In-Plane Ferroelectricity in Thin Flakes of Van der Waals Hybrid Perovskite

Two-Dimensional Layered Germanium Iodide Perovskite Ferroelectric Semiconductors

The discovery of ferroelectricity in two-dimensional (2D) semiconductors has opened a new and exciting chapter in next-generation electronics and spintronics due to their lattice-dimensionality-induced unique behaviors and fascinating functionalities brought by spontaneous polarization. The emerging layered halide perovskites with 2D lattices provide a great platform for generating reduced symmetry and low-dimensional ferroelectricity. Herein, inspired by the approach of reduced lattice dimensionality, a series of 2D layered germanium iodide perovskite ferroelectric semiconductors A2CsGe2I7 [where A=PA (propylammonium), BA (butylammonium) and AA (amylammonium)] was firstly developed, which demonstrates remarkable semiconducting features including narrow direct band gap (~1.8 eV) and high conductivity over 32.23 nS/cm. Emphatically, these layered germanium iodide perovskites manifest large in-plane ferroelectric polarization over ~10.0 μC/cm2, mainly attributed to the large off-centering ion displacement induced by stereo-active lone-pairs of Ge2+. More specifically, in contrast to three-dimensional ferroelectric CsGeI3, the representative 2D layered BA2CsGe2I7 manifests a superior polarization-sensitive bulk photovoltaic effect with a polarization ratio of 1.68 and high short circuit current density up to 81.25 μA/cm2, which is superior to those of reported layered halide perovskite ferroelectrics. This work provides an exciting pathway for the development of 2D ferroelectric semiconductors as well as sheds light on their further applications in photoelectronic fields.

Reconfigurable Sequential-Logic-in-Memory Implementation Utilizing Ferroelectric Field-Effect Transistors

In modern digital systems, sequential logic circuits store and process information over time, whereas combinational logic circuits process only the current inputs. Conventional sequential systems, however, are complex and energy-inefficient due to the separation of volatile and nonvolatile memory components. This study proposes a compact, nonvolatile, and reconfigurable van der Waals (vdW) ferroelectric field-effect transistor (FeFET)-based sequential logic-in-memory (S-LiM) unit that performs sequential logic operations in two nonvolatile states. Unlike conventional edge computing systems that require separate combinational logic circuits, sequential logic circuits (such as latches for short-term data storage), and nonvolatile memory for long-term data storage, this innovative S-LiM unit integrates logic and memory into a single nonvolatile vdW FeFET device. The nonvolatile ferroelectric elements directly replace both sequential logic and memory in conventional systems, eliminating frequent data transfers, reducing static power, and increasing the storage density. Six distinct logic operations are implemented in a single vdW FeFET through voltage-controlled ferroelectric polarization, highlighting the unit's reconfigurability. The device shows significant potential for low-power edge computing, especially where frequent power cycling is necessary. Its nonvolatile polarization retains the state without the need for storing processes, enabling rapid recovery at startup, even after extended power-off periods of tens of minutes. These features make the vdW FeFET-based S-LiM unit ideal for energy-efficient, high-density, and low-power edge computing, especially in remote operations with unstable power supplies. This innovation contributes to the development of next-generation, low-power electronics with enhanced efficiency and storage density.

Two-Dimensional Ferroelectric Materials: From Prediction to Applications

Ferroelectric materials hold immense potential for diverse applications in sensors, actuators, memory storage, and microelectronics. The discovery of two-dimensional (2D) ferroelectrics, particularly ultrathin compounds with stable crystal structure and room-temperature ferroelectricity, has led to significant advancements in the field. However, challenges such as depolarization effects, low Curie temperature, and high energy barriers for polarization reversal remain in the development of 2D ferroelectrics with high performance. In this review, recent progress in the discovery and design of 2D ferroelectric materials is discussed, focusing on their properties, underlying mechanisms, and applications. Based on the work discussed in this review, we look ahead to theoretical prediction for 2D ferroelectric materials and their potential applications, such as the application in nonlinear optics. The progress in theoretical and experimental research could lead to the discovery and design of next-generation nanoelectronic and optoelectronic devices, facilitating the applications of 2D ferroelectric materials in emerging advanced technologies.

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.

