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The flexoelectric effect, which manifests itself as a strain-gradient-induced electrical polarization, has triggered great interest due to its ubiquitous existence in crystalline materials without the limitation of lattice symmetry. Here, we propose a flexoelectric photodetector based on a thin-film heterostructure. This prototypical device is demonstrated by epitaxial LaFeO3 thin films grown on LaAlO3 substrates. A giant strain gradient of the order of 106/m is achieved in LaFeO3 thin films, giving rise to an obvious flexoelectric polarization and generating a significant photovoltaic effect in the LaFeO3-based heterostructures with nanosecond response under light illumination. This work not only demonstrates a novel self-powered photodetector different from the traditional interface-type structures, such as the p–n and Schottky junctions but also opens an avenue to design practical flexoelectric devices for nanoelectronics applications.

A Bi2Te3/Bi2S3@Bi nanocomposite with a network microstructure was successfully synthesized via a hydrothermal method and spark plasma sintering. This composite was constructed from Bi2Te3 nanoparticles and Bi2S3@Bi nanowires, and its network structure is beneficial for obtaining excellent thermoelectric performance. A ZT peak of 1.2 at 450 K was realized for the nanocomposite sample.

Grain boundaries could exhibit exceptional electronic structure and exotic properties, which are determined by a local atomic configuration and stoichiometry that differs from the bulk. However, optical and plasmonic properties at the grain boundaries in metallic oxides have rarely been discussed before. Here, we show that non-stoichiometric grain boundaries in the newly discovered metallic SrNbO3 photocatalyst show exotic electronic, optical and plasmonic phenomena in comparison to bulk. Aberration-corrected scanning transmission electron microscopy and first-principles calculations reveal that a Nb-rich grain boundary exhibits an increased carrier concentration with quasi-1D metallic conductivity, and newly induced electronic states contributing to the broad energy range of optical absorption. More importantly, dielectric function calculations reveal extended and enhanced plasmonic excitations compared with bulk SrNbO3. Our results show that non-stoichiometric grain boundaries might be utilized to control the electronic and plasmonic properties in oxide photocatalysis.

Oxygen octahedral tilting has been recognized to strongly interact with spin, charge, orbital, and lattice degrees of freedom in perovskite oxides. Here, we observe a strain‐driven stripe‐like morphology of two supertetragonal (monoclinic Cc and Cm) phases in the strained BiFeO3/LaAlO3 thin films. The two supertetragonal phases have a similar giant axial ratio but differences in oxygen pyramid tilting mode. Especially, the competition between polar instability and oxygen pyramid tilting is identified using atomically resolved scanning transmission electron microscopy, leading to the polarization rotation across the phase boundary. In addition, microtwins are observed in the Cc phase. Our findings provide new insights of the coupling between ferroelectric polarization and oxygen pyramid tilting in oxide thin films and will help to design novel phase morphology with desirable ferroelectric polarization and properties for new applications in perovskite oxides.

The electrocaloric effect in ferroelectric materials has drawn much attention due to its potential applications in integrated circuit cooling and novel cooling devices. In contrast to the widely researched positive electrocaloric effect, the negative electrocaloric effect has received much less attention due to the lack of any effective methods for significant enhancement. In this work, we fabricated PbZrO3 thin film on a Pt/Si substrate by the sol-gel method. By controlling the interface conditions between the thin film and substrate, we induced defects into the interface and stabilized a transient ferroelectric phase in the PbZrO3 thin film. The emergence of the transient ferroelectric phase postpones the antiferroelectric-ferroelectric phase transition. As a result, a negative electrocaloric effect up to -18.5 K is estimated near room temperature, the highest one ever reported in this temperature range. This result suggests a new strategy to enhance the negative electrocaloric effect and may benefit the application of PbZrO3 thin films in cooling devices.

