Direct measurement of ferroelectric polarization in a tunable semimetal

2D Van der Waals Sliding Ferroelectrics Toward Novel Electronic Devices

Ferroelectric materials, celebrated for their switchable polarization, have undergone significant evolution since their early discovery in Rochelle salt. Initial challenges, including water solubility and brittleness, are overcome with the development of perovskite ferroelectrics, which enable the creation of stable, high-quality thin films suitable for semiconductor applications. As the demand for miniaturization in nanoelectronics has increased, research has shifted toward low-dimensional materials. Traditional ferroelectrics often lose their properties at the nanoscale; however, 2D van der Waals (vdW) ferroelectrics, including CuInP2S6 and α-In2Se3, have emerged as promising alternatives. The recent discovery of sliding ferroelectricity, where polarization is linked to the polar stacking configuration of originally non-polar monolayers, has significantly broadened the scope of 2D ferroelectrics. This review offers a comprehensive examination of stacking orders in 2D vdW materials, stacking-order-linked ferroelectric polarization structures, and their manifestations in metallic, insulating and semiconducting 2D vdW materials. Additionally, it explores the applications of 2D vdW sliding ferroelectrics, and discusses the future prospects in nanotechnology.

Piezoelectric Nanoarrays with Mechanical-Electrical Coupling Microenvironment for Innervated Bone Regeneration

The involvement of neurons in the peripheral nervous system is crucial for bone regeneration. Mimicking extracellular matrix cues provides a more direct and effective strategy to regulate neuronal activity and enhance bone regeneration. However, the simultaneous coupling of the intrinsic mechanical-electrical microenvironment of implants to regulate innervated bone regeneration has been largely neglected. Inspired by the mechanical and bioelectric properties of the bone microenvironment, this study constructed a mechanical-electrical coupling microenvironment (M-E) model based on barium titanate piezoelectric nanoarrays, which could effectively promote innervated bone regeneration. The study found that the mechanical microenvironment provided by the nanostructure, coupled with the electrical microenvironment provided by the piezoelectric properties, created a controllable M-E. In vitro cell experiments demonstrated that this coupled microenvironment activated Piezo2 and VGCC ion channels, promoted calcium influx in DRG neurons, and activated downstream PI3K-AKT and RAS pathways. This cascade of events led to the synthesis and release of CGRP in sensory nerves, ultimately enhancing the osteogenic differentiation of BMSCs. This work not only broadens the current understanding of biomaterials that mimic the bone extracellular matrix but also provides new insights into innervated bone regeneration.

Ferroelectricity with concomitant Coulomb screening in van der Waals heterostructures

Interfacial ferroelectricity emerges in non-centrosymmetric heterostructures consisting of non-polar van der Waals (vdW) layers. Ferroelectricity with concomitant Coulomb screening can switch topological currents or superconductivity and simulate synaptic response. So far, it has only been realized in bilayer graphene moiré superlattices, posing stringent requirements to constituent materials and twist angles. Here we report ferroelectricity with concomitant Coulomb screening in different vdW heterostructures free of moiré interfaces containing monolayer graphene, boron nitride (BN) and transition metal chalcogenide layers. We observe a ferroelectric hysteretic response in a BN/monolayer graphene/BN, as well as in BN/WSe2/monolayer graphene/WSe2/BN heterostructure, but also when replacing the stacking fault-containing BN with multilayer twisted MoS2, a prototypical sliding ferroelectric. Our control experiments exclude alternative mechanisms, such that we conclude that ferroelectricity originates from stacking faults in the BN flakes. Hysteretic switching occurs when a conductive ferroelectric screens the gating field electrically and controls the monolayer graphene through its polarization field. Our results relax some of the material and design constraints for device applications based on sliding ferroelectricity and should enable memory device or the combination with diverse vdW materials with superconducting, topological or magnetic properties.

