Molecular-Beam Epitaxy of Two-Dimensional In2Se3 and Its Giant Electroresistance Switching in Ferroresistive Memory Junction

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

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

Room temperature ferroelectricity and an electrically tunable Berry curvature dipole in III-V monolayers

Two-dimensional ferroelectric monolayers are promising candidates for compact memory devices and flexible electronics. Here, through first-principles calculations, we predict room temperature ferroelectricity in AB-type monolayers comprising group III (A = Al, In, Ga) and group V (B = As, P, Sb) elements. We show that their spontaneous polarization, oriented out-of-plane, ranges from 9.48 to 13.96 pC m-1, outperforming most known 2D ferroelectrics. We demonstrate an electric field tunable Berry curvature dipole and nonlinear Hall current in these monolayers. Additionally, we highlight their applicability in next-generation memory devices by forming efficient ferroelectric tunnel junctions, especially in InP, which supports high tunneling electroresistance. Our findings motivate further exploration of these monolayers for studying the interplay between the Berry curvature and ferroelectricity and for integrating these ferroelectric monolayers in next-generation electronic devices.

Growth of Quasi-Two-Dimensional CrTe Nanoflakes and CrTe/Transition Metal Dichalcogenide Heterostructures

Two-dimensional (2D) van der Waals layered materials have been explored in depth. They can be vertically stacked into a 2D heterostructure and represent a fundamental way to explore new physical properties and fabricate high-performance nanodevices. However, the controllable and scaled growth of non-layered quasi-2D materials and their heterostructures is still a great challenge. Here, we report a selective two-step growth method for high-quality single crystalline CrTe/WSe2 and CrTe/MoS2 heterostructures by adopting a universal CVD strategy with the assistance of molten salt and mass control. Quasi-2D metallic CrTe was grown on pre-deposited 2D transition metal dichalcogenides (TMDC) under relatively low temperatures. A 2D CrTe/TMDC heterostructure was established to explore the interface's structure using scanning transmission electron microscopy (STEM), and also demonstrate ferromagnetism in a metal-semiconductor CrTe/TMDC heterostructure.

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.

High tunneling electroresistance in ferroelectric tunnel junctions based on two-dimensional α-In2Se3/MoTe2 van der Waals heterostructures

Ferroelectric polarization-controlled band alignment can be realized in van der Waals heterostructures (vdWHs), which can be used to create new types of ferroelectric tunnel junctions (FTJs). In this work, we design six probable configurations of two-dimensional vdWHs based on a two-dimensional α-In2Se3 ferroelectric material which has two opposite polarization states P↑ and P↓, and the semiconductor MoTe2. First-principles calculations show robust ferroelectric polarization-controlled switching behavior between the high conductance state in configuration AA-P↓ and the low conductance state in configuration AA-P↑ in the most stable AA stacked vdWHs. Based on this vdWH, a two-dimensional transverse FTJ with AA-P↓ or AA-P↑ as the tunneling barrier and (In0.5Sn0.5)2Se3 monolayers (n-type doped) as electrodes is designed. The tunneling electroresistance ratio of the FTJs at the Fermi level reaches 1.22 × 104% when the tunneling barrier contains two repeating units N = 2 and can be greatly increased by increasing the thickness of the ferroelectric layer. Analysis of the work function, charge redistribution, and local density of states is performed to interpret the above phenomena. The findings suggest the great potential of the AA stacked α-In2Se3/MoTe2 vdWH in the design of high-performance FTJs and application in high-density non-volatile memory devices.

A Metal-Free Molecular Ferroelectric [4-Me-cyclohexylamine]ClO4 Introduced by Boat and Chair Conformations of Cyclohexylamine

Organic ferroelectrics have received a great deal of interest due to their exclusive properties. However, organic ferroelectrics have not been fully explored, which hinders their practical application. Here, we presented a novel metal-free organic molecular ferroelectric [4-MCHA][ClO4 ] (1) (4-MCHA=trans-4-methylcyclohexylamine), which exhibits an above-room-temperature of 328 K. Strikingly, the single crystal structure analysis of 1 shows that the driving force of phase transition is related to the interesting chair-boat conformation change of 4-MCHA cation, in addition to the order-disorder transition of ClO4 - anion. Using piezoelectric response force microscopy (PFM), the presence of domains and the implemented polarization switching were clearly observed, which explicitly determined the presence of room-temperature ferroelectricity of 1. As far as we know, the ferroelectric phase transition mechanism attributed to the conformational change in a trans isomeric cation is very rare. This research enriched the path of designing ferroelectric materials and smart materials.