Modern Cs-corrected high-resolution transmission electron microscopy (HRTEM) has pushed the resolution limit to sub-angstrom scale and has made possible the quantitative analyses of local aperiodic atomic structures. After images have been obtained, a Wiener filter is often used to improve the signal-to-noise ratio, especially for those samples containing both crystal and large amorphous components. However, a Wiener filter may introduce distortions in the original experimental images. From this point of view, having a fundamental understanding of the effect of a Wiener filter on the accuracy of atomic displacement measurements in aperiodic structures is important. In this work, we first review the principle of the Wiener filter and theoretically discuss the origin of the distortions induced in aperiodic structures by using a Wiener filter. Then, using hypothetical experimental systems that contains both aperiodic crystal structures and amorphous layers, we carried out synthetic experiments to quantitatively estimate the effect of the Wiener filter on the measurements of aperiodic displacements. Compared with the values for a non-filtered image, the signal-to-noise ratio was significantly improved, and the accuracy of the displacement measurement was not decreased when proper Wiener filter parameters were used. Such results are of great importance for the processing of HRTEM images.

Magnetic measurement by transmitted electrons at nanometer or even atomic scale is always an attractive and challenging issue in the transmission electron microscope. Electron magnetic circular dichroism, proposed in 2003 and realized in 2006, opens a new insight into the measurement of local magnetic properties. Later, it is developed into a powerful technique for quantitative magnetic measurement with site specificity and element specificity at high spatial resolution over years of efforts, both in the aspect of theory and experiments. The novel technique has been widely applied to the characterization of magnetic materials now. This present review gives an overview of its development and applications in the past fifteen years since its invention. The theory of electron magnetic circular dichroism and its development are reviewed. The diffraction geometry and experimental setups are summarized. The general way for quantitative measurement of magnetic parameters is presented with typical cases. Representative breakthroughs in method development and applications over a wide range of materials are then described. Finally, prospects for future development are briefly discussed.

Efficient photocatalysis is important for sustainable energy. Recently, an unconventional photocatalyst based on intrinsic plasmon, called intrinsic plasmonic photocatalyst (IPP), seems promising for higher efficiency in hydrogen evolution. This catalyst seems to benefit from the advantages of visible light absorption, plasmon-assisted hot carrier generation, and good catalytic stability over conventional semiconductor photocatalysts. In this work, we report the relative hydrogen evolution efficiency under visible light irradiation of a family of IPP based on alkaline earth niobates (MNbO3, where M = Ca, Sr, or Ba), with efficiency of CaNbO3 > SrNbO3 > BaNbO3. The contributions of electron–phonon coupling time constant and solar energy absorption to the hydrogen evolution efficiency are identified as key based on our comprehensive study and characterization of carrier density (1022 cm–3), plasmon absorption, carrier dynamics, and surface area. This study demonstrates a generic approach to create a family of IPPs and further validates the role of solar energy absorption by intrinsic plasmon resonance in the enhancement of photocatalytic efficiency. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version The NUS researchers acknowledge Singapore National Research Foundation under its Competitive Research Funding ’Oxide Electronics on silicon beyond Moore’ (NRF-CRP15-2015-01). X.R.W. acknowledges the support from the Nanyang Assistant Professorship grant from Nanyang Technological University, Academic Research Fund Tier 1 (RG108/17 and RG177/18) from Singapore Ministry of Education. P.Y. is supported by SSLS via NUS Core Support (C-380-003-003-001). The authors thank the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. The laboratory is a National Research Infrastructure under the National Research Foundation (NRF) Singapore. The authors also thank for the support from Dr. Zhongxin Chen for the photocatalytic reaction measurements. The computational work for this article was partially performed on resources of the National Supercomputing Centre, Singapore (https://www.nscc.sg).

The adsorption of water molecules on tricalcium silicate (C3S), which influences the initial hydration of C3S, is still unclear at the atomistic level. In the present paper, density functional theory is employed to depict the adsorption of a single water molecule on seven low-index M3-C3S surfaces. The calculations show that both molecular and dissociative adsorption can occur on the C3S surfaces and that the latter mode is preferential. All of the ionic O atoms on the C3S surfaces can adsorb the H atoms from dissociated water molecules, while only two-coordinated covalent O atoms on the surfaces can form O H chemical bonds. The electronic structures of the ionic and two-coordinated covalent O atoms in the first atomic layer of the C3S surfaces show similar charge density localization of the valence band maximum (VBM), which can describe the variations in the reactivity of the ionic O atoms in the bulk or exposed on the surface slab. The partial density of states (PDOS) analysis shows that the formation of new Ca O bonds is mainly due to the overlap of O-2s and Ca-3p orbitals and O-2p and Ca-3d orbitals. Furthermore, the position of the OH group generated from the dissociated water molecule is found to significantly affect the adsorption energy. The general order of the adsorption energy in terms of the position of the OH group is Ehollow > Ebridge > Etop. The findings in this study provide additional support for the fundamental understanding of C3S hydration.

