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We report the control of the interplane magnetic exchange coupling in CaIrO3 perovskite thin films and superlattices with SrTiO3. By analyzing the anisotropic magneto-transport data, we demonstrate...

Transition metal dichalcogenides (TMDs) have become a playground for exploring rich physical phenomena like superconductivity and charge-density-waves (CDW). Here, we report the synthesis of the atom-thin TaSe2 with a rare 3R phase and enhanced superconductivity. The 3R phase is achieved by an ambient pressure chemical vapor deposition (CVD) strategy and confirmed by the high-resolution aberration-corrected STEM. Low-temperature transport data reveal an enhanced superconducting transition temperature (Tc) of 1.6 K in the 3R-TaSe2, which undoubtedly breaks the traditional perception of TaSe2 crystal as a material with Tc close to 0 K. This work demonstrates the strength of ambient pressure CVD in the exploration of crystal polymorphism, highlights a decisive role of layer stacking order in the superconducting transition, and provides fresh insights on manipulating crystal structures to gain access to enhanced Tc. Ministry of Education (MOE) Accepted version

Designing multifunctional catalysts with high activity, stability, and low-cost for energy storage and conversion is a significant challenge. Herein, a trifunctional electrocatalyst is synthesized by anchoring individually dispersed Co atoms on N and S codoped hollow carbon spheres (CoSA/N,S-HCS), which exhibits outstanding catalytic activity and stability for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction. When equipped in liquid or flexible solid-state rechargeable Zn–air batteries, CoSA/N,S-HCS endows them with high power and energy density as well as excellent long-term cycling stability, outperforming benchmark batteries based on a commercial Pt/C + RuO dual catalyst system. Furthermore, a self-driven water splitting system powered by flexible Zn–air batteries is demonstrated using CoSA/N,S-HCS as the sole catalyst, giving a high H2 evolution rate of 184 µmol/h. The state-of-art experimental characterizations and theoretical calculations reveal synergistic cooperation between atomically dispersed Co-N active sites, nearby electron-donating S dopants, and the unique carbon support to single-atom catalysts (SACs). This work demonstrates a general strategy to design various multifunctional SAC systems with a tailored coordination environment. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This work was supported by an AME-IRG grant (AMEIRG18-0016) from Agency for Science, Technology and Research (A*STAR) of Singapore and an AcRF tier 2 grant (MOE2017-T2-2-005) from Ministry of Education (Singapore).

© 2020 Wiley-VCH GmbH Self-assembled membranes with periodic wrinkled patterns are the critical building blocks of various flexible electronics, where the wrinkles are usually designed and fabricated to provide distinct functionalities. These membranes are typically metallic and organic materials with good ductility that are tolerant of complex deformation. However, the preparation of oxide membranes, especially those with intricate wrinkle patterns, is challenging due to their inherently strong covalent or ionic bonding, which usually leads to material crazing and brittle fracture. Here, wrinkle-patterned BaTiO3 (BTO)/poly(dimethylsiloxane) membranes with finely controlled parallel, zigzag, and mosaic patterns are prepared. The BTO layers show excellent flexibility and can form well-ordered and periodic wrinkles under compressive in-plane stress. Enhanced piezoelectricity is observed at the sites of peaks and valleys of the wrinkles where the largest strain gradient is generated. Atomistic simulations further reveal that the excellent elasticity and the correlated coupling between polarization and strain/strain gradient are strongly associated with ferroelectric domain switching and continuous dipole rotation. The out-of-plane polarization is primarily generated at compressive regions, while the in-plane polarization dominates at the tensile regions. The wrinkled ferroelectric oxides with differently strained regions and correlated polarization distributions would pave a way toward novel flexible electronics.

This paper reports a new {315}α″ type deformation twinning system, in adition to {130} α″ in a full α″ Ti-23Nb (at.%) alloy. The {315}α″ twins were identified by trace analyzes based on electron backscatter diffraction (EBSD) results, meanwhile ex-situ EBSD measurements showed that both {315}α″ and {130}α″ deformation twins transform into {332}β twins of the β parent phase after annealing at 573 K. The crystallographic correspondences between the α″ twinning systems and the {332}β twins have been analyzed theorectically, of which {315}α″/{332}β and {130}α″/{332}β correlated deformation twins were confirmed experimentally. The calculations of twinning shear, shuffle and Schmid factor were carried out to survey the priority of occurrence of these α″ twins. It is concluded that the remarkable work-hardening and enhanced plasticity arise from both {315}α″ and {130}α″ deformation twins in the present full α″ Ti-23Nb alloy.

