Recent Progress in Two‐Dimensional Ferroelectric Materials

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Review

Recent Progress in Two-Dimensional Ferroelectric Materials

Zhao Guan, He Hu, Xinwei Shen, Pinghua Xiang, Ni Zhong,* Junhao Chu, and Chungang Duan

DOI: 10.1002/aelm.201900818

phase transitions by Ginzburg et al.[7] In the 1950s, ferroelectric thin films of perovskite structures were first depos-ited. After successful integration into a semiconductor, ferroelectric thin films exhibit as an important component in the application of electronics and microtransducers. However, most early experiments were performed using a range of film thickness from 100 nm to several micrometers. It has been known since the late 1950s that ferro-electric thin films exhibit quite different phase transition characteristics compared with those of bulk materials. The finite-size effect in ferroelectric films was ana-lyzed and applied to ferroelectrics based on a transverse Ising model.[8] When the thickness of a thin film is below the critical value of around several to tens of nanometers, the spontaneous polariza-tion for traditional perovskite oxide usu-ally disappears with a dropped transition temperature due to the reduced long-

range Coulomb coupling, significantly enhanced depolar-izing electrostatic field, surface-reconstruction caused by the surface energy, electron screening and interfacial bonding related strain, the chemical environment, etc.[9,10] However, with the rapid development of microelectronics, higher den-sity electronic devices are urgently needed and the scaling trends continue to shrink the feature size of materials. Con-trollable thinner, that is, nanoscale ferroelectric materials, are essential not only for understanding the size effect but also for electronic applications. Although the dilemma has posed unprecedented challenges to the entire industry, many efforts have been put forward to overcome the difficulties. As early as the 1980s, ferroelectricity was introduced in polyvinylidene fluoride[11] and its copolymers with trifluoroethylene P(VDF-TrFE).[12] In 1986, P(VDF-TrFE) films as thin as 60 nm with a saturated polarization of 100 mC m–2 were reported.[13] Later in 1991, Scott reported the phase transitions in ferroelec-tric films with a thickness of 70–400 nm both theoretically and experimentally.[14] Significantly, in 1998, a ferroelectric transition in a two monolayers thick random copolymer of vinylidene fluoride and trifluoroethylene was observed.[15] Then, in 2000, Blinov et al. discussed the unique ferroelec-tric properties in two monolayers (≈1 nm) of P(VDF-TrFE), which were fabricated by the Langmuir–Blodgett tech-nique and demonstrated a breakthrough in original critical thickness, thus encouraging researchers to further explore

The investigation of two-dimensional (2D) ferroelectrics has attracted significant interest in recent years for applications in functional electronics. Without the limitation of a finite size effect, 2D materials with stable layered structures and reduced surface energy may go beyond the presence of an enhanced depolarization field in ultrathin ferroelectrics, thereby opening a pathway to explore low-dimensional ferroelectricity, making ultra-high-density devices possible and maintaining Moore’s Law. Although many theoretical works on potential 2D ferroelectric materials have been conducted, much still needs to be accomplished experimentally, as it is rare for 2D ferroelec-tric materials to be proven and plenty of 2D ferroelectrics are waiting to be discovered. First, experimental and theoretical progress on 2D ferroelectric materials, including in-plane and out-of-plane, is reviewed, followed by a general introduction to various characterization methods. Intrinsic mecha-nisms associated with promising 2D ferroelectric materials, together with related applications, are also discussed. Finally, an outlook for future trends and development in 2D ferroelectricity are explored. Researchers can use this to obtain a basic understanding of 2D ferroelectric materials and to build a database of progress of 2D ferroelectrics.

Z. Guan, H. Hu, Dr. X. W. Shen, Prof. P. H. Xiang, Prof. N. Zhong, Prof. J. H. Chu, Prof. C. G. DuanKey Laboratory of Polar Materials and Devices (MOE) and Department of ElectronicsEast China Normal University500 Dongchuan Rd., Shanghai 200241, ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.201900818.

1. Introduction

Ferroelectric materials, which are functional materials with stable and switchable spontaneous polarization that can be reversed by an external electric field, are technologically important for many applications, including nonvolatile mem-ories,[1,2] field-effect transistors (FET),[3] solar cells, sensors, photonics devices, etc.[4,5] Ferroelectricity was first discov-ered in 1920 in the form of bulk single crystals of Rochelle salt by Valasek.[6] Then, a second ferroelectric material, monobasic potassium phosphate KH2PO4, was discovered in 1935 by Busch and Scherrer. It was not until the 1940s that the first phenomenological theory of ferroelectricity was developed based on the Landau theory of second-order

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two-dimensional (2D) ferroelectricity.[16] Since then, together with the theoretical development, plenty of studies on ultrathin traditional ferroelectric compounds, such as PbTiO3 (1.2 nm),[10] BaTiO3 (2.4 nm),[17] and BiFeO3 (BFO) (1 unit cell [u.c.]),[18] and nontraditional ferroelectrics, such as HfO2/ZrO2 with element doping,[19] have been reported, suggesting that it could be retained down to several unit cells and even a single unit cell with the development of thin film fabrication technology. So far, the possibilities of nanoscale traditional ferroelectrics have been well proven by theoretical predic-tions. However, it should be mentioned that either the careful selection of growth substrates with less lattice mismatch or a complex fabrication process is always required for fabricating high-quality ferroelectric thin films at the nanoscale, which still extensively limits the potential applications of ferroelec-tric thin film in modern nanoelectronics.

Over the past several years, 2D materials have attracted increasing interest due to their unique properties where the charge and heat transport is confined within a plane.[16,20–22] Worldwide scientific efforts have been focused on 2D mate-rials since 2004, when Novoselov et al. successfully exfoliated graphene mechanically from graphite.[23] Since then, thou-sands of 2D materials beyond graphene have been discovered, including transition metal dichalcogenides (TMDs),[24,25] hex-agonal boron nitride,[26] MXenes,[27,28] Xenes,[29] organic mate-rials,[30] etc.[31] Compared with bulk forms, van der Waals 2D layered materials with strong intralayer chemical bonds but weak interlayer interactions usually lose some symmetry due to the reduction of dimensions, which might go beyond the limitations in 3D materials, providing us a potential pathway to obtain low-dimensional ferroelectricity. So far, low-dimen-sional materials with atomic thicknesses are playing an important role in various applications, including electronics/optoelectronics,[32] catalysis,[33] chemical and biological sen-sors,[34] energy storage and conversion,[35] supercapacitors,[36] solar cells,[37] biomedicine,[38] lithium ion batteries,[39] etc., which is beneficial for sustaining Moore’s Law for a longer period of time. Recently, the theoretical works have success-fully predicted many 2D materials with intrinsic ferroelec-tricity, such as 1T-MoS2, In2Se3, CuInP2S6, and MX (M = Ge, Sn; X = S, Se, Te). More inspiringly, the Curie temperature of CuInP2S6 and MX calculated by theory indicates stable fer-roelectricity at room temperature. Therefore, it is expected that, if ferroelectricity could be obtained in 2D materials, the integration of 2D ferroelectric materials for semicon-ductor wafers would promote their prosperous evolutions in microelectronics.

Nevertheless, it is currently still a large challenge to observe stable and strong out-of-plane (OOP) and in-plane (IP) spon-taneous polarization in 2D materials with a few unit cells. Although the existence of 2D ferroelectricity was predicted long before in 1976,[21] it was not until 2015 that 2D ferroelectricity was found in a real sense experimentally.[40] In fact, realistic 2D ferroelectric materials still suffer from severe tests where pristine structures of many known 2D materials are exclusively OOP centrosymmetric and the intrinsic ferroelectric mecha-nisms within 2D materials are still scanty and controversial. Therefore, taking a global look at the history of 2D ferroelec-tricity and its future development is necessary. Recently, some

Zhao Guan received her B.S. degree from Shandong Normal University in 2015. She is a Ph.D. candidate in the Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University. She is currently working on domain dynamics, 2D fer-roelectrics, and nonvolatile memories based on piezore-sponse force microscopy.

Ni Zhong received her B.S. degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences, and her Ph.D. from NARA Institute of Science and Technology (NAIST), Japan. In 2012, she joined the Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University as an associate professor. She is

currently focusing on ferroelectric thin films/2D ferroelec-trics/strongly correlated material and novel devices for next-generation computing systems.

Chungang Duan earned his Ph.D. degree in theoretical physics from the Institute of Physics, Chinese Academy of Sciences, Beijing, China, in 1998. He then worked at the University of Nebraska, USA, from 1998 to 2007. In 2008, he joined the East China Normal University as a full professor. He is now the director of the Key Lab of

Polar Materials and Devices, Ministry of Education, China. His group is currently working on functional materials for applications in information storage, energy conversion, and neuromorphic computing.

reviews have been reported to introduce 2D piezoelectric, fer-roelectrics, or multiferroic materials.[22,41,42] Here, we give a forward-looking review of the ferroelectricity in 2D mate-rials both theoretically and experimentally. After describing the history of 2D ferroelectrics, including the predicted or proven 2D ferroelectric materials, we will introduce various characterization/calculation methods and the typical explana-tions for identifying and characterizing 2D ferroelectrics in the





hopes of determining the underlying intrinsic mechanisms of 2D ferroelectricity. At the end, we discuss the applications of 2D ferroelectric materials and give an outlook to inspire more exciting studies.

2. 2D Ferroelectric Materials

2.1. Why Have So Many 2D Materials Been Predicted Theoretically as Ferroelectrics?

As early as 1976, 2D ferroelectricity was discussed theoretically by Feder.[21] However, the existence of the depolarization field/size effect makes ultrathin ferroelectric materials a challenge. The critical thickness in traditional ferroelectric materials is limited above a few unit cells and the Curie temperature also significantly decreases as the film becomes thinner.[43] Since graphene was mechanically exfoliated from graphite in 2004,[23] the outstanding behaviors of 2D materials make them another promising candidates to open up the possibility of 2D ferro-electricity, rendering a better combination of nanocircuits and a higher integration density. Plenty of calculations have been proposed to predict promising 2D ferroelectric materials, offering a promising guidance of experimental observations. For an overview of the theoretical progresses for 2D ferro-electric materials, we make a brief summary in the following sections.

