Abstract 摘要
In recent years, an increasing number of 2D van der Waals (vdW) materials are theory-predicted or laboratory-validated to possess in-plane (IP) and/or out-of-plane (OOP) spontaneous ferroelectric polarization. Due to their dangling-bond-free surfaces, interlayer charge coupling, robust polarization, tunable energy band structures, and compatibility with silicon-based technologies, vdW ferroelectric materials exhibit great promise in ferroelectric memories, neuromorphic computing, nanogenerators, photovoltaic devices, spintronic devices, and so on. Here, the very recent advances in the field of vdW ferroelectrics (FEs) are reviewed. First, theories of ferroelectricity are briefly discussed. Then, a comprehensive summary of the non-stacking vdW ferroelectric materials is provided based on their crystal structures and the emerging sliding ferroelectrics. In addition, their potential applications in various branches/frontier fields are enumerated, with a focus on artificial intelligence. Finally, the challenges and development prospects of vdW ferroelectrics are discussed.
近年来,越来越多的二维范德华(vdW)材料被理论预测或实验室验证具有面内(IP)和/或面外(OOP)自发铁电极化。由于 vdW 铁电材料的无悬键表面、层间电荷耦合、强极化、可调能带结构以及与硅基技术的兼容性,它们在铁电存储器、神经形态计算、纳米发电机、光伏器件、自旋电子器件等方面展现出巨大的应用前景。在此,我们将对 vdW 铁电(FEs)领域的最新进展进行综述。首先,简要讨论了铁电理论。然后,根据非堆叠 vdW 铁电材料的晶体结构和新出现的滑动铁电材料,对它们进行了全面总结。此外,还列举了它们在各个分支/前沿领域的潜在应用,重点是人工智能。最后,讨论了 vdW 铁电材料面临的挑战和发展前景。
1 Introduction 1 引言
Ferroelectrics (FEs) are materials having spontaneous electric polarization states that can be reversed by the application of an external electric field. The earliest research on ferroelectrics began in 1921 when Valasek J observed an electric hysteresis loop similar to the hysteresis loop in Rochelle salt.[1] Later, with the discovery of BaTiO3[2] and PbTiO3-PbZrO3,[3] FE materials have been widely studied. With the development of miniaturization and integration of FE components, the thickness of FE film is required to be as thin as possible. However, the depolarization field becomes stronger as the FE film becomes thinner, leading to the disappearance of FE characteristics once the thickness is below a critical size. Although, after decades of development, FE polarization could be preserved in traditional FEs with a limit thickness of down to several unit cells (2.4 nm for BaTiO3[4] and 1.2 nm for PbTiO3[5]) even a single unit cell (1 unit cell for BiFeO3[6] and 1.5 unit cell for PbZr0.2Ti0.8O3[7]), it is necessary to have realistic ferroelectric-electrode interfaces with sufficient carrier densities to eliminate the depolarizing field or a suitable substrate to provide a compressive strain that can bring 180° stripe domains to stabilize the ferroelectric phase. In addition, lattice-matched interfaces are mandatory to ensure good-enough performances when fabricating functional devices, and preparation processes (like the annealing temperatures) with high compatibility with the standard silicon-based technologies are strongly required. Those requirements increase the difficulty of preparation and integration and seriously restrict their applications in modern nano-electronic and nano-optoelectronic devices.
铁电体(FE)是一种具有自发电极化状态的材料,施加外部电场可使其极化状态逆转。最早的铁电研究始于 1921 年,当时 Valasek J 观察到一种类似于罗谢尔盐磁滞回线的电磁滞回线。1后来,随着BaTiO32和PbTiO3-PbZrO3 的发现,3铁电材料被广泛研究。随着 FE 元件微型化和集成化的发展,要求 FE 薄膜的厚度越薄越好。然而,FE 膜越薄,去极化场就越强,一旦厚度低于临界尺寸,FE 特性就会消失。虽然经过数十年的发展,传统 FE 的极限厚度可低至几个单元格(BaTiO34为 2.4 nm,PbTiO35 为 1.2 nm),甚至单个单元格(BiFeO36为 1 个单元格,PbZr0.2Ti0 为 1.5 个单元格),但 FE 极化仍能保持。8O37),因此必须要有逼真的铁电-电极界面,同时要有足够的载流子密度来消除去极化场,或者要有合适的衬底来提供压缩应变,从而产生 180° 的条纹畴来稳定铁电相。此外,在制造功能器件时,为了确保足够好的性能,必须要有晶格匹配的界面,并且强烈要求制备过程(如退火温度)与标准硅基技术高度兼容。这些要求增加了制备和集成的难度,严重限制了它们在现代纳米电子和纳米光电器件中的应用。
The discovery of vdW FE materials may, to a large extent, solve the above problems.[8, 9] The atoms inside the inner layers interact with each other through strong chemical bonds, while the adjacent layers are connected by relatively weak van der Waals forces. There is no dangling bond on their surface, which reduces the adsorption of impurity atoms and is beneficial for generating neat heterogeneous interfaces. So far, room-temperature (RT) ferroelectric polarization switching in few-layer thickness, even in a monolayer limit, has been experimentally observed in a variety of vdW FEs, such as CuInP2S6 (CIPS),[10] In2Se3,[11,12] monolayer SnS,[13] and 1T-MoTe2.[14] These 2D vdW FE materials not only possess the properties that traditional FEs have, such as stable spontaneous polarization and strong nonlinearity but also have some unique advantages, including wide-range adjustability of bandgaps and nontrivial quantum topological properties. These characteristics make vdW FEs a platform for high-density integrated electronic and optoelectronic devices and emerging novel applications like spintronics.[15
8,9内层原子通过强化学键相互作用,而相邻层之间则通过相对较弱的范德华力连接。其表面没有悬空键,从而减少了杂质原子的吸附,有利于生成整齐的异质界面。迄今为止,已在多种 vdW FE(如CuInP2S6(CIPS)、10 In2Se3、[11,12单层SnS13和1T-MoTe2 等)中实验观察到几层厚度(甚至单层极限)的室温(RT)铁电极化转换。14这些二维 vdW FE 材料不仅具有传统 FE 所具有的特性,如稳定的自发极化和强非线性,而且还具有一些独特的优势,包括带隙的大范围可调性和非三维量子拓扑特性。这些特性使 vdW FE 成为高密度集成电子和光电器件以及自旋电子学等新兴应用的平台。]
Traditionally, the essence of ferroelectricity lies in an intrinsic lattice symmetry breaking. Among 20 types of piezoelectric point groups without symmetric centers, crystals of 10 types of point groups exhibit pyroelectric effects. Ferroelectrics are a subclass of pyroelectrics, and they must belong to the 10 point groups. This also applies to some vdW FEs, such as black phosphorus (BP), black phosphorus analogs MX (M = Ge, Sn; X = S, Se, Te),[16, 17] α-In2Se3,[18] and β-InSe.[19] A typical example is the distorted monolayer 1T-MoS2, where a trimer configuration of Mo atoms breaks the lattice symmetry, resulting in ferroelectricity.[20, 21
传统上,铁电性的本质在于内在晶格对称性的破坏。在 20 种没有对称中心的压电点群中,有 10 种点群的晶体表现出热释电效应。铁电是热释电的一个子类,它们必须属于这 10 种点群。这也适用于一些 vdW FE,如黑磷 (BP)、黑磷类似物 MX(M = Ge、Sn;X = S、Se、Te)16、17α-In2Se3[18]和β-InSe[19]。一个典型的例子是扭曲单层 1T-MoS2,其中 Mo 原子的三聚体构型打破了晶格对称性,从而产生了铁电性。]
However, vdW materials with centrosymmetric lattice structures, such as graphene (Gr), hexagonal boron nitride (h-BN), and transition-metal dichalcogenides (TMDs), can be ferroelectric by vdW stacking, which is named as sliding FEs. Here, ferroelectricity is induced through interlayer sliding between two adjacent layers of vdW materials with centrosymmetric structures. Sliding FEs were first proposed theoretically in bilayer and multilayer h-BN with specific stacking architectures by first-principles calculation in 2017.[22] Subsequently, several direct experimental evidence was reported in bilayer and trilayer WTe2,[23] β-InSe,[19] AB/BA stacking h-BN,[24, 25] twisted monolayer/multilayer graphene,[26] twisted MoS2,[27] and MoS2/WS2 heterobilayers.[28] Interlayer sliding FEs exhibit novel physical properties, such as ferroelectric metal,[22] Moiré ferroelectric domain,[25, 26] ferroelectric nonlinear anomalous hall effect,[29] unconventional multiferroic couplings,[30] and ferroelectric spin-textured,[31] which may provide a platform to realize new functions, such as a fatigueless data-writing and reading strategy for ultra-high-density and low-heat-dissipation electronic devices.
然而,具有中心对称晶格结构的 vdW 材料,如石墨烯(Gr)、六方氮化硼(h-BN)和过渡金属二卤化物(TMD),可以通过 vdW 堆叠实现铁电性,这被命名为滑动铁电。在这里,铁电性是通过具有中心对称结构的相邻两层 vdW 材料之间的层间滑动诱导产生的。随后,在双层和三层WTe2、[23]β-InSe、19AB/BA 堆垛 h-BN、24,25扭曲的单层/多层石墨烯、26扭曲的MoS2、[27]MoS2/WS2异质层中,也报道了一些直接的实验证据。28层间滑动 FE 具有新颖的物理性质,如铁电金属、22Moiré 铁电畴、25,26铁电非线性反常霍尔效应、29非常规多铁氧体耦合30和铁电自旋纹理31等,可为实现新功能提供平台,如用于超高密度和低热耗散电子器件的无疲劳数据写入和读取策略。
Recent breakthroughs in vdW FEs, particularly in sliding FEs, are impressive. In the review written by Wang et al.,[15] 2D FE materials were summarized according to the origin of spontaneous polarization, and their unique properties due to reduced lattice dimensionality and promising applications were also discussed. Xue et al.[32] summarized the vdW FEs mainly from two aspects of the unique properties and the novel devices. Wu et al.[33] focused on the theoretical models, experimental investigations, related physics, and potential applications of sliding ferroelectrics. Here, we present a comprehensive overview of vdW FEs. First, we briefly discuss the modern ferroelectricity theories and their crucial role in the calculation and prediction of vdW ferroelectrics. Further, we systematically introduce the discovery, origin, and latest progress of vdW FEs. In particular, we provide a detailed summary of the development of sliding ferroelectrics from both the theoretical and experimental aspects. Then, vdW FEs-based devices are discussed, including electronic memories, optoelectronic memories, topology memories, neuromorphic computing, and ferroelectric negative capacitance field effect transistors. Finally, the existing major challenges as well as the prospects are outlined.
最近在 vdW FE,尤其是滑动 FE 方面取得的突破令人印象深刻。Wang 等人撰写的综述15根据自发极化的起源总结了二维 FE 材料,并讨论了它们因晶格尺寸减小而具有的独特性质和应用前景。Xue 等人32主要从独特性质和新型器件两个方面对 vdW FE 进行了总结。Wu 等人33重点介绍了滑动铁电的理论模型、实验研究、相关物理和潜在应用。在此,我们将对 vdW 铁电进行全面概述。首先,我们简要讨论了现代铁电理论及其在 vdW 铁电计算和预测中的关键作用。然后,我们系统地介绍了 vdW 铁电的发现、起源和最新进展。特别是,我们从理论和实验两方面详细总结了滑动铁电的发展。然后,讨论了基于 vdW FEs 的器件,包括电子存储器、光电存储器、拓扑存储器、神经形态计算和铁电负电容场效应晶体管。最后,概述了现有的主要挑战和前景。
2 Theories of Van der Waals Ferroelectrics
2 范德华铁电理论
Since the discovery of FE materials, several phenomenological or quantitative concepts, including Landau theory[34] and soft mode theory,[35] have been used to describe and compare their properties, which are also suitable for vdW ferroelectric systems. Some crucial indicators are used to describe the ferroelectricity of the materials, commonly including the hysteresis loop, Curie temperature (Tc), space-inversion symmetry breaking, electric dipole, spontaneous polarization, dielectric susceptibility, double well model, and transition energy barrier, among others. The hysteresis loop in the polarization-electric field curve is an essential indicator for judging whether a material is ferroelectric or not. Tc refers to the phase transition temperature between the paraelectric phase and the ferroelectric phase. Spontaneous polarization and dielectric susceptibility are used to compare the strength of ferroelectric polarization. These quantitative concepts make it possible to compare experimental data with theoretical results intuitively, enabling us to have a deeper understanding of the origin of ferroelectricity. It is necessary to comprehend the mechanism from both macroscopic and microscopic perspectives to guide the design of materials and devices.
