Reconfigurable two-dimensional optoelectronic devices enabled by local ferroelectric polarization

Tuning Exciton Emission via Ferroelectric Polarization at a Heterogeneous Interface between a Monolayer Transition Metal Dichalcogenide and a Perovskite Oxide Membrane

We demonstrate the integration of a thin BaTiO3 (BTO) membrane with monolayer MoSe2 in a dual-gate device that enables in situ manipulation of the BTO ferroelectric polarization with a voltage pulse. While two-dimensional (2D) transition metal dichalcogenides (TMDs) offer remarkable adaptability, their hybrid integration with other families of functional materials beyond the realm of 2D materials has been challenging. Released functional oxide membranes offer a solution for 2D/3D integration via stacking. 2D TMD excitons can serve as a local probe of the ferroelectric polarization in BTO at a heterogeneous interface. Using photoluminescence (PL) of MoSe2 excitons to optically read out the doping level, we find that the relative population of charge carriers in MoSe2 depends sensitively on the ferroelectric polarization. This finding points to a promising avenue for future-generation versatile sensing devices with high sensitivity, fast readout, and diverse applicability for advanced signal processing.

2D materials-based photodetectors combined with ferroelectrics

Photodetectors are essential optoelectronic devices that play a critical role in modern technology by converting optical signals into electrical signals, which are one of the most important sensors of the informational devices in current 'Internet of Things' era. Two-dimensional (2D) material-based photodetectors have excellent performance, simple design and effortless fabrication processes, as well as enormous potential for fabricating highly integrated and efficient optoelectronic devices, which has attracted extensive research attention in recent years. The introduction of spontaneous polarization ferroelectric materials further enhances the performance of 2D photodetectors, moreover, companying with the reduction of power consumption. This article reviews the recent advances of materials, devices in ferroelectric-modulated photodetectors. This review starts with the introduce of the basic terms and concepts of the photodetector and various ferroelectric materials applied in 2D photodetectors, then presents a variety of typical device structures, fundamental mechanisms and potential applications under ferroelectric polarization modulation. Finally, we summarize the leading challenges currently confronting ferroelectric-modulated photodetectors and outline their future perspectives.

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Polarizable Nonvolatile Ferroelectric Gating in Monolayer MoS2 Phototransistors

Given the requirements for power and dimension scaling, modulating channel transport properties using high gate bias is unfavorable due to the introduction of severe leakages and large power dissipation. Hence, this work presents an ultrathin phototransistor with chemical-vapor-deposition-grown monolayer MoS2 as the channel and a 10.2 nm thick Al:HfO2 ferroelectric film as the dielectric. The proposed device is meticulously modulated utilizing an Al:HfO2 nanofilm, which passivates traps and suppresses charge Coulomb scattering with Al doping, efficiently improving carrier transport and inhibiting leakage current. Furthermore, a bipolar pulses excitable polarization method is developed to induce a nonvolatile electrostatic field. The MoS2 channel is fully depleted by the switchable and stable floating gate originating from remanent polarization, leading to a high detectivity of 2.05 × 1011 Jones per nanometer of gating layer (Jones nm-1) and photocurrent on/off ratio >104 nm-1, which are superior to the state-of-the-art phototransistors based on two-dimensional (2D) materials and ferroelectrics. The proposed polarizable nonvolatile ferroelectric gating in a monolayer MoS2 phototransistor promises a potential route toward ultrasensitive photodetectors with low power consumption that boast of high levels of integration.

Vapor Phase Growth of Air-Stable Hybrid Perovskite FAPbBr3 Single-Crystalline Nanosheets

Organic-inorganic hybrid perovskites have attracted tremendous attention owing to their fascinating optoelectronic properties. However, their poor air stability seriously hinders practical applications, which becomes more serious with thickness down to the nanoscale. Here we report a one-step vapor phase growth of HC(NH2)2PbBr3 (FAPbBr3) single-crystalline nanosheets of tunable size up to 50 μm and thickness down to 20 nm. The FAPbBr3 nanosheets demonstrate high stability for over months of exposure to air with no degradation in surface roughness and photoluminescence efficiency. Besides, the FAPbBr3 photodetectors exhibit superior overall performance as compared to previous devices based on nonlayered perovskite nanosheets, such as an ultralow dark current of 24 pA, an ultrahigh responsivity of 1033 A/W, an external quantum efficiency over 3000%, a rapid response time around 25 ms, and a high on/off ratio of 104. This work provides a strategy to tackle the challenges of hybrid perovskites toward integrated optoelectronics with requirements of nanoscale thickness, high stability, and excellent performance.

