Magnet-in-ferroelectric crystals exhibiting photomultiferroicity.

Growing crystallographically incommensurate and dissimilar organic materials is fundamentally intriguing but challenging for the prominent cross-correlation phenomenon enabling unique magnetic, electronic, and optical functionalities. Here, we report the growth of molecular layered magnet-in-ferroelectric crystals, demonstrating photomanipulation of interfacial ferroic coupling. The heterocrystals exhibit striking photomagnetization and magnetoelectricity, resulting in photomultiferroic coupling and complete change of their color while inheriting ferroelectricity and magnetism from the parent phases. Under a light illumination, ferromagnetic resonance shifts of 910 Oe are observed in heterocrystals while showing a magnetization change of 0.015 emu/g. In addition, a noticeable magnetization change (8% of magnetization at a 1,000 Oe external field) in the vicinity of ferro-to-paraelectric transition is observed. The mechanistic electric-field-dependent studies suggest the photoinduced ferroelectric field effect responsible for the tailoring of photo-piezo-magnetism. The crystallographic analyses further evidence the lattice coupling of a magnet-in-ferroelectric heterocrystal system.

Intrinsic-strain-induced ferroelectric order and ultrafine nanodomains in SrTiO3.

Nano ferroelectrics holds the potential application promise in information storage, electro-mechanical transformation, and novel catalysts but encounters a huge challenge of size limitation and manufacture complexity on the creation of long-range ferroelectric ordering. Herein, as an incipient ferroelectric, nanosized SrTiO3 was indued with polarized ordering at room temperature from the nonpolar cubic structure, driven by the intrinsic three-dimensional (3D) tensile strain. The ferroelectric behavior can be confirmed by piezoelectric force microscopy and the ferroelectric TO1 soft mode was verified with the temperature stability to 500 K. Its structural origin comes from the off-center shift of Ti atom to oxygen octahedron and forms the ultrafine head-to-tail connected 90° nanodomains about 2 to 3 nm, resulting in an overall spontaneous polarization toward the short edges of nanoparticles. According to the density functional theory calculations and phase-field simulations, the 3D strain-related dipole displacement transformed from [001] to [111] and segmentation effect on the ferroelectric domain were further proved. The topological ferroelectric order induced by intrinsic 3D tensile strain shows a unique approach to get over the nanosized limitation in nanodevices and construct the strong strain-polarization coupling, paving the way for the design of high-performance and free-assembled ferroelectric devices.

Fluid fibers in true 3D ferroelectric liquids.

We demonstrate an exceptional ability of a high-polarization 3D ferroelectric liquid to form freely suspended fluid fibers at room temperature. Unlike fluid threads in modulated smectics and columnar phases, where translational order is a prerequisite for forming liquid fibers, recently discovered ferroelectric nematic forms fibers with solely orientational molecular order. Additional stabilization mechanisms based on the polar nature of the mesophase are required for this. We propose a model for such a mechanism and show that these fibers demonstrate an exceptional nonlinear optical response and exhibit electric field-driven instabilities.

Photovoltaic effect by soft phonon excitation.

SignificanceThe quantum-mechanical geometric phase of electrons provides various phenomena such as the dissipationless photocurrent generation through the shift current mechanism. So far, the photocurrent generations are limited to above or near the band-gap photon energy, which contradicts the increasing demand of the low-energy photonic functionality. We demonstrate the photocurrent through the optical phonon excitations in ferroelectric BaTiO3 by using the terahertz light with photon energy far below the band gap. This photocurrent without electron-hole pair generation is never explained by the semiclassical treatment of electrons and only arises from the quantum-mechanical geometric phase. The observed photon-to-current conversion efficiency is as large as that for electronic excitation, which can be well accounted for by newly developed theoretical formulation of shift current.

Tunable Multistate Ferroelectricity of Unit-Cell-Thick BaTiO3 Revived by a Ferroelectric SnS Monolayer via Interfacial Sliding.

Stabilization of multiple polarization states at the atomic scale is pivotal for realizing high-density memory devices beyond prevailing bistable ferroelectric architectures. Here, we show that two-dimensional ferroelectric SnS or GeSe is able to revive and stabilize the ferroelectric order of three-dimensional ferroelectric BaTiO3, even when the latter is thinned to one unit cell in thickness. The underlying mechanism for overcoming the conventional detrimental critical thickness effect is attributed to facile interfacial inversion symmetry breaking by robust in-plane polarization of SnS or GeSe. Furthermore, when invoking interlayer sliding, we can stabilize multiple polarization states and achieve efficient interstate switching in the heterostructures, accompanied by dynamical ferroelectric skyrmionic excitations. When invoking sliding and twisting, the moiré domains exhibit nontrivial polar vortexes, which can be laterally displaced via different sliding schemes. These findings provide an intuitive avenue for simultaneously overcoming the standing critical thickness issue in bulk ferroelectrics and weak polarization issue in sliding ferroelectricity.