Intrinsic Out-Of-Plane and In-Plane Ferroelectricity in 2D AgCrS2 with High Curie Temperature

2D ferroelectric materials have attracted extensive research interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, the available 2D ferroelectric materials are scarce and most of them are limited by the uncontrollable preparation. Herein, a novel 2D ferroelectric material AgCrS2 is reported that are controllably synthesized in large-scale via salt-assist chemical vapor deposition growth. By tuning the growth temperature from 800 to 900 °C, the thickness of AgCrS2 nanosheets can be precisely modulated from 2.1 to 40 nm. Structural and nonlinear optical characterizations demonstrate that AgCrS2 nanosheet crystallizes in a non-centrosymmetric structure with high crystallinity and remarkable air stability. As a result, AgCrS2 of various thicknesses display robust ferroelectric polarization in both in-plane (IP) and out-of-plane (OOP) directions with strong intercorrelation and high ferroelectric phase transition temperature (682 K). Theoretical calculations suggest that the ferroelectricity in AgCrS2 originates from the displacement of Ag atoms in AgS4 tetrahedrons, which changes the dipole moment alignment. Moreover, ferroelectric switching is demonstrated in both lateral and vertical AgCrS2 devices, which exhibit exotic nonvolatile memory behavior with distinct high and low resistance states. This study expands the scope of 2D ferroelectric materials and facilitates the ferroelectric-based nonvolatile memory applications.

Ultrafast Switching of Sliding Polarization and Dynamical Magnetic Field in van der Waals Bilayers Induced by Light

Sliding ferroelectricity is a unique type of polarity recently observed in van der Waals bilayers with a suitable stacking. However, electric-field control of sliding ferroelectricity is hard and could induce large coercive electric fields and serious leakage currents that corrode the ferroelectricity and electronic properties, which are essential for modern two-dimensional electronics and optoelectronics. Here, we proposed laser-pulse deterministic control of sliding polarization in bilayer hexagonal boron nitride by first principles and molecular dynamics simulation with machine-learned force fields. The laser pulses excite shear modes that exhibit certain directional movements of lateral sliding between bilayers. The vibration of excited modes under laser pulses is predicted to overcome the energy barrier and achieve the switching of sliding polarization. Furthermore, it is found that three possible sliding transitions-between AB (BA) and BA (AB) stacking-can lead to the occurrence of dynamical magnetic fields along three different directions. Remarkably, the magnetic fields are generated by the simple linear motion of nonmagnetic species, without any need for more exotic (circular, spiral) pathways. Such predictions of deterministic control of sliding polarization and multistates of dynamical magnetic field thus expand the potential applications of sliding ferroelectricity in memory and electronic devices.

Organic-Inorganic Hybrid Perovskite Ferroelectric Nanosheets Synthesized by a Room-Temperature Antisolvent Method

Over the past years, the application potential of ferroelectric nanomaterials with unique physical properties for modern electronics is highlighted to a large extent. However, it is relatively challenging to fabricate inorganic ferroelectric nanomaterials, which is a process depending on a vacuum atmosphere at high temperatures. As significant complements to inorganic ferroelectric nanomaterials, the nanomaterials of molecular ferroelectrics are rarely reported. Here a low-cost room-temperature antisolvent method is used to synthesize free-standing 2D organic-inorganic hybrid perovskite (OIHP) ferroelectric nanosheets (NSs), that is, (CHA)2PbBr4 NSs (CHA = cyclohexylammonium), with an average lateral size of 357.59 nm and a thickness ranging from 10 to 70 nm. This method shows high repeatability and produces NSs with excellent crystallinity. Moreover, ferroelectric domains in single NSs can be clearly visualized and manipulated using piezoresponse force microscopy (PFM). The domain switching and PFM-switching spectroscopy indicate the robust in-plane ferroelectricity of the NSs. This work not only introduces a feasible, low-cost, and scalable method for preparing molecular ferroelectric NSs but also promotes the research on molecular ferroelectric nanomaterials.

Chiral multiferroicity in two-dimensional hybrid organic-inorganic perovskites

Chiral multiferroics offer remarkable capabilities for controlling quantum devices at multiple levels. However, these materials are rare due to the competing requirements of long-range orders and strict symmetry constraints. In this study, we present experimental evidence that the coexistence of ferroelectric, magnetic orders, and crystallographic chirality is achievable in hybrid organic-inorganic perovskites [(R/S)-β-methylphenethylamine]2CuCl4. By employing Landau symmetry mode analysis, we investigate the interplay between chirality and ferroic orders and propose a novel mechanism for chirality transfer in hybrid systems. This mechanism involves the coupling of non-chiral distortions, characterized by defining a pseudo-scalar quantity, ξ = p ⋅ r ( p represents the ferroelectric displacement vector and r denotes the ferro-rotational vector), which distinguishes between (R)- and (S)-chirality based on its sign. Moreover, the reversal of this descriptor's sign can be associated with coordinated transitions in ferroelectric distortions, Jahn-Teller antiferro-distortions, and Dzyaloshinskii-Moriya vectors, indicating the mediating role of crystallographic chirality in magnetoelectric correlations.

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.