2.1.1. Recent Progress in Promising 2D Ferroelectric Materials

For van der Waals compounds, in 2014, piezoelectricity was revealed in a free-standing single layer of MoS2 by experimental evidence, where inversion symmetry breaking only existed in odd-numbered layers.[44] In the same year, the distorted 1T monolayer MoS2 (d1T-MoS2) was suggested as a ferroelec-tric material. Because of the degeneracy of the Fermi surface and a strong electron-phonon coupling with the opening of the bandgap, the ferroelectricity in distorted 1T-MoS2 was induced by the predominantly Mo displacement in the plane of the centrosymmetric 1T-MoS2, which could be seen from the broken inversion symmetry in the charge density difference (Figure 1a).[45] Later in 2016, similar calculations were extended to the monolayers of the 1T transition TMDs MX2 (M = Mo, W; X = S, Se, Te) with a distorted octahedral coordinated struc-ture (t-MX2), as shown in Figure 1b.[46] They found all t-MX2 monolayers with d2 metal ions show spontaneous dielectric polarization, which makes TMDs materials promising in 2D ferroelectricity.

In 2015, several low-buckled honeycomb AB binary mon-olayers (A and B belong to groups IV or III–V) were predicted to be ferroelectrics, where the valley-related properties and the coupling of the spin and valley physics are generated by the loss of inversion symmetry. The presence of the buckled honeycomb lattices, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with a trigonal symmetry, makes the ferroelectricity in the honey-comb binary compounds possible (Figure 2a).[47] In 2017, In2Se3 and other III2–VI3 van der Waals materials were pre-

dicted to be room temperature ferroelectric materials with both OOP and IP electric polarization. The vertical polariza-tion originates from the different interlayer spacing between the central Se layer and the two In layers, which is caused by the inequivalent In-Se bonds in two layers (Figure 2b). Due to the absence of IP inversion symmetry, this branch of 2D materials family could also present IP electric polarization.[48] As for In2Se3, the several experimental reports for its ferro-electricity with different phases that have been illustrated, though the claimed intrinsic mechanisms are not consistent, which we will discuss in section 4. In the same year, the ver-tical ferroelectricity induced by interlayer translation in a graphitic binary compound bilayer of BN, AlN, ZnO, MoS2, GaSe, etc., was discussed by Li et al. As shown in Figure 3a, a strong charge transfer from the upper layer to the down layer induced by the closer interlayer B-N distance could give rise to vertical polarization.[49] Such interlayer translation, which could be obtained by twist, strain, or an electric field, sheds some light on the experimental studies of 2D ferroelectrics and 2D ferroelectric applications, including the superlattice, nanogenerators, FET, etc.[50]

In addition to the above structures we mentioned in this section with two-element components, the transition metal thi-ophosphate (TMTP) family also attracts investigations for 2D ferroelectricity. In 2017, Xu et al. reported that the monolayer AgBiP2Se6 could exhibit OOP polarization with compensated ferroelectric ordering with a thickness of 6 Å. As shown in Figure 2c, the anti-ferroelectric distortion is induced by the off-centering Ag+ and Bi3+ ions, resulting in two stable ferroelectric phases with opposite directions.[51] Similar to AgBiP2Se6, other TMTP materials, including CuInP2S6,[40,52,53] CuInP2Se6,[54,55] etc., are also typical and promising materials for researchers to study. Among them, CuInP2S6 is a typical member of the TMTP family with intralayer ferroelectric ordering, which has already been proven to be a ferroelectric material experimen-tally and will be discussed in the next section.

To make the story more interesting, in addition to compound materials with multiple elements, elemental materials are also anticipated in the banquet, including elemental tellurium (Te) multilayers,[56] group V elemental monolayers (As, Sb, Bi),[57] etc. As for Te multilayers, the interlayer interaction-induced IP ion-displacement breaks the centrosymmetry. According to the calculation, the spontaneous IP polarization could persist up to 600 K, and it shows valley-dependent spin textures that could be controlled by polarization, making elemental ferroelectric mate-rials promising candidates for electronic and spintronic appli-cations.[56] Additionally, a 2D elemental group-V monolayer with a puckered lattice structure similar to phosphorene shows a robust IP spontaneous polarization. Due to the reduced sp3 hybridization, the elemental group-V monolayer has a buckled structure, which is shown in Figure 3b. The shift of the atoms denoted as a red color balls breaks the centrosymmetry, and thus induces two different phases with two opposite polariza-tion directions. Additionally, the intrinsic mechanisms of IP polarization in these two groups of elemental materials are dif-ferent, which we will discuss in section 4.

Except for the intrinsic 2D ferroelectric materials modulated by external stimulations, such as electricity or force, manually functionalized non-van der Waals 2D ferroelectric materials also





play an important role in the 2D ferroelectric family. In 2013, hydroxylized graphene, denoted as graphanol, was predicted as the first 2D van der Waals ferroelectric material with a high polar-ization. Proton displacement-induced dipole moments make hydroxylized graphene systems promising candidates for high-capacity cathode materials.[58] Compared with graphanol, 2D silicene, germanene, and stanene could also be functionalized by hydroxyl, methyl, or ligands.[59] Moreover, covalently function-alized 2D materials have also been designed to be ferroelectric

materials with high mobility and modest bandgaps through self-assembled monolayers. The ligand with a dipole moment shown in Figure 4a was discussed as the origin of ferroelectricity. The transformation of free carriers at the interface between electrons and holes could induce polarization switching.[60] These func-tionalized ferroelectric 2D materials, which could be designed as heterostructure devices, such as ferroelectric FET (FeFET), topo-logical transistors, and ferroelectric tunneling junctions, have great potential to anticipate future multifunctional devices.


Figure 2. a) Valence bands at the valley without and with spin-orbit coupling. Reproduced with permission.[47] Copyright 2015, American Physical Society. b) 3D crystal structure of layered In2Se3 and a side view of several structures in one quintuple layer In2Se3. The black arrows indicate the directions of the spontaneous polarization in the FE-ZB0 and FE-WZ0 structures. Reproduced with permission.[48] Copyright 2016, Nature Publishing Group. c) Top view of the structure of monolayer AgBiP2Se6, schematic side views of the two distorted phases with different polarization directions, and the phonon spectra of the ferroelectric phase and paraelectric phase of monolayer AgBiP2Se6. Reproduced with permission.[51] Copyright 2017, The Royal Society of Chemistry.

Figure 1. 2D ferroelectric materials in theory. a) Atomic structure comparison of centrosymmetric and distorted 1T MoS2, the band structure, and the charge densities of the ferroelectric d1T state with up polarization and the c1T state. Reproduced with permission.[45] Copyright 2014, American Physical Society. b) Distortion modes at the structural transition and zigzag chains within the centrosymmetric and trimerized MX2 phases. Reproduced with permission.[46] Copyright 2016, American Physical Society.




2.1.2. 2D Multiferroic Materials

Multiferroic materials are defined as materials that simultane-ously possess more than one primary ferroic property in the same phase, including ferromagnets, ferroelectrics, and fer-roelastics.[61] The functionalities of both orders and possible novel properties do not exist in either state alone, which make multiferroics interesting and caused them to attract growing attentions in the past decades.[62] Efforts to combine ferromag-netic and ferroelectric orders into one phase have been made for quite some time because of the mutual exclusion between them. However, with increasing discoveries of novel mecha-nisms driving multiferroicity, including the lone-pair mecha-nism, spin-driven mechanism,[63] charge ordering,[64] etc., multiferroics are expected to become involved in the funda-mental scientific studies.[65]

It is worth mentioning that many 2D ferroelectric mate-rials are predicted to be multiferroics, in which ferromag-netism, ferroelasticity, or even anti-ferroelectricity orders coexist, giving the materials great potential in applications of spintronics and nonvolatile memories. However, it was not until recently that an increasing amount of 2D multiferroic

materials have been predicted theoretically, which are either the coupling of ferroelectricity and ferroelasticity[66] or of ferromagnetism and ferroelectricity.[49,66–68] In 2016, the coexistence of ferroelectricity and ferroelasticity in GeS and GeSe monolayers was predicted.[69] The switchable pathway of the parallel zigzag chains with dipole moments along the armchair direction makes the existence of ferroelectricity in SnS and SnSe possible. As shown in Figure 4b, ferroe-lastic switching could be obtained by either applying a strain along the –y direction or an external electric field to obtain a phase change of 90°.[69] There is no doubt that this work pushes group-IV monochalcogenides into the focus of more properties, including domain wall motion,[70] optical second harmonic generation (SHG),[71] etc.[72] Later in 2017, Yang et al. discussed multiferroics in chemically functionalized phosphorene, including halogen- or hydrogen-intercalated bilayer graphene, silicene, germanene, and MoS2. The cou-pling between the OOP ferroelectricity and ferromagnetism came from the hopping of halogen adatoms between bilayers against the depolarization field and gave a possible approach to realizing high-density storage. The combination of elec-trical writing and magnetic reading also made them ideal


Figure 3. 2D ferroelectric materials in theory. a) Ferroelectric switching of BN bilayer and stacking configurations of a graphene/BN heterobilayer with distinct interlayer potentials. Reproduced with permission.[49] Copyright 2017, American Chemical Society. b) Top and side views of group-V elemental monolayer with undistorted and distorted structures and free energy contour for an As monolayer. Reproduced with permission.[57] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 4. a) Side and top views of methyl-terminated germanene/stanine, Sn (P, As, Sb)–CH2OCH3, silicon (111) passivated by –SH and –Cl and graphene nanostripes functionalized by –OH and –F. Reproduced with permission.[60] Copyright 2016, American Chemical Society. b) Pathway of fer-roelectric switching of SnS/SnSe monolayer. Reproduced with permission.[69] Copyright 2016, American Chemical Society.




energy-efficient nonvolatile memories.[73] These works have inspired many researchers to study the 2D ferroelectrics in the promising group IV monochalcogenides.[69,70,74–77] The outstanding performance in both the ferroelectric and mul-tiferroic fields indicates that 2D materials play an important role in low-dimensional high-energy physics and devices. In future nanoelectronics, ideal nonvolatile memories may be based on 2D multiferroic materials that combine efficient electric writing with energy-saving magnetic reading, facili-tating the extension of Moore’s Law.