Interlayer Coupling Controlled Ordering and Phases in Polar Vortex Superlattices.

The recent discovery of polar topological structures has opened the door for exciting physics and emergent properties. There is, however, little methodology to engineer stability and ordering in these systems, properties of interest for engineering emergent functionalities. Notably, when the surface area is extended to arbitrary thicknesses, the topological polar texture becomes unstable. Here we show that this instability of the phase is due to electrical coupling between successive layers. We demonstrate that this electrical coupling is indicative of an effective screening length in the dielectric, similar to the conductor-ferroelectric interface. Controlling the electrostatics of the superlattice interfaces, the system can be tuned between a pure topological vortex state and a mixed classical-topological phase. This coupling also enables engineering coherency among the vortices, not only tuning the bulk phase diagram but also enabling the emergence of a 3D lattice of polar textures.

A Time-Domain Perspective on the Structural and Electronic Response in Epitaxial Ferroelectric Thin Films on Silicon.

This operando study of epitaxial ferroelectric Pb(Zr0.48Ti0.52)O3 capacitors on silicon substrates studies their structural response via synchrotron-based time-resolved X-ray diffraction during hysteresis-loop measurements in the 2-200 kHz range. At high frequencies, the polarization hysteresis loop is rounded and the classical butterfly-like strain hysteresis acquires a flat dumbbell shape. We explain these observations from a time-domain perspective: The polarization and structural motion within the unit cell are coupled to the strain by the piezoelectric effect and limited by domain wall velocity. The solution of this coupled oscillator system is derived experimentally from the simultaneously measured electronic and structural data. The driving stress σFE(t) is calculated as the product of the measured voltage U(t) and polarization P(t). Unlike the electrical variables, σFE(t) and η(t) of the ferroelectric oscillate at twice the frequency of the applied electrical field. We model the measured frequency-dependent phase shift between η(t) and σFE(t).

Synergistic Effect of Ionic Liquid and Embedded QDs on 2D Ferroelectric Perovskite Films with Narrow Phase Distribution for Self-Powered and Broad-Band Photodetectors.

2D layered metal halide perovskites (MHPs) are a potential material for fabricating self-powered photodetectors (PDs). Nevertheless, 2D MHPs produced via solution techniques frequently exhibit multiple quantum wells, leading to notable degradation in the device performance. Besides, the wide band gap in 2D perovskites limits their potential for broad-band photodetection. Integrating narrow-band gap materials with perovskite matrices is a viable strategy for broad-band PDs. In this study, the use of methylamine acetate (MAAc) as an additive in 2D perovskite precursors can effectively control the width of the quantum wells (QWs). The amount of MAAc greatly affects the phase purity. Subsequently, PbSe QDs were embedded into the 2D perovskite matrix with a broadened absorption spectrum and no negative effects on ferroelectric properties. PM6:Y6 was combined with the hybrid ferroelectric perovskite films to create a self-powered and broad-band PD with enhanced performance due to a ferro-pyro-phototronic effect, reaching a peak responsivity of 2.4 A W-1 at 940 nm.

Switching Intrinsic Magnetic Skyrmions with Controllable Magnetic Anisotropy in van der Waals Multiferroic Heterostructures.

Magnetic skyrmions, topologically nontrivial whirling spin textures at nanometer scales, have emerged as potential information carriers for spintronic devices. The ability to efficiently create and erase magnetic skyrmions is vital yet challenging for such applications. Based on first-principles studies, we find that switching between intrinsic magnetic skyrmion and high-temperature ferromagnetic states can be achieved in the two-dimensional van der Waals (vdW) multiferroic heterostructure CrSeI/In2Te3 by reversing the ferroelectric polarization of In2Te3. The core mechanism of this switching is traced to the controllable magnetic anisotropy of CrSeI influenced by the ferroelectric polarization of In2Te3. We propose a useful descriptor linking the presence of magnetic skyrmions to magnetic parameters and validate this connection through studies of a variety of similar vdW multiferroic heterostructures. Our work demonstrates that manipulating magnetic skyrmions via tunable magnetic anisotropies in vdW multiferroic heterostructures represents a highly promising and energy-efficient strategy for the future development of spintronics.

