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Magnetoelectric (ME) materials have been attracting extensive attention due to their potential applications. It is important to realize strong ME effect in one single material at sufficiently high temperature, and thus substantial efforts in searching for ME materials with strong ME coupling together and a comprehensive understanding of the underlying mechanism is necessary. Herein we focus on the Mn-doped linear ME material Co4-xMnxNb2O9, in which a large electric polarization driven by magnetic field emerges below its Neel temperature TN, by presenting the detailed characterizations on the structure, magnetism, and ME effect. It is revealed that an increasing Mn substitution x allows largely enhanced TN and gradually suppressed ME coefficient αME. This ME coupling suppression is attributed to the weak spin-orbit interaction of Mn2+ with respect to Co2+. It is indicated that the Dzyaloshinskii-Moriya (DM) interaction plays an important role in the ME coupling. The measured largest αME is found in For Co0.8Mn3.2Nb2O9, reaching up to 9.9 ps/m at 10 K while the TN reaches up to 102 K, exceeding most of the linear ME materials that have been reported.

Ferroelectric tunnel junction (FTJ) has attracted considerable attention for its potential applications in nonvolatile memory and neuromorphic computing. However, the experimental exploration of FTJs with high ON/OFF ratios is a challenging task due to the vast search space comprising of ferroelectric and electrode materials, fabrication methods and conditions and so on. Here, machine learning (ML) is demonstrated to be an effective tool to guide the experimental search of FTJs with high ON/OFF ratios. A dataset consisting of 152 FTJ samples with nine features and one target attribute (i.e., ON/OFF ratio) is established for ML modeling. Among various ML models, the gradient boosting classification model achieves the highest prediction accuracy. Combining the feature importance analysis based on this model with the association rule mining, it is extracted that the utilizations of {graphene/graphite (Gra) (top), LaNiO3 (LNO) (bottom)} and {Gra (top), Ca[Formula: see text]Ce[Formula: see text]MnO3 (CCMO) (bottom)} electrode pairs are likely to result in high ON/OFF ratios in FTJs. Moreover, two previously unexplored FTJs: Gra/BaTiO3 (BTO)/LNO and Gra/BTO/CCMO, are predicted to achieve ON/OFF ratios higher than 1000. Guided by the ML predictions, the Gra/BTO/LNO and Gra/BTO/CCMO FTJs are experimentally fabricated, which unsurprisingly exhibit [Formula: see text]1000 ON/OFF ratios ([Formula: see text]8540 and [Formula: see text]7890, respectively). This study demonstrates a new paradigm of developing high-performance FTJs by using ML.

Solid solution 0.94Na0.5Bi0.5TiO3–6BaTiO3 (NBT–6BT) is considered to be one kind of lead‐free piezoelectric materials with excellent electrical properties due to the existence of morphotropic phase boundary (MPB). However, its relatively lower depolarization temperature is a long‐standing bottleneck for the application of NBT‐based piezoelectric ceramics. In this work, the influence of thermal quenching on depolarization temperature and electrical properties of rare‐earth Ho‐doped NBT–6BT lead‐free ceramics was investigated. It was shown that the relative high piezoelectric performance, as well as an improvement of depolarization temperature (Td), can be realized by thermal quenching. The results showed that the quenching process induced high concentration of oxygen vacancy, giving rise to the change of octahedra mode and enhanced lattice distortion, which is benefit to the temperature stability of piezoelectric and ferroelectric properties. Furthermore, up‐conversion photoluminescence (PL) of Ho‐doped NBT–6BT could be effectively tuned by the introduction of oxygen vacancy, suggesting a promising potential in optical–electrical multifunctional devices.

Ferroelectric topological objects (e.g. vortices, skyrmions) provide a fertile ground for exploring emerging physical properties that could potentially be utilized in future configurable nanoelectronic devices. Here, we demonstrate quasi-one-dimensional metallic high conduction channels along two types of exotic topological defects, i.e. the topological cores of (i) a quadrant vortex domain structure and (ii) a center domain (monopole-like) structure confined in high quality BiFeO3 nanoisland array, abbreviated as the vortex core and the center core. We unveil via phase-field simulations that the superfine (< 3 nm) metallic conduction channels along center cores arise from the screening charge carriers confined at the core whereas the high conductance of vortex cores results from a field-induced twisted state. These conducting channels can be repeatedly and reversibly created and deleted by manipulating the two topological states via an electric field, leading to an apparent electroresistance effect with an on/off ratio higher than 103. These results open up the possibility of utilizing these functional one-dimensional topological objects in high-density nanoelectronic devices such as ultrahigh density nonvolatile memory., manuscript (21 pages, 4 figures) plus supplementary information

