Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3

Symmetry Breaking via Anion-Molecule Synergy: Fluorine and Crystalline Water Directed Construction of Noncentrosymmetric Sr(IO2F2)2·2H2O

Noncentrosymmetric (NCS) compounds are pivotal for advancing functional materials in nonlinear optics (NLO), ferroelectrics, and piezoelectrics. Herein, we report the first NCS alkaline-earth metal fluorooxoiodate, Sr(IO2F2)2·2H2O, synthesized via hydrothermal methods. This compound crystallizes in the polar space group P212121 and features a wide bandgap of 4.52 eV and a short ultraviolet cutoff edge of 240 nm. Structural evolution from centrosymmetric (CS) Sr(IO3)2·H2O and Ba(IO2F2)2 to NCS Sr(IO2F2)2·2H2O was achieved through strategic fluorination and hydration. Experimental and computational analyses reveal that Sr(IO3)2·H2O exhibits exceptional birefringence of 0.126 at 1064 nm and a wide bandgap of 4.53 eV, positioning it as a promising UV birefringent optical material. In contrast, Sr(IO2F2)2·2H2O demonstrates moderate birefringence of 0.04 at 1064 nm and a nonlinear response of 0.64 × KDP by calculated. This work highlights the fluorine-water synergy as a powerful strategy for designing functional crystals with tailored optical properties.

Realization of Tunable X-ray Detection in 2D Chiral-Polar Hybrid Perovskites

Metal halide perovskites have emerged as outstanding X-ray detection absorption candidates, benefiting from their excellent carrier mobility, strong radiation absorption capability, and ease of preparation. However, the realization of highly anisotropic X-ray detection with a tunable performance has rarely been investigated. Two-dimensional (2D) chiral-polar hybrid perovskites with the natural multiple quantum wells (MQWs) structure and the bulk photovoltaic effect (BPVE) present a prospective solution to meet the requirement of highly anisotropic X-ray detection. The pair of 2D chiral-polar perovskites has been synthesized in this work, termed (R-β-MPA)PAPbI4 and (S-β-MPA)PAPbI4 (1R and 1S, MPA = methylphenethylamine and PA = propylamine), aiming to achieve unique anisotropic X-ray detection resulting from the coupling of the distinctive MQWs structure and the BPVE. As expected, photodetectors based on high-quality single crystals of 1R result in a high dichroism ratio (Imax/Imin ≈ 7) for self-powered polarized-light detection. Strikingly, these photodetectors achieve high anisotropic ratios, reaching up to 9 under X-ray irradiation. This work reveals a strong correlation between X-ray detection performance and the coupling of specially oriented crystals with the BPVE, opening new possibilities for the realization of tunable X-ray detection.

Configurable kinetics of polarization switching via ion migration in ferroionic CuInP2S6

Ferroelectric materials are promising for developing non-volatile memory, neuromorphic computing, and photovoltaic technologies. Taking advantage of variable switching kinetics provides an important strategy for designing multifunctional ferroelectric devices. However, the conventional ferroelectrics due to the unmovable atomic species generally own a single switching kinetics, thus versatile and configurable switching kinetics still remain challenging. In this work, we systematically investigate the switching kinetics of the van der Waals ferroionic CuInP2S6 through polarization-determined ferroelectric photovoltaic behaviors. Based on the time- and field-dependent polarization switching and numerical simulation, we discover three switching modes, including intralayer switching, interlayer switching and intralayer-interlayer coupling switching in CuInP2S6. Through designing the poling voltage amplitude and width, we achieve the configurable kinetic control of polarization switching in CuInP2S6, enabling tunable binary, gradual and accumulative switching with defined poling voltages in a single device. The work demonstrated here is instructive for the development of nanoscale multifunctional ferroelectric devices.

Enhanced Self-Powered Photodetection Performance of p-Si/n-BaTiO3 Film through the Photovoltaic-Pyroelectric Coupled Effect

Although the novel photovoltaic effects exhibited by ferroelectric materials have been applied for harnessing solar energy, the wide bandgaps often lead to low power conversion efficiencies, below 0.5%, as they absorb only 8-20% of the solar spectrum. In addition to harvesting solar energy, these ferroelectric materials have shown promise for photodetector applications, particularly for sensing near-UV irradiation. This study presents a novel self-powered broadband photodetector based on BaTiO3 thin film. The device, fabricated to incorporate the pyroelectric effect into the heterojunction, achieved responsivities and detectivities of 1.35, 0.91, 0.12, and 0.08 mA/W, as well as 2.25 × 1010, 0.04 × 1010, 0.003 × 1010, and 0.002 × 1010 Jones, respectively, at 365, 456, 532, and 632 nm, respectively, which surpass the performance reported for any other 4-stage pyroelectric-effect-based self-powered BaTiO3-based photodetector. The device also exhibited high photosensitivities of 7161%, 21900%, 3183%, and 1346% at the corresponding wavelengths at 0 V. By utilizing the light-induced coupled photovoltaic-pyroelectric effect, the photodetector obtained a remarkable enhancement in the responsivity and detectivity of over 2091%, in contrast to the photovoltaic effect. In addition, the photocurrent response caused by the photovoltaic-pyroelectric effect is thoroughly defined, and the impacts of light wavelength, power intensity, and bias voltage are explored. This study presents a promising strategy to increase the photocurrent of ferroelectric-based photodetectors, paving the way for advancements in their adoption in various optoelectronic devices for industrial and innovative applications.

Bandgap of Epitaxial Single-Crystal BiFe1-xMnxO3 Films Grown Directly on SrTiO3/Si(001)

We report the growth and optical characterization of single-crystal BiFe1-xMnxO3 thin films directly on SrTiO3/Si(001) substrates using molecular beam epitaxy. X-ray diffraction confirmed epitaxial growth, film crystallinity, and sharp interface quality. Scanning electron microscopy and energy dispersive X-ray spectroscopy verified uniform film morphology and successful Mn incorporation. Spectroscopic ellipsometry revealed a systematic bandgap reduction with increasing Mn concentration, from 2.7 eV in BiFeO3 to 2.58 eV in BiFe0.74Mn0.26O3, consistent with previous reports on Mn-doped BiFeO3. These findings highlight the potential of BiFe1₋xMnxO3 films for bandgap engineering, advancing their integration into silicon-compatible multifunctional optoelectronic and photovoltaic applications.

