Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

Quantifying the photocurrent fluctuation in quantum materials by shot noise

The DC photocurrent can detect the topology and geometry of quantum materials without inversion symmetry. Herein, we propose that the DC shot noise (DSN), as the fluctuation of photocurrent operator, can also be a diagnostic of quantum materials. Particularly, we develop the quantum theory for DSNs in gapped systems and identify the shift and injection DSNs by dividing the second-order photocurrent operator into off-diagonal and diagonal contributions, respectively. Remarkably, we find that the DSNs can not be forbidden by inversion symmetry, while the constraint from time-reversal symmetry depends on the polarization of light. Furthermore, we show that the DSNs also encode the geometrical information of Bloch electrons, such as the Berry curvature and the quantum metric. Finally, guided by symmetry, we apply our theory to evaluate the DSNs in monolayer GeS and bilayer MoS2 with and without inversion symmetry and find that the DSNs can be larger in centrosymmetric phase.

Carrier transport engineering in a polarization-interface-free ferroelectric PN junction for photovoltaic effect

The carrier transport performances play key roles in the photoelectric conversion efficiency for photovoltaic effect. Hence, the low carrier mobility and high photogenerated carrier recombination in ferroelectric materials depress the separation of carriers. This work designs a ferroelectric polarization-interface-free PN junction composed with P-type semiconductor BiFeO3 (BFO) derived from the variable valence of Fe and N-type semiconductor BiFe0.98Ti0.02O3 (BFTO) through Ti donor doping. The integration of the ferroelectricity decides the PN junction without polarization coupling like the traditional heterojunctions but only existing carrier distribution differential at the interface. The carrier recombination in PN junction is significantly reduced due to the driving force of the built-in electric field and the existence of depletion layer, thereby enhancing the switching current 3 times higher than that of the single ferroelectric films. Meanwhile, the carrier separation at the interface is significantly engineered by the polarization, with open circuit voltage and short circuit current of photovoltaic effect increased obviously. This work provides an alternative strategy to regulate bulk ferroelectric photovoltaic effects by carrier transport engineering in the polarization-interface-free ferroelectric PN junction.

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

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

The Integration of Two-Dimensional Materials and Ferroelectrics for Device Applications

In recent years, there has been growing interest in functional devices based on two-dimensional (2D) materials, which possess exotic physical properties. With an ultrathin thickness, the optoelectrical and electrical properties of 2D materials can be effectively tuned by an external field, which has stimulated considerable scientific activities. Ferroelectric fields with a nonvolatile and electrically switchable feature have exhibited enormous potential in controlling the electronic and optoelectronic properties of 2D materials, leading to an extremely fertile area of research. Here, we review the 2D materials and relevant devices integrated with ferroelectricity. This review starts to introduce the background about the concerned themes, namely 2D materials and ferroelectrics, and then presents the fundamental mechanisms, tuning strategies, as well as recent progress of the ferroelectric effect on the optical and electrical properties of 2D materials. Subsequently, the latest developments of 2D material-based electronic and optoelectronic devices integrated with ferroelectricity are summarized. Finally, the future outlook and challenges of this exciting field are suggested.

Giant intrinsic photovoltaic effect in one-dimensional van der Waals grain boundaries

The photovoltaic effect lies at the heart of eco-friendly energy harvesting. However, the conversion efficiency of traditional photovoltaic effect utilizing the built-in electric effect in p-n junctions is restricted by the Shockley-Queisser limit. Alternatively, intrinsic/bulk photovoltaic effect (IPVE/BPVE), a second-order nonlinear optoelectronic effect arising from the broken inversion symmetry of crystalline structure, can overcome this theoretical limit. Here, we uncover giant and robust IPVE in one-dimensional (1D) van der Waals (vdW) grain boundaries (GBs) in a layered semiconductor, ReS2. The IPVE-induced photocurrent densities in vdW GBs are among the highest reported values compared with all kinds of material platforms. Furthermore, the IPVE-induced photocurrent is gate-tunable with a polarization-independent component along the GBs, which is preferred for energy harvesting. The observed IPVE in vdW GBs demonstrates a promising mechanism for emerging optoelectronics applications.

X-ray-Induced Pyroelectric Effect in a Perovskite Ferroelectric Drives Low Detection Limit Self-Powered Responses

The light-induced pyroelectric effect (LPE) has shown a great promise in the application of optoelectronic devices, especially for self-powered detection and imaging. However, it is quite challenging and scarce to achieve LPE in the X-ray region. For the first time, we report X-ray LPE in a single-phase ferroelectric of (NPA)2(EA)2Pb3Br10 (1, NPA = neopentylamine, EA = ethylamine), adopting a two-dimensional trilayered perovskite motif, which has a large spontaneous polarization of ∼3.7 μC/cm2. Its ferroelectricity allows for significant LPE in the wavelength range of ordinary visible light. Strikingly, the X-ray LPE is observed in 1, which endows remarkable self-powered X-ray responses at 0 bias, including sensitivity up to 225 μC Gy-1 cm-2 and a low detection limit of ∼83.4 nGy s-1, being almost 66 times lower than the requirement for medical diagnostics (∼5.5 μGy s-1). This work not only develops a new mode for X-ray detection but also provides valuable insights for future photoelectric device application.

Origin of the switchable photocurrent direction in BiFeO3 thin films

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO3) photoactive thin films. Depending on the applied bias our BiFeO3 films exhibit photocurrents more usually found in p- or n-type semiconductor photoelectrodes. In order to understand the anomalous behaviour ambient photoemission spectroscopy and Kelvin-probe techniques have been used to determine the band structure of the BiFeO3. We found that the Fermi level (Ef) is at -4.96 eV (vs. vacuum) with a mid-gap at -4.93 eV (vs. vacuum). Our photochemically determined flat band potential (Efb) was found to be 0.3 V vs. NHE (-4.8 V vs. vacuum). These band positions indicate that Ef is close to mid-gap, and Efb is close to the equilibrium with the electrolyte enabling either cathodic or anodic band bending. We show an ability to control switching from n- to p-type behaviour through the application of external bias to the BiFeO3 thin film. This ability to control majority carrier dynamics at low applied bias opens a number of applications in novel optoelectronic switches, logic and energy conversion devices.

