Defect engineering in perovskite oxides enables novel functionalities by inducing and controlling lattice defects, effectively breaking lattice symmetry, stabilizing polar states and inducing ferroelectricity in non-polar/paraelectric or ferro/antiferromagnetic oxide thin films. A polar state can be stabilized in SrMnO3 (SMO) thin films by displacing Mn ions. However, conventional epitaxial strain engineering necessitates deposition on diverse single crystalline substrates with varying misfit strains and optimization of thin film growth conditions, posing challenges in achieving polar states in SMO thin films. In this study, polar distortion was achieved by inducing defects in SMO epitaxial thin films grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 substrates La0.7Sr0.3MnO3 (LSMO) electrode. Scanning transmission electron microscopy analysis revealed that Mn ion displacement and the c/a ratio increased on moving from the SMO/LSMO interface to the top surface of the SMO film. Electron energy loss spectroscopy depth profiles revealed variations in oxygen stoichiometry and Mn3+/Mn4+ ratio across the cross section of the SMO film. Consequently, a polar state was stabilized through strain gradient induced by defect chemistry in SMO thin films. Our study demonstrates that defect engineering can be effectively utilized in the realization of electric field-controlled magnetic devices at room temperature.

Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here we investigate the impact of dimensionality of epitaxially grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designing oxide superlattices with a correlated ferromagnetic SrRuO3 and nonmagnetic SrTiO3 layers, we observed modulated ferromagnetic behavior with the change of the SrRuO3 thickness. Especially, for three-unit-cell-thick layers, we observe a significant 1500% improvement of the coercive field in the anomalous Hall effect, which cannot be solely attributed to the dimensional crossover in ferromagnetism. The atomic-scale heterostructures further reveal the systematic modulation of anisotropy for the lattice structure and orbital hybridization, explaining the enhanced magnetic anisotropy. Our findings provide valuable insights into engineering the anisotropic hybridization of synthetic magnetic crystals, offering a tunable spin order for various applications.

The functionalities and diverse metastable phases of multiferroic BiFeO 3 （BFO） thin films depend on the misfit strain.Although mixed phase-induced strain relaxation in multiphase BFO thin films is well known,it is unclear whether a singlecrystalline BFO thin film can accommodate misfit strain without the involvement of its polymorphs.Thus,understanding the strain relaxation behavior is key to elucidating the lattice strain-property relationship.In this study,a correlative strain analysis based on dark-field inline electron holography（DIH） and quantitative scanning transmission electron microscopy（STEM） was performed to reveal the structural mechanism for strain accommodation of a single-crystalline BFO thin film.The nanoscale DIH strain analysis results indicated a random combination of multiple strain states that acted as a primary strain relief,forming irregularly strained nanodomains.The STEM-based bond length measurement of the corresponding strained nanodomains revealed a unique strain accommodation behavior achieved by a statistical combination of multiple modes of distorted structures on the unit-cell scale.The globally integrated strain for each nanodomain was estimated to be close to-1.5%,irrespective of the nanoscale strain states,which was consistent with the fully strained BFO film on the SrTiO 3 substrate.Density functional theory calculations suggested that strain accommodation by the combination of metastable phases was energetically favored compared to single-phase-mediated relaxation.This discovery allows a comprehensive understanding of strain accommodation behavior in ferroelectric oxide films,such as BFO,with various low-symmetry polymorphs.

Polarity discontinuity across LaAlO3/SrTiO3 (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t c) and disappear simultaneously above t c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

Elemental doping is a universal strategy in controlling the functionalities of materials that strongly correlate with naturally formed atomic defects. Element-resolved chemical mapping for atomic defects has answered numerous problems on the relations between defective structures and properties. However, tracking small amounts of dopants in multicomponent bulks and clarifying their doping behaviors remain challenging. Using advanced X-ray spectroscopy, the excess Cu doping behavior in ternary Bi2Te2.7Se0.3 bulk alloy is visualized with the unprecedented detectability from sub-one atomic percent of concentration. The low content of 0.2 at.% Cu preferentially occupies the Bi site, while Cu atoms are found in three crystallographic sites of Bi2Te3 structure and van der Waals gap at high Cu content of 1.2 at.%. These behaviors explain the nontrivial role of Cu dopants on carrier generation processes and relevant thermoelectric properties of Bi2Te2.7Se0.3. The atomic-level identification should also stimulate the elucidation of diverse properties in doped nanomaterials and quantum phenomena in doped topological insulators. (C) 2021 Elsevier Ltd. All rights reserved.

