Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In₂Se₃ confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In₂Se₃ and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In₂Se₃. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 10⁵ cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.     

Inhomogeneous Friction Behaviour of Nanoscale Phase Separated Layered CuInP2S6

Mechanical friction leads to wear and energy dissipation, and its control is of high importance in new-generation miniature electromechanical devices. 2D materials such as graphene are considered to be excellent solid lubricants due to their ultralow friction and have attracted considerable research interest. Unique friction properties are discovered in various other 2D materials. However, the friction of functional van der Waals materials which have potential applications in novel nanoelectronics, like ferroelectric copper indium thiophosphate, has barely been studied. Herein, the study reports on the observation of inhomogeneous friction behavior existing in copper-deficient CuInP₂S₆ (Cu_0.2In_1.26P₂S₆), which exhibits a nanoscale phase separation of polar and non-polar crystalline phases. The paraelectric In_4/3P₂S₆ phase exhibits higher friction than the ferroelectric CuInP₂S₆ phase, while phase boundaries between the two phases, interestingly, display the lowest friction. The origin of this phenomenon is attributed to different lattice strains of phases together with the presence of large strains at the nanoscale phase boundaries, which also manifests in the nonuniform tip-sample adhesion force. The findings provide new insights into nanoscale device design and wear behavior of a phase-separated van der Waals ferroelectric, which may help to reduce the power consumption of friction-exhibiting devices and extend their service life.     

Manipulation of Magnetic Skyrmion in a 2D van der Waals Heterostructure via Both Electric and Magnetic Fields

As a promising candidate for the much-desired low power consumption spintronic devices, 2D magnetic van der Waals material also provides a versatile platform for the design and control of topological spin textures. In this work on WTe₂/CrCl₃ bilayer van der Waals heterostructures, a complete Néel-type skyrmion–bimeron–ferromagnet phase transition is demonstrated, accompanied by the evolution of the topological number. This cyclic transition, mediated by a perpendicular magnetic field, is largely driven by the competition between the out-of-plane magnetocrystalline anisotropy and magnetic dipole–dipole interaction. In the presence of a driving current, the Néel-type skyrmion gains a higher velocity yet larger skyrmion Hall angle, in comparison to the bimeron. By incorporating a ferroelectric CuInP₂S₆ monolayer as a substrate, writing and erasing of skyrmions may be regulated using a ferroelectric polarization. This work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.     

Intact Metal/Metal Halide van der Waals Junction Enables Reliable Memristive Switching with High Endurance

Organic–inorganic or inorganic metal halide materials have emerged as a promising candidate for a resistive switching material owing to their ability to achieve low operating voltage, high on–off ratio, and multi-level switching. However, the high switching variation, limited endurance, and poor reproducibility of the device hinder practical use of the memristors. In this study, a universal approach to address the issues using a van der Waals metal contact (vdWC) is reported. By transferring the pre-deposited metal contact onto the active layers, an intact junction between the metal halide and contact layer is formed without unintended damage to the active layer caused by a conventional physical deposition process of the metal contacts. Compared with the thermally evaporated metal contact (EVC), the vdWC does not degrade the optoelectronic quality of the underlying layer to enable memristors with reduced switching variation, significantly enhanced endurance, and reproducibility relative to those based on the EVC. By adopting various metal halide active layers, versatile utility of the vdWC is demonstrated. Thus, this vdWC approach can be a useful platform technology for the development of high-performance and reliable memristors for future computing.     

Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a‐SrTiO3) Memristors

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a‐STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal‐oxide‐metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a‐STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano‐contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano‐switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a‐STO/metal stacks highlight the impact of mechanical stress on the modulation of non‐linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.     

Phase-change memory devices with stacked Ge-chalcogenide/Sn-chalcogenide layers

Non-volatile memory devices with two stacked layers of chalcogenide materials comprising the active memory device have been investigated for their potential as phase-change memories. The devices tested consisted of GeTe/SnTe, Ge₂Se₃/SnTe, and Ge₂Se₃/SnSe stacks. All devices exhibited resistance switching behavior. The polarity of the applied voltage with respect to the SnTe or SnSe layer was critical to the memory switching properties, most likely due to the voltage induced movement of either Sn or Te into the Ge-chalcogenide layer.     