Tuning ferroelectric photovoltaic performance in R3c-CuNbO3 through compressive strain engineering: a first-principles study

Most ferroelectric oxides exhibit relatively wide bandgaps, which pose limitations on their suitability for photovoltaics application. CuNbO3 possesses potential ferroelectric properties with an R3c polar structure that facilitate the separation of charge carriers under illumination, promoting the generation of photovoltaic effects. The optical and ferroelectric properties of R3c-CuNbO3, as well as the effect of strain on the properties are investigated by first-principles calculation in this paper. The calculated results indicate that R3c-CuNbO3 possesses a moderate band gap to absorb visible light. The interaction of Cu-O and Nb-O bonds is considered to have a crucial role in the photovoltaic properties of CuNbO3, contributing to the efficient absorption of visible light. The bandgap of CuNbO3 becomes smaller and the density of states near the conduction and valence bands becomes relatively uniform in distribution under compressive conditions, which improves the photoelectric conversion efficiency to 29.9% under conditions of bulk absorption saturation. The ferroelectric properties of CuNbO3 are driven by the Nb-O bond interactions, which are not significantly weakened by the compressive strain. CuNbO3 is expected to be an excellent ferroelectric photovoltaic material by modulation of compressive strain due to the stronger visible light absorption and excellent ferroelectric behavior.

Ferroelectric Resistance Switching in Epitaxial BiFeO3/La0.7Sr0.3MnO3 Heterostructures

BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) epitaxial heterostructures were successfully synthesized by pulsed laser deposition on (001)-oriented SrTiO3 single-crystal substrates with Au top electrodes. Stable bipolar resistive switching characteristics regulated by ferroelectric polarization reversal was observed in the Au/BFO/LSMO heterostructures. The conduction mechanism was revealed to follow the Schottky emission model, and the Schottky barriers in high-resistance and low-resistance states were estimated based on temperature-dependent current-voltage curves. Further, the observed memristive behavior was interpreted via the modulation effect on the depletion region width and the Schottky barrier height caused by ferroelectric polarization reversal, combining with the oxygen vacancies migration near the BFO/LSMO interface.

Building Block-Inspired Hybrid Perovskite Derivatives for Ferroelectric Channel Layers with Gate-Tunable Memory Behavior

Ferroelectric photovoltaics driven by spontaneous polarization (Ps ) holds a promise for creating the next-generation optoelectronics, spintronics and non-volatile memories. However, photoactive ferroelectrics are quite scarce in single homogeneous phase, owing to the severe Ps fatigue caused by leakage current of photoexcited carriers. Here, through combining inorganic and organic components as building blocks, we constructed a series of ferroelectric semiconductors of 2D hybrid perovskites, (HA)2 (MA)n-1 Pbn Br3n+1 (n=1-5; HA=hexylamine and MA=methylamine). It is intriguing that their Curie temperatures are greatly enhanced by reducing the thickness of inorganic frameworks from MAPbBr3 (n=∞, Tc =239 K) to n=2 (Tc =310 K, ΔT=71 K). Especially, on account of the coupling of room-temperature ferroelectricity (Ps ≈1.5 μC/cm2 ) and photoconductivity, n=3 crystal wafer was integrated as channel field effect transistor that shows excellent a large short-circuit photocurrent ≈19.74 μA/cm2 . Such giant photocurrents can be modulated through manipulating gate voltage in a wide range (±60 V), exhibiting gate-tunable memory behaviors of three current states ("-1/0/1" states). We believe that this work sheds light on further exploration of ferroelectric materials toward new non-volatile memory devices.

Stereochemically Active Lone Pair Induced Polar Tri-layered Perovskite for Record-Performance Polarized Photodetection

Bulk photovoltaic effect, a promising optoelectronic phenomenon for generating polarized dependent steady-state photocurrent, has been widely applied in various photodetectors. However, incorporating stereochemically active lone pair to construct bulk photovoltage in organic-inorganic hybrid perovskite (OIHP) is still elusive and challenging. Herein, bulk photovoltage (1.2 V) has been successfully achieved by introducing the stereo-chemically active lone pair perovskitizer to construct a polar tri-layered hybrid perovskite, namely, (IBA)2 MHy2 Pb3 Br10 (1, IBA=iso-butylamine, MHy=methylhydrazine). Strikingly, owning to the promising bulk photovoltage, 1-based detectors exhibit an ultra-highly sensitive polarized photodetection (polarization ratio of up to 24.6) under self-powered mode. This ratio surpasses all the reported two-dimension OIHP single-crystal photodetectors. In addition, detectors exhibit outstanding responsivity (≈200 mA W-1 ) and detectivity (≈2.4×1013 Jones). More excitingly, further investigation confirms that lone pair electrons in MHy+ result in the separation of positive and negative charges to produce directional dipoles, which further directional alignment to generate bulk photovoltage, thereby resulting in polarization-dependent photocurrent. Our findings provide a new demonstration for polar multilayer materials' construction and may open opportunities for a host of high-sensitive polarized photodetection.

0D van der Waals interfacial ferroelectricity

The dimensional limit of ferroelectricity has been long explored. The critical contravention is that the downscaling of ferroelectricity leads to a loss of polarization. This work demonstrates a zero-dimensional ferroelectricity by the atomic sliding at the restrained van der Waals interface of crossed tungsten disufilde nanotubes. The developed zero-dimensional ferroelectric diode in this work presents not only non-volatile resistive memory, but also the programmable photovoltaic effect at the visible band. Benefiting from the intrinsic dimensional limitation, the zero-dimensional ferroelectric diode allows electrical operation at an ultra-low current. By breaking through the critical size of depolarization, this work demonstrates the ultimately downscaled interfacial ferroelectricity of zero-dimensional, and contributes to a branch of devices that integrates zero-dimensional ferroelectric memory, nano electro-mechanical system, and programmable photovoltaics in one.