Additionally, ferrovalley materials with spontaneous valley polarization have also been recently proposed in 2D ferroelec-trics. As a new member of the ferroic family, ferrovalley mate-rials originated from at least two orders, including ferromag-netism[78] and ferroelectricity,[79] show the potential to break through the restriction of the external fields, becoming a new frontier for next-generation nonvolatile information storage.

2.1.3. Potential 2D Ferroelectric Materials

2D piezoelectric materials with noncentrosymmetric struc-tures are always the most potential candidates for finding ferroelectricity. Different from ferroelectricity, where spon-taneous polarization can be controlled by an external elec-tric field, piezoelectricity is a property of electric polarization caused by externally applied and uniform mechanical stress. In 2012, 2D monolayer TMDs materials, including BN, MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2, were discovered as piezoelectric materials based on density functional theory (DFT) calculations.[80] Later in 2014, Wu et al. provided direct evidence of the piezoelectric effect for monolayer MoS2, which was attributed to the displacement of Mo cations and S anions.[81] In the same year, Zhu et al. reported experimental evidence of piezoelectricity in a suspended free-standing MoS2 membrane with two terminals clamped on the metal electrodes. Breaking and recovery of the inversion symmetry of the 2D crystal orientation was attributed to a finite and zero piezoelectric response in MoS2 in the odd and even numbered layers, respectively.[44] Inspired by these works, other mate-rials, including layered hexagonal boron nitride (h-BN),[82,83] 2D graphene nitride,[84] doped graphene,[85] Janus MoSSe,[86] etc., have been predicted and proven to be piezoelectric mate-rials as well. Additionally, a review related to 2D piezoelec-tricity has been published in 2018, giving us a useful database for the related research community.[42]

Closely related to piezoelectricity, flexoelectricity, which is polarization induced by a strain gradient, also cannot be ignored in discovering or creating 2D ferroelectrics. The advan-tage of flexoelectricity is that the noncentrosymmetric structure is not a must. A centrosymmetric material could also possess flexoelectricity, making 2D ferroelectric materials transferred from nonferroelectrics possible. There are plenty of reports concerning flexoelectricity in solids, which perhaps give a new pathway to obtaining 2D ferroelectric materials.[87,88]

To conclude, from the discussion in Section 2.1, we can obtain a simple recognition of 2D ferroelectric materials in theory. Achieving ferroelectricity in low-dimensional materials has been a long sought-after goal in physical studies due to its

importance in fundamental science and the great potential for nanoscale device applications. It is highly desirable to discover or design novel atom-thick 2D materials that have IP polariza-tion, OOP polarization, or both as potential candidates. There are currently two major general directions in searching for 2D fer-roelectricity: 1) find atomistic monolayer, bilayer, and multilayer materials with intrinsically broken centrosymmetry; and 2) trans-form nonintrinsic-ferroelectric 2D materials into ferroelectrics by functioning the asymmetry structure, including the twisting angle in the van der Waals bilayer,[49,89] passivation,[60] controlling equiaxial strain,[90] etc.[22] However, whatever pathway is used to obtain 2D ferroelectrics, broken centrosymmetry is the key to finding or creating a new 2D ferroelectric materials. Among the many predicted 2D ferroelectrics, the class of phosphorene and phosphorene analogs with large spontaneous polarization, the desired bandgap, high carrier mobility, and small domain wall energy has promising nanoelectronics materials for prac-tical devices. Furthermore, metal ferroelectrics are also a promi-nent topic since the discovery of ferroelectricity in metal WTe2 in 2018, even if the concept had been proposed as early as 1973. The coexistence of metallic and ferroelectric behaviors in one material also makes the coupling of multiferroic orders less elu-sive, which we will discuss in detail in the next section.[91]

To have a clearer overview of the development of theoretical 2D ferroelectric materials, a list that includes the polarization magnitude, polarization direction, Curie temperature, critical thickness, etc., is summarized in Table 1. Note that many more articles have been published; what we present here is just the tip of the iceberg.

2.2. How Many 2D Materials Have Been Proven to Be Ferroelectrics?

Benefitting from the improvement in fabrication and char-acterization techniques, ferroelectricity could be observed in ultrathin films down to several unit cells. However, in contrast with the development of theoretical calculations, the experi-mental observation of 2D ferroelectric materials is still rare and the origins of 2D ferroelectricity also remain unclear and controversial due to the realistic limitations of weak or dimin-ished ferroelectric signals. So far, no more than ten 2D ferro-electric materials have been discovered experimentally except for α-In2Se3, CuInP2S6, SnTe, BA2PbCl4, d1T-MoTe2, and WTe2. By combining different advanced techniques, an increasing amount of ferroelectric proofs have been found to illustrate 2D ferroelectricity with various intrinsic mechanisms. Here we give a brief introduction of the latest experimentally reported 2D ferroelectric materials. These proven 2D ferroelectric mate-rials are also summarized in Table 2.

2.2.1. Copper Indium Thiophosphate

The TMTP family is a broad class of van der Waals layered solids, which offers a large number of ferroelectric bulk materials[92] and usually takes on the form of ABP2X6, where A/B is either a monovalent/trivalent or divalent/divalent combination and X is a chalcogenide, including S, Se, Te, etc. Among them, the atomic






Table 1. Predicted 2D ferroelectric materials.

Materials Year IP OOP Polarization Thickness TC Mechanisms Refs.

A-B stacked bilayer graphene

nanoribbons with armchair

edges

2015 ✓ –0.12e per spin Bilayer – Interlayer bias voltage and edge states [175]

Armchair phosphorene

nanoribbons

2016 ✓ 0.155–6.08 µC cm–2 Monolayer/bilayer – Electron polarization [170]

As 2018 ✓ 0.46 E-10 C m–1 Monolayer 478 K Structure distortion [57]

Sb 2018 ✓ 0.75 E-10 C m–1 Monolayer 680 K Structure distortion [57]

Bi 2018 ✓ 0.51(FE)/1.41(AFE) E-10

C m–1

Monolayer 463 K Structure distortion [57]

Te 2018 ✓ 1.02 E-10 C m–1 per layer Multilayer >RT Ion displacement [56]

d1T-MoS2 2014 ✓ 0.18 µC cm–2 0.5 nm >RT Structure distortion [45]

t-MoS2 2016 ✓ 0.23 µC cm–2 Monolayer – Structure distortion [46]

MoS2 2017 ✓ 0.97 E-12 C m–1 Bilayer – Interlayer translation [49]

t-MoSe2 2016 ✓ 0.15 µC cm–2 Monolayer – Structure distortion [46]

t-MoTe2 2016 ✓ 0.10 µC cm–2 Monolayer – Structure distortion [46]

WS2 2016 ✓ 0.18 µC cm–2 Monolayer – Structure distortion [46]

WSe2 2016 ✓ 0.21 µC cm–2 Monolayer – Structure distortion [46]

WTe2 2016 ✓ 0.11 µC cm–2 Monolayer – Structure distortion [46]

WTe2 2018 ✓ 5.1 E-2 µC cm–2 Bilayer and

multilayer>350 K Interlayer translation [174]

BiN 2017 ✓ 580 pC m–1 Monolayer 500 K Structure distortion [201]

SbN 2018 ✓ 7.81 E-10 C m–1 Bilayer 1700 K Ion displacement/structure distortion [172]

BiP 2018 ✓ 5.35 E-10 C m–1 Bilayer 800–900 K Ion displacement/structure distortion [172]

InSe 2017 ✓ 0.24 E-12 C m–1 Bilayer – Interlayer translation [49]

GaN 2017 ✓ 9.72 E-12 C m–1 Bilayer – Interlayer translation [49]

GaSe 2017 ✓ 0.46 E-12 C m–1 Bilayer- – Interlayer translation [49]

SiC 2017 ✓ 6.17 E-12 C m–1 Bilayer – Interlayer translation [49]

BN 2017 ✓ 2.08 E-12 C m–1 Bilayer – Interlayer translation [49]

AlN 2017 ✓ 10.29 E-12 C m–1 Bilayer – Interlayer translation [49]

ZnO 2017 ✓ 8.22 E-12 C m–1 Bilayer – Interlayer translation [49]

Buckled honeycomb AB

monolayer

2015 ✓ 0.88–11.45 E-12 C m–1 Monolayer – Buckled honeycomb lattice [47]

In2Se3 and other III2–VI3 van

der Waals materials

2017 ✓ ✓ – Single/multi-qua-

nintuple layer

RT Structure distortion [48]

Phosphorus oxides (PxOy) 2016 ✓ ✓ 1.2–2.4 E-12 C m–1 0.14–0.32 nm – Presence of lone-pair electrons, col-

lective oxygen displacements along

the y axis

[202]

GeS 2016 ✓ ✓ 4.41 E-10 C m–1 Monolayer >RT Structure distortion [69]

GeS 2017 ✓ 48.4 µC cm–2 Monolayer RT Ion displacement [70]

GeS 2016 ✓ 5.06 E-10 C m–1 Monolayer-odd-

number-layer

6400 K Structure distortion [74]

GeSe 2016 ✓ ✓ 3.4 E-10 C m–1 Monolayer >RT Structure distortion [69]

GeSe 2017 ✓ 35.7 µC cm–2 Monolayer RT Ion displacement [70]

GeSe 2016 ✓ 3.67 E-10 C m–1 Monolayer-odd-

number-layer


Monolayer β-GeSe 2018 ✓ 0.16 nC m–1 Monolayer 212 K Structure distortion [76]

SnS 2016 ✓ ✓ 2.47 E-10 C m–1 Monolayer >RT Structure distortion [69]

SnS 2017 ✓ 26 µC cm–2 Monolayer RT Ion displacement [70]

SnS 2016 ✓ 2.62 E-10 C m–1 Monolayer-odd-

number-layer


(Continued)





Materials Year IP OOP Polarization Thickness TC Mechanisms Refs.

