Mechanical writing of ferroelectric polarization

A review of progress in theoretical modeling of polarization dynamics in ferroelectric materials

Ferroelectric materials are considered candidates for functional device application since their discovery in 1920. The functionality is realized by polarization evolution itself or the resulting effects. Studies on ferroelectrics have been going on over a century with a rough journey, because they have the excellent physical properties and also the fatal disadvantages for the device applications, where polarization microstructure and the dynamics are always the core issues. The demand for miniaturization, low energy consumption, and intelligence of devices leads to the advancement of the studies on the polarization microstructure and dynamics towards microscopic and ultrafast scales, as well as precise manipulation. This review mainly focuses on the inherent logic of the development of the theoretical modeling on the polarization dynamics. We would like to discuss the historical background of the development of theoretical models and their limitations, following the historical trajectory how to understand the multiscale nature of polarization microstructure and dynamics and the developing demand of functional devices applications, based on which the prospect and future development direction of theoretical modeling are proposed.

Nanoscale Polarization-Dependent Young's Modulus of Ferroelectric BaTiO3 Single Crystals

Flexoelectric fields produced by strong strain gradients at the nanoscale couple to ferroelectric polarization, promoting changes in the mechanical properties of ferroelectric materials as a function of the direction of the ferroelectric polarization. In this work, we calculate the asymmetry in the Young's modulus found in oppositely polarized out-of-plane domains of BaTiO3 single crystals by means of contact resonance AFM, and we evaluate its impact on the electromechanical response as measured by piezoresponse force microscopy, both using band excitation modes. We analyze the electromechanical response of the different BaTiO3 domains using k-means to build up their mean elastic and electromechanical features and quantify the flexoelectrically induced modulation of the Young's modulus. Finally, we discuss the use of contact Kelvin probe force microscope measurements to decouple electrostatic artifacts from purely piezoelectric ones when flexoelectric coupling appears.

Photoinduced Deterministic Polarization Switching in CuInP2S6 for Multifunctional Optoelectronic Logic Gates

Optoelectronic logic gates (OELGs) converting photonic inputs into electric output based on Boolean logic are promising for next-generation computations, and it is highly desirable to be able to control the current polarity by light for multifunctional devices. Here we report a new strategy for OELGs based on bipolar photoconduction intrinsic to ferroelectric materials, simplifying the device configuration considerably while enabling multiple logic operations. We demonstrate this concept in two-dimensional (2D) ferroelectric CuInP2S6 (CIPS), taking advantage of the fact that its polarization switching is intimately coupled with Cu cation migration, and thus can be deterministically driven by both above- and below-bandgap illumination via the photothermal effect. This in turn switches the polarity of the photocurrent arising from the bulk photovoltaic effect (BPVE), which is sensitive to the light intensity, enabling the execution of "OR", "XOR", and "NOT" logic operations in a single device with a simple sandwich structure.

Reversible Ferroelectric Polarization Modulation of Chiral Molecular Ferroelectrics by Circularly Polarized Light

The optical modulation of ferroelectric polarization constitutes a transformative, non-contact strategy for the precise manipulation of ferroelectric properties, heralding advancements in optically stimulated ferroelectric devices. Despite its potential, progress in this domain is constrained by material limitations and the intricate nature of the underlying mechanisms. Recent studies have achieved efficient regulation of ferroelectric polarization and thermal conductivity in chiral ferroelectric thin films through the application of left- and right-handed circularly polarized light (LCP and RCP). Differential absorption of circularly polarized light (CPL) induces nonequilibrium carrier dynamics, generating distinctive interfacial electrostatic fields that enable precise control of ultrathin ferroelectric films. For (R)-BINOL-DIPASi and (S)-BINOL-DIPASi (C26H26O2Si), polarization changes surpass 23%, exhibiting opposite response under LCP and RCP excitation. In R chiral films, remnant polarization decreases from 1.05 µC cm- 2 under LCP to 0.85 µC cm- 2 under RCP, whereas in S chiral films, polarization increases from 0.85 µC cm- 2 under LCP to 0.98 µC cm- 2 under RCP. This reversible modulation facilitates reliable switching between ON and OFF states, presenting the potential of chiral ferroelectric materials for flexible, high-speed integrated photonic sensor technologies.

A Lithium Complex Ferroelectric with High Curie Temperature: [Li(1,10-Phenanthroline)2]I

Paraelectric-ferroelectric phase transitions have been mainly acknowledged by the order-disorder type in most molecular ferroelectrics or the displacive type in inorganic ferroelectrics. However, reports regarding symmetry-breaking ferroelectric phase transitions induced by distortion in the coordination geometry have been scarce. In this study, we present a lithium complex molecular ferroelectric [Li(1,10-phenanthroline)2]I, which undergoes an Aizu mmmFmm2 type symmetry-breaking ferroelectric phase transition at 371 K. Its ferroelectricity has been identified by the geometric distortion of the lithium ions coordination environment and the concomitant directional movement of iodide ions. Besides, the ferroelectric loop of [Li(1,10-phenanthroline)2]I can be readily obtained from a polarized polycrystalline pellet at room temperature; in addition, its nonlinear optical signal is comparable to that of KH2PO4 (KDP). To the best of our knowledge, [Li(1,10-phenanthroline)2]I represents the first lithium-based organic complex ferroelectric exhibiting a coordination geometry-triggered symmetry-breaking ferroelectric phase transition. This work will provide significant inspiration for the design of novel molecular ferroelectrics.

Polytype switching by super-lubricant van der Waals cavity arrays

Expanding the performance of field-effect devices is a key challenge of the ever-growing chip industry at the core of current technologies1. Non-volatile multiferroic transistors that control atomic movements rather than purely electronic distribution are highly desired2. Recently, a field-effect control over structural transitions was achieved in commensurate stacking configurations of honeycomb van der Waals (vdW) polytypes by sliding boundary strips between oppositely polarized domains3-6. This ferroelectric hysteretic response, however, relied on pre-existing dislocation strips between relatively large micron-scale domains, severely limiting practical implementations3,7,8. Here we report the robust electric switching of single-domain polytypes in nanometre-scale islands embedded in super-lubricant vdW arrays. We etch cavities into a thin layered spacer and then encapsulate it with functional flakes. The flakes above/under the lattice-mismatched spacer sag and touch at each cavity to form islands of commensurate and metastable polytype configurations. By imaging the polarization of the polytypes, we observe nucleation and annihilation of boundary strips and geometry-adaptable ferroelectric hysteresis loops. Using mechanical stress, we further control the position of boundary strips, modify marginal twist angles and nucleate patterns of polar domain. This super-lubricant arrays of polytype (SLAP) concept suggests 'slidetronics' device applications such as elastic-coupled neuromorphic memory cells and non-volatile multiferroic tunnelling transistors and programmable response by designing the size, shape and symmetry of the islands and of the arrays9.