Electrets are dielectric materials that have a quasi-permanent dipole polarization. A single-molecule electret is a long-sought-after nanoscale component because it can lead to miniaturized non-volatile memory storage devices. The signature of a single-molecule electret is the switching between two electric dipole states by an external electric field. The existence of these electrets has remained controversial because of the poor electric dipole stability in single molecules. Here we report the observation of a gate-controlled switching between two electronic states in Gd@C82. The encapsulated Gd atom forms a charged centre that sets up two single-electron transport channels. A gate voltage of ±11 V (corresponding to a coercive field of ~50 mV A–1) switches the system between the two transport channels with a ferroelectricity-like hysteresis loop. Using density functional theory, we assign the two states to two different permanent electrical dipole orientations generated from the Gd atom being trapped at two different sites inside the C82 cage. The two dipole states are separated by a transition energy barrier of 11 meV. The conductance switching is then attributed to the electric-field-driven reorientation of the individual dipole, as the coercive field provides the necessary energy to overcome the transition barrier. A Gd@C82 molecule shows electric polarization switching behaviour under a gate bias voltage, thus demonstrating a single-molecule electret device.

Electrical manipulation of skyrmions attracts considerable attention for its rich physics and promising applications. To date, such a manipulation is realized mainly via spin-polarized current based on spin-transfer torque or spin-orbital torque effect. However, this scheme is energy-consuming and may produce massive Joule heating. To reduce energy dissipation and risk of heightened temperatures of skyrmion-based devices, an effective solution is to use electric field instead of current as stimulus. Here, we realize an electric-field manipulation of skyrmions in a nanostructured ferromagnetic/ferroelectrical heterostructure at room temperature via an inverse magneto-mechanical effect. Intriguingly, such a manipulation is non-volatile and exhibits a multi-state feature. Numerical simulations indicate that the electric-field manipulation of skyrmions originates from strain-mediated modification of effective magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Our results open a direction for constructing low-energy-dissipation, non-volatile, and multi-state skyrmion-based spintronic devices., Accepted by Nature Communications 11, 3577 (2020)

The magnetic properties of the spin-5/2 double molybdate LiFe(MoO$_4$)$_2$ have been characterized by heat capacity, magnetic susceptibility, and neutron powder diffraction techniques. Unlike the multiferroic system LiFe(MoO$_4$)$_2$ which exhibits two successive magnetic transitions, LiFe(MoO$_4$)$_2$ undergoes only one antiferromagnetic transition at $T_N$ ~ 23.8 K. Its antiferromagnetic magnetic structure with the commensurate propagation vector k = (0, 0.5, 0) has been determined. Density functional theory calculations confirm the antiferromagnetic ground state and provide a numerical estimate of the relevant exchange coupling constants., Comment: 24 pages, 8 figures

Multiferroics are rare in nature due to the mutual exclusive origins of magnetism and ferroelectricity. The simultaneous coexistence of robust magnetism/ferroelectricity and strong magnetoelectric coupling in single multiferroics is hitherto unreported, which may also be attributed to their potential conflictions. In this paper, we show the first-principles evidence of such desired coexistence in ultrathin-layer CuCrS2 and CuCrSe2. The vertical ferroelectricity is neither induced by an empty d shell nor spin-driven, giving rise to an alternative possibility of resolving those intrinsic exclusions and contradictions. Compared with their bulk phases, the ferromagnetism in the thin-layer structures (two–six layers) can be greatly stabilized due to the enhanced carrier density and orbital shifting by vertical polarization, and the Curie temperatures of both ferromagnetism and ferroelectricity can be above room temperature. Moreover, a considerable net magnetization can be reversed upon ferroelectric switching, where the change in spin-resolved band structure also renders efficient ‘magnetic reading + electrical writing’. The thickness-different layers may even exhibit diversified types of magnetoelectric coupling, which both enriches the physics of multiferroics and facilitates their practical applications.

Magnetic skyrmion in chiral magnet exhibits a variety of unique topological properties associated with its innate topological structure. This inspires a number of ongoing searching for new topological magnetic textures. In this work, we used micromagnetic simulations and Monte Carlo simulations to investigate an exotic Néel-type magnetic kinks in square-shaped nanostructures of chiral magnets, which performs rather stably in the absence of magnetic field. The individual magnetic kink can reside in one of the four possible corners, and carry possibly upward or downward core polarity, constituting eight degenerate states. In addition, these kinks also exhibit unique behaviors of generation, stability and dynamics, as revealed by micromagnetic simulations. It was found that such kinks can be created, annihilated, displaced, and polarity-reversed on demand by applying a spin-polarized current pulse, and are easily switchable among the eight degenerate states. In particularly, the kinks can be switched toward the ferromagnetic-like states and backward reversibly by applying two successive current pulses, indicating the capability of writing and deleting the kink structures. These findings predict the existence of Néel-type magnetic kinks in the square-shaped nanostructures, as well as provide us a promising approach to tailor the kinks by utilizing the corners of the nanostructures, and control these states by spin-polarized currents. The present work also suggests a theoretical guide to explore other chiral magnetic textures in nanostructures of polygon geometries.

