Integration of Ultrathin Hafnium Oxide with a Clean van der Waals Interface for Two-Dimensional Sandwich Heterostructure Electronics.

Integrating high-κ dielectrics with a small equivalent oxide thickness (EOT) with two-dimensional (2D) semiconductors for low-power consumption van der Waals (vdW) heterostructure electronics remains challenging in meeting both interface quality and dielectric property requirements. Here, we demonstrate the integration of ultrathin amorphous HfOx sandwiched within vdW heterostructures by the selective thermal oxidation of HfSe2 precursors. The self-cleaning process ensures a high-quality interface with a low interface state density of 1011-1012 cm-2 eV-1. The synthesized HfOx displays excellent dielectric properties with an EOT of ∼1.5 nm, i.e., a high κ of ∼16, an ultralow leakage current of 10-6 A/cm2, and an impressively high breakdown field of 9.5 MV/cm. This facilitates low-power consumption vdW heterostructure MoS2 transistors, demonstrating steep switching with a low subthreshold swing of 61 mV/decade. This one-step integration of high-κ dielectrics into vdW sandwich heterostructures holds immense potential for developing low-power consumption 2D electronics while meeting comprehensive dielectric requirements.

Quasi-Zero-Dimensional Ferroelectric Polarization Charges-Coupled Resistance Switching with High-Current Density in Ultrascaled Semiconductors.

Ferroelectric memristors hold immense promise for advanced memory and neuromorphic computing. However, they face limitations due to low readout current density in conventional designs with low-conductive ferroelectric channels, especially at the nanoscale. Here, we report a ferroelectric-mediated memristor utilizing a 2D MoS2 nanoribbon channel with an ultrascaled cross-sectional area of <1000 nm2, defined by a ferroelectric BaTiO3 nanoribbon stacked on top. Strikingly, the Schottky barrier at the MoS2 contact can be effectively tuned by the charge transfers coupled with quasi-zero-dimensional polarization charges formed at the two ends of the nanoribbon, which results in distinctive resistance switching accompanied by multiple negative differential resistance showing the high-current density of >104 A/cm2. The associated space charges in BaTiO3 are minimized to ∼3.7% of the polarization charges, preserving nonvolatile polarization. This achievement establishes ferroelectric-mediated nanoscale semiconductor memristors with high readout current density as promising candidates for memory and highly energy-efficient in-memory computing applications.

Electromechanical Performance Optimization of Ni-Doped Bi(Sc, Zr)O3-PbTiO3 Ceramics and Microstructure Analysis.

BiScO3-PbTiO3-based ceramics show great potential in high-temperature piezoelectric applications. However, their high dielectric loss tan δ and low mechanical quality factor Qm have to be optimized. In this paper, a ceramic system of (1-y)Bi(Sc0.975Zr0.025)O3-yPb(Ti1-xNix)O3 (BSZ-yPT-xNi, x = 0, 0.015, 0.025, and 0.035 and y = 0.62, 0.63, 0.64, and 0.65) was systematically investigated. Increase in x or y values leads to the enhancement of the tetragonal phase and tetragonal lattice distortion. The rhombohedral/tetragonal morphotropic phase boundary (MPB) locates in the vicinity of y = 0.64 for the x = 0 and 0.015 samples, and y = 0.63 for the x = 0.025 and 0.035 samples. For these MPB samples, the substitution of Ni2+ for Ti4+ causes domain refinement, evolving from the submicrometer lamellar domains to hierarchical domains, and finally to the high-density stripe-like nanodomains, which benefits the domain wall motion and makes the coercive field reduced. However, the alignment of defect dipoles (NiTi''-VO••) after the sufficient poling and aging treatment induces the noticeable internal bias field, which increases with the addition of Ni2+. Apparent piezoelectric "hardening" occurred, evidenced by the increase in Qm and the reduction in tan δ. Among the MPB samples, the x = 0.025/y = 0.63 ceramic shows the superior comprehensive electromechanical performance with the d33 of 380 pC/N, kt of 0.51, Qm of 112, and tan δ of 0.010. Besides, excellent temperature stability was achieved with the d33 of 367-380 pC/N, Qm of 106-112, and tan δ ≤ 0.010 in the temperature range of 25-250 °C.