Ferroelectric-Optoelectronic Hybrid System for Photodetection

Photodetectors (PDs), as functional devices based on photon-to-electron conversion, are an indispensable component for the next-generation Internet of Things system. The research of advanced and efficient PDs that meet the diverse demands is becoming a major task. Ferroelectric materials can develop a unique spontaneous polarization due to the symmetry-breaking of the unit cell, which is switchable under an external electric field. Ferroelectric polarization field has the intrinsic characteristics of non-volatilization and rewritability. Introducing ferroelectrics to effectively manipulate the band bending and carrier transport can be non-destructive and controllable in the ferroelectric-optoelectronic hybrid systems. Hence, ferroelectric integration offers a promising strategy for high-performance photoelectric detection. This paper reviews the fundamentals of optoelectronic and ferroelectric materials, and their interactions in hybrid photodetection systems. The first section introduces the characteristics and applications of typical optoelectronic and ferroelectric materials. Then, the interplay mechanisms, modulation effects, and typical device structures of ferroelectric-optoelectronic hybrid systems are discussed. Finally, in summary and perspective section, the progress of ferroelectrics integrated PDs is summed up and the challenges of ferroelectrics in the field of optoelectronics are considered.

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.

Next-Generation Photodetectors beyond Van Der Waals Junctions

With the continuous advancement of nanofabrication techniques, development of novel materials, and discovery of useful manipulation mechanisms in high-performance applications, especially photodetectors, the morphology of junction devices and the way junction devices are used are fundamentally revolutionized. Simultaneously, new types of photodetectors that do not rely on any junction, providing a high signal-to-noise ratio and multidimensional modulation, have also emerged. This review outlines a unique category of material systems supporting novel junction devices for high-performance detection, namely, the van der Waals materials, and systematically discusses new trends in the development of various types of devices beyond junctions. This field is far from mature and there are numerous methods to measure and evaluate photodetectors. Therefore, it is also aimed to provide a solution from the perspective of applications in this review. Finally, based on the insight into the unique properties of the material systems and the underlying microscopic mechanisms, emerging trends in junction devices are discussed, a new morphology of photodetectors is proposed, and some potential innovative directions in the subject area are suggested.

Electret Modulation Strategy to Enhance the Photosensitivity Performance of Two-Dimensional Molybdenum Sulfide

Due to the limited light absorption efficiency of atomic thickness layers and the existence of quenching effects, photodetectors solely made of transition metal dichalcogenides (TMDs) have exhibited an unsatisfactory detection performance. In this article, electret/TMD hybridized devices were proposed by vertically coupling a MoS2 channel and the PTFE film, which reveals an optimized photodetection behavior. Negative charges were generated in the PTFE layer through the corona charging method, akin to applying a negative bias on the MoS2 channel in lieu of a traditional voltage-driven back gate. Under a charging voltage of -6 kV, PTFE/MoS2 devices reveal improved photodetection performance (Rhybrid = 67.95A/W versus Ronly = 3.37 A/W, at 470 nm, 1.20 mW cm-2) and faster recovery speed (τd(hybrid) = 2000 ms versus τd(only) = 2900 ms) compared to those bare MoS2 counterparts. The optimal detection performance (2 orders of magnitude) was obtained when the charging voltage was -2 kV, limited by the minimum of the carrier density in MoS2 channels. This study provides an alternative strategy to optimize optoelectronic devices based on the 2D components through non-voltage-driven gating.

Reconfigurable Neuromorphic Computing: Materials, Devices, and Integration

Neuromorphic computing has been attracting ever-increasing attention due to superior energy efficiency, with great promise to promote the next wave of artificial general intelligence in the post-Moore era. Current approaches are, however, broadly designed for stationary and unitary assignments, thus encountering reluctant interconnections, power consumption, and data-intensive computing in that domain. Reconfigurable neuromorphic computing, an on-demand paradigm inspired by the inherent programmability of brain, can maximally reallocate finite resources to perform the proliferation of reproducibly brain-inspired functions, highlighting a disruptive framework for bridging the gap between different primitives. Although relevant research has flourished in diverse materials and devices with novel mechanisms and architectures, a precise overview remains blank and urgently desirable. Herein, the recent strides along this pursuit are systematically reviewed from material, device, and integration perspectives. At the material and device level, one comprehensively conclude the dominant mechanisms for reconfigurability, categorized into ion migration, carrier migration, phase transition, spintronics, and photonics. Integration-level developments for reconfigurable neuromorphic computing are also exhibited. Finally, a perspective on the future challenges for reconfigurable neuromorphic computing is discussed, definitely expanding its horizon for scientific communities.