Construction of a highly efficient and durable electrode plays the key dominant function in determining the performance of supercapacitors. Metal phosphides have been considered as a class of promi...

In this work, electrospinning N-doped carbon nanofibers containing iron carbide (Fe₃C@N-CFs) are synthesized and employed as the cathode in the flexible Al-air battery. Benefiting from the excellent catalytic activity of the iron carbide which is uniformly encapsulated in the N-doped carbon matrix, as well as the large specific surface area of the cross-linked network nanostructure, the as-prepared Fe₃C@N-CFs show outstanding catalytic activity and stability toward oxygen reduction reaction. The as-fabricated all-solid-state Al-air batteries with Fe₃C@N-CF catalyst show a stable discharge voltage (1.61 V) for 8 h, giving a capacity of 1287.3 mA h g⁻¹. Ministry of Education (MOE) Authors acknowledge the support of Ministry of Education, Singapore (MOE2016-T2-2-138), for research conducted at the National University of Singapore.

The oxygen evolution reaction (OER) is a pivotal half-reaction for next-generation energy storage and conversion technologies, for example, in metal–air batteries and water splitting. Herein, we re...

The ternary diamond-like compound Cu2SnSe3 is a potential thermoelectric material. Its Cu-Se conducting network leaves Sn as a likely doping site to optimize the carrier concentration without much deterioration to the carrier mobility. Thus, the precise determination on the intricate phase structures of Cu2SnSe3 is critical. Here, we first use the atomically resolved scanning transmission electron microscopy and reveal an unusual phase coexistence (monoclinic and orthorhombic phases) in Ag-doped Cu2SnSe3. Owing to coexisting phases and the order–disorder transition in the orthorhombic phase, the Ag-doped Cu2SnSe3 shows an unusual three-stage behavior in its temperature-dependent electrical transport properties and achieves record high power factors in this system. The observed three-stage behavior due to the phase coexistence is supported by the first principle calculations, Hall measurements, and optical diffuse reflectance measurements. To fully understand the band structures and the coherent interface between these coexisting phases, a band contact model is proposed that could well explain the three-stage electrical transport behavior. Moreover, the phase coexistence is observed at the nanoscale regime, thus providing a high density of phase boundaries. Such coexisting nanophases play an important role in lowering the lattice thermal conductivity. As a result, the ZT value obtained in Cu2Sn0.93Ag0.07Se3 is double that of undoped Cu2SnSe3.

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.

Large polarons have been of significant recent technological interest as they screen and protect electrons from point-scattering centers. Anatase TiO2 is a model system for studying large polarons as they can be studied systematically over a wide range of temperature and carrier density. The electronic and magneto transport properties of reduced anatase TiO2 epitaxial thin films are analyzed considering various polaronic effects. Unexpectedly, with increasing carrier concentration, the mobility increases, which rarely happens in common metallic systems. We find that the screening of the electron–phonon (e–ph) coupling by excess carriers is necessary to explain this unusual dependence. We also find that the magnetoresistance could be decomposed into a linear and a quadratic component, separately characterizing the carrier transport and trapping as a function of temperature, respectively. The various transport behaviors could be organized into a single phase diagram, which clarifies the evolution of large p...

Metal nitrides are widely recognized as a class of desirable supercapacitor electrode materials owing to their high electrical conductivity and structural stability. Embedding metal nanoparticles in nitrides can further enhance the conductivity for electron transport. Herein, a heterostructure consisting of Ni-doped Co–Co2N is synthesized by simple thermal annealing of metal–organic framework (MOF)-derived NiCo2O4 in ammonia atmosphere, in the process of which the MOF-derived two-dimensional (2D) nanoflake arrays were well retained, and the metal–metal nitride heterostructure was well established when annealed at 350 °C. Benefiting from the MOF-derived 2D nanoflake morphology and the metal-metal nitride heterostructure, the Ni-doped Co–Co2N delivers a specific capacity of 361.93 C/g. A full cell test has been conducted using Ni-doped Co–Co2N as the positive electrode and porous carbon as the negative electrode, and it shows an energy density of 20.4 Wh/kg at the power density of 9.85 kW/kg with 82.4% of i...

Ferroelectric thin film has attracted great interest for nonvolatile memory applications and can be used in either ferroelectric Schottky diodes or ferroelectric tunneling junctions due to its promise of fast switching speed, high on-to-off ratio, and nondestructive readout. Two-dimensional α-phase indium selenide (In2Se3), which has a modest band gap and robust ferroelectric properties stabilized by dipole locking, is an excellent candidate for multidirectional piezoelectric and switchable photodiode applications. However, the large-scale synthesis of this material is still elusive, and its performance as a ferroresistive memory junction is rarely reported. Here, we report the low-temperature molecular-beam epitaxy (MBE) of large-area monolayer α-In2Se3 on graphene and demonstrate the use of α-In2Se3 on graphene in ferroelectric Schottky diode junctions by employing high-work-function gold as the top electrode. The polarization-modulated Schottky barrier formed at the interface exhibits a giant electrore...