Bismuth and rare earth elements have been identified as effective substituent elements in the iron garnet structure, allowing an enhancement in magneto-optical response by several orders of magnitude in the visible and near-infrared region. Various mechanisms have been proposed to account for such enhancement, but testing of these ideas is hampered by a lack of suitable experimental data, where information is required not only regarding the lattice sites where substituent atoms are located but also how these atoms affect various order parameters. Here, we show for a Bi-substituted lutetium iron garnet how a suite of advanced electron microscopy techniques, combined with theoretical calculations, can be used to determine the interactions between a range of quantum-order parameters, including lattice, charge, spin, orbital, and crystal field splitting energy. In particular, we determine how the Bi distribution results in lattice distortions that are coupled with changes in electronic structure at certain lattice sites. These results reveal that these lattice distortions result in a decrease in the crystal-field splitting energies at Fe sites and in a lifted orbital degeneracy at octahedral sites, while the antiferromagnetic spin order remains preserved, thereby contributing to enhanced magneto-optical response in bismuth-substituted iron garnet. The combination of subangstrom imaging techniques and atomic-scale spectroscopy opens up possibilities for revealing insights into hidden coupling effects between multiple quantum-order parameters, thereby further guiding research and development for a wide range of complex functional materials.

Here we have developed an approach of three-dimensional (3D) measurement of magnetic moment vectors in three Cartesian directions using electron magnetic chiral dichroism (EMCD) at atomic scale. Utilizing a subangstrom convergent electron beam in the scanning transmission electron microscopy (STEM), beam-position-dependent chiral electron energy-loss spectra (EELS), carrying the EMCD signals referring to magnetization in three Cartesian directions, can be obtained during the scanning across the atomic planes. The atomic resolution EMCD signals from all of three directions can be separately obtained simply by moving the EELS detector. Moreover, the EMCD signals can be remarkably enhanced using a defocused electron beam, relieving the issues of low signal intensity and signal-to-noise-ratio especially at atomic resolution. Our proposed method is compatible with the setup of the widely used atomic resolution STEM-EELS technique and provides a straightforward way to achieve 3D magnetic measurement at atomic scale on newly developing magnetic-field-free TEM.

Li1.3Al0.3Ti1.7(PO4)3 (LATP) is a promising candidate as solid electrolyte and Li+ conductive component in the composite electrodes of all-solid-state Li-ion batteries. For both applications, reducing the sintering temperature of LATP while preserving its electrochemical properties is highly desired. This work is dedicated to reducing the sintering temperature of LATP from conventionally around 1000 °C to a low temperature of 775 °C with adding an extra 10 wt % of Li2CO3 to the precursors by a reactive sintering process. Comparative investigations with the stoichiometric LATP prepared by the same sintering method indicate that the combination effect of reactive sintering and Li2CO3-excess promotes the liquid phase sintering within LATP yielding a high relative density of 95.3%, whereas the stoichiometric LATP can only achieve a comparable relative density at 875 °C. Furthermore, the reactive sintering assisted Li2CO3-excess LATP exhibits a significantly higher ionic conductivity of 0.65 mS cm−1 at 25 °C and lower total activation energy of 0.334 eV compared with that of the stoichiometric LATP. Correlative studies on the microstructure and the separated specific grain/grain boundary conductivities for the two samples reveal that the improvement of Li+ conductivity for Li-excess LATP is attributed to its smaller total grain boundary thickness.