Advanced Etching Techniques of LiNbO3 Nanodevices

Single LiNbO3 (LNO) crystals are widely utilized in surface acoustic wave devices, optoelectronic devices, and novel ferroelectric memory devices due to their remarkable electro-optic and piezoelectric properties, and high saturation and remnant polarizations. However, challenges remain regarding their nanofabrication that hinder their applications. The prevailing etching techniques for LNO encompass dry etching, wet etching, and focused-ion-beam etching, each having distinct merits and demerits. Achieving higher etching rates and improved sidewall angles presents a challenge in LNO nanofabrication. Building upon the current etching researches, this study explores various etching methods using instruments capable of generating diverse plasma densities, such as dry etching in reactive ion etching (RIE) and inductively coupled plasma (ICP), proton exchange-enhanced etching, and wet chemical etching following high-temperature reduction treatment, as well as hybrid dry and wet etching. Ultimately, after employing RIE dry etching combined with wet etching, following a high-temperature reduction treatment, an etching rate of 10 nm/min and pretty 90° sidewall angles were achieved. Furthermore, high etching rates of 79 nm/min with steep sidewall angles of 83° were obtained using ICP dry etching. Additionally, using SiO2 masks, a high etching rate of 108 nm/min and an etching selectivity ratio of 0.86:1 were achieved. Distinct etching conditions yielded diverse yet exceptional results, providing multiple processing paths of etching for the versatile application of LNO.

Macroscopic Piezoelectricity of Halide Perovskite Single Crystals and Their Highly Sensitive Self-Powered X-ray Detectors

The FAxMA1-xPbI3 single crystal has excellent semiconductor photoelectric performance and good stability; however, there have been conflicting opinions regarding its macroscopic piezoelectricity. Here, the FAxMA1-xPbI3 (x = 0-0.1) single crystals (FAx SCs) exhibit a high macroscopic piezoelectric d33 coefficient of over 10 pC/N. The single crystal transforms from a tetragonal ferroelectric phase to a cubic paraelectric phase at x = 0.1-0.125. Furthermore, the fully polarized MAPbI3 and FA0.05 SCs were applied to prepare self-powered X-ray detectors with vertical structures. The sensitivity of the detector reaches 5.1 × 104 μC·Gy-1·cm-2 under a 0 V bias voltage, and its detection limit is as low as 50 nGy/s. This work provides an approach to designing self-powered and high-quality detectors with piezoelectric semiconductors.

Anion-induced robust ferroelectricity in sulfurized pseudo-rhombohedral epitaxial BiFeO3 thin films via polarization rotation

Polarization rotation caused by various strains, such as substrate and/or chemical strain, is essential to control the electronic structure and properties of ferroelectric materials. This study proposes anion-induced polarization rotation with chemical strain, which effectively improves ferroelectricity. A method for the sulfurization of BiFeO3 thin films by introducing sulfur anions is presented. The sulfurized films exhibited substantial enhancement in room-temperature ferroelectric polarization through polarization rotation and distortion, with a 170% increase in the remnant polarization from 58 to 100.7 μC cm-2. According to first-principles calculations and the results of X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy, this enhancement arose from the introduction of S atoms driving the re-distribution of the lone-pair electrons of Bi, resulting in the rotation of the polarization state from the [001] direction to the [110] or [111] one. The presented method of anion-driven polarization rotation might enable the improvement of the properties of oxide materials.

Strategies for enhancing performance of perovskite bismuth ferrite photocatalysts (BiFeO3): A comprehensive review

Organic pollutants pose a significant threat to water safety, and their degradation is of paramount importance. Photocatalytic technology has emerged as a promising approach for environmental remediation, and Bismuth ferrite (BiFeO3) has been shown to exhibit remarkable potential for photocatalytic degradation of water pollutants, with its excellent crystal structure properties and visible light photocatalytic activity. This review presents an overview of the crystal properties and photocatalytic mechanism of perovskite bismuth ferrite (BiFeO3), as well as a summary of various strategies for enhancing its efficiency in photocatalytic degradation of organic pollutants. These strategies include pure phase preparation, microscopic modulation, composite modification of BiFeO3, and the integration of Fenton-like reactions and external field-assisted methods to improve its photocatalytic performance. The review emphasizes the impact of each strategy on photocatalytic enhancement. By providing comprehensive strategies for improving the efficiency of BiFeO3 photocatalysis, this review inspires new insights for efficient degradation of organic pollutants using BiFeO3 photocatalysis and contributes to the development of photocatalysis in environmental remediation.

Mixing cage cations in 2D metal-halide ferroelectrics enhances the ferro-pyro-phototronic effect for self-driven photopyroelectric detection

The ferro-pyro-phototronic (FPP) effect, coupling photoexcited pyroelectricity and photovoltaics, paves an effective way to modulate charge-carrier behavior of optoelectronic devices. However, reports of promising FPP-active systems remain quite scarce due to a lack of knowledge on the coupling mechanism. Here, we have successfully enhanced the FPP effect in a series of ferroelectrics, BA2Cs1-xMAxPb2Br7 (BA = butylammonium, MA = methylammonium, 0 ≤ x ≤ 0.34), rationally assembled by mixing cage cations into 2D metal-halide perovskites. Strikingly, chemical alloying of Cs+/MA+ cations leads to the reduction of exciton binding energy, as verified by the x = 0.34 component; this facilitates exciton dissociation into free charge-carriers and boosts photo-activities. The crystal detector thus displays enhanced FPP current at zero bias, almost more than 10 times higher than that of the x = 0 prototype. As an innovative study on the FPP effect, this work affords new insight into the fundamental principle of ferroelectrics and creates a new strategy for self-driven photodetection.