Superconductivity from Domain Wall Fluctuations in Sliding Ferroelectrics

Bilayers of two-dimensional van der Waals materials that lack an inversion center can show a novel form of ferroelectricity, where certain stacking arrangements of the two layers lead to an interlayer polarization. Under an external out-of-plane electric field, a relative sliding between the two layers can occur, accompanied by an interlayer charge transfer and a ferroelectric switching. We show that the domain walls that mediate ferroelectric switching are a locus of strong attractive interactions between electrons. The attraction is mediated by the ferroelectric domain wall fluctuations, effectively driven by the soft interlayer shear phonon. We comment on the possible relevance of this attraction mechanism to the recent observation of an interplay between sliding ferroelectricity and superconductivity in bilayer T_{d}-MoTe_{2}. We also discuss the possible role of this mechanism in the superconductivity of moiré bilayers.

Reversible Tuning Electrical Properties in Ferroelectric SnS with NH3 Adsorption and Desorption

Two-dimensional (2D) ferroelectrics usually exhibit instability or a tendency toward degradation when exposed to the ambient atmosphere, and the mechanism behind this phenomenon remains unclear. To unravel this affection mechanism, we have undertaken an investigation utilizing NH3 and two-dimensional ferroelectric SnS. Herein, the adsorption and desorption of NH3 molecules can reversibly modulate the electrical properties of SnS, encompassing I-V curves and transfer curves. The response time for NH3 adsorption is approximately 1.12 s, which is much quicker than that observed in other two-dimensional materials. KPFM characterizations indicate that air molecules' adsorption alters the surface potentials of SiO2, SnS, metal electrodes, and contacts with minimal impact on the electrode contact surface potential. Upon the adsorption of NH3 molecules or air molecules, the hole concentration within the device decreases. These findings elucidate the adsorption mechanism of NH3 molecules on SnS, potentially fostering the advancement of rapid gas sensing applications utilizing two-dimensional ferroelectrics.

Negative Capacitance Field Effect Transistors based on Van der Waals 2D Materials

Steep subthreshold swing (SS) is a decisive index for low energy consumption devices. However, the SS of conventional field effect transistors (FETs) has suffered from Boltzmann Tyranny, which limits the scaling of SS to sub-60 mV dec-1 at room temperature. Ferroelectric gate stack with negative capacitance (NC) is proved to reduce the SS effectively by the amplification of the gate voltage. With the application of 2D ferroelectric materials, the NC FETs can be further improved in performance and downscaled to a smaller dimension as well. This review introduces some related concepts for in-depth understanding of NC FETs, including the NC, internal gate voltage, SS, negative drain-induced barrier lowering, negative differential resistance, single-domain state, and multi-domain state. Meanwhile, this work summarizes the recent advances of the 2D NC FETs. Moreover, the electrical characteristics of some high-performance NC FETs are expressed as well. The factors which affect the performance of the 2D NC FETs are also presented in this paper. Finally, this work gives a brief summary and outlook for the 2D NC FETs.

Spontaneous Inversion Symmetry Breaking and Emergence of Berry Curvature and Orbital Magnetization in Topological ZrTe_{5} Films

ZrTe_{5} has recently attracted much attention due to the observation of intriguing nonreciprocal transport responses which necessitate the lack of inversion symmetry (I). However, there has been debate on the exact I-asymmetric structure and the underlying I-breaking mechanism. Here, we report a spontaneous I breaking in ZrTe_{5} films, which initiates from interlayer sliding and is stabilized by subtle intralayer distortion. Moreover, we predict significant nonlinear anomalous Hall effect (NAHE) and kinetic magnetoelectric effect (KME), which are attributed to the emergence of Berry curvature and orbital magnetization in the absence of I symmetry. We also explicitly manifest the direct coupling between sliding ferroelectricity, NAHE, and KME based on a sliding-dependent k·p model. By studying the subsurface sliding in ZrTe_{5} multilayers, we speculate that surface nonlinear Hall current and magnetization would emerge on the natural cleavage surface. Our findings elucidate the sliding-induced I-broken mechanism in ZrTe_{5} films and open new avenues for tuning nonreciprocal transport properties in Van der Waals layered materials.