Reversible shuffle twinning yields anisotropic tensile superelasticity in ceramic GeSe

Superelasticity is a reversible, nonlinear strain response to stress stimuli beyond the linear elastic regime. It is commonly associated with a martensitic transformation in its host material, usually a metal or polymer. Except for the ceramic crystals ZrO2 and BaTiO3, which show superelasticity under compressive stress, inorganic materials with covalent or ionic bonding usually do not exhibit superelastic behaviour because of large energy barriers for structural transitions. Here we show anisotropic tensile superelasticity in the ceramic crystal GeSe, which originates from reversible shuffle twinning. Through in situ transmission electron microscopy mechanical testing, we trace the evolution from a linear elastic behaviour to a nonlinear superelastic plateau in stress-strain curves and concurrently observe the generation of stripy-shaped twin domains along the <110> direction. Density functional theory calculations paired with molecular dynamics simulations reveal a release of elastic potential energy upon the shuffle twinning process from a Z-shaped to an anti-Z-shaped bond configuration, which is responsible for the observed tensile superelasticity. This mechanism makes the observed superelasticity highly directional. In line with the anisotropic Young's modulus and Poisson's ratio in GeSe, experiments confirm that superelastic response emerges only when we apply strain along or close to the zigzag direction. We expect to find similar anisotropic superelasticity in ceramic semiconductors with similar crystal structure such as SnSe, SnS or GeS.

First-principles analysis of the photocurrent in a monolayer α-selenium p-n junction optoelectronic device

Two-dimensional monoelemental materials have emerged as promising candidates for use in the development of next-generation optoelectronic devices. In this work, we investigate the photovoltaic effect of monolayer (ML) α-selenium p-n junctions by using ab initio quantum transport simulations. Our research results indicate that the photocurrent of the ML α-selenium p-n junction optoelectronic device exhibits anisotropy. The maximum photoresponsivity (178.49 a02 per photon) in the armchair (ARM) direction is one-half that (341.72 a02 per photon) in the zigzag (ZZ) direction. When stress is applied, the most significant modulation of photoresponsivity occurs in the ZZ direction, reaching a value of 613.21 a02 per photon. When a gate voltage is applied, the most significant modulation of photoresponsivity occurs in the ARM direction, reaching a value of -684.88 a02 per photon. When a thermal difference is applied, the most significant modulation of photoresponsivity occurs in the ARM direction, reaching a value of 412.14 a02 per photon. Thus, ML α-selenium in the ZZ direction can be used for photodetection and photosensing, while ML α-selenium in the ARM direction can be used for photosensing. Both strain engineering and temperature differences cause a blueshift in the photocurrent as a function of energy. Our work paves the way for research into low-dimensional monoelemental-material optoelectronic devices.

In-Sensor Polarization Convolution Based on Ferroelectric-Reconfigurable Polarization-Sensitive Photodiodes

In-sensor computing can enhance the imaging system performance by putting part of the computations into the sensor. While substantial advancements have been made in latency, spectral range, and functionalities, the strategy for in-sensor light polarization computing has remained unexplored. Here, it is shown that ferroelectric-reconfigurable polarization-sensitive photodiodes (FPPDs) based on BiFeO3 nanowire arrays can perform in-sensor computations on polarization information. This innovation leverages the anisotropic photoresponse from the 1D structure of nanowires and the non-volatile reconfigurability of ferroelectrics. The devices show programmable anisotropic ratios as high as 5219, surpassing most state-of-the-art polarization-sensitive photodetectors and commercial polarization image sensors. Employing tunable photoresponse as kernel, FPPDs can perform convolutions to directly extract feature maps containing polarization information, which raises the recognition accuracy on road-scene objects under adverse weather up to 89.6%. The research highlights the potential of FPPDs as a highly efficient vision sensor and extends the boundaries of advanced intelligent imaging systems.

2D Van Der Waals Ferroelectric Materials and Devices for Neuromorphic Computing

2D van der Waals (vdW) ferroelectric materials are emerging as transformative components in modern electronics and neuromorphic computing. The atomic-scale thickness, coupled with robust ferroelectric properties and seamless integration into vdW engineering, offers unprecedented opportunities for the development of high-performance and low-power devices. Notably, 2D ferroelectric devices excel in enabling multistate storage and neuromorphic functionalities in emulating synapses or retinas, positioning them as prime candidates for next-generation in-sensor-and-memory units. Despite ongoing challenges such as scalability, material stability, and uniformity, rapid interdisciplinary advancements and advancing nanofabrication processes are driving the field forward. This review delves into the fundamental principles of 2D ferroelectricity, highlights typical materials, and examines key device structures along with their applications in non-von Neumann architecture development and neuromorphic computing. By providing an in-depth overview, this work underscores the potential of 2D ferroelectric materials to revolutionize the future of electronics.

Nanostructure engineering for ferroelectric photovoltaics

Ferroelectric photovoltaics have attracted increasing attention since their discovery in the 1970s, due to their above-bandgap photovoltage and polarized-light-dependent photocurrent. However, their practical applications have been limited by their weak visible light absorption and low photoconductivity. Intrinsic modification of the material, such as bandgap tuning through chemical doping, has proven effective, but usually leads to the degradation of ferroelectricity. Recently, various nanostructures, such as multilayer heterojunctions, nanoparticles, vertically aligned nanocomposites and polar nanoregions, have been developed to enhance photovoltaic performance. These approaches enable the nanoassembly of materials in a lower-dimension manner to optimize the bulk photovoltaic effect whilst effectively preserving or even inducing ferroelectricity. This review highlights the fabrication processes of these emerging ferroelectric nanostructures and evaluates their photovoltaic performance.

Unprecedented Ultraviolet Circularly Polarized Light-Dependent Anomalous Photovoltaics in Chiral Hybrid Perovskites

Circularly Polarized Light (CPL)-dependent anomalous photovoltaic effect (APVE), characterized by light helicity-manipulated steady photocurrent and above-bandgap photovoltage, has demonstrated significant potential in the fields of photoelectronic and photovoltaics. However, exploiting CPL-dependent APVE in chiral hybrid perovskites, a promising family with intrinsic chiroptical activity and non-centrosymmetric structure, remains challenging. Here, leveraging the flexible structural design of chiral alternating cations intercalation-type perovskites, CPL-dependent APV, for the first time, is achieved in chiral perovskites. Specifically, by introducing lone pair electrons into the organic layers to greatly amplify the polarization, [(R)-PPA](MOPA)PbBr4 (2-R) (PPA = 1-phenylpropylammonium, MOPA = 3-methoxypropylammonium) exhibit intrinsic APVE with an above-bandgap photovoltage of 6.50 V (Eg = 3.01 eV) under ultraviolet (UV) light illumination. Strikingly, profiting from the natural chiral optical activity of chiral perovskites, unprecedented UV CPL-dependent APV is realized in 2-R, driving the high distinguishability between right-hand and left-hand CPLs with a large anisotropy factor (gIph) of 0.33. This study pioneers the realization of CPL-dependent APV within chiral perovskite, promising significant advancements in optoelectronic device technologies.