Mechanical and electromechanical properties of 2D materials studied via in situ microscopy techniques

Two-dimensional (2D) materials with van der Waals stacking have been reported to have extraordinary mechanical and electromechanical properties, which give them revolutionary potential in various fields. However, due to the atomic-scale thickness of these 2D materials, their fascinating properties cannot be effectively characterized in many cases using conventional measurement techniques. Based on typical microscopy techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM), a range of in situ microscopy techniques have been developed to systematically quantify the mechanical and electromechanical properties of 2D materials. This review highlights the advancements of in situ microscopy techniques for studying elasticity and fracture, adhesion and separation, structural superlubricity, as well as c-axis piezoresistivity and rotation angle-related transport of 2D materials. The methods and results of various microscopy experiments, including nanoindentation using AFM, pressurized bubble tests, self-retraction experiments, pull-to-peel methods and so on, are compared, and their respective advantages and limitations are discussed. Finally, we summarize the current challenges in these microscopy techniques and outline development opportunities.

Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electonic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

Dual-role ion dynamics in ferroionic CuInP2S6: revealing the transition from ferroelectric to ionic switching mechanisms

Due to its "ferroionic" nature, CuInP2S6 combines switchable ferroelectric polarization with highly mobile Cu ions, allowing for multiple resistance states. Its conductive mechanism involves ferroelectric switching, ion migration, and corresponding intercoupling, which are highly sensitive to external electric field. Distinguishing the dominant contribution of either ferroelectric switching or ion migration to dynamic conductivity remains a challenge and the conductive mechanism is not clear yet. Here, based on polarization switching analyses and first-principles calculations, this work demonstrates that the Cu ion migration pathways enable the formation of a quadruple-well state, determining the conductive mechanism. Accordingly, it favors the manipulation of Cu ion transport in the intralayer and interlayer in a controlled manner, and makes a transition from ferroelectric-dominated to ion-migration-dominated conductivity, by tailoring the electric fields. This work deepens the understanding of ion migration dynamics and conductive switching in ferroionic systems, which is critical for the advancement of memristor-based neuromorphic computing.

Anomalous reverse mechanical polarization switching in negative piezoelectric CuInP2S6

van der Waals ferroelectric CuInP2S6 (CIPS) has drawn significant attention not only because of its unique properties but also owing to its technological potential for nanoelectronics. Mechanical polarization switching provides a new approach to modulating polarization states through flexoelectricity. This approach is particularly favourable for CIPS to avoid surface damage under an electric field due to the coupling between polarization switching and ionic motion. Here, we report anomalous downward-to-upward polarization switching under tip force in CIPS nanoflakes, which is believed to stem from the competition between piezoelectric and flexoelectric fields induced by tip pressure, together with the unique quadruple-well state present in CIPS. This work provides novel insights into the polarization switching mechanism of CIPS, elucidating the interplay between competing piezoelectric and flexoelectric fields, and it may pave the way for the design of electromechanical devices based on flexoelectric engineering.

Synthesizing ordered polar patterns in nonpolar SrTiO3 nanofilms via wrinkle-induced flexoelectricity

Ordered polar structures in oxide nanofilms play a pivotal role in the development of nanoelectronic applications. Hitherto, ordered polar structures have been restricted to a limited number of ferroelectric materials, and there is no effective scheme to induce and manipulate ordered polar patterns in centrosymmetric nonpolar nanofilms due to the absence of spontaneous symmetry breaking. Here, we circumvent these limitations by utilizing the wrinkle-induced strain gradient modulation associated with flexoelectricity as a general means of inducing and manipulating ordered polar patterns in nonpolar nanofilms. Leveraging the surface instability caused by strain mismatch between oxide nanofilms and pre-strained compliant substrate, we successfully fabricate striped SrTiO3 wrinkles, where well-ordered strain gradients and corresponding periodic polar patterns are readily achieved. Through in-situ piezoresponse force microscopy experiments, we show that the generated polar patterns can be manipulated by varying strain boundaries. Furthermore, the atomistic resolution images and first-principles calculations reveal that such wrinkle-induced ordered polar patterns primarily emerge from the flexoelectric coupling between the local polarization and strain gradients. These findings provide implications for manipulating polar structures by strain gradient and flexoelectric engineering, which in turn enable the realization of nontrivial polar structures in a broader range of materials.

Mechanical Writing of Polar Skyrmionic Topological States via Extrinsic Dzyaloshinskii-Moriya-like Flexoelectricity in Ferroelectric Thin Films

Exploring complex topological structures in condensed matter has shown promising applications in nanotechnology. Although polar topologies such as chiral vortices and skyrmions have been observed in ferroelectric heterostructures, their existence in simple systems has posed challenges due to the absence of intrinsic noncollinear interaction (like Dzyaloshinskii-Moriya interaction in ferromagnetics). Here, we demonstrate that a nanoindentation mechanically switches local polarizations to stable polar topologies, including skyrmions, within a room-temperature PbTiO3 thin film via the flexoelectric effect as a noncollinear (Dzyaloshinskii-Moriya-like) driving force using phase-field simulations. In addition, by moving the indenter, the continuous polarization switching leads to the "writing" of arbitrary polar patterns (such as donut-like skyrmionium). Furthermore, the written topologies can be "erased" by applying a voltage with the same conducted indenter. Therefore, this study shows the writing and erasing process of room-temperature polar topologies in a ferroelectric thin film, which significantly advances their potential applications.

