Phase-controllable large-area two-dimensional In2Se3 and ferroelectric heterophase junction

Polymorphic phases in 2D In2Se3: fundamental properties, phase transition modulation methodologies and advanced applications

Two-dimensional (2D) In2Se3, which is a multifunctional semiconductor, exhibits multiple crystallographic phases, each of which possesses distinct electronic, optical, and thermal properties. This inherent phase variability makes it a promising candidate for a wide range of applications, including memory devices, photovoltaics, and photodetectors. This review comprehensively explores the latest progress of various polymorphic phases of 2D In2Se3, emphasizing their unique properties, characterization methods, phase modulation strategies, and practical applications. Commencing with a rigorous examination of the structural attributes inherent in its various phases, we introduce sophisticated techniques for its characterization. Subsequently, modulation strategies, encompassing variations in temperature, application of electric fields, induced stress, and alterations in pressure, are explored, each exerting an influence on the phase transitions in 2D In2Se3. Finally, we highlight recent advancements and applications resulting from these phase transitions, including homoepitaxial heterophase structures, optical modulators, and phase change memory (PCM). By synthesizing insights into phase properties, modulation strategies, and potential applications, this review endeavours to provide a comprehensive understanding of the significance and prospects of In2Se3 in the semiconductor field.

Atomistic Mechanisms of the Crystallographic Orientation-Dependent Cu1.8S Conductive Channel Formation in Cu2S-Based Memristors

Achieving multiple types of resistive switching in a single material with controlled ionic motion is a key challenge in neuromorphic computing, traditionally addressed by combining materials with distinct switching behaviors. Here, Cu2-xS is identified as a promising candidate to overcome this limitation due to its hierarchical phase transitions. Using in situ biasing experiments, reversible and non-reversible phase transitions (and resistive switching) are demonstrated in γ-Cu2S by controlling the compliance current. The formation of parallel high-digenite Cu1.8S channels, orientated along the γ-Cu2S [201] crystallographic direction, drives the nonvolatile resistive switching. These channels emerge via an intermediate δ-Cu2S phase and are stabilized at room temperature by residual strains, alongside β-Cu2S phase. The work clarifies the complex, electrically triggered phase transformations in γ-Cu2S, and highlights the potential of Cu2-xS as a versatile material for neuromorphic computing.

Pseudosymmetric Epitaxy for Scalable Growth of Uniform Two-Dimensional Ferroelectric α-In2Se3 Monolayer

The 2D ferroelectric semiconductor α-In2Se3 offers compelling opportunities for next-generation ultrathin electronics, but the controllable growth of a monolayer with large-scale uniformity and single phase has proven challenging. Here, we demonstrate the pseudosymmetry epitaxial growth of a uniform centimeter-scale α-In2Se3 monolayer by leveraging a fluorophlogopite mica (F-mica) substrate with its pseudohexagonal surface atom configuration, in a confined space chemical vapor deposition setup. Transmission electron microscopy and in-plane XRD reveal the pseudohexagonal symmetry of an F-mica surface and establish the in-plane epitaxial relation of (100) α-In2Se3∥(010) F-mica with a 4 × 4 α-In2Se3 unit cell matching the 3 × 3 F-mica surface. Second-harmonic generation and piezoresponse force microscopy confirm the homogeneity and polarization of the films. A ferroelectric semiconductor junction array based on the α-In2Se3 films exhibits consistent and reliable multipattern memorization and an enhanced On/Off ratio over 105. Our strategies offer critical insights into pseudosymmetric epitaxy of 2D materials and pave the way for advanced ultrathin ferroelectric memory technologies.

Broadband photoresponse based on a Te/CuInP2S6 ferroelectric field-effect transistor

Narrow bandgap two-dimensional (2D) semiconductors have garnered significant attention for their potential applications in next-generation optoelectronic devices. However, only few previous studies have manipulated electronic polarization, such as ferroelectric polarization and spin polarization, in conjunction with photodetectors. In this work, we designed Te ferroelectric field-effect transistors (Fe-FETs) that exhibit a clear counterclockwise hysteresis loop in transfer characteristic curves. The device achieves an ultrabroad band photoresponse from 637 nm to 10.6 μm and a high photoresponsivity (R) of 10.2 A W-1 under 1 V bias. Importantly, under 637 nm laser irradiation, the device shows a very fast speed with a rise time (τr) of 3.86 μs and decay time (τd) of 6.28 μs. The proposed Te Fe-FET device provides a strategy for designing high-performance photodetectors with extensive applications.

2D Reconfigurable Memory for Integrated Optical Sensing and Multifunctional Image Processing

Recently, the growing demand for data-centric applications has significantly accelerated progress to overcome the "memory wall" caused by the separation of image sensing, memory, and computing units. However, despite advancements in novel devices driving the development of the in-sensor computing paradigm, achieving seamless integration of optical sensing, storage, and image processing within a single device remains challenging. This study demonstrates an in-sensor computing architecture using a ferroelectric-defined reconfigurable α-In2Se3 phototransistor. The three polarization states of the device exhibit a linear and distinguishable photoresponse, with a maximum photoresponse current difference of 2.17 × 10-6 A and a retention time exceeding 500 s. The nonvolatile weight and synaptic properties are programmed by external electrical stimulation, enabling 112 distinct conductance states with a nonlinearity of 0.12. Additionally, the device supports efficient optical writing, electrical erasing, optoelectronic logic, and decoding via combined optoelectronic control. In-sensor computation for image edge detection is simulated by embedding a nonvolatile Prewitt convolution kernel into a 3 × 3 device array. The integrated structure and array design highlight the strong potential of 2D ferroelectric semiconductors for in-sensor computing, providing a promising platform for next-generation multifunctional artificial vision systems.