SnSe 2016 ✓ ✓ 1.87 E-10 C m–1 Monolayer >RT Structure distortion [69]

SnSe 2017 ✓ 18.1 µC cm–2 Monolayer RT Ion displacement [70]

SnSe 2016 ✓ 1.51 E-10 C m–1 Monolayer-odd-

number-layer


SiTe 2017 ✓ 42.0 µC cm–2 Monolayer 570 K Interatomic interactions [75]

GeTe 2017 ✓ 32.8 µC cm–2 Monolayer 570 K Interatomic interactions [75]

GeTe 2017 ✓ – – <1 u.c. – Structure distortion [109]

SnTe 2017 ✓ 19.4 µC cm–2 Monolayer 570 K Interatomic interactions [75]

SnTe 2017 ✓ – – <1 u.c. – Structure distortion [109]

PbTe 2017 ✓ – – <1 u.c. – Structure distortion [109]

Rashba lead chalcogenide

PbX (X = S, Se, Te)

monolayers

2018 ✓ 0.03 µC cm–2 Monolayer – Out-of-plane strain [203]

2D hyperferroelectric metals

(CrN, CrB2)

2017 ✓ 6.2 E-12 C m–1 Monolayer – Spin-phonon coupling and metal–

metal bonding

[114]

Transition metal halide

monolayer (CrBr3)

2018 ✓ 0.92 E-10 C m–1 Monolayer 50 K Structure distortion [173]

Monolayer metal trihalides

(monolayer CrI3 with I

vacancies)

2018 ✓ – Monolayer RT I vacancies [171]

Unzipped graphene oxide

monolayers

2016 ✓1.67–2.92 pC m–1

6.95, 1.96 µC cm–2

Monolayer – Oxidation induced by foldable bonds

between COC bond

[204]

GaTeCl 2018 ✓ 187–578 pC m–1 Monolayer – Structural distortion/spontaneous

strain

[205]

AgBiP2Se6 2017 ✓ 0.2 µC cm–2 Monolayer >RT Ion-displacement/ferroelectric

ordering

[51]

TMPCs-CuMP2X6 (M = Cr, V;

X = S, Se)

2018 ✓ 0.65–0.79 pC m–1 Monolayer RT Structure distortion [68]

Chalcogen diphosphates

(MCDs, CuInP2Se6)

2017 ✓ 0.32–1.515 µC cm–2 >6 layers (4 nm) 150 K Structure distortion [54]

Oxygen-functionalized

scandium carbide Mxene

(Sc2CO2)

2017 ✓ ✓ 1.6 µC cm–2 Monolayer RT Redistribution of charge carriers [28]

Bismuth oxychalcogenides

(Bi2O2Se, Bi2O2Te, Bi2O2S)

2017 ✓ ✓56.1/7.1 µC cm–2

0.12/0.15 E-10 C m–1

– RT Structure distortion/interlayer dis-

placement of adjacent Bi2O2 layers

[66]

2D perovskite oxide thin films 2018 ✓ – 1–5 u.c. RT Second-order Jahn–Teller effect, sur-

face effect, trilinear coupling between

two rotational modes, and the A-site

displacement

[122]

Graphanol 2013 – – 41.1, 43.7, 67.7, 27 µC

cm–2

Single and

multilayers

500 K Proton hoping and rotation [58]

Hydroxyl-functionalized

graphene

2013 – – 6.6 µC cm–2 Atomic scale 700 K Hydroxyl decoration/structure

distortion

[206]

Covalently functionalized

monolayers

2016 ✓ – 0.31–1.17 E-10 C m–1 – 350 K Ligand with a dipole moment [60]

Halogen-decorated

phosphorene

2017 ✓ 0.11 E-10 C m–1 Monolayer and

bilayer

572 K Horizontal displacement of the

halogen/covalent bonded

[73]

Hydrogenated carbon nitride

(g-C6N8H)

2017 ✓ – 10.2, 12.7 µC cm–2 0.357 nm 250 K, 700 K Multimode proton-transfer [67]

Bismuth layers functionalized

by CH2OH (Bi-CH2OH)

2018 ✓ 0.0263–0.256 E-10 C m–1 Atomically thin RT Ligand molecule rotation mechanism [176]

TC, Curie temperature; Ref.: references; RT, room temperature; u.c., unit cell.

Table 1. Continued.




structure of copper indium thiophosphate (CuInP2S6) contains a sulfur framework, in which Cu, In, and P-P triangular patterns are filled with the octahedral voids, as shown in Figure 5a. In 1994, the first-order paraelectric-ferroelectric phase transition at 315 K in bulk CuInP2S6 was first observed via calorimetry, X-ray powder diffraction, and dielectric measurements.[93] In 1997, this material was suggested to be ferroelectric with a hop-ping mechanism on the basis of X-ray and neutron diffraction studies. The hopping mechanism involving thermally activated Cu(I) migration along the polar axis in the paraelectric phases was predicted to demonstrate the origin of ferroelectricity, while other polarization effects were excluded by the thermal and fre-quency dependence measurements on the real and imaginary parts of the complex permittivity and modulus.[94] Later in 2015, Belianinov et al. reported ferroelectric CuInP2S6 down to ≈50 nm experimentally, as evidenced by domain structures, rewritable polarization, and hysteresis loops, opening up the opportunities to obtain ferroelectric CuInP2S6 down to the nanoscale.[40] In the same year, Susner et al. showed enhanced ferroelectric proper-ties in CuInP2S6 as thin as 20 nm by chemical and structural modification through composition tuning-like periodic modula-tion of the Cu/In ratio, as shown in Figure 5b, offering a new pathway to produce 2D IP heterostructured materials.[95] Only after 1 year, the size effect in 10 nm CuInP2S6 was discussed,[53] while room temperature ferroelectricity in ultrathin CuInP2S6 was reported by Liu et al. in the same year.[52] The switchable polarization in a film as thin as 4 nm with a transition tem-perature of ≈320 K, as shown in Figure 5c, was reported. The spontaneous polarization was suggested to originate from the displacement of copper sublattice from the centrosymmetric positions to the indium sublattice. The anti-alignment dipole largely reduces the depolarization field inside CuInP2S6, indi-cating the stability of polarization down to ultrathin thicknesses. The TMTP family is a promising candidate for discovering intra-layer ferroelectricity since the metal atoms could move to a side of the octahedral hole and create dipole moments, which would make the materials ferroelectric or antiferroelectric due to the

alignment of different moments. Meanwhile, this has poten-tial not only in ferrovalley materials combined with magnetic materials but also in multiferroics, which have great prospects. However, for CuInP2S6, the ionic displacement would also cause a change in the topography. In 2018, Balke et al. studied ionic conduction in CuInP2S6 through local volume changes with the function of temperature and frequency.[96] The displacement induced by the driving field increases with an increasing tem-perature and drops with an increasing frequency, largely indi-cating that the ordering of the Cu ions in two off-center sites in the crystal lattice is the most possible explanation for CuInP2S6 ferroelectricity. Although a systematic comparison measure-ment has been made in this article, the larger electromechanical response and higher contact resonance frequency contribute to a change in the topography, making the experimental results more complex to explain. In fact, the change in topography is usually an inevitable problem during 2D ferroelectricity studies, which would make the observed evidence less reliable and sharply increases the ability to explain the phenomena. The nanoscale thickness and different interface conditions could make the 2D materials less hard and more fragile, dramatically increasing the difficulties in studying 2D ferroelectricity. Therefore, over-coming or avoiding the influence of topography changes will be a key point in obtaining further promising 2D ferroelectric materials for a long time.

2.2.2. Indium Selenide

2D materials based on III–VI semiconductors, crystallized with a layered structure, have been extensively investigated. Indium selenide (In2Se3) is one of the most important III–VI compounds, where a single layer of In2Se3 consists of alter-nating Se or In atomic layer via covalent bonds and an Se-In-Se-In-Se quintuple layer is constituted by stacking single layers together through weak van der Waals force, which is shown in Figure 6b.[97] It usually possesses five known forms


Table 2. Proved 2D ferroelectric materials.

Materials Year IP OOP Thickness Curie temperature Mechanisms Ref.b)

CuInP2S6 2015 ✓ 50 nm 315 K Structure distortion [40]

CuInP2S6 2016 ✓ 4 nm 315 K Structure distortion [52]

SnTe 2016 ✓ 0.63 nm 270 K Structure distortion [108]

α-In2Se3 2017 ✓ 10 nm RT Structure distortion [101]

α-In2Se3 2017 ✓ ✓ 2–6 nm RT Dipole effect [97]

In2Se3 2018 ✓ 3 nm 700 K Dipole locking/covalent bond configuration [102]

α-In2Se3 2018 ✓ 6 nm RT Structure distortion [105]

α-In2Se3 2018 ✓ 5 nm RT Dipole effect [104]

β′-In2Se3 2018 ✓ 45 nm 473 K Structure distortion [107]

2H α-In2Se3 2018 ✓ ✓ 1.2 nm RT Inequivalent interlayer spacing [106]

WTe2 2018 ✓ 2–3 layers 350 K Electron-hole correlation effects [115]

BA2PbCl4 2018 ✓ 1.7 nm 453 K Structure distortion [123]

d1T-MoTe2 2019 ✓ 0.8 nm RT Structure distortion [120]

RT, room temperature.