All Light Controlled Five State Logic Gates on a Ferroelectric Ceramic Chip

Differentiating photoelectric response in a single material with a simple approach is desirable for all-in-one optoelectronic logical devices. In ferroelectric materials, significantly distinct photoelectric features should be observed if they are in diverse polarization states, unveiling a possible pathway to realize multifunctional optoelectronic logic gates through ferroelectric polarization design. In this study, the Ti3+ self-doping strategy is first applied to 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ferroelectric ceramics (BZT-BCT-xT) through co-firing metallic Ti powders and BZT-BCT powders to enhance photoelectric output. Subsequently, on the BZT-BCT-3T ceramic surface that has optimal photoelectric properties, three individual regions are separated and heterogeneously polarized by using a novel planar three-electrodes structure. Intriguing illumination region-dependent photocurrent directions are demonstrated in this as-fabricated device. Based on this, five basic optoelectronic logic gates are integrated into single ferroelectric ceramic via fully light-controlled methods, including "AND", "OR", "NOT", "NAND" and "NOR". These gates can be readily switched by simply altering the output electrodes or light intensity of 405 nm LED modulating light. This work not only puts forward an innovative strategy for designing ferroelectric optoelectronic logic gates, but also provides feasibility for more ferroelectric materials to be applied in logical devices.

Low-Bandgap Ferroelectric h-LuMnO3 Thin Films for Photovoltaic Applications

This work explores the deposition of hexagonal (h-) LuMnO3 thin films in the P63cm phase and investigates the conditions under which the synergy of ferroelectric and photoactive properties, can be achieved to confirm the potential of this material for applications in the development of next-generation photovoltaic devices. Single-phase h-LuMnO3 was successfully deposited on different substrates, and the thermal stability of the material was confirmed by Micro-Raman spectroscopy analysis from 77 to 850 K, revealing the suitable ferro- to para-electric transition near 760 K. Optical measurements confirm the relatively narrow band gap at 1.5 eV, which corresponds to the h-LuMnO3 system. The presence of domain structures and the signature of hysteresis loops consistent with ferroelectric behaviour were confirmed by piezoresponse force microscopy. In addition, light-dependent photocurrent measurements revealed the photoactive sensitivity of the material.

Ultrafast photocurrent hysteresis in photoferroelectric α-In2Se3 diagnosed by terahertz emission spectroscopy

Nonvolatile control over the physical state of polar materials through all-optical methods has been a long-standing objective pursued in optoelectronics. Photoferroelectric semiconductors exhibit immense potential in capturing multimodal nonvolatile states, attributed to their spontaneous and reversible in-plane and out-of-plane polarizations. Herein, we uncover an unprecedented nonvolatile, zero-bias, ultrafast photocurrent hysteresis response with an innovative all-optical approach, discerned by analyzing in-plane and out-of-plane terahertz (THz) waves emitted from photoferroelectric α-In2Se3. The mechanism underlying such ultrafast photocurrent hysteresis arises from anomalous linear and circular photovoltaic effects synchronously fueled by a localized rearrangement of polarization. By harnessing the anisotropic photoferroelectric kinetics-induced relative phase between the in-plane and out-of-plane polarizations, we further demonstrate the flexible selection of chirality, tunable rotational angle, and optimizable ellipticity of THz waves. Our findings present a unique ultrafast and nondestructive strategy for investigating photoferroelectric hysteresis, empowering dynamic polarization manipulation of THz waves for a wide range of THz applications.

Bifunctional Design of Ferroelectric-Order and Band-Engineering in Cu:KTN Crystal for Extended Self-Powered Photoelectric Response

Photoelectric conversion in ferroelectric crystals can support many important applications in modern on-chip technology, but suffering from two problems, low responsive current and narrow responsive range. Especially, wide-gap ferroelectric oxides are only active at short-wavelength ultraviolet region with weak photocurrent at nanoampere levels. Here, a bifunctional design strategy of ferroelectric-order and electronic-band to improve the photocurrent and extend the responsive range simultaneously, is proposed. In a Cu-doped KTa1- xNbxO3 (KTN) perovskite crystal, a conductive channel is constructed by "head-to-head" ferroelectric domains, associated with the emergence of micrometer-scale supercells. In addition, the introduction of Cu+ ion can induce defect levels, thus extending the responsive range beyond the inherent absorption of pure KTN. Through rational device optimization, a record self-powered responsivity of 5.23 mA W-1 is realized in Cu:KTN photodetector, which is two orders of magnitude higher than undoped KTN crystal. The temperature-dependent light diffraction and photocurrent show that the ferroelectric-order is dominated in this photoresponse behavior. Moreover, Cu:KTN detector is active in the broadband range from 390 to 1030 nm, covering ultraviolet, visible, and near-infrared regions. This work provides an effective method for the design of next-generation self-powered photodetectors with ultrahigh responsivity and ultrawide responsive range.

2D Van der Waals Sliding Ferroelectrics Toward Novel Electronic Devices

Ferroelectric materials, celebrated for their switchable polarization, have undergone significant evolution since their early discovery in Rochelle salt. Initial challenges, including water solubility and brittleness, are overcome with the development of perovskite ferroelectrics, which enable the creation of stable, high-quality thin films suitable for semiconductor applications. As the demand for miniaturization in nanoelectronics has increased, research has shifted toward low-dimensional materials. Traditional ferroelectrics often lose their properties at the nanoscale; however, 2D van der Waals (vdW) ferroelectrics, including CuInP2S6 and α-In2Se3, have emerged as promising alternatives. The recent discovery of sliding ferroelectricity, where polarization is linked to the polar stacking configuration of originally non-polar monolayers, has significantly broadened the scope of 2D ferroelectrics. This review offers a comprehensive examination of stacking orders in 2D vdW materials, stacking-order-linked ferroelectric polarization structures, and their manifestations in metallic, insulating and semiconducting 2D vdW materials. Additionally, it explores the applications of 2D vdW sliding ferroelectrics, and discusses the future prospects in nanotechnology.