Interfacial magnetism at transition-metal oxide interfaces is of tremendous interest. The emergence of ferromagnetism and a matrix-dependent charge transfer between Co and Mn ions in the interdiffusion region of compressively strained LaMnO3–LaCoO3 superlattices (SLs) were studied. The SLs exhibit considerable ferromagnetism, which is negligible in both constituents. This is explained by the matrix-dependent charge transfer between Co and Mn ions. That is, the valence state of diffused Co ions into LaMnO3 shifts from 3+ to 2+ by adopting one electron from Mn3+, inducing the same amount of Mn4+, while the Mn ions that diffuses into LaCoO3 remains Mn3+. Matrix-dependent charge transfer effect was induced by the interplay between intrinsic and extrinsic effect in transition-metal oxides superlattices, which could be used to site-selectively modulate the electronic and magnetic states.

Exploration of cheap, efficient, and highly durable transition-metal-based electrocatalysts is critically important for the renewable energy chain, including both energy storage and energy conversi...

Controllable synthesis of single atom catalysts (SACs) with high loading remains challenging due to the aggregation tendency of metal atoms as the surface coverage increases. Here we report the synthesis of graphene supported cobalt SACs (Co1/G) with a tuneable high loading by atomic layer deposition. Ozone treatment of the graphene support not only eliminates the undesirable ligands of the pre-deposited metal precursors, but also regenerates active sites for the precise tuning of the density of Co atoms. The Co1/G SACs also demonstrate exceptional activity and high selectivity for the hydrogenation of nitroarenes to produce azoxy aromatic compounds, attributable to the formation of a coordinatively unsaturated and positively charged catalytically active center (Co–O–C) arising from the proximal-atom induced partial depletion of the 3d Co orbitals. Our findings pave the way for the precise engineering of the metal loading in a variety of SACs for superior catalytic activities., Controllable synthesis of single atom catalysts with sufficiently high metal loading remains challenging due to the tendency of agglomeration. Here the authors synthesize a series of stable atomically dispersed cobalt atoms on graphene with high Co loadings via the regeneration of active sites by atomic layer deposition.

Molybdenum diselenide (MoSe2) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe2 cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe2 on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe2 film can be grown with electron mobility ∼15 cm2/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe2 grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry.

Earth-abundant, efficient and durable electrocatalysts for water splitting are vitally important for a sustainable energy system. Here we report the rational design of hollow Mo-doped CoP (Mo-CoP) nanoarrays, which simultaneously combine electronic structure modification through doping with a high density of reaction sites through nanostructuring. With this strategy the Mo-CoP arrays give significantly improved hydrogen evolution reaction (HER) performance, and also, when in situ transformed into Mo-doped CoOOH (Mo-CoOOH) arrays, excellent activity towards the oxygen evolution reaction (OER) is obtained. The origin of the improvement is determined by atomic-resolution imaging combined with density functional theory (DFT). An electrolyzer using Mo-CoP and Mo-CoOOH can be powered by a single AA battery (~1.5 V), and maintains a stable water-splitting current for 20 h, superior to most reported electrocatalysts in alkaline media, offering great promise for practical applications.

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron–hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO3 film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron–hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems., An ambipolar ferromagnet with both electron- and hole-doped ferromagnetism in a single material would facilitate understanding of ferromagnetic semiconductors for spintronic applications. Here the authors demonstrate ambipolar ferromagnetism in LaMnO3, using ionic liquid gating enabled electrostatic doping to produce electron–hole asymmetry.

Dislocations in crystals naturally break the symmetry of the bulk, introducing local atomic configurations with symmetries such as fivefold rings. But dislocations do not usually nucleate aperiodic structure along their length. Here we demonstrate the formation of extended binary quasicrystalline precipitates with Penrose-like random-tiling structures, beginning with chemical ordering within the pentagonal structure at cores of prismatic dislocations in Mg–Zn alloys. Atomic resolution observations indicate that icosahedral chains centered along [0001] pillars of Zn interstitial atoms are formed templated by the fivefold rings at dislocation cores. They subsequently form columns of rhombic and elongated hexagonal tiles parallel to the dislocation lines. Quasicrystalline precipitates are formed by random tiling of these rhombic and hexagonal tiles. Such precipitation may impact dislocation glide and alloy strength., Quasicrystal precipitation in alloys should be favored at dislocations, which break crystal symmetry, but almost all precipitates at dislocations are periodic. Here, the authors show rare quasicrystal formation along dislocations in a magnesium zinc alloy with atomic resolution.