The hierarchical microstructures of high-entropy alloys (HEAs) can result in highly complex magnetic textures and properties. Here, we use high spatial resolution correlative magnetic, structural and chemical imaging to investigate magnetic textures in phase separated AlCoxCr1 – xFeNi (x = 0.5 and 1) HEAs. The AlCoFeNi HEA, which contains nm-sized A2 precipitates in a B2 matrix, supports large magnetic domains with small-angle magnetization variations. In contrast, the AlCo(Cr)FeNi HEA, which undergoes hierarchical phase separation, contains an unexpected distribution of magnetic vortices within individual A2 precipitates in a weakly ferromagnetic B2 host, in addition to weakly ferromagnetic or nonmagnetic B2 precipitates in large magnetic domains of the A2 phase, as well as Fe-Co-rich inter-phase A2 regions that have strong magnetization. The coercivity is attributed to a complicated magnetization reversal process, which includes the successive reversal of the magnetic vortices. These results provide important insight for the rational design of HEAs with unique and tailored magnetic properties.

Using interlayer interaction to control functional heterostructures with atomic-scale designs has become one of the most effective interface-engineering strategies nowadays. Here, we demonstrate the effect of a crystalline LaFeO3 buffer layer on amorphous and crystalline LaAlO3/SrTiO3 heterostructures. The LaFeO3 buffer layer acts as an energetically favored electron acceptor in both LaAlO3/SrTiO3 systems, resulting in modulation of interfacial carrier density and hence metal-to-insulator transition. For amorphous and crystalline LaAlO3/SrTiO3 heterostructures, the metal-to-insulator transition is found when the LaFeO3 layer thickness crosses 3 and 6 unit cells, respectively. Such different critical LaFeO3 thicknesses are explained in terms of distinct characteristic lengths of the redox-reaction-mediated and polar-catastrophe-dominated charge transfer, controlled by the interfacial atomic contact and Thomas-Fermi screening effect, respectively. Our results not only shed light on the complex interlayer charge transfer across oxide heterostructures but also provides a new route to precisely tailor the charge-transfer process at a functional interface., 40 Pages, 14 Figures, 1 Table

Electrical control of material properties based on ionic liquids (IL) has seen great development and emerging applications in the field of functional oxides, mainly understood by the electrostatic and electrochemical gating mechanisms. Compared to the fast, flexible, and reproducible electrostatic gating, electrochemical gating is less controllable owing to the complex behaviors of ion migration. Here, the interface‐dependent oxygen migration by electrochemical gating is resolved at the atomic scale in the LaAlO3–SrTiO3 system through ex situ IL gating experiments and on‐site atomic‐resolution characterization. The difference between interface structures leads to the controllable electrochemical oxygen migration by filling oxygen vacancies. The findings not only provide an atomic‐scale insight into the origin of interface‐dependent electrochemical gating but also demonstrate an effective way of engineering interface structure to control the electrochemical gating., The interface‐dependent oxygen migration by electrochemical gating is resolved at the atomic scale in the LaAlO3–SrTiO3 system through ex situ ionic liquid gating experiments and on‐site atomic‐resolution characterization. The difference between interface structures leads to the controllable electrochemical oxygen migration by filling oxygen vacancies.

Aberration-corrected scanning transmission electron microscopy (STEM) provides real space imaging and spectroscopy at atomic resolution with a new level of sensitivity to structure, bonding, elemental valence and even dynamics. It has developed into the most powerful characterization and even fabrication platform for all materials, especially for functional materials with complex structural features that dynamically respond to external fields. Several representative examples of functional materials are shown, including piezoelectric and photoelectric materials, functional oxide interfaces, metal–organic framework (MOF)-derived transition metal oxide/phosphide catalysts and two-dimensional (2D) materials. In piezoelectric systems, atomic-resolution polarization mapping by Z-contrast imaging shows the intimate coexistence of two ferroelectric phases inside nanodomains. Quantitative STEM imaging and EELS spectroscopy reveal the underlying mechanism of the exotic coupling between polarization and charge at a functional oxide interface. Under the low-voltage STEM, defects in 2D materials are clearly visible, for example novel edges and single atom dopants. The dynamic behavior of single atoms in vacancies of monolayer MoS2 is shown to highlight the electron-beam induced nanofabrication ability of aberration-corrected STEM. Thanks to the rapid development of segmented/pixelated detectors, differential phase contrast and 4D STEM techniques are promising for the near future, providing more opportunities to understand the link between structure and functionality.