As one typical giant permittivity material in the last 20 years, CaCu3Ti4O12 is need of a clarification between structures and the permittivity in the development of industrialization. Besides, it presents other electrical, antiferromagnetic, and photo-related properties. One model for all phenomena, all doubt for one model. We highlight the importance of intrinsic structures, by identifying the insulating anisotropy of grain boundaries, the undesirable loss effect of twinning boundaries and the dielectric insensitivity of A-site atomic Ca/Cu disorders. Based on both B-site specific titanium ions and A-site copper ions' polaron-like 3d electrons, we have established a comprehensive and self-consistent model of binary dielectric polarizing species, mainly with the Hamiltonian in the Ising model analysis of dielectric polarization. The review points to a new way of applying quantum physics to deepen dielectric researches in transitional metal oxides, by exploiting strong ionic and electronic correlations beyond classical dielectric physics.

Constructing earth-abundant and efficient electrocatalysts for overall water splitting are still in the research and exploration stage. In this work, the Fe-doped NiSe2 polyhedron nanocrystals grown on Ni foam (Ni0.75Fe0.25Se2@NF) with controllable surface morphology are developed as high-performance bifunctional electrocatalysts. The surfaces of the optimized polyhedron nanocrystals provide high density of catalytic active sites. NiSe2 is originally a catalytic active substance, and the Fe doping in NiSe2 can greatly enhance the conductivity. The optimized Ni0.75Fe0.25Se2@NF (6 h) electrode are performed in an alkaline solution, exhibiting outstanding OER and HER activity with small overpotentials of 210 mV and −117 mV to reach current density of 10 mA cm−2 and −10 mA cm−2, respectively. Remarkably, the Ni0.75Fe0.25Se2@NF (6 h) electrode serve as cathode and anode in water splitting electrolyzer also demonstrates highly efficient overall water electrolysis performance, with a relatively low cell voltage of 1.61 V at 10 mA cm−2 (1.57 V for RuO2(+)//Pt/C(−) couple) and excellent long-term stability of 50 h for overall water electrolysis.

Zeolite imidazolate frameworks (ZIFs) are a novel kind of zeolite materials with regular porous network structures, belonging to mental-organic frameworks (MOFs). Thanks to their high porosity and large specific surface area, ZIFs could have potential applications in the field of photocatalysis. In this paper, visible light responsive photocatalyst of ZIF-67 was synthesized by a one-step hydrothermal method under a mild condition. We explored three organic dyes in aqueous solutions were degraded under visible light irradiation. And corresponding photocatalytic performances of ZIF-67 with controllable synthetic conditions were evaluated systematically. It was found that ZIF-67(9 h) had the highest degradation efficiency. Besides, the ZIF-67(9 h) were heated in Ar can maintain their crystal structure and microporous form, but exposed more sites to improve photocatalytic activity with partial bond breakage. More than 75% methyl orange (MO), 85% methyl blue (MB) and 54% Rhodamine B (RhB) can be degraded with a 0.5 g L−1 ZIF-67(9 h)(Ar) under visible light irradiation. Moreover, the ZIF-67(9 h)(Ar) demonstrates a strong pH changing tolerance in both acid and alkaline aqueous solutions, the degradations can be well performed in a wide pH range from 3.0 to 11.0.

Using first-principles calculations based the density functional theory, we investigate the electronic structures and superconducting properties of iron-based superconducting films MFeAs (M = Li/Na) under the different strains. The calculations show that when the 1–5% compressive and tensile strains are applied, although the striped antiferromagnetic ground state of the MFeAs films remain unvaried, the Fermi surface nesting between electron-hole pockets, the electronic density of state at Fermi energy and magnetic moments of Fe ions all undergo the significant variations. The applications of compressive strains improve the Fermi surface nesting, decrease the localized magnetic moments of Fe ions, and increase the electronic density of states at the Fermi level of films. The variations suggest that the compressive strains favour the superconductivity by facilitating the spin fluctuations in films. However, the energy bands and electronic properties of MFeAs films under tensile strains show the opposite variations, and tend to suppress the superconductivity of films. Our results provide a simple solution and a comprehensive explanation for the improved superconductivity of MFeAs films by compressive strains.

Organic solar cells (OSCs) have reached a milestone with 15.6% efficiency through the development of active and interfacial materials. However, anode modifiers in the inverted structure, especially the interface of organic active layer/inorganic MoO3, are important but seldom reported. Herein, to improve this contact, a simple small molecule DPA2T with oligo-oxyethylene chains is reported. It not only shows a good solubility in alcoholic solvents but also an effective π-π stacking in solid-state. Furthermore, the implement of DPA2T in OSCs enhances the fill factor and the short circuit current resulting in an improvement of the overall power conversion efficiency.