Quantitative Characterization of Elemental Segregation in Inconel 718 Superalloy by Micro-Beam X-ray Fluorescence Spectroscopy and Its Correlation Study.

Inconel 718 (IN718) nickel-based superalloy is widely used in aerospace and nuclear applications owing to its excellent comprehensive mechanical properties, oxidation resistance, and hot corrosion resistance. However, the elemental segregation caused by heterogeneous solidification during casting has great influence on the mechanical properties. Therefore, accurately characterizing the segregation behavior is necessary. Traditional quantitative characterization of elemental segregation uses various sampling methods, in which only macroscopic segregation results are obtained. In this study, micro-beam X-ray fluorescence (µ-XRF) is used for the quantitative characterization of element micro-segregation in IN718 superalloy. The concentration distributions of Cr, Fe, Mo, Nb, and Ti in IN718 alloy are determined with optimized testing parameters, and the degree of elemental segregation in different regions of the analytical area is calculated. It is found that the segregation degree of Nb and Ti in the testing area is larger than other alloying elements. The correlation between the microstructure distribution and the segregation degree of Nb and Ti has been studied using scanning electron microscopy (SEM) combined with energy-dispersive spectrometry (EDS). There is severe segregation of Nb and Ti in areas where Nb-containing precipitates are accumulated. The distribution of abnormal signals of Nb with a high fluorescence intensity has a close relationship with the area of precipitates-enriched Nb.

Achieving Ultrahigh Energy Storage Performance for NaNbO3-Based Lead-Free Antiferroelectric Ceramics via the Coupling of the Stable Antiferroelectric R Phase and Nanodomain Engineering.

NaNbO3(NN)-based lead-free eco-friendly antiferroelectric (AFE) ceramics with an extremely high maximum polarization (Pm) are believed to be a promising alternative to traditional lead-based ceramics. Nevertheless, the high energy dissipation resulting from the large polarization hysteresis, which arises from the AFE-ferroelectric (FE) phase transition, poses a great challenge to the application of this promising ceramic. Herein, an excellent recoverable energy storage density (Wrec) was attained by intentionally designing a (0.86 - x) NaNbO3-0.14CaTiO3-xBiMg2/3Nb1/3O3 (NN-CT-xBMN) relaxor antiferroelectric ceramic, attributed to the synergistic effect of the stable AFE R phase and nanodomain engineering to overcome the bottleneck. The obtained results illustrate that the inclusion of BMN causes the transition from AFE microdomains to nanodomains and stabilizes the relaxor AFE orthorhombic R phase, which generates a highly stable polarization field response with low hysteresis and delays the AFE-FE phase transition, thus improving energy storage density. As a consequence, a high Wrec of 5.41 J cm-3 with an excellent conversion efficiency η of 86.7% was obtained in the NN-CT-0.08BMN ceramic. Moreover, the NN-CT-0.08BMN ceramic exhibits superior stability in temperature (25-150 °C), frequency (1-600 Hz), and fatigue behavior (10°-104 cycles) together with a large current density (CD = 810 A cm-2), ultrahigh power density (PD = 118 MW cm-3), and ultrafast discharge rate (t0.9 < 0.7 µs). This superior energy storage density, coupled with outstanding stability, suggests that the NN-CT-0.08BMN ceramic has the potential to be a promising candidate for pulsed power applications and power electronics.

Improved Thermal Stability and Film Uniformity of Halide Perovskite by Confinement Effect brought by Polymer Chains of Polyvinyl Pyrrolidone.

Polyvinyl pyrrolidone (PVP) is doped to PbI2 and organic salt during two-step growth of halideperovskite. It is observed that PVP molecules can interact with both PbI2 and organic salt, reduce the aggregation and crystallization of the two, and then slow down the coarsening rate of perovskite. As doping concentration increases from 0 to 1 mM in organic salt, average crystallite size of perovskite decreases monotonously from 90 to 34 nm; Surface fluctuation reduces from 259.9 to 179.8 nm at first, and then increases; Similarly, surface roughness decreases from 45.55 to 26.64 nm at first, and then rises. Accordingly, a kind of "confinement effect" is resolved to crystallite growth and surface fluctuation/roughness, which helps to build compact and uniform perovskite film. Density of trap states (t-DOS) is cut down by ≈60% at moderate doping  (0.2 mM). Due to the "confinement effect", power conversion efficiency of perovskite solar cells is improved from 19.46 (±2.80) % to 21.50 (±0.99) %, and further improved to 24.11% after surface modification. Meanwhile, "confinement effect" strengthens crystallite/grain boundaries and improves thermal stability of both film and device. T80 of device increases to 120 h, compared to 50 h for reference ones.