Centimeter-Size Single Crystals of Halide Perovskite Photoferroelectric Solid Solution with Ultrahigh Pyroelectricity Boosted Photodetection

Photoferroelectrics, especially emerging halide perovskite ferroelectrics, have motivated tremendous interests owing to their fascinating bulk photovoltaic effect (BPVE) and cross-coupled functionalities. However, solid solutions of halide perovskite photoferroelectrics with controllable structure and enhanced performance are scarcely explored. Herein, through mixing cage cation, a homogeneous halide perovskite photoferroelectric PA2 FAx MA1-x Pb2 Br7 solid solution (PA, FA and MA are CH3 CH2 CH2 NH3 + , NH2 CHNH2 + and CH3 NH3 + , 0≤x≤1) has been developed, which demonstrates tunable Curie temperature in a wide range of 263-323 K and excellent optoelectrical features. As the component adjusted to x=0.7, the bulk crystal demonstrates ultrahigh pyroelectric coefficient up to 1.48 μC cm-2  K-1 around room temperature. Strikingly, benefiting from the light-induced pyroelectricity and remarkable BPVE, a self-powered and sensitive photodetector based solid solution crystals with boosted responsivity and detectivity over than 1300 % has been achieved. This pioneering work sheds light on the exploration of photoferroelectric solid solutions towards high-performance photoelectronic devices.

Tuning the Multiferroic Properties of BiFeO_{3} under Uniaxial Strain

More than twenty years ago, multiferroic compounds combining in particular magnetism and ferroelectricity were rediscovered. Since then, BiFeO_{3} has emerged as the most outstanding multiferroic by combining at room temperature almost all the fundamental or applicative properties that may be desired: electroactive spin wave excitations called electromagnons, conductive domain walls, or a low band gap of interest for magnonic devices. All these properties have so far only been discontinuously strain engineered in thin films according to the lattice parameter imposed by the substrate. Here we explore the ferroelectricity and the dynamic magnetic response of BiFeO_{3} bulk under continuously tunable uniaxial strain. Using elasto-Raman spectroscopy, we show that the ferroelectric soft mode is strongly enhanced under tensile strain and driven by the volume preserving deformation at low strain. The magnonic response is entirely modified with low energy magnon modes being suppressed for tensile strain above pointing out a transition from a cycloid to an homogeneous magnetic state. Effective Hamiltonian calculations show that the ferroelectric and the antiferrodistortive modes compete in the tensile regime. In addition, the homogeneous antiferromagnetic state becomes more stable compared to the cycloidal state above a +2% tensile strain close to the experimental value. Finally, we reveal the ferroelectric and magnetic orders of BiFeO_{3} under uniaxial strain and how the tensile strain allows us to unlock and to modify in a differentiated way the polarization and the magnetic structure.

Artificial SiNz:H Synapse Crossbar Arrays with Gradual Conductive Pathway for High-Accuracy Neuromorphic Computing

Inspired by its highly efficient capability to deal with big data, the brain-like computational system has attracted a great amount of attention for its ability to outperform the von Neumann computation paradigm. As the core of the neuromorphic computing chip, an artificial synapse based on the memristor, with a high accuracy in processing images, is highly desired. We report, for the first time, that artificial synapse arrays with a high accuracy in image recognition can be obtained through the fabrication of a SiNz:H memristor with a gradient Si/N ratio. The training accuracy of SiNz:H synapse arrays for image learning can reach 93.65%. The temperature-dependent I-V characteristic reveals that the gradual Si dangling bond pathway makes the main contribution towards improving the linearity of the tunable conductance. The thinner diameter and fixed disconnection point in the gradual pathway are of benefit in enhancing the accuracy of visual identification. The artificial SiNz:H synapse arrays display stable and uniform biological functions, such as the short-term biosynaptic functions, including spike-duration-dependent plasticity, spike-number-dependent plasticity, and paired-pulse facilitation, as well as the long-term ones, such as long-term potentiation, long-term depression, and spike-time-dependent plasticity. The highly efficient visual learning capability of the artificial SiNz:H synapse with a gradual conductive pathway for neuromorphic systems hold great application potential in the age of artificial intelligence (AI).

Giant room-temperature nonlinearities in a monolayer Janus topological semiconductor

Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials-the Janus transition metal dichalcogenides in the 1 T' phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T' MoSSe (e.g., > 50 times higher than 2H MoS2 for 18th order harmonic generation; > 20 times higher than 2H MoS2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.

Giant Polarization Sensitivity via the Anomalous Photovoltaic Effect in a Two-Dimensional Perovskite Ferroelectric

Polarization sensitivity, which shows great potential in photoelectric detection, is expected to be significantly improved by the ferroelectric anomalous photovoltaic (APV) effect. However, it is challenging to explore new APV-active ferroelectrics due to severe polarization fatigue induced by the leakage current of photoexcited carriers. For the first time, we report a strong APV effect in a 2D hybrid perovskite ferroelectric assembled by alloying mixed organic cations, (HA)₂(EA)₂Pb₃Br₁₀ (1, where HA⁺ is n-hexylammonium and EA⁺ is ethylammonium), which has a large spontaneous polarization ∼3.8 μC/cm² and high a Curie temperature ∼378 K. Its ferroelectricity allows a strong APV effect with an above-bandgap photovoltage up to 7.4 V, which exceeds its bandgap (∼2.7 eV). Most strikingly, based on the dependence on polarized-light angle, this strong APV effect renders the highest level of polarization sensitivity with a giant current ratio of ∼25, far beyond other 2D single-phase materials. This study sheds light on the exploration of APV-active ferroelectrics and inspires their future high-performance optoelectronic device applications.

High-Temperature Dielectric Relaxation and Electric Conduction Mechanisms in a LaCoO3-Modified Na0.5Bi0.5TiO3 System

This research study examines the high-temperature dielectric relaxation and electric conduction mechanisms in (x)LaCoO₃-(1 - x)Na_0.5Bi_0.5TiO₃ samples, where x is 0.05, 0.10, and 0.15. The findings demonstrate that all the samples exhibit two dielectric transitions: first, a frequency-dispersive shoulder at a lower temperature (T_s) around 425-450 K, which is associated with polar nanoregions (PNRs), and second, from ferroelectric to paraelectric transition at the Curie temperature (T_c) approximately between 580 and 650 K. The impedance analysis reveals the negative temperature coefficient of resistance behavior of the specimens. The broad and asymmetric relaxation peaks obtained from modulus spectroscopy demonstrate a wide range of relaxations, suggesting non-Debye-type behavior. Furthermore, the conductivity studies provide insights into understanding the transport phenomena in the samples. The oxygen vacancies resulting from the addition of LaCoO₃ into the Na_0.5Bi_0.5TiO₃ ceramics are responsible for the relaxation and conduction processes, and the charge carrier is doubly ionized oxygen ion vacancies. All samples except for LCNBT10 at 1 kHz exhibit a negative magnetodielectric response.