The present work demonstrates ferroelectric to antiferroelectric transition aided electric field control of positive and negative electrocaloric effects (ECE). In this context, epitaxial PbZrO3 (PZO) thin films were grown on (111) Nb:SrTiO3 single crystal substrates by the sol–gel method, and the epitaxial growth was confirmed by atomic resolution scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) images. The structural and elemental analysis revealed a gradual increase of natural lead volatilization from the middle to the top in PZO thin films. The electrocaloric effect (ECE) was calculated using Maxwell equations corresponding to 10 kHz and 100 Hz P–E data and was found to be −10.5 K (negative ECE) at 375 K and 30 K (positive ECE) at 300 K. Intriguingly, both negative and positive ECE were found in the same phase transition sequence, that is, from an antiferroelectric phase to a ferroelectric phase. The frequency dependence of ECE is found to be dependent on the polarization switching dynamics and kinetics. These results are not only scientifically important for providing an easy way to combine the negative and positive ECE together to further enhance the cooling efficiency but are also technically important for future ECE devices.

Interface‐driven magnetic effects and phenomena associated with spin–orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO3/SrTiO3 (LMO/STO) polar–nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first‐principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version M.L. and C.T. contributed equally to this work. The authors thank Prof. J. M. D. Coey for discussion on the experimental results. This work is supported by the Lee Kuan Yew Postdoctoral Fellowship through a Singapore Ministry of Education Tier 1 (Grant R-284- 000-158-114). X.R.W. acknowledges support from the Nanyang Assistant Professorship grant from Nanyang Technological University and Academic Research Fund Tier 1 (RG108/17 and RG177/18) and Tier 3 (MOE2018-T3-1-002) from the Singapore Ministry of Education. S.J.P. acknowledges support from the National University of Singapore and the Ministry of Education under its Tier 2 Grant (MOE2017-T2-1-129). A.A. acknowledges the support from NUS Academic Research Fund (No. R-144-000-391-114 and No. R-144-000-403- 114) and the Singapore National Research Foundation (NRF) under the Competitive Research Programs (CRP Grant No. NRF-CRP15-2015-01).

Magnetic skyrmions are topologically stable vortex-like spin structures that are promising for next generation information storage applications. Materials that host magnetic skyrmions, such as MnSi and FeGe with the noncentrosymmetric cubic B20 crystal structure, have been shown to stabilize skyrmions upon nanostructuring. Here, we report a chemical vapor deposition method to selectively grow nanowires (NWs) of cubic FeGe out of three possible FeGe polymorphs for the first time using finely ground particles of cubic FeGe as seeds. X-ray diffraction and transmission electron microscopy (TEM) confirm that these micron-length NWs with ∼100 nm to 1 μm diameters have the cubic B20 crystal structure. Although Fe13Ge8 NWs are also formed, the two types of NWs can be readily differentiated by their faceting. Lorentz TEM imaging of the cubic FeGe NWs reveals a skyrmion lattice phase under small applied magnetic fields (∼0.1 T) at 233 K, a skyrmion chain state at lower temperatures (95 K) and under high magnetic fi...