Improved Crystallization of Lead Halide Perovskite in Two-Step Growth Method by Polymer-Assisted "Slow-Release Effect".

Fast reaction between organic salt and lead iodide always leads to small perovskite crystallites and concentrated defects. Here, polyacrylic acid is blended with organic salt, so as to regulate the crystallization in a two-step growth method. It is observed that addition of polyacrylic acid retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI2 , by which "slow-release effect" is defined. Such effect improves crystallization of perovskite. X-ray diffraction study shows that, after addition of 2 mm polyacrylic acid, average crystallite size of perovskite increases from ≈40 to ≈90 nm, meanwhile, grain size increases. Thermal admittance spectroscopy study shows that trap density is reduced by nearly one order (especially for deep energy levels). Due to the improved crystallization and reduced trap density, charge recombination is obviously reduced, while lifetime of charge carriers in perovskite film and devices are prolonged, according to time-resolved photoluminescence and transient photo-voltage decay curve tests, respectively. Accordingly, power conversion efficiency of the device is promoted from 19.96 (±0.41)% to 21.84 (±0.25)% (with a champion efficiency of 22.31%), and further elevated to 24.19% after surface modification by octylammonium iodide.

Achieving Ultrahigh Energy Storage Density of La and Ta Codoped AgNbO3 Ceramics by Optimizing the Field-Induced Phase Transitions.

Energy storage capacitors are extensively used in pulsed power devices because of fast charge/discharge rates and high power density. However, the low energy storage density and efficiency of dielectric capacitors limit their further commercialization in modern energy storage applications. Lead-free AgNbO3-based antiferroelectric (AFE) ceramics are considered to be one of the most promising environmentally friendly materials for dielectric capacitors because of their characteristic double polarization-electric field hysteresis loops with small remanent polarization and large maximum polarization. An enhancement of these characteristics allows achieving a synergistic improvement of both the energy storage density and efficiency of the antiferroelectric materials. This work reports on a feasible codoping strategy enabling the preparation of AgNbO3-based ceramics with high energy storage performance. An introduction of La3+ and Ta5+ ions into the AgNbO3 perovskite lattice was found to increase the structural stability of the antiferroelectric phase at the expense of a reduction of local polar regions, resulting in the shifting of the electric field-induced antiferroelectric-ferroelectric phase transition toward higher fields. An ultrahigh recoverable energy storage density of 6.73 J/cm3 and high energy storage efficiency of 74.1% are obtained for the Ag0.94La0.02Nb0.8Ta0.2O3 ceramic subjected to a unipolar electric field of 540 kV/cm. These values represent the best energy performance in reported lead-free ceramics so far. Hence, the La3+/Ta5+ codoping has been shown to be a good route to improve the energy storage properties of AgNbO3 ceramics.

Quantitative Distribution Characterization and Correlation Study of Composition, Structure and Hardness of Rim Region in Railway Wheel.

The railway wheel is the key component of high-speed railway train. To assure the safety in service, higher requirements are put forward in this study for the composition, microstructure uniformity, and comprehensive properties of wheel materials. In this paper, the high throughput quantitative distribution characterization methods of composition, microstructure, inclusions and Vickers hardness of high-speed railway wheel materials based on the spark source original position analysis technique, high throughput scanning electron microscope (SEM) combined with image batch processing technology, and automatic two-dimensional quantitative distribution analysis technique of inclusions and micro hardness have been studied. The distribution trend of the content of nine elements, size and quantity of sulfides and oxides, ferrite area fraction, and Vickers hardness from the wheel tread surface to the radial depth of about 50 mm below the surface has been discussed. The influence of inclusions distribution on the element segregation and the effect of rim-chilling process with different water spraying angle on the distribution of microstructure and micro hardness have been investigated. It was found that unsynchronized cooling on both sides of the rim altered the phase behavior of ferrite and pearlite and obvious inhomogeneity distribution of ferrite appeared, which led to the asymmetrical Vickers hardness in areas near or away from the flange. Based on the quantitative characterization of area fraction and micro hardness on the same location of wheel rim, a statistical mapping relationship between ferrite area fraction and Vickers hardness was established.