Strain-Engineered Ferroelectricity in 2H Bilayer MoS2

The exploration of two-dimensional (2D) materials exhibiting out-of-plane ferroelectric and piezoelectric properties through interlayer twist/translation or strain, known as sliding ferroelectricity, has become a focal point in the quest for low-power electronic devices, capitalizing on weak van der Waals interactions. Herein, we delve into the behavior of strained bilayer molybdenum disulfide (2L-MoS2) transferred onto a nanocone-patterned substrate. An intriguing observation is the emergence of unexpected vertical ferroelectricity in MoS2, irrespective of whether it was prepared using chemical vapor deposition or mechanical exfoliation from the bulk crystal. Such an observation underscores the versatility and reproducibility of the emerging ferroelectricity across different preparation methods. Furthermore, the piezoelectric coefficients recorded are exceptionally high, with the values of 37.54 and 24.80 pm V-1 for monolayer and bilayer MoS2, respectively, outperforming most currently discovered 2D piezoelectrics. The presence of room-temperature out-of-plane ferroelectricity in strained 2L-MoS2 is confirmed through first-principles calculations and piezoresponse force microscopy. This ferroelectric behavior can be attributed to the symmetry breaking and interlayer sliding within the strained 2L-MoS2 structure. Our findings not only deepen the understanding of ferroelectricity in 2D materials but also offer insights for the design of 2D ferroelectrics, thereby enabling diverse functionalities and applications in ferroelectricity.

Intertwined Flexoelectricity and Stacking Ferroelectricity in Marginally Twisted hBN Moiré Superlattice

Moiré superlattices in twisted van der Waals homo/heterostructures present a fascinating interplay between electronic and atomic structures, with potential applications in electronic and optoelectronic devices. Flexoelectricity, an electromechanical coupling between electric polarization and strain gradient, is intrinsic to these superlattices because of the lattice misfit strain at the atomic scale. However, due to its weak magnitude, the effect of flexoelectricity on moiré ferroelectricity has remained underexplored. Here, the role of flexoelectricity in shaping and modulating the moiré ferroelectric patterns in twisted hBN homojunction is unveiled. Enhanced flexoelectric effects induce unique stacking ferroelectric domains with hollow triangular structures. Interlayer bubbles influence domain shape and periodicity through local electric field modulation, and tip-stress enables the reversible manipulation of domain area and polarization direction. These findings highlight the impact of flexoelectric effects on moiré ferroelectricity, offering a new tuning knob for its manipulation.

Dynamical Manipulation of Polar Topologies from Acoustic Phonon Excitations

Since the recent discovery of polar topologies, a recurrent question has been how to remotely tune them. Many efforts have focused on the pumping of polar optical phonons from optical methods, but with limited success, as only switching between specific phases has been achieved so far. Additionally, the correlation between optical pulse characteristics and the resulting phase is poorly understood. Here, we propose an alternative approach and demonstrate the deterministic and dynamical tailoring of polar topologies using acoustic phonon excitations. Our second-principles simulations reveal that by pumping specific longitudinal and transverse acoustic phonons, various topological textures can be induced in materials like BaTiO3 or PbTiO3. This method leverages the strong coupling between polarization and strain in these materials, enabling predictable and dynamic control of polar patterns. Our findings open up an alternative possibility for the manipulation of polar textures, suggesting a promising research direction.

Flexomagnetoelectric Effect in Sr_{2}IrO_{4} Thin Films

Symmetry engineering is explicitly effective to manipulate and even create phases and orderings in strongly correlated materials. Flexural stress is universally practical to break the space-inversion or time-reversal symmetry. Here, by introducing strain gradient in a centrosymmetric antiferromagnet Sr_{2}IrO_{4}, the space-inversion symmetry is broken accompanying a nonequivalent O p-Ir d orbital hybridization along the z axis. Thus, an emergent polar phase and out-of-plane magnetic moment have been simultaneously observed in these asymmetric Sr_{2}IrO_{4} thin films, which both are absent in its ground state. Furthermore, upon the application of a magnetic field, such polarization can be controlled by modifying the occupied d orbitals through spin-orbit interaction, giving rise to a flexomagnetoelectric effect. This Letter provides a general strategy to artificially design multiple symmetries and ferroic orderings in strongly correlated systems.

The Effect of Multi-Fields Synergy from Electric/Light/Thermal/Force Technologies on Photovoltaic Performance of Ba0.06Bi0.47Na0.47TiO3 Ferroelectric Ceramics via the Mg/Co Substitution at A/B Sites

Currently, it is widely reported that the photovoltaic effect in ferroelectric materials can be promoted by the application of a piezoelectric force, an external electric field, and intense light illumination. Here, a semiconducting ferroelectric composition is introduced, (1-x) Ba0.06Bi0.47Na0.47TiO3-xMgCoO3 (abbreviated as xMgCo, where x = 0.02-0.08), synthesized through Mg/Co ions codoping. This process effectively narrows the optical bandgaps to a spectrum of 1.38-3.06 eV. Notably, the system exhibits a substantial increase in short-circuit photocurrent density (Jsc), by the synergy of the electric, light, and thermal fields. The Jsc can still be further enhanced by the extra introduction of a force field. Additionally, the Jsc also shows an obvious increase after the high field pre-poling. The generation of a considerable number of oxygen vacancies due to the Co2+/Co3+ mixed valence state (in a 1:3 ratio) contributes to the reduced optimal bandgap. The integration of Mg2+ ion at the A-site restrains the loss and sustains robust ferroelectricity (Pr = 24.1 µC cm-2), high polarizability under an electric field, and a significant piezoelectric coefficient (d33 = 102 pC N-1). This study provides a novel perspective on the physical phenomena arising from the synergy of multiple fields in ferroelectric photovoltaic materials.

Possible ferro-electro-magnetic behaviours of graphene-based materials: hydrogenated graphene, MWCNTs and reduced graphene-oxide (GO)

This study investigated the electric polarization and magnetic behaviours of various graphene-based materials, including hydrogenated graphene (H-graphene), multi-wall carbon nanotubes (MWCNTs), and reduced graphene oxide (r-GO). Results showed that MWCNTs exhibit higher magnetization, with a magnetic squareness (M r/M s) of approximately ≈0.5, compared to H-graphene (≈0.25). H-graphene exhibits the highest electric polarization compared to MWCNTs/r-GO, whereas r-GO demonstrates the lowest levels of polarization and magnetization compared to H-graphene/MWCNTs. The valence band maximum (4.08 eV for MWCNTs, 4.26 eV for H-graphene, and 4.78 eV for r-GO) in quasi-localized states at the Fermi level results in defects in the graphene-based lattice, which are associated with dipole moment and lead to alterations in magnetic behaviours. Different density of states (DOS) is attributed from the ultra-violet photoelectron spectra and the small variations in the Fermi edge is observed in H-graphene, MWCNTs, and r-GO are responsible for the observed magnetisation and polarizations. The unique polarization/magnetization behaviours present an opportunity for potential exploitation in storage and information processing technologies in the science and engineering community.