Strong bulk photovoltaic effect in engineered edge-embedded van der Waals structures

Bulk photovoltaic effect (BPVE), a second-order nonlinear optical effect governed by the quantum geometric properties of materials, offers a promising approach to overcome the Shockley-Quiesser limit of traditional photovoltaic effect and further improve the efficiency of energy harvesting. Here, we propose an effective platform, the nano edges embedded in assembled van der Waals (vdW) homo- or hetero-structures with strong symmetry breaking, low dimensionality and abundant species, for BPVE investigations. The BPVE-induced photocurrents strongly depend on the orientation of edge-embedded structures and polarization of incident light. Reversed photocurrent polarity can be observed at left and right edge-embedded structures. Our work not only visualizes the unique optoelectronic effect in vdW nano edges, but also provides an effective strategy for achieving BPVE in engineered vdW structures.

All-ferroelectric implementation of reservoir computing

Reservoir computing (RC) offers efficient temporal information processing with low training cost. All-ferroelectric implementation of RC is appealing because it can fully exploit the merits of ferroelectric memristors (e.g., good controllability); however, this has been undemonstrated due to the challenge of developing ferroelectric memristors with distinctly different switching characteristics specific to the reservoir and readout network. Here, we experimentally demonstrate an all-ferroelectric RC system whose reservoir and readout network are implemented with volatile and nonvolatile ferroelectric diodes (FDs), respectively. The volatile and nonvolatile FDs are derived from the same Pt/BiFeO₃/SrRuO₃ structure via the manipulation of an imprint field (E_imp). It is shown that the volatile FD with E_imp exhibits short-term memory and nonlinearity while the nonvolatile FD with negligible E_imp displays long-term potentiation/depression, fulfilling the functional requirements of the reservoir and readout network, respectively. Hence, the all-ferroelectric RC system is competent for handling various temporal tasks. In particular, it achieves an ultralow normalized root mean square error of 0.017 in the Hénon map time-series prediction. Besides, both the volatile and nonvolatile FDs demonstrate long-term stability in ambient air, high endurance, and low power consumption, promising the all-ferroelectric RC system as a reliable and low-power neuromorphic hardware for temporal information processing.

Visible-Photoactive Perovskite Ferroelectric-Driven Self-Powered Gas Detection

Chemiresistive sensing has been regarded as the key monitoring technique, while classic oxide gas detection devices always need an external power supply. In contrast, the bulk photovoltage of photoferroelectric materials could provide a controllable power source, holding a bright future in self-powered gas sensing. Herein, we present a new photoferroelectric ([n-pentylaminium]₂[ethylammonium]₂Pb₃I₁₀, 1), which possesses large spontaneous polarization (∼4.8 μC/cm²) and prominent visible-photoactive behaviors. Emphatically, driven by the bulk photovoltaic effect, 1 enables excellent self-powered sensing responses for NO₂ at room temperature, including extremely fast response/recovery speeds (0.15/0.16 min) and high sensitivity (0.03 ppm^-1). Such figures of merit are superior to those of typical inorganic systems (e.g., ZnO) using an external power supply. Theoretical calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements confirm the great selectivity of 1 for NO₂. As far as we know, this is the first realization of ferroelectricity-driven self-powered gas detection. Our work sheds light on the self-powered sensing systems and provides a promising way to broaden the functionalities of photoferroelectrics.

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.

Self-Powered Bipolar Photodetector Based on a Ce-BaTiO3 PTCR Semiconductor for Logic Gates

Ferroelectric materials bring new opportunities for self-powdered photodetectors, taking advantage of their anomalous bulk photovoltaic effect. However, ferroelectric-based photodetectors suffer from relatively poor responsivity and detectivity due to obstacles of low electrical conductivity and low photoelectric conversion ability. The present work proposes a strategy based on heterovalent ion Ce-doping into BaTiO₃ (Ce-BTO) that gives rise to a good room temperature conductivity combined with a significant PTCR (positive temperature coefficient of resistivity) effect. By utilizing a Ce-BTO PTCR semiconductor, a high-performance self-powered photodetector ITO/Ce-BTO/Ag is fabricated, demonstrating a polarity-switchable photoresponse with the change of wavelength due to the competition between hot electrons induced by the Ag plasmonic effect and electron-hole pairs separated by a Schottky barrier. Moreover, benefiting from the reduced bandgap and the introduced impurity states, good responsivity (9.85 × 10^-5 A/W) and detectivity (1.25 × 10¹⁰ Jones) as well as fast response/recovery time (83/47 ms) is achieved under 450 nm illumination. Finally, four representative logic gates ("OR", "AND", "NOR", and "NAND") are demonstrated with one photodetector via the bipolar photoresponse. This work opens an avenue to promote the application of PTCR semiconductors in optoelectronics, offering a conceivable means toward high-performance self-powered photodetectors.

Ferroelectric perovskite-type films with robust in-plane polarization toward efficient room-temperature chemiresistive sensing

Ferroelectric materials have become key components for versatile device applications, and their thin films are highly desirable for integrating the miniaturized devices. Despite substantial endeavors, it is still challenging to achieve effective chemiresistive sensing in the ferroelectric films. Here, for the first time, we have exploited ferroelectric thin films of 2D hybrid perovskite BA2EA2Pb3I10 (1), to fabricate the high-performance chemiresistor gas sensors. The spin-coated films of 1 exhibit high orientation and good crystallinity, thus preserving robust in-plane spontaneous polarization (P s ∼2.0 μC/cm2) and low electric coercivity. Notably, such ferroelectric film-based sensors after electric poling enable the dramatic room-temperature sensing responses to NO2 gas, including high sensitivity (0.05 ppm-1), extremely low detection limit (1 ppm) and fast responding rate (∼6 s). Besides, the chemiresistive responses are remarkably enhanced by threefold (up to 320%) through electric poling. It is proposed that this behavior closely involves with strong in-plane ferroelectric polarization of 1 that generates a built-in electric field inhibiting the recombination of charge carriers. As far as we know, this ferroelectric-based film chemiresisor is one of the best room-temperature sensors for NO2 gas among all the existing candidate materials. These findings highlight great potential of ferroelectrics toward effective chemiresistive performances, and also establish a bright direction to explore their future device applications.