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

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

Tetra-Coordinated W2 S3 for Efficient Dual-pH Hydrogen Production

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising catalysts for the hydrogen evolution reaction (HER) that play a crucial role in renewable energy technologies. Breaking the inherent structural paradigm limitations of 2D TMDs is the key to exploring their fascinating physical and chemical properties, which is expected to develop a revolutionary HER catalyst. Herein, we unambiguously present metallic W2 S3 instead of energetically favorable WS2 via a unique stoichiometric growth strategy. Benefiting from the excellent conductivity and hydrophilicity of the tetra-coordinated structure, as well as an appropriate Gibbs free energy value and an enough low energy barrier for water dissociation, the W2 S3 as catalyst achieves Pt-like HER activity and high long-term stability in both acidic and alkaline electrolytes. For application in proton exchange membrane (PEM) and anion exchange membrane (AEM) electrolysers, W2 S3 as the cathode catalyst yields excellent bifunctionality index (ɳ@ 1 A cm - 2 , PEM ${_{{\rm{@1 {\rm A} cm}}^{{\rm{ - }}{\rm{2}}} {\rm{, PEM}}} }$ =1.73 V, ɳ@ 1 A cm - 2 , AEM ${_{{\rm{@1 {\rm A} cm}}^{{\rm{ - }}{\rm{2}}} {\rm{, AEM}}} }$ =1.77 V) and long-term stability (471 h@PEM with a decay rate of 85.7 μV h-1 , 360 h@AEM with a decay rate of 27.1 μV h-1 ). Our work provides significant insight into the tetra-coordinated W2 S3 and facilitates the development of advanced electrocatalysts for sustainable hydrogen production.

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

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

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.

Ferroic Phases in Two-Dimensional Materials

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

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Erasable Domain Wall Current-Dominated Resistive Switching in BiFeO3 Devices with an Oxide-Metal Interface

Electric transport in the charged domain wall (CDW) region has emerged as a promising phenomenon for the development of next-generation ferro-resistive memory with ultrahigh data storage density. However, accurately measuring the conductivity of CDWs induced by polarization reversal remains challenging due to the polarization modulation of the Schottky barrier at the thin film-electrode interface, which could partially contribute to the collected "on" current of the device. Here, we propose carefully selecting an electrode that can suppress the effect of interfacial barrier modulation induced by polarization reversal, allowing the collected current mainly from the conductive CDWs. The experiment was conducted on epitaxial BiFeO₃(001) thin-film devices with vertical and horizontal geometries. Piezo-response force microscopy scanning showed the local polarization experienced 180° rotation to form CDWs under the vertical electric field. However, devices with SrRuO₃ epitaxial top electrodes still exhibit an interfacial barrier-dominated diode behavior, with the "on" current proportional to the electrode area. To identify the CDW current, more interfacial defects were introduced by the deposition of Pt top electrodes, which significantly enhanced charge injection for the compensation of the reversed polarization driven by the electric field, leading to the suppressed polarization modulation of the Schottky barrier height. It was observed that the current flow through Pt electrodes is significantly lower compared to that of SRO electrodes and appears to be primarily influenced by the electrode perimeter instead of the electrode area, indicating CDW-dominated conduction behavior in these devices. Planar nanodevices were further fabricated to support the quantitative investigation of the Pt electrode size-dependent "on" current with a linear fit of the current magnitude versus the CDW cross-sectional area. This work constitutes an essential part of understanding the role of the CDW current in ferro-resistive memory devices.

Periodic Ferroelectric Stripe Domains in α-In2Se3 Nanoflakes Grown via Reverse-Flow Chemical Vapor Deposition

The two-dimensional (2D) layered semiconductor α-In₂Se₃ has aroused great interest in atomic-scale ferroelectric transistors, artificial synapses, and nonvolatile memory devices due to its distinguished 2D ferroelectric properties. We have synthesized α-In₂Se₃ nanosheets with rare in-plane ferroelectric stripe domains at room temperature on mica substrates using a reverse flow chemical vapor deposition (RFCVD) method and optimized growth parameters. This stripe domain contrast is found to be strongly correlated to the stacking of layers, and the interrelated out-of-plane (OOP) and in-plane (IP) polarization can be manipulated by mapping the artificial domain structure. The acquisition of amplitude and phase hysteresis loops confirms the OOP polarization ferroelectric property. The emergence of striped domains enriches the variety of the ferroelectric structure types and novel properties of 2D In₂Se₃. This work paves a new way for the controllable growth of van der Waals ferroelectrics and facilitates the development of novel ferroelectric memory device applications.