Achieving high power conversion efficiencies (PCEs) in ferroelectric photovoltaics (PVs) is a longstanding challenge. Although recently ferroelectric thick films, composite films, and bulk crystals have all been demonstrated to exhibit PCEs >1%, these systems still suffer from severe recombination because of the fundamentally low conductivities of ferroelectrics. Further improvement of PCEs may therefore rely on thickness reduction if the reduced recombination could overcompensate for the loss in light absorption. Here, a PCE of up to 2.49% (under 365-nm ultraviolet illumination) was demonstrated in a 12-nm Pb(Zr0.2Ti0.8)O3 (PZT) ultrathin film. The strategy to realize such a high PCE consists of reducing the film thickness to be comparable with the depletion width, which can simultaneously suppress recombination and lower the series resistance. The basis of our strategy lies in the fact that the PV effect originates from the interfacial Schottky barriers, which is revealed by measuring and modeling the thickness-dependent PV characteristics. In addition, the Schottky barrier parameters (particularly the depletion width) are evaluated by investigating the thickness-dependent ferroelectric, dielectric and conduction properties. Our study therefore provides an effective strategy to obtain high-efficiency ferroelectric PVs and demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. An approach to boost the power conversion efficiencies (PCEs) of ferroelectric photovoltaics (PVs) is proposed based on the Schottky barrier effect. This approach leverages the thinning of a ferroelectric film to somewhere close to the depletion width, which can simultaneously suppress the recombination and lower the series resistance. Using this approach, we achieve a PCE up to 2.49% (under 365-nm ultraviolet illumination) in the 12-nm Pb(Zr0.2Ti0.8)O3 ultrathin films. Our study provides insightful guidance on how to design and tailor the ferroelectric films to achieve high PCEs, and also demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. Eliminating stray electrical effects in ultra-thin films can help optimize an unconventional solar energy technology. Ferroelectric materials have internal dipoles that spontaneously move photogenerated charges toward external circuits, producing higher power outputs than predicted by theory. Zhen Fan from South China Normal University in Guangzhou and colleagues now report that the dimensions of ferroelectric thin films distinctly affect how efficiently they convert sunlight into electricity. Measurements of solar cells containing lead-zirconium-titanate ferroelectrics with different thicknesses revealed a jump in conversion efficiencies when the film reached a thickness of 12 nanometers. Further analysis showed that this thickness correlates with the solar cell’s ‘depletion width’, a zone formed when metal electrodes contact the film. The electric field in the depletion zone complements the pushing actions of the ferroelectric dipoles, lowering electrical losses compared to thicker ferroelectric films.

Flexible transparent conductive electrodes are essential component for flexible optoelectronic devices and have been extensively studied in recent years, while most of the researches are focusing on the electrode itself, few topics in material green and recyclability. In this paper, we demonstrate a high-performance transparent conductive electrode (TCE), based on our previous cracking technology, combined with a green and recyclable substrate, a starch film. It not only shows low Rs (less than 1.0 Ω sq−1), high transparency (> 82%, figure of merit ≈ 10,000), but also provides an ultra-smooth morphology and recyclability. Furthermore, a series of biosensors on human joints are demonstrated, showing great sensitivity and mechanical stability. Electronic supplementary material The online version of this article (10.1186/s11671-019-2957-3) contains supplementary material, which is available to authorized users.

Hydrogenation of acetone on the (010) surface of hydrogen molybdenum bronzes was investigated by density functional theory (DFT) calculations with periodic slab models. The formation of H-bond between the carbonyl oxygen of acetone and the terminal OH group of the surface leads to a stable adsorption of acetone. The effect of hydrogen concentration in the bronzes on the hydrogenation of acetone was systematically investigated, indicating the hydrogenation reaction is a one-step concerted and exothermic process regardless of the hydrogen contents in the bronzes surface. The 8H surface with increased H-content shows a significantly exothermic reaction process and exhibits the smallest kinetic barrier compared with 4H or 6H surfaces. Additionally, the selectivity for hydrogenation acetone could increase owing the absence of C C bond activation. The findings in this study can help with designing of high-efficient and low-cost metal oxide catalysts for hydrogenation of unsaturated substances.

Up-conversion (UC) oxides comprise a huge family of materials with abundant optical functionalities and they merit extensive research. Some members of this family have been modified with proper rare earth (RE3+) ions and their importance has recently been established in non-contact temperature sensors, although the thermal sensitivity (S) value was still low. In this study, we investigated Nd3+ and Yb3+ sensitized Ba4La2Ti4Nb6O30 (BLTN:xNd3+/Yb3+) with a tungsten bronze structure for use as optical thermometers for the first time. Under excitation at 980 nm, obvious near-infrared emissions corresponding to 4F7/2/4S3/2 → 4I9/2, 4F5/2/2H9/2 → 4I9/2, and 4F3/2 → 4I9/2 transitions were observed, and the UC mechanism involved was elaborated. Furthermore, the UC thermometric properties were investigated according to the fluorescence intensity ratio technique and the maximum S values determined for BLTN:xNd3+/Yb3+ (x = 0.12 and 0.16) samples were both higher than 0.03 K–1. The potential of BLTN:xNd3+/Yb3+ for use in optical thermometers was estimated based on comparisons with other active materials.

Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of perovskite LaMO3/YMO3 superlattices (M = Cr, Mn, Co and Ni). It is found that ferroelectricity can emerge in LaMO3/YMO3 superlattices (M = Cr, Mn, Co), allowing them to be promising multiferroic candidates, while no ferroelectricity is found in the LaNiO3/YNiO3 superlattice. The electronic structure calculations indicate that the LaCrO3/YCrO3, LaMnO3/YMnO3, and LaCoO3/YCoO3 superlattices are insulators, and their magnetic ground states exhibit G-type antiferromagnetic (AFM), A-type AFM, and G-type AFM order, respectively, while the LaNiO3/YNiO3 superlattice is however a half-metallic ferromagnet. The electronic structure and magnetic ground state are discussed, based on the projected density of states data and Heisenberg model, respectively, and the magnetic phase transition temperature is evaluated based on mean-field theory. In the meantime, the spontaneous ferroelectric polarization of the LaMO3/YMO3 superlattices (M = Cr, Mn, Co) is determined respectively using the Born effective charge model and Berry phase method, and their hybrid improper ferroelectric character is predicted, with the net polarization mainly from the different displacements of the LaO layers and YO layers along the b-axis. It is suggested that alternative multiferroic materials can be obtained by properly designing superlattices that consist of two non-polar magnetic materials but exhibit tunable magnetic ground states and transition temperature and hybrid improper ferroelectricity.

Metal thio(seleno)phosphates MPX3 have attracted considerable attentions with wide spanned band gaps and rich magnetic properties. In this series, two neighboring members MnPS3 and NiPS3 differ in magnetic atoms, magnetic easy axes, spin anisotropy, as well as nearest-neighbor magnetic interactions. The competition between these components may cause intriguing physical phenomena. In this article, the evolution of magnetism of Mn1−x Ni x PS3 series is reported. Despite the incompatible antiferromagnetic orders of two end members, the antiferromagnetism persists as the ground state in the whole substitution region. The magnetic ordering temperature T N show nonmonotonic V-shape behavior, and the reentrant spin glass phase at x= 0.5 is observed. In addition, abnormal bifurcation of T N occurs at x = 0.75, which may be due to the temperature-dependent spin reorientation or phase separation. The evolution of magnetism is further confirmed semi-quantitatively by our density functional theory calculations. Our study indicates that exotic magnetism can be intrigued when multi-degrees of freedom are involved in these low-dimensional systems, which call for more in-depth microscopic studies in future.

We propose a novel method to drive phase transitions by etching strained BiFeO3 thin films to nanoislands. Atomic force microscopy (AFM) measurements reveal that the amount of rhombohedral-like (R) phase increases as the BiFeO3 thin films with tetragonal-like (T) matrix are etched to nanoislands and larger fraction of R phase can be obtained with the size reduction from 1 μm to 200 nm, indicating the T to R phase transitions induced by partial release of substrate clamping. Using piezoresponse force microscopy (PFM), it is demonstrated that rhombohedral-like (R) to tetragonal-like (T) phase transitions can be reversibly achieved under DC electric field in BFO nanoislands. Large electromechanical response has been observed in BFO nanoislands as well. This approach can be extended to other strained oxide films and provide guidance for the development of high-performance electromechanical lead-free materials.

The ferroelectric polarization switching along an external electric field is most important for the applications of ferroelectric memories and piezoelectric sensors and actuators; however, the depolarization commonly occurs randomly and cannot be controlled exactly until now. Here, a tip bias introduces the polarization switching and a ∼10 μm-scale domain in a triglycine sulfate crystal, and then the polarization backswitching as a special depolarization introduces a series of ordered granular domains along a line being parallel to the c axis and through the tip which divides the original domain to two similar parts. Such backswitching is controlled by the surface charge change as a result of the interplay among polarization charges, mobile H+ ions at the surface, and the strong crystal anisotropy. The self-organized ferroelectric domains offer us a new freedom to design novel ferroelectric or piezoelectric devices in future.

Previous experiments have found that LaMnO3 thin films grown on SrTiO3 substrate exhibit insulating ferromagnetism different from that of the bulk LaMnO3. Utilizing first-principles calculations based on density functional theory, magnetic transitions, energy gaps and spin exchange interactions of LaMnO3 thin films with different epitaxial strains are investigated, and the effect of the Jahn-Teller distortion, rotation and tilt of oxygen octahedrons on magnetic and electrical properties are analyzed in detail. A relative narrow insulating ferromagnetic area without structural phase transition is obtained during the transition from metallic ferromagnetic phase to insulating A-type antiferromagnetic phase. If the magnetic transition of LaMnO3 film is attributed to Jahn-Teller distortion, then the metal–insulator transition should be ascribed to the tilt of the oxygen octahedron and large change of the O1-Mn-O2 angle in ab plane, which is helpful to understand the insulating ferromagnetic characters during the preparations of the LaMnO3 superlattices or heterostructures, also indicates that the insulating ferromagnetic properties of LaMnO3 films can be tuned by epitaxial strain-controlled oxygen octahedrons distortions.