自发现铁电材料以来,包括朗道理论34和软模式理论35在内的一些现象学或定量概念被用来描述和比较它们的特性,这些概念也适用于 vdW 铁电系统。一些关键指标被用来描述材料的铁电性,通常包括滞后环、居里温度(Tc)、空间反转对称破缺、电偶极子、自发极化、介电感应强度、双井模型和过渡能障等。极化-电场曲线中的滞后环是判断材料是否为铁电体的重要指标。Tc指的是介电相和铁电相之间的相变温度。自发极化和介电感应强度用于比较铁电极化的强度。这些定量概念可以直观地比较实验数据和理论结果,使我们能够更深入地了解铁电的起源。我们有必要从宏观和微观两个角度来理解其机理,以指导材料和器件的设计。
Landau theory serves as a conceptual bridge between observed macroscopic phenomena and microscopic models and has been successfully used in describing the phenomenological behavior of phase transitions in superconductors and ferromagnets. As a macroscopical thermodynamic theory, the basic principle of Landau theory is to expand the free energy into a Taylor series of the polarization parameters and establish the relationship between the expansion coefficients and the macroscopic measurable parameters. Therefore, it can be used to predict macroscopic measurable quantities with only a few system parameters. Landau–Devonshire theory has been an important theoretical tool for studying the phenomena of first- and second-order phase transitions in bulk FEs. Landau–Ginzburg theory with boundary conditions and nonuniform polarization has been developed to study thin films and multilayer ferroelectrics.
朗道理论是观察到的宏观现象与微观模型之间的概念桥梁,已成功用于描述超导体和铁磁体中相变的现象学行为。作为一种宏观热力学理论,朗道理论的基本原理是将自由能扩展为极化参数的泰勒级数,并建立扩展系数与宏观可测量参数之间的关系。因此,它只需几个系统参数就能用于预测宏观可测量量。朗道-德文郡理论是研究块状 FE 中一阶和二阶相变现象的重要理论工具。兰道-金兹堡理论具有边界条件和非均匀极化,已被用于研究薄膜和多层铁电体。
As shown in Figure 1, free energy, F, determines the equilibrium stability of a uniform system and can be expressed as a function of spontaneous polarization P,
如图1 所示,自由能F 决定了均匀系统的平衡稳定性,可表示为自发极化P 的函数,

铁电极化理论。
In 1940, H. S. Allen discovered that the α-β phase transition of Quartz was accompanied by a decrease in the frequency of a completely symmetric optical phonon mode.[36] Cochran and Anderson formally proposed the soft mode theory of ferroelectric phase transition.[37] This microscopic theory is commonly used to describe and forecast ferroelectricity, where spontaneous polarization is related to the tiny displacement of an atom vibrating non-harmonically near the equilibrium position. The basic concept of this theory is that the generation of ferroelectricity is related to the softening of a certain transverse optical mode in the center of the Brillouin zone. The freezing of the soft mode eigenvectors leads to the static displacement of the atoms, which contributes to spontaneous polarization. Soft mode theory plays an important role in predicting 2D vdW FEs.
1940 年,H. S. Allen 发现石英的α-β相变伴随着一个完全对称的光学声子模式频率的降低。36Cochran 和 Anderson 正式提出了铁电相变的软模式理论。37这一微观理论通常用于描述和预测铁电性,其中自发极化与平衡位置附近非谐振 动原子的微小位移有关。该理论的基本概念是:铁电性的产生与布里渊区中心的某种横向光学模式的软化有关。软模式特征向量的冻结会导致原子的静态位移,从而产生自发极化。软模式理论在预测二维 vdW FE 方面发挥着重要作用。
The contemporary theory of electric polarization is constructed based on the knowledge of the Berry phase.[38] The electric polarization vector
当代电极化理论是基于贝里相的知识构建的。38电极化矢量
3 Van der Waals Ferroelectric Materials
3 范德瓦耳斯铁电材料
In the past few years, various vdW materials and their homo/heterostructures have been reported to exhibit robust ferroelectricity. In this section, we will summarize the vdW ferroelectric materials that have been either theoretically predicted or experimentally verified. Generally speaking, vdW FE materials can be divided into two categories. The first is from the broken symmetry of the intrinsic crystalline units, which is similar to traditional FEs. The second originates from the special stacking orders between vdW layers where the individual building blocks can even be non-polar, and the polarization switchings are strongly related to the in-plane slidings of the adjacent vdW layers. To distinguish these two types, we call them non-stacking vdW ferroelectrics and sliding vdW ferroelectrics, respectively. As a palingenetic concept and research topic, sliding vdW ferroelectrics will be particularly discussed. The content covers their crystalline structures, the origins of ferroelectric phase transitions, and the methods of verifying ferroelectricity.
在过去几年中,有报道称各种 vdW 材料及其同/异质结构表现出强大的铁电性。在本节中,我们将总结已被理论预测或实验验证的 vdW 铁电材料。一般来说,vdW 铁电材料可分为两类。第一类源于固有晶体单元的对称性被打破,这与传统的铁电材料类似。第二类源于 vdW 层之间的特殊堆积顺序,其中单个构件甚至可以是非极性的,而极化切换与相邻 vdW 层的面内滑动密切相关。为了区分这两种类型,我们将它们分别称为非堆叠 vdW 铁电和滑动 vdW 铁电。作为一个物理概念和研究课题,我们将特别讨论滑动 vdW 铁电。内容包括它们的晶体结构、铁电相变的起源以及验证铁电性的方法。
3.1 Non-stacking Van der Waals Ferroelectrics
3.1 非堆积范德华铁电体
3.1.1 CuInP2S6 Series
3.1.1CuInP2S6系列
The ferroelectric ordering in lamellar CuInP2S6 with a polarization value of 3.01 µC cm−2 at RT was predicted dating back to 1997.[46] In 2015, Belianinov[47] reported the out-of-plane ferroelectricity in 100 nm thick CuInP2S6 at RT through piezoresponse force microscopy (PFM). In 2016, Liu et al.[10] observed stable ferroelectricity at RT even in 4 nm CuInP2S6 by analyzing phase and amplitude curves of PFM. They also determined that the ferroelectric transition temperature of CuInP2S6 is above 320 K. The OOP ferroelectricity of CIPS originates from the asymmetric Cu and In ion displacement.[39] Each CIPS monolayer has a closely packed octahedral framework formed by S anions, Cu, In cations, and P-P pairs that occupy the ionic sites close to the triangular S face, forming an alternating arrangement of CuS6 octahedra, InS6 octahedra, and P2S6 octahedra networks. Both Cu and In cations can displace vertically in opposite directions within each layer, leading to ferroelectricity along the OOP direction. At temperatures > ≈320 K (Tc), Cu ions are easier to move and can occupy both intralayer and interlayer positions, resulting in a disordered state.[48] Locally controlled Cu ion transport in CIPS was revealed with high reversibility by a biased scanning probe microscopy tip.[49] Furthermore, when a slight ionic modification is performed by an applied field, the piezoelectric response of CIPS is enhanced by ≈45%. In 2020, in-plane ferroelectricity in thin CIPS flakes was visualized by a PFM technique. The in-plane ferroelectricity comes from the structural phase transition from monoclinic (Cc phase) to trigonal (P31c phase) revealed by density functional theory (DFT) calculation and transmission electron microscopy (TEM) measurements.[40
462015 年,Belianinov47通过压电响应力显微镜(PFM)报道了 100nm厚的CuInP2S6在 RT 时的面外铁电性。2016 年,Liu 等人10通过分析压电显微镜的相位和振幅曲线,观察到 4 nm 厚的CuInP2S6在 RT 时也具有稳定的铁电性。CIPS 的 OOP 铁电性源于不对称的Cu和 In 离子位移。39每个 CIPS 单层都有一个由 S 阴离子、Cu、In 阳离子和 P-P 对形成的紧密排列的八面体框架,它们占据靠近三角形 S 面的离子位点,形成CuS6八面体、InS6八面体和P2S6八面体网络的交替排列。在每一层中,Cu 和 In 阳离子都能向相反方向垂直位移,从而导致沿 OOP 方向的铁电性。 在温度大于 ≈320 K(Tc)时,铜离子更容易移动,并可占据层内和层间位置,从而形成无序态48。偏置扫描探针显微镜尖端揭示了 CIPS 中局部受控的铜离子传输,且具有高度可逆性49。2020 年,CIPS 薄片中的面内铁电现象通过 PFM 技术得以显现。密度泛函理论(DFT)计算和透射电子显微镜(TEM)测量显示,面内铁电性来自于从单斜(Cc 相)到三棱(P31c 相)的结构相变。40]
Very interestingly, it has been revealed that CIPS possesses a negative longitudinal piezoelectric coefficient, which was only found in poly(vinylidene fluoride).[50, 51] Due to the high moving ability of Cu ions into the vdW gap, CIPS is featured with a uniaxial quadruple potential well with two energy minima in each polarization direction along the z-axis, corresponding to two distinct polarization phases, as revealed by DFT calculations, quantum molecular dynamics simulations, and PFM measurements.[51] The first phase corresponds to the Cu ions migration within the layer, has low polarization (±4.93 µC cm−2), and is very sensitive to strain, resulting in a negative longitudinal piezoelectric coefficient. While the high polarization state with a doubled polarization value (±11.26 µC cm−2), which is derived from the Cu ions migration in the vdW gap, features a positive longitudinal piezoelectric coefficient. Moreover, the Cu ions migration can be controlled using conductive scanning tips by applying different biases and mechanical stimuli.[52] The ferroelectric properties of CIPS have garnered extensive research attention, and its applications have continued to expand.[39, 53, 54] In addition to CIPS, other thiophosphate-layered materials have been found to possess ferroelectric properties, such as CuInP2Se6,[55] CuBiP2Se6,[56, 57] CuCrP2S6,[58] and AgBiP2Se6.[57
非常有趣的是,研究发现 CIPS 具有负纵向压电系数,而这只有在聚偏 氟乙烯中才有发现。50,51由于铜离子在 vdW 间隙中的高移动能力,CIPS 具有单轴四重势阱,在沿Z 轴的每个极化方向上都有两个能量最小值,这与 DFT 计算、量子分子动力学模拟和 PFM 测量所揭示的两个不同的极化阶段相对应。第一阶段对应于铜离子在层内的迁移,具有低极化(±4.93 µC cm-2),对应变非常敏感,从而产生负的纵向压电系数。而极化值加倍(±11.26 µC cm-2)的高极化态则源自 vdW 间隙中的铜离子迁移,具有正的纵向压电系数。52CIPS 的铁电特性引起了广泛的研究关注,其应用范围也在不断扩大。39、53、54除 CIPS 外,还发现其他硫磷酸盐层材料也具有铁电特性,如CuInP2Se6、55 CuBiP2Se6、56、57 CuCrP2S6、58和AgBiP2Se6。57]
3.1.2 In2Se3 and InSe
3.1.2In2Se3和 InSe
A single layer of In2Se3 consists of a quintuple atomic layer, Se-In-Se-In-Se, which possesses five phases (α, β, γ, δ, κ) depending on the layer-by-layer stacking orders, as shown in Figure 2a. Among them, the α phase is the most stable, while the β phase is metastable at RT. α-In2Se3 exhibits two distinct stacking arrangements, the hexagonal (2H) and rhombohedral (3R) structures, both of which exhibit intercorrelated IP and OOP ferroelectricity. The β phase is a high-temperature phase with a bulk-centrosymmetric structure and is the parent structure of β′-In2Se3 (room-temperature phases). β′-In2Se3 hosts 2D ferroelasticity[59, 60] and possesses in-plane antiferroelectric periodic nanostripes.[61] The antiferroelectric order in β′-In2Se3 was identified down to a single layer limit.[62] At 77 K, β′-In2Se3 transforms into β″-In2Se3, which is ferroelectric revealed by DFT calculations.[63, 64
如图2a 所示,单层In2Se3由 Se-In-Se-In-Se 五原子层组成,根据逐层堆积顺序的不同,有五种相(α、β、γ、δ、κ)。其中,α相最稳定,而β相在 RT 状态下是瞬变的。α-In2Se3有两种不同的堆叠排列,即六方(2H)和斜方(3R)结构,它们都表现出相互关联的 IP 和 OOP 铁电性。β相是具有体中心对称结构的高温相,也是β′-In2Se3(室温相)的母体结构。β′-In2Se3具有二维铁弹性59、60,并具有面内反铁电周期性纳米条纹61。62在 77 K 时,β′-In2Se3 转变为 β″-In2Se3,DFT 计算显示它是铁电体。63 , 64]

a)α-In2Se3、β-In2Se3 和γ-In2Se3 的晶格和单胞侧视图。b) 层状In2Se3的三维晶体结构,以及一个五层In2Se3FE-ZB′ 和 FE-WZ′ 结构中几种结构的侧视图。c) 电极化反转过程在 OOP 和 IP 方向上的初始和最终状态。d) 通过 PFM 研究在云母上生长的In2Se3的 IP 和 OOP 开关耦合。11Copyright 2018, American Chemical Society.