Depolarization induced III-V triatomic layers with tristable polarization states

The integration of ferroelectrics that exhibit high dielectric, piezoelectric, and thermal compatibility with the mainstream semiconductor industry will enable novel device types for widespread applications, and yet there are few silicon-compatible ferroelectrics suitable for device downscaling. We demonstrate with first-principles calculations that the enhanced depolarization field at the nanoscale can be utilized to soften unswitchable wurtzite III-V semiconductors, resulting in ultrathin two-dimensional (2D) sheets possessing reversible polarization states. A 2D sheet of AlSb consisting of three atomic planes is identified to host both ferroelectricity and antiferroelectricity, and the tristate switching is accompanied by a metal-semiconductor transition. The thermodynamic stability and potential synthesizability of the triatomic layer are corroborated with phonon spectrum calculations, ab initio molecular dynamics simulations, and variable-composition evolutionary structure search. We propose a 2D AlSb-based homojunction field effect transistor that supports three distinct and nonvolatile resistance states. This new class of III-V semiconductor-derived 2D materials with dual ferroelectricity and antiferroelectricity opens up the opportunity for nonvolatile multibit-based integrated nanoelectronics.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

Recent progress in the theoretical design of two-dimensional ferroelectric materials

Two-dimensional (2D) ferroelectrics (FEs), which maintain stable electric polarization in ultrathin films, are a promising class of materials for the development of various miniature functional devices. In recent years, several 2D FEs with unique properties have been successfully fabricated through experiments. They have been found to exhibit some unique properties either by themselves or when they are coupled with other functional materials (e.g., ferromagnetic materials, materials with 5d electrons, etc.). As a result, several new types of 2D FE functional devices have been developed, exhibiting excellent performance. As a type of newly discovered 2D functional material, the number of 2D FEs and the exploration of their properties are still limited and this calls for further theoretical predictions. This review summarizes recent progress in the theoretical predictions of 2D FE materials and provides strategies for the rational design of 2D FE materials. The aim of this review is to provide guidelines for the design of 2D FE materials and related functional devices.

Towards two-dimensional van der Waals ferroelectrics

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

Highly degenerate 2D ferroelectricity in pore decorated covalent/metal organic frameworks

Two-dimensional (2D) ferroelectricity, a fundamental concept in low-dimensional physics, serves as the basis of non-volatile information storage and various electronic devices. Conventional 2D ferroelectric (FE) materials are usually two-fold degenerate, meaning that they can only store two logical states. In order to break such limitation, a new concept of highly degenerate ferroelectricity with multiple FE states (more than 2) coexisting in a single 2D material is proposed. This is obtained through the asymmetrical decoration of porous covalent/metal organic frameworks (COFs/MOFs). Using first-principles calculations and Monte Carlo (MC) simulations, Li-decorated 2D Cr(pyz)₂ is systematically explored as a prototype of highly degenerate 2D FE materials. We show that 2D FE Li_0.5Cr(pyz)₂ and LiCr(pyz)₂ are four-fold and eight-fold degenerate, respectively, with sizable spontaneous electric polarization that can be switched across low transition barriers. In particular, the coupling between neighbouring electric dipoles in LiCr(pyz)₂ induces novel ferroelectricity-controlled ferroelastic transition and direction-controllable hole transport channels. Moreover, three-fold and six-fold degenerate ferroelectricity is also demonstrated in P-decorated g-C₃N₄ and Ru-decorated C₂N, respectively. Our work presents a general route to obtain highly degenerate 2D ferroelectricity, which goes beyond the two-state paradigm of traditional 2D FE materials and substantially broadens the applications of 2D FE compounds.