Self-Powered Filterless Narrowband UV Photodetection Triggered by Asymmetric Charge Carrier Generation in a Wide-Bandgap Halide Perovskite Ferroelectric

Narrowband photodetection with selective light detection in ultraviolet (UV) range is particularly pronounced in specialized applications such as targeted wavelength imaging and UV-phototherapy. In contrast to conventional strategies, ferroelectric materials with pronounced bulk photovoltaic effect (BPVE) provide a novel asymmetric carrier generation concept for achieving filterless spectrally selective photodetection. Herein, for the first time, the realization of self-powered filterless narrowband UV photodetection is demonstrated in bulk single crystals of a newly developed halide perovskite ferroelectric, 2FEA2EA2Pb2Cl10 (2FEEPC), which exhibits a wide bandgap of 3.36 eV and a remarkable spontaneous polarization (≈3.5 µC cm-2). Benefiting from the strongly distorted inorganic framework with asymmetric delocalized electronic states, 2FEEPC demonstrates a highly wavelength-dependent BPVE along the polar direction, showing a high narrowband detecting performance with a response peak at UV wavelength of 355 nm and a full-width-at-half-maximum (FWHM) of ≈15 nm. More interestingly, the ferro-pyro-phototronic effect of 2FEEPC endows a colossal gain of up to 600% for photopyroelectric current. This work presents an unprecedented approach to achieve self-powered filterless narrowband photodetection, thereby shedding light on the development of photosensitive ferroelectrics for spectrally photoelectric applications.

Two-Dimensional Transition Metal Dichalcogenides: A Theory and Simulation Perspective

Two-dimensional transition metal dichalcogenides (2D TMDs) are a promising class of functional materials for fundamental physics explorations and applications in next-generation electronics, catalysis, quantum technologies, and energy-related fields. Theory and simulations have played a pivotal role in recent advancements, from understanding physical properties and discovering new materials to elucidating synthesis processes and designing novel devices. The key has been developments in ab initio theory, deep learning, molecular dynamics, high-throughput computations, and multiscale methods. This review focuses on how theory and simulations have contributed to recent progress in 2D TMDs research, particularly in understanding properties of twisted moiré-based TMDs, predicting exotic quantum phases in TMD monolayers and heterostructures, understanding nucleation and growth processes in TMD synthesis, and comprehending electron transport and characteristics of different contacts in potential devices based on TMD heterostructures. The notable achievements provided by theory and simulations are highlighted, along with the challenges that need to be addressed. Although 2D TMDs have demonstrated potential and prototype devices have been created, we conclude by highlighting research areas that demand the most attention and how theory and simulation might address them and aid in attaining the true potential of 2D TMDs toward commercial device realizations.

Gate-Controlled Superconducting Switch in GaSe/NbSe2 van der Waals Heterostructure

The demand for low-power devices is on the rise as semiconductor engineering approaches the quantum limit, and quantum computing continues to advance. Two-dimensional (2D) superconductors, thanks to their rich physical properties, hold significant promise for both fundamental physics and potential applications in superconducting integrated circuits and quantum computation. Here, we report a gate-controlled superconducting switch in GaSe/NbSe2 van der Waals (vdW) heterostructure. By injecting high-energy electrons into NbSe2 under an electric field, a non-equilibrium state is induced, resulting in significant modulation of the superconducting properties. Owing to the intrinsic polarization of ferroelectric GaSe, a much steeper subthreshold slope and asymmetric modulation are achieved, which is beneficial for the device performance. Based on these results, a superconducting switch is realized that can reversibly and controllably switch between the superconducting and normal states under an electric field. Our findings highlight the significant high-energy injection effect from band engineering in 2D vdW heterostructures combining superconductors and ferroelectric semiconductors and demonstrate the potential for applications in superconducting integrated circuits.

Large Manipulation of Ferrimagnetic Curie Temperature by A-Site Chemical Substitution in ACu3Fe2Re2O12 (A = Na, Ca, and La) Half Metals

CaCu3Fe2Re2O12 and LaCu3Fe2Re2O12 quadruple perovskite oxides are well known for their high ferrimagnetic Curie temperatures and half-metallic electronic structures. By A-site chemical substitution with lower valence state Na+, an isostructural compound NaCu3Fe2Re2O12 with both A- and B-site ordered quadruple perovskite structures in Pn-3 symmetry was prepared using high-pressure and high-temperature techniques. The X-ray absorption study demonstrates the valence states to be Cu2+, Fe3+, and Re5.5+. A ferrimagnetic phase transition is found to take place at the Curie temperature TC ≈ 240 K, which is much less than that observed in A = Ca (560 K) and La (710 K) analogues. NaCu3Fe2Re2O12 possesses a larger saturated magnetic moment up to 9.4 μB/f.u. as well as a remarkably reduced coercive field less than 10 Oe at 2 K. Theoretical calculations suggest that NaCu3Fe2Re2O12 displays a half-metallic electronic band structure with complete spin polarization of conduction electrons in the minority-spin bands. The magnetic properties and electronic structures of the ACu3Fe2Re2O12 family are compared and discussed.

Beyond visible: giant bulk photovoltaic effect for broadband neuromodulation

The giant bulk photovoltaic effect in tellurene nanomaterials has been harnessed to enable broadband infrared neuromodulation, expanding the potential for safe, non-invasive neural stimulation and highlighting the importance of material innovation in advancing infrared photonic applications.

Interface-engineered non-volatile visible-blind photodetector for in-sensor computing

Ultraviolet (UV) detection is extensively used in a variety of applications. However, the storage and processing of information after detection require multiple components, resulting in increased energy consumption and data transmission latency. In this paper, a reconfigurable UV photodetector based on CeO2/SrTiO3 heterostructures is demonstrated with in-sensor computing capabilities achieved through interface engineering. We show that the non-volatile storage capability of the device could be significantly improved by the introduction of an oxygen reservoir. A photodetector array operated as a single-layer neural network was constructed, in which edge detection and pattern recognition were realized without the need for external memory and computing units. The location and classification of corona discharges in real-world environments were also simulated and achieved an accuracy of 100%. The approach proposed here offers promising avenues and material options for creating non-volatile smart photodetectors.