Using DFT calculations and molecular dynamic simulations, Ding et al. have demonstrated that ferroelectric Zincblende (FE-ZB′) and ferroelectric Wurtzite (FE-WZ′) structures (Figure 2b) are the most stable states, while the face-centered cubic (fcc′) structure is metastable.[18] Therefore, the FE-ZB′ and FE-WZ′ ground states are identified as the α phase In2Se3, while the fcc′ state is as the β phase In2Se3. The underlying basis for the emergence of spontaneous OOP polarization is that the interlayer spacing between the central Se-layer and the adjacent two In-layers is dramatically different, breaking the centrosymmetry. The IP electric polarization due to IP centrosymmetry breaking is illustrated in Figure 2c. The pioneer experimental evidence for OOP ferroelectric polarization of α-In2Se3 was given by PFM in 2017,[65] where domains with opposite polarizations were visualized and switched even for nanoflakes with thicknesses down to ≈10 nm. The tangled switching behavior of the IP and OOP polarization in α-In2Se3 with thickness ≥2 nm was demonstrated through PFM in a dual A.C. resonance tracking mode, as shown in Figure 2d.[11] The electric-field-induced polarization switching and hysteresis loops were observed down to bilayer and monolayer thickness.[12
Ding 等人利用 DFT 计算和分子动力学模拟证明,铁电锌斜方晶 (FE-ZB′) 和铁电钨斜方晶 (FE-WZ′) 结构(图2b)是最稳定的状态,而面心立方 (fcc′) 结构则是可转移的。因此,FE-ZB′和 FE-WZ′ 基态被确定为In2Se3的α相,而 fcc′ 态则被确定为In2Se3 的β相。自发 OOP 极化出现的根本原因是中心 Se 层与相邻两个 In 层之间的层间间距大不相同,打破了中心对称。图2c 展示了 IP 中心对称破缺导致的 IP 电极化。PFM 在 2017 年给出了α-In2Se3的 OOP 铁电极化的先驱实验证据65 ,即使在厚度小至 ≈10 nm 的纳米片上,也能观察到极化相反的畴并进行切换。如图2d 所示,在双 A.C. 共振跟踪模式下,通过 PFM 展示了厚度≥2 nm 的α-In2Se3中 IP 和 OOP 极化的纠缠切换行为。11电场诱导的极化转换和磁滞环在双层和单层厚度下都能观察到。12]
The metastable fcc′ structure possesses a rhombohedral structure and belongs to the R3m space group. Zheng et al.[59] have shown that the β′-In2Se3 phase is stable at RT, both in thin layers and bulk using optical microscopy (OM) and low-energy electron microscopy. This discovery challenges the previous perception that the β′-In2Se3 phase only exists at temperatures between 60 and 200 °C and is formed by cooling β-In2Se3 from 200 °C. A strong IP PFM magnitude and phase signals were revealed by PFM, which confirms the presence of in-plane ferroelectricity in β′-In2Se3.
可蜕变的 fcc′ 结构具有斜方体结构,属于 R3m 空间群。Zheng 等人59利用光学显微镜(OM)和低能电子显微镜研究表明,β′-In2Se3相在 RT 时在薄层和块体中都是稳定的。这一发现挑战了以往的看法,即β′-In2Se3相只存在于 60 至 200 ℃ 的温度范围内,并且是通过从 200 ℃ 冷却β-In2Se3形成的。PFM 显示了强烈的 IP PFM 幅值和相位信号,这证实了β′-In2Se3 中存在面内铁电性。
In addition to In2Se3, a wider family of III2-VI3 materials may be vdW FE materials. DFT calculations suggest that the ferroelectric phases, FE-ZB′ and FE-WZ′, are also the ground states of Al2S3, Al2Se3, Al2Te3, Ga2S3, Ga2Se3, Ga2Te3, In2S3, and In2Te3, when such materials are in the form of quintuple layers.[18] Polarization switching in 2D α‑Ga2Se3 nanoflakes with a 4 nm thickness has been observed with a high Tc of up to 450 K.[66] Additionally, the ferroelectric phase transition of Ga2Te3 single crystal was observed at ≈430 K, as the data revealed by the electric conductivity and Hall mobility measurements.[67
除了In2Se3 之外,还有更多的III2-VI3材料可能是 vdW FE 材料。DFT 计算表明,铁电相 FE-ZB′ 和 FE-WZ′ 也是Al2S3、Al2Se3、Al2Te3、Ga2S3、Ga2Se3、Ga2Te3、In2S3 和In2Te3 的基态,当这些材料为五叠层形式时。66此外,电导率和霍尔迁移率测量数据显示,Ga2Te3单晶在 ≈430 K 时发生了铁电相变。]
Stoichiometric InSe is another III-VI group compound that consists of four atomic layers, Se-In-In-Se, and can be divided into β, ε, and γ-phase based on their configurations. In β-InSe, the interlayer electron transfer can alter the interlayer potential, leading to simultaneous IP and OOP polarization. Exfoliated β-InSe nanoflakes exhibited obvious IP and OOP ferroelectricity. OOP polarization hysteresis loops were observed in a 7-nm-thick sample, while IP and OOP ferroelectric switching was achieved in a 10-nm-thick sample.[19] At present, α, β, γ-In2Se3, and β-InSe have been experimentally verified to have in-plane and out-plane ferroelectric polarization at RT.
化学计量 InSe 是另一种 III-VI 族化合物,由四层原子层 Se-In-In-Se 组成,根据其构型可分为β、ε 和 γ相。在β-InSe 中,层间电子转移可以改变层间电位,从而导致 IP 和 OOP 极化同时发生。剥离的β-InSe纳米片表现出明显的IP和OOP铁电性。19目前,α、β、γ-In2Se3 和β-InSe 已被实验验证在 RT 下具有面内和面外铁电极化。
3.1.3 MX (M = Ge, Sn; X = S, Se, Te)
3.1.3 MX(M = Ge、Sn;X = S、Se、Te)
Bulk MXs adopt a layered orthorhombic structure (space group Pnma), with a honeycomb-folded structure similar to BP. Fei et al. predicted that monolayer and odd-number layer MXs possess robust IP spontaneous polarization.[16] For monolayer MXs, there are two stable structures related to a spatial inversion, as shown in Figure 3a. These two stable structures are connected through a saddle point by the free-energy contour obtained using first-principle calculations. Wu et al. predicted the coexistence of ferroelectricity and ferroelasticity in the SnS and SnSe monolayers by first-principle calculations. In particular, phosphorene is a ferroelastic material possessing the highest reversible ferroelastic strain (Figure 3b)[17] among phosphorene analogs. In 2019, experimental evidence of IP ferroelectricity in few-layer SnS was given by PFM amplitude and phase hysteresis loops, along with robust current–voltage (I–V) hysteresis curves observed from 4 up to 298 K in bipolar devices (Figure 3c).[69] In 2020, robust IP ferroelectric switching was realized in two-terminal SnS devices and was identified to exist below a critical thickness of ≈15 layers (Figure 3d).[13
块状 MX 采用层状正交菱形结构(空间群 Pnma),具有与 BP 相似的蜂窝状折叠结构。Fei 等人预测单层和奇数层 MX 具有稳健的 IP 自发极化。16对于单层 MX,存在两种与空间反转有关的稳定结构,如图3a 所示。根据第一原理计算得到的自由能等值线,这两个稳定结构通过一个鞍点相连。Wu 等人通过第一原理计算预测了 SnS 和 SnSe 单层中铁电性和铁弹性的共存。其中,磷烯是一种铁弹性材料,在磷烯类似物中拥有最高的可逆铁弹性应变(图3b)17。2019 年,通过 PFM 振幅和相位滞后环,以及在双极器件中从 4 到 298 K 的稳健电流-电压(I-V)滞后曲线(图3c)69,实验证明了少层 SnS 中的 IP 铁电性。2020 年,在双端 SnS 器件中实现了稳健的 IP 铁电开关,并确定在临界厚度≈15 层以下存在 IP 铁电(图3d)13。]

a) 单层 IV 族单质的结构俯视图。两个扭曲的退化极性结构(B 和 B′)和高对称性非极性相(A)的示意侧视图。单层 SnSe 的自由能等值线图根据倾斜角(θ1和 θ2)而变化。b) 磷烯和单层 SnS/SnSe 的铁弹性开关路径。c) 由于 AB 叠层结构的反转对称性,相邻的 SnS 层具有反铁电性耦合。生长在石墨上的少层 SnS 的 PFM 形貌、振幅和相位图像。SnS 场效应晶体管 (FET) 器件的I-V磁滞曲线。美国化学学会 2019 年版权所有。d) 两端 SnS 器件。不同扫描范围的 Ag/9L-SnS 器件的 RT I-V。不同厚度 SnS 的铁电电阻开关。13Copyright 2020, Springer Nature.
IP ferroelectric polarization can be detected by scanning tunneling microscopy (STM) by measuring the electronic band bending induced by the bound charges at the edges or ferroelectric domain walls. Chang et al. demonstrated uncontrolled IP polarized switching of 1-unit cell SnTe growth on graphitized hexagonal SiC(0001) substrate through STM tip-induced domain motion.[70] IP polarization of the non-centrosymmetric SnSe monolayer was proved by dI/dV mapping images. The ferroelectric domains can be manipulated by applying bias voltage pulses using an STM tip situated at a lateral distance d0 away from the SnSe plate.[71
IP 铁电极化可通过扫描隧道显微镜 (STM) 测量边缘或铁电畴壁上的束缚电荷引起的电子带弯曲来检测。Chang 等人通过 STM 尖端诱导的畴运动,证明了在石墨化六方 SiC(0001) 衬底上生长的 1 单元 SnTe 不受控制的 IP 极化切换。使用 STM 针尖在离 SnSe 板横向距离d0处施加偏置电压脉冲,可以操纵铁电畴71。]
Ferroelectric switching in epitaxial GeTe film was observed by PFM in early 2014.[72] GeTe has been theoretically proposed as the father compound of ferroelectric Rashba semiconductors.[73] Their bulk bands have giant Rashba-like splitting arising from the ferroelectric polarization induced by inversion symmetry breaking. Thus, ferroelectric Rashba semiconductors allow for the ferroelectric control of spin. In 2018, ferroelectric control of the spin texture in GeTe was experimentally demonstrated through a surface engineering strategy by PFM and spin-angular resolved photoemission experiments.[74] In 2020, Zhang et al. showed that spin Hall conductivity (SHC) in GeTe can be further tuned by ferroelectric polarizations based on the DFT calculations.[75] In 2021, ferroelectric switching in GeTe was controlled through the rotation of lone-pair electrons by an electric field-driven phase transition.[76] The ferroelectric nanodomains of α-GeTe thin films epitaxially grown on Si(111) have been observed, where domain walls are only 71°, rather than the more common 90° and 180° domain walls.[70] The interface with the Si substrate is stabilized by relaxing lattice mismatch stress by misfit dislocations.[77] In 2021, soft mode dynamics were used to explore the origin of the ferroelectric phase transition of GeTe, which successfully reveals the dynamic nature of the phase transition between the rhombohedral and cubic structures.[41] Except for GeTe, other Ge-containing compounds present ferroelectricity, such as IP ferroelectric in few-layered GeS nanoflakes,[78] OOP polarization in monolayer γ-GeSe,[79] and multiferroic in GeSnTe2 via first-principle calculations.[80
2014 年初,PFM 在外延 GeTe 薄膜中观测到了铁电开关。72GeTe 被理论界认为是铁电拉什巴半导体的父化合物73。它们的体带具有巨大的类似拉什巴的分裂,产生于反转对称破缺诱导的铁电极化。因此,铁电 Rashba 半导体可以实现自旋的铁电控制。2018 年,通过 PFM 和自旋角分辨光发射实验的表面工程策略,实验证明了 GeTe 中自旋纹理的铁电控制。742020 年,Zhang 等人基于 DFT 计算表明,GeTe 中的自旋霍尔电导率(SHC)可通过铁电极化进一步调整。2021 年,GeTe 中的铁电转换是通过电场驱动的相变旋转孤对电子来控制的。76在 Si(111) 上外延生长的α-GeTe 薄膜的铁电纳米域已被观察到,其域壁仅为 71°,而不是更常见的 90°和 180°。772021 年,研究人员利用软模式动力学探索了 GeTe 铁电相变的起源,成功揭示了斜方体结构和立方体结构之间相变的动态性质。41除 GeTe 外,其他含 Ge 的化合物也具有铁电性,如通过第一原理计算发现的少层 GeS 纳米片的 IP 铁电性、单层γ-GeSe 的 OOP 极化78和GeSnTe2的多铁性。80]
3.1.4 Distortion-Induced Ferroelectrics in T-MX2 (M = Mo, W and X = S, Se, Te)
3.1.4 T-MX2(M = Mo、W,X = S、Se、Te)中的畸变诱导铁电体
In the monolayer 1T-MoS2, two lattice planes of S atoms are staggered so that each Mo site becomes the center of inversion, as shown in Figure 4a. The centrosymmetric 1T-MoS2, labeled as c1T, is unstable and undergoes distortion leading to a lower symmetry cell tripled structure, called d1T (Figure 4b). The distortion opens up a gap of 0.7 eV in the electronic structure and causes charge density to localize only on one of the S atoms, leading to robust ferroelectricity with the ordering of electric dipoles perpendicular to its plane.[20] The d1T structure with trimerization of Mo atoms has been observed in experiments.[81] The distortion from c1T to d1T applies equally well to other 1T-MX2, such as WS2, WSe2, MoSe2, MoTe2, and WTe2, as demonstrated by Bruyer et al.[82] Additionally, experimental evidence for ferroelectricity has been reported in a d1T-MoTe2 monolayer (Figure 4c,d)[21] and a distorted trigonal structural 1T′′-MoS2 with an 8 nm thickness (Figure 4e).[83] The distorted d1T-MoTe2 was realized through phase transition by laser irradiation from 2H-MoTe2 exfoliated on a Pt substrate. The examples mentioned above all belong to the phase transition from an octahedral to a distorted octahedral structure. This transformation not only induces ferroelectricity, but also produces nontrivial topological phenomena.[84, 85] Analogical to this, another type of distorted octahedral structure is observed in ReS2[86, 87] and ReSe2.[88] The “T” phase possesses a diamond-chain structure formed with the four transition-metal atom clusters linked with bridge bonds, as shown in Figure 4f. Recently, strain-induced ferroelectric domain orientation switching was achieved in ReS2.[89] The two equivalent orientation states of the “T” phase (O1 and O2) can be switched with each other through the mechanical bending of the substrate.