Single-layer molybdenum disulfide (MoS2) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS2 films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS2 on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO3, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin–orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline an...

Earth-abundant electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a wide pH range are highly desirable for sustainable energy conversion technologies, but challenging to develop. Herein, we report hollow CoP nanosphere-embedded carbon nanotube/nitrogen-doped carbon (NC-CNT/CoP) nanoarrays, in which a nanoscale Kirkendall effect generates few-layer graphene-coated hollow CoP nanospheres with abundant active sites. The integrated NC-CNT/CoP electrode behaves as an efficient pH-universal HER catalyst and, through in situ transformation, the derived materials show excellent OER performance. The NC-CNT/CoP-based electrolyzers achieve a current density of 10 mA cm−2 at low voltages of 1.63, 1.69, and 1.66 V in KOH, PBS, and H2SO4, respectively, which are similar to the values obtained using noble metal catalysts. Importantly, the integrated electrode exhibits superior stability than that of the benchmark noble metals in a wide pH range. This work presents a promising method for achieving nonprecious catalysts for efficient energy conversion.

This study concerns near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies of a Pt/A1203 diesel oxidation catalyst used in exhaust aftertreatment. We apply the technique to an ind ...

Water splitting provides a clean and renewable way to produce high-purity hydrogen; the current bottlenecks of which are the slow kinetics and poor stability of the available electrocatalysts. Herein, we report a class of open hollow Co–Pt bimetallic nanoclusters embedded in mesoporous N-doped carbon nanoflake arrays grown on flexible carbon cloth (Co–Pt/C NAs, 2.5 wt% Pt), which demonstrate outstanding electrocatalytic activity and ultra-long durability for both hydrogen and oxygen evolution in alkaline media (50 mV for hydrogen evolution and 320 mV for oxygen evolution, vs. RHE). The high performance arises from the unique nanostructure and the synergy of Co–Pt promoting water dissociation as demonstrated by density functional theory (DFT) calculation results. It can be directly utilized as a highly efficient bifunctional electrocatalyst for overall alkaline water splitting, and it outperforms noble-metal-based materials in terms of much lower operating voltage (1.54 V at 10 mA cm−2) and higher stability (no degradation at a constant current or voltage up to 120 h), representing a highly promising electrode for electrochemical energy conversion.

We show that self-assembly of protonated polymer chains and metal halide network gives rise to formation of two-dimensional (2D) hybrid perovskites, which incorporate protonated polyaniline as the conjugated organic cation component and PbI6 octahedra as the inorganic component. Single- and few-unit-cell-thick layers of these perovskites are obtained by mechanical exfoliation. Owing to the semiconducting behaviour of conjugated compounds, the atomically thin hybrid conjugated polymer perovskites reported here are expected to be considerably different from the quantum well systems based on unconjugated organic cation incorporating hybrid perovskites, in which the insulating organic layers act as potential barriers and the semiconducting inorganic layers act as potential wells. Besides, these materials are highly flexible in terms of chemical composition unlike conventional inorganic 2D materials, such as graphene, transition metal chalcogenides, transition metal oxides, boron nitride and black phosphorus. Different functional polymers/macromolecules could be self-assembled with a metal halide network in the same manner for the design of novel 2D hybrid perovskites for target applications. We also demonstrate that the conjugated polymer perovskites have tunable optical band gaps and they are highly stable against humidity.

High-performance organic photovoltaics (OPVs) are of great scientific and technological importance due to their potential large-scale industrial applications. Introducing semiconductor quantum dots has been proven to be an effective way to improve the power conversion efficiency (PCE) of OPVs. In this paper, we report a novel approach to fabricate atomically thin antimonene quantum sheets (AMQSs) possessing a uniform size (≈2.2 nm) via imidazolium ionic liquid-assisted exfoliation. In this method, the yield of AMQSs (1.1 mg mL−1) has been increased by nearly two orders of magnitude compared with that of previously reported methods. Furthermore, upon adding AMQSs into the light absorber in OPVs, the optimal device with 1.0 mg mL−1 AMQSs shows the highest PCE of 9.75%, resulting in over 25% enhancement in PCE compared to that of the reference device. It also leads to a noticeable enhancement in the short-circuit current density (Jsc) of 16.7% and the fill factor (FF) of 8.4%. The increased PCE is mainly due to the two-dimensional electronic structure of AMQSs that can enhance the light absorption, assist exciton dissociation and reduce charge recombination of OPVs. This work provides a new avenue toward mass production of two-dimensional quantum sheets and points to a new strategy for highly efficient OPVs.