Emerging 2D Ferroelectric Devices for In-Sensor and In-Memory Computing

The quantity of sensor nodes within current computing systems is rapidly increasing in tandem with the sensing data. The presence of a bottleneck in data transmission between the sensors, computing, and memory units obstructs the system's efficiency and speed. To minimize the latency of data transmission between units, novel in-memory and in-sensor computing architectures are proposed as alternatives to the conventional von Neumann architecture, aiming for data-intensive sensing and computing applications. The integration of 2D materials and 2D ferroelectric materials has been expected to build these novel sensing and computing architectures due to the dangling-bond-free surface, ultra-fast polarization flipping, and ultra-low power consumption of the 2D ferroelectrics. Here, the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing is reviewed. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices, are reviewed followed by the integration of perception, memory, and computing application. Notably, 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing. As an emerging device configuration, 2D ferroelectric devices have the potential to expand into the sensor-memory and computing integration application field, leading to new possibilities for modern electronics.

High-Temperature Molecular Ferroelectric [C4N2H14]2[Sb2I10] with Narrow Bandgap and Switchable Photoelectric Response

Organic-inorganic hybrid ferroelectrics have attracted considerable attention due to their outstanding piezoelectricity, mechanical flexibility, and robust nonlinear optical properties. But the species with above room-temperature (RT) ferroelectricity, visible-light bandgap, and high photoelectric performance are still scarce. Herein, a novel organic-inorganic hybrid ferroelectric [C4N2H14]2[Sb2I10] has been synthesized hydrothermally and employed as a light-absorbing layer in organic-inorganic hybrid solar cells. A polar monoclinic structure with a space group of Pn was resolved by single-crystal XRD. A direct band gap of 1.89 eV was revealed in [C4N2H14]2[Sb2I10] by UV-vis spectroscopy and density functional theory (DFT) studies. A dramatic enhancement in photoelectric performance has been achieved by turning the ferroelectric polarization, leading to a maximum Voc ∼ 0.52 V and Jsc ∼ 15.52 μA/cm2, which are 15-fold and 29-fold higher than those of the unpoled sample, respectively. This work may open new avenues for the application of molecular ferroelectrics in optoelectronic devices.

Switchable Photovoltaic Effect Induced by Light Intensity

Photovoltaic devices capable of reversible photovoltaic polarity through external signal modulation may enable multifunctional optoelectronic systems. However, such devices are limited to those induced by gate voltage, electrical poling, or optical wavelength by using complicated device architectures. Here, we show that the photovoltaic polarity is also switchable with the intensity of incident light. The modulation in light intensity induces photovoltaic polarity switching in geometrically asymmetric MoS2 Schottky photodiodes, explained by the asymmetric lowering of the Schottky barrier heights due to the trapping of photogenerated holes at the MoS2/Cr interface states. An applied gate voltage can further modulate the carrier concentration in the MoS2 channel, providing a method to tune the threshold light intensity of polarity switching. Finally, a bidirectional optoelectronic logic gate with "AND" and "OR" functions was demonstrated within a single device.

Photodetector Based on Elemental Ferroelectric Black Phosphorus-like Bismuth

Two-dimensional ferroelectric materials have emerged as a promising candidate for the development of next-generation photodetectors owing to their inherent photogalvanic effect (PGE) and strong light-matter interactions. Recently, the first-ever elemental-based ferroelectric material, black-phosphorus-like Bi (BP-Bi), has been successfully synthesized. In this work, we investigate the PGE of the monolayer (ML) BP-Bi by using ab initio quantum transport simulation. We find that the photocurrent of the ML BP-Bi in the ferroelectric direction (armchair) is significantly larger than that in the vertical ferroelectric direction [zigzag (ZZ)]. For example, despite the comparable optical absorption rates of BP-Bi in the armchair (ARM) and ZZ directions, the maximum photocurrent (133 mA/W) in the ARM direction is 2 orders of magnitude greater than that (4.70 mA/W) in the ZZ direction. The asymmetry is attributed to the breaking and existence of the mirror inversion symmetries along the ARM and ZZ directions, respectively. Our work paves the way for the research of the low-dimensional ferroelectric photodetector.

Photovoltaic effect in methylammonium lead triiodide single crystal

Due to the crystalline acentricity leading to the bulk photovoltaic effect (PV) the ferroelectrics (FEs) are considered as important candidates for creation of the PV cells overcoming the Shockley-Queisser limit of semiconductors. However, this research direction still requires more investigations to develop reliable pathways for PV efficiency optimization. The recent progress in the power conversion efficiency of the cells based on the organic-based compounds such as CH3NH3PbI3perovskite attracted much attention of the scientists. Unfortunately, manufacturing of these multilayer cells implies a very complicated technology and very high price of the devices. Under such circumstances investigations of the PV effect in the single crystals of FE perovskites look very promising. In this paper we report that due to the sample illumination with intensive UV light, CH3NH3PbI3single crystal is transformed from the pristine antiFE into the FE state. As a result, the PV effect characteristic of the FEs is realized in this material. The theoretically maximal value of the power conversion efficiency in this case was found to be one of the largest among the single crystals of this class of ferroics. We also considered the ways allowing to increase the PV efficiency of the potential solar cells based on such materials.

Out-of-plane polarization induces a picosecond photoresponse in rhombohedral stacked bilayer WSe2

Constructing van der Waals materials with spontaneous out-of-plane polarization through interlayer engineering expands the family of two-dimensional ferroelectrics and provides an excellent platform for enhancing the photoelectric conversion efficiency. Here, we reveal the effect of spontaneous polarization on ultrafast carrier dynamics in rhombohedral stacked bilayer WSe2. Using precise stacking techniques, a 3R WSe2-based vertical heterojunction was successfully constructed and confirmed by polarization-resolved second harmonic generation measurements. Through output characteristics and the scanning photocurrent map under zero bias, we reveal a non-zero short-circuit current in the graphene/3R WSe2/graphene heterojunction region, demonstrating the bulk photovoltaic effect. Furthermore, the out-of-plane polarization enables the 3R WSe2 heterojunction region to achieve an ultrafast intrinsic photoresponse time of approximately 3 ps. The ultrafast response time remains consistent across varying detection powers, demonstrating environmental stability and highlighting the potential in optoelectronic applications. Our study presents an effective strategy for enhancing the response time of photodetectors.