Tunnel junctions based on interfacial two dimensional ferroelectrics

Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements. We observe well-pronounced ambipolar switching behaviour in ferroelectric tunnelling junctions with composite ferroelectric/non-polar insulator barriers and support our experimental results with complementary theoretical modelling. Furthermore, we show that the switching behaviour is strongly influenced by the underlying domain structure, allowing the fabrication of diverse ferroelectric tunnelling junction devices with various functionalities. We show that to observe the polarisation reversal, at least one partial dislocation must be present in the device area. This behaviour is drastically different from that of conventional ferroelectric materials, and its understanding is an important milestone for the future development of optoelectronic devices based on sliding ferroelectricity.

Quantum-Geometric Origin of Out-of-Plane Stacking Ferroelectricity

Stacking ferroelectricity (SFE) has been discovered in a wide range of van der Waals materials and holds promise for applications, including photovoltaics and high-density memory devices. We show that the microscopic origin of out-of-plane stacking ferroelectric polarization can be generally understood as a consequence of a nontrivial Berry phase borne out of an effective Su-Schrieffer-Heeger model description with broken sublattice symmetry, thus elucidating the quantum-geometric origin of polarization in the extremely nonperiodic bilayer limit. Our theory applies to known stacking ferroelectrics such as bilayer transition-metal dichalcogenides in 3R and T_{d} phases, as well as general AB-stacked honeycomb bilayers with staggered sublattice potential. Our explanatory and self-consistent framework based on the quantum-geometric perspective establishes quantitative understanding of out-of-plane SFE materials beyond symmetry principles.

Ion adsorption promotes Frank-van der Merwe growth of 2D transition metal tellurides

Reliable synthesis methods for high-quality, large-sized, and uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) are crucial for their device applications. However, versatile approaches to growing high-quality, large-sized, and uniform 2D transition-metal tellurides are rare. Here, we demonstrate an ion adsorption strategy that facilitates the Frank-van der Merwe growth of 2D transition-metal tellurides. By employing this method, we grow MoTe2 and WTe2 with enhanced lateral size, reduced thickness, and improved uniformity. Comprehensive characterizations confirm the high quality of as-grown MoTe2. Moreover, various characterizations verify the adsorption of K+ and Cl- ions on the top surface of MoTe2. X-ray photoelectron spectroscopy (XPS) analysis reveals that the MoTe2 is stoichiometric without K+ and Cl- ions and exhibits no discernable oxidation after washing. This top surface control strategy provides a new controlling knob to optimize the growth of 2D transition-metal tellurides and holds the potential for generalized to other 2D materials.

Stacking engineering in layered homostructures: transitioning from 2D to 3D architectures

Artificial materials, characterized by their distinctive properties and customized functionalities, occupy a central role in a wide range of applications including electronics, spintronics, optoelectronics, catalysis, and energy storage. The emergence of atomically thin two-dimensional (2D) materials has driven the creation of artificial heterostructures, harnessing the potential of combining various 2D building blocks with complementary properties through the art of stacking engineering. The promising outcomes achieved for heterostructures have spurred an inquisitive exploration of homostructures, where identical 2D layers are precisely stacked. This perspective primarily focuses on the field of stacking engineering within layered homostructures, where precise control over translational or rotational degrees of freedom between vertically stacked planes or layers is paramount. In particular, we provide an overview of recent advancements in the stacking engineering applied to 2D homostructures. Additionally, we will shed light on research endeavors venturing into three-dimensional (3D) structures, which allow us to proactively address the limitations associated with artificial 2D homostructures. We anticipate that the breakthroughs in stacking engineering in 3D materials will provide valuable insights into the mechanisms governing stacking effects. Such advancements have the potential to unlock the full capability of artificial layered homostructures, propelling the future development of materials, physics, and device applications.

Nanotube Slidetronics

One-dimensional slidetronics is predicted for double-walled boron-nitride nanotubes. Local electrostatic polarization patterns along the body of the nanotube are found to be determined by the nature of the two nanotube walls, their relative configuration, and circumferential faceting modulation during coaxial interwall sliding. By careful choice of chiral indices, chiral polarization patterns can emerge that spiral around the nanotube circumference. The potential usage of the discovered slidetronic effect for low-dimensional nanogenerators is briefly discussed.