Organic-Inorganic Rare-Earth Double Perovskite Ferroelectric with Large Piezoelectric Response and Ferroelasticity for Flexible Composite Energy Harvesters.

Hybrid organic-inorganic perovskite (HOIP) ferroelectric materials have great potential for developing self-powered electronic transducers owing to their impressive piezoelectric performance, structural tunability and low processing temperatures. Nevertheless, their inherent brittle and low elastic moduli limit their application in electromechanical conversion. Integration of HOIP ferroelectrics and soft polymers is a promising solution. In this work, a hybrid organic-inorganic rare-earth double perovskite ferroelectric, [RM3HQ]2RbPr(NO3)6 (RM3HQ = (R)-N-methyl-3-hydroxylquinuclidinium) is presented, which possesses multiaxial nature, ferroelasticity and satisfactory piezoelectric properties, including piezoelectric charge coefficient (d33) of 102.3 pC N-1 and piezoelectric voltage coefficient (g33) of 680 × 10-3 V m N-1. The piezoelectric generators (PEG) based on composite films of [RM3HQ]2RbPr(NO3)6@polyurethane (PU) can generate an open-circuit voltage (Voc) of 30 V and short-circuit current (Isc) of 18 µA, representing one of the state-of-the-art PEGs to date. This work has promoted the exploration of new HOIP ferroelectrics and their development of applications in electromechanical conversion devices.

Enhancing Short-Range Interactions to Broaden the Temperature Range for Coexistence of Antiferroelectricity and Ferroelasticity.

Antiferroelectric (AFE) materials, characterized by double electric hysteresis loops, can be transformed to the ferroelectric (FE) phase under an external electric field, making them promising candidates for electronic energy storage and solid-state refrigeration. Additionally, the field-induced strain in AFE materials is contingent upon the direction of the electric field, rendering it with a switching characteristic. Although AFE materials have made progress in the field of energy storage and negative electrocaloric effect, the coexistence of AFE and ferroelasticity is still rarely reported. Here, two isomorphic organic-inorganic hybrid perovskites, HDAEPbCl4 and HDAEPbBr4 (HDAE is [2-(hydroxydimethylammonio)ethan-1-aminium]), exhibiting FE-AFE-PE (PE is paraelectric) phase transitions, are presented. Remarkably, the temperature range where AFE and ferroelasticity coexist is significantly broadened from 59.9 K to 115.1 K by strengthening short-range forces via halogen substitution. This discovery extends the family of FE, AFE, and ferroelastic materials, contributing to the development of multifunctional materials and advancing multifunctional material development.

Polymorphic Heterogeneous Polar Structure Enabled Superior Capacitive Energy Storage in Lead-Free Relaxor Ferroelectrics at Low Electric Field.

High-performance energy storage dielectrics capable of low/moderate field operation are vital in advanced electrical and electronic systems. However, in contrast to achievements in enhancing recoverable energy density (Wrec), the active realization of superior Wrec and energy efficiency (η) with giant energy-storage coefficient (Wrec/E) in low/moderate electric field (E) regions is much more challenging for dielectric materials. Herein, lead-free relaxor ferroelectrics are reported with giant Wrec/E designed with polymorphic heterogeneous polar structure. Following the guidance of Landau phenomenological theory and rational composition construction, the conceived (Bi0.5Na0.5)TiO3-based ternary solid solution that delivers giant Wrec/E of ≈0.0168 µC cm-2, high Wrec of ≈4.71 J cm-3 and high η of ≈93% under low E of 280 kV cm-1, accompanied by great stabilities against temperature/frequency/cycling number and excellent charging-discharging properties, which is ahead of most currently reported lead-free energy storage bulk ceramics measured at same E range. Atomistic observations reveal that the correlated coexisting local rhombohedral-tetragonal polar nanoregions embedded in the cubic matrix are constructed, which enables high polarization, minimized hysteresis, and significantly delayed polarization saturation concurrently, endowing giant Wrec/E along with high Wrec and η. These findings advance the superiority and feasibility of polymorphic nanodomains in designing highly efficient capacitors for low/moderate field-region practical applications.

Realization of the Giant Pyroelectric Response via Modulated Polar Structures.