Scalable Synthesis of High-Quality Ultrathin Ferroelectric Magnesium Molybdenum Oxide

The development of ultrathin, stable ferroelectric materials is crucial for advancing high-density, low-power electronic devices. Nonetheless, ultrathin ferroelectric materials are rare due to the critical size effect. Here, a novel ferroelectric material, magnesium molybdenum oxide (Mg2Mo3O8) is presented. High-quality ultrathin Mg2Mo3O8 crystals are synthesized using chemical vapor deposition (CVD). Ultrathin Mg2Mo3O8 has a wide bandgap (≈4.4 eV) and nonlinear optical response. Mg2Mo3O8 crystals of varying thicknesses exhibit out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at a 2 nm thickness. The Mg2Mo3O8 exhibits a relatively large remanent polarization ranging from 33 to 52 µC cm- 2, which is tunable by changing its thickness. Notably, Mg2Mo3O8 possesses a high Curie temperature (>980 °C) across various thicknesses. Moreover, the as-grown Mg2Mo3O8 crystals display remarkable stability under harsh environments. This work introduces nolanites-type crystal into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg2Mo3O8 expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

Van der Waals Ferroelectrics: Theories, Materials, and Device Applications

In recent years, an increasing number of 2D van der Waals (vdW) materials are theory-predicted or laboratory-validated to possess in-plane (IP) and/or out-of-plane (OOP) spontaneous ferroelectric polarization. Due to their dangling-bond-free surfaces, interlayer charge coupling, robust polarization, tunable energy band structures, and compatibility with silicon-based technologies, vdW ferroelectric materials exhibit great promise in ferroelectric memories, neuromorphic computing, nanogenerators, photovoltaic devices, spintronic devices, and so on. Here, the very recent advances in the field of vdW ferroelectrics (FEs) are reviewed. First, theories of ferroelectricity are briefly discussed. Then, a comprehensive summary of the non-stacking vdW ferroelectric materials is provided based on their crystal structures and the emerging sliding ferroelectrics. In addition, their potential applications in various branches/frontier fields are enumerated, with a focus on artificial intelligence. Finally, the challenges and development prospects of vdW ferroelectrics are discussed.

Surface Engineering of Perovskite Single Crystals by Atomic Layer Deposited Tin Oxide for Optical Communication

Perovskite single crystals with excellent physical properties have broad prospects in the field of optoelectronics. However, the presence of dangling bonds, surface dislocations, and chemical impurities results in high surface defect density and sensitivity to humidity. Unfortunately, there are relatively few surface engineering strategies for single perovskite single crystals. We present a strategy utilizing atomic layer deposited SnOx to passivate surface defects in perovskite single crystals. The photodetector prepared based on the modified FAPbBr3 single crystals exhibits a low dark current of 1.89 × 10-9 A at a 5 V bias, close to 4 times lower with respect to the pristine device, a high detectivity of 2.3 × 1010 jones, and a fast response time of 27 μs. Moreover, the photodetectors feature long-term operational stability because the presence of a dense SnOx capping layer hinders the ingress of moisture and diffusion of ions. We further demonstrate the promise of our perovskite single crystal detectors for real-time subaqueous optical communication.

Topological Nodal-Point Superconductivity in Two-Dimensional Ferroelectric Hybrid Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) with enhanced stability, high tunability, and strong spin-orbit coupling have shown great potential in vast applications. Here, we extend the already rich functionality of 2D HOIPs to a new territory, realizing topological superconductivity and Majorana modes for fault-tolerant quantum computation. Especially, we predict that room-temperature ferroelectric BA2PbCl4 (BA for benzylammonium) exhibits topological nodal-point superconductivity (NSC) and gapless Majorana modes on selected edges and ferroelectric domain walls when proximity-coupled to an s-wave superconductor and an in-plane Zeeman field, attractive for experimental verification and application. Since NSC is protected by spatial symmetry of 2D HOIPs, we envision more exotic topological superconducting states to be found in this class of materials due to their diverse noncentrosymmetric space groups, which may open a new avenue in the fields of HOIPs and topological superconductivity.

Ambient Degradation Anisotropy and Mechanism of van der Waals Ferroelectric NbOI2

The spontaneous centrosymmetry-breaking and robust room-temperature ferroelectricity in niobium oxide dihalides spurs a flurry of explorations into its promising second-order nonlinear optical properties, and promises potential applications in nonvolatile electro-optical and optoelectronic devices. However, the ambient stability of the niobium oxide dihalides remains questionable, which overshadows their future development. In this work, the chemical degradation of NbOI2 is comprehensively investigated using combined chemical and optical microscopies in conjunction with spectroscopies. We unveil the highly anisotropic degradation kinetics of NbOI2 driven by the hydrolysis process of the unstable dangling iodine bonds dominantly on the (010) facet and progressing along the c axis. Knowing its degradation mechanism, the NbOI2 flake can then be stabilized by the hexagonal boron nitride encapsulation, which isolates the air moisture. These findings provide direct insights into the ambient instability of NbOI2, and they deliver possible solutions to circumvent this issue, which are essential for its practical integration in photonic and electronic devices.

