical identification of these swapping bilayer defects in hexagonal GeSb2Te4 together with nanoscale atomic modeling and simulations. We identify the intermixing of Sb and Te in the bilayer to be the essential ingredient for the stability of the defects, and elucidate their abundance as due to the small energy cost. The bilayer defects are demonstrated to be ineffective in altering the electron localization nature that is relevant to transport properties of hexagonal GeSb2Te4. Our work paves the way for future studies of layer-switching dy...

Aberration-corrected scanning transmission electron microscopy was employed to investigate the microstructures and secondary phases in LaBaCo2O5.5 + δ (LBCO) thin films grown on SrTiO3 (STO) substrates. The as-grown films showed an epitaxial growth on the substrates with atomically sharp interfaces and orientation relationships of [100]LBCO//[100]STO and (001)LBCO//(001)STO. Secondary phases were observed in the films, which strongly depended on the sample fabrication conditions. In the film prepared at a temperature of 900 °C, nano-scale CoO pillars nucleated on the substrate, and grew along the [001] direction of the film. In the film grown at a temperature of 1000 °C, isolated nano-scale Co3O4 particles appeared, which promoted the growth of {111} twinning structures in the film. The orientation relationships and the interfaces between the secondary phases and the films were illustrated, and the growth mechanism of the film was discussed.