Direct methanol fuel cells (DMFCs) have been recognized as a promising type of power sources to solve the energy shortage and environmental pollution from fossil energy consumption. However, their commercialization is still hindered by two major problems, the high cost of Pt-based catalysts and the crossover of methanol from anodes to cathodes. In the second problem, the methanol molecules can poison the Pt-based catalysts and lower the cell voltages by reacting with oxygen. Fe-based catalysts containing Fe–N–C active sites are well known as low-cost candidates that are promising to replace the Pt-based. But they cannot prevent the methanol molecules from reacting with oxygen. For the first time, we discovers a new enhancement of the electrocatalysis of Fe–N–C nanofiber catalysts induced by a synergy of methanol and oxygen-containing groups in the catalysts. Its mechanism is revealed by first-principles calculations of density functional theory and then proven experimentally. More significantly, the synergy-induced enhancement (SIE) is further improved experimentally by 40.8 times and reaches 21.60±0.05%. This indicates that the SIE has an enormous upside potential. Moreover, the methanol molecules in the SIE react with not oxygen but epoxy, reducing the harm of the reaction of methanol and oxygen in DMFCs. Further, the SIE has been employed in fuel cells and realized enhancement of current density by 3.0±0.5% in anion-exchange membrane DMFC and by 5.95±0.07% in H2–O2 anion exchange membrane fuel cell (AEMFC). Therefore, the new SIE can simultaneously solve both of the two problems and thus facilitate the DMFC commercialization for easing the crises of energy and pollution.

Niobium-based oxides have a wide range of applications owing to their rich crystal and electronic structures. Defects at the atomic scale are always unavoidable and will affect their functionalities, especially when in the form of thin films. Here, atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy have been performed on various defects (point, line, planar defects and segregated phases) in alkaline and alkaline-earth niobate thin films: CaZrO3 modified (K, Na)NbO3 and strontium niobate (SNO), respectively. In CaZrO3 modified (K,Na)NbO3 thin films, a tetragonal tungsten bronze phase was found, with a sharp boundary with the perovskite phase. In SNO thin films, several kinds of point defects and antiphase boundaries are commonly observed. In addition, a strongly Sr deficient phase, SrNb2O6, precipitates inside the SrNbO3 phase with a coherent interface. The different oxidation states of Nb in SrNbO3 and SrNb2O6 were revealed from the O K edge. Our characterization of the point defects and extended defects in niobate thin films offers practical guidelines for thin film deposition or discovery of defect-based novel functionalities.

Defects in crystalline materials have a tremendous impact on their functional behavior. Controlling and tuning of these imperfections can lead to marked improvements in their physical, electrical, magnetic, and optical properties. Thanks to the development of aberration-corrected (scanning) transmission electron microscopy (STEM/TEM), direct visualization of defects at multiple length scales has now become possible, including those critically important defects at the atomic scale. Thorough understanding of the nature and dynamics of these defects is the key to unraveling the fundamental origins of structure-property relationships. Such insight can therefore allow the creation of new materials with desired properties through appropriate defect engineering. Herein, several examples of new insights obtained from representative functional materials are shown, including piezoelectrics/ferroelectrics, oxide interfaces, thermoelectrics, electrocatalysts, and 2D materials.

Ag-modified In2O3/ZnO bundles with micro/nano porous structures have been designed and synthesized with by hydrothermal method continuing with dehydration process. Each bundle consists of nanoparticles, where nanogaps of 10–30 nm are present between the nanoparticles, leading to a porous structure. This porous structure brings high surface area and fast gas diffusion, enhancing the gas sensitivity. Consequently, the HCHO gas-sensing performance of the Ag-modified In2O3/ZnO bundles have been tested, with the formaldehyde-detection limit of 100 ppb (parts per billion) and the response and recover times as short as 6 s and 3 s, respectively, at 300 °C and the detection limit of 100 ppb, response time of 12 s and recover times of 6 s at 100 °C. The HCHO sensing detect limitation matches the health standard limitation on the concentration of formaldehyde for indoor air. Moreover, the strategy to synthesize the nanobundles is just two-step heating and easy to scale up. Therefore, the Ag-modified In2O3/ZnO bundles are ready for industrialization and practical applications.

n-ZnO nanofilm/p-Si micropillar heterostructure light-emitting diode (LED) arrays for white light emissions are achieved and the light emission intensity of LED array is enhanced by 120% under -0.05% compressive strains. These results indicate a promising approach to fabricate Si-based light-emitting components with high performances enhanced by the piezo-phototronic effect, with potential applications in touchpad technology, personalized signatures, smart skin, and silicon-based photonic integrated circuits.