Interfacial electronic states and self-formed asymmetric Schottky contacts in polar α-In2Se3/Au contacts

In recent years, the two-dimensional (2D) semiconductor α-In2Se3 has great potential for applications in the fields of electronics and optoelectronics due to its spontaneous iron electrolysis properties. Through ab initio electronic structure calculations and quantum transport simulations, the interface properties and transport properties of α-In2Se3/Au contacts with different polarization directions are studied, and a two-dimensional α-In2Se3 asymmetric metal contact design is proposed. When α-In2Se3 is polarized upward, it forms an n-type Schottky contact with Au. While when α-In2Se3 is polarized downward, it forms a p-type Schottky contact with Au. More importantly, significant rectification effect is found in the asymmetric Au/α-In2Se3/Au field-effect transistor. The carrier transports under positive and negative bias voltages are found to be dominated by thermionic excitation and tunneling, respectively. These findings provide guidance for the further design of 2D α-In2Se3-based transistors.

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.

Ultrafast-Programmable 2D Homojunctions Based on van der Waals Heterostructures on a Silicon Substrate

The development of electrically ultrafast-programmable semiconductor homojunctions can lead to transformative multifunctional electronic devices. However, silicon-based homojunctions are not programmable so that alternative materials need to be explored. Here 2D, multi-functional, lateral homojunctions made of van der Waals heterostructures with a semi-floating-gate configuration on a p++ Si substrate feature atomically sharp interfaces and can be electrostatically programmed in nanoseconds, more than seven orders of magnitude faster than other 2D-based homojunctions. By applying voltage pulses with different polarities, lateral p-n, n+ -n and other types of homojunctions can be formed, varied, and reversed. The p-n homojunctions possess a high rectification ratio of up to ≈105 and can be dynamically switched between four distinct conduction states with the current spanning over nine orders of magnitude, enabling them to function as logic rectifiers, memories, and multi-valued logic inverters. Built on a p++ Si substrate, which acts as the control gate, the devices are compatible with Si technology.

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

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

Broadband, Polarization-Sensitive, and Self-Powered High-Performance Photodetection of Hetero-Integrated MoS2 on Lithium Niobate

High-performance photodetectors hold promising potential in optical communication and imaging systems. However, conventional counterparts are suffering narrow detection range, high power consumption, and poor polarization sensitivity. Characteristics originating from switchable polarization in ferroelectrics can be used to optimize the photo-to-electric procedure and improve the photodetection performance. In this regard, we constructed a configuration by integrating 2-dimensional molybdenum disulfide (MoS₂) with ferroelectric lithium niobate (LiNbO₃), resulting in the MoS₂/LiNbO₃ heterostructured photodetector. Benefiting from the pyroelectric effect of LiNbO₃, the limitation of bandgap on the detection range can be broken, thus broadening the response band of the detector to 365 to 1,064 nm, as well as enabling the self-powered characteristic. Meanwhile, high carrier mobility and decent light absorbance of MoS₂ introduce robust light-matter interactions with the underlying LiNbO₃, leading to ultrafast rise/fall times of ≈150 μs/250 μs and switching ratios of up to ≈190. Moreover, the highest responsivity, specific detectivity, and external quantum efficiency achieved were 17.3 A·W^-1, 4.3 × 10¹¹ Jones, and 4,645.78%, respectively. Furthermore, because of the anisotropy of the spontaneous-polarized LiNbO₃ substrate, the photocurrent of the device achieved a dichroic ratio of 7.42, comparing favorably to most MoS₂-based photodetectors. This work demonstrates the integration potential between ferroelectric LiNbO₃ and 2-dimensional materials for high-performance photodetection.

Contact-engineered reconfigurable two-dimensional Schottky junction field-effect transistor with low leakage currents

Two-dimensional (2D) materials have been considered promising candidates for future low power-dissipation and reconfigurable integrated circuit applications. However, 2D transistors with intrinsic ambipolar transport polarity are usually affected by large off-state leakage currents and small on/off ratios. Here, we report the realization of a reconfigurable Schottky junction field-effect transistor (SJFET) in an asymmetric van der Waals contact geometry, showing a balanced and switchable n- and p-unipolarity with the I_ds on/off ratio kept >10⁶. Meanwhile, the static leakage power consumption was suppressed to 10^-5 nW. The SJFET worked as a reversible Schottky rectifier with an ideality factor of ~1.0 and a tuned rectifying ratio from 3 × 10⁶ to 2.5 × 10^-6. This empowered the SJFET with a reconfigurable photovoltaic performance in which the sign of the open-circuit voltage and photo-responsivity were substantially switched. This polarity-reversible SJFET paves an alternative way to develop reconfigurable 2D devices for low-power-consumption photovoltaic logic circuits.

