Layer-dependent ferroelectricity in 2H-stacked few-layer α-In2Se3

Bulk photovoltaic effect in a two-dimensional ferroelectric semiconductor α-In2Se3

The bulk photovoltaic effect, arising from the separation of charge carriers driven by crystal symmetry, is an intriguing physical phenomenon that has been attracting broad interest in the field of photovoltaic applications due to its junction-free nature and potential to surpass the Shockley-Queisser limit. The photovoltaic applications of conventional ferroelectric materials with wide bandgaps (2.7-4 eV) are limited due to their low photocurrent densities and weak photovoltaic response in the visible light region. The emergence of two-dimensional ferroelectric semiconductors with coupled visible light absorption and spontaneous polarization characteristics is promising for the development of functional photoferroelectrics. Herein, we report the experimental demonstration of enhanced bulk photovoltaic response in a two-dimensional ferroelectric semiconductor α-In2Se3 under the excitation of visible light. The generated photovoltaic current density is nearly two orders of magnitude higher than that of conventional bulk ferroelectric materials. Our findings highlight the potential of two-dimensional ferroelectric semiconductor materials for bulk photovoltaic applications across a broad spectral region.

Enhanced Synaptic Memory Window and Linearity in Planar In2Se3 Ferroelectric Junctions

A synaptic memristor using 2D ferroelectric junctions is a promising candidate for future neuromorphic computing with ultra-low power consumption, parallel computing, and adaptive scalable computing technologies. However, its utilization is restricted due to the limited operational voltage memory window and low on/off current (ION/OFF) ratio of the memristor devices. Here, it is demonstrated that synaptic operations of 2D In2Se3 ferroelectric junctions in a planar memristor architecture can reach a voltage memory window as high as 16 V (±8 V) and ION/OFF ratio of 108, significantly higher than the current literature values. The power consumption is 10-5 W at the on state, demonstrating low power usage while maintaining a large ION/OFF ratio of 108 compared to other ferroelectric devices. Moreover, the developed ferroelectric junction mimicked synaptic plasticity through pulses in the pre-synapse. The nonlinearity factors are obtained 1.25 for LTP, -0.25 for LTD, respectively. The single-layer perceptron (SLP) and convolutional neural network (CNN) on-chip training results in an accuracy of up to 90%, compared to the 91% in an ideal synapse device. Furthermore, the incorporation of a 3 nm thick SiO2 interface between the α-In2Se3 and the Au electrode resulted in ultrahigh performance among other 2D ferroelectric junction devices to date.

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.

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.

Oxygen-Doped 2D In2Se3 Nanosheets with Extended In-Plane Lattice Strain for Highly Efficient Piezoelectric Energy Harvesting

With the emergence of electromechanical devices, considerable efforts have been devoted to improving the piezoelectricity of 2D materials. Herein, an anion-doping approach is proposed as an effective way to enhance the piezoelectricity of α-In2Se3 nanosheets, which has a rare asymmetric structure in both the in-plane and out-of-plane directions. As the O2 plasma treatment gradually substitutes selenium with oxygen, it changes the crystal structure, creating a larger lattice distortion and, thus, an extended dipole moment. Prior to the O2 treatment, the lattice extension is deliberately maximized in the lateral direction by imposing in situ tensile strain during the exfoliation process for preparing the nanosheets. Combining doping and strain engineering substantially enhances the piezoelectric coefficient and electromechanical energy conversion. As a result, the optimal harvester with a 0.9% in situ strain and 10 min plasma exposure achieves the highest piezoelectric energy harvesting values of ≈13.5 nA and ≈420 µW cm-2 under bending operation, outperforming all previously reported 2D materials. Theoretical estimation of the structural changes and polarization with gradual oxygen substitution supports the observed dependence of the electromechanical performance.

Photo-Assisted Ferroelectric Domain Control for α-In2Se3 Artificial Synapses Inspired by Spontaneous Internal Electric Fields

α-In2Se3 semiconductor crystals realize artificial synapses by tuning in-plane and out-of-plane ferroelectricity with diverse avenues of electrical and optical pulses. While the electrically induced ferroelectricity of α-In2Se3 shows synaptic memory operation, the optically assisted synaptic plasticity in α-In2Se3 has also been preferred for polarization flipping enhancement. Here, the synaptic memory behavior of α-In2Se3 is demonstrated by applying electrical gate voltages under white light. As a result, the induced internal electric field is identified at a polarization flipped conductance channel in α-In2Se3/hexagonal boron nitride (hBN) heterostructure ferroelectric field effect transistors (FeFETs) under white light and discuss the contribution of this built-in electric field on synapse characterization. The biased dipoles in α-In2Se3 toward potentiation polarization direction by an enhanced internal built-in electric field under illumination of white light lead to improvement of linearity for long-term depression curves with proper electric spikes. Consequently, upon applying appropriate electric spikes to α-In2Se3/hBN FeFETs with illuminating white light, the recognition accuracy values significantly through the artificial learning simulation is elevated for discriminating hand-written digit number images.