Among pyroelectric materials, Bi0.5Na0.5TiO3 (BNT)-based relaxors are particularly noteworthy due to their significant polarization fluctuation near the depolarization temperature (Td), resulting in a large pyroelectric response. What has been overlooked is the dynamic behavior of inherent polar structures, particularly the temperature-dependent evolution of polar nanoregions (PNRs), which significantly impacts the pyroelectric behavior. Herein, based on the large pyroelectric response origination (the ferroelectric-relaxor phase transition), the mixed nonergodic and ergodic relaxor (NR+ER) critical state is constructed, which is believed to trigger the easily fluctuating polarization state with excellent pyroelectric response. Composition engineering (with Li+, Sr2+, and Ta5+) strategically controls the relaxor process and modulates the dynamic behavior of inherent polar structures by the random field effect. The pyroelectric coefficient of more than 1441 µCm-2K-1 at room temperature (RT), more than 9221 µCm-2K-1 (RT), and ≈107911 µCm-2K-1 (Td) are achieved in the Li+-doped sample, the Sr2+-doped sample, and the (Li++Ta5+) co-doped sample, respectively. This work earns the highest RT pyroelectric coefficient in BNT-based relaxors, which is suitable for pyroelectric applications. Furthermore, it provides a strategy for modulating the pyroelectric performance of BNT-based relaxors.

Local Isomeric Polar Nanoclusters Enabled Superior Capacitive Energy Storage Under Moderate Fields in NaNbO3-Based Lead-Free Ceramics.

The high-field energy-storage performance of dielectric capacitors has been significantly improved in recent years, yet the high voltage risks of device failure and large cost of insulation technology increase the demand for high-performance dielectric capacitors at finite electric fields. Herein, a unique superparaelectric state filled with polar nanoclusters with various local symmetries for lead-free relaxor ferroelectric capacitors is subtly designed through a simple chemical modification method, successfully realizing a collaborative improvement of polarization hysteresis, maximum polarization, and polarization saturation at moderate electric fields of 20-30 kV mm-1. Therefore, a giant recoverable energy density of ≈5.0 J cm-3 and a high efficiency of ≈82.1% are simultaneously achieved at 30 kV mm-1 in (0.9-x)NaNbO3-0.1BaTiO3-xBiFeO3 lead-free ceramics, showing a breakthrough progress in moderate-field comprehensive energy-storage performances. Moreover, superior charge-discharge performances of high-power density ≈182 MW cm-3, high discharge energy density ≈4.3 J cm-3 and ultra-short discharge time <70 ns as well as excellent temperature stability demonstrate great application potentials for dielectric energy-storage capacitors in pulsed power devices. This work provides an effective and paradigmatic strategy for developing novel lead-free dielectrics with high energy-storage performance under finite electric fields.

Single-Crystal Ferroelectric-Based (K,Na)NbO3 Microcuboid/CuO Nanodot Heterostructures with Enhanced Photo-Piezocatalytic Activity.

Developing single-crystal-based heterostructured ferroelectrics with high-performance photo-piezocatalytic activity is highly desirable to utilize large piezopotentials and more reactive charges that can trigger the desired redox reactions. To that end, a single-crystal-based (K,Na)NbO3 (KNN) microcuboid/CuO nanodot heterostructure with enhanced photo-piezocataytic activity, prepared using a facile strategy that leveraged the synergy between heterojunction formation and an intense single-crystal-based piezoelectric effect, is reported herein. The catalytic rhodamine B degrading activity of KNN/CuO is investigated under light irradiation, ultrasonication, or co-excitation with both stimulations. Compared to polycrystalline KNN powders and bare KNN single-crystals, single-crystal-based KNN/CuO exhibits a higher piezocurrent density and an optimal energy band structure, resulting in 5.23 and 2.37 times higher piezocatalytic degradation activities, respectively. Furthermore, the maximum photo-piezocatalytic rate constant (≈0.093 min-1 ) of KNN/CuO under 25 min ultrasonication and light irradiation is superior to that of other KNN-based catalysts, and 1.6 and 48.6 times higher than individual piezocatalytic and photocatalytic reaction rate constants, respectively. The excellent photo-piezocatalytic activity is attributed to the enhanced charge-carrier separation and proper alignment of band structure to the required redox levels by the appropriate p-n heterojunction and high piezoelectric potential. This report provides useful insight into the relationships between heterojunctions, piezoelectric responses, and catalytic mechanisms for single-crystal-based heterostructured catalysts.

2D materials-based photodetectors combined with ferroelectrics.

Photodetectors are essential optoelectronic devices that play a critical role in modern technology by converting optical signals into electrical signals, which are one of the most important sensors of the informational devices in current 'Internet of Things' era. Two-dimensional (2D) material-based photodetectors have excellent performance, simple design and effortless fabrication processes, as well as enormous potential for fabricating highly integrated and efficient optoelectronic devices, which has attracted extensive research attention in recent years. The introduction of spontaneous polarization ferroelectric materials further enhances the performance of 2D photodetectors, moreover, companying with the reduction of power consumption. This article reviews the recent advances of materials, devices in ferroelectric-modulated photodetectors. This review starts with the introduce of the basic terms and concepts of the photodetector and various ferroelectric materials applied in 2D photodetectors, then presents a variety of typical device structures, fundamental mechanisms and potential applications under ferroelectric polarization modulation. Finally, we summarize the leading challenges currently confronting ferroelectric-modulated photodetectors and outline their future perspectives.