Van Der Waals Heterojunction Modulated Charge Collection for H2O2 Production Photocathode

Efficient photocarrier generation and collection are highly desirable for solar-fuel conversion systems. However, the latter is challenging for many photoelectrodes due to carrier losses happening in the semiconductor-electrolyte interface and the semiconductor-substrate interface. To overcome it, a novel Au/CuBi₂O₄ (CBO)/PtSe₂ van der Waals heterojunction photocathode has been developed to boost photocarrier collection. As a result, the ohmic contact at Au/CBO interface shows a low resistance for hole transfer. Meanwhile, while promoting electron transfer, the Van der Waals heterojunction of CBO/PtSe₂ interface with energy barrier-blocked holes. Concurrently, the composite photocathode exhibits significantly enhanced performance: a photocurrent of −0.59 mA cm⁻² at 0.5 V versus reversible hydrogen electrode and an H₂O₂ generation rate of 2.39 mol (Lhm²)⁻¹. This work demonstrates the charge regulation role of Van der Waals heterojunctions in the solar-fuel system. More broadly, Van der Waals heterojunctions provide an excellent bond-free approach to creating versatile optoelectronic devices.     

Double Negative Differential Transconductance Characteristic: From Device to Circuit Application toward Quaternary Inverter

Multi‐valued logic (MVL) computing, which uses more than three logical states, is a promising future technology for handling huge amounts of data in the forthcoming “big data” era. The feasibility of MVL computing depends on the development of new concept devices/circuits beyond the complementary metal oxide semiconductor (CMOS) technology. This is because many CMOS devices are required to implement basic MVL functions, such as multilevel NOT, AND, and OR. In this study, a novel MVL device is reported with a complementarily controllable potential well, featuring the negative differential transconductance (NDT) phenomenon. This NDT device implemented on the WS₂–graphene–WSe₂ van der Waals heterostructure is evolved to a double‐NDT device operating on the basis of two consecutive NDT phenomena via structural engineering and parallel device configuration. This double‐NDT device is intensively analyzed via atomic force microscopy, kelvin probe force microscopy, Raman spectroscopy, and temperature‐dependent electrical measurement to gain a detailed understanding of its operating mechanism. Finally, the operation of a quaternary inverter configured with the double‐peak NDT device and a p‐channel transistor through Cadence circuit simulation is theoretically demonstrated.     

MoS2 Van der Waals p–n Junctions Enabling Highly Selective Room‐Temperature NO2 Sensor

Van der Waals p–n junctions of 2D materials present great potential for electronic devices due to the fascinating properties at the junction interface. In this work, an efficient gas sensor based on planar 2D van der Waals junctions is reported by stacking n‐type and p‐type atomically thin MoS₂ films, which are synthesized by chemical vapor deposition (CVD) and soft‐chemistry route, respectively. The electrical conductivity of the van der Waals p–n junctions is found to be strongly affected by the exposure to NO₂ at room temperature (RT). The MoS₂ p–n junction sensor exhibits an outstanding sensitivity and selectivity to NO₂ at RT, which are unavailable in sensors based on individual n‐type or p‐type MoS₂. The sensitivity of 20 ppm NO₂ is improved by 60 times compared to a p‐type MoS₂ sensor, and an extremely low limit of detection of 8 ppb is obtained under ultraviolet irradiation. Complete and very fast sensor recovery is achieved within 30 s. These results are superior to most of the previous reports related to NO₂ detection. This work establishes an entirely new sensing platform and proves the feasibility of using such materials for the high‐performance detection of gaseous molecules at RT.     

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic-irradiated phase formed in Fe₃GeTe₂ by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe₃GeTe₂, the voltage-driven oxygen incorporated into Fe₃GeTe₂ from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all-solid, long-retention, and low-dissipation nano-electronic devices using van der Waals materials.     