(α, β, γ, δ, κ), which arise from different stacking orders of the layers. Among them, α phase is recognized as the most stable layered structure at room temperature. Additionally, the crystal structure of In2Se3 was also clarified by Ding et al., where FE-ZB′ and FE-WZ′ structures belong to the α phase while the β phase arranges with the ABCAB structure, as shown in Figure 2b.[48] Additionally, the transformation between the α and β phases is theoretically feasible.[48] It has been studied in optoelectronic and photovoltaic devices as well as phase change memories with layered In2Se3 for quite some time,[98] which was illustrated and summarized by Li et al. in 2019.[99]

As early as 1990, Abrahams suggested that the In2Se3 family may be ferroelectric based on a structural analysis.[100] A long

interval passed before Ding et al. predicted that the ground state structures of the intrinsic prototypical In2Se3 quintuple layer possess both spontaneous OOP and IP electric polarization due to the centrosymmetry breaking in 2017. They argued that polar-ization switching could be obtained by laterally shifting the cen-tral Se layer with a modest electric field through readily acces-sible kinetic pathways.[48] In the same year, Zhou et al. reported the experimental observation of OOP piezoelectricity and fer-roelectricity in 10 nm multilayered α-In2Se3 using a combina-tion of structural, optical, and electrical characterizations. Fer-roelectric domains are clearly visualized by piezoresponse force microscopy (PFM) with an obvious phase contrast and domain walls boundary as shown in Figure 6a, which is completely


Figure 5. Ferroelectricity in layered CuInP2S6. a) Atomic structure of CuInP2S6. Reproduced with permission.[52] Copyright 2016, Nature Publishing Group. b) PFM images as a function of the Cu/In ratio. Reproduced with permission.[95] Copyright 2015, American Chemical Society. c) AFM topog-raphy, PFM amplitude, phase for CuInP2S6 flakes with two to four layers on Au-coated SiO2/Si substrate and a PFM phase image with a written reverse bias. Reproduced with permission.[52] Copyright 2016, Nature Publishing Group.




irrelevant with the topography.[101] Shortly afterwards, Cui et al. reported layer dependent intralayer ferroelectricity in ultrathin 2D layered semiconducting α-In2Se3. They intrinsically dem-onstrated that the intercorrelated OOP and IP polarizations in In2Se3 is the result of the diode effect, where the reversal of the OOP polarization under a vertical electric field could also cause rotation of the IP polarization (Figure 6b).[97] Recently, Xiao et al. illustrated intrinsic OOP 2D ferroelectric ordering in atomically thin In2Se3 crystals with thicknesses down to 3 nm, and verified the polarization-locking mechanism with SHG measurement and PFM, as shown in Figure 7a.[102] The SHG intensity mappings on In2Se3 before and after PFM reversed poling indicate the reversal of IP crystal orientation and non-linear optical polarization. Later, Xue et al. studied OOP and IP piezoelectricity in both monolayer and multilayer hexagonal α-In2Se3 nanoflakes based on the works related to In2Se3 fer-roelectricity. Additionally, the layer-dependent responses, such as odd- or even-related ferroelectricity, are also taken into con-sideration.[103] Afterwards, a reversible spontaneous electric polarization and hysteresis was observed at room temperature in the form of a graphene/α-In2Se3 heterojunction, where the 2D layers had thicknesses of 5 nm (Figure 7b). In this work, Wan et al. attributed the switchable diode effect to OOP ferroe-lectricity, showing controllable rectifying I–V behavior.[104] Addi-tionally, Poh et al. also observed OOP polarization switching in MBE-grown 6 nm In2Se3 using high-resolution PFM. A typical ferroelectric butterfly loop and hysteresis loop were obtained during the experiments (Figure 8a).[105] Following on the previous work, stable OOP and IP ferroelectricity at room

temperature in 6 nm and 1.2 nm, respectively, hexagonal α-In2Se3 nanoflakes was reported by Xue et al. Obvious phase contrast and electrically switchable polarization were observed via PFM (Figure 8b) and the noncentrosymmetric structure was revealed by SHG. In this article, the origin of the ferroe-lectricity was attributed to the inequivalent interlayer spacing between the Se atom layer and two adjacent In atom layers.[106] To ensure that the results of the PFM are reliable, they also performed the same measurements on ultrathin β-In2Se3, and no similar high piezoresponses occurred. However, in another report, IP polarization was discovered in β′-In2Se3 down to 45 nm at room temperature using polarized light microscopy, PFM, scanning tunneling microscopy (STM), and micro-low-energy electron diffraction (Figure 7c). The 1D superstructure distortion aligning along one of the threefold rotational sym-metric directions of the hexagonal c plane is the origin of the IP ferroelectricity in β′-In2Se3 bulk crystal up to 200°C, opening up new possibilities for 2D van der Waals In2Se3.[107]

From these discoveries, we know that III–VI semiconduc-tors, especially In2Se3, play important role in 2D ferroelectric materials, even though their mechanisms have been claimed inconsistently, which we will discuss in the next section. Mean-while, different forms and stacking forms should also be taken into consideration during studies, which could help us explore 2D ferroelectricity both logically and deeply. Additionally, more tunable and high-efficient multifunctional electronics of 2D fer-roelectric materials could be expected by coupling with other 2D systems, such as heterostructures, and unique properties, such as multiferroics.


Figure 6. a) AFM topography, PFM phase, and amplitude images of a thin α-In2Se3 flake. Reproduced with permission.[101] Copyright 2017, American Chemical Society. b) Topography and IP phase images of the as-grown In2Se3 on mica with thicknesses from 2 to 6 nm and a PFM phase image after switching. Reproduced with permission.[97] Copyright 2018, American Chemical Society.




2.2.3. Tin Telluride

2D group-IV tellurides XTe (X = Si, Ge, Sn) with hinge-like struc-tures, such as phosphorene, are promising materials for future nanoscale ferroelectric applications. In 2016, Chang et al. dem-onstrated stable spontaneous polarization in a 1 u.c. tin telluride (SnTe) at liquid helium temperatures. Stripe domains, lattice distortion, band-bending, and electrical polarization manipula-tion were observed by using STM and scanning tunneling spec-troscopy (STS). As shown in Figure 9, a slight distortion from a perfect square to a parallelogram was attributed as the origin of its ferroelectricity. The band-bending pattern on a single domain island determined that the polarization of 1 u.c. SnTe film has an IP component along the [110] diagonal. Meanwhile, 2–4 u.c. SnTe

films also show robust ferroelectricity at room temperature. On the basis of the these results, a nonvolatile ferroelectric random access memory (FeRAM) device was designed, in which the ON/OFF ratio can reach as high as 3000, indicating a wide range of applications in nonvolatile high-density memories, nanosensors and electronic devices.[108] Based on the evidence of 2D ferroelec-tric SnTe, more theoretical predictions on the possible ferroelec-tric phase in group-IV tellurides have been reported,[75,109] which calls for a deeper understanding of the internal mechanisms and physics. Compared with group-IV chalcogenides, which have large IP polarization but usually exist in odd-numbered layers, group-IV tellurides with a similar phosphorene structure might be more exciting for achieving integrated ferroelectric applica-tions since they are not limited by the even-odd number layers.


Figure 7. a) Polarized domain patterned by PFM and SHG mapping before and after electric poling. Reproduced with permission.[102] Copyright 2018, American Physical Society. b) Domain, hysteresis loops, and polarization switching of 2D ferroelectric In2Se3 layer as thin as 5 nm. Reproduced with permission.[104] Copyright 2018, The Royal Society of Chemistry. c) Out-of-plane and in-plane PFM images of β′-In2Se3. Adapted with permission.[107] Copyright The Authors. Published by the American Association for the Advancement of Science.




2.2.4. Tungsten Ditelluride

Metals filled with conduction electrons generally cannot exhibit ferroelectricity since the static field can be screened. However, in 1965, “ferroelectric metal” was introduced by Anderson

and Blount, where the continuous structure transition in the metal must undergo a transition from the nonpolar to polar phase.[91] Although this concept was cast early, some doubtable voices and the rare discovery of ferroelectric metals quelled the idea[110] until the recent definite cases where metals with


Figure 8. a) PFM amplitude and phase images of MBE-grown 6 nm In2Se3. Polarization switching and hysteresis loops indicate the ferroelectricity. Reproduced with permission.[105] Copyright 2018, American Chemical Society. b) Pristine and DC bias written OOP and IP PFM amplitudes and phases. Reproduced with permission.[106] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 9. a) The stripe domain of a 1 u.c. SnTe film. b) Fourier transform of an area crossing a domain boundary. c) dI/dV spectra at 4.7 K. d) STM image of single-domain island. e) Spatially resolved dI/dV spectra along the up and down directions. Reproduced with permission.[108] Copyright 2016, The American Association for the Advancement of Science.




polar structure were identified. In 2013, ferroelectric transi-tion in metallic LiOsO3 was experimentally discovered. Based on the results of resistivity measurements and neuron diffrac-tion data, metallic LiOsO3 undergoes a continuous transition at 140 K from centrosymmetric to noncentrosymmetric phase accompanied by a shift of Li ions along the [111] axis.[111] In addition to LiOsO3, Cd2ReO7, (Sr, Ca)Ru2O6, etc., have also been claimed.[112] However, different from the illustration of Anderson et al., Filippetti et al. predicted a native ferroelectric metal Bi5Ti5O17 with a layered perovskite structure in 2016. Such a truly ferroelectric metal with a native switchable polari-zation and native metallicity could coexist in the same phase, which challenges the common wisdom regarding the polar metal.[113] Later in 2017, Luo et al. proposed the concept of 2D hyperferroelectric metals based on the first-principles calcula-tion of 2D CrN and CrB2, revealing that the spin-phonon cou-pling and metal–metal interactions are two new mechanisms for competing with the depolarization field for stabilizing the OOP polarization and extending the switchable ferroelectric metallic system.[114] Excitingly, after 1 year, the work on the top-ological semimetal tungsten ditelluride (WTe2) again brought native ferroelectric metal to the public view.[115] Figure 10a shows the bistability in the conductance G of undoped trilayer and bilayer devices, indicating ferroelectric switching. The bistable conductance of WTe2 in precisely the same interval measurements at a series of temperatures indicates that the critical temperature is 350 K (Figure 10b). Additionally, the electron-hole correlation effect was also raised to explain the fact that conductance is sensitive to polarization. Instead of lat-tice distortion, a relative motion of the electron cloud related to the ion cores was described as a possible ferroelectric mecha-nism in WTe2. Metal ferroelectrics must have two features, that is, a continuous structure transition and the breaking of inversion symmetry. The polar distortion is now not difficult to

characterize based on advanced techniques, while the mecha-nisms of symmetry breaking in metallic systems also require different approaches to understand the fundamental chemical and physical factors. The design principle for new polar metals with novel coupling or properties is remarkable.