Epitaxial Growth of Large Area Two-Dimensional Ferroelectric α-In2Se3

Two-dimensional (2D) ferroelectric materials have attracted intensive attention in recent years for academic research. However, the synthesis of large-scale 2D ferroelectric materials for electronic applications is still challenging. Here, we report the successful synthesis of centimeter-scale ferroelectric In₂Se₃ films by selenization of In₂O₃ in a confined space chemical vapor deposition method. The as-grown homogeneous thin film has a uniform thickness of 5 nm with robust out-of-plane ferroelectricity at room temperature. Scanning transmission electron microscopy and Raman spectroscopy reveal that the thin film is 2H stacking α-In₂Se₃ with excellent crystalline quality. Electronic transport measurements of In₂Se₃ highlight the current-voltage hysteresis and polarization modulated diode effect due to the switchable Schottky barrier height (SBH). First-principles calculations reveal that the polarization modulated SBH is originated from the competition between interface charge transfer and polarized charge. The large area growth of epitaxial In₂Se₃ opens up potential applications of In₂Se₃ in novel nanoelectronics.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

Oscillatory Order-Disorder Transition during Layer-by-Layer Growth of Indium Selenide

It is important to understand the polymorph transition and crystal-amorphous phase transition in In₂Se₃ to tap the potential of this material for resistive memory storage. By monitoring layer-by-layer growth of β-In₂Se₃ during molecular beam epitaxy (MBE), we are able to identify a cyclical order-disorder transition characterized by a periodic alternation between a glassy-like metastable subunit cell film consisting of n < 5 sublayers (nth layers = the number of subunit cell layers), and a highly crystalline β-In₂Se₃ at n = 5 layers. The glassy phase shows an odd-even alternation between the indium-cluster layer (n = 1, 3) and an In-Se solid solution (n = 2, 4), which suggests the ability of In and Se atoms to diffuse, aggregate, and intermix. These dynamic natures of In and Se atoms contribute to a defect-driven memory resistive behavior in current-voltage sweeps that is different from the ferroelectric switching of α-In₂Se₃.

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.

UV-light-assisted gas sensor based on PdSe2/InSe heterojunction for ppb-level NO2 sensing at room temperature

The fabrication of van der Waals (vdWs) heterostructures mainly extends to two-dimensional (2D) materials. Nevertheless, the current processes for obtaining high-quality 2D films are mainly exfoliated from their bulk counterparts or by high-temperature chemical vapor deposition (CVD), which limits industrial production and is often accompanied by defects. Herein, we first fabricated the type-II p-PdSe₂/n-InSe vdWs heterostructure using the ultra-high vacuum laser molecular beam epitaxy (LMBE) technique combined with the vertical 2D stacking strategy, which is reproducible and suitable for high-volume manufacturing. This work found that the introduction of 365 nm UV light illumination can significantly improve the electrical transport properties and NO₂ sensing performance of the PdSe₂/InSe heterojunction-based device at room temperature (RT). The detailed studies confirm that the sensor based on the PdSe₂/InSe heterojunction delivers the comparable sensitivity (R_a/R_g = ∼2.6 at 10 ppm), a low limit of detection of 52 ppb, and excellent selectivity for NO₂ gas under UV light illumination, indicating great potential for NO₂ detection. Notably, the sensor possesses fast response and full recovery properties (275/1078 s) compared to the results in the dark. Furthermore, the mechanism of enhanced gas sensitivity was proposed based on the energy band alignment of the PdSe₂/InSe heterojunction with the assistance of investigating the surface potential variations. This work may pave the way for the development of high-performance, room-temperature gas sensors based on 2D vdWs heterostructures through the LMBE technique.

Structural Phase Transitions between Layered Indium Selenide for Integrated Photonic Memory

The primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy-hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In₂ Se₃ ) triggered by a single nanosecond pulse is demonstrated. The high-resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer "shear glide" and isosymmetric phase transition, switching between the α- and β-structural states contains low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular-beam-epitaxy-grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3 GHz, introduced by a 1.5 µm-long In₂ Se₃ -covered layer, resulted from the combinations of material absorption and scattering.