Integration of Self-Passivated Topological Electrodes for Advanced 2D Optoelectronic Devices

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal-semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi₂ Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe₂ channel realizes a high responsivity of 20.5 A W^-1 , an excellent detectivity of 2.18 × 10¹²  Jones, and a fast rise/decay time of 41.66/38.81 ms. Furthermore, high-resolution visible-light imaging capability of the WSe₂ device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS₂ and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

General Theory for Bilayer Stacking Ferroelectricity

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

Carrier Modulation in 2D Transistors by Inserting Interfacial Dielectric Layer for Area-Efficient Computation

2D materials with atomic thickness display strong gate controllability and emerge as promising materials to build area-efficient electronic circuits. However, achieving the effective and nondestructive modulation of carrier density/type in 2D materials is still challenging because the introduction of dopants will greatly degrade the carrier transport via Coulomb scattering. Here, a strategy to control the polarity of tungsten diselenide (WSe₂ ) field-effect transistors (FETs) via introducing hexagonal boron nitride (h-BN) as the interfacial dielectric layer is devised. By modulating the h-BN thickness, the carrier type of WSe₂ FETs has been switched from hole to electron. The ultrathin body of WSe₂ , combined with the effective polarity control, together contribute to the versatile single-transistor logic gates, including NOR, AND, and XNOR gates, and the operation of only two transistors as a half adder in logic circuits. Compared with the use of 12 transistors based on static Si CMOS technology, the transistor number of the half adder is reduced by 83.3%. The unique carrier modulation approach has general applicability toward 2D logic gates and circuits for the improvement of area efficiency in logic computation.

Measuring Band Modulation of MoS2 with Ferroelectric Gates

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS₂ modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS₂ layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Porous carbon-based metal-free monolayers towards highly stable and flexible wearable thermoelectrics and microelectronics

In the search for high mechanical strength and flexibility, ultrahigh semiconducting speed is crucial for the next generation of microelectronic and wearable electronics. Herein, we propose two 2D graphene-like macrocyclic complex carbon-based monolayers, namely g-MC-A and g-MC-B. Both monolayers are dynamically stable according to phonon dispersion and ab initio molecular dynamics simulations. The yield stress of these two layers reaches half that of graphene, revealing remarkably high mechanical strength. Besides, both monolayers are semiconductors. The electron mobility of g-MC-A is high: up to 10⁴ cm² V^-1 s^-1, comparable to black phosphorene. Furthermore, these two monolayers exhibit excellent inherent conductivity with anisotropic characteristics. Interestingly, an extra valley is observed near the conduction band edge for both layers, further simulation predicted both metal-free monolayers will exhibit ZT > 1, implying high thermoelectric performance. Therefore, these two C-based metal-free layers have promising applications in mechanical enhancement, microelectronics, wearable electronics and thermoelectric devices.

A Reconfigurable Two-WSe2 -Transistor Synaptic Cell for Reinforcement Learning

Reward-modulated spike-timing-dependent plasticity (R-STDP) is a brain-inspired reinforcement learning (RL) rule, exhibiting potential for decision-making tasks and artificial general intelligence. However, the hardware implementation of the reward-modulation process in R-STDP usually requires complicated Si complementary metal-oxide-semiconductor (CMOS) circuit design that causes high power consumption and large footprint. Here, a design with two synaptic transistors (2T) connected in a parallel structure is experimentally demonstrated. The 2T unit based on WSe₂ ferroelectric transistors exhibits reconfigurable polarity behavior, where one channel can be tuned as n-type and the other as p-type due to nonvolatile ferroelectric polarization. In this way, opposite synaptic weight update behaviors with multilevel (>6 bit) conductance states, ultralow nonlinearity (0.56/-1.23), and large G_max /G_min ratio of 30 are realized. By applying positive/negative reward to (anti-)STDP component of 2T cell, R-STDP learning rules are realized for training the spiking neural network and demonstrated to solve the classical cart-pole problem, exhibiting a way for realizing low-power (32 pJ per forward process) and highly area-efficient (100 µm² ) hardware chip for reinforcement learning.

Ferroelectric Polarized in Transistor Channel Polarity Modulation for Reward-Modulated Spike-Time-Dependent Plasticity Application

Reward signals reflect the developmental tendency of reinforcement learning (RL) agents. Reward-modulated spike-time-dependent plasticity (R-STDP) is an efficient and concise information processing feature in RL. However, the physical construction of R-STDP normally demands complex circuit design engineering, resulting in large power consumption and large area. In this work, we studied the role of ferroelectric polarization in the modulation of carbon nanotube transistor channel polarity. Furthermore, we applied a modulating channel method to construct a 2T synaptic component by spin-coating technology. Based on the nonvolatility of ferroelectric polarization, the synaptic component constructed has the characteristics of reconfigurable polarity. One channel could be modulated to n-type and the other to p-type. One modulated channel was used to perform the STDP function when the reward signal arrived, and the other modulated channel was used to perform the anti-STDP function when the punishment signal arrived. Finally, R-STDP learning rules are implemented on hardware. This work provides a strategy for hardware construction of RL.