Discovery of molecular ferroelectric catalytic annulation for quinolines

Ferroelectrics as emerging and attractive catalysts have shown tremendous potential for applications including wastewater treatment, hydrogen production, nitrogen fixation, and organic synthesis, etc. In this study, we demonstrate that molecular ferroelectric crystal TMCM-CdCl3 (TMCM = trimethylchloromethylammonium) with multiaxial ferroelectricity and superior piezoelectricity has an effective catalytic activity on the direct construction of the pharmacologically important substituted quinoline derivatives via one-pot [3 + 2 + 1] annulation of anilines and terminal alkynes by using N,N-dimethylformamide (DMF) as the carbon source. The recrystallized TMCM-CdCl3 crystals from DMF remain well ferroelectricity and piezoelectricity. Upon ultrasonic condition, periodic changes in polarization contribute to the release of free charges from the surface of the ferroelectric domains in nano size, which then quickly interacts with the substrates in the solution to trigger the pivotal redox process. Our work advances the molecular ferroelectric crystal as a catalytic route to organic synthesis, not only providing valuable direction for molecular ferroelectrics but also further enriching the executable range of ferroelectric catalysis.

Enhanced photodetection through a perovskite BaTiO3 dielectric in a Si-MoS2 heterojunction

The present investigation deals with the effect of a BaTiO3 (BTO) dielectric layer on the performance of MoS2/p-Si heterojunction photodetectors. The MoS2/p-Si junction demonstrates a responsivity of ∼80 A W-1 and detectivity of ∼1012 Jones. The inclusion of a dielectric BTO layer significantly enhances the performance of MoS2/p-Si photodetectors, leading to a remarkable improvement with a very high responsivity of ∼603 A W-1 and detectivity of ∼1013 Jones. The I-V characteristics of the MoS2/p-Si and MoS2/BTO/p-Si junctions under illumination can be understood by considering their respective energy band diagrams. This addition alters the energy band alignment, leading to higher conduction band offset and valence band offset values. The large photocurrent in forward bias in the MoS2/BTO/p-Si junction may be attributed to the presence of photogenerated electrons in the depletion region of BTO. BTO exhibits characteristics such as a long carrier diffusion length and low recombination rates, contributing to a reduction in carrier recombination within the photodetector for which the photocurrent of the MoS2/BTO/p-Si heterojunction can be improved significantly. The enhanced performance of the MoS2/BTO/p-Si junction, characterized by higher responsivity and detectivity, underscores the potential of this heterojunction for advanced photodetection applications, suggesting promising avenues for further research and development in the field of photodetectors. A comparative study with the available literature reveals that the excellent responsivity of ∼603 A W-1 and detectivity of ∼1013 Jones of the presently studied MoS2/BTO/p-Si heterojunction appear highly promising for various futuristic device applications.

Optical Control of In-Plane Domain Configuration and Domain Wall Motion in Ferroelectric and Ferroelastic Materials

The sensitivity of ferroelectric domain walls to external stimuli makes them functional entities in nanoelectronic devices. Specifically, optically driven domain reconfiguration with in-plane polarization is advantageous and thus is highly sought. Here, we show the existence of in-plane polarized subdomains imitating a single domain state and reversible optical control of its domain wall movement in a single-crystal of ferroelectric BaTiO3. Similar optical control in the domain configuration of nonpolar ferroelastic material indicates that long-range ferroelectric polarization is not essential for the optical control of domain wall movement. Instead, flexoelectricity is found to be an essential ingredient for the optical control of the domain configuration, and hence, ferroelastic materials would be another possible candidate for nanoelectronic device applications.

Reversible Mechanical Switching of Ferroelastic Stripe Domains in Multiferroic Thin Films

The application potential of ferroelectric thin films largely relies on the controllability of their domain structure. Among the various proposed strategies, mechanical switching is being considered as a potential alternative to replace electrical switching for control of the domain structure of ferroelectric thin films via, e.g., the flexoelectric effect. So far, studies on mechanical switching are confined to out-of-plane polarization switching in ferroelectric thin films, which are in pristine or prepoled single-domain states. In this work, we report reversible in-plane mechanical switching of the monoclinic phase (MC phase) stripe domains in BiFeO3 thin films can be realized by scanning tip force. Via controlling the fast scan direction of the scanning probe microscopy tip and the magnitude of the tip force, the effective trailing field induced by the local tip force can be rotated to consequently switch the net in-plane polarization of the two-variant stripe domain patterns by either 90° or 180°. Moreover, the monoclinic to rhombohedral (MC-R) phase transition occurs during mechanical switching with the distribution of R-phase domains dependent on the switching paths. These results extend our current understanding of the mechanical switching behavior in ferroelectric thin films and should be instructive for their future applications.

Free-standing two-dimensional ferro-ionic memristor

Two-dimensional (2D) ferroelectric materials have emerged as significant platforms for multi-functional three-dimensional (3D) integrated electronic devices. Among 2D ferroelectric materials, ferro-ionic CuInP2S6 has the potential to achieve the versatile advances in neuromorphic computing systems due to its phase tunability and ferro-ionic characteristics. As CuInP2S6 exhibits a ferroelectric phase with insulating properties at room temperature, the external temperature and electrical field should be required to activate the ferro-ionic conduction. Nevertheless, such external conditions inevitably facilitate stochastic ionic conduction, which completely limits the practical applications of 2D ferro-ionic materials. Herein, free-standing 2D ferroelectric heterostructure is mechanically manipulated for nano-confined conductive filaments growth in free-standing 2D ferro-ionic memristor. The ultra-high mechanical bending is selectively facilitated at the free-standing area to spatially activate the ferro-ionic conduction, which allows the deterministic local positioning of Cu+ ion transport. According to the local flexoelectric engineering, 5.76×102-fold increased maximum current is observed within vertical shear strain 720 nN, which is theoretically supported by the 3D flexoelectric simulation. In conclusion, we envision that our universal free-standing platform can provide the extendable geometric solution for ultra-efficient self-powered system and reliable neuromorphic device.