如图4a 所示,在单层1T-MoS2 中,S 原子的两个晶格平面交错排列,因此每个 Mo 位点都成为反转中心。中心对称的1T-MoS2 结构(称为 c1T)并不稳定,会发生变形,从而形成对称性较低的单元三重结构(称为 d1T)(图4b)。这种畸变在电子结构中打开了一个 0.7 eV 的间隙,并导致电荷密度只集中在其中一个 S 原子上,从而产生垂直于其平面的电偶极子有序排列的强铁电性20。此外,d1T-MoTe2单层(图4c、d)21和厚度为 8 纳米的扭曲三 角结构 1T′′-MoS2(图4e)也有铁电性实验证据。83扭曲的d1T-MoTe2是通过激光照射在铂基底上剥离的2H-MoTe2实现相变的。上述例子都属于从八面体结构到扭曲八面体结构的相变。如图4f 所示 ,"T "相具有由四个过渡金属原子簇通过桥键连接而成的菱形链结构。最近,在ReS2 中实现了应变诱导的铁电畴取向切换。[89]"T "相的两种等效取向状态(O1 和 O2)可通过基底的机械弯曲相互切换。

a) 单层MoS2结构俯视图,该结构为中心对称的 1T 多面体形式,Mo 原子为八面体配位。带状结构以及 d1T 与 c1T 的比较。d1T-MoS2 的电子结构。电荷密度差的反转对称性被打破,证实了铁电性。a,b) 经许可后转载。e) 施加机械应力后原始 8 纳米厚 1T"-MoS2薄片的地形图、PFM 振幅和相位、开尔文探针力显微镜(KPFM)和电 力显微镜(EFM)图。f)ReS2或ReSe2 的等效"T "取向态 O1 和 O2 之间的铁弹性切换。具有畴结构的ReS2的高倍扫描透射电子显微镜图像。通过光学显微镜、极性光学显微镜、地形图以及 IP PFM 相位和振幅观察多域ReS2薄片的铁电畴。经授权转载。89版权所有 2022 年,Wiley-VCH GmbH。
3.1.5 Other vdW Ferroelectrics
3.1.5 其他 vdW 铁电体
In addition to the above-mentioned categories, there are other types of vdW FE materials, such as MXene (Hf2CF2[44] and M2XT2[90]), vdW hybrid perovskite BA2PbCl4,[91] M2Ge2Y6 (M = Hf, Y = Se, Te) monolayers,[92] 1D vdW FEs NbOX3[93] (X = Cl, Br, and I), and 1D vdW spiral-chain structure Te.[94] Some of these materials are in the stage of theoretical prediction, and further experiments are needed to verify their hypotheses.
除上述类别外,还有其他类型的 vdW FE 材料,如 MXene(Hf2CF2[44]和M2XT290)、vdW 混合包晶BA2PbCl4、91 M2Ge2Y6(M = Hf,Y = Se,Te)单层、921D vdW FENbOX393(X = Cl、Br 和 I)以及 1D vdW 螺旋链结构 Te等94。其中一些材料还处于理论预测阶段,需要进一步的实验来验证其假设。
3.2 Sliding Van der Waals Ferroelectrics
3.2 滑动范德华铁电体
Theoretical research on AA and AB stacking bilayer h-BN and bilayer graphene can be traced back to 2011, which was originally used to explain the Moiré superlattice, charge density wave, and other phenomena.[95, 96] In 2017, vertical ferroelectric polarization was theoretically proposed in bilayer and multilayer of h-BN, AlN, ZnO, MoS2, GaSe, etc.[22] The significance of this theoretical work was highlighted by direct experimental evidence, where the sliding ferroelectricity was verified in bilayer and trilayer WTe2[23] in 2018, β-InSe[19] in 2019, AB/BA stacking h-BN[24, 25] in 2021, and bulk γ-InSe[97] in 2023. The concept of sliding ferroelectricity and a new term “slidetronics” were raised. Next, we will prepare a detailed summary of the development and research on sliding FEs.
关于AA和AB堆叠双层h-BN和双层石墨烯的理论研究可以追溯到2011年,最初用于解释莫伊雷超晶格、电荷密度波等现象。95,962017年,理论上提出了h-BN、AlN、ZnO、MoS2、GaSe等双层和多层的垂直铁电极化22。2018 年在双层和三层WTe2[23]、2019 年在β-InSe19、2021 年在 AB/BA 堆叠h-BN24,25和2023 年在体γ-InSe97中验证了滑动铁电性,这些直接的实验证据凸显了这一理论工作的意义。会上提出了滑动铁电的概念和新名词 "滑动电子学"。接下来,我们将对滑动铁电的发展和研究进行详细总结。
3.2.1 The Mechanism of Sliding Ferroelectrics
3.2.1 滑动铁电的机理
The origin of ferroelectricity in the BN bilayer is displayed in Figure 5a, in which the N (B) atom in the upper layer is directly right over the hexagon center (the N atom) in the lower layer.[22] For this configuration, the upper and lower layers of the bilayer do not overlap, resulting in an interlamellar net charge transfer from the upper to the lower layer (black arrows) and a vertical polarization upward. This polarization can be switched by dragging along the armchair direction for one BN bond length, making the N atoms in the upper layer right over the B atoms of the lower layer. In the ferroelectric switching pathway calculated using the NEB method, an energy barrier of ≈9 meV per unit cell must be crossed from one stable state to another. This unique sliding ferroelectricity can also exist in a graphene/BN hetero-bilayer, ascribed to the small lattice constant difference (2%) between graphene and h-BN. Distinguishing from the abovementioned right-over bilayer BN, a small twisted angle or a slight difference in strain can give rise to the large-scale modulation of periodic ferroelectric domains.[22] As displayed in Figure 5b, the polarization is vanished in AA stacking domains and is either upward (downward) in AB-up (AB-down) domains.
图5a 显示了 BN 双层铁电性的起源,其中上层的 N(B)原子直接位于下层六边形中心(N 原子)的正上方。22在这种构型下,双电层的上层和下层不会重叠,从而导致层间净电荷从上层转移到下层(黑色箭头),并产生向上的垂直极化。这种极化可以通过沿 armchair 方向拖动一个BN键长度来切换,使上层的 N 原子正好位于下层的 B 原子之上。在使用 NEB 方法计算出的铁电转换路径中,从一个稳定状态到另一个稳定状态必须跨越每个单位晶胞≈9 meV 的能量势垒。这种独特的滑动铁电性也可以存在于石墨烯/BN 异质双电层中,原因是石墨烯和 h-BN 之间的晶格常数差异很小(2%)。22如图5b 所示,在 AA 叠层中极化消失,而在 AB 上(AB 下)层中极化向上(向下)。

a) BN 双层的铁电转换途径;b) 上层和下层之间出现小扭转角或微小应变差时的铁电 Moiré 超晶格。AB-上层、AB-下层和 AA 区分别用黄色、绿色和红色圆圈标出。a,b) 经授权转载。
Generally, three types of vdW stacking mechanisms can induce ferroelectricity. The first type involves sliding between homogeneously stacked layers. The second type occurs due to lattice mismatch in heterogeneously stacked layers, resulting in small local stresses. The third type is observed in twisted stacking vdW materials, where interlayer sliding leads to periodic modulation of the Moiré ferroelectric domain. In all cases, the ferroelectric behavior is primarily caused by atomic rearrangement due to sliding, with minimal or negligible relative displacements between atoms within each layer. The relative displacement of atoms between adjacent layers caused by sliding leads to the redistribution of electronic clouds. The driving force behind interlayer charge transfer is the asymmetry in the atomic distribution between corresponding positions in the upper and lower layers, resulting in localized accumulation of electronic clouds. The asymmetric distribution of electronic clouds breaks the spatial lattice symmetry, which is the essence of sliding ferroelectricity. The ferroelectricity in sliding cases is different from the aforementioned cases, such as charge inversion polarization switching driven by Cu ion migration in CuInP2S6, atomic displacements in In2Se3, or cation trimerization in 1T-MoS2 monolayer. It is also distinct from the lattice instability described in ferroelectric theories.[37] Based on this understanding, sliding ferroelectricity can potentially occur by stacking of any vdW materials, not only in 2D vdW material, but also in 1D vdW material.