Electrospinning Synthesis of Na0.5Bi0.5TiO3 Nanofibers for Dielectric Capacitors in Energy Storage Application.

Dielectric composites based on ferroelectric ceramics nanofibers are attracting increasing attention in capacitor application. In this work, the sol-gel method and electrospinning technology are utilized to prepare one-dimensional Na0.5Bi0.5TiO3 (NBT) nanofibers, and the influence of electrospinning process parameters such as spinning voltage, liquid supply rate, and collector speed on the morphology and structure of nanofibers are systematically explored. The final optimized parameters include the applied voltage of 20 kV, the solution flow rate of 1 mL/h, and the collector's rotation speed of 1500 rpm. The optimized NBT nanofibers are introduced into the PVDF polymer matrix for energy storage application. Owing to the enhanced interfacial polarization between PVDF matrix and NBT nanofibers with a high aspect ratio, the NBT-PVDF nanocomposites achieve a high discharge energy density of 14.59 J cm-3 and an energy efficiency of 53.69% at 490 kV mm-1, which are higher than those of pure PVDF, i.e., 10.26 J cm-3 and 48.17% at 420 kV mm-1, respectively. The results demonstrate that the strategy of synthesizing NBT nanofibers using the electrospinning method is of great potential for high-performance dielectric capacitor application.

Reconfigurable Quasi-Nonvolatile Memory/Subthermionic FET Functions in Ferroelectric-2D Semiconductor vdW Architectures.

The functional reconfiguration of transistors and memory in homogenous ferroelectric devices offers significant opportunities for implementing the concepts of in-memory computing and logic-memory monolithic integration. Thus far, reconfiguration is realized through programmable doping profiles in the semiconductor channel using multiple-gate operation. This complex device architecture limits further scaling to match the overall chip requirements. Here, reconfigurable memory/transistor functionalities in a ferroelectric-gated van der Waals transistor by controlling the behavior of ferroelectric oxygen vacancies at the interface are demonstrated. Short- and long-term memory functions are demonstrated by modulating the border oxygen vacancy distribution and the associated charge dynamics. The quasi-nonvolatile long-term memory exhibits data retention of over 105 s and endurance of up to 5 × 105 cycles, verifying its applicability as a potential device platform for neuromorphic networks. More importantly, by modulating the ferroelectricity of the interfacial domains with the interactions of oxygen vacancies, a hysteresis-free logic transistor is realized with a subthermionic subthreshold swing down to 46 mV dec-1 , which resembles a negative-capacitance field-effect transistor. The new concept of achieving functional reconfiguration with prior device performance in a single-gate ferroelectric field-effect transistor is of great advantage in future integrated circuit applications.

Polarisation tuneable piezo-catalytic activity of Nb-doped PZT with low Curie temperature for efficient CO2 reduction and H2 generation.

The reduction of CO2 into useful hydrocarbon chemicals has attracted significant attention in light of the depletion in fossil resources and the global demand for sustainable sources of energy. In this paper, we demonstrate piezo-catalytic electrochemical reduction of CO2 by exploiting low Curie temperature, T c ∼ 38 °C, Nb-doped lead zirconate titanate (PZTN) piezoelectric particulates. The large change in spontaneous polarisation of PZTN due to the acoustic pressures from to the application of ultrasound in the vicinity of the T c creates free charges for CO2 reduction. The effect of applied acoustic power, particulate agglomeration and the impact of T c on piezo-catalytic performance are explored. By optimization of the piezo-catalytic effect a promising piezo-catalytic CO2 reduction rate of 789 µmol g-1 h-1 is achieved, which is much larger than the those obtained from pyro-catalytic effects. This efficient and polarisation tunable piezo-catalytic route has potential to promote the development of CO2 reduction via the utilization of vibrational energy for environmental improvement.