Phase Modulation of 2D Semiconducting GaTe from Hexagonal to Monoclinic through Layer Thickness Control and Strain Engineering

Phase engineering offers a novel approach to modulate the properties of materials for versatile applications. Two-dimensional (2D) GaTe, an emerging III-VI semiconductor, can exist in hexagonal (h) or monoclinic (m) phases with fascinating phase-dependent properties (e.g., isotropic or anisotropic electrical transport). However, the key factors governing GaTe phases remain obscure. Herein, we achieve phase modulation of GaTe by tuning two previously overlooked factors: layer thickness and strain. The precise layer-controlled synthesis of GaTe from a monolayer (1L) to >10L is achieved via molecular beam epitaxy. A layer-dependent phase transition from h-GaTe (1-5L) to m-GaTe (>10L) is unambiguously unveiled by scanning tunneling microscopy/spectroscopy, driven by system energy minimization according to density functional theory calculations. Local phase transitions from ultrathin h-GaTe to m-GaTe are also obtained via introduced tensile strain. This work clarifies the factors influencing GaTe phases, providing valuable guidance for the phase engineering of other 2D materials toward the desired properties and applications.

Resistive Switching in α-In2Se3 Lateral Field-Effect Transistors

Ferroelectric semiconducting field-effect transistors (FeS-FETs) based on two-dimensional materials exhibit nonvolatile resistive switching, making them promising candidates for next-generation memory and neuromorphic computing. However, the mechanisms governing resistive switching in α-In2Se3 lateral devices remain unresolved, particularly regarding the relative contributions of channel and contact resistance. In this study, Kelvin probe force microscopy (KPFM) was employed to spatially resolve the gate-poling-dependent contact and channel resistances in α-In2Se3 FeS-FETs, while scanning photocurrent microscopy (SPCM) was used to quantify changes in effective Schottky barrier height at the metal contacts. Both contact and channel resistances were found to increase (decrease) with positive (negative) poling, with the contact resistance modulation correlating with changes in Schottky barrier height. Control experiments on as-exfoliated multidomain flakes confirmed that spontaneous polarization influences both channel and contact resistances. However, typical clockwise resistive switching characteristics can be observed even in the absence of detectable ferroelectric polarization switching. Furthermore, typical gate-poling conditions lead to the formation of stacking defects observed by ex situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The observed defects can impede domain wall motion, providing a rationale for the lack of an abrupt switching threshold and a possible mechanism of coupling in-plane fields to out-of-plane polarization. We conclude that resistive switching in α-In2Se3 lateral channel devices is often influenced by both reversible polarization switching and irreversible defect formation, highlighting the need for improved domain wall control and defect mitigation strategies to enhance FeS-FET performance for reliable memory applications.

A review of synaptic devices based on organic ferroelectric materials

This article reviews advancements in synaptic devices using organic ferroelectric materials, particularly PVDF and its copolymers. As AI and big data progress, traditional computing faces limitations in storage and power consumption, leading to neuromorphic computing's rise. Ferroelectric memristors exhibit excellent controllability and retention, making them ideal for simulating synapses. The article discusses fabrication methods, performance optimization, and challenges such as thermal stability and integration costs. Ultimately, optimized PVDF-based devices could significantly enhance low-power, high-performance computing and drive innovations in AI and the internet of things.

Femtosecond Laser Manipulation of Multistage Phase Switching in Two-Dimensional In2Se3 Visualized via an In Situ Transmission Electron Microscope

Phase transitions critically determine material properties for applications, making them central to material science. The two-dimensional (2D) van der Waals material In2Se3 has been extensively studied as a model system for multiphase switching due to its intricate phase transition behaviors and outstanding ferroelectric properties for device applications. However, the lack of an efficient method for precise phase control and the poorly defined conditions for multiphase transitions have severely hindered its practical use. Here, we report that the femtosecond (fs) laser can serve as a potent tool for fast and precisely manipulating multiphase transitions in In2Se3 thin flakes. Using a transmission electron microscope capable of in situ fs laser irradiation, we realize controllable fast phase switching between four phases of 2D In2Se3 by controlling the laser fluence, including the transition from the ferroelectric α phase to the antiferroelectric β' or paraelectric β phase, reversible switching between antiferroelectric β' and paraelectric β phases at room temperature, as well as reversible transformation between the ferroelectric α' phase and antiferroelectric β' or paraelectric β phase at liquid nitrogen temperature. Notably, these multiphase transitions are accompanied by rapid formation and annihilation of domain structures and superlattices, resulting in fast changes in electric conductivity. Our first-principles calculations verify the multiphase transition pathways and reveal that the conductivity change stems from electronic band structure variation among the different phases. This work systematically investigates the phase transition behaviors in In2Se3 through spatially and temporally resolved characterization methods, providing foundational insights into memory device optimization.

Phase-Engineered In2Se3 Ferroelectric P-N Junctions in Phototransistors for Ultra-Low Power and Multiscale Reservoir Computing

Two-dimensional (2D) ferroelectric field-effect transistors (Fe-FETs) based on p-n junctions are the basic units of future neuromorphic hardware. The In2Se3 semiconductor with ferroelectric, photoelectric, and phase transition properties possesses great application potential for in-sensor computing, but its ferroelectric p-n junction (FePNJ) is not well investigated. Here, we present an optoelectronic synapse made of uniformly full-coverage α-In2Se3/WSe2 FePNJ, achieving ultralow-power classification recognition and multiscale signal processing. Using chemical vapor deposition (CVD), we can obtain β'-In2Se3/WSe2 subferroelectric p-n junctions by direct growth on SiO2/Si substrate and α-In2Se3/WSe2 FePNJ by phase transition. Modulated by the synergistic effect of the polarization electric field and the built-in electric field, the FePNJ exhibits significantly enhanced and highly tunable synaptic effects (memory retention >2500 s and >8 multilevel current states under single optical/electrical pulses), along with power consumption down to atto-joule levels. Utilizing these photoelectric properties, we constructed an all-ferroelectric in-sensor reservoir computing system, comprising both reservoir and readout networks, achieving ultralow-power handwritten digit recognition. We also created a multiscale reservoir computing system through the gate-voltage-modulated relaxation time scale of the FePNJ, which can efficiently detect motions in the 1 to 100 km h-1 speed range.