一般来说,有三种 vdW 堆叠机制可以诱导铁电性。第一种是均匀堆叠层之间的滑动。第二种是由于异质堆叠层中的晶格失配,导致局部应力较小。第三种类型出现在扭曲堆叠的 vdW 材料中,层间滑动导致莫伊雷铁电畴的周期性调制。在所有情况下,铁电行为主要是由滑动导致的原子重排引起的,每层内原子间的相对位移极小或可以忽略不计。滑动引起的相邻层间原子相对位移会导致电子云的重新分布。层间电荷转移的驱动力是上层和下层相应位置之间原子分布的不对称性,从而导致电子云的局部积累。电子云的非对称分布打破了空间晶格对称性,这就是滑动铁电的本质。滑动情况下的铁电性不同于上述情况,如CuInP2S6 中由铜离子迁移驱动的电荷反转极化转换、In2Se3 中的原子位移或1T-MoS2单层中的阳离子三聚化。37基于这种理解,任何 vdW 材料的堆叠都有可能产生滑动铁电,不仅在二维 vdW 材料中如此,在一维 vdW 材料中也是如此。
3.2.2 The Experimental Progress of Sliding Ferroelectrics
TMDs-Based
In 2018, Fei et al. observed a robust OOP electric polarization that can be switched using gate electrodes at RT in bilayer and trilayer 1T-WTe2, but not in monolayer, through a precisely designed device structure (Figure 6a).[23] They further stated that the electron-hole correlation effect, rather than lattice instability, drives the spontaneous polarization in WTe2. Therefore, the polarization could principally involve a relative motion of the electron cloud relative to the ion cores, rather than a lattice distortion, just like the charge transfer in a bilayer BN.[22] In 2019, Yang et al.[42] proved that the polarization switching of bilayer WTe2 stems from the interlayer vertical charge transfer caused by in-plane slippage (Figure 6b) through first-principle calculation. Additionally, they also indicated that the polarization of trilayer WTe2 can be switched from downward to upward by displacing the middle layer by a distance of a+b, where a is the horizontal distance between Te0 and Te2, and b is the horizontal distance between Te1 and Te3, as shown in Figure 5c (left). In five layers, the second and fourth layers are displaced by a+b simultaneously (Figure 6c (right)). Moreover, the Moiré superlattice pattern (Figure 6d) of FE domains can be formed and tuned in bilayer WTe2 with a small twist angle. It is worth mentioning that the ferroelectricity of WTe2 persists in samples thicker than three layers or even in bulk crystals. The coexistence of native metallicity and ferroelectricity in bulk crystalline WTe2 at room temperature was observed.[98]

In 2020, Xiao et al.[99] demonstrated interlayer sliding in a few-layer WTe2 using an identical device as that reported in Fei's work.[23] In this WTe2 device, three stacking orders can be switched by controlled OOP electric fields and electrostatic doping. The layer-parity-selective nonlinear Hall memory behavior was observed, in which the Berry curvature and its dipole sign reverse in the trilayer while being invariant in four layers during the ferroelectric transition. These findings open up an avenue toward exploring the coupling between topology, electron correlations, and ferroelectricity in hidden stacking orders. A spin field effect transistor (spin-FET) design based on bilayer WTe2 was proposed, in which ferroelectric polarization can be used to control the spin texture, effectively improving the spin-polarized injection rate.[100] The coexistence of ferroelectric and topological orders may also occur in trilayer Bi2Te3, a general paradigm for designing 2D ferroelectric topological insulators.[101]
In 2022, Jarillo-Herrero's group[102] employed a dual-gated vdW heterostructures (vdWHs) device to find the sliding ferroelectricity in the family of rhombohedral-stacking TMDs. The built-in interlayer potentials (ΔVP) were extracted and compared. ΔVP of four rhombohedral-stacking bilayer TMDs (≈55 mV) is about half of that in parallel-stacked bilayer h-BN.[103] The experimentally obtained ΔVP value is in reasonable agreement with those obtained from first-principle calculations. The switchable ferroelectric behavior (interfacial charge transfer and movement of domain walls) of thin MoS2 with small twist angles was demonstrated by studying field-driven domain evolution in both structural (back-scattered electron channel contrast imaging, BSECCI, Figure 7a) and electronic (Kelvin probe force microscopy and electronic transport measurements, Figure 7b,c) properties.[27] To experimentally observe this ferroelectricity, a special heterojunction structure was constructed on marginally twisted MoS2 encapsulated in h-BN from both sides, placed on a graphite back gate and covered with a graphene top gate. As a result, domain switching was visualized by BSECCI under different values of the transverse electric field applied in situ. The transverse field can penetrate the twisted bilayer without introducing any noticeable carrier density due to the special heterojunction device structure. Based on this, the electronic transport properties of such ferroelectric field-effect transistor devices consisting of marginally twisted pairs of mono-, bi-, and tri-layers of MoS2 were studied. It was shown that parallelly aligned twisted 1L/1L and 3L/3L devices produced ferroelectricity. However, for 2L, this alignment leads to predominantly centrosymmetric 2H stacking, which has no electric polarization (Figure 7c). In the latest research by Rogée et al., unexpected OOP ferroelectricity and piezoelectricity were demonstrated in untwisted, commensurate, and epitaxial MoS2/WS2 heterobilayers synthesized by one-step chemical vapor deposition process.[28] The piezoelectric constant (d33, 1.95–2.09 pm V−1) was measured that was six times larger than that of monolayer In2Se3. This results from the symmetry breaking of interlayer sliding without needing twist angles or Moiré domains.

h-BN-Based
In 2021, Jarillo-Herrero's group[103] demonstrated that twisted bilayer BN exhibits staggered OOP Moiré ferroelectricity domains. They used PFM to directly detect the switching of the Moiré domains that strongly depends on the stacking order (Figure 8a). Similarly, the stacking order ferroelectric domains in twisted bilayer graphene, twisted-WSe2, Gr/BN, and WSe2/MoSe2 have also been successively visualized.[104] The polarization switching was probed through the resistance change of an adjacently stacked graphene inserted in two parallelly stacked bilayer BN sheets or two twisted bilayer BN sheets (Figure 8b). The dual-gated devices have a complicated structure of top gate/h-BN/graphene/0° parallel stacked bilayer BN (P-BBN)/h-BN/bottom gate, as shown in the insets of Figure 8b. Here, the key reason that polarization can be detected is that graphene is very sensitive to the charge carriers produced by the polarization of P-BBN. A prominent hysteresis was observed on the bottom gate while no hysteresis was detected on the top gate. Using this structure, they compared the ferroelectric switching in parallelly stacked and twisted bilayer BN devices (Figure 8c). They found that the coercive field and switch speed of the twisted bilayer BN is much smaller and slower than those of P-BBN. In 2021,[24] stable ferroelectric order at the interface between two naturally grown flakes of BN was detected by PFM operating in a Kelvin-probe mode. Alternating domains of inverted normal polarization were measured. Reversible polarization switching was achieved by scanning a particular location with up and down polarization domains after biasing the tip by voltages of −20 and 10 V, where the tip biasing caused a lateral shift of one lattice site between the domains.

Graphene-Based
The latest research works have discovered bistable stacking states, called O state and Ō state (Figure 9a–c), in twisted monolayer/multilayer graphene with a twist angle of 0.14° using conductive atomic force microscopy (c-AFM). These states have reversed stacking orders and distinct strain soliton networks, which can be switched repeatedly by locally perturbing them with electrical or mechanical forces.[26] This can be triggered by lifting and reapproaching the tip to the sample surface with a compressive load followed by scanning under normal load, as illustrated in Figure 9b. The switching behavior can propagate through the strain soliton networks, as shown in Figure 9c, and it can even indirectly perturb away from the closed-loop soliton, making the whole network completely switched. However, continuous wrinkles can arrest or even completely block this propagation behavior.

Other Materials
Recent research has shown an increasing interest in systems with sliding ferroelectric properties.[33] In the past 2 years, both homogeneous and heterogeneous vdW stacking systems have been theoretically predicted to have spontaneous ferroelectric polarization, covering β-ZrI2,[45,105, 106] trilayer Bi2Te3,[101] MoA2N4 (A = Si or Ge) bilayers,[107] 1T′-ReS2 multilayers,[108] 3R bilayer BX (X = P, As, Sb),[109] 2D topological insulators,[101] and so on. Very recently, mesoscopic sliding ferroelectricity was observed in multiwall WS2 nanotubes.[110] Table 1 summarizes the corresponding polarization parameters, FE mechanism, layer thickness limit, polarization direction, and other relevant factors of these systems. In addition to sliding FEs, other 2D vdW ferroelectric heterostructures have also been extensively studied, for example, strain tunable ferroelectricity of SnSe/SnTe heterostructures,[111] In2Se3/Bi2Se3 heterobilayer,[112] and mixed-valence β-SnS/SnS2 heterostructure.[113, 114] For a more comprehensive comparison, we have also included such heterojunctions in Table 1. In addition, the parameters for some typical non-stacking vdW FE materials are also provided for a direct comparison. A comprehensive summary of non-stacking vdW FE materials can be found in Ref. [115].
Materials | Thickness | Twist angle | Direction | Tc | Polarization | Energy Barrier |
Interfacial Potential/Energy |
Mechanisms | Method | Yearref. |
---|---|---|---|---|---|---|---|---|---|---|
h-BN | BL | 0° | OOP |
2.08 pC m−1 (2D) 0.68 µC cm−2 (3D) |
9 meV per u.c. | 0.23 V | Slide | DFT | 2017[22] | |
Gr/h-BN MoS2 InSe GaSe |
1L/1L BL BL BL |
0° 0° 0° 0° |
OOP OOP OOP OOP |
1.5 pC m−1 (Up) 0.33 pC m−1 (Down) 0.97 pC m−1 0.24 pC m−1 0.46 pC m−1 |
0.17 V −0.04 V |
Slide | DFT | 2017[22] | ||
BLG/h-BN | 1L/1L | OOP |
0.18 µC cm−2 (D1) 0.05 µC cm−2 (D2) |
Slide | Hall-test | 2020[116] | ||||
h-BN |
ML/ML BL |
<1° 0.33° |
OOP OOP |
3 mC cm−2 (3D) | 0.2 eV | EFM/KPFM | 2021[117] | |||
h-BN | BL |
0.33° 0.67° 1.05° |
DFT | 2021[118] | ||||||
h-BN | BL | 0° | OOP |
0.27 V (AB) 0.18 V (BA) |
Slide | SFTJs | 2022[119] | |||
h-BN | BL | 0.6° | OOP | >RT |
0.68 µC cm−2(3D) 2.25 pC m−1(2D) 2.08 pC m−1 |
PFM/FeFET DFT |
2021[103] | |||
Gr | BL/BL | 0–0.5° | LPFM/DFT | 2021[120] | ||||||
Gr | 1L/ML | 0.14° | OOP | Slide | c-PFM/DFT/MD | 2022[26] | ||||
Gr | BLG | OOP | ≈5 pC m−1 | Slide | Devices | 2022[121] | ||||
Td-MoTe2 | BL | 0° | IP/OOP | ≈260 K |
3.6 × 1011 e cm−2 (=0.058 C cm−2) |
16.9 meV per u.c. | Slide | TEM/STM | 2019[122] | |
MoS2 | BL | 0-2° | OOP | >RT | 3.8 × 106 e m−1 | 63 mV (Th) | Slide | BSECCI/KPFM | 2022[27] | |
MoSe2 WSe2 WS2 MoS2 h-BN |
BL |
0.25° 0.2° 0.1° |
OOP | >RT |
57 mV (66) Ex (Th) 56 mV (66) 56 mV (56) 47 mV (64) 109 mV (100) |
Slide | 2022[102] | |||
WTe2 | BL | 0° | OOP | 350 K | 0.03 µC m−2 | 0.6 meV per f.u. | Slide | DFT | 2018[42] | |
WTe2 | BL, TL | 0° | OOP | 350 K | 2 × 1011 e cm−2(20 K) | Slide | Devices | 2018[23] | ||
TMDs | BL | 0° | Slide | DFT | 2022[123] | |||||
WSe2 | BL | 1° | OOP | Slide | DFT | 2021[124] | ||||
β-InSe | ML | 0° | IP/OOP | >803 K | PFM | 2019[125] | ||||
ZrI2 | BL | 0° | IP/OOP | 476 K (Th) |
0.094 µC m−2 (IP) 0.021 µC m−2 (OOP) |
1.6 meV per f.u. (OOP) | Slide | DFT | 2021[45] | |
β-ZrI2 | BL | 0° | IP/OOP | 0.39 µC cm−2 |
28 meV per u.c (=7 meV per f.u.) |
Slide | DFT | 2021[105] | ||
β-ZrI2 | BL | 0° | OOP | ≈400 K |
0.243 µC cm−2 (Th) 0.24 µC cm−2 (Ex) |
5.3 meV per u.c. | Slide | DFT | 2021[106] | |
Bi2Te3 | TL | 0° | IP/OOP |
3.10 × 1010 e cm−2 (IP) 5.1 × 109 e cm−2(OOP) |
69.46 meV per u.c. | 34 meV | Slide | DFT | 2021[101] | |
MoSi2N4 MoGe2N4. CrSi2N4 Wsi2N4, |
BL | 0° | OOP |
3.36 pC m−1 3.05 pC m−1 2.49 pC m−1 3.44 pC m−1 |
20.02 meV per u.c. 33.23 meV per u.c. |
Slide | DFT | 2021[107] | ||
1T′-ReS2 | 2-7L | 0° | OOP | ≈405 K | 0.07–0.68 pC m−1 (0 K) | ≈17.1 meV | Slide | PFM/SHG/FTJ | 2022[108] | |
BP BAs BSb BN[126] |
BL BL |
0° |
OOP OOP |
1.6 pC m−1 2.2 pC m−1 13.9 pC m−1 2 pC m−1 |
19.5 meV per f.u. 28.4 meV per f.u. 128.5 meV per f.u. |
0.15 V 0.18 V 0.47 V |
Slide | DFT | 2022[109] | |
Phosphorene Arsenene Antimonene Phosphorene Arsenene Antimonene α-In2Se3[127] γ-GeSe[79] |
BL 1L 1L |
0° |
OOP OOP OOP |
0.09 µC cm−2 (3D) 0.22 µC cm−2 (3D) 0.24 µC cm−2 (3D) 0.53 pC m−1 (2D) 1.33 pC m−1 (2D) 1.45 pC m−1 (2D) |
10 meV per f.u. 18 meV per f.u. 38 meV per f.u. 50 meV per f.u. 887 meV per f.u. |
0.05 eV 0.1 eV 0.12 eV |
Slide | DFT | 2021[128] | |
T-FeCl2 | BL, TL | OOP |
2.7 nC cm−2 5.4 nC cm−2 |
4.2 meV per f.u. 26.8 meV per f.u. |
7 × 10−4 eV | Slide | DFT | 2022[129] | ||
VS2 | BL | 0° | OOP | 0.202 µC cm−2 | Slide | DFT | 2020[130] | |||
WS2 nanotubes | Multiwall | Slide | TEM | 2022[110] | ||||||
1T-TiTe2/1T-TiSe2 | 1L/1L | 0.5° | >RT | STM | 2022[131] | |||||
WSSe/In2Se3 | 1L/1L | DFT | 2022[132] | |||||||
SnSe/SnTe SnSe/SnTe/SnSe |
1L/1L 1L/1L/1L |
0° 0° |
IP IP |
172.1 pC m−1 138.5 pC m−1 |
7.593 meV | Strain | DFT | 2021[111] | ||
SnTe | 2L | IP | 40 pC m−1 | Intrinsic | 2019[133] | |||||
β-SnS/SnS2 | 1L/1L | 0° | IP/OOP | 10 pC m−1 | 36 meV per f.u | Bending | DFT | 2021[113] |
- Annotation:1L = Monolayer, BL = Bilayer, TL = Trilayer, ML = Multilayer, Ex/Th = Experiment/Theory, u.c = Unitcell, f. u. = formula unit, 3D = Three Dimensions, EFM = Electrostatic Force Microscopy, AB = AB-stacks, BLG = Bernal-stacked Bilayer Graphene, LPFM = Lateral Piezoelectric Force Microscopy, MD = Molecular Dynamics simulation, SHG = Second Harmonic Generation, D1(2) = Device1(2).