Charge-Ferroelectric Transition in Ultrathin Na0.5 Bi4.5 Ti4 O15 Flakes Probed via a Dual-Gated Full van der Waals Transistor.

Ferroelectric field-effect transistors (FeFETs) have recently attracted enormous attention owing to their applications in nonvolatile memories and low-power logic electronics. However, the current mainstream thin-film-based ferroelectrics lack good compatibility with the emergent 2D van der Waals (vdW) heterostructures. In this work, the synthesis of thin ferroelectric Na0.5 Bi4.5 Ti4 O15 (NBIT) flakes by a molten-salt method is reported. With a dry-transferred NBIT flake serving as the top-gate dielectric, dual-gate molybdenum disulfide (MoS2 ) FeFETs are fabricated in a full vdW stacking structure. Barrier-free graphene contacts allow the investigation of intrinsic carrier transport of MoS2 governed by lattice scattering. Thanks to the high dielectric constant of ≈94 in NBIT, a metal to insulator transition with a high electron concentration of 3.0 × 1013 cm-2 is achieved in MoS2 under top-gate modulation. The electron field-effect mobility as high as 182 cm2 V-1 s-1 at 88 K is obtained. The as-fabricated MoS2 FeFET exhibits clockwise hysteresis transfer curves that originate from charge trapping/release with either top-gate or back-gate modulation. Interestingly, hysteresis behavior can be controlled from clockwise to counterclockwise using dual-gate. A multifunctional device utilizing this unique property of NBIT, which is switchable electrostatically between short-term memory and nonvolatile ferroelectric memory, is envisaged.

Demonstration of Enhanced Piezo-Catalysis for Hydrogen Generation and Water Treatment at the Ferroelectric Curie Temperature.

Hydrogen can contribute significantly to the energy mix of the near future, as it is an attractive replacement for fossil fuels due to its high energy density and low greenhouse gas emission. A fascinating approach is to use the polarization change of a ferroelectric due to an applied stress or temperature change to achieve piezo- or pyro-catalysis for both H2 generation and wastewater treatment. We exploit low Curie temperature (Tc) ferroelectrics for polarization-driven electrochemical reactions, where the large changes in polarization and high activity of a ferroelectric near its Tc provides a novel avenue for such materials. We present experimental evidence for enhanced water splitting and rhodamine B degradation via piezo-catalysis by ultrasonic excitation at its Tc. Such work provides an effective strategy for water splitting/treatment systems that employ low Tc ferroelectrics under the action of mechanical stress or/and thermal fluctuations.

Author Correction: Significantly Enhanced Energy Storage Density by Modulating the Aspect Ratio of BaTiO3 Nanofibers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Suppressed polarization by epitaxial growth of SrTiO3 on BaTiO3 nanoparticles for high discharged energy density and efficiency nanocomposites.

In order to meet the increasing demand of integration and miniaturization of electronic components, capacitors with high energy density are urgently needed. In this work, a strategy of suppressing interfacial polarization for obtaining enhanced energy density and efficiency polymer based nanocomposites is proposed. This strategy is conducted by epitaxial growth of a SrTiO3 layer with a moderate dielectric constant on the surface of a BaTiO3 core to form a kind of novel filler and compositing with the P(VDF-HFP) matrix to prepare dielectric nanocomposites. The SrTiO3 shell could effectively confine the mobility of charge carriers to enhance the dielectric strength of the composites and improve the energy efficiency by reducing the Maxwell-Wagner-Sillars (MWS) interfacial polarization and space charge polarization between the BaTiO3@SrTiO3 fillers and the P(VDF-HFP) matrix due to their similar crystal structure and lattice parameter. The nanocomposite containing 1 vol% BaTiO3@SrTiO3 nanoparticles achieved a discharged energy density of 13.89 J cm-3 and an energy efficiency of 63% at 494.7 kV mm-1, which are superior to 9.96 J cm-3 and 50% of BaTiO3/P(VDF-HFP) nanocomposites with the same loading, respectively, and its discharged energy density is 69% higher than 8.2 J cm-3 of the neat P(VDF-HFP) at 401.5 kV mm-1. This work provides an effective way for nanocomposite capacitors with high energy density and efficiency.