Domain nucleation kinetics and polarization-texture-dependent electronic properties in two-dimensional α-In2Se3 ferroelectrics

Two-dimensional (2D) ferroelectric semiconductors, such as α-In2Se3 with switchable spontaneous polarization and superior optoelectronic properties, exhibit large potential for functional device applications. The electric transport properties and device performance of 2D α-In2Se3 are strongly sensitive to the ferroelectric domain structures and polarization textures, but they are rarely explored at the atomic scale. Herein, by a combination of first-principles calculations and a developed domain switching theory, we report the domain nucleation kinetics and polarization-texture dependent electronic properties in α-In2Se3 ferroelectrics. Our calculated results reveal that the reversed domains characterized by armchair boundaries tend to form triangular or stripped shape. The energy barrier for propagating domain boundaries is ∼1.42 eV and can be reduced by loading external electric field, which is responsible for driving the evolution of domain structures. Moreover, the domain switching leads to notable changes in the band gap and carrier spatial distribution of α-In2Se3 monolayer, resulting in higher electric resistance of multi-polarization domain structures than that of single-polarization state. The domain structures of multilayer α-In2Se3 follow a layer-by-layer switching mechanism, which causes the transition of electronic structures from self-doped p-n junctions to type-II semiconductor homojunctions. This study not only provides an in-depth insight into the domain switching mechanisms of α-In2Se3 but also opens up the possibility to tailor their electronic and transport properties.

Orbital multiferroicity in pentalayer rhombohedral graphene

Ferroic orders describe spontaneous polarization of spin, charge and lattice degrees of freedom in materials. Materials exhibiting multiple ferroic orders, known as multiferroics, have important parts in multifunctional electrical and magnetic device applications1-4. Two-dimensional materials with honeycomb lattices offer opportunities to engineer unconventional multiferroicity, in which the ferroic orders are driven purely by the orbital degrees of freedom and not by electron spin. These include ferro-valleytricity corresponding to the electron valley5 and ferro-orbital-magnetism6 supported by quantum geometric effects. These orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements and electric control of the valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low-temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tanΘH > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E, we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite field E · B (where B is the magnetic field), but not to the individual fields. Tuning E would switch each ferroic order independently and achieve non-volatile switching of them together. Our observations demonstrate a previously unknown type of multiferroics and point to electrically tunable ultralow-power valleytronic and magnetic devices.

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.

Non-volatile control of topological phase transition in an asymmetric ferroelectric In2Te2S monolayer

The coupling of topological electronic states and ferroelectricity is highly desired due to their abundant physical phenomenon and potential applications in multifunctional devices. However, it is difficult to achieve such a phenomenon in a single ferroelectric (FE) monolayer because the two polarized states are topologically equivalent. Here, we demonstrate that the symmetry of polarized states can be broken by constructing a Janus structure in a FE monolayer. We illustrate such a general idea by replacing a layer of Te atoms in the In2Te3 monolayer with S atoms. Using first-principles calculations, we show that the In2Te2S monolayer has two asymmetric polarized states, which are characterized by a metal and semiconductor, respectively. Importantly, as the spin-orbit coupling is included, a band gap (50.4 meV) is created in the metallic state, resulting in a non-trivial topological phase. Thus, it proves to be a feasible method to engineer non-volatile FE control of topological order in a single-phase system. We also demonstrate the underlying physical mechanism of topological phase transition, which is unveiled to be related to the weakened intrinsic electric field resulting from charge transfer. These interesting results provide a general way to design asymmetric FE materials and shed light on their potential application in non-volatile multifunctional devices.

Atypical Sliding and Moiré Ferroelectricity in Pure Multilayer Graphene

Most nonferroelectric two-dimensional materials can be endowed with so-called sliding ferroelectricity via nonequivalent homobilayer stacking, which is not applicable to monoelement systems like pure graphene bilayer with inversion symmetry at any sliding vector. Herein, we show first-principles evidence that multilayer graphene with N>3 can all be ferroelectric, where the polarizations of polar states stem from the symmetry breaking in stacking configurations of across layer instead of adjacent layer, which are electrically switchable via interlayer sliding. The nonpolar states can also be electrically driven to polar states via sliding, and more diverse states with distinct polarizations will emerge in more layers. In contrast to the ferroelectric moiré domains with opposite polarization directions in twisted bilayers reported previously, the moiré pattern in some multilayer graphene systems (e.g., twisted monolayer-trilayer graphene) possess nonzero net polarizations with domains of the same direction separated by nonpolar regions, which can be electrically reversed upon interlayer sliding. The distinct moiré bands of two polar states should facilitate electrical detection of such sliding moiré ferroelectricity during switching.