4 The Applications Based on Van der Waals Ferroelectrics
Traditional FEs encounter a series of problems when fabricating low-dimensional devices, including interface problems caused by lattice mismatch, size effect caused by body miniaturization, and compatibility issues during integration. 2D vdW FE materials possess properties similar to traditional ones, such as stable spontaneous polarization and strong optical nonlinearity. Additionally, vdW FEs own special properties of vdW materials, such as atomically sharp interfaces without dangling bonds, layered structures, and ultrathin thickness. These characteristics make vdW FEs promising advanced high-performance devices.
In this section, we begin by presenting the recent developments of vdW ferroelectric-based memories, including electronic and optoelectronic memories. In particular, recent advances in vdW-FE-based neuromorphic devices are summarized. Then, we review the applications of vdW FEs in low-power-consumption logic devices based on ferroelectric negative capacitance field effect transistors, solar cells based on ferroelectric photovoltaic effects, and nanogenerators based on piezo/triboelectric and pyroelectric effect. Finally, we provide an overview of the emerging topological memories based on vdW FEs.
4.1 Electronic Memories
A ferroelectric tunnel junction (FTJ) is a two-terminal device composed of a ferroelectric layer sandwiched between two metal electrodes. Because the barrier height between the FE and electrode can be tuned in a nonvolatile manner by changing the ferroelectric polarization, the tunneling current state can be programmed in a nonvolatile manner and used as information storage media. FTJs are widely used as ferroelectric random access memories. In the 2D case, FTJs based on CIPS ferroelectric tunneling barrier layer, in which monolayer Gr and chromium were used as the asymmetric contact electrodes, exhibited a giant tunneling electroresistance of above 107 between the on and off states,[134] which greatly surpassed conventional FTJs.
A ferroelectric field effect transistor (Fe-FET) is a transistor where a FE layer is employed replacing the traditional insulator of a Metal Oxide Field Effect Transistor (MOSFET). Different ferroelectric polarization states can induce different current levels of the channel, which can be used as nonvolatile memories. A Fe-FET is usually featured with a counterclockwise hysteresis loop in the transfer curve (drain current-gate voltage) due to the ferroelectric polarization reversal when switching the gate voltage. Fe-FETs based on conventional FEs usually have poor performance because of the severe interfacial states and bad integration capability with silicon-based technologies due to the high-temperature process. However, these problems can be addressed for vdW FEs thanks to the dangling-bond-free surfaces. A vivid example is the MoS2/h-BN/Gr/CIPS vdW Fe-FET, where the MoS2 was used as the channel material, h-BN as the gate dielectric, and Gr as the gate contact.[135] The vdW Fe-FET exhibited a low subthreshold swing of sub-60 mV dec−1, a memory window larger than 3.8 V, and a current ratio of over 107. A prototype gate-programmable memory was constructed by stacking a few layers of Gr, CuInP2S6, MoS2, and h-BN, which combines a metal-FE-semiconductor junction (serving as a two-terminal memristor) and a MOSFET.[136] In this vdW vertical architecture, the Fermi level in the semiconducting layer can be gated to tune the carrier density, which simultaneously affects the ferroelectric switching behavior of the memristor with on-off ratios reaching 105. In another case, an all-ferroelectric FET utilizing a ferroelectric dielectric material of CIPS and a ferroelectric semiconductor of α-In2Se3 as dielectric and channel layers, respectively, exhibited a wide hysteresis window, a high on/off current ratio (higher than 106), a long stable retention time (>104 s), and a stable cyclic endurance (>104 cycles).[137]
In 2019, Si et al. reported a ferroelectric semiconductor FET (FeS-FET) in which a 2D FE semiconductor, α-In2Se3, was used as the channel layer. The FeS-FET device exhibits highlighted performances, such as a high on/off ratio (over 108), a maximum on-current of 862 µA µ−1m−1, and a low supply voltage.[138] A gate terminal was used to regulate the ferroelectric polarization inside the channel, thereby influencing the accumulation of charge carriers and current states. Unlike traditional Fe-FET, where only the polarization bound charges at the gate insulator/semiconductor interface can affect the electrostatics, in a FeS-FET, the polarization charges accumulated at both the bottom and top surfaces of the FE semiconductor layer can affect the electrostatics. More importantly, FeS-FETs have a simpler device structure with fewer interfaces compared with FE-FETs, which can promote the practical applications of FEs.
Due to the weak interlayer coupling in vdW FEs, the dipole alignment between the neighboring layers can be modulated by external voltages. Stable ferroelectric and antiferroelectric (AFE) states can be realized in In2X3 (X = S, Se, Te) bilayers predicted by first-principles calculations. The stable ferroelectric and antiferroelectric states are isolated by sizable energy barriers, which can be used as nonvolatile memory states. Three types of dipole ordering exist in bilayer In2X3: a tail-to-tail AFE state (AFE-T), a head-to-head AFE state (AFE-H), and a FE state. The transform from the AFE-T (FE) state to the FE (AFE-T) state should stride over two metastable states with a transition barrier of ≈30–40 meV. Using a generalized gradient approximation, the two AFE states have moderate band gaps (≈1 eV), while the FE state has a very small one (0.037 eV) due to the depolarization electric field produced by the OOP polarization. The electronic transport of IP and OOP tunnelling across the In2S3 bilayers was also explored using quantum-mechanical modelling, which predicted a giant TER effect with multiple nonvolatile resistance states. Recently, these were proven experimentally in a 2H-stacked α-In2Se3 ferroelectric memory.[139] In this device, a three-resistance-state memory was demonstrated with a switching speed of milli-seconds, an endurance of >300 cycles, and a good retention of >4500 s. Furthermore, the three states exhibited distinctive photoresponse properties, demonstrating great potential in nonvolatile memories and optoelectronic devices.
Yang et al.[119] developed vdW sliding FTJs (SFTJs) using a bilayer h-BN as an ultrathin tunnel barrier. Calculated transport behavior by DFT combined with the nonequilibrium Green function method displayed that there was a very small TER when the Au electrodes were directly in contact with the h-BN, and a giant TER of ≈10 000% when graphene was inserted between the Au electrode and the h-BN (Figure 10a). The decisive reason is that the graphene intercalation layer diminishes the strong hybridization between Au and the adjacent h-BN and restores the bilayer h-BN ferroelectricity. This key discovery allows the weak signals from sliding FEs to be measured in actual devices.[23, 102, 103] The OOP ferroelectric switch in an untwisted hetero-bilayer WS2/MoS2 was considered with an interlayer sliding mechanism by calculating differential charge densities between the up and down polarizations.[28] The direction of polarization is switchable by lateral sliding one layer across roughly one-third of the unit cell (Figure 10b,c). In a corresponding FTJ device, OOP polarization reversal was observed by applying positive/negative voltages (3.5 V), accompanied by switching between a high-resistance state and a low-resistance state. These sliding-induced multiple polarization states were recently demonstrated in dual-gate ferroelectric FETs based on 3R-MoS2.[140]

4.2 Optoelectronic Memories and Photodetectors
vdW FE semiconductors offer significant advantages over traditional ferroelectric materials and have attracted widespread attention in the field of optoelectronic memories and photodetectors. These materials can serve as light-absorbing and ferroelectric functional layers as well as conducting channels simultaneously, which can significantly simplify the device structure. Their bandgap ranges widely from sub-electronvolt to several electronvolts, therefore, devices based on different vdW FE materials may cover a very wide spectrum. For instance, α-In2Se3 has a bandgap ranging from 1.44 eV (48 nm thick) to 1.64 eV (8 nm thick).[141] Relevant reports have shown that α-In2Se3 exhibits good light response to UV, visible, and IR lights. The bandgap of CIPS is ≈2.9 eV,[142] β-InSe is 1.2829 eV,[143] SnSe is 0.90 eV,[144] GeSe monolayer is 1.16 eV,[145] and Te is from 0.31 (bulk) to 1.17 eV (bilayer).[146]
As an example, α-In2Se3 was used as a channel layer in photodetectors with high sensitivity approaching 20 photons and excellent endurance of more than 106 cycles due to the ferroelectric-semiconducting coupling property.[147] A simplified two-terminal α-In2Se3 device can achieve both optical writing and electrical erasing processes.[148] Visible light irradiation can induce ferroelectric domain reversals with an optical writing and electrical erasing endurance of over 800 cycles. The optically controlled ferroelectric domain reversal is attributed to the combined interaction between ferroelectric bound charges, compensation charges, imprint fields, and photogenerated carriers. The nonvolatile optoelectronic memory was also demonstrated in a two-terminal planar architecture based on α-In2Se3 by Xue et al.[149]
2D ferroelectric vdWHs have been widely studied as optoelectronic memories and high-performance photodetectors. Ferroelectric α-In2Se3/MoS2 vdWHs showed an enhanced carrier separation effect, and the photocurrent density under visible light radiation was up to 0.5 mA cm2 exceeding that of silicon thin film devices.[150] The electronic transport and photoluminescent behavior of the photosensitive layer can be also modulated by the built-in field effect of polarization switching in MoS2/CIPS vdW heterostructures.[151] The reconfigurable photoresponse can be extended to 1550 nm based on a ferroelectric heterostructure composed of BP/α-In2Se3.[152] The device shows distinguished capability in terms of high responsivity (4.73 × 104 A W−1), large specific detectivity (≈2.09 × 1012 Jones), high external quantum efficiency (9.21 × 106%), and notable photo-on-off ratio (4.82 × 103).