Insights into Strain Engineering: From Ferroelectrics to Related Functional Materials and Beyond

Ferroelectrics have become indispensable components in various application fields, including information processing, energy harvesting, and electromechanical conversion, owing to their unique ability to exhibit electrically or mechanically switchable polarization. The distinct polar noncentrosymmetric lattices of ferroelectrics make them highly responsive to specific crystal structures. Even slight changes in the lattice can alter the polarization configuration and response to external fields. In this regard, strain engineering has emerged as a prevalent regulation approach that not only offers a versatile platform for structural and performance optimization within ferroelectrics but also unlocks boundless potential in various functional materials. In this review, we systematically summarize the breakthroughs in ferroelectric-based functional materials achieved through strain engineering and progress in method development. We cover research activities ranging from fundamental attributes to wide-ranging applications and novel functionalities ranging from electromechanical transformation in sensors and actuators to tunable dielectric materials and information technologies, such as transistors and nonvolatile memories. Building upon these achievements, we also explore the endeavors to uncover the unprecedented properties through strain engineering in related chemical functionalities, such as ferromagnetism, multiferroicity, and photoelectricity. Finally, through discussions on the prospects and challenges associated with strain engineering in the materials, this review aims to stimulate the development of new methods for strain regulation and performance boosting in functional materials, transcending the boundaries of ferroelectrics.

Flexoelectric Engineering of Bulk Photovoltaic Photodetector

The bulk photovoltaic effect (BPVE) offers an interesting approach to generate a steady photocurrent in a single-phase material under homogeneous illumination, and it has been extensively investigated in ferroelectrics exhibiting spontaneous polarization that breaks inversion symmetry. Flexoelectricity breaks inversion symmetry via a strain gradient in the otherwise nonpolar materials, enabling manipulation of ferroelectric order without an electric field. Combining these two effects, we demonstrate active mechanical control of BPVE in suspended 2-dimensional CuInP2S6 (CIPS) that is ferroelectric yet sensitive to electric field, which enables practical photodetection with an order of magnitude enhancement in performance. The suspended CIPS exhibits a 20-fold increase in photocurrent, which can be continuously modulated by either mechanical force or light polarization. The flexoelectrically engineered photodetection device, activated by air pressure and without any optimization, possesses a responsivity of 2.45 × 10-2 A/W and a detectivity of 1.73 × 1011 jones, which are superior to those of ferroelectric-based photodetection and comparable to those of the commercial Si photodiode.

Reversible flexoelectric domain engineering at the nanoscale in van der Waals ferroelectrics

The universal flexoelectric effect in solids provides a mechanical pathway for controlling electric polarization in ultrathin ferroelectrics, eliminating potential material breakdown from a giant electric field at the nanoscale. One challenge of this approach is arbitrary implementation, which is strongly hindered by one-way switching capability. Here, utilizing the innate flexibility of van der Waals materials, we demonstrate that ferroelectric polarization and domain structures can be mechanically, reversibly, and arbitrarily switched in two-dimensional CuInP2S6 via the nano-tip imprinting technique. The bidirectional flexoelectric control is attributed to the extended tip-induced deformation in two-dimensional systems with innate flexibility at the atomic scale. By employing an elastic substrate, artificial ferroelectric nanodomains with lateral sizes as small as ~80 nm are noninvasively generated in an area of 1 μm2, equal to a density of 31.4 Gbit/in2. Our results highlight the potential applications of van der Waals ferroelectrics in data storage and flexoelectronics.

Robust multiferroicity and magnetic modulation of the ferroelectric imprint field in heterostructures comprising epitaxial Hf0.5Zr0.5O2 and Co

Magnetoelectric multiferroics, either single-phase or composites comprising ferroelectric/ferromagnetic coupled films, are promising candidates for energy efficient memory computing. However, most of the multiferroic magnetoelectric systems studied so far are based on materials that are not compatible with industrial processes. Doped hafnia is emerging as one of the few CMOS-compatible ferroelectric materials. Thus, it is highly relevant to study the integration of ferroelectric hafnia into multiferroic systems. In particular, ferroelectricity in hafnia, and the eventual magnetoelectric coupling when ferromagnetic layers are grown atop of it, are very much dependent on quality of interfaces. Since magnetic metals frequently exhibit noticeable reactivity when grown onto oxides, it is expected that ferroelectricity and magnetoelectricity might be reduced in multiferroic hafnia-based structures. In this article, we present excellent ferroelectric endurance and retention in epitaxial Hf0.5Zr0.5O2 films grown on buffered silicon using Co as the top electrode. The crucial influence of a thin Pt capping layer grown on top of Co on the ferroelectric functional characteristics is revealed by contrasting the utilization of Pt-capped Co, non-capped Co and Pt. Magnetic control of the imprint electric field (up to 40% modulation) is achieved in Pt-capped Co/Hf0.5Zr0.5O2 structures, although this does not lead to appreciable tuning of the ferroelectric polarization, as a result of its high stability. Computation of piezoelectric and flexoelectric strain-mediated mechanisms of the observed magnetoelectric coupling reveal that flexoelectric contributions are likely to be at the origin of the large imprint electric field variation.

Revealing Strong Flexoelectricity and Optoelectronic Coupling in 2D Ferroelectric CuInP2S6 Via Large Strain Gradient

The interplay between flexoelectric and optoelectronic characteristics provides a paradigm for studying emerging phenomena in various 2D materials. However, an effective way to induce a large and tunable strain gradient in 2D devices remains to be exploited. Herein, we propose a strategy to induce large flexoelectric effect in 2D ferroelectric CuInP2S6 by constructing a 1D-2D mixed-dimensional heterostructure. The strong flexoelectric effect is induced by enormous strain gradient up to 4.2 × 106 m-1 resulting from the underlying ZnO nanowires, which is further confirmed by the asymmetric coercive field and the red-shift in the absorption edge. The induced flexoelectric polarization efficiently boosts the self-powered photodetection performance. In addition, the improved photoresponse has a good correlation with the induced strain gradient, showing a consistent size-dependent flexoelectric effect. The mechanism of flexoelectric and optoelectronic coupling is proposed based on the Landau-Ginzburg-Devonshire double-well model, supported by density functional theory (DFT) calculations. This work provides a brand-new method to induce a strong flexoelectric effect in 2D materials, which is not restricted to crystal symmetry and thus offers unprecedented opportunities for state-of-the-art 2D devices.