Emerging Ferroelectric Uniaxial Lamellar (Smectic AF) Fluids for Bistable In-Plane Polarization Memory

The emerging matter category of liquid-matter ferroelectrics, i.e., ferroelectric nematics, demonstrates an unprecedented combination of fluidity and spontaneous polarization. However, unlike traditional ferroelectrics, the field-switched polarization at zero-field cannot be conserved, so the memory effect remains challenging. Here we report another new type of ferroelectric liquid crystal state, dubbed the ferroelectric smectic A phase, where the polarization is longitudinally coupled to the smectic quasi-layer order. With higher packing density, the phase exhibits higher values of refractive anisotropy and spontaneous polarization compared to the ferroelectric nematics. A delicate balance between the liquid crystal elasticity and flow viscosity enables both the switching and memory of the polarization field, thus opening the door toward realizing liquid-matter ferroelectric memory devices.

Ferroelectric Tuning of ZnO Ultraviolet Photodetectors

The ferroelectric field effect transistor (Fe-FET) is considered to be one of the most important low-power and high-performance devices. It is promising to combine a ferroelectric field effect with a photodetector to improve the photodetection performance. This study proposes a strategy for ZnO ultraviolet (UV) photodetectors regulated by a ferroelectric gate. The ZnO nanowire (NW) UV photodetector was tuned by a 2D CuInP₂S₆ (CIPS) ferroelectric gate, which decreased the dark current and enhanced the responsivity and detectivity to 2.40 × 10⁴ A/W and 7.17 × 10¹¹ Jones, respectively. This strategy was also applied to a ZnO film UV photodetector that was tuned by a P(VDF-TrFE) ferroelectric gate. Lower power consumption and higher performance can be enabled by ferroelectric tuning of ZnO ultraviolet photodetectors, providing new inspiration for the fabrication of high-performance photodetectors.

Ferroelectrics-Integrated Two-Dimensional Devices toward Next-Generation Electronics

Ferroelectric materials play an important role in a wide spectrum of semiconductor technologies and device applications. Two-dimensional (2D) van der Waals (vdW) ferroelectrics with surface-insensitive ferroelectricity that is significantly different from their traditional bulk counterparts have further inspired intensive interest. Integration of ferroelectrics into 2D-layered-material-based devices is expected to offer intriguing working principles and add desired functionalities for next-generation electronics. Herein, fundamental properties of ferroelectric materials that are compatible with 2D devices are introduced, followed by a critical review of recent advances on the integration of ferroelectrics into 2D devices. Representative device architectures and corresponding working mechanisms are discussed, such as ferroelectrics/2D semiconductor heterostructures, 2D ferroelectric tunnel junctions, and 2D ferroelectric diodes. By leveraging the favorable properties of ferroelectrics, a variety of functional 2D devices including ferroelectric-gated negative capacitance field-effect transistors, programmable devices, nonvolatile memories, and neuromorphic devices are highlighted, where the application of 2D vdW ferroelectrics is particularly emphasized. This review provides a comprehensive understanding of ferroelectrics-integrated 2D devices and discusses the challenges of applying them into commercial electronic circuits.

High-performance hierarchical O-SnS/I-ZnIn2S4 photodetectors by leveraging the synergy of optical regulation and band tailoring

Low light absorption and limited carrier lifetime are critical obstacles inhibiting further performance improvement of 2D layered material (2DLM) based photodetectors, while scalable fabrication is an ongoing challenge prior to commercialization from the lab to market. Herein, wafer-scale SnS/ZIS hierarchical nanofilms, where out-of-plane SnS (O-SnS) is modified onto in-plane ZIS (I-ZIS), have been achieved by pulsed-laser deposition. The derived O-SnS/I-ZIS photodetector exhibits markedly boosted sensitivity as compared to a pristine ZIS device. The synergy of multiple functionalities contributes to the dramatic improvement, including the pronounced light-trapping effect of O-SnS by multiple scattering, the high-efficiency spatial separation of photogenerated electron-hole pairs by a type-II staggered band alignment and the promoted carrier transport enabled by the tailored electronic structure of ZIS. Of note, the unique architecture of O-SnS/I-ZIS can considerably expedite the carrier dynamics, where O-SnS promotes the electron transfer from SnS to ZIS whilst the I-ZIS enables high-speed electron circulation. In addition, the interlayer transition enables the bridging of the effective optical window to telecommunication wavelengths. Moreover, monolithic integration of arrayed devices with satisfactory device-to-device variability has been encompassed and a proof-of-concept imaging application is demonstrated. On the whole, this study depicts a fascinating functional coupling architecture toward implementing chip-scale integrated optoelectronics.