Twist-assisted intrinsic toughening in two-dimensional transition metal dichalcogenides

Material fractures are typically irreversible, marking a one-time event leading to failure. Great efforts have been made to enhance both strength and fracture toughness of bulk materials for engineering applications, such as by introducing self-recovery and secondary breaking behaviours. In low-dimensional structures, two-dimensional materials often exhibit exceptional strength but accompanied by extreme brittleness. Here we discover that the toughness of two-dimensional materials can be enhanced without sacrificing strength-by simply twisting the layers. Through in situ scanning transmission electron microscopy, supported by nanoindentation and theoretical analysis, we reveal that twisted bilayer structures enable sequential fracture events: initial cracks heal to form stable grain boundaries, which then shield subsequent fracture tips from stress concentration. This process consumes additional energy compared with conventional fracture, with toughness enhancement tunable through twist angle adjustment. The intrinsic toughening mechanism via twisting, along with the emerging electronic properties of twistronics that are currently attracting substantial attention, presents an exciting opportunity for future devices.

Heterophase Junction Effect on Photogenerated Charge Separation in Photocatalysis and Photoelectrocatalysis

ConspectusThe conversion of solar energy into chemical energy is promising to address energy and environmental crises. For solar conversion processes, such as photocatalysis and photoelectrocatalysis, a deep understanding of the separation of photogenerated charges is pivotal for advancing material design and efficiency enhancement in solar energy conversion. Formation of a heterophase junction is an efficient strategy to improve photogenerated charge separation of photo(electro)catalysts for solar energy conversion processes. A heterophase junction is formed at the interface between the semiconductors possessing the same chemical composition with similar crystalline phase structures but slightly different energy bands. Despite the small offset of Fermi levels between the different phases, a built-in electric field is established at the interface of the heterophase junction, which can be the driving force for the photogenerated charge separation at the nanometer scale. Notably, slight variations in the energy band of the two crystalline phases result in small energy barriers for the photogenerated carrier transfer. Moreover, the structural similarity of the two different crystalline phases of a semiconductor could minimize the lattice mismatch at the heterophase junction, distinguishing it from a p/n junction or heterojunction formed between two very different semiconductors.This Account provides an overview of the understanding, design, and application of heterophase junctions in photocatalysis and photoelectrocatalysis. It begins with a conceptualization of the heterophase junction and reviews recent advances in the synthesis of semiconductors with a heterophase junction. The phase transformation method with the advantage of forming a heterophase junction with an atomically matched interface and the secondary seed growth method for unique structures with excellent electronic and optoelectronic properties are described. Furthermore, the mechanism of the heterophase junction for improving the photogenerated charge separation is discussed, followed by a comprehensive discussion of the structure-activity relationship for the heterophase junction. The home-built spatially resolved and time-resolved spectroscopies offer direct imaging of the built-in electric field across the heterophase junction and then the direct detection of the photogenerated charge transfer between the two crystalline phases driven by the built-in electric field. Such an efficient interfacial charge transfer results in the improvement of the photogenerated charge separation, a higher yield of long-lived charges, and thus the inhibition of the charge recombination. Benefiting from these insights, structural design strategies for the heterophase junction, such as precise tuning of band alignment, exposed heterophase amounts, phase alignment, and interface structure, have been developed. Finally, the challenges, opportunities, and perspectives of heterophase junctions in the design of advanced photo(electro)catalyst systems for solar energy to chemical energy conversion will be discussed.

Flexible Temperature Sensor with 2D In2Se3 Ferroelectric-Semiconductor Field Effect Transistor Exhibiting Record High Sensitivity

In this study, a In2Se3 ferroelectric-semiconductor field-effect transistor is reported for highly sensitive temperature sensing. The high thermal sensitivity of 696%/°C can be achieved by integrating semiconducting behavior and ferroelectricity in In2Se3 thin film. By inducing a polarization up state in the ferroelectric-semiconductor by applying gate bias, the carriers within the In2Se3 semiconductor are depleted, suppressing channel formation. Under polarization up state, off-currents are very sensitive to temperature variation, following variable range hopping conduction. The drain currents are found to be in proportional toexp [ T 0 T ] 1 4 $\exp {{[ {\frac{{{{T}_0}}}{{\mathrm{T}}}} ]}^{\frac{1}{4}}}$ with T0 of 1.27 × 1012 K. To achieve variable range hopping transport, defective In2Se3 is deposited at low temperatures (240 °C) using spray pyrolysis. Fabricated In2Se3 temperature sensor on flexible polyimide substrate can detect a broad range of temperatures (RT to 200 °C) with high stability, exhibiting excellent temperature sensing performance. In addition, the very thin flexible temperature sensor array is demonstrated for large area spatial temperature sensing.

Vacancy Defects in 2D Ferroelectric In2Se3 and the Conductivity Modulation by Polarization-Defect Coupling

α-In2Se3 is a promising two-dimensional (2D) ferroelectric semiconductor with unique phase transition behaviors and intrinsic n-type conductivity. However, the origin of this conductivity and the impact of defects on the phase transition remain unclear. In this study, we employed the WLZ method to calculate vacancies' formation energy and ionization energy in monolayer α-In2Se3 and identified the defect-bound band edge states. Our results reveal a strong polarization-defect coupling effect, where the bottom-layer selenium vacancy drives intrinsic n-type conductivity in the sample with upward polarization while reversing the polarization-induced deep p-type defect. Furthermore, we demonstrate that a vacancy stabilizes the ferroelectric phase and reduces the phase transition rate to the paraelectric phase. Finally, we propose a defect-engineered ferroelectric field-effect transistor model that controls the resistance by leveraging the polarization-defect coupling effect. This work highlights the significant roles of vacancy defects in 2D α-In2Se3, offering strategies to design In2Se3 electronic devices at the nanoscale.

Unconventional (anti)ferroelectricity in van der Waals group-IV monochalcogenides

Fundamentally, ferroelectrics must belong to a noncentrosymmetric space group, limiting the exploration of more new ferroelectric materials. We circumvent this limitation by triggering structure distortion and inducing ferroelectricity in centrosymmetric van der Waals group-IV monochalcogenide GeSe semiconductor that features unexpected intrinsic out-of-plane antiferroelectricity. Double-type and single-type hysteresis loops from electric measurements, bonding distortion observed in in-situ atomic imaging, and perpendicular polarization uncovered by first-principles calculations, confirm the intrinsic out-of-plane antiferroelectricity and the antiferroelectric-ferroelectric transition induced by the vertical external electric-field. The hidden out-of-plane antiferroelectricity and field induced ferroelectric polarization in spatial-inversion symmetric GeSe makes it a new member of van der Waals layered semiconductors with both in-plane and out-of-plane ferroelectricity, and possibly, can be extended to all group-IV monochalcogenides and other centrosymmetric van der Waals layered materials.