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.

s In this work, we highlight the significances of Controlled Surface Sulfidation (CSS) for high-current-density hydrogen evolution reaction (HER) with bimetallic NiMoO4 microcuboids supported on Ni foam. It reveals that a new phase 1T’-MoS2 is derived in-situ from the surface NiMoO4 microcuboids during CSS in addition to pyrite-phase NiS2 and 2H-MoS2 as obtained from full sulfidation. We denote the controlled sulfided sample as “CSS-NiS2/MoS2” and the fully sulfided sample as “FS-NiS2/MoS2”, respectively. The CSS-NiS2/MoS2 electrode only required low overpotentials of 12, 47, 112 mV to drive -10, -20, -100 mA cm−2, respectively, which surpasses the FS-NiS2/MoS2 considerably (54, 90, 195 mV at -10, -20, -100 mA cm−2, respectively). Notably, it delivers a high current density of -500 and -1000 mA cm−2 at low overpotentials of only 200 and 264 mV, respectively. The durability of the high-current-density activity of CSS-NiS2/MoS2 is also proven over 50 h of stability test. The excellent performances of CSS-NiS2/MoS2 may be synergistically contributed from the active phases and the design dual hierarchical (i.e., 2D-nano/1D-microhybrids) structure. The CSS may serve as an effective strategy to modulate the electrochemical properties of materials holding great promises for the applications of next-generation energy storage and conversion.

The assembly of graphene-based materials into nanofibres is of intense technological interest for numerous applications ranging from tissue engineering and drug delivery to fuel cells and space elevators. We demonstrate a composite nanofibre synthesis process using functionalised graphene structures in liquid medium (two-dimensional [2D] electrolytes) as building blocks. The approach consists in simultaneous scrolling and reacting 2D electrolytes, leading to a dimensional reduction of 2D materials into one-dimensional nanostructures. The spontaneous self-assembly and cross-linking processes allow to produce nanofibres without the need of fibrillation techniques, such as wet-spinning or external templates.

The magnetic Weyl fermion originates from the time reversal symmetry (TRS)-breaking in magnetic crystalline structures, where the topology and magnetism entangle with each other. Therefore, the magnetic Weyl fermion is expected to be effectively tuned by the magnetic field and electrical field, which holds promise for future topologically protected electronics. However, the electrical field control of the magnetic Weyl fermion has rarely been reported, which is prevented by the limited number of identified magnetic Weyl solids. Here, the electric field control of the magnetic Weyl fermion is demonstrated in an epitaxial SrRuO3 (111) thin film. The magnetic Weyl fermion in the SrRuO3 films is indicated by the chiral anomaly induced magnetotransport, and is verified by the observed Weyl nodes in the electronic structures characterized by the angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through the ionic-liquid gating experiment, the effective manipulation of the Weyl fermion by electric field is demonstrated, in terms of the sign-change of the ordinary Hall effect, the nonmonotonic tuning of the anomalous Hall effect, and the observation of the linear magnetoresistance under proper gating voltages. The work may stimulate the searching and tuning of Weyl fermions in other magnetic materials, which are promising in energy-efficient electronics.

Atomically dispersed Pt species are advocated as a promising electrocatalyst for the oxygen reduction reaction (ORR) to boost noble metal utilization efficiency. However, when assembled on various substrates, isolated Pt single atoms are often demonstrated to proceed through the two‐electron ORR pathway due to the unfavorable O─O bond cleavage thermodynamics in the absence of catalytic ensemble sites. In addition, although their distinct local coordination environments at the exact single active sites are intensively explored, the interactions and synergy between closely neighboring single atom sites remain elusive. Herein, atomically dispersed Pt monomers strongly interacting on a Mo2C support is demonstrated as a model catalyst in the four‐electron ORR, and the beneficial interactions between two closely neighboring and yet non‐contiguous Pt single atom sites (named as quasi‐paired Pt single atoms) are shown. Compared to isolated Pt single atom sites, the quasi‐paired Pt single atoms deliver a superior mass activity of 0.224 A mg−1Pt and near‐100% selectivity toward four‐electron ORR due to the synergistic interaction from the two quasi‐paired Pt atom sites in modulating the binding mode of reaction intermediates. Our first‐principles calculations reveal a unique mechanism of such quasi‐paired configuration for promoting four‐electron ORR.