Chemical Vapor Deposition Synthesis of Intrinsic High-Temperature Ferroelectric 2D CuCrSe2

Ultrathin 2D ferroelectrics with high Curie temperature are critical for multifunctional ferroelectric devices. However, the ferroelectric spontaneous polarization is consistently broken by the strong thermal fluctuations at high temperature, resulting in the rare discovery of high-temperature ferroelectricity in 2D materials. Here, a chemical vapor deposition method is reported to synthesize 2D CuCrSe2 nanosheets. The crystal structure is confirmed by scanning transmission electron microscopy characterization. The measured ferroelectric phase transition temperature of ultrathin CuCrSe2 is about ≈800 K. Significantly, the switchable ferroelectric polarization is observed in ≈5.2 nm nanosheet. Moreover, the in-plane and out-of-plane ferroelectric response are modulated by different maximum bias voltage. This work provides a new insight into the construction of 2D ferroelectrics with high Curie temperature.

Asymmetric conducting route and potential redistribution determine the polarization-dependent conductivity in layered ferroelectrics

Precise control of the conductivity of layered ferroelectric semiconductors is required to make these materials suitable for advanced transistor, memory and logic circuits. Although proof-of-principle devices based on layered ferroelectrics have been demonstrated, it remains unclear how the polarization inversion induces conductivity changes. Therefore, function design and performance optimization remain cumbersome. Here we combine ab initio calculations with transport experiments to unveil the mechanism underlying the polarization-dependent conductivity in ferroelectric channel field-effect transistors. We find that the built-in electric field gives rise to an asymmetric conducting route formed by the hidden Stark effect and competes with the potential redistribution caused by the external field of the gate. Furthermore, leveraging our mechanistic findings, we control the conductivity threshold in α-In2Se3 ferroelectric channel field-effect transistors. We demonstrate logic-in-memory functionality through the implementation of electrically self-switchable primary (AND, OR) and composite (XOR, NOR, NAND) logic gates. Our work provides mechanistic insights into conductivity modulation in a broad class of layered ferroelectrics, providing foundations for their application in logic and memory electronics.

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

Artificial Visual Systems Fabricated with Ferroelectric van der Waals Heterostructure for In-Memory Computing Applications

Rapid developments in artificial neural network techniques and retina-inspired artificial visual systems are required to realize the sensing, processing, and memorization of an optical signal in a single device. Herein, a ferroelectric field-effect transistor fabricated with CuInP2S6 and α-In2Se3 van der Waals heterostructures is proposed and demonstrated for the development of an artificial visual system. The dipole polarizations are coupled and bidirectionally locked inside the ferroelectric α-In2Se3 along the in-plane and out-of-plane directions and are controlled by the gate voltages. Furthermore, light-induced polarization can change the order of polarization of the dipoles inside α-In2Se3. We demonstrate that using the combined control of these electrical and optical signals, the device may function like a retina-inspired vision system. The device can operate across a wide wavelength range (405-850 nm) and detect very low incident light (0.03 mW/cm2). Color recognition, high paired-pulse facilitation (∼170%), and short- to long-term memory transitions through quick learning are observed using this device. Additionally, this device demonstrates different complex processing abilities, including pattern recognition, light adaptation, optical logic operation, and event learning. The proposed ferroelectric heterostructure-based artificial visual system can serve as an essential bridge for fulfilling the future requirements of all-in-one sensing and memory-processing devices.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

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

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

Giant tunneling electroresistance in a 2D bilayer-In2Se3-based out-of-plane ferroelectric tunnel junction

Ferroelectric tunnel junctions (FTJs) have great potential in nonvolatile memory devices and have been extensively studied in recent years. Compared with conventional FTJs based on perovskite-type oxide materials as the barrier layer, two-dimensional (2D) van der Waals ferroelectric materials are advantageous in improving the performance of FTJs and achieving miniaturization of FTJ devices due to the features such as atomic thickness and ideal interfaces. In this work, we present a 2D out-of-plane ferroelectric tunnel junction (FTJ) constructed using graphene and bilayer-In₂Se₃. Using density functional calculations combined with the nonequilibrium Green's function technique, we investigate the electron transport properties in the graphene/bilayer-In₂Se₃ (BIS) vdW FTJ. Our calculations show that the FTJ we constructed can be switched from ferroelectric to antiferroelectric by changing the relative dipole arrangement of the BIS to form multiple nonvolatile resistance states. Since the charge transfer between the layers varies for the four different polarization states, the TER ratios range from 10³% to 10¹⁰%. The giant tunneling electroresistance and multiple resistance states in the 2D BIS-based FTJ suggest that it has great potential for application in nanoscale nonvolatile ferroelectric memory devices.