Competing Ferroelectric Polarization and Defect Migration Induced Resistive Switching in β'-In2Se3

Layered β'-In2Se3 has garnered significant attention due to its intriguing multiferroic properties. Until now, most studies have focused on a material-level understanding, with limited exploration of device-level properties. This work systematically investigates the in-plane resistive switching behavior of β'-In2Se3. Besides resistive switching resulting from ferroelectric polarization reversal, the critical role of defect migration is unveiled in determining the overall electrical characteristics of β'-In2Se3 devices. Specifically, we elucidate the contribution of electric-field-induced Se vacancy migration to resistive switching through time-dependent current evolution, in situ electric force microscopy, and density functional theory calculations. By considering the interplay between free carriers, bound charges, and mobile defects, a comprehensive physical picture of the complex resistive switching behavior of β'-In2Se3 devices is established. This work provides crucial insights into understanding and manipulating the resistive switching behavior of 2D vdW ferroelectric devices.

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.

Probing and manipulating the Mexican hat-shaped valence band of In2Se3

Ferroelectrics based on van der Waals semiconductors represent an emergent class of materials for disruptive technologies ranging from neuromorphic computing to low-power electronics. However, many theoretical predictions of their electronic properties have yet to be confirmed experimentally and exploited. Here, we use nanoscale angle-resolved photoemission electron spectroscopy and optical transmission in high magnetic fields to reveal the electronic band structure of the van der Waals ferroelectric indium selenide (α-In2Se3). This indirect bandgap semiconductor features a weakly dispersed valence band, which is shaped like an inverted Mexican hat. Its form changes following an irreversible structural phase transition of α-In2Se3 into β-In2Se3 via a thermal annealing in ultra-high vacuum. Density functional theory supports the experiments and reveals the critical contribution of spin orbit coupling to the form of the valence band. The measured band structure and its in situ manipulation offer opportunities for precise engineering of ferroelectrics and their functional properties beyond traditional semiconducting systems.

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.

Ferroelectric Perovskite/MoS2 Channel Heterojunctions for Wide-Window Nonvolatile Memory and Neuromorphic Computing

Ferroelectric materials commonly serve as gate insulators in typical field-effect transistors, where their polarization reversal enables effective modulation of the conductivity state of the channel material, thereby realizing non-volatile memory. Currently, novel 2D ferroelectrics unlock new prospects in next-generation electronics and neuromorphic computation. However, the advancement of these materials is impeded by limited selectivity and narrow memory windows. Here, new concepts of 2D ferroelectric perovskite/MoS2 channel heterostructures field-effect transistors are presented, in which 2D ferroelectric perovskite features customizable band structure, few-layered ferroelectricity, and submillimeter-size monolayer wafers. Further studies reveal that these devices exhibit unique charge polarity modulation (from n- to p-type channel) and remarkable nonvolatile memory behavior, especially record-wide hysteresis windows up to 177 V, which enables efficient imitation of biological synapses and achieves high recognition accuracy for electrocardiogram patterns. This result provides a device paradigm for future nonvolatile memory and artificial synaptic applications.

Sliding ferroelectric memories and synapses based on rhombohedral-stacked bilayer MoS2

Recent advances have uncovered an exotic sliding ferroelectric mechanism, which endows to design atomically thin ferroelectrics from non-ferroelectric parent monolayers. Although notable progress has been witnessed in understanding the fundamental properties, functional devices based on sliding ferroelectrics remain elusive. Here, we demonstrate the rewritable, non-volatile memories at room-temperature with a two-dimensional (2D) sliding ferroelectric semiconductor of rhombohedral-stacked bilayer MoS2. The 2D sliding ferroelectric memories (SFeMs) show superior performances with a large memory window of >8 V, a high conductance ratio of above 106, a long retention time of >10 years, and a programming endurance greater than 104 cycles. Remarkably, flexible SFeMs are achieved with state-of-the-art performances competitive to their rigid counterparts and maintain their performances post bending over 103 cycles. Furthermore, synapse-specific Hebbian forms of plasticity and image recognition with a high accuracy of 97.81% are demonstrated based on flexible SFeMs.

Large-Area Aligned Growth of Low-Symmetry 2D ReS2 on a High-Symmetry Surface

The large-scale preparation of two-dimensional (2D) materials is pivotal in unlocking their extensive potential for next-generation semiconductor device applications. Wafer-scale single crystals of a high-symmetry 2D material (e.g., graphene and molybdenum disulfide) can be achieved by seamlessly stitching the aligned domains. However, achieving the alignment of low-symmetry 2D materials remains a great challenge and is rarely reported. Rhenium disulfide (ReS2), one of the low-symmetry 2D materials, shows considerable promise for optoelectronics, especially polarization-sensitive applications. Here, we report large-area chemical vapor deposition synthesis of highly oriented, low-symmetry monolayer ReS2 flakes on a high-symmetry Au(111) surface, followed by seamless stitching into a centimeter-scale continuous 2D film. Cross-sectional scanning transmission electron microscopy reveals that the aligned monolayer ReS2 flakes are guided by step edges on Au(111) surfaces along the [011̅] direction. Additionally, 2D ReS2 can flatten Au surfaces during its growth through surface step bunching. The growth of the ReS2 monolayer demonstrates its ability to extend across Au surface steps and facets. Thus, we have established a reliable and robust synthesis route that accommodates different surface roughness conditions. The aligned and scalable film growth of low-symmetry 2D ReS2 significantly contributes to the in-depth understanding of epitaxial growth mechanisms for low-symmetry 2D materials, holding promise for advancing their future applications.