The catalytic and magnetic properties of molybdenum disulfide (MoS2) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS2 film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS2 films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS2 films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated ed...

High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ∼ 1.5 at 723 K and a record high average ZT > 1.0 at 300–873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu2Te inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ∼37 μW cm–1 K–2 at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ∼0.38 Wm–1 K–1 in PbTe-5.5%Cu2Te, approaching the theoretical minimum value of ∼0.36 Wm–1 K–1. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties.

Engineering ferromagnetism, by modulating its magnitude or anisotropy, is an important topic in the field of magnetism and spintronics. Among different types of magnetic materials, ferromagnetic insulators, in which magnetic moment unusually coexists with localized electrons, are of particular interest. Here, we report a remarkable interfacial enhancement of the ferromagnetism by adding one unit-cell LaAlO3 adjacent to an insulating LaMnO3 ultrathin film. The enhancement of ferromagnetism is explained in terms of charge transfer at the interface, as evidenced by X-ray absorption spectroscopy and ab initio calculations. This study demonstrates an effective and dramatic approach to modulate the functionality of ferromagnetic insulators, contributing to the arsenal of engineering techniques for future spintronics.

The two current reigning paradigms enabling nanotechnology are scanning probe microscopy and molecular machine devices that date back to seminal experiments by Eigler and visionary work by Drexler, respectively. The nanoscience and nanotechnology community is seeing the emergence of a third paradigm—the use of the atomically focused beam of a scanning transmission electron microscope (STEM) to control and direct matter on the atomic scale. Beam-induced modifications involving one atom or a small group of atoms can be induced and monitored in real time with atomic resolution. Combined with the development of beam-control electronics, big data acquisition, and analytical tools such as artificial intelligence-based feedback systems, electron and ion microscopies are at the brink of a transition from purely imaging tools to tools capable of creating structures with atomic precision and high throughput. In this issue of MRS Bulletin, we present recent advances in electron- and ion-beam-based atomic fabrication on surfaces, in layered materials, and finally in three dimensions—the ultimate dream and possibly the final frontier of nanoscience.

The correction of aberrations in the scanning transmission electron microscope (STEM) has simultaneously improved both spatial and temporal resolution, making it possible to capture the dynamics of single atoms inside materials, and resulting in new insights into the dynamic behavior of materials. In this article, we describe the different beam–matter interactions that lead to atomic excitations by transferring energy and momentum. We review recent examples of sequential STEM imaging to demonstrate the dynamic behavior of single atoms both within materials, at dislocations, at grain and interface boundaries, and on surfaces. We also discuss the effects of such dynamic behavior on material properties. We end with a summary of ongoing instrumental and algorithm developments that we anticipate will improve the temporal resolution significantly, allowing unprecedented insights into the dynamic behavior of materials at the atomic scale.

Metal phosphides are promising electrode materials for supercapacitors, owing to their conductive properties and high capacity. However, they are suffering from the short term cycling stability. Herein, an efficient sulfidisation strategy is successfully established to improve the overall electrochemical performance of metal phosphides. In detail, sulfur-doped CoP nanotube arrays aligned on carbon cloth are developed through a sulfidisation reaction process with CoP, which can provide significant improvement in electrochemical performance over that of bare CoP. The sulfur-doped CoP nanotube arrays not only show a capacity which is 1.78 times higher than that of CoP, but also demonstrate excellent cycle stability retaining 99% of the original capacity after 10,000 charge and discharge cycles, which is much better than CoP alone. Furthermore, a hybrid supercapacitor has been fabricated using sulfur-doped CoP as the positive electrode, which can deliver a maximum energy density of 39 W h/kg at a power density of 0.8 kW/kg. The full cell also shows excellent cycling stability by maintaining 86.4% of the original capacitance after 50,000 cycles, which is much better than previously reported metal phosphide-based supercapacitors.

In this chapter we will review a few examples of applications of atomic resolution aberration corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to complex oxide materials. These are most challenging systems where subtle changes in structure or chemistry may result in colossal responses in macroscopic physical behavior. Here, we will review how atomic resolution compositional mapping can be achieved in manganite thin films and single crystals, highlighting the importance of considering artifacts during quantification. Besides, minor changes in near edge fine structure may take place when the crystalline environment, and hence nearest neighbor configuration, is modified. These can also be tracked by atomic resolution EELS, as will be shown through the study of binary Fe oxides. Also, examples regarding the study of distributions of point defects such as O vacancies in cobaltite thin films will be discussed. In these materials, a combination of epitaxial strain and defects may promote physical behaviors not present in bulk, such as the stabilization of unexpected spin state superlattices. Last, a study of extended defects such as dislocation lines will be reviewed. In particular, we will show how chemical segregation at dislocation cores in yttria-stabilized zirconia grain boundaries results in the generation of static O vacancies that affect the local electrostatic potential and hence, the macroscopic ionic conduction properties.