Ferroic Phases in Two-Dimensional Materials

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

Ferroelectricity in Epitaxial Perovskite Oxide Bi2WO6 Films with One-Unit-Cell Thickness

Retaining ferroelectricity in ultrathin films or nanostructures is crucial for miniaturizing ferroelectric devices, but it is a challenging task due to intrinsic depolarization and size effects. In this study, we have shown that it is possible to stably maintain in-plane polarization in an extremely thin, one-unit-cell thick epitaxial Bi2WO6 film. The use of a perfectly lattice-matched NdGaO3 (110) substrate for the Bi2WO6 film minimizes strain and enhances stability. We attribute the residual polarization in this ultrathin film to the crystal stability of the Bi-O octahedral framework against structural distortions. Our findings suggest that ferroelectricity can surpass the critical thickness limit through proper strain engineering, and the Bi2WO6/NdGaO3 (110) system presents a potential platform for designing low-energy consumption, nonvolatile ferroelectric memories.

Tailoring angle dependent ferroelectricity in nanoribbons of group-IV monochalcogenides

Ferroelectricity is significant in low dimensional structures due to the potential applications in multifunctional nanodevices. In this work, the tailoring angle dependent ferroelectricity is systematically investigated for the nanoribbons and nanowires of puckered group-IV monochalcogenides MX (M =Ge,Sn; X =S,Se). Based on first-principles calculations, it is found that the ferroelectricity of nanoribbon and nanowire strongly depends on the tailoring angle. Firstly, the critical width for the bare nanoribbon of group-IV monochalcogenide is obtained and discussed. As the nanowires are concerned, the ferroelectricity will disappear when the tailoring angle becomes small. At last, H-passivation on the edge and the strain engineering are employed to improve the ferroelectricity of nanoribbon, and it is obtained that H-passivation is beneficial to the enhancement of polarization for nanoribbons tailored near the armchair direction, while the polarization of nanoribbons tailored along the diagonal direction will decrease when the edges are passivated with H atoms, and the tensile strain along the length direction always favors the improvement of ferroelectricity of the considered nanoribbons. Therefore, tailoring angle has great influence on the ferroelectricity of nanoribbons and nanowires, which may be used as an effective way to tune the ferroelectricity and further the electronic structures of nanostructures in the field of nanoelectronics.

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.

Towards two-dimensional van der Waals ferroelectrics

The discovery of ferroelectricity in two-dimensional (2D) van der Waals (vdW) materials has brought important functionalities to the 2D materials family, and may trigger a revolution in next-generation nanoelectronics and spintronics. In this Perspective, we briefly review recent progress in the field of 2D vdW ferroelectrics, focusing on the mechanisms that drive spontaneous polarization in 2D systems, unique properties brought about by the reduced lattice dimensionality and promising applications of 2D vdW ferroelectrics. We finish with an outlook for challenges that need to be addressed and our view on possible future research directions.

General Theory for Bilayer Stacking Ferroelectricity

Two-dimensional (2D) ferroelectrics, which are rare in nature, enable high-density nonvolatile memory with low energy consumption. Here, we propose a theory of bilayer stacking ferroelectricity (BSF), in which two stacked layers of the same 2D material, with different rotation and translation, exhibit ferroelectricity. By performing systematic group theory analysis, we find all the possible BSF in all 80 layer groups (LGs) and discover the rules about the creation and annihilation of symmetries in the bilayer. Our general theory can not only explain all the previous findings (including sliding ferroelectricity), but also provide a new perspective. Interestingly, the direction of the electric polarization of the bilayer could be totally different from that of the single layer. In particular, the bilayer could become ferroelectric after properly stacking two centrosymmetric nonpolar monolayers. By means of first-principles simulations, we predict that the ferroelectricity and thus multiferroicity can be introduced to the prototypical 2D ferromagnetic centrosymmetric material CrI_{3} by stacking. Furthermore, we find that the out-of-plane electric polarization in bilayer CrI_{3} is interlocked with the in-plane electric polarization, suggesting that the out-of-plane polarization can be manipulated in a deterministic way through the application of an in-plane electric field. The present BSF theory lays a solid foundation for designing a large number of bilayer ferroelectrics and thus colorful platforms for fundamental studies and applications.