Epitaxial Growth of Large Area Two-Dimensional Ferroelectric α-In2Se3

Two-dimensional (2D) ferroelectric materials have attracted intensive attention in recent years for academic research. However, the synthesis of large-scale 2D ferroelectric materials for electronic applications is still challenging. Here, we report the successful synthesis of centimeter-scale ferroelectric In₂Se₃ films by selenization of In₂O₃ in a confined space chemical vapor deposition method. The as-grown homogeneous thin film has a uniform thickness of 5 nm with robust out-of-plane ferroelectricity at room temperature. Scanning transmission electron microscopy and Raman spectroscopy reveal that the thin film is 2H stacking α-In₂Se₃ with excellent crystalline quality. Electronic transport measurements of In₂Se₃ highlight the current-voltage hysteresis and polarization modulated diode effect due to the switchable Schottky barrier height (SBH). First-principles calculations reveal that the polarization modulated SBH is originated from the competition between interface charge transfer and polarized charge. The large area growth of epitaxial In₂Se₃ opens up potential applications of In₂Se₃ in novel nanoelectronics.

Octahedral Distortion and Displacement-Type Ferroelectricity with Switchable Photovoltaic Effect in a 3d^{3}-Electron Perovskite System

Because of the half-filled t_{2g}-electron configuration, the BO_{6} octahedral distortion in a 3d^{3} perovskite system is usually very limited. In this Letter, a perovskitelike oxide Hg_{0.75}Pb_{0.25}MnO_{3} (HPMO) with a 3d^{3} Mn^{4+} state was synthesized by using high pressure and high temperature methods. This compound exhibits an unusually large octahedral distortion enhanced by approximately 2 orders of magnitude compared with that observed in other 3d^{3} perovskite systems like RCr^{3+}O_{3} (R=rare earth). Essentially different from centrosymmetric HgMnO_{3} and PbMnO_{3}, the A-site doped HPMO presents a polar crystal structure with the space group Ama2 and a substantial spontaneous electric polarization (26.5  μC/cm^{2} in theory) arising from the off-center displacements of A- and B-site ions. More interestingly, a prominent net photocurrent and switchable photovoltaic effect with a sustainable photoresponse were observed in the current polycrystalline HPMO. This Letter provides an exceptional d^{3} material system which shows unusually large octahedral distortion and displacement-type ferroelectricity violating the "d^{0}-ness" rule.

Superior Performances of Self-Driven Near-Infrared Photodetectors Based on the SnTe:Si/Si Heterostructure Boosted by Bulk Photovoltaic Effect

The upsurge of new materials that can be used for near-infrared (NIR) photodetectors operated without cooling is crucial. As a novel material with a small bandgap of ≈0.28 eV, the topological crystalline insulator SnTe has attracted considerable attention. Herein, this work demonstrates self-driven NIR photodetectors based on SnTe/Si and SnTe:Si/Si heterostructures. The SnTe/Si heterostructure has a high detectivity D* of 3.3 × 10¹² Jones. By Si doping, the SnTe:Si/Si heterostructure reduces the dark current density and increases the photocurrent by ≈1 order of magnitude simultaneously, which improves the detectivity D* by ≈2 orders of magnitude up to 1.59 × 10¹⁴ Jones. Further theoretical analysis indicates that the improved device performance may be ascribed to the bulk photovoltaic effect (BPVE), in which doped Si atoms break the inversion symmetry and thus enable the generation of additional photocurrents beyond the heterostructure. In addition, the external quantum efficiency (EQE) measured at room temperature at 850 nm increases by a factor of 7.5 times, from 38.5% to 289%. A high responsivity of 1979 mA W^-1 without bias and fast rising time of 8 µs are also observed. The significantly improved photodetection achieved by the Si doping is of great interest and may provide a novel strategy for superior photodetectors.

Ferroelectric photovoltaic response engineered by lattice strain derived from local metal-ion dipoles

An unfavorable inverse relationship between polarization, bandgap, and leakage always limits the ferroelectric photovoltaic performances. This work proposes a strategy of lattice strain engineering different from traditional lattice distortion by introducing a (Mg_2/3Nb_1/3)³⁺ ion group into the B site of BiFeO₃ films to construct local metal-ion dipoles. A giant remanent polarization of 98 µC/cm², narrower bandgap of 2.56 eV, and the decreased leakage current by nearly two orders of magnitude are synchronously obtained in the BiFe_0.94(Mg_2/3Nb_1/3)_0.06O₃ film by engineering the lattice strain, breaking through the inverse relationship among these three. Thereby, the open-circuit voltage and the short-circuit current of the photovoltaic effect reach as high as 1.05 V and 2.17 µA /cm², respectively, showing an excellent photovoltaic response. This work provides an alternative strategy to enhance ferroelectric photovoltaic performances by lattice strain derived from local metal-ion dipoles.

Observation of the photovoltaic effect in a van der Waals heterostructure

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS₂/MoS₂ vdW heterostructure. Light excitation of WS₂/MoS₂ at a wavelength of 633 nm yields a photocurrent without applying bias voltages, and the excitation power dependence of the photocurrent shows characteristic crossover from a linear to square root dependence. Photocurrent mapping has clearly shown that the observed photovoltaic effect arises from the WS₂/MoS₂ region, not from Schottky junctions at electrode contacts. Kelvin probe microscopy observations show no slope in the electrostatic potential, excluding the possibility that the photocurrent originates from an unintentionally formed built-in potential.