A Quaternary van der Waals Ferromagnetic Semiconductor AgVP2Se6

The recent realization of 2D magnetism in van der Waals (vdWs) magnets holds promise for future information technology. However, the vdWs semiconducting ferromagnets, which remain rare, are especially important in developing 2D magnetic devices with new functionalities due to the possibility of simultaneous control of the carrier charge and spin. Metal thiophosphate (MTP), a multifunctional vdWs material system that combines the sought‐after properties of complex oxides, is a promising vdWs magnet system. Here, single crystals of a novel vdWs ferromagnetic semiconductor MTP AgVP₂Se₆ with a room‐temperature resistivity of 1 Ω m are successfully synthesized. Due to the nature of vdWs bonding along the c‐axis, the magnetic properties of the few‐layer AgVP₂Se₆ with different thicknesses are characterized on the exfoliated samples. The AgVP₂Se₆ flakes exhibit significant thickness‐dependent magnetic properties, and a rectangular hysteresis loop with a large coercive field of 750 Oe at 2 K and an undiminished Curie temperature of 19 K are observed in the 6.7 nm AgVP₂Se₆ flake. The discovered vdWs ferromagnet AgVP₂Se₆ with semiconducting behavior will provide alternative platforms for exploring 2D magnetism and potential applications in spintronic devices.     

Progressive NO2 Sensors with Rapid Alarm and Persistent Memory-Type Responses for Wide-Range Sensing Using Antimony Triselenide Nanoflakes

Antimony triselenide (Sb₂Se₃) nanoflake-based nitrogen dioxide (NO₂) sensors exhibit a progressive bifunctional gas-sensing performance, with a rapid alarm for hazardous highly concentrated gases, and an advanced memory-type function for low-concentration (<1 ppm) monitoring repeated under potentially fatal exposure. Rectangular and cuboid shaped Sb₂Se₃ nanoflakes, comprising van der Waals planes with large surface areas and covalent bond planes with small areas, can rapidly detect a wide range of NO₂ gas concentrations from 0.1 to 100 ppm. These Sb₂Se₃ nanoflakes are found to be suitable for physisorption-based gas sensing owing to their anisotropic quasi-2D crystal structure with extremely enlarged van der Waals planes, where they are humidity-insensitive and consequently exhibit an extremely stable baseline current. The Sb₂Se₃ nanoflake sensor exhibits a room-temperature/low-voltage operation, which is noticeable owing to its low energy consumption and rapid response even under a NO₂ gas flow of only 1 ppm. As a result, the Sb₂Se₃ nanoflake sensor is suitable for the development of a rapid alarm system. Furthermore, the persistent gas-sensing conductivity of the sensor with a slow decaying current can enable the development of a progressive memory-type sensor that retains the previous signal under irregular gas injection at low concentrations.     

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

GeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi₂Te₃ alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.     

Anisotropic Properties of Quasi-1D In4Se3: Mechanical Exfoliation,Electronic Transport,and Polarization-Dependent Photoresponse

Theoretical and experimental investigations of various exfoliated samples taken from layered In₄Se₃ crystals are performed. In spite of the ionic character of interlayer interactions in In₄Se₃ and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few-nanometer-thick flakes of In₄Se₃ by mechanical exfoliation of its bulk crystals. The In₄Se₃ flakes exfoliated on Si/SiO₂ have anisotropic electronic properties and exhibit field-effect electron mobilities of about 50 cm² V⁻¹ s⁻¹ at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS₂ and TiS₃. In₄Se₃ devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry-dependent band structure calculations for the most expected ac cleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In₄Se₃ crystals is possible, while the fast anisotropic photoresponse makes In₄Se₃ a competitive electronic material, in the TMC family, for emerging optoelectronic device applications.     

Visualizing Thermally Activated Memristive Switching in Percolating Networks of Solution-Processed 2D Semiconductors

Memristive systems present a low-power alternative to silicon-based electronics for neuromorphic and in-memory computation. 2D materials have been increasingly explored for memristive applications due to their novel biomimetic functions, ultrathin geometry for ultimate scaling limits, and potential for fabricating large-area, flexible, and printed neuromorphic devices. While the switching mechanism in memristors based on single 2D nanosheets is similar to conventional oxide memristors, the switching mechanism in nanosheet composite films is complicated by the interplay of multiple physical processes and the inaccessibility of the active area in a two-terminal vertical geometry. Here, the authors report thermally activated memristors fabricated from percolating networks of diverse solution-processed 2D semiconductors including MoS₂, ReS₂, WS₂, and InSe. The mechanisms underlying threshold switching and negative differential resistance are elucidated by designing large-area lateral memristors that allow the direct observation of filament and dendrite formation using in situ spatially resolved optical, chemical, and thermal analyses. The high switching ratios (up to 10³) that are achieved at low fields (≈4 kV cm⁻¹) are explained by thermally assisted electrical discharge that preferentially occurs at the sharp edges of 2D nanosheets. Overall, this work establishes percolating networks of solution-processed 2D semiconductors as a platform for neuromorphic architectures.     