Growth of two-dimensional van der Waals layered single-crystal (SC) films is highly desired to manifest intrinsic material sciences and unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) film has not been established to date. Here, we report the SC growth of TMdC monolayers in a centimeter scale via atomic sawtooth gold surface as a universal growth template. Atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission-electron-microscopy. Anisotropic adsorption energy of TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of Miller indices. Growth using atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2, WSe2, MoS2, MoSe2/WSe2 heterostructure, and W1-xMoxS2 alloy. Our strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures in a wafer scale, to further facilitate the applications of TMdCs in post silicon technology.

Herein, we report on a novel method for transferring two-dimensional (2D) materials grown on Au substrates using sulfur intercalation between the 2D materials and the Au surfaces. The strong nature of the S-Au bond allows intercalation of sulfur atoms into their interface, under a sulfur-rich atmosphere, at 600 degrees C. The relaxed interfacial interaction achieved via intercalation is carefully confirmed by recovering phonon mode and work function of tungsten disulfide (WS2) in Raman spectra and Kelvin probe force microscopy, and, more importantly, by observing the expansion of the interfacial distance, from 0.24 to 0.44 nm, using cross-sectional transmission electron microscopy. The released interactions facilitate delamination of WS2 from the Au surface, using an electrochemical bubbling method. The resultant Au foil then is reused for repeated WS2 growth. The successful transfer of other 2D materials, including molybdenum disulfide and hexagonal boron nitride, is also demonstrated. Our strategy advances the use of Au substrates for growing wafer-scale 2D monolayers.

To realize ultrafast and energy-efficient electronic devices, reducing the switching voltage slope for ON and OFF states that scales the supply voltage and device dimensions is critical. Novel device architectures based on two-dimensional (2D) materials have overcome the fundamental thermionic limit of the switching slope (60 mV/dec); however, a versatile switching device required for highly integrated memory and neuromorphic applications has not been achieved with such exceptional switching slope characteristics. Here, we demonstrate a switching voltage slope down to 0.62 mV/dec in a threshold switching device based on a vertical heterojunction of silver/hexagonal boron nitride (h-BN)/graphene. The sub-1 mV/dec switching slope for the first time, maintaining a high ON/OFF ratio (up to 1010), originates from the unique coupling between the migrated silver atoms and the chemically-inert graphene electrode through the 2D insulating h-BN. Moreover, our original switching device enables the evolution from a conventional volatile (threshold switching) to non-volatile memristive state by adequate voltage spikes, which is ideal for selector applications in highly integrated crossbar array architecture and in a novel synaptic device for neuromorphic computing.

Introduction of defect structures into Fe-doped TiO2 nanoparticles (Fe@TiO2 NPs) has been shown to endow NPs with improved photocatalytic properties. However, current strategies for the preparation of defect-containing NPs require high temperature or complicated treatments, which can induce unwanted phase transition. In this paper, we report a facile method to introduce surface oxygen vacancies into anatase-type Fe@TiO2 NPs without altering the crystalline phase via simple pH treatments at moderate temperatures. Furthermore, we present the effects of pH on the formation of surface oxygen vacancies. The optimized treatment under basic conditions is revealed to promote the formation of oxygen vacancies on the surface of anatase Fe@TiO2 NPs and effectively reduces the particle size by more than 25%, thereby causing a significant enhancement in the photocatalytic activities of the NPs (e.g. similar to 3.5 times better in photocatalytic degradation rate of 4-CP as compared to acid-treated Fe@TiO2). Comprehensive structural and chemical characterizations reveal that the point defects are predominantly formed on the surface of anatase NPs, and their population can be maximized by use of basic pH conditions. Our results pave a way toward the facile and efficient engineering of surface defect structures on catalytic metal oxide NPs for the design of high-performance photocatalysts.

A Correction to this paper has been published: https://doi.org/10.1038/s41565-020-00821-z.

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

Electrides have emerged as promising materials with exotic properties, such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride ([Hf2S]2+∙2e-) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf2S layer on the cleaved [Hf2S]2+∙2e- surface reacts with oxygen to form an outermost amorphous HfO2 layer with ~10-nm thickness, passivating the [Hf2S]2+∙2e- electride. The excess electrons in the [Hf2S]2+∙2e- electride are transferred through the thin HfO2 passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.