Mechanical Control of Polar Patterns in Wrinkled Thin Films via Flexoelectricity

Intriguing topological polar structures in oxide nanofilms have drawn growing attention owing to their immense potential applications in nanoscale electronic devices. Here, we report a novel route to mechanically manipulate polar structures via flexoelectricity in wrinkled thin films. Our results present a flexoelectric polar transition from a nonpolar state to uniaxial polar stripes, biaxial meronlike or antimeronlike polar structures, and polar labyrinths by varying wrinkle morphologies. The evolution mechanisms and the outstanding mechanical tunability of these flexoelectric polar patterns were investigated theoretically and numerically. This strategy based on flexoelectricity for generating nontrivial polar structures will no longer rely on the superlattice structure and can be widely applicable to all centrosymmetric or noncentrosymmetric materials, providing a broader range of material and structure candidates for polar topologies.

Nanotube ferroelectric tunnel junctions with an ultrahigh tunneling electroresistance ratio

Low-dimensional ferroelectric tunnel junctions are appealing for the realization of nanoscale nonvolatile memory devices due to their inherent advantages of device miniaturization. Those based on current mechanisms have limitations, including low tunneling electroresistance (TER) effects and complex heterostructures. Here, we introduce an entirely new TER mechanism to construct a nanotube ferroelectric tunnel junction with ferroelectric nanotubes as the tunneling region. When rolling a ferroelectric monolayer into a nanotube, due to the coexistence of its intrinsic ferroelectric polarization with the flexoelectric polarization induced by bending, a metal-insulator transition occurs depending on the radiative polarization states. For the pristine monolayer, its out-of-plane polarization is tunable by an in-plane electric field, and the conducting states of the ferroelectric nanotube can thus be tuned between metallic and insulating states via axial electric means. Using α-In2Se3 as an example, our first-principles density functional theory calculations and nonequilibrium Green's function formalism confirm the feasibility of the TER mechanism and indicate an ultrahigh TER ratio that exceeds 9.9 × 1010% of the proposed nanotube ferroelectric tunnel junctions. Our findings provide a promising approach based on simple homogeneous structures for high density ferroelectric microelectric devices with excellent ON/OFF performance.

Advancements and Prospects of Flexoelectricity

The flexoelectric effect, as a novel form of the electromechanical coupling phenomenon, has attracted significant attention in the fields of materials science and electronic devices. It refers to the interaction between strain gradients and electric dipole moments or electric field intensity gradients and strain. In contrast to the traditional piezoelectric effect, the flexoelectric effect is not limited by material symmetry or the Curie temperature and exhibits a stronger effect at the nanoscale. The flexoelectric effect finds widespread applications ranging from energy harvesting to electronic device design. Utilizing the flexoelectric effect, enhanced energy harvesters, sensitive sensors, and high-performance wearable electronic devices can be developed. Additionally, the flexoelectric effect can be utilized to modulate the optoelectronic properties and physical characteristics of materials, holding the potential for significant applications in areas such as optoelectronic devices, energy storage devices, and flexible electronics. This review provides a comprehensive overview of the historical development, measurement of flexoelectric coefficients, enhancement mechanisms, and current research progress of the flexoelectric effect. Additionally, it offers a perspective on future prospects of the flexoelectric effect.

Flexophotovoltaic Effect and Above-Band-Gap Photovoltage Induced by Strain Gradients in Halide Perovskites

We have measured the flexophotovoltaic effect of single crystals of halide perovskites MAPbBr_{3} and MAPbI_{3}, as well as the benchmark oxide perovskite SrTiO_{3}. For halide perovskites, the flexophotovoltaic effect is found to be orders of magnitude larger than for SrTiO_{3}, and indeed large enough to induce photovoltages bigger than the band gap. Moreover, we find that in MAPbI_{3} the flexophotovoltaic effect is additional to a native bulk photovoltaic response that is switchable and ferroelectric-like. The results suggest that strain gradient engineering can be a powerful tool to modify the photovoltaic output even in already well-established photovoltaic materials.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

A 2D ferroelectric vortex pattern in twisted BaTiO3 freestanding layers

The wealth of complex polar topologies1-10 recently found in nanoscale ferroelectrics results from a delicate balance between the intrinsic tendency of the materials to develop a homogeneous polarization and the electric and mechanical boundary conditions imposed on them. Ferroelectric-dielectric interfaces are model systems in which polarization curling originates from open circuit-like electric boundary conditions, to avoid the build-up of polarization charges through the formation of flux-closure11-14 domains that evolve into vortex-like structures at the nanoscale15-17 level. Although ferroelectricity is known to couple strongly with strain (both homogeneous18 and inhomogeneous19,20), the effect of mechanical constraints21 on thin-film nanoscale ferroelectrics has been comparatively less explored because of the relative paucity of strain patterns that can be implemented experimentally. Here we show that the stacking of freestanding ferroelectric perovskite layers with controlled twist angles provides an opportunity to tailor these topological nanostructures in a way determined by the lateral strain modulation associated with the twisting. Furthermore, we find that a peculiar pattern of polarization vortices and antivortices emerges from the flexoelectric coupling of polarization to strain gradients. This finding provides opportunities to create two-dimensional high-density vortex crystals that would enable us to explore previously unknown physical effects and functionalities.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

Wnt7a is Required for Regeneration of Dystrophic Skeletal Muscle

Intramuscular injection of Wnt7a has been shown to accelerate and augment skeletal muscle regeneration and to ameliorate dystrophic progression in mdx muscle, a model for Duchenne muscular dystrophy (DMD). However, loss-of-function studies to investigate the requirement for Wnt7a in muscle regeneration has not been evaluated. Here, we assessed muscle regeneration and function in wild type (WT) and mdx mice where Wnt7a was specifically deleted in muscle using a conditional Wnt7a floxed allele and a Myf5-Cre driver. We found that both WT and mdx mice with deletion of Wnt7a in muscle, exhibited marked deficiencies in muscle regeneration at 21 d following cardiotoxin (CTX) induced injury. Unlike WT, deletion of Wnt7a in mdx resulted in a marked decrease in specific force generation prior to CTX injury. However, both WT and mdx muscle lacking Wnt7a displayed decreased specific force generation following CTX injection. Notably the regeneration deficit observed in mdx mice lacking Wnt7a in muscle was rescued by a single tail vein injection of an extracellular vesicle preparation containing Wnt7a (Wnt7a-EVs). Therefore, we conclude that the regenerative capacity of muscle in mdx mice is due to the upregulation of endogenous Wnt7a following injury, and that systemic delivery of Wnt7a-EVs represents a therapeutic strategy for treating DMD.