Harnessing Plasticity in an Amine-Borane as a Piezoelectric and Pyroelectric Flexible Film.

We demonstrate that trimethylamine borane can exhibit desirable piezoelectric and pyroelectric properties. The material was shown to be able operate as a flexible film for both thermal sensing, thermal energy conversion and mechanical sensing with high open circuit voltages (>10 V). A piezoelectric coefficient of d33 ≈10-16 pC N-1 , and pyroelectric coefficient of p≈25.8 µC m-2 K-1 were achieved after poling, with high pyroelectric figure of merits for sensing and harvesting, along with a relative permittivity of ϵ 33 σ ≈ 6.3.

Superior Thermal Stability of High Energy Density and Power Density in Domain-Engineered Bi0.5Na0.5TiO3-NaTaO3 Relaxor Ferroelectrics.

Thermal-stable dielectric capacitors with high energy density and power density have attracted increasing attention in recent years. In this work, (1 - x)Bi0.5Na0.5TiO3-xNaTaO3 [(1 - x)BNT-xNT, x = 0-0.30] lead-free relaxor ferroelectric ceramics are developed for capacitor applications. The x = 0.20 ceramic exhibits superior thermal stability of discharged energy density (WD) with a variation of less than 10% in an ultrawide temperature range of -50 to 300 °C, showing a significant advantage compared with the previously reported ceramic systems. The WD reaches 4.21 J/cm3 under 38 kV/mm at room temperature. Besides, a record high of power density (PD ≈ 89.5 MW/cm3) in BNT-based ceramics is also achieved in x = 0.20 ceramic with an excellent temperature insensitivity within 25-160 °C. The x = 0.20 ceramic is indicated to be an ergodic relaxor ferroelectric with coexisted R3c nanodomains and P4bm polar nanoregions at room temperature, greatly inducing large maximum polarization, maintaining low remnant polarization, and thus achieving high WD and PD. Furthermore, the diffuse phase transition from R3c to P4bm phase on heating is considered to be responsible for the superior thermal stability of the high WD and PD. These results imply the large potential of the 0.80BNT-0.20NT ceramic in temperature-stable dielectric capacitor applications.

Interface design for high energy density polymer nanocomposites.

This review provides a detailed overview on the latest developments in the design and control of the interface in polymer based composite dielectrics for energy storage applications. The methods employed for interface design in composite systems are described for a variety of filler types and morphologies, along with novel approaches employed to build hierarchical interfaces for multi-scale control of properties. Efforts to achieve a close control of interfacial properties and geometry are then described, which includes the creation of either flexible or rigid polymer interfaces, the use of liquid crystals and developing ceramic and carbon-based interfaces with tailored electrical properties. The impact of the variety of interface structures on composite polarization and energy storage capability are described, along with an overview of existing models to understand the polarization mechanisms and quantitatively assess the potential benefits of different structures for energy storage. The applications and properties of such interface-controlled materials are then explored, along with an overview of existing challenges and practical limitations. Finally, a summary and future perspectives are provided to highlight future directions of research in this growing and important area.

High energy density in PVDF nanocomposites using an optimized nanowire array.

TiO2 nanowire arrays are often utilized to prepare high performance polymer nanocomposites, however, the contribution to the energy density is limited due to their non-ferroelectric characteristics. A nanocomposite with an optimized nanowire array combining the ferroelectric properties of lead zirconate titanate (PZT) with TiO2, readily forming nanowires (denoted as a TiO2-P nanowire array), is prepared to enhance the permittivity. Poly(vinylidene fluoride) (PVDF) is used as the polymer matrix due to its high breakdown strength, e.g. 600-700 kV mm-1. As a result, the permittivity and breakdown electric field reach 53 at 1 kHz and 550 kV mm-1, respectively. Therefore, the nanocomposites achieve a higher discharge energy density of 12.4 J cm-3 with excellent cycle stability, which is the highest among nanocomposites based on a nanowire array as a filler in a PVDF matrix. This work provides not only a feasible approach to obtain high performance dielectric nanocomposites, but also a wide range of potential applications in the energy storage and energy harvesting fields.