Ferroelectric Domain Modulation with Tip-Poling Engineering in BiFeO3 Films

BiFeO3 (BFO) films with ferroelectricity are the most promising candidates regarding the next generation of storage devices and sensors. The comprehensive understanding of ferroelectric switchable properties is challenging and critical to robust domain wall nanoelectronics. Herein, the domain dynamic was explored in detail under external bias conditions using scanning probe microscopy, which is meaningful for the understanding of domain dynamics and the foundation of ferroelectric devices. The results show that domain reversal occurred under external electric fields with sufficient energy excitation, combined with the existence of a charged domain wall. These findings extend the domain dynamic and current paths in ferroelectric films and shed light on the potential applications for ferroelectric devices.

Ferroelectric, flexoelectric and photothermal coupling in PVDF-based composites for flexible photoelectric sensors

A ferroelectric polyvinylidene fluoride (PVDF) film with excellent flexibility possesses great potential for photodetection and wearable devices. However, the relatively weak photo-absorption and the consequent small photocurrent limit its photofunctional properties. Herein, we embedded a strongly visible-light active material system, 0.5Ba(Zr0.08Ti0.8Mn0.12)O3-0.5(Ba0.7Ca0.3)TiO3 (BZTM-BCT) loaded with Ag and Au nanoparticles, into a PVDF film, which demonstrates a significantly higher photovoltaic response in the whole visible light range with a β-phase content of over 90%. In a state of "bending + poling", the PVDF/BZTM-BCT:Au film presents an optimal response for photoelectric properties by exhibiting a photocurrent that is 57 times higher than that of a pure PVDF film when illuminated with 405 nm LED light at 100 mW cm-2. Photoexcitation and thermal excitation jointly contribute to the generation of free carriers, while the flexoelectric and ferroelectric coupling electric field provides a greater driving force for carrier separation and transport. More interestingly, composite film-based photoelectric sensors can simultaneously respond to light and the movement and deformation of contacted things, indicating its potential in versatile applications. Overall, this work puts forward a new route for designing new flexible multifunctional photoelectric devices.

Exploring current conduction dynamics in multiferroic BiFeO3 thin films prepared via modified chemical solution method

The current study describes current conduction mechanisms in BiFeO3 thin films prepared by using a modified chemical solution-based technique. In particular, in these films, X-ray photoelectron spectroscopy revealed that the defect causing Fe2+ ions reduced by about 18% compared to the solution-based synthesis methods reported before, indicating better film quality. The leakage current density was measured to be 1.7 × 10-6 A/cm2 at an applied voltage of 1.5 V, which is one order of magnitude less than the previously reported work in a similar system. Oxygen annealing was found to be effective in further reducing the current conduction, making these films a suitable choice for device applications. The current-voltage curves exhibited three different behaviours of current conduction mechanisms in these films. In particular, when the applied electric field was increased, a transition from Ohmic conduction to trap-filled space charge limited conduction was observed. The presented investigations demonstrate the importance of deposition methods in determining the suitability of thin films for device applications.

Multistate Ferroelectric Diodes with High Electroresistance Based on van der Waals Heterostructures

Some van der Waals (vdW) materials exhibit ferroelectricity, making them promising for novel nonvolatile memories (NVMs) such as ferroelectric diodes (FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been integrated with graphene for memory devices. Here we demonstrate FeDs with self-rectifying, hysteretic current-voltage characteristics based on vertical heterostructures of 10 nm thick CIPS and graphene. By using vdW indium-cobalt top electrodes and graphene bottom electrodes, we achieve a high electroresistance (on- and off-state resistance ratios) of ∼106, an on-state rectification ratio of 2500 for read/write voltages of 2 V/0.5 V, and a maximum output current density of 100 A/cm2. These metrics compare favorably with state-of-the-art FeDs. Piezoresponse force microscopy measurements show that stabilization of intermediate net polarization states in CIPS leads to stable multibit data retention at room temperature. The combination of two-terminal design, multibit memory, and low-power operation in CIPS-based FeDs is potentially interesting for compute-in-memory and neuromorphic computing applications.

Switchable Photoelectric Response in High-Temperature Leadless Molecular Ferroelectric [C4N2H14][BiI5]

Lead-free molecular ferroelectrics have garnered considerable attention for their promising potential, but such species with narrow band gap and sensitive photoelectric response are yet inadequate. Herein, we demonstrated the bulk ferroelectric photovoltaic effect in a novel lead-free molecular ferroelectric [C4N2H14][BiI5] with a Curie temperature (Tc) of 366 K and a narrow band gap (Eg) of 1.92 eV. The transformation of the crystal structure from the polar space group P21 to the nonpolar space group P21/m was elucidated using single-crystal X-ray diffraction. Room-temperature (RT) hysteresis loop reveals the intrinsic ferroelectricity of [C4N2H14][BiI5] with a relative small coercive field (Ec ∼ 0.27 kV/cm), saturation polarization (Ps ∼ 1.87 μC/cm2), and remanent polarization (Pr ∼ 1.61 μC/cm2). [C4N2H14][BiI5]-based solar device exhibits significant PV effects with a steady-state photocurrent (Jsc) of 3.54 μA/cm2 and a photovoltage (Voc) of 0.34 V under AM 1.5 G illumination, which can be significantly improved by adjusting the ferroelectric polarization, reaching a maximum Jsc of 140 μA/cm2 and Voc of 0.51 V. This work offers a promising avenue for lead-free molecular ferroelectric materials in the field of optoelectronic devices.

Strain evolution from the ferroelectric to the relaxor state in (0.67 - x)BiFeO3-0.33BaTiO3-xBi(Mg0.5Zr0.5)O3 lead-free ceramics

The BiFeO3-BaTiO3 solid solution exhibits enhanced electric properties due to its modified phase structure with relaxor characteristics and reduced leakage current. Despite these advancements, the underlying mechanism behind the phase transition from a ferroelectric to a relaxor state in BF-BT ceramics remains largely unexplored. Here, the evolution of strain in (0.67 - x)BiFeO3-0.33BaTiO3-xBi(Mg0.5Zr0.5)O3 ceramics is investigated, with a focus on the strain transition from a ferroelectric to a relaxor phase. A strengthening of relaxor behavior is observed in the modified rhombohedral (R) and pseudocubic (PC) phase structure, resulting in optimal strain (Suni = 0.25%, Spos = 0.24%) at x = 0.04. The enhanced strain is attributed to the promotion of domain switching and the presence of strong random fields, with polar nanoregions integrating into a long-range ordered matrix. Furthermore, a gradual increase in strain with rising temperature is noted, driven by increased polarization and the expansion of ferroelectric domains. This study underscores the critical role of structural modifications in augmenting the electric response of BF-BT ceramics, thereby advancing the development of lead-free piezoelectric materials.