Mesoscopic sliding ferroelectricity enabled photovoltaic random access memory for material-level artificial vision system

Intelligent materials with adaptive response to external stimulation lay foundation to integrate functional systems at the material level. Here, with experimental observation and numerical simulation, we report a delicate nano-electro-mechanical-opto-system naturally embedded in individual multiwall tungsten disulfide nanotubes, which generates a distinct form of in-plane van der Waals sliding ferroelectricity from the unique combination of superlubricity and piezoelectricity. The sliding ferroelectricity enables programmable photovoltaic effect using the multiwall tungsten disulfide nanotube as photovoltaic random-access memory. A complete "four-in-one" artificial vision system that synchronously achieves full functions of detecting, processing, memorizing, and powering is integrated into the nanotube devices. Both labeled supervised learning and unlabeled reinforcement learning algorithms are executable in the artificial vision system to achieve self-driven image recognition. This work provides a distinct strategy to create ferroelectricity in van der Waals materials, and demonstrates how intelligent materials can push electronic system integration at the material level.

Two-Dimensional Organic-Inorganic Room-Temperature Multiferroics

Organic-inorganic multiferroics are promising for the next generation of electronic devices. To date, dozens of organic-inorganic multiferroics have been reported; however, most of them show a magnetic Curie temperature much lower than room temperature, which drastically hampers their application. Here, by performing first-principles calculations and building effective model Hamiltonians, we reveal a molecular orbital-mediated magnetic coupling mechanism in two-dimensional Cr(pyz)₂ (pyz = pyrazine) and the role that the valence state of the molecule plays in determining the magnetic coupling type between metal ions. Based on these, we demonstrate that a two-dimensional organic-inorganic room-temperature multiferroic, Cr(h-fpyz)₂ (h-fpyz = half-fluoropyrazine), can be rationally designed by introducing ferroelectricity in Cr(pyz)₂ while keeping the valence state of the molecule unchanged. Our work not only reveals the origin of magnetic coupling in 2D organic-inorganic systems but also provides a way to design room-temperature multiferroic materials rationally.

Rational Design of Molecular Ferroelectrics with Negatively Charged Domain Walls

Ferroelectric domains and domain walls are unique characteristics of ferroelectric materials. Among them, charged domain walls (CDWs) are a special kind of peculiar microstructure that highly improve conductivity, piezoelectricity, and photovoltaic efficiency. Thus, CDWs are believed to be the key to ferroelectrics' future application in fields of energy, sensing, information storage, and so forth. Studies on CDWs are one of the most attractive directions in conventional inorganic ferroelectric ceramics. However, in newly emerged molecular ferroelectrics, which have advantages such as lightweight, easy preparation, simple film fabrication, mechanical flexibility, and biocompatibility, CDWs are rarely observed due to the lack of free charges. In inorganic ferroelectrics, doping is a traditional method to induce free charges, but for molecular ferroelectrics fabricated by solution processes, doping usually causes phase separation or phase transition, which destabilizes or removes ferroelectricity. To realize stable CDWs in molecular systems, we designed and synthesized an n-type molecular ferroelectric, 1-adamantanammonium hydroiodate. In this compound, negative charges are induced by defects in the I^- vacancy, and CDWs can be achieved. Nanometer-scale CDWs that are stable at temperatures as high as 373 K can be "written" precisely by an electrically biased metal tip. More importantly, this is the first time that the charge diffusion of CDWs at variable temperatures has been investigated in molecular ferroelectrics. This work provides a new design strategy for n-type molecular ferroelectrics and may shed light on their future applications in flexible electronics, microsensors, and so forth.

High-TC Two-Dimensional Ferroelectric CuCrS2 Grown via Chemical Vapor Deposition

Two-dimensional (2D) ferroelectrics have attracted intensive attention. However, the 2D ferroelectrics remain rare, and especially few of them represent high ferroelectric transition temperature (T_C), which is important for the usability of ferroelectrics. Herein, CuCrS₂ nanoflakes are synthesized by salt-assisted chemical vapor deposition and exhibit switchable ferroelectric polarization even when the thickness is downscaled to 6 nm. On the contrary, a CuCrS₂ nanoflake shows a T_C as high as ∼700 K, which can be attributed to the robust tetrahedral bonding configurations of Cu cations. Such robustness can be further clarified by a theoretically predicted high order-disorder transition barrier and structure evolution from 600 to 800 K. Additionally, the interlocked out-of-plane (OOP) and in-plane (IP) ferroelectric domains are observed and two kinds of devices based on OOP and IP polarizations are demonstrated.