Revisiting the Ferroelectric Photovoltaic Properties of Vertical BiFeO3 Capacitors: A Comprehensive Study

The ferroelectric photovoltaic effect has been extensively studied for possible applications in energy conversion and photo-electrics. The reversible spontaneous polarization gives rise to a switchable photovoltaic behavior. However, despite its long history, the origin of the ferroelectric photovoltaic effect still lacks a full understanding since multiple mechanisms such as bulk and Schottky-barrier-related interface effects are involved. Herein, we report a comprehensive study on the photovoltaic response of BiFeO₃-based vertical heterostructures, using multiple strategies to clarify its origin. We found that, under white light illumination, polarization-modulated Schottky barrier at the interface is the dominating mechanism. By varying the top metal contacts, only the photovoltaic effect of the polarization downward state is strongly modulated, suggesting selective interface contribution in different polarization states. A Schottky-barrier-free device shows negligible photovoltaic effect, suggesting the lack of bulk photovoltaic effect in vertical heterostructures under white light illumination.

Nonthermal Ultrafast Optical Control of Magnetization Dynamics by Linearly Polarized Light in Metallic Ferromagnet

Coherent optical control of the magnetization in ferromagnetic (FM) mediums using ultrafast nonthermal effect paves a promising avenue to improve the speed and repetition rate of the magnetization manipulation. Whereas previously, only heat-induced or helicity-dependent magnetization dynamics are demonstrated in metallic ferromagnets. Here, the linearly-polarized light control of magnetization is demonstrated in FM Co coupled with ferroelectric (FE) BiFeO₃ by tuning the light polarization direction. It is revealed that in the Co/BiFeO₃ heterostructure excited by femtosecond laser pulses, the magnetization precession amplitude follows a sinusoidal dependence on the laser polarization direction. This nonthermal control of coherent magnetization rotation is attributed to the optical rectification effect in the BiFeO₃ layer, which yields a FE polarization depending on the light polarization, and the subsequent modulation of magnetic energy in Co by the electrostriction-induced strain. This work demonstrates an effective route to nonthermally manipulate the ultrafast magnetization dynamics in metallic ferromagnets.

Effect of Poling on Photocatalysis, Piezocatalysis, and Photo-Piezo Catalysis Performance of BaBi4Ti4O15 Ceramics

This study focuses on analyzing the poling effect of BaBi4Ti4O15 (BBT) on the basis of photo and piezo-catalysis performance. BBT powder is prepared via a solid state reaction followed by calcination at 950 °C for 4 h. BBT is characterized by an X-ray diffractometer, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The optical bandgap of BBT is evaluated with the help of Tauc's plot and found to be 3.29 eV, which comes in the photon energy range of ultra-violet radiation. BBT powder is poled by using Corona poling in the presence of 2 kV mm-1 of electric field. An aqueous solution of methyl blue (MB) dye in the presence of UV radiation is used to evaluate the photo/piezocatalysis performance. Photocatalysis, piezocatalysis, and photo-piezo catalysis degradation efficiencies of poled and unpoled BBT powder are tested for 120 min of UV light irradiation. Photo-piezocatalysis shows degradation efficiencies of 62% and 40% for poled and unpoled BBT powder, respectively. Poling of BBT powder shows significant enhancement in degradation performance of MB dye in aqueous solution. Scavenger tests are also performed to identify reactive species.

Switchable Giant Bulk Photocurrents and Photo-spin-currents in Monolayer PT-Symmetric Antiferromagnet MnPSe3

Converting light into steady currents and spin-currents in two-dimensional (2D) platform is essential for future energy harvesting and spintronics. We show that the giant and modulable bulk photovoltaic effects (BPVEs) can be achieved in air-stable 2D antiferromagnet (AFM) monolayer MnPSe₃, with nonlinear photoconductance >4000 nm·μA/V² and photo-spin-conductance >2000 (nm·μA/V²ℏ/2e) in the visible spectrum. The propagation and the spin-polarizations of photocurrents can be switched via simply rotating the Néel vector. We unveil that the PT-symmetry, mirror symmetries, and spin-orbital-couplings are the keys for the observed sizable and controllable 2D BPVEs. All the results provide insights into the BPVEs of 2D AFM and suggest that the layered MnPSe₃ is an outstanding 2D platform for energy device and photo-spintronics.

Three-dimensional narrow-bandgap perovskite semiconductor ferroelectric methylphosphonium tin triiodide for potential photovoltaic application

A novel A-site three-dimensional organic-inorganic halide perovskites (3D OIHP) ferroelectric, methylphosphonium tin triiodide (MPSnI₃), featuring a narrow bandgap of 1.43 eV, was synthesized. The integration of ferroelectricity with initially moderate efficiency (2.23%) may afford a promising platform to investigate the ferroelectric photovoltaic effect in organic-inorganic halide perovskite solar cells.

On the photovoltaic effect asymmetry in ferroelectrics

Despite symmetrical polarization, the magnitude of a light-induced voltage is known to be asymmetric with respect to poling sign in many photovoltaic (PV) ferroelectrics (FEs). This asymmetry remains unclear and is often attributed to extrinsic effects. We show here for the first time that such an asymmetry can be intrinsic, steaming from the superposition of asymmetries of internal FE bias and electro-piezo-strictive deformation. This hypothesis is confirmed by the observed decrease of PV asymmetry for smaller FE bias. Moreover, the both PV effect and remanent polarization are found to increase under vacuum-induced expansion and to decrease for gas-induced compression, with tens percents tunability. The change in cations positions under pressure is analysed through the first-principle density functional theory calculations. The reported properties provide key insight for FE-based solar elements optimization.

Wear-Resisting and Stable 4H-SiC/Cu-Based Tribovoltaic Nanogenerators for Self-Powered Sensing in a Harsh Environment

Tribovoltaic nanogenerators (TVNGs) are an emerging class of devices for high-entropy energy conversion and mechanical sensing that benefit from their outstanding real-time direct current output characteristics. Here, a self-powered TVNG was fabricated using a small-area 4H-SiC semiconductor wafer and a large-area copper foil. Thus, the cost of materials remains low compared to devices employing large-scale semiconductors. The 4H-SiC/metal-TVNGs (SM-TVNGs) presented here are sensitive to vertical force and sliding velocity, making them appropriate for mechanical sensing. Notably, owing to the modulated bindingtons and surface states, these SM-TVNGs performed well in a harsh environment, namely, in high-temperature and high-humidity conditions. In addition, the SM-TVNGs exhibited an excellent wear-resisting property. On these bases, we designed a self-powered and real-time monitoring device able to estimate the number of staff present in various areas of a deep mining site, a high-temperature and high-humidity environment. This work not only discloses basic physics behind the tribovoltaic effect but also sheds light on possible applications of SM-TVNGs for wear-resisting and stable mechanical sensors in harsh environments.