Dielectric Contrast Tailoring for Polarized Photosensitivity toward Multiplexing Optical Communications and Dynamic Encrypt Technology

A selective-area oxidation strategy is developed to polarize high-symmetry 2D layered materials (2DLMs). The dichroic ratio of the derived O-WS₂/WS₂ photodetector reaches ∼8, which is competitive among state-of-the-art polarization photodetectors. Finite-different time-domain simulations consolidate that the polarization-sensitive photoresponse is associated with anisotropic spacial confinement, which gives rise to distinct dielectric contrasts for linearly polarized light of various directions and thus the polarization-dependent near-field distribution. Furthermore, selective-area oxidation treatment brings about dual effects, comprising the in situ formation of seamless in-plane WS₂ homojunctions by thickness tailoring and the formation of out-of-plane O-WS₂/WS₂ heterojunctions. As a consequence, the recombination of photocarriers is markedly suppressed, resulting in outstanding photosensitivity with the optimized responsivity, external quantum efficiency, and detectivity of 0.161 A/W, 49.4%, and 1.4 × 10¹¹ Jones for an O-WS₂/WS₂ photodetector in a self-powered mode. A scheme of multiplexing optical communications is revealed by harnessing the intensity and polarization state of light as independent transmission channels. Furthermore, dynamic encryption by leveraging the polarization state as a secret key is proposed. In the end, broad universality is reinforced through the induction of linear dichroism within 2D WSe₂ crystals. On the whole, this study provides an additional perspective on polarization optoelectronics based on 2DLMs.

Asymmetric Ferroelectric-Gated Two-Dimensional Transistor Integrating Self-Rectifying Photoelectric Memory and Artificial Synapse

Ferroelectric field-effect transistors (Fe-FET) are promising candidates for future information devices. However, they suffer from low endurance and short retention time, which retards the application of processing memory in the same physical processes. Here, inspired by the ferroelectric proximity effects, we design a reconfigurable two-dimensional (2D) MoS₂ transistor featuring with asymmetric ferroelectric gate, exhibiting high memory and logic ability with a program/erase ratio of over 10⁶ and a self-rectifying ratio of 10³. Interestingly, the robust electric and optic cycling are obtained with a large switching ratio of 10⁶ and nine distinct resistance states upon optical excitation with excellent nonvolatile characteristics. Meanwhile, the operation of memory mimics the synapse behavior in response to light spikes with different intensity and number. This design realizes an integration of robust processing memory in one single device, which demonstrates a considerable potential of an asymmetric ferroelectric gate in the development of Fe-FETs for logic processing and nonvolatile memory applications.

Ultra-sensitive polarization-resolved black phosphorus homojunction photodetector defined by ferroelectric domains

With the further miniaturization and integration of multi-dimensional optical information detection devices, polarization-sensitive photodetectors based on anisotropic low-dimension materials have attractive potential applications. However, the performance of these devices is restricted by intrinsic property of materials leading to a small polarization ratio of the detectors. Here, we construct a black phosphorus (BP) homojunction photodetector defined by ferroelectric domains with ultra-sensitive polarization photoresponse. With the modulation of ferroelectric field, the BP exhibits anisotropic dispersion changes, leading an increased photothermalelectric (PTE) current in the armchair (AC) direction. Moreover, the PN junction can promote the PTE current and accelerate carrier separation. As a result, the BP photodetector demonstrates an ultrahigh polarization ratio (PR) of 288 at 1450 nm incident light, a large photoresponsivity of 1.06 A/W, and a high detectivity of 1.27 × 10¹¹ cmHz^1/2W⁻¹ at room temperature. This work reveals the great potential of BP in future polarized light detection.

Integrated polarization-sensitive photodetectors are important for sensing applications and optical communication. Here, the authors report the realization of 2D black phosphorus homojunction photodetectors defined by ferroelectric substrates, showing polarization ratios up to 288 and high responsivity in the near-infrared.