Vacancy dynamics and ordering underpin the electrochemical functionality of complex oxides and strongly couple to their physical properties. In the field of the epitaxial thin films, where connection between chemistry and film properties can be most clearly revealed, the effects related to oxygen vacancies are attracting increasing attention. In this article, we report a direct, real-time, atomic level observation of the formation of oxygen vacancies in the epitaxial LaCoO3 thin films and heterostructures under the influence of the electron beam utilizing scanning transmission electron microscopy (STEM). In the case of LaCoO3/SrTiO3 superlattice, the formation of the oxygen vacancies is shown to produce quantifiable changes in the interatomic distances, as well as qualitative changes in the symmetry of the Co sites manifested as off-center displacements. The onset of these changes was observed in both the [100]pc and [110]pc orientations in real time. Additionally, annular bright field images directly show the formation of oxygen vacancy channels along [110]pc direction. In the case of 15 u.c. LaCoO3 thin film, we observe the sequence of events during beam-induced formation of oxygen vacancy ordered phases and find them consistent with similar processes in the bulk. Moreover, we record the dynamics of the nucleation, growth, and defect interaction at the atomic scale as these transformations happen. These results demonstrate that we can track dynamic oxygen vacancy behavior with STEM, generating atomic-level quantitative information on phase transformation and oxygen diffusion.

Lead-based piezoelectric materials are currently facing global restrictions due to their lead toxicity. Thus it is urgent to develop lead-free substitutes with high piezoelectricity and temperature stability, among which, potassium-sodium niobate [(K,Na)NbO3, KNN] has the most potential. It is very difficult to simultaneously achieve high piezoelectric performance and reliable stability in KNN-based systems. In particular, the structural/physical origin for their high piezoelectricity is still unclear, which hinders property optimization. Here we report the achievement of high temperature stability (less than 10% variation for electric field-induced strain from 27 °C to 80 °C), good fatigue properties (stable up to 106 cycles) as well as an enhanced piezoelectric coefficient (d33) of 525 pC N−1 in (1 − x)(K1−yNay)(Nb1−zSbz)O3–xBi0.5(Na1−wKw)0.5HfO3 (KNNS–BNKH) ceramics through manipulating the rhombohedral–tetragonal (R–T) phase boundary. The structural origin of their high piezoelectric performance can be attributed to a hierarchical nanodomain architecture, where the local structure inside nanodomains comprises R and T nanotwins. The physical origin can be attributed to low domain wall energy and nearly vanishing polarization anisotropy, facilitating easy polarization rotation among different states. We believe that the new breakthrough will open a window for the practical applications of KNN-based ceramics.

Owing to the lack of an intrinsic driving force for two-dimensional (2D) anisotropic growth of non-layered inorganic complexes, spontaneous direct growth into a lamellar structure is considered to be an effective way to obtain ultrathin/atomically thick nanosheets. Here, we demonstrate a fast chemical synthesis, in which non-layered nickel boron oxide (Ni–Bi) in situ forms ultrathin 2D nanosheet arrays vertically aligned on both sides of graphene support, constituting a trilayered sandwich heterostructure. The realization of such an open framework heterostructure circumvents the aggregation issue and thus maximizes the exposure of the active sites of the nanosheets. In addition, the direct integration of the eletrochemically active material on conductive support ensures fast charge transport in the thickness dimension along with high structural integrity and durability. Proof-of-concept catalysis performance reveals exceptional photo/electro-catalytic reactivity and stability of the 2D-on-2D layered hybrid. This work highlights the capability and ease of fabricating ultrathin 2D heterostructures that are potentially useful for a wide range of environment-related applications.

Nowadays, extensive endeavors have been made to explore the potential thermoelectric performance of SnTe through either enhancing the power factor or reducing its thermal conductivity. Herein, we report that both the power factor and thermal conductivity of SnTe can be simultaneously improved through introducing its analogues (PbTe, PbSe, PbS, SnSe, and SnS) into the SnTe matrix. We found that the power factor of SnTe could be greatly enhanced over the whole temperature range through modifying its band structure via alloying with its analogues, which is supported by density functional calculations. Meanwhile when the solubility limit of PbTe is exceeded, aberration-corrected scanning transmission electron microscopy observations reveal a hierarchical architecture with multi-scale structural defects, resulting in a very low thermal conductivity through scattering phonons of a wide range of wavelength. The combination of simultaneously enhanced power factor and depressed thermal conductivity leads to a maximum ZT ∼ 1.1 at 900 K and a high average ZT ∼ 0.52 at 300–900 K for Sn0.97Bi0.03Te–3% PbTe, generating a high conversion efficiency of ∼10%. The present results indicate that SnTe is a robust candidate for medium-temperature thermoelectric power generation.