In this work, we reported a bi-functional ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) films which show energy storage properties and photovoltaic effects. The PZT films were crystallized into pure pseudo-cubic structure with the (001) preferred orientation. The polarization characterizations revealed a large recoverable energy storage density (∼11.2 J/cm3) and a high energy conversion efficiency (∼68%) with good thermal stability (25 °C–100 °C) and fast response (>10 Hz). Meanwhile, the Pt/PZT/Pt capacitor presented a significant photovoltaic effect. The open circuit voltage and the short circuit current were -0.92 V and 139 μA/cm2 at original state, respectively, while they were slightly increased to -1.01 V and 155 μA/cm2 in polarization-up state, and slightly decreased to -0.77 V and 90 μA/cm2 in polarization-down state. The maximum power conversion efficiency was enhanced one order of magnitude than that of polycrystalline PZT film reported elsewhere. Our study opens up a pathway toward developing bi-functional ferroelectric devices with simultaneously high energy storage density and high photovoltaic performance.

The compass-type anisotropy appears naturally in chiral magnets with strong spin-orbit coupling. In this work, we investigate the critical roles of compass anisotropy in modulating various spin textures of chiral magnets, by Monte Carlo simulations. The simulated results reveal a gradual helical reorientation and varying symmetry of skyrmion crystal structures as a function of compass anisotropy. Furthermore, an extended continuum spin model with the lattice discretization anisotropy is proposed to interpret the dependences of helical and skyrmion crystal structures on the compass anisotropy. It is demonstrated that specific helical propagating directions are favored by the high-order lattice anisotropy arising from spin interactions in discretized lattice. Besides that, some threshold values for the helical structures are identified by analytical approach.

Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li2Co(WO4)2 samples. Under zero magnetic field, two successive magnetic transitions at T N1 ∼ 9.4 K and T N2 ∼ 7.4 K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li2Co(WO4)2 enters an incommensurate magnetic state with a temperature dependent k between T N1 and T N2. The magnetic propagation vector locks-in to a commensurate value k = (1/2, 1/4, 1/4) below T N2. The antiferromagnetic structure is refined at 1.7 K with Co2+ magnetic moment 2.8(1) μ B, consistent with our first-principles calculations.

Hole transport materials (HTMs) play a pivotal role in the hole extraction and transportation processes in perovskite solar cells (PSCs). Although a great number of HTMs based on various planar conjugated cores have been explored, the development of branches is rather limited. In this work, branches of 9-phenyl-9H-carbazole derivatives have been incorporated in different N-alkylated dithieno[3,2-b:2′,3′-d]pyrrol (DTP) core to construct HTMs (TM-9 to TM-14). Among them, TM-13 with 9-(4-methoxyphenyl)-9H-carbazole branch exhibites excellent performance in hole extraction and transportation ability. Consequently, the PSCs based on TM-13 have achieved the power conversion efficiency (PCE) of 20.44%, which is comparable to the PCE of 20.62% with the widely used Spiro-OMeTAD HTM. More importantly, the synthetic cost of TM-13 (62 $/g) is much lower than that of Spiro-OMeTAD (400 $/g), suggesting its promising commercialization for low-cost and efficient HTMs for PSCs.

Ru-doped porous hollow SnO2 nanotubes were prepared by simple electrospinning method, and their crystal structures, morphologies, and formation mechanism of porous hollow nanotubes were fully elucidated. The sensor based on the porous hollow Ru-doped SnO2 nanotubes shows an excellent sensing response of ~340 with ultrafast response time of 0.58 s at 250 ℃ to 100 ppm acetone. Moreover, the sensor exhibits good stability and excellent selectivity. Finally, the mechanism of the enhanced gas sensing properties was explained as the comprehensive effects of the larger specific surface area resulted from the hollow porous nanotubes structure, the ultra-fine SnO2 grain size, and the catalyst of RuO2. This research suggests the Ru-doped SnO2 nanotubes have the potential to be used as highly reliable and high-performance sensing layer for acetone gas sensors.

We propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically, where ferroelectrics with strain-sensitive Curie temperature are necessary. We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics (e.g. organic PhMDA) can be ideal candidates, which are also flexible and lead-free. Owing to the specific features of hydrogen bonding, their proton hopping barrier will drastically increase with prolonged proton transfer distance, while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds. Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%. Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction, and in another direction, an unprecedented ultra-high piezoelectric coefficient of 2058 pC/N can be obtained. This value is even underestimated and can be greatly enhanced when applying a smaller strain. Aside from sensors, they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients.

The magnetism and spin exchange coupling of monolayer CrOCl with different strains are investigated systematically using first principles. It is found that the magnetic ground state can be changed from ferromagnetic (FM) to antiferromagnetic (AFM), and the Curie temperature (TC) is enhanced significantly by applying the uniaxial strain along a- or b-axis direction. The variations of spin exchange coupling are explained according to the Goodenough–Kanamori–Anderson (GKA) and Bethe–Slater Interaction (BSI) rules. The strain-dependent magnetic state is mainly attributed to the competition between direct exchange interactions of cation–cation and indirect superexchange ones of cation–anion–cation in monolayer CrOCl. The different competitions in a- and b-axis direction determine the different critical intervals R of magnetic transitions, where R is the distance of the two nearest-neighbor (NN) Cr3+ ions. The AFM-FM transition occurs at R/r3d = 2.9 and 3.75 in a-axis direction, while it happens at R/r3d = 2.65 along b-axis direction. These results indicate that the sensitive relevancy between the external strain and magnetic coupling makes monolayer CrOCl a promising candidate for spintronics.