The anisotropic transport behavior of β-InSe, attributed to its non-symmetrical point group of D46 h, makes it a promising option for creating filter-free polarization-sensitive photodetectors. 2D β-InSe can achieve high-performance polarization-sensitive photodetectors with a maximum to minimum dark current ratio of 3.76 at two orthogonal orientations and a high photocurrent anisotropic ratio of 0.70 at 1 V bias voltage.[153] The ferroelectricity of β-InSe has been demonstrated, but its potential application in ferroelectric optoelectronic memories has been rarely reported, which presents an intriguing direction for further exploration. While there have been several reports on the electronic memories of SnS, studies of its application as optoelectronic memories are still scarce. As an example, a 14-monolayer SnS FET yields an on/off ratio of 3.41 × 106, a subthreshold swing of 180 mV dec−1, and a field effect mobility of 1.48 cm2 V−1 s−1. The SnS photodetectors show a broad photoresponse from 365 to 820 nm, a detectivity of 9.78 × 1010 cm2 Hz1/2 W−1, and rapid response under an extremely weak 365 nm illumination.[154] This study provides valuable insights into the layer-dependent optoelectronic properties of 2D SnS, which hold great potential for next-generation 2D optoelectronic devices. Other vdW ferroelectric materials that have been verified to exhibit ferroelectricity face similar limitations. Therefore, there is still a wide scope for exploration in this area.
Ferroelectric materials serving as gate dielectric layers can generate localized polarization fields to modulate the carrier concentration inside the channels, which can effectively suppress the dark currents and significantly improve the performance of photodetectors.[155] For example, photodetectors based on emerging low-dimensional materials usually show a high dark current. To improve the performance, a continuous gate voltage is usually applied to suppress the dark current and enhance the final performance, which will cause additional power consumption. One solution is to use FEs as the dielectrics. The strong nonvolatile polarization electric field can effectively control the concentration of carriers and decrease the dark current. A similar idea can be applied to 2D FE devices, which has not been realized so far.
4.3 Artificial Intelligence/Neuromorphic Computing
The vdW FEs are also attractive as memristive devices for mimicking neurons and synapses, mainly due to their robust nonvolatile ferroelectric polarizations.[156-159] Neuromorphic devices based on vdW FE semiconductors show an optimistic prospect. In 2021, Wang et al.[160] demonstrated a vdW ferroelectric channel (α-In2Se3) transistor that integrates nonvolatile memory and neural computation functions. A 30 nm Al2O3 thin film was used as the bottom dielectric layer, h-BN on Al2O3 for interface optimization, 2D α-In2Se3 as the channel, and another h-BN as the top dielectric layer. The devices showed remarkable performance, including fast nonvolatile storage with a writing speed of 40 ns, a neuromorphic computing capability, a high-precision image recognition of 94.74%, an ultra-low energy consumption of 234/40 fJ per event for excitation/inhibition, and an improved endurance through the internal electric field. Unlike conventional Fe-FETs, both polarization bound and free charge carriers exist in this ferroelectric channel, therefore, the accumulated free charge carriers on the top and bottom surfaces of the channel induce a long retention time and an improved endurance. In another case, a sandwich structure of Ag/α-In2Se3/ITO memristor[161] exhibited typical bipolar resistive switching behavior with excellent endurance, fast switching speeds (16.4 and 18.0 ns for SET and RESET process), and continuous tunability under alternating-current pulse stimulations. The reinforcement learning behavior was faithfully simulated, laying the foundation for further brain-like computing and artificial intelligence applications. Very recently, reservoir computing based on a planar α-In2Se3 device for temporal and sequential information processing has been successfully fabricated and demonstrated, showing potential for artificial synapses and recurrent neural networks.[162] The IP FE analog synaptic devices with a lateral configuration of Pt/SnS/Pt exhibited high stability, high linearity in the potentiation/depression process, and long retention at RT. In the artificial neural network simulation, the accuracy of pattern recognition reaches 92.1%.[163] Similarly, vertical two-terminal devices of Au/SnSe/NSTO[164] displayed continuous conductance tunability by gradually switching the polarization of the ferroelectric SnSe layer. Stimulus pulses with a nanosecond duration were applied to the device to mimic the ability of rapid learning and long-term memory of the human brain. The bio-synaptic functions, including spike-timing-dependent plasticity, short-term plasticity, and long-term plasticity, were simulated with an energy consumption of as low as 66 fJ.
An optoelectronic synapse based on α-In2Se3 has been developed with heterosynaptic plasticity and tunable relaxation timescales via light intensity or back-gate voltage.[165] In addition, a monolithic neuromorphic machine vision system based on α-In2Se3 FeS-FET was reported by Cai et al., in 2022.[166] The FeS-FET-based sensor exhibits a retina-like adaption behavior with a broadband light response covering from 275 to 808 nm. A 3 × 3 sensor array realized a broadband-dim-image binary classification task with a high recognition accuracy of 93.0% and a 20% accuracy promotion over an incomplete system. In another case, logic, in-memory computing, optoelectric logic, and nonvolatile computing functions were realized in a single FeS-FET based on α-In2Se3, showing potential for function integration.[167]
2D FE vdWHs have also been widely studied as optoelectronic in-memory computing sensors for neuromorphic networks.[168, 169] A ferroelectric p-n junction consisting of MoTe2 and α-In2Se3 as p-type and ferroelectric n-type channels, respectively, was studied.[170] The nonvolatile reconfigurable built-in potential related to the ferroelectric polarization in α-In2Se3 induces nonvolatile rectification and photovoltaic effect, as well as driving-voltage-free optoelectronic synaptic function. Similarly, an optoelectronic synaptic device based on ferroelectric α-In2Se3 (n-type)/GaSe (p-type) heterojunction features biological behaviors of synapse, memory, and logic functions.[171] Here, the source and drain were defined as presynaptic and postsynaptic terminals, while the top gate was used as a third terminal. The photoluminescence peaks of α-In2Se3 and GaSe were located at 870 and 620 nm, respectively, indicating that the device absorbed both visible and near-infrared light. Synapses plasticity behavior, such as paired-pulse facilitation and long-term potentiation/depression, were achieved by ferroelectric switching. The retina-like function was achieved by selectively converting light stimuli into electrical signals, which enable the device to recognize colors. To mimic the classical conditioned reflex, Pavlov's dog experiment was successfully carried out. Light pulses were identified as an unconditional stimulus (food) to cause an unconditioned reflex (salivation), while electrical pulses were used as a conditioned stimulus (bell) to trigger a conditioned reflex. After training, electrical pulses alone could induce an above-threshold postsynaptic current, and a forgetting behavior was observed with increasing time. In addition, in a photonic neuromorphic device based on WSe2/In2Se3 heterostructure, the light-stimulated synaptic behavior extends from visible light to the short-wavelength infrared region (up to 1800 nm) with a low power consumption of 258 fJ per switching under a bias of −0.1 V.[172]
4.4 Ferroelectric Negative Capacitance Field Effect Transistors
Ferroelectric negative capacitance field effect transistors (Fe-NCFETs) can effectively reduce the subthreshold swing (SS) to below 60 mV dec−1, which can dramatically decrease the dynamic power consumption of logic circuits and address the energy wall problem. However, research on vdW-FE-based NCFETs has been limited. For instance, MoS2/CuInP2S6 NCFETs have been demonstrated with an average SS of below 60 mV dec−1 for over seven decades of drain current, and with a minimum SS of 28 mV dec−1.[173] A step further, vdW FE heterojunctions field effect transistors MoS2/CuInP2S6 with sub-5 nm gate lengths were demonstrated with a hysteresis-free subthermionic switching over five-to-six orders of magnitude of drain current and a minimum SS of as low as 6.1 mV dec−1.[174] These findings indicate that vdW FE materials and their heterostructures hold promise for highly integrated electronic devices with increased energy efficiency.
4.5 Photovoltaic and Bulk Photovoltaic Effects
The photovoltaic effect involves two basic steps: the generation of photo carriers through absorbing light by a semiconductor; the separation and collection of photo carriers under an inherent electric field, resulting in a net current flow in a particular direction. By using α-In2Se3 as the n-type ferroelectric semiconductor layer, MoTe2 as the p-type semiconductor layer, and 2D Ruddlesden–Popper perovskites as the intrinsic light absorption layer, the p-i-n FE vdWHs produces a nonvolatile tunable photovoltaic effect with a high short-circuit current density (468 mA cm−2), which is one to two orders higher than that of typical lateral vdWHs.[175]
Conventional photovoltaic devices rely on p-n junctions or Schottky junctions. It means that the photovoltaic voltage is limited by the bandgaps during photoelectric conversion. Hence, there is a theoretical upper limit of the photoelectric conversion efficiency, known as the Shockley–Queisser (SQ) limit.[176] The SQ limit is expected to be broken by the bulk photovoltaic effect (BPVE), which usually occurs in non-centrosymmetric crystalline materials.[177, 178] Photogenerated electrons and holes are separated spontaneously in a single component of crystal material, without requiring a p-n junction. Currently, studies on BPVE mainly focus on 3D ferroelectric perovskite oxide crystals, such as BiFeO3,[179] BaTiO3,[180] and LiNbO3.[181] However, these materials are usually semiconductors with large bandgaps and very low short-circuit currents, which limit their practical applications. To obtain higher photocurrent densities in bulk photovoltaic devices, theoretical studies point out that the high density of electronic states and the shift current associated with the Berry phase in low-dimensional materials are the key factors.[182] Researchers are trying to find polar semiconductor materials with high ferroelectric polarization strengths, narrow bandgaps, and high output powers. vdW ferroelectrics have wide-range bandgaps, which indicate that they are a potential platform for BPVE. In 2021, Li et al.[183] reported the outstanding bulk photovoltaic effect in vdW FE CIPS with a photocurrent density (2.6 × 10−5 A cm−2) of two orders of magnitude higher than that of conventional bulk ferroelectric perovskite oxides.[184, 185] They also demonstrated a crossover from 2D to 3D bulk photovoltaic effect with the observation of a dramatic decrease in photocurrent density when the thickness of the CIPS exceeds the free path length at ≈40 nm. These results highlight the potential of developing solar cells with ultrathin 2D FE materials.
