Van der Waals Ferroelectrics: Theories, Materials, and Device Applications
范德华铁电：理论、材料和器件应用

3.1 Non-stacking Van der Waals Ferroelectrics
3.1 非堆积范德华铁电体

3.1.2 In₂Se₃ and InSe
3.1.2_In2Se3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]}

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 In₂Se_3, 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 T_c 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。]

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 d_I/d_V 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, WSe_2, 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。

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 BN 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BN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）。

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 (ΔV_P) were extracted and compared. ΔV_P of four rhombohedral-stacking bilayer TMDs (≈55 mV) is about half of that in parallel-stacked bilayer h-BN.^[103] The experimentally obtained ΔV_P 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].

Table 1. Summary of vdW sliding FEs and comparisons with typical non-stacking vdW FE materials.

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).

4.1 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 D⁴_6 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 Hz^1/2 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 $$ can reach as huge as 100 $$⁻¹ near the band edge by theoretical calculation (Figure 11a),^[75] indicating the potential of GeTe in ferromagnet-free spintronic devices.^[199] The origin of the large SHC can be attributed to the Rashba effect and intrinsic spin-orbit coupling. Spin-pumping experiments (pump a spin current from the Fe layer into the GeTe and detect its transverse charge current) indicate that a spin-to-charge conversion exists in epitaxial Fe/GeTe(111) heterostructures, unveiling the potential of GeTe for spin-orbitronics.^[200] In addition, the CIPS monolayer is a potential material for valleytronics, as its IP helical and nonhelical pseudospin texture is induced by the Rashba and Dresselhaus effect, respectively.^[201]

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.