Stacking selected polarization switching and phase transition in vdW ferroelectric α-In2Se3 junction devices

The structure and dynamics of ferroelectric domain walls are essential for polarization switching in ferroelectrics, which remains relatively unexplored in two-dimensional ferroelectric α-In2Se3. Interlayer interactions engineering via selecting the stacking order in two-dimensional materials allows modulation of ferroelectric properties. Here, we report stacking-dependent ferroelectric domain walls in 2H and 3R stacked α-In2Se3, elucidating the resistance switching mechanism in ferroelectric semiconductor-metal junction devices. In 3R α-In2Se3, the in-plane movement of out-of-plane ferroelectric domain walls yield a large hysteresis window. Conversely, 2H α-In2Se3 devices favor in-plane domain walls and out-of-plane domain wall motion, producing a small hysteresis window. High electric fields induce a ferro-paraelectric phase transition of In2Se3, where 3R In2Se3 reaches the transition through intralayer atomic gliding, while 2H In2Se3 undergoes a complex process comprising intralayer bond dissociation and interlayer bond reconstruction. Our findings demonstrate tunable ferroelectric properties via stacking configurations, offering an expanded dimension for material engineering in ferroelectric devices.

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.

Enhanced Electrical Polarization in van der Waals α-In2Se3 Ferroelectric Semiconductor Field-Effect Transistors by Eliminating Surface Screening Charge

A van der Waals (vdW) α-In2Se3 ferroelectric semiconductor channel-based field-effect transistor (FeS-FET) has emerged as a next-generation electronic device owing to its versatility in various fields, including neuromorphic computing, nonvolatile memory, and optoelectronics. However, screening charges cause by the imperfect surface morphology of vdW α-In2Se3 inhibiting electrical polarization remain an unresolved issue. In this study, for the first time, a method is elucidated to recover the inherent electric polarization in both in- and out-of-plane directions of the α-In2Se3 channel based on post-exfoliation annealing (PEA) and improve the electrical performance of vdW FeS-FETs. Following PEA, an ultra-thin In2Se3-3xO3x layer formed on the top surface of the α-In2Se3 channel is demonstrated to passivate surface defects and enhance the electrical performance of FeS-FETs. The on/off current ratio of the α-In2Se3 FeS-FET has increased from 5.99 to 1.84 × 106, and the magnitude of ferroelectric resistance switching has increased from 1.20 to 26.01. Moreover, the gate-modulated artificial synaptic operation of the α-In2Se3 FeS-FET is demonstrated and illustrate the significance of the engineered interface in the vdW FeS-FET for its application to multifunctional devices. The proposed α-In2Se3 FeS-FET is expected to provide a significant breakthrough for advanced memory devices and neuromorphic computing.

Phase Tailoring of In2Se3 Toward van der Waals Vertical Heterostructures via Selenization of γ-InSe Semiconductor

The polymorphic nature of In2Se3 leads to excellent phase-dependent physical properties including ferroelectricity, photoelectricity, and especially the intriguing phase change ability, making the precise phase modulation of In2Se3 of fundamental importance but very challenging. Here, the growth of In2Se3 with desired-phase is realized by temperature-controlled selenization of van der Waals (vdW) layered bulk γ-InSe. Detailed results of Raman spectroscopy, scanning electron microscopy (SEM), and state-of-the-art spherical aberration-corrected transmission electron microscopy (Cs-TEM) clearly and consistently show that β-In2Se3, 3R α-In2Se3, and 2H α-In2Se3 can be perfectly obtained at ≈270, ≈300, and ≈600 °C, respectively. Further comprehensive atomic imaging analyses confirm that the seeding material, InSe, plays a critical role in the low-temperature epitaxial growth of vdW-layered In2Se3, and, more interestingly, β-In2Se3 acts as an intermediate phase between 3R and 2H α-In2Se3 transitions. This investigation not only provides a simple yet versatile strategy for the phase modulation of In2Se3, but also sheds light on the temperature-dependent phase evolution of In2Se3.

Multimodal 2D Ferroelectric Transistor with Integrated Perception-and-Computing-in-Memory Functions for Reservoir Computing

Emerging neuromorphic hardware promises energy-efficient computing by colocating multiple essential functions at the individual component level. The implementation is challenging due to mismatch between the characteristics of multifunctional devices and neural networks. Here, we demonstrate an artificial synapse based on a 2D α-phase indium selenide that exhibits integrated perception-and-computing-in-memory functions in a single-transistor setup, serving as a basic building block for reservoir computing. Extending to the array architecture enables concurrent image-sensing and memory. Further, we implement multimode deep-reservoir computing with adjustable nonlinear transformation and multisensory fusion using this core device. In the lane-keeping-assistance task for an unmanned vehicle, the system demonstrates ∼104 times lower energy consumption and significantly boosted data throughput compared to the state-of-the-art graphics processors. The demonstrated perception-and-computing-in-memory (PCIM) functions at a single-transistor level shows the feasibility of implementing ultrascalable, resource-efficient hardware for brain-inspired computing.

High-Performance Ultra-Broadband Photodetector Based on Fe3O4/CrSiTe3 Heterostructures

Photodetectors based on advanced materials with a broad spectral photoresponse, high sensitivity, huge integration ability, room-temperature operation, and stable environmental stability are highly desired for diversified applications of imaging, sensing, and communication. Herein, a high-performance ultra-broadband photodetector based on an ultrathin two-dimensional (2D) Fe3O4 nanoflake heterostructure with high sensitivity was designed. The photodetector response light was from visible 405 nm to long-wave infrared (LWIR) 10.6 μm in ambient air. The competitive performances, including a high photoresponsivity (R) of 182.8 A W-1, fast speed with the rise time τr = 8.8 μs, and decay time τd = 4.1 μs, were demonstrated in the visible range. Notably, the device exhibits an excellent uncooled LWIR detection ability, with a high R of 1.4 A W-1 realized at a 1.5 V bias. In the full spectral range, the noise equivalent power is lower than 0.79 pW Hz-1/2, and specific detectivity (D*) is higher than 4.9 × 108 cm Hz1/2 W-1 in ambient air. This work provides alternative ultrathin 2D materials for future infrared optoelectronic devices.

Flash-within-flash synthesis of gram-scale solid-state materials

Sustainable manufacturing that prioritizes energy efficiency, minimal water use, scalability and the ability to generate diverse materials is essential to advance inorganic materials production while maintaining environmental consciousness. However, current manufacturing practices are not yet equipped to fully meet these requirements. Here we describe a flash-within-flash Joule heating (FWF) technique-a non-equilibrium, ultrafast heat conduction method-to prepare ten transition metal dichalcogenides, three group XIV dichalcogenides and nine non-transition metal dichalcogenide materials, each in under 5 s while in ambient conditions. FWF achieves enormous advantages in facile gram scalability and in sustainable manufacturing criteria when compared with other synthesis methods. Also, FWF allows the production of phase-selective and single-crystalline bulk powders, a phenomenon rarely observed by any other synthesis method. Furthermore, FWF MoSe2 outperformed commercially available MoSe2 in tribology, showcasing the quality of FWF materials. The capability for atom substitution and doping further highlights the versatility of FWF as a general bulk inorganic materials synthesis protocol.