Tactile tribotronic reconfigurable p-n junctions for artificial synapses

The emulation of biological synapses with learning and memory functions and versatile plasticity is significantly promising for neuromorphic computing systems. Here, a robust and continuously adjustable mechanoplastic semifloating-gate transistor is demonstrated based on an integrated graphene/hexagonal boron nitride/tungsten diselenide van der Waals heterostructure and a triboelectric nanogenerator (TENG). The working states (p-n junction or n⁺-n junction) can be manipulated and switched under the sophisticated modulation of triboelectric potential derived from mechanical actions, which is attributed to carriers trapping and detrapping in the graphene layer. Furthermore, a reconfigurable artificial synapse is constructed based on such mechanoplastic transistor that can simulate typical synaptic plasticity and implement dynamic control correlations in each response mode by further designing the amplitude and duration. The artificial synapse can work with ultra-low energy consumption at 74.2 fJ per synaptic event and the extended synaptic weights. Under the synergetic effect of the semifloating gate, the synaptic device can enable successive mechanical facilitation/depression, short-/long-term plasticity and learning-experience behavior, exhibiting the mechanical behavior derived synaptic plasticity. Such reconfigurable and mechanoplastic features provide an insight into the applications of energy-efficient and real-time interactive neuromodulation in the future artificial intelligent system beyond von Neumann architecture.

Heterojunction Nanomedicine

Exogenous stimulation catalytic therapy has received enormous attention as it holds great promise to address global medical issues. However, the therapeutic effect of catalytic therapy is seriously restricted by the fast charge recombination and the limited utilization of exogenous stimulation by catalysts. In the past few decades, many strategies have been developed to overcome the above serious drawbacks, among which heterojunctions are the most widely used and promising strategy. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction nanomedicine, such as semiconductor-semiconductor heterojunctions (including type I, type II, type III, PN, and Z-scheme junctions) and semiconductor-metal heterojunctions (including Schottky, Ohmic, and localized surface plasmon resonance-mediated junctions). The catalytic mechanisms and properties of the above junction systems are also discussed in relation to biomedical applications, especially cancer treatment and sterilization. This review concludes with a summary of the challenges and some perspectives on future directions in this exciting and still evolving field of research.

Discovery of Robust Ferroelectricity in 2D Defective Semiconductor α-Ga2 Se3

2D ferroelectrics with robust polar order in the atomic-scale thickness at room temperature are needed to miniaturize ferroelectric devices and tackle challenges imposed by traditional ferroelectrics. These materials usually have polar point group structure regarding as a prerequisite of ferroelectricity. Yet, to introduce polar structure into otherwise nonpolar 2D materials for producing ferroelectricity remains a challenge. Here, by combining first-principles calculations and experimental studies, it is reported that the native Ga vacancy-defects located in the asymmetrical sites in cubic defective semiconductor α-Ga₂ Se₃ can induce polar structure. Meanwhile, the induced polarization can be switched in a moderate energy barrier. The switched polarization is observed in 2D α-Ga₂ Se₃ nanoflakes of ≈4 nm with a high switching temperature up to 450 K. Such polarization switching could arise from the displacement of Ga vacancy between neighboring asymmetrical sites by applying an electric field. This work removes the point group limit for ferroelectricity, expanding the range of 2D ferroelectrics into the native defective semiconductors.

Solid-Ionic Memory in a van der Waals Heterostructure

Defect states dominate the performance of low-dimensional nanoelectronics, which deteriorate the serviceability of devices in most cases. But in recent years, some intriguing functionalities are discovered by defect engineering. In this work, we demonstrate a bifunctional memory device of a MoS₂/BiFeO₃/SrTiO₃ van der Waals heterostructure, which can be programmed and erased by solely one kind of external stimuli (light or electrical-gate pulse) via engineering of oxygen-vacancy-based solid-ionic gating. The device shows multibit electrical memory capability (>22 bits) with a large linearly tunable dynamic range of 7.1 × 10⁶ (137 dB). Furthermore, the device can be programmed by green- and red-light illuminations and then erased by UV light pulses. Besides, the photoresponse under red-light illumination reaches a high photoresponsivity (6.7 × 10⁴ A/W) and photodetectivity (2.12 × 10¹³ Jones). These results highlighted solid-ionic memory for building up multifunctional electronic and optoelectronic devices.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

2D Bi2Se3 materials for optoelectronics

2D layered materials with diverse exciting properties have recently attracted tremendous interest in the scientific community. Layered topological insulator Bi2Se3 comes into the spotlight as an exotic state of quantum matter with insulating bulk states and metallic Dirac-like surface states. Its unique crystal and electronic structure offer attractive features such as broadband optical absorption, thickness-dependent surface bandgap and polarization-sensitive photoresponse, which enable 2D Bi2Se3 to be a promising candidate for optoelectronic applications. Herein, we present a comprehensive summary on the recent advances of 2D Bi2Se3 materials. The structure and inherent properties of Bi2Se3 are firstly described and its preparation approaches (i.e., solution synthesis and van der Waals epitaxy growth) are then introduced. Moreover, the optoelectronic applications of 2D Bi2Se3 materials in visible-infrared detection, terahertz detection, and opto-spintronic device are discussed in detail. Finally, the challenges and prospects in this field are expounded on the basis of current development.