Self-supported hollow nanoarrays with hierarchical pores and rich reaction sites are promising for advanced electrocatalysis. Herein, we report a rational design of novel CoS2 nanotube arrays assembled on a flexible support which can be directly utilized as an efficient bifunctional electrocatalyst for overall water splitting. Uniform wire-like metal–organic framework (MOF) nanoarrays were first fabricated and a sulfidation process by thermal treatment was carried out to transform the MOF arrays into CoS2 nanotube arrays. The unique hollow CoS2 tubular arrays are shown to provide high surface area for an efficient electrochemical reaction, and the well-defined electrical/mechanical connection to the substrate enhances both mass and electron transfer. The CoS2 nanotube arrays exhibited a high electrochemical activity in catalyzing both oxygen and hydrogen evolution reactions, in terms of low onset potential, high current density and excellent stability. Using the CoS2 nanotube arrays as catalysts, a water-splitting current density of 10 mA cm−2 in alkaline solution is achieved with a cell voltage of 1.67 V, and the stable current can be maintained for 20 h even when the electrode is in a bent state.

A more accurate solution thermodynamic analysis of colloidal molybdenum disulfide (MoS2) nanosheets dispersed in 91 combinations of organic solvent (alcohols and amines)/water mixtures is made poss...

Unprotected E-hydrazone esters are prized building blocks for the preparation of 1H-indazoles and countless other N-containing biologically active molecules. Despite previous advances, efficient and stereoselective synthesis of these compounds remains nontrivial. Here, we show that Pt single atoms anchored on defect-rich CeO2 nanorods (Pt1/CeO2), in conjunction with the alcoholysis of ammonia borane, promotes exceptionally E-selective hydrogenation of α-diazoesters to afford a wide assortment of N-H hydrazone esters with an overall turnover frequency of up to 566 hours-1 upon reaction completion. The α-diazoester substrates could be generated in situ from readily available carboxylic esters in one-pot hydrogenation reaction. Utility is demonstrated through concise, scalable synthesis of 1H-indazole-derived pharmaceuticals and their 15N-labeled analogs. The present protocol highlights a key mechanistic nuance wherein simultaneous coordination of a Pt site with the diazo N═N and ester carbonyl motifs plays a central role in controlling stereoselectivity, which is supported by density functional theory calculations. National Research Foundation (NRF) Published version

Dislocations are always observed during crystal growth, and it is usually desirable to reduce the dislocation density in high-quality crystals. Here, the annihilation process of the 30 degrees Shockley partial dislocation pairs in CdTe is studied by first-principles calculations. We found that the dislocations can glide relatively easily due to the weak local bonding. Our systematic study of the slipping mechanism of the dislocations suggests that the energy barrier for the annihilation process is low. Band structure calculations reveal that the band bending caused by the charge transfer between the two dislocation cores depends on the core-core distance. A simple linear model is proposed to describe the mechanism of formation of the dislocation pair. More importantly, we demonstrate that hole injection can affect the core structure, increase the mobility, and eventually trigger a spontaneous mutual annihilation, which could be employed as a possible facile way to reduce the dislocation density.

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

We report enhanced thermoelectric performance of SnTe by further increasing its intrinsic high carrier concentration caused by Sn vacancies in contrast to the traditional method. Along with In(2)Te(3) alloying, which results in an enhanced Seebeck coefficient, Li(2)Te is added to further increase the carrier concentration in order to maintain high electrical conductivity. Finally, a relatively high PF(ave) of ∼28 μW cm(−1) K(−2) in the range between 300 and 873 K is obtained in an optimized SnTe-based compound. Furthermore, nanoprecipitates with extremely high density are constructed to scatter phonons strongly, resulting in an ultralow lattice thermal conductivity of ∼0.45 W m(−1) K(−1) at 873 K. Given that the Z value is temperature dependent, the (ZT)(eng) and (PF)(eng) values are adopted to accurately predict the performance of this material. Taking into account the Joule and Thomson heat, output power density of ∼5.53 W cm(−2) and leg efficiency of ∼9.6% are calculated for (SnTe)(2.94)(In(2)Te(3))(0.02)-(Li(2)Te)(0.045) with a leg length of 4 mm and cold- and hot-side temperatures of 300 and 870 K, respectively.

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 PbZrO