Recent progresses in transmission electron microscopy studies of two-dimensional ferroelectrics

The rich potential of two-dimensional materials endows them with superior properties suitable for a wide range of applications, thereby attracting substantial interest across various fields. The ongoing trend towards device miniaturization aligns with the development of materials at progressively smaller scales, aiming to achieve higher integration density in electronics. In the realm of nano-scaling ferroelectric phenomena, numerous new two-dimensional ferroelectric materials have been predicted theoretically and subsequently validated through experimental confirmation. However, the capabilities of conventional tools, such as electrical measurements, are limited in providing a comprehensive investigation into the intrinsic origins of ferroelectricity and its interactions with structural factors. These factors include stacking, doping, functionalization, and defects. Consequently, the progress of potential applications, such as high-density memory devices, energy conversion systems, sensing technologies, catalysis, and more, is impeded. In this paper, we present a review of recent research that employs advanced transmission electron microscopy (TEM) techniques for the direct visualization and analysis of ferroelectric domains, domain walls, and other crucial features at the atomic level within two-dimensional materials. We discuss the essential interplay between structural characteristics and ferroelectric properties on the nanoscale, which facilitates understanding of the complex relationships governing their behavior. By doing so, we aim to pave the way for future innovative applications in this field.

Nonvolatile Electro-optic Response of Graphene Driven by Ferroelectric Polarization

Two-dimensional materials (2DMs) have exhibited remarkably tunable optical characteristics, which have been applied for significant applications in communications, sensing, and computing. However, the reported tunable optical properties of 2DMs are almost volatile, impeding them in the applications of multifarious emerging frameworks such as programmable operation and neuromorphic computing. In this work, nonvolatile electro-optic response is developed by the graphene-Al2O3-In2Se3 heterostructure integrating with microring resonators (MRRs). In such compact devices, the optical absorption coefficient of graphene is substantially tuned by the out-of-plane ferroelectric polarization in α-In2Se3, resulting in a nonvolatile optical transmission in MRRs. This work demonstrates that integrating graphene with ferroelectric materials paves the way to develop nonvolatile devices in photonic circuits for emerging applications such as optical neural networks.

Atomic-Scale Scanning of Domain Network in the Ferroelectric HfO2 Thin Film

Ferroelectric HfO2-based thin films have attracted much interest in the utilization of ferroelectricity at the nanoscale for next-generation electronic devices. However, the structural origin and stabilization mechanism of the ferroelectric phase are not understood because the film is typically nanocrystalline with active yet stochastic ferroelectric domains. Here, electron microscopy is used to map the in-plane domain network structures of epitaxially grown ferroelectric Y:HfO2 films in atomic resolution. The ferroelectricity is confirmed in free-standing Y:HfO2 films, allowing for investigating the structural origin for their ferroelectricity by 4D-STEM, high-resolution STEM, and iDPC-STEM. At the grain boundaries of <111>-oriented Pca21 orthorhombic grains, a high-symmetry mixed-(R3m, Pnm21) phase is induced, exhibiting enhanced polarization due to in-plane compressive strain. Nanoscale Pca21 orthorhombic grains and their grain boundaries with mixed-(R3m, Pnm21) phases of higher symmetry cooperatively determine the ferroelectricity of the Y:HfO2 film. It is also found that such ferroelectric domain networks emerge when the film thickness is beyond a finite value. Furthermore, in-plane mapping of oxygen positions overlaid on ferroelectric domains discloses that polarization is suppressed at vertical domain walls, while it is active when domains are aligned horizontally with subangstrom domain walls. In addition, randomly distributed 180° charged domain walls are confined by spacer layers.

Spatially Resolved Light-Induced Ferroelectric Polarization in α-In2Se3/Te Heterojunctions

Light-induced ferroelectric polarization in 2D layered ferroelectric materials holds promise in photodetectors with multilevel current and reconfigurable capabilities. However, translating this potential into practical applications for high-density optoelectronic information storage remains challenging. In this work, an α-In2Se3/Te heterojunction design that demonstrates spatially resolved, multilevel, nonvolatile photoresponsivity is presented. Using photocurrent mapping, the spatially localized light-induced poling state (LIPS) is visualized in the junction region. This localized ferroelectric polarization induced by illumination enables the heterojunction to exhibit enhanced photoresponsivity. Unlike previous reports that observe multilevel polarization enhancement in electrical resistance, the device shows nonvolatile photoresponsivity enhancement under illumination. After polarization saturation, the photocurrent increases up to 1000 times, from 10-12 to 10-9 A under the irradiation of a 520 nm laser with a power of 1.69 nW, compared to the initial state in a self-driven mode. The photodetector exhibits high detectivity of 4.6×1010 Jones, with a rise time of 27 µs and a fall time of 28 µs. Furthermore, the device's localized poling characteristics and multilevel photoresponse enable spatially multiplexed optical information storage. These results advance the understanding of LIPS in 2D ferroelectric materials, paving the way for optoelectronic information storage technologies.

Quasi-equilibrium growth of inch-scale single-crystal monolayer α-In2Se3 on fluor-phlogopite

Epitaxial growth of two-dimensional (2D) materials with uniform orientation has been previously realized by introducing a small binding energy difference between the two locally most stable orientations. However, this small energy difference can be easily disturbed by uncontrollable dynamics during the growth process, limiting its practical applications. Herein, we propose a quasi-equilibrium growth (QEG) strategy to synthesize inch-scale monolayer α-In2Se3 single crystals, a semiconductor with ferroelectric properties, on fluor-phlogopite substrates. The QEG facilitates the discrimination of small differences in binding energy between the two locally most stable orientations, realizing robust single-orientation epitaxy within a broad growth window. Thus, single-crystal α-In2Se3 film can be epitaxially grown on fluor-phlogopite, the cleavage surface atomic layer of which has the same 3-fold rotational symmetry with α-In2Se3. The resulting crystalline quality enables high electron mobility up to 117.2 cm2 V-1 s-1 in α-In2Se3 ferroelectric field-effect transistors, exhibiting reliable nonvolatile memory performance with long retention time and robust cycling endurance. In brief, the developed QEG method provides a route for preparing larger-area single-crystal 2D materials and a promising opportunity for applications of 2D ferroelectric devices and nanoelectronics.