Hierarchically porous indium oxide nanolamellas with two levels of nanopores have been designed and synthesized by low-temperature dehydration. Each nanolamella consists of nanoparticles, where nanogaps of 5–50 nm are present between the nanoparticles and concave nanopits of 3 nm exist in the surface of each nanoparticle. The nanopits bring negative curvatures to the surfaces of the nanoparticles, leading to a high density of atomic steps and then enhancing the surface activity. Consequently, the indium oxide nanolamellas have exhibited a formaldehyde-detection limit of 80 ppb (parts per billion) with response and recover times as short as 5 s and 1.3 s respectively. The 80-ppb detection limit is lower than the previously reported values from gas-sensing semiconductors and the health standard limitation on the concentration of formaldehyde in indoor air. The detection signal also has an excellent linearity with the formaldehyde concentration. Moreover, the strategy to synthesize the nanolamellas is just two-step heating and easy to scale up. Therefore, the hierarchically porous indium oxide nanolamellas are ready for industrialization and practical applications.

Materials with the morphotropic phase boundary (MPB) exhibit an ultrahigh mechanical response to electrical inputs, which has been widely used in applications such as sensors and actuators. Recently, the rare-earth element doped $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) was found to possess a MPB between a rhombohedral polar phase and an orthorhombic antipolar phase with enhanced piezoelectric response, enabling it to be an attractive alternative to toxic Pb-based piezoelectric materials. Despite theoretical and experimental efforts, the phase transition behavior under electric fields has not been directly confirmed, leaving a gap in the understanding of the origin of enhanced piezoelectricity. Here, we have demonstrated an irreversible electric-field induced phase transition from the antipolar phase to the polar phase in Sm-doped BFO with the pre-MPB composition, and a reversible phase transition between the polar phase and the antipolar/nonpolar phase in Sm-doped BFO with the MPB composition. In situ transmission electron microscopy technique combined with thermodynamic calculation based on the Ginzburg-Landau-Devonshire theory indicates that the electric-field induced reversible phase transition leads to enhanced piezoelectric response and double P-E hysteresis loops. These results provide us a deep insight into the mechanism of exotic electromechanical response in the rare-earth element doped BFO system with the composition near the MPB.

Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane "Equation missing" single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.

Epitaxially grown functional perovskites on silicon (001) and the ferroelectricity of a 3.2 nm thick BaTiO3 barrier layer are demonstrated. The polarization-switching-induced change in tunneling resistance is measured to be two orders of magnitude. The obtained results suggest the possibility of integrating ferroelectric tunnel junctions as binary data storage media in non-volatile memory cells on a silicon platform. ASTAR (Agency for Sci., Tech. and Research, S’pore)

The electrocaloric effect has drawn much attention due to its potential application in cooling devices. A negative electrocaloric effect is predicted to be induced in defect-doped ferroelectrics by computational results [A. Grunebohm and T. Nishimatsu, Phys. Rev. B 93, 134101 (2016) and Ma et al., Phys. Rev. B 94, 094113 (2016)], but it need to be confirmed by experimental results. In this work, we prepared a 1 mol. % Mn-doped Pb(Zr0.2,Ti0.8)O3 ceramics (Pb((Zr0.2,Ti0.8)0.99,Mn0.01)O3), and the electrocaloric effect of the defect-containing ferroelectric ceramics has been investigated by both direct and indirect methods. The indirect method shows a similar negative electrocaloric effect signal as the computational results predicted, while the direct method gives a positive electrocaloric effect. The absence of the negative electrocaloric effect obtained by the direct method may originate from: (a) the unavailability and the improper prediction of the Maxwell relation, (b) an improper assumption of fixed defects in the computational models, and (c) the offset of heat loss due to the application of a large electric field. In addition, we find a giant positive electrocaloric effect of 0.55 K at room temperature in the aged ceramics where no phase transition takes place. We attribute this abnormal electrocaloric effect to the restoration force of the defect dipoles. Our results not only provide insights into the origin of the negative electrocaloric effect, but also offer opportunities for the design of electrocaloric materials.