An effective approach to alleviate the volume expansion of alloying material and magnify the capacity of sodium-ions batteries anode by anchoring the SnS nanoparticles densely on porous carbon nanotubes film.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe2/SnSe2 heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe2/SnSe2 heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm2, 3-4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe2 contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe2 contact in the WSe2/SnSe2 device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.

Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Tii-xHfxNiSni-ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects. (C) 2019 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

The large-scale crossbar array is a promising architecture for hardware-amenable energy efficient three-dimensional memory and neuromorphic computing systems. While accessing a memory cell with negligible sneak currents remains a fundamental issue in the crossbar array architecture, up-to-date memory cells for large-scale crossbar arrays suffer from process and device integration (one selector one resistor) or destructive read operation (complementary resistive switching). Here, we introduce a self-selective memory cell based on hexagonal boron nitride and graphene in a vertical heterostructure. Combining nonvolatile and volatile memory operations in the two hexagonal boron nitride layers, we demonstrate a self-selectivity of 10(10) with an on/off resistance ratio larger than 10(3). The graphene layer efficiently blocks the diffusion of volatile silver filaments to integrate the volatile and non-volatile kinetics in a novel way. Our self-selective memory minimizes sneak currents on large-scale memory operation, thereby achieving a practical readout margin for terabit-scale and energy-efficient memory integration.

Journal Article Atomic and Electronic Reconstruction at the a-LAO/STO Interface by E-Beam Induced Crystallization Get access Gwangyeob Lee, Gwangyeob Lee Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, South KoreaDepartment of Materials Science and Engineering, Yonsei University, Seoul, South Korea Search for other works by this author on: Oxford Academic Google Scholar Shin-ik Kim, Shin-ik Kim Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul, South Korea Search for other works by this author on: Oxford Academic Google Scholar Gyeongtak Han, Gyeongtak Han Department of Energy Science, Sungkyunkwan University, Suwon, South Korea Search for other works by this author on: Oxford Academic Google Scholar Young-Min Kim, Young-Min Kim Department of Energy Science, Sungkyunkwan University, Suwon, South KoreaIBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Suwon, South Korea Search for other works by this author on: Oxford Academic Google Scholar Seung-Hyub Baek, Seung-Hyub Baek Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul, South Korea Search for other works by this author on: Oxford Academic Google Scholar Do Hyang Kim, Do Hyang Kim Department of Materials Science and Engineering, Yonsei University, Seoul, South Korea Search for other works by this author on: Oxford Academic Google Scholar Hye Jung Chang Hye Jung Chang Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, South KoreaDivision of Nano & Information Technology, KIST School, University of Science and Technology, Seoul, South Korea Corresponding author: almacore@kist.re.kr Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1894–1895, https://doi.org/10.1017/S1431927619010201 Published: 01 August 2019

Two-dimensional (2D) van der Waals (vdW) heterostructures exhibit novel physical and chemical properties, allowing the development of unprecedented electronic, optical, and electrochemical devices. However, the construction of wafer-scale vdW heterostructures for practical applications is still limited due to the lack of well-established growth and transfer techniques. Herein, we report a method for the fabrication of wafer-scale 2D vdW heterostructures with an ultraclean interface between layers via the aid of a freestanding viscoelastic polymer support layer (VEPSL). The low glass transition temperature ( Tg) and viscoelastic nature of the VEPSL ensure absolute conformal contact between 2D layers, enabling the easy pick-up of layers and attaching to other 2D layers. This eventually leads to the construction of random sequence 2D vdW heterostructures such as molybdenum disulfide/tungsten disulfide/molybdenum diselenide/tungsten diselenide/hexagonal boron nitride. Furthermore, the VEPSL allows the conformal transfer of 2D vdW heterostructures onto arbitrary substrates, irrespective of surface roughness. To demonstrate the significance of the ultraclean interface, the fabricated molybdenum disulfide/graphene heterostructure employed as an electrocatalyst yielded excellent results of 73.1 mV·dec-1 for the Tafel slope and 0.12 kΩ of charge transfer resistance, which are almost twice as low as that of the impurity-trapped heterostructure.