Ultrafast high-endurance memory based on sliding ferroelectrics

The persistence of voltage-switchable collective electronic phenomena down to the atomic scale has extensive implications for area- and energy-efficient electronics, especially in emerging nonvolatile memory technology. We investigate the performance of a ferroelectric field-effect transistor (FeFET) based on sliding ferroelectricity in bilayer boron nitride at room temperature. Sliding ferroelectricity represents a different form of atomically thin two-dimensional (2D) ferroelectrics, characterized by the switching of out-of-plane polarization through interlayer sliding motion. We examined the FeFET device employing monolayer graphene as the channel layer, which demonstrated ultrafast switching speeds on the nanosecond scale and high endurance exceeding 1011 switching cycles, comparable to state-of-the-art FeFET devices. These characteristics highlight the potential of 2D sliding ferroelectrics for inspiring next-generation nonvolatile memory technology.

Free-standing two-dimensional ferro-ionic memristor

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

Electronic structure orientation as a map of in-plane antiferroelectricity in β'-In2Se3

Antiferroelectric (AFE) materials are excellent candidates for sensors, capacitors, and data storage due to their electrical switchability and high-energy storage capacity. However, imaging the nanoscale landscape of AFE domains is notoriously inaccessible, which has hindered development and intentional tuning of AFE materials. Here, we demonstrate that polarization-dependent photoemission electron microscopy can resolve the arrangement and orientation of in-plane AFE domains on the nanoscale, despite the absence of a net lattice polarization. Through direct determination of electronic transition orientations and analysis of domain boundary constraints, we establish that antiferroelectricity in β'-In2Se3 is a robust property from the scale of tens of nanometers to tens of micrometers. Ultimately, the method for imaging AFE domain organization presented here opens the door to investigations of the influence of domain formation and orientation on charge transport and dynamics.

Electronic Metal-Support Interactions Boost *OOH Intermediate Generation in Cu/In2Se3 for Electrochemical H2O2 Production

Two-electron oxygen reduction reaction (2e- ORR) is a promising method for the synthesis of hydrogen peroxide (H2O2). However, high energy barriers for the generation of key *OOH intermediates hinder the process of 2e- ORR. Herein, we prepared a copper-supported indium selenide catalyst (Cu/In2Se3) to enhance the selectivity and yield of 2e- ORR by employing an electronic metal-support interactions (EMSIs) strategy. EMSIs-induced charge rearrangement between metallic Cu and In2Se3 is conducive to *OOH intermediate generation, promoting H2O2 production. Theoretical investigations reveal that the inclusion of Cu significantly lowers the energy barrier of the 2e- ORR intermediate and impedes the 4e- ORR pathway, thus favoring the formation of H2O2. The concentration of H2O2 produced by Cu/In2Se3 is ~2 times than In2Se3, and Cu/In2Se3 shows promising applications in antibiotic degradation. This research presents a valuable approach for the future utilization of EMSIs in 2e- ORR.

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.

Melting-free integrated photonic memory with layered polymorphs

Chalcogenide-based nonvolatile phase change materials (PCMs) have a long history of usage, from bulk disk memory to all-optic neuromorphic computing circuits. Being able to perform uniform phase transitions over a subwavelength scale makes PCMs particularly suitable for photonic applications. For switching between nonvolatile states, the conventional chalcogenide phase change materials are brought to a melting temperature to break the covalent bonds. The cooling rate determines the final state. Reversible polymorphic layered materials provide an alternative atomic transition mechanism for low-energy electronic (small domain size) and photonic nonvolatile memories (which require a large effective tuning area). The small energy barrier of breaking van der Waals force facilitates low energy, fast-reset, and melting-free phase transitions, which reduces the chance of element segregation-associated device failure. The search for such material families starts with polymorphic In2Se3, which has two layered structures that are topologically similar and stable at room temperature. In this perspective, we first review the history of different memory schemes, compare the thermal dynamics of phase transitions in amorphous-crystalline and In2Se3, detail the device implementations for all-optical memory, and discuss the challenges and opportunities associated with polymorphic memory.

Can 2D Semiconductors Be Game-Changers for Nanoelectronics and Photonics?

2D semiconductors have interesting physical and chemical attributes that have led them to become one of the most intensely investigated semiconductor families in recent history. They may play a crucial role in the next technological revolution in electronics as well as optoelectronics or photonics. In this Perspective, we explore the fundamental principles and significant advancements in electronic and photonic devices comprising 2D semiconductors. We focus on strategies aimed at enhancing the performance of conventional devices and exploiting important properties of 2D semiconductors that allow fundamentally interesting device functionalities for future applications. Approaches for the realization of emerging logic transistors and memory devices as well as photovoltaics, photodetectors, electro-optical modulators, and nonlinear optics based on 2D semiconductors are discussed. We also provide a forward-looking perspective on critical remaining challenges and opportunities for basic science and technology level applications of 2D semiconductors.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection

A two-dimensional (2D) ferroelectric semiconductor, which is coupled with photosensitivity and room-temperature ferroelectricity, provides the possibility of coordinated conductance modulation by both electric field and light illumination and is promising for triggering the revolution of optoelectronics for monolithic multifunctional integration. Here, we report that semiconducting Sn2P2S6 crystals can be achieved in a 2D morphology using a chemical vapor transport approach with the assistant of space confinement and experimentally demonstrate the robust ferroelectricity in atomic-thin Sn2P2S6 nanosheet at room temperature. The intercorrelated programming of ferroelectric order along out-of-plane (OOP) and in-plane (IP) directions enables a tunable bulk photovoltaic (BPV) effect through multidirectional electrical control. By combining the capability of anisotropic in-plane optical absorption, a highly integrated Sn2P2S6 optoelectronic device vertically sandwiched with graphene electrodes yields the polarization-dependent open-circuit photovoltage with a dichroic ratio of 2.0 under 405 nm light illumination. The reintroduction of ferroelectric Sn2P2S6 to the 2D asymmetric semiconductor family provides possibilities to hardware implement of the self-powered polarization-sensitive photodetection and spotlights the promising applications for next-generation photovoltaic devices.