A non-destructive and efficient transfer method for preparing 2D materials samples for transmission electron microscopy study

Studying two-dimensional (2D) materials using transmission electron microscopy (TEM) is necessary and very important in many aspects. However, some 2D materials are not resistant to acids or alkalis, which are widely used in normal wet transfer techniques to transfer the exfoliated 2D nanosheets onto the TEM grids. On the other hand, dry stamping method can damage the holey carbon film on the TEM grids. In this article, we present a non-destructive, efficient, and widely applicable transfer method for preparing the TEM samples of the exfoliated 2D materials. Our method only uses the heat-release tape, PMMA, and blue Nitto tape. Neither acid nor alkali is involved in our method, therefore, impurities and damage can be avoided to the greatest extent. The method is also very efficient and can be accomplished in less than 30 min after the exfoliation of the 2D materials. This method is particularly useful for preparing the TEM samples of the 2D materials that are not resistant to acids and alkalis. The present method is also applicable to various 2D materials and various substrates.

Temperature-Driven α-β Phase Transformation and Enhanced Electronic Property of 2H α-In2Se3

In recent years, thin layered indium selenide (In2Se3) has attracted rapidly increasing attention due to its fascinating properties and promising applications. Here, we report the temperature-driven α-β phase transformation and the enhanced electronic property of 2H α-In2Se3. We find that 2H α-In2Se3 transforms to β-In2Se3 when it is heated to a high temperature, and the transformation temperature increases from 550 to 650 K with the thickness decreasing from 67 to 17 nm. Additionally, annealing the sample below the phase transformation temperature can effectively improve the electronic property of a 2H α-In2Se3 field-effect transistor, including increasing the on-state current, decreasing the off-state current, and improving the subthreshold swing. After annealing, not only the contact resistance decreases significantly but also the mobility at 300 K increases more than 2 times to 45.83 cm2 V-1 s-1, which is the highest among the reported values. Our results provide an effective method to improve the electrical property and the stability of the In2Se3 nanodevices.

Giant Bandgap Engineering in Two-Dimensional Ferroelectric α-In2Se3

Bandgap engineering is an efficient strategy for controlling the physical properties of semiconductor materials. For flexible two-dimensional (2D) materials, strain provides a nondestructive and adjustable method for bandgap adjustment. Here, we propose that, in 2D materials with out-of-plane ferroelectricity, the antibonding nature of the valence band maximum and conduction band minimum and polarized charge distribution induced by ferroelectricity give rise to giant changes of the bandgap under curvature strain field. This hypothesis was proven by scanning tunneling microscopy/spectroscopy measurements on monolayer α-In₂Se₃ that revealed that the bandgap of α-In₂Se₃ increases significantly due to bending. Both experiments and theoretical calculations indicated that the bandgap increases monotonically with the degree of bending of the α-In₂Se₃ layer. Our work suggests that bending is an effective method for tuning the gaps of 2D ferroelectric materials, providing a new platform for bandgap engineering under the combination of ferroelectricity and strain field.

Growth of 2D Materials at the Wafer Scale

Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered 2D materials in high-end electronics and optoelectronics. Most vdW 2D materials are initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. This review briefly introduces several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer scale are reviewed in depth. In particular, three strategies are summarized that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy is reviewed. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

A van der Waals Ferroelectric Tunnel Junction for Ultrahigh-Temperature Operation Memory

Facing the constant scaling down and thus increasingly severe self-heating effect, developing ultrathin and heat-insensitive ferroelectric devices is essential for future electronics. However, conventional ultrathin ferroelectrics and most 2D ferroelectric materials (2DFMs) are not suitable for high-temperature operation due to their low Curie temperature. Here, by using few-layer α-In₂ Se₃ , a special 2DFM with high Curie temperature, van der Waals (vdW) ferroelectric tunnel junction (FTJ) memories that deliver outstanding and reliable performance at both room and high temperatures are constructed. The vdW FTJs offer a large on/off ratio of 10⁴ at room temperature and still reveal excellent on/off ratio at an ultrahigh temperature of 470 K, which will fail down other 2DFMs. Moreover, long retention and reliable cyclic endurance at high temperature are achieved, showing robust thermal stability of the vdW FTJ memory. The observations of this work demonstrate an exciting promise of α-In₂ Se₃ for reliable service in high temperature either from self-heating or harsh environments.