In this work, a series of efficient visible-light-driven WO3/Cu (II) nanofiber photocatalysts, with superior photocatalytic activities for the degradation of dyes and aniline, were prepared by electrospinning and impregnation methods. The impregnation method brought about homogeneous distribution of Cu (II) species on the surface of WO3 nanofibers. The prepared WO3/Cu (II) nanofiber photocatalysts exhibited enhanced visible-light absorption and reduced charge recombination, as compared to pure WO3 nanofibers, due to the interfacial charge transfer (IFCT) effect between the Cu (II) species and WO3. The combined benefits of the grafting Cu (II) species on the surface of WO3, in terms of optical, surface, and texture properties, led to a significant improvement in the photocatalytic activity for the degradation of Rhodamine B (Rh B) and aniline under conditions of visible light irradiation. Upon the irradiation with visible light in the presence of WO3/Cu (II)-2% photocatalyst, the concentration of Rhodamine B (Rh B) decreased from 10 mg/L to 1.5 mg/L, and that of aniline solution decreased from 5 mg/L to 0.728 mg/L over a period of 3 h. And after running five cycles, the concentration of aniline solution could decrease from 5 mg/L to 0.89 mg/L by WO3/Cu (II)-2% photocatalyst.

The discovery of two-dimensional (2D) ferroic materials has stimulated substantial efforts in developing emergent functionalities by synthesizing van der Waals (vdW) heterostructures, and one promising effect is the nonvolatile electrical control of magnetism in magnetoelectric (ME) heterostructures consisting of coupled 2D vdW ferromagnetic and ferroelectric layers. In this paper, it is proposed that the asymmetric interfacial coupling in such heterostructures may seriously distort the ferroelectric double-well potential, thus destabilizing the ferroelectricity. We investigate this consequence in $\mathrm{F}{\mathrm{e}}_{3}\mathrm{GeT}{\mathrm{e}}_{2}/\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ vdW heterostructure using the first-principles calculations. It is revealed that one of the two potential wells for ferroelectric monolayer $\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ is suppressed by the asymmetric interfacial coupling between electric polarization and the built-in electric field induced by intrinsic charge transfer, while the ferroelectric bistability can be recovered when the $\ensuremath{\alpha}\ensuremath{-}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ layer is thicker than three unit cells. Therefore, this work presents an alternative mechanism of critical thickness for the 2D ferroelectricity in ME vdW heterostructures.

Ferroelectric domain walls have been promised for some potential applications due to their unique properties. In particular, the electrical conductivity of charged domain walls (DWs) allows a new dimension to ferroelectric functionalities. In this work, we construct two representative types of charged DWs, i.e. head-to-head (HH) wall and tail-to-tail (TT) wall, and employ the first-principles method to study the electronic structure of these charged walls in BaTiO3 and the interactions between them and oxygen vacancies. It is revealed that the HH walls show the n-type conductivity, but the TT walls show the p-type conductivity. While embedded oxygen vacancies attract the TT wall and repel the HH wall, the interaction between the walls and oxygen vacancies depends on the vacancy occupation. This interaction enhances the conductivity of HH walls and reduces the conductivity of TT walls, and in particular a TT wall in binding with oxygen vacancies will drive the transition of p-type wall conductivity into n-type wall conductivity. The interaction of these walls with oxygen vacancies is discussed using the electrostatic model. This work represents a comprehensive understanding of electrical transport of charged DWs in ferroelectrics and possible roadmaps for manipulation.

Among numerous noble-metal-free electrocatalysts, molybdenum sulfides are recognized as promising candidates for hydrogen evolution reaction (HER). Owing to abundant under-coordinated sulfur atoms serving as catalytically active centers, intensive spectra studies have revealed that the HER property of amorphous molybdenum sulfide (a-MoSx) is superior to crystal molybdenum sulfide (c-MoS2). In this work, nitrogen-doped active carbon (NAC) is obtained through plasma treatment and amorphous MoSx/NAC hybrid catalyst films are electrodeposited on carbon fiber papers (CFP) which are employed as porous three-dimensional electrodes with low resistivity. The incorporation of NAC increases the electrochemically active surface area, enhances the electron transport, and facilitates the reaction kinetics. Moreover, the functionality durability benefits from synergistic effect between a-MoSx and NAC. Thus, the obtained hybrid catalyst delivers the excellent HER activity, requiring only 203 mV overpotential to achieve a geometrical current density of 100 mA cm−2 with a Tafel slope of ∼43.9 mV per decade.