Emergent ferroelectricity in subnanometer binary oxide films on silicon

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

Searching for High-Quality Halide Perovskite Single Crystals toward X-ray Detection

Metal halide perovskite materials, which combine outstanding physical properties, large absorption coefficient, tailored composition, and low-cost solution-processing, have aroused wide attention for use in various optoelectronic devices. Recently, perovskite single crystals have been rapidly outpacing traditional semiconductor materials in the field of radiation detection. As a prerequisite, achieving high-quality single crystals under controllable solution-phase growth must be tackled to fulfill their full potential as a new paradigm in this stagnated field. This Perspective summarizes the advances in X-ray detectors based on lead halide perovskite single crystals, presenting a comprehensive picture of the relationship among composition engineering, synthesis, and device properties. Additionally, we share our thoughts on several outstanding challenges of perovskite single crystals as promising X-ray detectors and propose possible approaches to the unresolved issues. We anticipate that this Perspective can open up new opportunities to improve their optoelectronic properties, which confers fascinating photonics applications with above and beyond state-of-the-art performance.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

First observation on emergence of strong room-temperature ferroelectricity and multiferroicity in 2D-Ti3C2Tx free-standing MXene film

Two-dimensional (2D) multiferroics are key candidate materials towards advancement of smart technology. Here, we employed a simple synthesis approach to address the long-awaited dream of developing ferroelectric and multiferroic 2D materials, especially in the new class of materials called MXenes. The etched Ti₃C₂T_x MXene was first synthesized after HF-treatment followed by a delamination process for successful synthesis of free-standing Ti₃C₂T_x film. The free-standing film was then exposed to air at room-temperature and heated at different temperatures to form a TiO₂ layer derived from the Ti₃C₂T_x MXene itself. The ferroelectric measurement showed a clear polarization hysteresis loop at room-temperature. Also, due to the reported ferromagnetic behavior of Ti₃C₂T_x MXene, our composite could show multiferroic properties at room-temperature. The magnetoelectric coupling test was also performed that showed a clear, switchable spontaneous polarization under applied magnetic field. TiO₂ is reported to be an incipient ferroelectric that assumes a ferroelectric phase in composite form. The structural and morphological analysis confirmed successful synthesis of free-standing film and the Raman spectroscopy revealed the formation of different phases of TiO₂ and the observed ferroelectricity could be due to structural deformation as a result of the formation of this new phase. The measured value of remanent polarization is 0.5 μC cm⁻². This is the first report on the existence of a ferroelectric phase and multiferroic coupling in 2D free-standing MXene film at room-temperature which opens-up the possibility of 2D material-based electric and magnetic data storage applications at room-temperature.

Two-dimensional (2D) multiferroics are key candidate materials towards advancement of smart technology.

A bright future for engineering piezoelectric 2D crystals

The piezoelectric effect, mechanical-to-electrical and electrical-to-mechanical energy conversion, is highly beneficial for functional and responsive electronic devices. To fully exploit this property, miniaturization of piezoelectric materials is the subject of intense research. Indeed, select atomically thin 2D materials strongly exhibit the piezoelectric effect. The family of 2D crystals consists of over 7000 chemically distinct members that can be further manipulated in terms of strain, functionalization, elemental substitution (i.e. Janus 2D crystals), and defect engineering to induce a piezoelectric response. Additionally, most 2D crystals can stack with other similar or dissimilar 2D crystals to form a much greater number of complex 2D heterostructures whose properties are quite different to those of the individual constituents. The unprecedented flexibility in tailoring 2D crystal properties, coupled with their minimal thickness, make these emerging highly attractive for advanced piezoelectric applications that include pressure sensing, piezocatalysis, piezotronics, and energy harvesting. This review summarizes literature on piezoelectricity, particularly out-of-plane piezoelectricity, in the vast family of 2D materials as well as their heterostructures. It also describes methods to induce, enhance, and control the piezoelectric properties. The volume of data and role of machine learning in predicting piezoelectricity is discussed in detail, and a prospective outlook on the 2D piezoelectric field is provided.

Atomic-Scale Visualization of Polar Domain Boundaries in Ferroelectric In2Se3 at the Monolayer Limit

Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for broad applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of polar domain boundaries in two-dimensional (2D) ferroelectric β'-In₂Se₃ down to the monolayer limit. We observe a double-barrier energy potential with a width of about 3 nm across the 60° tail-to-tail domain boundaries in monolayer β'-In₂Se₃. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Room-Temperature Ferroelectricity in 2D Metal-Tellurium-Oxyhalide Cd7Te7Cl8O17 via Selenium-Induced Selective-Bonding Growth

Two-dimensional (2D) ferroelectric materials have attracted increasing interest due to meeting the requirements of integration, miniaturization, and multifunction of devices. However, the exploration of intrinsic 2D ferroelectric materials is still in the early stage, for which the related reports are still limited, especially fewer ones prepared by chemical vapor deposition (CVD). Here, the ultrathin metal-tellurium-oxyhalide Cd₇Te₇Cl₈O₁₇ (CTCO) flakes as thin as 3.8 nm are realized via the selenium-induced selective-bonding CVD method. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be ascribed to the induction of selective bonding of a hydrogen atom in H₂O molecules by the introduction of selenium, leading to the generation of strong oxidants. Excitingly, switchable out-of-plane ferroelectric polarization was observed in CTCO flakes down to 6 nm at room temperature, which may be caused by mobile Cl vacancies. This work has implications for the synthesis and applications of 2D ferroelectric materials.