Magnetic anisotropy and ferroelectric-driven magnetic phase transition in monolayer Cr2Ge2Te6

Monolayer Cr₂Ge₂Te₆ (ML-CGT) has attracted broad interest due to its novel electronic and magnetic properties. However, there are still controversies on the origin of its intrinsic magnetism. Here, by exploring the electronic and magnetic properties of ML-CGT, we find that the magnetic shape anisotropy (MSA) is vital for establishing the long-range ferromagnetism, except for the contribution from magnetocrystalline anisotropy energy (MCA). Electronic band analysis, combined with atomic- and orbital-resolved magnetic anisotropy from a second-order perturbation theory, further reveals that the MCA of ML-CGT is mainly originated from hybridized Te-p_y and -p_z orbitals. The MSA from magnetic Cr atoms in ML-CGT is larger than MCA, resulting in an in-plane magnetic anisotropy. Noticeably, by constructing a heterostructure (HTS) with ferroelectric Sc₂CO₂, CGT undergoes an in-plane to out-of-plane spin reorientation via ferroelectric polarization switching, accompanied with an electronic property transition from semiconductor to half-metal. The Curie temperature of CGT/Sc₂CO₂ HTS can be enhanced to 92.4 K under the ferroelectric polarization, which is much higher than that of pristine ML-CGT (34.7 K). These results not only clarify the contradiction of magnetic mechanism of ML-CGT in previous experimental and theoretical works, but also open the door for realizing nonvolatile magnetic memory devices based on a multifunctional ferromagnetic/ferroelectric HTS.

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

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

Electric-Field-Induced Room-Temperature Antiferroelectric-Ferroelectric Phase Transition in van der Waals Layered GeSe

Searching van der Waals ferroic materials that can work under ambient conditions is of critical importance for developing ferroic devices at the two-dimensional limit. Here we report the experimental discovery of electric-field-induced reversible antiferroelectric (AFE) to ferroelectric (FE) transition at room temperature in van der Waals layered α-GeSe, employing Raman spectroscopy, transmission electron microscopy, second-harmonic generation, and piezoelectric force microscopy consolidated by first-principles calculations. An orientation-dependent AFE-FE transition provides strong evidence that the in-plane (IP) polarization vector aligns along the armchair rather than zigzag direction in α-GeSe. In addition, temperature-dependent Raman spectra showed that the IP polarization could sustain up to higher than 700 K. Our findings suggest that α-GeSe, which is also a potential ferrovalley material, could be a robust building block for creating artificial 2D multiferroics at room temperature.

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.

Two-dimensional multiferroics

Due to unprecedented application prospects such as high-density and low-power multistate storage, spintronics and nanoelectronics, two-dimensional (2D) multiferroics, coupled with at least two ferroic orders, have gotten a lot of interest in recent years. Multiple functions can be achieved in 2D multiferroics via coupling phenomena such as magnetoelectricity, piezoelectricity, and magnetoelasticity, which offers technical support for the creation of multifunctional devices. The research progress of 2D ferromagnetic-ferroelectric multiferroic materials, ferroelectric-ferroelastic multiferroic materials, and ferromagnetic-ferroelastic materials in recent years is reviewed in this paper. The categorization of 2D multiferroics is explored in terms of the multiple sources of ferroelectricity. The top-down approaches and the bottom-up methods used to fabricate 2D multiferroics materials are introduced. Finally, the authors outline potential research prospects and application scenarios for 2D multiferroic materials.

Two-dimensional ferroelectric metal for electrocatalysis

The coexistence of metallicity and ferroelectricity has been an intriguing and controversial phenomenon as these two material properties are considered incompatible in bulk. We clarify the concept of the ferroelectric metal by revisiting the original definitions for ferroelectric and metal. Two-dimensional (2D) ferroelectrics with out-of-plane polarization can be engineered via layer stacking to a genuine ferroelectric metal characterized by switchable polarization and non-zero density of states at the Fermi level. We demonstrate that 2D ferroelectric metals can serve as electrically-tunable, high-quality electrocatalysts.

Controllable Distribution and Reversible Migration of Charges in BiFeO3-Based Films on Si Substrates

In ferroelectric-based integrated devices, there are usually buffer layers between ferroelectric films and semiconductor substrates. Here, Bi_x%FeO_3-δ (x = 95, 100, and 105) (BFOx) films are prepared directly on n-Si substrates by the sol-gel method, and the variation of the hysteresis loops with Bi content and heat treatment is investigated. With the help of the dielectric measurement and the composition analysis, a PN heterojunction is believed to exist at the BFOx/Si interface. The Bi/Fe ratio determines not only the type and concentration of charged defects in the films but also the height of the interface barrier and its binding effect on mobile charges. Furthermore, the distribution and the migration of charges can be regulated reversibly by heat treatment. This work reveals the interaction between ferroelectric films and semiconductor substrates, providing an important reference for the design and application of ferroelectric/semiconductor heterostructures.