We investigate the effect of commonly used solution-processed TiOx, SnO2 and ZnO interlayers on the perovskite film crystallization process. We find that the ZnO/perovskite interface can efficiently catalyze the perovskite crystallization even without thermal annealing. Funding Agencies|ERC [717026]; Carl Tryggers Stiftelse; European Commission [691210]; China Scholarship Council; VINNMER Marie Curie Fellowships

A series of barbituric acid doped carbon nitride (CN-BA) photocatalysts were successfully prepared by copolymerizing dicyandiamide with barbituric acid (BA). Under AM1.5 simulated sunlight, CN-BA photocatalysts exhibit enhanced photocatalytic activity compared to pure carbon nitride for the degradation of aniline. The highest activity is obtained with 2% doped CN-BA photocatalyst. Results on the photodegradation of aniline indicate that for the optimized CN-BA photocatalyst, the concentration of aniline solution was reduced gradually from 16 mg/L to 1.354 mg/L in 2 h. This corresponds to a 6 times higher photodegradation efficiency than pure carbon nitride samples. The enhanced photocatalytic activity of CN-BA relies on the enhanced surface area, the higher light absorption and the reduced recombination of the photo-generated electron-hole pairs. This interpretation results from multiple characterizations with EPR, BET, N2 adsorption, Solid-state 13C NMR, UV–vis DRS, FESEM, and TEM. Under simulated sunlight irradiation, CN-BA is excited and generates electron-hole pairs. The photo-generated electrons in the CN-BA conduction band react with the molecular oxygen to form O2−. Part of the O2− transforms into OH, which further oxides aniline. Meanwhile, photo-generated holes in the valence band of CN-BA can benefit to the formation of OH or directly oxide aniline.

The intrinsic magnetism of MoS2 has been extensively investigated via simulations, but few reliable experimental results have been explored. Herein, we develop zigzag-edge rich layered structural MoS2 pyramids via chemical vapor deposition, triggering exceptional ferromagnetism. The magnetic measurements revealed the robust ferromagnetism of MoS2 pyramids compared with MoS2 flakes. The existence of ferromagnetism was mostly attributed to the presence of abundant zigzag-edges in the layered pyramids, confirmed by transmission electron microscopy, vibrating sample magnetometry, and magnetic force microscopy. Moreover, a clearly identified remnant and switchable magnetic moment was revealed for the first time in the MoS2 pyramid. This study provides sound evidence with the zigzag-edge induced ferromagnetism of the MoS2 materials, promising potential magnetic and spintronic applications.

MgAgSb alloys have been demonstrated to be one of the two best classes of low-temperature thermoelectric materials. In this study, a series of dopants to the slightly Ag- and Sb-deficient MgAg0.97Sb0.99 alloy are studied to explore their consequences on the power factor (PF) and thermoelectric figure-of-merit (ZT) of the alloy. The systematic study reveals that starting from the MgAg0.97Sb0.99 alloy, the donor dopant degrades both the PF and ZT, whereas the acceptor dopant enhances the PF without affecting the ZT. Both the donor and acceptor make the total thermal conductivity higher. A full-scale thermoelectric computation based on first-principles calculations and the Boltzmann transport theory is carried out to validate the experimental results. The calculated results for the donors and acceptors are semi-quantitatively consistent with our experimental results. The present study suggests that carrier doping alone may not be sufficient to significantly improve the overall thermoelectric performance of the MgAg0.97Sb0.99 alloy.

Halide perovskite CH3NH3PbI3 (MAPbI3) is a promising candidate material for future high efficiency solar cells, while suffering from low stability that hinders their commercial applications. Here, we investigated the microscopic mechanisms underlying the photovoltaic degradation behaviors in MAPbI3 based solar cells, by using a time sequence photoconductive atomic force microscope spectroscopic (pcAFMs) in a moist environment. From the evolution of pcAFM maps, one can see two distinguished different evolution stages, the initial inter-diffusion stage that increases the overall photocurrent at the first few hours, which is followed by a decomposition stage that leads to the continuous decline of photocurrents with elongated durations. Moreover, an estimated lifetime map was also derived, which offers an overview insight into the different microscopic degradation mechanisms. In particular, some major decomposition mechanisms have been revealed, including an overall slow decomposition in all the grains, fast spreading of initial failure region, as well as newly formed failure regions occurring at grain junctions. The unveiling of these mechanisms might help improve the performance of halide perovskite solar cells, and provides a paradigmatic example for probing the microscopic photovoltaic mechanisms by using the powerful pcAFM technique.

Organic-inorganic hybrid perovskite solar cells (PSCs) are one of the most promising technologies in the field of photovoltaics due to its high-power conversion efficiency (PCE) and easy fabrication process. However, its moisture stability has posed a crucial hurdle towards its further commercialization. In this paper, we have introduced an interfacial layer, cerotic acid (CA), inspired from the honeycomb, to modify the surface of perovskite films, thus improving moisture stability. The PCE of the CA modified PSCs retained ~81% of its initial value after aging 30 days at a relative humidity of 35%, in sharp contrast with the pristine devices, only 19% retention of its initial value. In addition, the C=O group in CA was found effectively passivate the unsaturated Pb sites in perovskite films and further contributes to the PCE of devices. Overall, this work has demonstrated a simple, potentially low-cost, and environmentally friendly method to improve not only the photovoltaic performance but also the moisture stability.