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.

Enabling Room-Temperature Triferroic Coupling in Dual Transition-Metal Dichalcogenide Monolayers Via Electronic Asymmetry

Triferroic compounds are the ideal platform for multistate information devices but are rare in the two-dimensional (2D) form, and none of them can maintain macroscopic order at room temperature. Herein, we propose a general strategy for achieving 2D triferroicity by imposing electric polarization into a ferroelastic magnet. Accordingly, dual transition-metal dichalcogenides, for example, 1T'-CrCoS₄, are demonstrated to display room-temperature triferroicity. The magnetic order of 1T'-CrCoS₄ undergoes a magnetic transition during the ferroic switching, indicating robust triferroic magnetoelectric coupling. In addition, the negative out-of-plane piezoelectricity and strain-tunable magnetic anisotropy make the 1T'-CrCoS₄ monolayer a strong candidate for practical applications. Following the proposed scheme, a new class of 2D room-temperature triferroic materials is introduced, providing a promising platform for advanced spintronics.

Coupled ferroelectricity and superconductivity in bilayer Td-MoTe2

Achieving electrostatic control of quantum phases is at the frontier of condensed matter research. Recent investigations have revealed superconductivity tunable by electrostatic doping in twisted graphene heterostructures and in two-dimensional semimetals such as WTe₂ (refs. ^1-5). Some of these systems have a polar crystal structure that gives rise to ferroelectricity, in which the interlayer polarization exhibits bistability driven by external electric fields^6-8. Here we show that bilayer T_d-MoTe₂ simultaneously exhibits ferroelectric switching and superconductivity. Notably, a field-driven, first-order superconductor-to-normal transition is observed at its ferroelectric transition. Bilayer T_d-MoTe₂ also has a maximum in its superconducting transition temperature (T_c) as a function of carrier density and temperature, allowing independent control of the superconducting state as a function of both doping and polarization. We find that the maximum T_c is concomitant with compensated electron and hole carrier densities and vanishes when one of the Fermi pockets disappears with doping. We argue that this unusual polarization-sensitive two-dimensional superconductor is driven by an interband pairing interaction associated with nearly nested electron and hole Fermi pockets.

Interlayer Registry Dictates Interfacial 2D Material Ferroelectricity

We discover that the complex ferroelectric response of layered materials toward interlayer sliding is fully dictated by the interlayer lattice registry. Importantly, the entire sliding polarization landscape of two-dimensional (2D) layered material interfaces is fully described via a simple and intuitive geometric measure, termed the polarization registry index (PRI), that quantifies the degree of interlayer commensurability. Beyond the understanding of the fundamental origin of 2D ferroelectricity, the developed tool also provides highly efficient characterization and rationalization of existing experimental and computational evidence of 2D interfacial ferroelectricity, as well as the prediction of emergent controllable polarization in new noncentrosymmetric layered systems.

Cumulative polarization in conductive interfacial ferroelectrics

Ferroelectricity in atomically thin bilayer structures has been recently predicted1 and measured2-4 in two-dimensional materials with hexagonal non-centrosymmetric unit-cells. The crystal symmetry translates lateral shifts between parallel two-dimensional layers to sign changes in their out-of-plane electric polarization, a mechanism termed 'slide-tronics'4. These observations have been restricted to switching between only two polarization states under low charge carrier densities5-12, limiting the practical application of the revealed phenomena13. To overcome these issues, one should explore the nature of polarization in multi-layered van der Waals stacks, how it is governed by intra- and interlayer charge redistribution and to what extent it survives the addition of mobile charge carriers14. To explore these questions, we conduct surface potential measurements of parallel WSe2 and MoS2 multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields that provide a means to design multi-state 'ladder-ferroelectrics'. Furthermore, we find that the internal polarization remains notable on electrostatic doping of mobile charge carrier densities as high as 1013 cm-2, with substantial in-plane conductivity. Using density functional theory calculations, we trace the extra charge redistribution in real and momentum spaces and identify an eventual doping-induced depolarization mechanism.