Bulk Photovoltage Effect in Ferroelectric BaTiO3

Due to the unusual charge separation mechanism and anomalous photovoltaic effects, the bulk photovoltage effect in ferroelectric semiconductors has attracted a great deal of attention in solar energy conversion, especially in attempts to utilize nonthermalized carriers. Among the various mechanisms that have been proposed for interpreting the photovoltaic effect, a shift mechanism was derived from quantum phenomena, which have been modeled and studied for many years. However, the concurrent shift and ballistic mechanism make investigating the ferroelectric bulk photovoltage effect complex and challenging. Here, taking a tetragonal ferroelectric BaTiO₃ single crystal as a prototype, we report an approach for distinguishing the shift and ballistic mechanism-induced surface photovoltage. The results indicate different effects on the charge separation of the ballistic mechanism and shift mechanisms, as evidenced by surface photovoltage measurement. Interestingly, the shift and ballistic mechanisms afford charge separation in two opposite directions but on the same order of magnitude under monochromatic superband illumination. Our results provide facile and efficient methods for clarifying the shift and ballistic mechanisms in ferroelectrics.

Brief Theoretical Overview of Bi-Fe-O Based Thin Films

This paper will provide a brief overview of the unique multiferroic material Bismuth ferrite (BFO). Considering that Bismuth ferrite is a unique material which possesses both ferroelectric and magnetic properties at room temperature, the uniqueness of Bismuth ferrite material will be discussed. Fundamental properties of the material including electrical and ferromagnetic properties also will be mentioned in this paper. Electrical properties include characterization of basic parameters considering the electrical resistivity and leakage current. Ferromagnetic properties involve the description of magnetic hysteresis characterization. Bismuth ferrite can be fabricated in a different form. The common forms will be mentioned and include powder, thin films and nanostructures. The most popular method of producing thin films based on BFO materials will be described and compared. Finally, the perspectives and potential applications of the material will be highlighted.

Physics inspired compact modelling of [Formula: see text] based memristors

With the advent of the Internet of Things, nanoelectronic devices or memristors have been the subject of significant interest for use as new hardware security primitives. Among the several available memristors, BiFe[Formula: see text] (BFO)-based electroforming-free memristors have attracted considerable attention due to their excellent properties, such as long retention time, self-rectification, intrinsic stochasticity, and fast switching. They have been actively investigated for use in physical unclonable function (PUF) key storage modules, artificial synapses in neural networks, nonvolatile resistive switches, and reconfigurable logic applications. In this work, we present a physics-inspired 1D compact model of a BFO memristor to understand its implementation for such applications (mainly PUFs) and perform circuit simulations. The resistive switching based on electric field-driven vacancy migration and intrinsic stochastic behaviour of the BFO memristor are modelled using the cloud-in-a-cell scheme. The experimental current-voltage characteristics of the BFO memristor are successfully reproduced. The response of the BFO memristor to changes in electrical properties, environmental properties (such as temperature) and stress are analyzed and consistant with experimental results.

Strain-Manipulated Photovoltaic and Photoelectric Effects of the MAPbBr3 Single Crystal

Lead halide perovskite materials, such as MAPbBr₃ and MAPbI₃, show excellent semiconductor properties, and thus, they have attracted a lot of attention for applications in solar cells, photodetectors, etc. Here, a periodic strain can dynamically manipulate the build-in electric field (E_bi) of the depletion region with piezoelectricity at the Au/MAPbBr₃ interface. As a result, the photovoltaic short-circuit current density (J_sc) and the open-circuit voltage (V_oc) are increased by 670 and 82%, respectively, by applying an external strain upon an asymmetric solar-cell-like Au/MAPbBr₃/Ga structure. Furthermore, the equivalent piezoelectric d₃₃ values of ∼3.5 pC/N are confirmed in the Au/MAPbBr₃/Au structure with both the sinusoidal strain and the 405 nm light illumination with 220 mW/cm² upon one semitransparent Au electrode. This study not only proves that pressure can effectively enhance the energy conversion efficiency of the halide perovskite-based solar cells and light detectors but also supposes a multifunctional sensor, which can detect light intensity, sense dynamic pressure, explore accelerated speed, etc. simultaneously.

Multifunctionality of Li2 SrNb2 O7 : Memristivity, Tunable Rectification, Ferroelasticity, and Ferroelectricity

Layered Li₂ SrNb₂ O₇ , an inorganic oxide in its bulk single-crystalline form, is experimentally demonstrated to exhibit multiple structural facets such as ferroelasticity, ferroelectricity, and antiferroelectricity. The transition from a room temperature (RT) centrosymmetric structure to a low-temperature out-of-plane ferroelectric and in-plane antiferroelectric structure and the low-temperature (LT) ferroelectric domain configuration are unveiled in TEM, piezoresponse force microscopy, and polarization loop studies. Li₂ SrNb₂ O₇  also exhibits highly tunable ferroelasticity and excellent Li⁺ in-plane conduction, which leads to a giant in-plane memristor behavior and an in-plane electronic conductivity increase by three orders of magnitude by electric poling at room RT). The accumulation of Li⁺ vacancies at the crystal-electrode interface is visualized using in situ optical microscopy. The Li-ionic biased state shows a clear in-plane rectification effect combined with a significant relaxation upon time at RT. Relaxation can be fully suppressed at LTs such as 200 K, and utilizing an electric field cooling, a stable rectification can be achieved at 200 K. The results shed light on the selective control of multifunctionalities such as ferroelasticity, ferroelectricity, and ionic-migration-mediated effects (a memristor effect and rectification) in a single-phase bulk material utilizing, for example, different directions, temperatures, frequencies, and magnitudes of electric field.