Reveal long-lived hot electrons in 2D indium selenide and ferroelectric-regulated carrier dynamics of InSe/α-In2Se3/InSe heterostructure

As typical representatives of group III chalcogenides, InSe, α-In2Se3, and β'-In2Se3 have drawn considerable interest in the domain of photoelectrochemistry. However, the microscopic mechanisms of carrier dynamics in these systems remain largely unexplored. In this work, we first reveal that hot electrons in the three systems have different cooling rate stages and long-lived hot electrons, through the utilization of density functional theory calculations and nonadiabatic molecular dynamics simulations. Furthermore, the ferroelectric polarization of α-In2Se3 weakens the nonadiabatic coupling of the nonradioactive recombination, successfully competing with the narrow bandgap and slow dephasing process, and achieving both high optical absorption efficiency and long carrier lifetime. In addition, we demonstrate that the ferroelectric polarization of α-In2Se3 not only enables the formation of the double type-II band alignment in the InSe/α-In2Se3/InSe heterostructure, with the top and bottom InSe sublayers acting as acceptors and donors, respectively, but also eliminates the hindrance of the built-in electric field at the interface, facilitating an ultrafast interlayer carrier transfer in the heterojunction. This work establishes an atomic mechanism of carrier dynamics in InSe, α-In2Se3, and β'-In2Se3 and the regulatory role of the ferroelectric polarization on the charge carrier dynamics, providing a guideline for the design of photoelectronic materials.

Wafer-Scale Two-Dimensional Semiconductors for Deep UV Sensing

2D semiconductors (2SEM) can transform many sectors, from information and communication technology to healthcare. To date, top-down approaches to their fabrication, such as exfoliation of bulk crystals by "scotch-tape," are widely used, but have limited prospects for precise engineering of functionalities and scalability. Here, a bottom-up technique based on epitaxy is used to demonstrate high-quality, wafer-scale 2SEM based on the wide band gap gallium selenide (GaSe) compound. GaSe layers of well-defined thickness are developed using a bespoke facility for the epitaxial growth and in situ studies of 2SEM. The dominant centrosymmetry and stacking of the individual van der Waals layers are verified by theory and experiment; their optical anisotropy and resonant absorption in the UV spectrum are exploited for photon sensing in the technological UV-C spectral range, offering a scalable route to deep-UV optoelectronics.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

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

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

In-Plane Ferrielectric Order in van der Waals β'-In2Se3

van der Waals ferroic materials exhibit rich potential for implementing future generation functional devices. Among these, layered β'-In2Se3 has fascinated researchers with its complex superlattice and domain structures. As opposed to ferroelectric α-In2Se3, the understanding of β'-In2Se3 ferroic properties remains unclear because ferroelectric, antiferroelectric, and ferroelastic characteristics have been separately reported in this material. To develop useful applications, it is necessary to understand the microscopic structural properties and their correlation with macroscopic device characteristics. Herein, using scanning transmission electron microscopy (STEM), we observed that the arrangement of dipoles deviates from the ideal double antiparallel antiferroelectric character due to competition between antiferroelectric and ferroelectric structural ordering. By virtue of second-harmonic generation, four-dimensional STEM, and in-plane piezoresponse force microscopy, the long-range inversion-breaking symmetry, uncompensated local polarization, and net polarization domains are unambiguously verified, revealing β'-In2Se3 as an in-plane ferrielectric layered material. Additionally, our device study reveals analogous resistive switching behaviors of different types owing to polarization switching, defect migration, and defect-induced charge trapping/detrapping processes.

Large-Area Growth of Ferroelectric 2D γ-In2 Se3 Semiconductor by Spray Pyrolysis for Next-Generation Memory

In2 Se3 , 2D ferroelectric-semiconductor, is a promising candidate for next-generation memory device because of its outstanding electrical properties. However, the large-area manufacturing of In2 Se3 is still a big challenge. In this work, spray pyrolysis technique is introduced for the growth of large-area In2 Se3 thin film. A polycrystalline γ-In2 Se3 layer can be grown on 15 cm × 15 cm glasss at the substrate temperature of 275 °C. The In2 Se3 ferroelectric-semiconductor field effect transistor (FeS-FET) on glass substrate demonstrates a large hysteresis window of 40.3 V at the ±40 V of gate voltage sweep and excellent uniformity. The FeS-FET exhibits an electron field effect mobility of 0.97 cm2 V-1 s-1 and an on/off current ratio of >107 in the transfer curves. The memory behavior of the large-area, In2 Se3 FeS-FETs for next-generation memory is demonstrated.

The tunneling electroresistance effect in a van der Waals ferroelectric tunnel junction based on a graphene/In2Se3/MoS2/graphene heterostructure

In recent years, α-In2Se3 has attracted great attention in miniaturizing nonvolatile random memory devices because of its room temperature ferroelectricity and atomic thickness. In this work, we construct two-dimensional (2D) van der Waals (vdW) heterostructures α-In2Se3/MoS2 with different ferroelectric polarization and design a 2D graphene (Gr)/In2Se3/MoS2/Gr ferroelectric tunnel junction (FTJ) with the symmetric electrodes. Our calculations show that the band alignment of the heterostructures can be changed from type-I to type-II accompanied by the reversal of the ferroelectric polarization of In2Se3. Furthermore, the ferroelectricity persists in Gr/In2Se3/MoS2/Gr vdW FTJs, and the presence of dielectric layer MoS2 in the FTJs enables the effective modulation of the tunneling barrier by altering the ferroelectric polarization of α-In2Se3, which results in two distinct conducting states denoted as "ON" and "OFF" with a large tunneling electroresistance (TER) ratio exceeding 105%. These findings suggest the importance of ferroelectric vdW heterostructures in the design of FTJs and propose a promising route for applying the 2D ferroelectric/semiconductor heterostructures with out-of-plane polarization in high-density ferroelectric memory devices.