Ferroelectrically Modulated and Enhanced Photoresponse in a Self-Powered α-In2Se3/Si Heterojunction Photodetector

Photodetectors have been applied to pivotal optoelectronic components of modern optical communication, sensing, and imaging systems. As a room-temperature ferroelectric van der Waals semiconductor, 2D α-In₂Se₃ is a promising candidate for a next-generation optoelectronic material because of its thickness-dependent direct bandgap and excellent optoelectronic performance. Previous studies of photodetectors based on α-In₂Se₃ have been rarely focused on the modulated relationship between the α-In₂Se₃ intrinsic ferroelectricity and photoresponsivity. Herein, a simple integrated process and high-performance photodetector based on an α-In₂Se₃/Si vertical hybrid-dimensional heterojunction was constructed. Our photodetector in the ferroelectric polarization up state accomplishes a self-powered, highly sensitive photoresponse with an on/off ratio of 4.5 × 10⁵ and detectivity of 1.6 × 10¹³ Jones, and it also shows a fast response time with 43 μs. The depolarization field generated by the remanent polarization of ferroelectrics in α-In₂Se₃ provides a strategy for enhancement and modulation of photodetection. The negative correlation was discovered because the enhancement photoresponsivity factor of ferroelectric modulation competes with the photovoltaic behavior within the α-In₂Se₃/Si heterojunction. Our research highlights the great potential of the high-efficiency heterojunction photodetector for future object recognition and photoelectric imaging.

Reducing Threshold of Ferroelectric Domain Switching in Ultrathin Two-Dimensional CuInP2S6 Ferroelectrics via Electrical-Mechanical Coupling

Room-temperature out-of-plane two-dimensional ferroelectrics have promising applications in miniaturized non-volatile memory appliances. The feasible manipulation of polarization switching significantly influences the memory performance of ferroelectrics. However, conventional high-voltage-induced polarization switching inevitably generates charge injection or electric breakdown, and large-mechanical-loading-induced polarization switching may damage the structure of ferroelectrics. Hence, decreasing critical voltage/loading for ferroelectric polarization reversal is highly required. Herein, using atomic force microscopy experiments, the ferroelectric domain switching via both electric field and mechanical loading was demonstrated for an ultrathin (∼4.1 nm) CuInP₂S₆ nanoflake. The relevant threshold voltage/loading for polarization switching was ∼ -5 V/1095 nN, resulting from the electric field and flexoelectric effect, respectively. Finally, the electrical-mechanical coupling was adopted to reduce the threshold voltage/loading of CuInP₂S₆ significantly. It can be explained by the Landau-Ginzburg-Devonshire double-well model. This effective way for easily tuning the polarization states of CuInP₂S₆ opens up new prospects for mechanically written and electrically erased memory devices.

Integrated Memory Devices Based on 2D Materials

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Magnetoelectric coupling effects on the band alignments of multiferroic In2Se3-CrI3 trilayer heterostructures

Due to unique magnetoelectric coupling effects, two-dimensional (2D) multiferroic van der Waals heterostructures (vdWHs) are promising for next-generation information processing and storage devices. Here, we design theoretically multiferroic In₂Se₃/CrI₃ trilayer vdWHs with different stacking patterns. For the CrI₃/In₂Se₃/CrI₃ trilayer vdWHs, whether ferroelectric upward or downward polarization, type-I and type-II band alignments are formed for spin-up and spin-down channels. However, for the CrI₃/In₂Se₃/In₂Se₃ trilayer vdWHs, downward polarization induces the type-III band alignment, which is typical for spin-tunnel transistors. Moreover, nonvolatile ferroelectric polarization and stacking patterns can induce the conversion between a unipolar semiconductor and a bipolar (unipolar) half-metal. These results provide a possible route to realize nanoscale multifunctional spintronic devices based on 2D multiferroic systems.

Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications

Nanostructured In₂Se₃ compounds have been widely used in electronics, optoelectronics, and thermoelectrics. Recently, the revelation of ferroelectricity in low-dimensional (low-D) In₂Se₃ has caused a new upsurge of scientific interest in nanostructured In₂Se₃ and advanced functional devices. The ferroelectric, thermoelectric, and optoelectronic properties of In₂Se₃ are highly correlated with the crystal structure. In this review, we summarize the crystal structures and electronic band structures of the widely interested members of the In₂Se₃ compound family. Recent achievements in the preparation of low-D In₂Se₃ with controlled phases are discussed in detail. General principles for obtaining pure-phased In₂Se₃ nanostructures are described. The excellent ferroelectric, optoelectronic, and thermoelectric properties having been demonstrated using nanostructured and heterostructured In₂Se₃ with different phases are also summarized. Progress and challenges on the applications of In₂Se₃ nanostructures in nonvolatile memories, photodetectors, gas sensors, strain sensors, and photovoltaics are discussed in detail. In the last part of this review, perspectives on the challenges and opportunities in the preparation and applications of In₂Se₃ materials are presented.