2.2.5. Molybdenum Ditelluride (MoTe2)

TMD materials, which have the same formula MX2 (M is a transition metal: Mo, W; X is a chalcogenide atom: S, Se, and Te), have attracted tremendous attention due to their promising applications in electronics, optoelectronics, and valleytronics.[25] These compounds typically have abundant structural phases, including 2H, 1T, 1T′, and Td phases. The 2H phase, a trig-onal prismatic coordination, is a stable semiconductor and the most common structure among all phases. The 1T phase is an unstable metallic octahedral coordination. Usually there are some methods to transfer the 2H phase to the 1T phase, that is, from semiconducting to metallic.[116] In contrast, the 1T′ phase (monoclinic structure) could be interpreted as a centrosymmetric distorted 1T phase with zigzag M-M chains. The Td phase is very similar to the 1T′ phase but has a non-centrosymmetric orthorhombic structure. Both the 1T′ and Td phases are half-metallic states, which could give researchers more opportunities to obtain promising and unique properties, including a large magnetoresistance effect,[117] quantum spin Hall effect,[118] Weyl semimetal states,[119] etc.

Interestingly, another phase, the distorted 1T (d1T) phase or trimerized (t) phase, has become the definition for 2D ferroelec-tric structures. The emergence of ferroelectricity in monolayer d1T-MoS2 or t-MoS2 has also been reported theoretically.[45,46] The difference among 1T′, Td, and d1T phases is their method of distortion, that is, different motion directions of the atoms.


Figure 10. a) Structure of 2D 1T′ WTe2 and the device geometry. Evidence for ferroelectric switching in WTe2, including the conductance G of undoped trilayer device T1 and bilayer device B1. b) The graphene conductance at a series of temperature, indicating the two conductance states, is associated with different out-of-plane polarization states of WTe2. Reproduced with permission.[115] Copyright 2018, Springer Nature Limited.




The trimerization of Mo in d1T or t phase causes the breaking of inversion centrosymmetry, leading to a switchable polariza-tion. Although the ferroelectricity in d1T-MoS2 still exists in theory due to its improper origin, other materials in this TMDs family, such as d1T-MoTe2, have been proven to be 2D ferro-electric materials. In 2019, Yuan et al. reported an experimental observation of room temperature ferroelectricity in monolayer d1T-MoTe2.[120] They used X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to confirm the elemental composition and compare the phase differences, as shown in Figure 11a. Then, the atomic structure differences in the 2H and d1T phases were illustrated by high-resolution transmis-sion electron microscopy (HRTEM), where some Te vacancies were observed in the d1T phase (Figure 11b,c). To prove the fer-roelectricity in d1T-MoTe2, PFM is used to obtain the stable hys-teresis loops and electrically switchable polarization, as shown in Figure 11d,g. No phase contrast in the 2H phase makes the results obtained from the d1T structure more reliable. However, the origin of the ferroelectricity is usually the most difficult and controversial part to discuss. To determine the intrinsic reason for the induced ferroelectricity in d1T-MoTe2, further evidence, including HRTEM and SHG, is presented in Figure 11e,f. The SHG signals confirm the noncentrosymmetry and the HRTEM images indicate the trimerized structure, which is consistent with the charge density in the two phases according to their

calculation (Figure 11h). The vertical displacements of the Te atoms–induced trimerized structure cause spontaneous polari-zation, opening a novel pathway to explain 2D ferroelectricity and explore promising applications. In this report, the I–V characteristic of d1T-MoTe2 on Pt (Figure 11i) and graphene makes d1T-MoTe2 a promising material in electronic devices.

2.2.6. Bis(benzylammonium) Lead Tetrachloride

Before 2D ferroelectric materials become a prominent topic, ultrathin traditional ferroelectric films were always a long-sought project since the increasingly wide range of commercial ferroelectric devices are mostly based on perovskite-structure ferroelectric materials. However, most of the attention of pre-vious works on perovskite oxide thin films is focused on the critical thickness for spontaneous polarization, especially for OOP polarization. The reduced long-range Coulomb coupling, the significantly enhanced depolarizing electrostatic field, and the thermal fluctuation make ultrathin conventional films with thicknesses down to the 2D limitation struggle in the ferro-electric field. In fact, the development of ultrathin traditional ferroelectrics has never stopped. Except for employing van der Waals ferroelectric materials, exploring IP ferroelectricity in compounds is also a good choice for obtaining nanoscale


Figure 11. a) Raman spectra obtained from experiments and calculations. b) HRTEM image of 2H and c) d1T-MoTe2. d) PFM phase image of mon-olayer d1T-MoTe2 poled by ±8 V. e) Top-view HRTEM image of d1T-MoTe2 and f) the atomic structure of monolayer d1T-MoTe2. g) PFM phase hysteresis and butterfly loops of monolayer d1T-MoTe2. h) Top view of the charge density difference between the 1T and d1T phases. i) I–V characteristics of monolayer d1T-MoTe2 on Pt. Reproduced with permission.[120] Copyright 2019, Nature Publishing Group.




ferroelectrics. In 2004, Fong et al. reported studying the stable ferroelectricity in ultrathin perovskite PbTiO3 film as thin as 1.2 nm at room temperature.[10] In 2017, a 1.5 u.c. ferroelec-tric material PbZr0.2Ti0.8O3 (PZT) was reported by Gao et al. via quantitative annular bright field imaging.[121] In 2018, three kinds of ferroelectric states with three different mecha-nisms were investigated in 2D perovskite oxide thin films.[122] In the same year, bis(benzylammonium) lead tetrachloride (BA2PbCl4), one of the members in the Ruddlesden–Popper family (An+1BnX3n+1), was proven to be ferroelectric at room temperature via PFM.[123] Angular-resolved PFM was per-formed to determine the polarization vector of the flake by rotating the sample with respect to the cantilever axis. The magnitude of the piezoresponse is displayed using the combi-nation of amplitude and phase signals so that IP polarization axis of BA2PbCl4 was verified, which is labeled by the arrows

shown in Figure 12a. After confirming the existence of the IP ferroelectricity in a single-unit cell and two van der Waals layer flakes, the switchability of the ferroelectric polarization was tested by using coplanar electrodes. The domain walls could not only be different due to the excessive holes and little elec-trons injection from electrodes but could also be lithographed by scanning along the dotted arrow with a biased AFM probe (Figure 12b). The superior performance of the ferroelectricity was attributed to the off-center organic cation and the displace-ment of the Pb2+ from the octahedral center along the polar axis. BA2PbCl4 could also behave well in flexible applications where upward and downward bending produces opposite signs of the current, making this 2D hybrid perovskite dem-onstrate potential applications in nonvolatile information processing and storage and other innovative devices. The results of this new discovery have encouraged people to seek


Figure 12. a) Surface topography and different azimuth angle angular-resolved PFM study of BA2PbCl4 flakes. The blue arrows denote the identified polarization vector. b) Lateral PFM images of 50 nm BA2PbCl4 flakes using coplanar electrodes and 40 nm BA2PbCl4 flakes using the scanning probe. Reproduced with permission.[123] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.




a deeper understanding of the suppression of ferroelectricity in perovskite oxide thin films. Instead of exploring the OOP polarization, robust IP ferroelectricity might be the start to obtain unique 2D conventional ferroelectrics. The discovery of weaker thickness-dependent ferroelectric orders should be the important point for breaking the limitation of the size effect. In 2019, 1 u.c. thick tetragonal BiFeO3 ultrathin films at room temperature were demonstrated, which further scaled down the critical thickness for traditional ferroelectric materials. It was suggested that the crystallographic structure, tetragonality, interfacial chemical environments, and ionic polarization of oxide electrode contributed to reducing the ferroelectrics crit-ical thickness, making it promise for high-density memories and miniaturizing devices.[18]

In conclusion, although the realistic evidence is not suf-ficient, the works to date have already opened up new oppor-tunities to observing and understanding ferroelectricity in potential 2D materials, including van der Waals materials and traditional ferroelectrics, which have shed light on the world of 2D ferroelectrics. Group-IV materials, such as chalcogenides and tellirides, play an important role in theoretical 2D ferro-electric materials, which still have rare progress experimen-tally. The phosphorene structure usually gives these materials more chances to obtain robust 2D ferroelectricity. Additionally, the TMDs family is also a great promising candidate for dis-covering 2D ferroelectricity. The improper origin or interlayer translation probably makes achieving 2D ferroelectrics easier. However, compared with interlayer translation, intralayer polarization excludes the odd-even number layers limitation. Most 2D ferroelectric materials found experimentally, including SnTe, MoTe2, WTe2, BA2PbCl4, etc., are ferroelectric whether they are odd or even layered. Based on the urgent demand of miniaturization of next-generation memory and logical devices, more experimental observations are required to add new mem-bers to the 2D ferroelectric family since approaches to verify the breaking inversion symmetry and the polarization switching are already available. What we discussed here is just the begin-ning of this rising field.