Giant Flexoelectricity in Bent Semiconductor Thinfilm

We elucidate the flexoelectricity of semiconductors in the high strain gradient regime, the underlying mechanism of which is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high-strain-gradient-induced band gap closure. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film. Therefore, upon the band gap closure, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to a pronounced change of polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

Switchable tribology of ferroelectrics

Switchable tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe-down domains have lower friction coefficients and show slower wear rates than up domains and can be used as smart masks. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is widely applicable across various ferroelectrics with different chemical compositions and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm-cm) size. These findings demonstrate that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.

Dual Breaking of Molecular Orbitals and Spatial Symmetry in an Optically Controlled Ferroelectric

As particles carry quantified energy, photon radiation enables orbital transitions of energy levels, leading to changes in the spin state of electrons. The resulting switchable structural bistability may bring a new paradigm for manipulating ferroelectric polarization. However, the studies on molecular orbital breaking in the ferroelectric field remain blank. Here, for the first time, a new mechanism of ferroelectrics-dual breaking of molecular orbitals and spatial symmetry, demonstrated in a photochromic organic crystal with light-induced polarization switching, is formally proposed. By alternating the ultraviolet/visible light irradiation, the states of electron spin and the radial distribution p atomic orbitals experience a change, showing a reversible switch from "shoulder-to-shoulder" form to a "head-to-head" form. This reflects a reversible conversion between π and σ bonds, which induces and couples with the variation of spatial symmetry. The intersection of spatial symmetry breaking and molecular orbital breaking in ferroelectrics present in this work will be more conducive to data encryption and anticounterfeiting.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Writing-Speed Dependent Thresholds of Ferroelectric Domain Switching in Monolayer α-In2 Se3

An electrical-biased or mechanical-loaded scanning probe written on the ferroelectric surface can generate programmable domain nanopatterns for ultra-scaled and reconfigurable nanoscale electronics. Fabricating ferroelectric domain patterns by direct-writing as quickly as possible is highly desirable for high response rate devices. Using monolayer α-In2 Se3 ferroelectric with ≈1.2 nm thickness and intrinsic out-of-plane polarization as an example, a writing-speed dependent effect on ferroelectric domain switching is discovered. The results indicate that the threshold voltages and threshold forces for domain switching can be increased from -4.2 to -5 V and from 365 to 1216 nN, respectively, as the writing-speed increases from 2.2 to 10.6 µm s-1 . The writing-speed dependent threshold voltages can be attributed to the nucleations of reoriented ferroelectric domains, in which sufficient time is needed for subsequent domain growth. The writing-speed dependent threshold forces can be attributed to the flexoelectric effect. Furthermore, the electrical-mechanical coupling can be employed to decrease the threshold force, achieving as low as ≈189±41 nN, a value smaller than those of perovskite ferroelectric films. Such findings reveal a critical issue of ferroelectric domain pattern engineering that should be carefully addressed for programmable direct-writing electronics applications.

Ferroelectricity in hafnia controlled via surface electrochemical state

Ferroelectricity in binary oxides including hafnia and zirconia has riveted the attention of the scientific community due to the highly unconventional physical mechanisms and the potential for the integration of these materials into semiconductor workflows. Over the last decade, it has been argued that behaviours such as wake-up phenomena and an extreme sensitivity to electrode and processing conditions suggest that ferroelectricity in these materials is strongly influenced by other factors, including electrochemical boundary conditions and strain. Here we argue that the properties of these materials emerge due to the interplay between the bulk competition between ferroelectric and structural instabilities, similar to that in classical antiferroelectrics, coupled with non-local screening mediated by the finite density of states at surfaces and internal interfaces. Via the decoupling of electrochemical and electrostatic controls, realized via environmental and ultra-high vacuum piezoresponse force microscopy, we show that these materials demonstrate a rich spectrum of ferroic behaviours including partial-pressure-induced and temperature-induced transitions between ferroelectric and antiferroelectric behaviours. These behaviours are consistent with an antiferroionic model and suggest strategies for hafnia-based device optimization.

Ultralow Tip-Force Driven Sizable-Area Domain Manipulation through Transverse Flexoelectricity

Deterministic control of ferroelectric domain is critical in the ferroelectric functional electronics. Ferroelectric polarization can be manipulated mechanically with a nano-tip through flexoelectricity. However, it usually occurs in a very localized area in ultrathin films, with possible permanent surface damage caused by a large tip-force. Here it is demonstrated that the deliberate engineering of transverse flexoelectricity offers a powerful tool for improving the mechanical domain switching. Sizable-area domain switching under an ultralow tip-force can be realized in suspended van der Waals ferroelectrics with the surface intact, due to the enhanced transverse flexoelectric field. The film thickness range for domain switching in suspended ferroelectrics is significantly improved by an order of magnitude to hundreds of nanometers, being far beyond the limited range of the substrate-supported ones. The experimental results and phase-field simulations further reveal the crucial role of the transverse flexoelectricity in the domain manipulation. This large-scale mechanical manipulation of ferroelectric domain provides opportunities for the flexoelectricity-based domain controls in emerging low-dimensional ferroelectrics and related devices.

A Two-Dimensional Hybrid Lead Bromide Ferroelectric Semiconductor with an Out-of-Plane Polarization

A two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) material with out-of-plane ferroelectricity is the key to the miniaturization of vertical-sandwich-type ferroelectric optoelectronic devices. However, 2D OIHP ferroelectrics with out-of-plane polarization are still scarce, and effective design strategies are lacking. Herein, we report a novel 2D Dion-Jacobson perovskite ferroelectric semiconductor synthesized by a rigid-to-flexible cationic tailoring strategy, achieving an out-of-plane polarization of 1.7 μC/cm2 and high photoresponse. Integrating out-of-plane ferroelectricity with excellent photoelectric properties affords a promising platform to investigate ferroelectricity-related effects in vertical optoelectronic devices.