CMOS-compatible Hf0.5Zr0.5O2-based ferroelectric memory crosspoints fabricated with damascene process.

We report the fabrication of Hf0.5Zr0.5O2(HZO) based ferroelectric memory crosspoints using a complementary metal-oxide-semiconductor-compatible damascene process. In this work, we compared 12 and 56µm2crosspoint devices with the 0.02 mm2round devices commonly used as a benchmark. For all devices, a 9 nm thick ferroelectric thin film was deposited by plasma-enhanced atomic layer deposition on planarized bottom electrodes. The wake-up appeared to be longer for the crosspoint memories compared to 0.02 mm2benchmark, while all the devices reached a 2Prvalue of ∼50µC cm-2after 105cycles with 3 V/10µs squared pulses. The crosspoints stand out for their superior endurance, which was increased by an order of magnitude. Nucleation limited switching experiments were performed, revealing a switching time <170 ns for our 12 and 56µm2devices, while it remained in theµs range for the larger round devices. The downscaled devices demonstrate notable advantages with a rise in endurance and switching speed.

Revealing the Polarizations of Molecular Ferroelectrics via SHG Polarimetry at the Nanoscale.

Multifarious molecular ferroelectrics with multipolar axial characteristics have emerged in recent years, enriching the scenarios for energy harvesting, sensing, and information processing. The increased polar axes have enhanced the urgency of distinguishing different polarization states in material design, mechanism exploration, etc. However, conventional methods hardly meet the requirements of in situ, fast, microscale, contactless, and nondestructive features due to their inherent limitations. Herein, SHG polarimetry is introduced to probe the multioriented polarizations on a nanosized multiaxial molecular ferroelectric, i.e., TMCM-CdCl3 nanoplates, as an example. Combined with the analysis of the second-order susceptibility tensor, SHG polarimetry could serve as an effective method to detect the polarization orders and domain distributions of molecular ferroelectrics. Profiting from the full-optical feature, SHG polarimetry can even be performed on samples covered by transparent mediums, 2D materials, or thin metal electrodes. Our research might spark further fundamental studies and expand the application boundaries of next-generation ferroelectric materials.

Role of Defects in the Breakdown Phenomenon of Al1-xScxN: From Ferroelectric to Filamentary Resistive Switching.

Aluminum scandium nitride (Al1-xScxN), with its large remanent polarization, is an attractive material for high-density ferroelectric random-access memories. However, the cycling endurance of Al1-xScxN ferroelectric capacitors is far below what can be achieved in other ferroelectric materials. Understanding the nature and dynamics of the breakdown mechanism is of the utmost importance for improving memory reliability. The breakdown phenomenon in ferroelectric Al1-xScxN is proposed to be an impulse thermal filamentary-driven process along preferential defective pathways. For the first time, stable and robust bipolar filamentary resistive switching in ferroelectric Al1-xScxN is reported. A hot atom damage defect generation model illustrates how filament formation and ferroelectric switching are connected. The model reveals the tendency of the ferroelectric wurtzite-type Al1-xScxN system to reach internal symmetry with bipolar electric field cycling. Defects generated from bipolar electric field cycling influence both the energy barrier between the polarization states and that required for the filament formation.

Thermodynamics of Light-Induced Nanoscale Polar Structures in Ferroelectric Superlattices.

We study the thermodynamics of nanoscale polar structures in PbTiO3/SrTiO3 ferroelectric superlattices induced by above-bandgap optical excitation using a phase-field model explicitly considering both structural and electronic processes. We demonstrate that the light-excited carriers provide the charge compensation of polarization bound charges and the lattice thermal energy, both of which are key to the thermodynamic stabilization of a previously observed supercrystal, a three-dimensionally periodic nanostructure, within a window of substrate strains, while different mechanical and electrical boundary conditions can stabilize a number of other nanoscale polar structures by balancing the competing short-range exchange interactions responsible for the domain wall energy and long-range electrostatic and elastic interactions. The insights into the light-induced formation and richness of nanoscale structures from this work offer theoretical guidance for exploring and manipulating the thermodynamic stability of nanoscale polar structures employing a combination of thermal, mechanical, and electrical stimuli as well as light.