Interface coupling between complex oxides offers unique possibilities to tailor materials properties and stabilize novel ground states. Understanding the structural reconstruction of the corner-shared octahedral framework and the charge redistribution are crucial for controlling interfacial properties in oxide electronics. Here, we study the interfacial oxygen octahedral behavior in La0.7Sr0.3MnO3/SrTiO3 heterostructure, by directly imaging the oxygen octahedra at the atomic scale and extracting the structural parameters. We combine these experimental results with electronic structure calculations to elucidate the effect of reconstructed MnO6 octahedral geometry on increased interfacial magnetization and conductivity. The Mn valence profiles near the interface are quantitatively analyzed and compared at variant temperatures, revealing the insulating nature of interfacial manganite with reduced Mn valence. This study suggests a pathway to manipulate the interfacial properties and creation of new ground states in complex oxide heterostructures by tuning competing structural and electronic parameters.

Alloy 3J40 is a nickel-based alloy with high hardness and high elasticity. Alloy 3J40 was widely used as bears and conical pivots in many fields, such as, navigation, aviation and antiaircraft gun due to its excellent properties of elasticity, strength, hardness, corrosion resistance, nonmagnetic characteristics and so on. However, after aging treatment, the hardness of 3J40 alloy was uneven, there were large differences in hardness between the air-cooled and the furnace-cooling alloys, and the properties were unstable. Previous researches basely focused on the distribution, size, shape and other aspects of inclusions, to improve the plasticity and thermal processing properties of the alloy. Studies about properties and microstructure of the alloy devoted to the studies of the traditional Ni3Al-γ-phase. Up to now no public research report had been found about the α-Cr phase strengthening mechanism of the 3J40 alloy. In this paper, the effect of α-Cr phase on the strengthen mechanism in 3J40 alloy was investigated. By thermodynamic phase equilibrium calculation, the possible phases, which may precipitate after heat treatment, have been predicted. Relationships between the mechanical properties and the ageing heat treatment parameters, such as, ageing temperature, ageing time and cooling rate, had been studied. Microstructure of the alloy under different heat-treatment process has been analyzed, especially cared about the morphology, particles size and quantity of the α-Cr phase. And the strengthening mechanism was also investigated. The results showed that the microstructure of the aged 3J40 alloy were composed of γ-based phase, precipitated γ’ phase, (Ni3Al) phase, and α-Cr phase in granular and laminar. And the main contribution for the strengthen mechanism may be associated with the α-Cr phase laminar.

It is well known that the magnetic state of the La0.7Sr0.3MnO3 (LSMO) thin-film heterostructure is strongly correlated with the lattice, spin, orbital, and charge states, since these influence the electric and magnetic transport properties even on the unit-cell level. Therefore, understanding the material's magnetic properties on the nanoscale is important for the development of novel applications. The recently developed electron magnetic-circular dichroism (EMCD) technique allows the determination of atomic site-specific magnetic information via the use of transmitted electrons; however, its sensitivity is not high enough to quantitatively acquire magnetic information in many weak magnetism systems. Here, we utilized a dynamical diffraction-effect assisted EMCD technique to quantitatively determine the spin and orbital magnetic moment of LSMO/SrTiO3 thin films on the nanometer scale using a transmission electron microscope. Further, data processing was optimized to enhance the intensity of the EMCD signa...

Transport efficiency of pure spin current across the ferromagnetic films adjacent with a nonmagnetic metal is strongly dependent on the spin mixing conductance, which is very sensitive to atomic-level interface conditions. Here, by the means of advanced electron microscopy techniques, atomic structure, electronic structure, and magnetic properties at Y3Fe5O12 (YIG)/Pt interface are detailed characterized to correlate the microstructure and magnetic properties with interfacial transport properties. It is found that the order-disorder structure transformation at the interface is accompanied with oxygen deficiency, thus the reduced iron valence and the break of magnetic atom-O-magnetic atom bridges, which is responsible for superexchange interaction and magnetic order. It is also found that the magnetic moment of interfacial iron ions is decreased. The disorder interfacial layer with suppressed magnetism finally contributes to the declined spin transport efficiency. Our results provide the knowledge to control and manipulate the interfacial structure and properties in order to obtain higher spin transport efficiency.