Pressure Control of Nonferroelastic Ferroelectric Domains in ErMnO3

Mechanical pressure controls the structural, electric, and magnetic order in solid-state systems, allowing tailoring of their physical properties. A well-established example is ferroelastic ferroelectrics, where the coupling between pressure and the primary symmetry-breaking order parameter enables hysteretic switching of the strain state and ferroelectric domain engineering. Here, we study the pressure-driven response in a nonferroelastic ferroelectric, ErMnO3, where the classical stress-strain coupling is absent and the domain formation is governed by creation-annihilation processes of topological defects. By annealing ErMnO3 polycrystals under variable pressures in the MPa regime, we transform nonferroelastic vortex-like domains into stripe-like domains. The width of the stripe-like domains is determined by the applied pressure as we confirm by three-dimensional phase field simulations, showing that pressure leads to oriented layer-like periodic domains. Our work demonstrates the possibility to utilize mechanical pressure for domain engineering in nonferroelastic ferroelectrics, providing a lever to control their dielectric and piezoelectric responses.

Nanoscale multistate resistive switching in WO3 through scanning probe induced proton evolution

Multistate resistive switching device emerges as a promising electronic unit for energy-efficient neuromorphic computing. Electric-field induced topotactic phase transition with ionic evolution represents an important pathway for this purpose, which, however, faces significant challenges in device scaling. This work demonstrates a convenient scanning-probe-induced proton evolution within WO₃, driving a reversible insulator-to-metal transition (IMT) at nanoscale. Specifically, the Pt-coated scanning probe serves as an efficient hydrogen catalysis probe, leading to a hydrogen spillover across the nano junction between the probe and sample surface. A positively biased voltage drives protons into the sample, while a negative voltage extracts protons out, giving rise to a reversible manipulation on hydrogenation-induced electron doping, accompanied by a dramatic resistive switching. The precise control of the scanning probe offers the opportunity to manipulate the local conductivity at nanoscale, which is further visualized through a printed portrait encoded by local conductivity. Notably, multistate resistive switching is successfully demonstrated via successive set and reset processes. Our work highlights the probe-induced hydrogen evolution as a new direction to engineer memristor at nanoscale.

Wnt binding to Coatomer proteins directs secretion on exosomes independently of palmitoylation

Wnt proteins are secreted hydrophobic glycoproteins that act over long distances through poorly understood mechanisms. We discovered that Wnt7a is secreted on extracellular vesicles (EVs) following muscle injury. Structural analysis identified the motif responsible for Wnt7a secretion on EVs that we term the Exosome Binding Peptide (EBP). Addition of the EBP to an unrelated protein directed secretion on EVs. Disruption of palmitoylation, knockdown of WLS, or deletion of the N-terminal signal peptide did not affect Wnt7a secretion on purified EVs. Bio-ID analysis identified Coatomer proteins as candidates responsible for loading Wnt7a onto EVs. The crystal structure of EBP bound to the COPB2 coatomer subunit, the binding thermodynamics, and mutagenesis experiments, together demonstrate that a dilysine motif in the EBP mediates binding to COPB2. Other Wnts contain functionally analogous structural motifs. Mutation of the EBP results in a significant impairment in the ability of Wnt7a to stimulate regeneration, indicating that secretion of Wnt7a on exosomes is critical for normal regeneration in vivo . Our studies have defined the structural mechanism that mediates binding of Wnt7a to exosomes and elucidated the singularity of long-range Wnt signalling.

Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO₃, their ferroelectric key properties, including large spontaneous polarization (P_s), low coercive field (E_c), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI₃ (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large P_s of 24.14 μC/cm² (on a par with BaTiO₃), a low E_c below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH₂PO₄ (KDP)). Revealed by the first-principles calculations, its large P_s originates from the synergistic effects of the stereochemically active 4s² lone pair of Ge²⁺ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low E_c. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

Erasable Domain Wall Current-Dominated Resistive Switching in BiFeO3 Devices with an Oxide-Metal Interface

Electric transport in the charged domain wall (CDW) region has emerged as a promising phenomenon for the development of next-generation ferro-resistive memory with ultrahigh data storage density. However, accurately measuring the conductivity of CDWs induced by polarization reversal remains challenging due to the polarization modulation of the Schottky barrier at the thin film-electrode interface, which could partially contribute to the collected "on" current of the device. Here, we propose carefully selecting an electrode that can suppress the effect of interfacial barrier modulation induced by polarization reversal, allowing the collected current mainly from the conductive CDWs. The experiment was conducted on epitaxial BiFeO₃(001) thin-film devices with vertical and horizontal geometries. Piezo-response force microscopy scanning showed the local polarization experienced 180° rotation to form CDWs under the vertical electric field. However, devices with SrRuO₃ epitaxial top electrodes still exhibit an interfacial barrier-dominated diode behavior, with the "on" current proportional to the electrode area. To identify the CDW current, more interfacial defects were introduced by the deposition of Pt top electrodes, which significantly enhanced charge injection for the compensation of the reversed polarization driven by the electric field, leading to the suppressed polarization modulation of the Schottky barrier height. It was observed that the current flow through Pt electrodes is significantly lower compared to that of SRO electrodes and appears to be primarily influenced by the electrode perimeter instead of the electrode area, indicating CDW-dominated conduction behavior in these devices. Planar nanodevices were further fabricated to support the quantitative investigation of the Pt electrode size-dependent "on" current with a linear fit of the current magnitude versus the CDW cross-sectional area. This work constitutes an essential part of understanding the role of the CDW current in ferro-resistive memory devices.

Spherical ferroelectric solitons

Spherical ferroelectric domains, such as electrical bubbles, polar skyrmion bubbles and hopfions, share a single and unique feature-their homogeneously polarized cores are surrounded by a vortex ring of polarization whose outer shells form a spherical domain boundary. The resulting polar texture, typical of three-dimensional topological solitons, has an entirely new local symmetry characterized by a high polarization and strain gradients. Consequently, spherical domains represent a different material system of their own with emergent properties drastically different from that of their surrounding medium. Examples of new functionalities inherent to spherical domains include chirality, optical response, negative capacitance and giant electromechanical response. These characteristics, particularly given that the domains naturally have an ultrafine scale, offer new opportunities in high-density and low-energy nanoelectronic technologies. This Perspective gives an insight into the complex polar structure and physical origin of these spherical domains, which facilitates the understanding and development of spherical domains for device applications.