Ionic Photovoltaics-in-Memory in van der Waals Material

The photovoltaic effect is gaining growing attention in the optoelectronics field due to its low power consumption, sustainable nature, and high efficiency. However, the photovoltaic effects hitherto reported are hindered by the stringent band-alignment requirement or inversion symmetry-breaking, and are challenging for achieving multifunctional photovoltaic properties (such as reconfiguration, nonvolatility, and so on). Here, a novel ionic photovoltaic effect in centrosymmetric CdSb2Se3Br2 that can overcome these limitations is demonstrated. The photovoltaic effect displays significant anisotropy, with the photocurrent being most apparent along the CdBr2 chains while absent perpendicular to them. Additionally, the device shows electrically-induced nonvolatile photocurrent switching characteristics. The photovoltaic effect is attributed to the modulation of the built-in electric field through the migration of Br ions. Using these unique photovoltaic properties, a highly secure circuit with electrical and optical keys is successfully implemented. The findings not only broaden the understanding of the photovoltaic mechanism, but also provide a new material platform for the development of in-memory sensing and computing devices.

In-Sublattice Carrier Transition Enabled Polarimetric Photodetectors with Reconfigurable Polarity Transition

Miniaturized polarimetric photodetectors based on anisotropic two-dimensional materials attract potential applications in ultra-compact polarimeters. However, these photodetectors are hindered by the small polarization ratio values and complicated artificial structures. Here, a novel polarization photodetector based on in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, with a giant and reconfigurable PR value, is demonstrated. The unique periodic sublattice structure of CdSb2Se3Br2 features an in-sublattice carrier transition preferred along Sb2Se3 chains. Leveraging on the in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, gate voltage has an anisotropic modulation effect on the band alignment of heterostructure along sublattice. Consequently, the heterostructure exhibits a polarization-tunable photo-induced threshold voltage shift, which provides reconfigurable PR values from positive (unipolar regime) to negative (bipolar regime), covering all possible numbers (1→+∞/-∞→-1). Using this anisotropic photovoltaic effect, gate-tunable polarimetric imaging is successfully implemented. This work provides a new platform for developing next-generation highly polarimetric optoelectronics.

Switchable Bulk Photovoltaic Effect in Intrinsically Ferroelectric 3D All-Inorganic CsPbBr3 Perovskite Nanocrystals

Ferroelectric all-inorganic halide perovskite nanocrystals with both spontaneous polarization and visible light absorption are promising candidates for designing ferroelectric photovoltaic applications. It remains a challenge to realize ferroelectric photovoltaic devices with all-inorganic halide perovskites that can be operated in the absence of an external electric field. Here we report that a popular all-inorganic halide perovskite nanocrystal, CsPbBr3, exhibits a ferroelectricity-driven photovoltaic effect under visible light in the absence of an external electric field. Pristine CsPbBr3 nanocrystals exhibit intrinsic ferroelectric key properties with a notable saturated polarization of ∼0.15 μC/cm2 and a high Curie temperature of 462 K, driven by the stereochemical activity of the Pb(II) lone pair. Furthermore, application of an external electric field allows the photovoltaic effect to be enhanced and the spontaneous polarization to be switched with the direction of the electric field. CsPbBr3 nanocrystals exhibit a robust fatigue performance and a prolonged photoresponse under continuous illumination in the absence of an external electric field. These findings establish all-inorganic halide perovskite nanocrystals as potential candidates for designing photoferroelectric devices by coupling optical functionalities and ferroelectric responses.

Bulk photovoltaic effect in ferroelectric nematic liquid crystals

The bulk photovoltaic (BPV) effect in ferroelectric liquid crystals is of increasing scientific interest owing to its great potential for light-energy conversion. The ferroelectric nematic phase exhibits a huge spontaneous polarization that can be aligned to a preferred direction. In this Letter, we investigate the tensorial properties of the BPV effect in the planarly aligned ferroelectric nematic phase of the liquid crystalline material RM734. A steady-state short-circuit photocurrent of ~160 pA and an open-circuit photovoltage of ~50 mV were observed in a cell with a thickness of 5.5 µm under the illumination of ultraviolet light without any bias voltage. Based on the photocurrent measurements in different electrode configurations, the non-zero elements of the BPV tensor were obtained. The BPV effect is attributed to the combination of the spontaneous polarization and the asymmetric distribution of photoinduced charge carriers. This study not only provides an understanding of the bulk PV mechanism in soft ferroelectrics but also promises a wide range of unprecedented, to the best of our knowledge, benefits for light harvesting to engineer marketable photovoltaic devices.

Developing fatigue-resistant ferroelectrics using interlayer sliding switching

Ferroelectric materials have switchable electrical polarization that is appealing for high-density nonvolatile memories. However, inevitable fatigue hinders practical applications of these materials. Fatigue-free ferroelectric switching could dramatically improve the endurance of such devices. We report a fatigue-free ferroelectric system based on the sliding ferroelectricity of bilayer 3R molybdenum disulfide (3R-MoS2). The memory performance of this ferroelectric device does not show the wake-up effect at low cycles or a substantial fatigue effect after 106 switching cycles under different pulse widths. The total stress time of the device under an electric field is up to 105 s, which is long relative to other devices. Our theoretical calculations reveal that the fatigue-free feature of sliding ferroelectricity is due to the immobile charge defects in sliding ferroelectricity.

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.

Bulk Photovoltaic Effect in Polar 3D Perovskitoid Enables Self-Powered Polarization-Sensitive Photodetection

The family of polar hybrid perovskites, in which bulk photovoltaic effects (BPVEs) drive steady photocurrent without bias voltage, have shown promising potentials in self-powered polarization-sensitive photodetection. However, reports of BPVEs in 3D perovskites remain scare, being mainly hindered by the limited dipole moment or lack of symmetry breaking. Herein, a polar 3D perovskitoid, (BDA)Pb2Br6 (BDA = NH3C4H8NH3), where the spontaneous polarization (Ps)-induced BPVE drives self-powered photodetection of polarized-light is reported. Emphatically, the edge-sharing Pb2Br10 dimer building unit allows the optical anisotropy and polarity in 3D (BDA)Pb2Br6, which triggers distinct optical absorption dichroism ratio of ≈2.80 and BPVE dictated photocurrent of 3.5 µA cm-2. Strikingly, these merits contribute to a polarization-sensitive photodetection with a high polarization ratio (≈4) under self-powered mode, beyond those of 2D hybrid perovskites and inorganic materials. This study highlights the potential of polar 3D perovskitoids toward intelligent optoelectronic applications.

Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

Intermediate multidomain state in single-crystalline Mn-doped BiFeO3 thin films during ferroelectric polarization switching

A intermediate multidomain state and large crystallographic tilting of 1.78° for the (hh0)pc planes of a (001)pc-oriented single-domain Mn-doped BiFeO3 (BFMO) thin film were found when an electric field was applied along the [110]pc direction. The anomalous crystallographic tilting was caused by ferroelastic domain switching of the 109° domain switching. In addition, ferroelastic domain switching occurred via an intermediate multidomain state. To investigate these switching dynamics under an electric field, we used in situ fluorescent X-ray induced Kossel line pattern measurements with synchrotron radiation. In addition, in situ inverse X-ray fluorescence holography (XFH) experiments revealed that atomic displacement occurred under an applied electric field. We attributed the atomic displacement to crystallographic tilting induced by a converse piezoelectric effect. Our findings provide important insights for the design of piezoelectric and ferroelectric materials and devices.

The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

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.

Colossal Ferroelectric Photovoltaic Effect in Inequivalent Double-Perovskite Bi2FeMnO6 Thin Films

Double perovskite films have been extensively studied for ferroelectric order, ferromagnetic order, and photovoltaic effects. The customized ion combinations and ordered ionic arrangements provide unique opportunities for bandgap engineering. Here, a synergistic strategy to induce chemical strain and charge compensation through inequivalent element substitution is proposed. A-site substitution of the barium ion is used to modify the chemical valence and defect density of the two B-site elements in Bi2FeMnO6 double perovskite epitaxial thin films. We dramatically increased the ferroelectric photovoltaic effect to ∼135.67 μA/cm2 from 30.62 μA/cm2, which is the highest in ferroelectric thin films with a thickness of less than 100 nm under white-light LED irradiation. More importantly, the ferroelectric polarization can effectively improve the photovoltaic efficiency of more than 5 times. High-resolution HAADF-STEM, synchrotron-based X-ray diffraction and absorption spectroscopy, and DFT calculations collectively demonstrate that inequivalent ion plays a dual role of chemical strain (+1.92 and -1.04 GPa) and charge balance, thereby introducing lattice distortion effects. The reduction of the oxygen vacancy density and the competing Jahn-Teller distortion of the oxygen octahedron are the main phenomena of the change in electron-orbital hybridization, which also leads to enhanced ferroelectric polarization values and optical absorption. The inequivalent strategy can be extended to other double perovskite systems and applied to other functional materials, such as photocatalysis for efficient defect control.

Strain-Prompted Giant Flexo-Photovoltaic Effect in Two-Dimensional Violet Phosphorene Nanosheets

As a second-order nonlinear optical phenomenon, the bulk photovoltaic (BPV) effect is expected to break through the Shockley-Queisser limit of thermodynamic photoelectron conversion and improve the energy conversion efficiency of photovoltaic cells. Here, we have successfully induced a strong flexo-photovoltaic (FPV) effect, a form of BPV effect, in strained violet phosphorene nanosheets (VPNS) by utilizing strain engineering at the h-BN nanoedge, which was first observed in nontransition metal dichalcogenide (TMD) systems. This BPV effect was found to originate from the disruption of inversion symmetry induced by uniaxial strain applied to VPNS at the h-BN nanoedge. We have revealed the intricate relationship between the bulk photovoltaic effect and strain gradients in VPNS through thickness-dependent photovoltaic response experiments. A bulk photovoltaic coefficient of up to 1.3 × 10-3 V-1 and a polarization extinction ratio of 21.6 have been achieved by systematically optimizing the height of the h-BN nanoedge and the thickness of VPNS, surpassing those of reported TMD materials (typically less than 3). Our results have revealed the fundamental relationship between the FPV effect and the strain gradients in low-dimensional materials and inspired further exploration of optoelectronic phenomena in strain-gradient engineered materials.

Halide Perovskite Inducing Anomalous Nonvolatile Polarization in Poly(vinylidene fluoride)-based Flexible Nanocomposites

Ferroelectric materials have important applications in transduction, data storage, and nonlinear optics. Inorganic ferroelectrics such as lead zirconate titanate possess large polarization, though they are rigid and brittle. Ferroelectric polymers are light weight and flexible, yet their polarization is low, bottlenecked at 10 μC cm-2. Here we show poly(vinylidene fluoride) nanocomposite with only 0.94% of self-nucleated CH3NH3PbBr3 nanocrystals exhibits anomalously large polarization (~19.6 μC cm-2) while retaining superior stretchability and photoluminance, resulting in unprecedented electromechanical figures of merit among ferroelectrics. Comprehensive analysis suggests the enhancement is accomplished via delicate defect engineering, with field-induced Frenkel pairs in halide perovskite stabilized by the poled ferroelectric polymer through interfacial coupling. The strategy is general, working in poly(vinylidene fluoride-co-hexafluoropropylene) as well, and the nanocomposite is stable. The study thus presents a solution for overcoming the electromechanical dilemma of ferroelectrics while enabling additional optic-activity, ideal for multifunctional flexible electronics applications.

A Multistate Non-Volatile Photoelectronic Memory Device Based on Ferroelectric Tunnel Junction with Modulable Visible Light Photoresponse

Recently, certain ferroelectric tunnel junctions (FTJs) exhibit non-volatile modulations on photoresponse as well as tunneling electroresistance (TER) effects related to ferroelectric polarization states. From the opposite perspective, the corresponding polarization states can be read by detecting the levels of the photocurrent. In this study, we fabricate a novel amorphous selenium (a-Se)/PbZr0.2Ti0.8O3 (PZT)/Nb-doped SrTiO3 (NSTO) heterojunction, which exhibits a high TER of 3 × 106. Unlike perovskite oxide FTJs with a limited ultraviolet response, the introduction of a narrow bandgap semiconductor (a-Se) enables self-powered photoresponse within the visible light range. The self-powered photoresponse characteristics can be significantly modulated by ferroelectric polarization. The photocurrent after writing polarization voltages of +4 and -5 V exhibits a 1200% increase. Furthermore, the photocurrent could be clearly distinguished after writing stepwise polarization voltages, and then a multistate information storage is designed with nondestructive readout capacity under light illumination. This work holds great significance in advancing the development of ferroelectric multistate photoelectronic memories with high storage density and expanding the design possibilities for FTJs.