Recent Advances in Layered Two-Dimensional Ferroelectrics from Material to Device

With the advent of the post Moore era, modern electronics require further device miniaturization of all electronic components, particularly ferroelectric memories, due to the need for massive data storage. This demand stimulates the exploration of robust switchable ferroelectric polarizations at the atomic scale. In this scenario, van der Waals ferroelectrics have recently gained increasing attention because of their stable layered structure at nanometer thickness, offering the opportunity to realize two-dimensional ferroelectricity that is long-sought in conventional thin film ferroelectrics. In this review, recent advancements are summarized in layered ferroelectrics with highlights of the fundamentals of intrinsic two-dimensional ferroelectricity, the emergence of artificial stacking ferroelectricity, and related protype devices with exotic functions. In addition, the unique polarization control in van der Waals ferroelectrics is discussed. Although great challenges remain unsolved, these studies undoubtedly advance the integration of 2D ferroelectrics in electronics.     

Surface Modification of a Titanium Carbide MXene Memristor to Enhance Memory Window and Low-Power Operation

With the demand for low-power-operating artificial intelligence systems, bio-inspired memristor devices exhibit potential in terms of high-density memory functions and the emulation of the synaptic dynamics of the human brain. The 2D material MXene attracts considerable interest for use in resistive-switching memory and artificial synapse devices owing to its excellent physicochemical properties in memristor devices. However, few memristive and synaptic MXene devices that display increased switching performances are reported, with no significant results. Herein, the conductivity of MXene (Ti₃C₂T_x) is engineered via etching and oxidation to enhance the switching performance of the device. The exceptional properties of partially oxidized MXene memristors include large memory windows and low threshold biases, and the complex spike-timing-dependent plasticity synaptic rules are also emulated. The low threshold potential distribution, reliable retention time (10⁴ s), and distinct resistance states with a high ON–OFF ratio (>10⁴) are the main memory-related features of this device. The experimentally determined switching potentials of the optimized device are also uniformly distributed, according to a statistical probability-based approach. This investigation may promote the essential material properties for use in high-density non-volatile memory storage and artificial synapse systems in the field of innovative nanoelectronic devices.     

Conduction Mechanisms in Multiferroic Multilayer BaTiO<Subscript>3</Subscript>/NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/BaTiO<Subscript>3</Subscript> Memristors

Memristive devices and materials are extensively studied as they offer diverse properties and applications in digital, analog and bio-inspired circuits. In this paper, we present an important class of memristors, multiferroic memristors, which are composed of multiferroic multilayer BaTiO₃/NiFe₂O₄/BaTiO₃ thin films, fabricated by a spin-coating deposition technique on platinized Si wafers. This cost-effective device shows symmetric and reproducible current–voltage characteristics for the actuating voltage amplitude of ±10 V. The origin of the conduction mechanism was investigated by measuring the electrical response in different voltage and temperature conditions. The results indicate the existence of two mechanisms: thermionic emission and Fowler–Nordheim tunnelling, which alternate with actuating voltage amplitude and operating temperature.     

Highly Enhanced Polarization Switching Speed in HfO2-based Ferroelectric Thin Films via a Composition Gradient Strategy

The next-generation semiconductor memories are essentially required for the advancements in modern electronic devices. Ferroelectric memories by HfO₂-based ferroelectric thin films (FE-HfO₂) have opened promising directions in recent years. Nevertheless, improving the polarization switching speed of FE-HfO₂ remains a critical task. In this study, it is demonstrated that the composition-graded Hf_1-xZr_xO₂ (HZO) ferroelectric thin film has more than two times faster polarization switching speed than the conventional composition-uniform one. Meanwhile, it has excellent ferroelectricity and improved endurance characteristics. It is also discovered that when the HZO thin film has a gradient composition, the polarization-switching dynamics shifts from the nucleation-limited-switching mechanism to the domain-wall growth mechanism. Moreover, the transition of switching dynamics is responsible for the faster speed and better endurance of the composition-graded HZO thin film. These findings not only reveal the physical mechanisms of this material system but also provide a new strategy for memory devices having faster speed and higher endurance.