3. How to Obtain Proofs Indicating 2D Ferroelectricity

3.1. Calculation Methods to Predict 2D Ferroelectrics

Most currently reported simulations on 2D ferroelectricity depend on first principle calculations based on DFT, as imple-mented in the Vienna Ab initio Simulation Package.[124] The pseudopotentials/ultrasoft pseudopotentials are generated by using the projector-augmented wave with a plane-wave basis set, which is employed for electron–ion interactions.[125] The exchange correlation potential is treated in the Perdew–Burke–Ernzerhof form of the generalized gradient approximation or local density approximation to approximate the exchange and correlation potential.[126] For electronic band structure compu-tation, the Brillouin zone is sampled in the Monkhorst–Pack scheme, while the more accurate screened exchange hybrid Heyd–Scuseria–Ernzerhof functional is used.[127] To eliminate the interaction between adjacent monolayers, a sufficiently large

vacuum thickness (≈20 Å) along the z axis is always adopted in the theoretical calculations. The polarization switching pro-cess (energy barrier of ferroelectric/ferroelastic phase transi-tion) is investigated with the nudged elastic band method.[128] The Berry phase method (based on the Kohn–Sham wave func-tions from DFT calculations) rigorously defines the sponta-neous polarization of a periodic solid and provides a route for its computation in electronic structure codes, which is used in the calculations of the ferroelectric polarization.[129] The DFT+U method is adopted to give a better description of the strongly correlated effect.[130]Inspired by the successful DFT+U method, the orbital selective external potential method,[131] describing an external orbital-independent potential, is proposed to simu-late the potential to influence the specific atomic orbit for the-oretical purposes. The van der Waals interaction is modeled using the Lennard-Jones potential.[132]

3.2. Advanced Techniques Proving 2D Ferroelectricity

According to the experimental development of 2D ferroelec-tric materials, it is very challenging to probe the ferroelectric properties in ultrathin films due to the much-reduced vertical signals compared with the bulk, indicating that more sensi-tive approaches are required to monitor the 2D properties. Generally, more sensitive probes, including synchrotron X-ray scattering,[10] ultraviolet Raman spectroscopy,[133] polarized SHG,[82,134,135] PFM,[136,137] etc., are employed to detect weak signals.[108] Here we briefly introduce four of them.

3.2.1. STM

STM provides a direct technique to probe the local electronic structure, and STS could offer information on the electronic density of states of different materials. The principle of the STM is straightforward. It essentially consists of scanning a metal tip over the surface at a constant tunnel current. The dis-placements of the conductive tip caused by the voltages applied to the piezo drives would yield the picture of the surface topog-raphy. The high resolution of the STM strongly depends on the tunnel current on the distance between the two tunnel electrodes, that is, the metal tip and the scanned surface.[138] The STS reveals a robust electronic gap-like feature around the zero bias, as well as significant spatial inhomogeneity at higher biases above the gap edge.[139,140] In fact, this method has been used in many different aspects, including character-izing the inhomogeneous electronic structure,[141] atomic struc-ture,[139,142] electron–phonon/phonon–electron interaction,[143] the effect of twisting on the band structure,[144] etc.

In 2016, Chang et al. only used STM and STS and discovered robust IP ferroelectricity in atomic-thick SnTe.[108] They utilized STM to observe the domain formation and lattice distortion, and then obtained the clear signatures of band bending via STS. Additionally, polarization switching was also manipulated by applying a voltage pulse between the STM tip and the 1 u.c. SnTe. This discovery opened a new opportunity to explore ferro-electricity in 2D materials, although it is still difficult to observe spontaneous polarization at room temperature at a thickness of





1 u. 1 µc. Because of the high requirement and difficulties of STM measurements, no other STM results proving 2D ferro-electricity have been reported since.

3.2.2. PFM

PFM based on the converse piezoelectric effect was first dem-onstrated by Güthner and Dransfeld in 1992 by investigating a ferroelectric polymer film, where they imaged the generated domain pattern by locally poled domains with a tip.[145] In con-ventional PFM, the domain structure is mapped by scanning the electromechanical response caused by sample surface defor-mation with a conductive AFM tip, while a small alternating current electric bias is applied to monitor the piezoelectric strain. The results of the amplitude and phase signals reflect the absolute magnitude of the local piezoelectric response and the direction of the ferroelectric polarization.[146] The high-res-olution imaging, nanoscale manipulation, and local measure-ments make PFM an important tool in nanoferroics studies. The exploration of new emergent phenomena, including func-tional domain walls,[147] tunneling electroresistance (TER),[17,148] flexoelectricity,[88] photovoltaics,[149] topological states,[150] etc.,[151] make PFM a valuable platform for conducting funda-mental issues and emergent electronic properties and nano-structures, indicating that PFM is not just a simple technique limited in domain patterning. In addition, a series of review articles regarding PFM can be found, which offer us different outlook for future trends and developments in PFM.[136,152–154] There are typically three typical PFM modes: vector PFM, switching spectroscopy PFM (SS-PFM), and lithography.[153]

1) Vector PFM: In vector PFM, the real-space reconstruction of polarization orientation comes from three components of the piezoresponse: vertical PFM plus at least two orthogonal lat-eral PFMs. It is performed by imaging both the OOP and IP PFM and enables discrimination between different ferroelec-tric domains.[155]

2) SS-PFM: PFM spectroscopy refers to locally generated butter-fly-like amplitude loops and the sharp phase hysteresis loops in ferroelectric materials. From these hysteresis loops, infor-mation on local ferroelectric behavior, such as imprint, local work of switching, and nucleation biases, can be obtained.[156] SS-PFM demonstrates real-space imaging of the energy dis-tribution of nucleation centers in ferroelectrics, thus resolv-ing the structural origins of the Landauer paradox.

3) Lithography: For ferroelectric applications, PFMs can be used to modify the ferroelectric polarization of the sample through the application of an electric field. Switchable polarization can be obtained if the applied electric field is larger than the coercive field. This technique could be used to “write” sin-gle domains, domain arrays, and complex patterns without changing the surface topography. With the PFM technique, plenty of fundamental studies based on nanoscale ferroelec-tric materials, including electric domain patterning,[154] cor-relation between polarization and electronic transport,[157] etc.,[158] have been conducted.

In the past several years, the results obtained from PFM are usually reliable in characterizing ferroelectric properties, and it

has become a standard for imaging ferroelectric domain pat-terns, as seen from an impressive increase in the number of PFM-related papers published annually.[153,159] However, in recent years, PFM has faced some challenges where ferroe-lectric-like hysteresis loops and PFM images were observed in various materials, including paraferroelectrics, transition metal oxide thin films and Li-ion conductive glass ceramic, etc.,[160] which could mistake the analysis and interpretation of PFM data and make PFM results become necessary proofs rather than sufficient evidence. PFM is a tool to detect the electrically modulated mechanical strain, and thus, any results caused by a deformation could make it show ferroelectric or ferroelectric-like phenomena, which is the reason that many PFM results obtained from nonferroelectric materials are similar to those of ferroelectrics. In fact, there are several factors, including the electrochemical response,[161] static electric effect,[162] envi-ronmental influence,[163] charge injection,[152] etc.,[164,165] that are attributed to these ferroelectric-like PFM responses. Some researchers have tried to offer some approaches to differentiate ferroelectric and nonferroelectric results, indicating that PFM could be applied to study ferroelectrics and could also be used in a broader nonferroelectric field.[165]

Therefore, PFM not only has developed as a versatile, easy-to-handle, nondestructive tool in studying the fundamental issues and emergent electronic properties but also as a poten-tial manner to probe the local electromechanics of nonferro-electric systems. Since signals of 2D materials are difficult to observe, PFM is a proper candidate for detecting weak signals. In fact, in most recent studies on 2D ferroelectricity, PFM was chosen as the versatile tool. In combination with other tech-niques, including conducting AFM, magnetic force microscopy, transmission electron microscopy (TEM), etc., identifying 2D ferroelectricity by using PFM makes things much easier and more promising.

3.2.3. SHG

Optical SHG has shown to be a rather simple, nondestructive (electrode-free), and inherently high spatial, spectral and tem-poral resolutions probe to study the structure, symmetry, and morphology of interfaces and ultrathin films.[134,166] It is a non-linear optical process based on the principle where a second-order nonlinear laser-excited coherent optical process involves two photons with the same frequency interacting with a non-linear material and generating a new photon with twice the energy of the initial photons. SHG was first demonstrated by Franken et al. in 1961[167] and was considered and formulated by Bloembergen and Pershan in 1962.[168] Because of the nonzero second harmonic coefficient, only noncentrosymmetric struc-tures could emit SHG light, which is known to be a sensitive probe of the breakdown of crystallographic symmetry. The detailed principle of SHG is described in the supplementary materials of the report of Xiao et al.[102] Due to the simplicity, surface specificity, and versatility, SHG has become a unique tool for surface studies and has received much attention.

Recently, large SHGs were discovered in a number of 2D materials, such as monolayer/multilayer MoS2, MoSe2, WS2, WSe2, h-BN, GaSe, InSe, and AgInP2S6.[71,82,169] Most recently,





Liu et al.,[52] Chang et al.,[108] Xiao et al.,[102] and Yuan et al.[120] utilized the SHG technique to explore CuInP2S6, SnTe, In2Se3, and MoTe2, respectively, and proved their ferroelectricity via combination with other methods. The results of SHG are usually used to prove the breaking of centrosymmetry, which is the primary condition if 2D ferroelectricity exists. Thus, SHG is also an excellent approach to discover 2D ferroelec-tric materials with weak ferroelectric signals and ferroelectric-paraelectric phase transitions (from a centrosymmetric to a non centrosymmetric type).