4.6 Piezo/triboelectric and Pyroelectric Effects
2D materials possessing piezoelectricity have been widely studied for applications in energy harvesters, actuators, and sensors, due to their ultrathin body thickness and remarkable mechanical endurance against large strains. Moreover, piezoelectric characteristics have been demonstrated to exist even at atomic thickness for 2D materials. Theoretical and experimental studies of various 2D-layered materials as piezo/triboelectric harvesters have been reviewed in detail.[186] Ferroelectric crystals have pyroelectric and piezoelectric properties, and their potential applications in currently validated vdW FEs remain unexplored apart from CIPS.[187] The nature of vdW material endows the low-dimensional CIPS with excellent bendability. Moreover, it also has robust intrinsic OOP ferroelectric polarization at RT with a high d33 piezoelectric coefficient of 17.4 pm V−1 in a few-layer thickness, overmatching other 2D-layered piezoelectrics. The piezoelectric nanogenerator based on a single CIPS nanoflake demonstrates a maximum piezoelectric current of 1.7 nA and a voltage response of 12 mV. The high piezoelectric responses make them promising for future strain-modulable nanoelectronic and piezotronic devices integrated with silicon-based chips. Research on the piezoelectricity of other vdW FEs with excellent IP and/or OOP polarization performances also needs to be put on the agenda. vdW ferroelectric materials also display pyroelectricity, which is widely used in thermal imaging, nanogenerator, and energy harvesters. Recently, Jiang et al. observed a large enhancement of the pyroelectric coefficient when the free-standing In2Se3 sheets become thinner.[188] In addition, CIPS nanoflakes, even with a bilayer thickness can have temporary pyroelectric surface charges.[189]
4.7 Catalysis and Energy Conversion
The ferroelectric property plays a significant role in catalysis, which is mainly due to the following aspects: 1) Polarization modulation can alter the electronic structure and surface reactivity of the catalyst, thereby tuning the reaction rate and selectivity. 2) Polarization modulation can optimize the distribution of surface active sites and the structure of catalytic centers, thereby improving catalytic performance. 3) Ferroelectric polarization can regulate charge transfer and distribution on the catalytic surface, influencing electron transfer steps in catalytic reactions, which will enhance the electron conductivity and reaction rate of the catalyst. 4) Ferroelectric materials can absorb, store, and release energy through polarization switching and reversal. This energy storage effect provides an additional energy supply in catalytic reactions, promoting the progress of the reactions. Catalysis based on ferroelectrics offers several advantages, including enhanced light adsorption, increased catalytic efficiency due to intrinsic polarization, reduced electron-hole pair recombination, and improved selectivity via the ferroelectric switch.[190] Ju et al. utilized transition metal-decorated α-In2Se3 monolayers as ferroelectric single-atom catalysts for electrochemical CO2 reduction. The polarization alters the reaction barriers and pathways of CO2 reduction, leading to the formation of different final products. The primary reasons for these phenomena are the ferroelectric-induced adjustments in the empty and occupied d-orbitals of the adsorbed metal atom, electron transfer between In2Se3 and the transition metal, and the CO2 adsorption energies.[191] 2D FE In2Se3 can be used as an electrocatalyst for the hydrogen evolution reaction (HER) catalyst. By constructing Co-In2Se3 vertical heterostructures, the HER activity can be turned on and off by ferroelectrically switching the polarity of In2Se3.[192] The electrocatalytic performance of transition metals embedded in nitrogen-doped graphene catalysts for HER can be effectively regulated by placing them on the surface of ferroelectric In2Se3. The modulation of adsorption energies and electron transfer occurs through the reversal of polarization, inducing electron redistribution, changes in magnetic moments, and shifts in band states near the Fermi level.[193] Calculations indicate that the catalytic performance in single-atom catalysis of CO oxidation using a Pt/In2Se3 monolayer can also be effectively regulated by ferroelectric switching.[194] The intrinsic electric fields in 2D M2X3(M = Al, Ga, In; X = S, Se, Te) ferroelectric materials boost the solar-to-hydrogen efficiency for photocatalytic water splitting.[195] 2D In2X3 (X = S, Se, Te) are promising photocatalysts for overall water splitting under the infrared light spectrum.[196] Differing from traditional photocatalysts, the inherent polarization of In2X3 systems results in a spontaneous built-in electric field, facilitating the spatial separation of photogenerated electrons and holes, thereby enhancing the overall photocatalytic efficiency. Ju et al.’s research on photocatalytic water splitting demonstrates that the photocatalytic activities and energy conversion efficiency can be effectively modulated by the ferroelectric-paraelectric phase transition of an AgBiP2Se6 monolayer.[197]
4.8 Spintronics and Valleytronics
The polarization of vdW FE is closely related to the Berry curvature of the band structure in the Brillouin zone. The Berry curvature is also associated with various topological quantum phenomena, such as non-trivial topology states in Weyl semimetals, spontaneous valley polarization, and Rashba splitting. Thus, the coupled ferroelectric and topological properties have motivated researchers to explore their potential applications. A typical example is group-V monolayers.[198] GeTe is a ferroelectric Rashba semiconductor with a giant Rashba-like splitting, which allows for ferroelectric control of spin without a magnetic field. The magnitude of SHC

More interestingly, a nonvolatile Berry curvature memory[99] was proposed on interlayer sliding ferroelectric few-layer WTe2. In this topological memory, the sign reversal of the large Berry curvature and its dipole enables synchronized annihilation with the switching of the ferroelectric polarization through layer-parity-selective interlayer sliding. In addition, a spin-FET (Figure 11b)[100] based on bilayer WTe2 was proposed, where ferromagnetic metal electrodes covered the multilayer WTe2 regions, and the gate electrodes were used to switch the dielectric polarization of the bilayer WTe2 channel. In this device, ferromagnetic electrodes efficiently inject a highly spin-polarized current into the WTe2, while the gate electrodes allow for control of the spin texture by modulating the ferroelectric polarizations, which effectively improve the spin-polarized injection rate. Usually, the intertwined ferroelectric and topological physics are realized in ferroelectric topological insulators (FETI) in 3D materials, but are rather scarce in 2D. It is worth mentioning that Liang et al.[101] proposed a general paradigm to design 2D vdW ferroelectric topological insulators by sliding the topological multilayers (Bi2Te3) based on first-principle calculations. Bi2Te3 is a topological insulator with a topologically protected chiral boundary state. The IP and OOP ferroelectricity can be induced through specific interlayer sliding, thus enabling the coexistence of ferroelectric and topological orders. If the IP and OOP mirror symmetries, as well as the inversion symmetry, are broken, the ferroelectricity switch will occur, yielding a 2D FETI (in Figure 11c,d). This general scheme opens an avenue toward exploring the coupling between topology, electron correlations, and ferroelectricity in hidden stacking orders and a new electrically controlled memory in spintronic devices.
Ferroelectricity and antiferromagnetism are coupled in bilayer VS2 through a ferrovalley predicated by first-principle calculations.[130] The switching of four different ferroelectric-antiferromagnetic configurations provide a new possibility for realizing multistate storage (Figure 11e,f). Notably, this phenomenon is universal in bilayer ferroelectric-antiferromagnetic ferrovalley materials. Recently, coexisting ferroelectric and ferromagnetic orders with intrinsic anisotropies were discovered in vdW multiferroic CuCrP2S6.[202] It is worth mentioning that vdW FE materials are feasible carriers that can be coupled with layered ferromagnetic (FM) materials[203] to form FM/FE bilayer heterostructures, in which FE polarization reversal may induce semimetal-semiconductor transition (MnCl3/CuInP2S6),[204] magnetization switching (CrOBr/In2Se3),[205] topological transition (β-Sb/In2Se3),[206] and enhanced magneto-crystalline anisotropy (Cr2Si2Te6/In2Se3).[207] The ability of ferroelectricity to control charge, spin, and valley degrees of freedom presents a promising opportunity for the post Moore's era due to the advantages of high-storage density and low-energy consumption.
5 Summary and Outlooks
A considerable number of vdW FEs have Tc higher than room temperature, which makes vdW FEs feasible for investigation and valuable for practical applications.[115] In addition, heterogeneous stacking vdW materials could form a large branch of sliding FE that will greatly expand the vdW FEs library. And most of the sliding FEs, particularly those formed by basic constituent materials, such as Gr, h-BN, and TMDs, exhibit high stability at room temperature. On the other hand, electric polarization is an important parameter used to evaluate and compare the properties of FEs. The polarization of sliding FEs is generally located on a few tenths to several pC/m, as summarized in Table 1. For example, bilayer BN has a polarization of 2.08 pC m−1 (0.68 µC cm−2), bilayer MoS2 is 0.97 pC m−1, bilayer InSe is 0.24 pC m−1, and bilayer BP is 1.6 pC m−1. These values are comparable with those of monolayer vdW FEs that polarization is caused by structure distortion, such as 0.23 µC cm−2 for monolayer t-MoS2 and 0.18 µC cm−2 for monolayer WS2. They are generally smaller than those of conventional FEs with a similar thickness, such as 49 (88) µC cm−2 for 2 u.c. (3 u.c.) BFO film6 and 16 (22) µC cm−2 for 1.5 u.c. (4 u.c) thick PbZr0.2Ti0.8O3 film on bare SrTiO37. However, as mentioned above, this does not affect the application prospects of vdW FE materials, since they have their special advantages in compatibility and integration compared to traditional ferroelectrics. In addition, the polarization switching energy barriers of vdW FEs are smaller than those of conventional ones, for example, 0.6 meV per f.u. for bilayer WTe2 and 19.5 meV/f.u for bilayer BP, and ≈75 meV per f.u. for PbTiO3 at zero in-plane strain.[208] This means that the electric polarization states of thin vdW FEs are more easy to be flipped.
Sliding FEs exhibit rich novel physical properties. Two- and three-layer WTe2 were demonstrated to be a ferroelectric metal, which can be used as electrodes to reduce the contact resistance at the metal-vdW material interfaces. Moiré ferroelectricity domains remind us of the Moiré pattern, periodic charge density waves, superconductivity in magic-angle bilayer graphene, and Moiré phonons. In addition, the up and down ferroelectricity domain networks can propagate by applying a transverse external electric field. Ferroelectricity-topology coupling may give rise to brand-new physical phenomena and scientific frontiers. For example, a Berry curvature memory was designed where the Berry curvature dipole was controlled by ferroelectricity in hidden stacking orders. Sliding FEs also provide a broad platform for the study of ferroelectric spintronics and valleytronics, so that the topological states can be controlled nonvolatile manner. Bilayer TMD heterostructures work as a natural platform for interlayer exciton condensates, enabling a viable strategy to modulate the interlayer excitons using ferroelectric switching via the stacking order of the layers.
Inspired by the fact that robust/stable ferroelectric flipping can be realized even in a single layer and plenty of new phenomena and effects can be observed, research on vdW FEs is in full swing. Researchers have high expectations for the applications in advanced electronic and optoelectronic devices with high-performance and low-energy consumption, such as high-density information storage, intelligent computing for mega data analysis, and non-magnetic-control spintronic devices for quantum computers. However, there are still many challenges on the way to practical applications: i) The very first and key obstacle is that the preparation of high-quality large-area single crystal thin films is still very difficult. Although large-area single-crystal films of a variety of 2D materials have been synthesized recently, the synthesis of vdW FEs has not been carried out effectively. ii) vdW FEs have advantages for use in electronic and optoelectronic memories and neuromorphic devices. However, typical cases mainly focus on α-In2Se3 and CIPS, while devices based on other vdW FEs with robust polarization, such as β-InSe, GeSe, SnS, and SnSe, are rarely reported. iii) An important challenge is to discover air-stable and insensitive vdW ferroelectric materials under ambient conditions, allowing them to operate for a long lifetime. iv) Sliding FEs and the related new findings are attractive, but they are still at the initial stage of theoretical predictions and basic experimental confirmations. v) Ferroelectric-topological coupling may give rise to magical physical phenomena and contribute to new scientific frontiers, such as spintronics and valleytronics. But the research in this area is still at the stage of theoretical calculation, and its practical application remains to be examined with time and experiments.
More studies are needed not only in diverse underlying mechanisms and copious material properties but also in exploring practical applications. It has been ≈20 years since the first re-discovery of 2D materials, and the field is still experiencing the stage from laboratory to industry. It is no doubt that vdW FEs can enrich the 2D library and provide more functionality and possibility. But, to break through bottlenecks and achieve scientific and technological breakthroughs, various subjects and fields, such as physics, chemistry, mathematics, materials science, engineering, and mechanics, should be involved in exploring vdW FEs.
Acknowledgements
This work was supported by the National Key R&D Program of China (nos. 2021YFA1201500 and 2018YFA0703700), the National Natural Science Foundation of China (nos. 12204122, 62274046, 91964203, 62204217, and 61974036), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB44000000 and XDB36000000), and the CAS Key Laboratory of Nanosystem and Hierarchical Fabrication. The authors also gratefully acknowledge the support of the Youth Innovation Promotion Association CAS. [Correction added on April 10, 2024, after first online publication: Figure 11 was updated.]
Conflict of Interest
The authors declare no conflict of interest.
Biographies
Shuhui Li received her Bachelor's degree from Zhengzhou University in 2017 and her Ph.D. degree from the Institute of Physics (IOP), Chinese Academy of Sciences (CAS) in 2021. She is currently engaged in her postdoctoral work at the National Center for Nanoscience and Technology (NCNST), University of Chinese Academy of Sciences. Her current research interest is the controllable synthesis and function of devices based on 2D-layered materials.
Feng Wang received his Bachelor's degree from Yanshan University (2010) and Master's degree from Zhejiang University (2013). He then worked as a research assistant in the Department of Mechanical and Automation Engineering at the Chinese University of Hong Kong. He received his Ph.D. degree from the National Center for Nanoscience and Technology (NCNST) in 2017. After 2 years of postdoctoral work at the Institute of Semiconductors, Chinese Academy of Sciences, he joined NCNST in 2019. His current research interest is developing 2D materials and their heterostructures for electronic and optoelectronic applications.
Xueying Zhan received her B.S. degree at Huazhong University of Science and Technology (HUST), China, in 2010. She then obtained her Master's degree in Optoelectronic Engineering from the College of Optoelectronic Science and Engineering at HUST, China, in 2013. From then on, she worked as an engineer at the National Center for Nanoscience and Technology (NCNST), Chinese Academy of Sciences. Her major research interests include controllable synthesis and applications of low-dimensional and composite nanomaterials in the areas of optoelectronics and photocatalysis.
Jun He received his Ph.D. degree in Semiconductor Physics from the Institute of Semiconductors, Chinese Academy of Sciences, in 2003. He worked successively at the Applied Physics Department of Technische Universiteit Eindhoven, Netherlands; the Material Department of University of California, Santa Babara, and the California Nano System Institute (CNSI) at the University of California, Los Angeles, USA. He became a full professor of the National Center for Nanoscience and Technology (NCNST) in 2010. Now, he is the dean of the School of Physical Science and Technology, Wuhan University. His main research interest is the synthesis, characterization, and devices of low-dimensional semiconductor materials.
Zhenxing Wang received his B.S. and Ph.D. degrees from the University of Science and Technology of China in 2002 and 2009, respectively. After 2 years of postdoctoral work at Peking University, he joined the National Center for Nanoscience and Technology (NCNST) in 2011, where he is currently a full professor. His current research interest focuses on low-dimensional materials and optoelectronic devices.