Nonvolatile Reconfigurable 2D Schottky Barrier Transistors

Nonvolatile reconfigurable transistors can be used to implement highly flexible and compact logic circuits with low power consumption in maintaining the configuration. In this paper, we build nonvolatile reconfigurable transistors based on 2D CuInP₂S₆/MoTe₂ heterostructures. The ferroelectric polarization-induced electron and hole doping in the heterostructure are investigated. By introducing the ferroelectric doping into the source/drain contacts, we demonstrate reconfigurable Schottky barrier transistors, whose polarity (n-type or p-type) can be dynamically programmed, where the configuration is nonvolatile in nature. These transistors exhibit a tunable photoresponse, where the n-n doping state leads to negative photocurrent, whereas the p-p doping state gives rise to a positive photocurrent. The transistor with asymmetric (n-p or p-n) contacts exhibits a strong photovoltaic effect. These reconfigurable logic and optoelectronic transistors will enable a new type of device fabric for future computing systems and sensing networks.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft electronics.

Substitutionally Doped MoSe2 for High-Performance Electronics and Optoelectronics

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe₂ is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe₂ p-n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (10⁴ ). The analysis of dark current reveals the domination of the Shockley-Read-Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W^-1 ), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈10⁷ ). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe₂ inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Memristor-based biomimetic compound eye for real-time collision detection

The lobula giant movement detector (LGMD) is the movement-sensitive, wide-field visual neuron positioned in the third visual neuropile of lobula. LGMD neuron can anticipate collision and trigger avoidance efficiently owing to the earlier occurring firing peak before collision. Vision chips inspired by the LGMD have been successfully implemented in very-large-scale-integration (VLSI) system. However, transistor-based chips and single devices to simulate LGMD neurons make them bulky, energy-inefficient and complicated. The devices with relatively compact structure and simple operation mode to mimic the escape response of LGMD neuron have not been realized yet. Here, the artificial LGMD visual neuron is implemented using light-mediated threshold switching memristor. The non-monotonic response to light flow field originated from the formation and break of Ag conductive filaments is analogue to the escape response of LGMD neuron. Furthermore, robot navigation with obstacle avoidance capability and biomimetic compound eyes with wide field-of-view (FoV) detection capability are demonstrated.

Giant Transport Anisotropy in ReS_{2} Revealed via Nanoscale Conducting-Path Control

The low in-plane symmetry in layered 1T'-ReS_{2} results in strong band anisotropy, while its manifestation in the electronic properties is challenging to resolve due to the lack of effective approaches for controlling the local current path. In this work, we reveal the giant transport anisotropy in monolayer to four-layer ReS_{2} by creating directional conducting paths via nanoscale ferroelectric control. By reversing the polarization of a ferroelectric polymer top layer, we induce a conductivity switching ratio of >1.5×10^{8} in the ReS_{2} channel at 300 K. Characterizing the domain-defined conducting nanowires in an insulating background shows that the conductivity ratio between the directions along and perpendicular to the Re chain can exceed 5.5×10^{4} in monolayer ReS_{2}. Theoretical modeling points to the band origin of the transport anomaly and further reveals the emergence of a flat band in few-layer ReS_{2}. Our work paves the path for implementing highly anisotropic 2D materials for designing novel collective phenomena and electron lensing applications.

2D materials-based homogeneous transistor-memory architecture for neuromorphic hardware

[Figure: see text].

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Assembly of Close-Packed Ferroelectric Polymer Nanowires via Interface-Epitaxy with ReS2

The flexible, transparent, and low-weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization-enabled device functionalities, large-scale fabrication of polymer thin films with well-controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir-Blodgett, spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) thin films composed of close-packed crystalline nanowires via interface-epitaxy with 1T'-ReS₂ . Upon controlled thermal treatment, uniform P(VDF-TrFE) films restructure into about 10 and 35 nm-wide (010)-oriented nanowires that are crystallographically aligned with the underlying ReS₂ , as revealed by high-resolution transmission electron microscopy. Piezoresponse force microscopy studies confirm the out-of-plane polar axis of the nanowire films and reveal coercive voltages as low as 0.1 V. Reversing the polarization can induce a conductance switching ratio of >10⁸ in bilayer ReS₂ , over six orders of magnitude higher than that achieved by an untreated polymer gate. This study points to a cost-effective route to large-scale processing of high-performance ferroelectric polymer thin films for flexible energy-efficient nanoelectronics.