Nonvolatile magnetoelectric coupling in two-dimensional ferromagnetic-bilayer/ferroelectric van der Waals heterostructures

One of the promising research topics on two-dimensional (2D) van der Waals (vdW) material based devices is the nonvolatile electrical control of magnetism. Usually, it is very hard to tune ferromagnetic or antiferromagnetic ordering by ferroelectric polarization due to strong exchange coupling. The existence of vdW layer spacing, however, which is ubiquitous in 2D materials, makes interlayer magnetic exchange coupling much weaker than interlayer coupling. In this work, we design a multiferroic heterostructure composed of a CrOBr ferromagnetic bilayer and an In₂Se₃ ferroelectric monolayer. The weaker interlayer exchange coupling of the CrOBr bilayer makes it easier to be regulated by ferroelectric polarization, enabling reversible nonvolatile electric control of shifts between ferromagnetic and antiferromagnetic ordering. The unique electrically controlled interlayer magnetic coupling for tuning the overall magnetism may be available for the practical application of 2D vdW bilayer magnets in high-sensitivity sensors and high-density data storage.

A two-dimensional multiferroic metal with voltage-tunable magnetization and metallicity

We design a multiferroic metal that combines seemingly incompatible ferromagnetism, ferroelectricity, and metallicity by hole doping a two-dimensional (2D) ferroelectric with high density of states near the Fermi level. The strong magnetoelectric effect is demonstrated in hole-doped and arsenic-doped monolayer α-In₂Se₃ using first-principles calculations. Taking advantage of the oppositely charged surfaces created by an out-of-plane polarization, the 2D magnetization and metallicity can be electrically switched on and off in an asymmetrically doped monolayer. The substitutional arsenic defect pair exhibits an intriguing electric field-tunable charge disproportionation process accompanied by an on-off switch of local magnetic moments. The charge ordering process can be controlled by tuning the relative strength of on-site Coulomb repulsion and defect dipole-polarization coupling via strain engineering. Our design principle relying on no transition metal broadens the materials design space for 2D multiferroic metals.

A review of ultra-thin ferroelectric films

Ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly non-volatile memories and field-effect transistors. With the rapid development of micro-electronics technology, there is an urgent requirement for higher density electronic devices, which need ultra-thin ferroelectric materials films. However, as ferroelectric films have becomes thinner and thinner, electrical spontaneous polarization signals have been found in a few atomic layers or even monolayer structures. The mechanisms of detection and formation of these signals are not well understood and various controversial interpretations have emerged. In this review, we summarized the recent research progress in the ultra-thin film ferroelectric material, such as HfO₂, CuInP₂S₆, In₂Se₃, MoTe₂and BaTiO₃. Various key aspects of ferroelectric materials are discussed, including crystal structure, ferroelectric mechanism, characterization, fabrication methods, applications, and future outlooks. We hope this review will offer ideas for further improvement of ferroelectric properties of ultra-thin films and promotes practical applications.

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.

Emerging 2D Memory Devices for In-Memory Computing

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Out-of-plane and in-plane ferroelectricity of atom-thick two-dimensional InSe

Two-dimensional (2D) ferroelectric materials are promising substitutes of three-dimensional perovskite based ferroelectric ceramic materials. Yet most studies have been focused on the construction of non-centrosymmetric 2D van der Waals materials and only a few are constructed experimentally. Herein, we experimentally demonstrate the co-existence of voltage-tunable out-of-plane (OOP) and in-plane (IP) ferroelectricity in few-layer InSe prepared by a solution-processable method and fabricate ferroelectric semiconductor channel transistors. The reversible polarization can initiate instant switch of resistance with high ON/OFF ratios and a comparable subthreshold swing of 160 mV/dec under gate modulation. The origins of such unique OOP and IP ferroelectricity of the centrosymmetric structure are theoretically analyzed.

Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas

Atomically thin indium selenide (InSe) is a representative two-dimensional (2D) family that have recently attracted extensive interest for their intriguing emerging physics and potential optoelectronic applications with high-performance. Here, by utilizing molecular beam epitaxy and scanning tunneling microscopy, we report a controlled synthesis of InSe thin films down to the monolayer limit and characterization of their electronic properties at atomic scale. Highly versatile growth conditions are developed to fabricate well crystalline InSe films, with a reversible and controllable phase transformation between InSe and In₂Se₃. The band gap size of InSe films, as enhanced by quantum confinement, increases with decreasing film thickness. Near various categories of lattice imperfections, the band gap becomes significantly enlarged, resulting in a type-I band alignments for lateral heterojunctions. Such band gap enhancement, as unveiled from our first-principles calculations, is ascribed to the local compressive strain imposed by the lattice imperfections. Moreover, InSe films host highly conductive 2D electron gas, manifesting prominent quasiparticle scattering signatures. The 2D electron gas is self-formed via substrate doping of electrons, which shift the Fermi level above the confinement-quantized conduction band. Our study identifies InSe ultrathin film as an appealing system for both fundamental research and potential applications in nanoelectrics and optoelectronics.

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

Atomically thin two-dimensional (2D) van der Waals materials have exhibited many exotic layer-dependent physical properties including electronic structure, magnetic order, etc. Here, we report a striking even-odd layer dependent oscillation in the ferroelectric polarization of 2H-stacked few-layer α-In₂Se₃ nanoflakes. As characterized by piezoresponse force microscopy (PFM), when the in-plane (IP) electric polarization of 2H-stacked α-In₂Se₃ films is electrically aligned, the out-of-plane (OOP) polarization of the odd-layer (OL) samples is obviously larger than that of the even-layer (EL) ones. Similarly, samples with electrically aligned OOP polarization also show even-odd layer dependent IP polarization. Such an even-odd oscillation, as confirmed by the density functional theory calculations, can be attributed to the strong intercorrelation of the IP and OOP electric polarization of the α-In₂Se₃ monolayers and the special 2H-stacking structure of a 180 degree IP rotation with respect to the adjacent layers. Moreover, a negative differential resistance, interestingly, is induced by the polarization flip with a small coercive field of ∼1.625 kV cm^-1, and its peak-to-valley ratio can be tuned up to ∼7 by the gate. This work demonstrates that the delicate stacking geometry of multilayer α-In₂Se₃ can bring an interesting even-odd ferroelectric effect, enriching the layer-dependent physical properties of the 2D materials family.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

Asymmetric Metal/α-In2Se3/Si Crossbar Ferroelectric Semiconductor Junction

A ferroelectric semiconductor junction is a promising two-terminal ferroelectric device for nonvolatile memory and neuromorphic computing applications. In this work, we propose and report the experimental demonstration of asymmetric metal/α-In₂Se₃/Si crossbar ferroelectric semiconductor junctions (c-FSJs). The depletion in doped Si is used to enhance the modulation of the effective Schottky barrier height through the ferroelectric polarization. A high-performance α-In₂Se₃ c-FSJ is achieved with a high on/off ratio > 10⁴ at room temperature, on/off ratio > 10³ at an elevated temperature of 140 °C, retention > 10⁴ s, and endurance > 10⁶ cycles. The on/off ratio of the α-In₂Se₃ asymmetric FSJs can be further enhanced to >10⁸ by introducing a metal/α-In₂Se₃/insulator/metal structure.

Strain-Modulated Photoelectric Responses from a Flexible α-In2Se3/3R MoS2 Heterojunction

Semiconducting piezoelectric α-In₂Se₃ and 3R MoS₂ have attracted tremendous attention due to their unique electronic properties. Artificial van der Waals (vdWs) heterostructures constructed with α-In₂Se₃ and 3R MoS₂ flakes have shown promising applications in optoelectronics and photocatalysis. Here, we present the first flexible α-In₂Se₃/3R MoS₂ vdWs p-n heterojunction devices for photodetection from the visible to near infrared region. These heterojunction devices exhibit an ultrahigh photoresponsivity of 2.9 × 10³ A W^-1 and a substantial specific detectivity of 6.2 × 10¹⁰ Jones under a compressive strain of - 0.26%. The photocurrent can be increased by 64% under a tensile strain of + 0.35%, due to the heterojunction energy band modulation by piezoelectric polarization charges at the heterojunction interface. This work demonstrates a feasible approach to enhancement of α-In₂Se₃/3R MoS₂ photoelectric response through an appropriate mechanical stimulus.