Spatially Resolved Polarization Manipulation of Ferroelectricity in Twisted hBN

Robust room-temperature interfacial ferroelectricity has been formed in the 2D limit by simply twisting two atomic layers of non-ferroelectric hexagonal boron nitride (hBN). A thorough understanding of this newly discovered ferroelectric system is required. Here, twisted hBN is used as a tunneling junction and it is studied at the nanometer scale using conductive atomic force microscopy. Three properties unique to this system are discovered. First, the polarization dependence of the tunneling resistance contrasts with the conventional theory. Second, the ferroelectric domains can be controlled using mechanical stress, highlighting the original meaning of the emergent "slidetronics". Third, ferroelectric hysteresis is highly spatially dependent. The hysteresis is symmetric at the domain walls. A few nanometers away, the hysteresis shifts completely to the positive or negative side, depending on the original polarization. These findings reveal the unconventional ferroelectricity in this 2D system.

Steep-slope transistors enabled with 2D quantum coupling stacks

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS₂transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS₂devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe₂/MoS₂devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

Giant ferroelectric polarization in a bilayer graphene heterostructure

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 10¹³cm⁻², which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m⁻¹ is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena.

Interfacial ferroelectricity may emerge in moiré superlattices. Here, the authors find that the polarized charge is much larger than the capacity of the moiré miniband and the associated anomalous screening exists outside the band.

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.

Interfacial ferroelectricity in marginally twisted 2D semiconductors

Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of new metamaterials. Here we demonstrate a room temperature ferroelectric semiconductor that is assembled using mono- or few-layer MoS₂. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarization arranged into a twist-controlled network. The last can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The observed interfacial charge transfer, movement of domain walls and their bending rigidity agree well with theoretical calculations. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential avenue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.

Interfacial ferroelectricity in rhombohedral-stacked bilayer transition metal dichalcogenides

van der Waals materials have greatly expanded our design space of heterostructures by allowing individual layers to be stacked at non-equilibrium configurations, for example via control of the twist angle. Such heterostructures not only combine characteristics of the individual building blocks, but can also exhibit physical properties absent in the parent compounds through interlayer interactions¹. Here we report on a new family of nanometre-thick, two-dimensional (2D) ferroelectric semiconductors, where the individual constituents are well-studied non-ferroelectric monolayer transition metal dichalcogenides (TMDs), namely WSe₂, MoSe₂, WS₂ and MoS₂. By stacking two identical monolayer TMDs in parallel, we obtain electrically switchable rhombohedral-stacking configurations, with out-of-plane polarization that is flipped by in-plane sliding motion. Fabricating nearly parallel-stacked bilayers enables the visualization of moiré ferroelectric domains as well as electric field-induced domain wall motion with piezoelectric force microscopy. Furthermore, by using a nearby graphene electronic sensor in a ferroelectric field transistor geometry, we quantify the ferroelectric built-in interlayer potential, in good agreement with first-principles calculations. The new semiconducting ferroelectric properties of these four new TMDs opens up the possibility of studying the interplay between ferroelectricity and their rich electric and optical properties^2-5.

Identifying the Transition Order in an Artificial Ferroelectric van der Waals Heterostructure

Two-dimensional semiconducting ferroelectrics can enable new technology for low-energy electronic switching. However, conventional ferroelectric materials are usually electrically insulating and suffer from severe depolarization effects when downscaled to atomic thickness. Following recent work, we show that robust ferroelectricity can be obtained from nonferroelectric semiconducting 2H-WSe₂ by creating R-stacked bilayers with broken inversion symmetry. Here, we identify that the phase transition order of this artificial ferroelectric heterostructure is first-order, with a discontinuous jump in the order parameter across the phase transition temperature. The Curie temperature has been experimentally determined as 353 K. Using the Landau-Devonshire theory, we further determine the Curie-Weiss temperatures to be 351.2 K. We additionally demonstrate the robustness of this artificial ferroelectric material using consecutive polarization measurements, where no appreciable deterioration was detected.