Effective Piezo-Phototronic Enhancement of Flexible Photodetectors Based on 2D Hybrid Perovskite Ferroelectric Single-Crystalline Thin-Films

2D hybrid perovskites are very attractive for optoelectronic applications because of their numerous exceptional properties. The emerging 2D perovskite ferroelectrics, in which are the coupling of spontaneous polarization and piezoelectric effects, as well as photoexcitation and semiconductor behaviors, have great appeal in the field of piezo-phototronics that enable to effectively improve the performance of optoelectronic devices via modulating the electro-optical processes. However, current studies on 2D perovskite ferroelectrics focus on bulk ceramics that cannot endure irregular mechanical deformation and limit their application in flexible optoelectronics and piezo-phototronics. Herein, we synthesize ferroelectric EA₄ Pb₃ Br₁₀ single-crystalline thin-films (SCFs) for integration into flexible photodetectors. The in-plane multiaxial ferroelectricity is evident within the EA₄ Pb₃ Br₁₀ SCFs through systematic characterizations. Flexible photodetectors based on EA₄ Pb₃ Br₁₀ SCFs are achieved with an impressive photodetection performance. More importantly, optoelectronic EA₄ Pb₃ Br₁₀ SCFs incorporated with in-plane ferroelectric polarization and effective piezoelectric coefficient show great promise for the observation of piezo-phototronic effect, which is capable of greatly enhancing the photodetector performance. Under external strains, the responsivity of the flexible photodetectors can be modulated by piezo-phototronic effect with a remarkable enhancement up to 284%. Our findings shed light on the piezo-phototronic devices and offer a promising avenue to broaden functionalities of hybrid perovskite ferroelectrics.

Two-Dimensional van der Waals Ferroelectrics: Scientific and Technological Opportunities

Recent breakthroughs in two-dimensional (2D) van der Waals ferroelectrics have been impressive, with a series of 2D ferroelectrics having been realized experimentally. The discovery of ferroelectric order in atom-thick layers not only is important for exploring the interplay between dimensionality and ferroelectric order but may also enable ultra-high-density memory, which has attracted significant interest. However, understanding of 2D ferroelectrics goes beyond simply their atomic-scale thickness. In this Perspective, I suggest possible innovations that may resolve a number of conventional issues and greatly transform the roles of ferroelectrics in nanoelectronics. The major obstacles in the commercialization of nanoelectronic devices based on current ferroelectrics involve their insulating and interfacial issues, which hinder their combination with semiconductors in nanocircuits and reduce their efficiency in data reading/writing. In comparison, the excellent semiconductor performance of many 2D ferroelectrics may enable computing-in-memory architectures or efficient ferroelectric photovoltaics. In addition, their clean van der Waals interfaces can greatly facilitate their integration into silicon chips, as well as the popularization of nondestructive data reading and indefatigable data writing. Two-dimensional ferroelectrics also give rise to new physics such as interlayer sliding ferroelectricity, Moiré ferroelectricity, switchable metallic ferroelectricity, and unconventional robust multiferroic couplings, which may provide high-speed energy-saving data writing and efficient data-reading strategies. The emerging 2D ferroelectric candidates for optimization will help resolve some current issues (e.g., weak vertical polarizations), and further exploitation of the aforementioned advantages may open a new era of nanoferroelectricity.

Giant Ferroelectric Resistance Switching Controlled by a Modulatory Terminal for Low-Power Neuromorphic In-Memory Computing

Ferroelectrics have been demonstrated as excellent building blocks for high-performance nonvolatile memories, including memristors, which play critical roles in the hardware implementation of artificial synapses and in-memory computing. Here, it is reported that the emerging van der Waals ferroelectric α-In₂ Se₃ can be used to successfully implement heterosynaptic plasticity (a fundamental but rarely emulated synaptic form) and achieve a resistance-switching ratio of heterosynaptic memristors above 10³ , which is two orders of magnitude larger than that in other similar devices. The polarization change of ferroelectric α-In₂ Se₃ channel is responsible for the resistance switching at various paired terminals. The third terminal of α-In₂ Se₃ memristors exhibits nonvolatile control over channel current at a picoampere level, endowing the devices with picojoule read-energy consumption to emulate the associative heterosynaptic learning. The simulation proves that both supervised and unsupervised learning manners can be implemented in α-In₂ Se₃ neutral networks with high image recognition accuracy. Moreover, these heterosynaptic devices can naturally realize Boolean logic without an additional circuit component. The results suggest that van der Waals ferroelectrics hold great potential for applications in complex, energy-efficient, brain-inspired computing systems and logic-in-memory computers.

Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BA₂PbCl₄ (BA = C₆H₅CH₂NH₃⁺)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb²⁺ ions that is essential to the ferroelectricity in 2D BA₂PbCl₄. The nonpolar ground state of the closely related compounds BA₂PbBr₄ and BA₂PbI₄ can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA₂PbF₄ displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

Although only a few 2D materials have been predicted to possess ferroelectricity, 2D ferroelectrics are expected to play a dominant role in the upcoming nano era as important functional materials. The ferroelectric properties of 2D ferroelectrics are significantly different than those of traditional bulk ferroelectrics owing to their intrinsic size and surface effects. To date, 2D ferroelectrics have been reported to exhibit diverse properties ranging from bulk photovoltaic and piezoelectric/pyroelectric effects to the spontaneous valley and spin polarization. These properties are either dependent on ferroelectric polarization or coupled with it for easy electric control, thus making 2D ferroelectrics applicable to multifunctional nanodevices. At present, cumulative efforts are being made to explore 2D ferroelectrics in theories, experiments, and applications. Herein, such theories and methods are briefly introduced. Subsequently, intrinsic and extrinsic origins of 2D ferroelectricity are separately summarized. In addition, invented or laboratory-validated 2D ferroelectric-based applications are listed. Finally, the existing challenges and prospects of 2D ferroelectrics are discussed.

Strong Temperature Effect on the Ferroelectric Properties of CuInP2S6 and Its Heterostructures

Van der Waals (vdW) ferroelectric insulator CuInP₂S₆ (CIPS) has attracted intense research interest due to its unique ferroelectric and piezoelectric properties. In this paper, we systematically investigate the temperature and frequency dependence of the ferroelectric properties of CIPS. We find that there is a large imprint in the CIPS capacitor, which can be attributed to the fixed dipoles induced by defects. At high temperatures and low frequencies, the amplitude and direction of the imprint become tunable by the preset pulse, as the copper ions are more mobile and these dipoles become switchable. When the polarization in CIPS changes direction, the graphene/CIPS/graphene ferroelectric diode exhibits switchable resistance since the Fermi level in graphene is modulated by the polarization in CIPS. For CIPS/MoTe₂ dual-gate transistor, a temperature-dependent nonvolatile memory window is observed, which can be attributed to the interplay between ferroelectric polarization and interface traps. This research provides experimental groundwork for vdW ferroelectric materials, expands the understanding of ferroelectricity in CIPS, and opens up exciting opportunities for novel electronic devices based on vdW ferroelectric materials.

Microscopic Manipulation of Ferroelectric Domains in SnSe Monolayers at Room Temperature

Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and nonvolatile electronic devices based on vertical and coplanar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene and the demonstration of controlled room-temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which are difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.

Thickness-Dependent In-Plane Polarization and Structural Phase Transition in van der Waals Ferroelectric CuInP2 S6

van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-density storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in-plane polarization in vdW ferroelectrics CuInP₂ S₆ single crystals is reported, whereas below a critical thickness between 90 and 100 nm, the in-plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.

Intrinsic interaction between in-plane ferroelectric polarization and surface adsorption

The chemical properties of a ferroelectric surface are polarization dependent, the underlying nature of which is, however, far from being completely understood. One of the reasons is that when the polarization direction is perpendicular to the surface, the depolarization field may induce electronic or atomic reconstruction and thus change the chemistry of the ferroelectric surface in complicated ways. Instead, the in-plane polarization results in no depolarization field. Therefore, the chemical properties of a ferroelectric surface can be more intrinsically reflected by the interplay between the in-plane polarization and the surface adsorption. By using first-principles calculations, we study the effect of the strain-induced in-plane polarization on the adsorption of a series of molecules on the reduced rutile TiO2(110) surface. We reveal that it is the surface doping caused by the charge transfer between the adsorbates and the TiO2(110) surface that dominates the polarization-induced change of the adsorption energy, as a result of screening long-range Coulomb interactions. The electrostatic interaction between the polarization of the substrate and the polar molecule is of relatively less importance. We propose that charge transfer effects generally occur for ferroelectric surfaces with no localized surface states.

Frustrated Dipole Order Induces Noncollinear Proper Ferrielectricity in Two Dimensions

Within Landau theory, magnetism and polarity are homotopic, displaying a one-to-one correspondence between most physical characteristics. However, despite widely reported noncollinear magnetism, spontaneous noncollinear electric dipole order as a ground state is rare. Here, a dioxydihalides family is predicted to display noncollinear ferrielectricity, induced by competing ferroelectric and antiferroelectric soft modes. This intrinsic of dipoles generates unique physical properties, such as Z_{2}×Z_{2} topological domains, atomic-scale dipole vortices, and negative piezoelectricity.

Fluorinated 2D Lead Iodide Perovskite Ferroelectrics

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead-iodide-based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃ NH₃ PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4-difluorocyclohexylammonium)₂ PbI₄ , 1, is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization-induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.