3.2.4. TEM

TEM is a technique in which such features as the crystal struc-ture, defects, etc., could be observed with high spatial resolu-tion via the interactions between the high energy beam of electrons and the atoms of a very thin sample. The technique mainly covers three aspects, including imaging, diffraction, and spectroscopy.

For imaging, a row of atoms/lattice as well as defects, including grain boundaries, twins, dislocations, stacking faults, etc., could be seen from HRTEM, while the atomic structure could be obtained from aberration-corrected scanning TEM (AC-TEM). There are usually two imaging modes that could be used in TEM, including the bright field mode where only the transmitted elec-trons can pass the objective aperture, and the dark field mode where only one diffracted beam is allowed to pass the objective aperture. For diffraction, the selected area electron diffraction (SAED) could give information about the crystal structure of the specimens with high spatial resolution and is usually used to dif-ferentiate the different phases. Take, for example, the report in 2018 by Cui et al., who utilized SAED to confirm the existence of ferroelectricity in α-In2Se3 in two flakes with different thickness. The results show that α-In2Se3 ferroelectricity was only observed in the monolayer flake, which is agreeable with the theoretical prediction.[97] For spectroscopy, energy dispersive spectrometer (EDS) and electron energy loss spectroscopy (EELS) are one of the great advantages of TEM. We could obtain a quantitative X-ray microanalysis, that is, elements valence states from EDS and light elements analysis such as chemical quantitative map-ping with high spatial resolution from EELS.

Due to the weak vertical ferroelectric signals in the 2D limit, topography changes and complex ferroelectric-like behaviors, PFM alone usually can offer necessary but insufficient evi-dence to prove 2D ferroelectricity. It is hard to confirm the 2D ferroelectricity through only hysteresis loops and poled phase contrast if the domain structure cannot be observed clearly. In this case, TEM usually plays an important role to make things clear. HRTEM could be used to observe the phases/phase transition[120] while the atomic structure of a specimen could be observed via atomic resolution STEM. In 2018, Xue et al. utilized high-resolution STEM and illustrated that one quin-tuple layer of In2Se3 consists of the alternately arranged Se-In-Se-In-Se atomic planes. Via combination of other techniques, including SHG, PFM, Raman, etc., they showed evidence of OOP and IP piezoelectricity in both the monolayer and mul-tilayer α-In2Se3.[103] In 2019, Yuan et al. utilized HRTEM and AC-TEM to deeply understand the ferroelectric mechanism,

which is usually most confusing component of 2D ferro-electricity studies. The trimerized structure of d1T-MoTe2 was confirmed by HRTEM, where the distance between Te and Te atoms is 3.4 Å. The ion displacement mechanism was claimed as a few Te atoms move toward the Mo plane by approximately 0.6 Å in OOP, while the other atoms largely remain still, which has been observed by AC-STEM.[120]

In conclusion, studies usually combine different techniques to study the 2D ferroelectricity due to its challenging nature. SHG is usually used to justify the centrosymmetry, while PFM offers evidence of ferroelectricity. To further understand intrinsic mechanisms, atomic structures are usually shown to prove the right specimens’ phases and possible ferroelectric mechanisms, including ion displacement, interlayer transla-tion, etc.

4. Intrinsic Mechanisms of 2D Ferroelectricity

Ferroelectric materials are typified as the polar system with electrically switchable macroscopic polarization, arising from spontaneous alignment of electric dipoles. Generally, the emer-gence of ferroelectric polarization requires an asymmetric structure, and the mechanisms within ferroelectric materials can be divided into two types: ion displacement caused by opposite moving positive and negative charged ions, and elec-tronic polarization induced by asymmetric spin exchange inter-actions, as shown in Figure 13a.[170] In fact, the forming of net electric dipole is always the key factor for achieving ferroelec-tricity in 2D materials. Various mechanisms have been reported to date since the pathways used to induce or produce the elec-tric dipole are different. Additionally, other mechanisms, which do not result from polar distortion, have been proposed to find novel approaches to obtain 2D ferroelectrics. Meanwhile, non-intrinsic ferroelectricity induced by artificial designation of 2D materials also makes things more interesting. Based on the var-ious descriptions, we attempt to summarize two major mecha-nisms of ferroelectricity in 2D materials to simplify the entire story: intralayer bonding and interlayer interactions.

4.1. Intralayer Bonding

4.1.1. Structure Distortion

For ion displacement ferroelectrics, positive and negative charged ions move in opposite directions, namely, an intrinsic structural distortion as shown in Figure 13a. Among different illustrations, they could be displayed as the displacements of protons,[58] unequal lattice constants and relative atomic dis-placement,[109] distorted phosphorene lattice structure,[57] sur-face vacancy-induced distortion,[171] etc. However, despite the various explanations, what essentially remains the same is the occurrence of the dipole moments. Take the report of Chang et al.,[108] for example, who attributed the slightly distorted lat-tice (from a perfect square to a parallelogram) to the stable IP spontaneous polarization in atomic-thick SnTe by giving evi-dence of STM, as shown in Figure 9. The parallelogram is elon-gated along the [110] and its equivalent orientations, and thus





the elongated diagonals for two adjacent domains are perpen-dicular to one another. Similar to the SnTe, SHG-proved 2D ferroelectricity in CuInP2S6 also originates from the distortion of the structure.[52] The nonzero SHG reveals the broken inver-sion symmetry and ferroelectric order, which is demonstrated in detail in Ref. [52], which reflect the hexagonal ordering of the displaced Cu and In sublattices in the ferroelectric phase. Beyond this, noncentrosymmetric R3m symmetry of the α-In2Se3 was also confirmed experimentally via STEM and SHG. Se atoms move toward the neighboring In atoms as observed by STEM, breaking the inversion symmetry of each quintuple layer and indicating the origin of ferroelectricity in α-In2Se3.[101] In fact, ion displacement is a popular explanation for ferroelectricity in traditional 3D ferroelectric materials. For 2D ferroelectrics, the mechanism also plays an important role in studying or designing ferroelectric polarization. The occur-rence of the electric dipole is usually the mark of ferroelectricity. So far, many distorted ferroelectric structures are predicted in theory, mostly based on DFT calculations, including a d1T-MoS2,[45] group IV monochalcogenides,[69,70,74] decorated phos-phorene,[73] phosphorene-like structures,[172] etc. With the rapid development of 2D ferroelectrics, today an increasing number of structure-distortion induced 2D ferroelectricity have been dis-covered and proven by experimental methods, which has made the intrinsic mechanism in 2D ferroelectrics clearer.

4.1.2. Dipole Locking/Covalent Bond Configuration

2D ferroelectric materials with dipole locking are based on the unique covalent bond reconstruction (Figure 13b). Distinct from conventional ferroelectric switching, where small uniaxial atomic distortions occur without covalent bond breaking and

forming, the breaking of old covalent bonds and forming of new bonds is the mark of this mechanism. The intrinsically intercor-related OOP and IP polarization against the depolarization field opens up new approaches to explore 2D ferroelectric physics.

As mentioned in Section 2.2.2, Xiao et al. demonstrated the mechanism of dipole locking, leading to the observation of intrinsic 2D OOP ferroelectricity in atomically thin In2Se3 crys-tals with thicknesses down to 3 nm.[102] They found the switching of vertical electric field could control IP lattice orientation with the middle Se atom moving laterally and In-Se covalent bonds breaking and forming. Additionally, Cui et al. also attributed the dipole effect, where the reversal of the OOP polarization caused by the vertical electric field induces the rotation of the IP polarization to the ferroelectricity in 2D-layered semiconductor In2Se3.[97] How-ever, the mechanism in In2Se3 described by Ding et al. is attrib-uted to the polar displacement rather than the dipole coupling, which means the mechanisms in In2Se3 is no longer simple.[48]

Similar to the interactions between OOP and IP polarization, ferroelectric and ferromagnetism coupling might appear in 2D multiferroics with the effect of covalent bonds reconstruc-tion, which could be designed for incorporating into circuits. In 2017, Yang et al. showed that covalent-bonded halogen-dec-orated phosphorene systems could achieve electric writing and magnetic reading with robust OOP polarization against depo-larization field.[73] In this case, unique covalent bonds configu-ration in 2D materials may become a potential pathway to break the barrier in orders coupling, especially for multiferroics.

4.1.3. Electronic Polarization/Spin Exchange Interactions

For electronic polarization, the net electric dipole is induced by the asymmetric spin exchange interactions, while the


Figure 13. Mechanisms inducing 2D ferroelectricity. Intralayer interaction including a) schematic images of ion displacement and electron polariza-tion. Reproduced with permission.[170] Copyright 2016, American Chemical Society. b) Dipole locking with the breaking and forming of covalent bonds. Reproduced with permission.[102] Copyright 2018, American Physical Society. c) The geometric structure of the BN bilayers, indicating the ferroelectric switching pathways. Reproduced with permission.[49] Copyright 2017, American Chemical Society.




dispersion of electrons in 2D materials could be manipulated by the external electric field, as shown in Figure 13a. Hu et al. predicted that IP atomic-thick ferroelectricity could be induced by the vertical electric field in both monolayer and bilayer n-A-PNRs (n: label the width; A-PNR: armchair phosphorene nanoribbons) where n = odd number.[170] Take a 9-A-PNR as an example: the differential charge density, the significant charge accumulation on the PP bonds connecting different half-layers and the reduced charge on PP bonds connecting the same half-layer demonstrate the significant charge redistribu-tion along the ribbon direction, leading to the emergence of polarization. It is perpendicular to the direction of the external field and lies in the plane. This report first shows the IP fer-roelectricity with the electron polarization induced by spin ordering, which may motivate new exciting results and appli-cations. Huang et al. revealed multiferroics in charged CrBr3 monolayer nanostructures combined with a large electric dipole and ferromagnetic ordering.[173] In these systems, the IP ferroe-lectricity is ensured by the combination of the charge order and orbital order, which are induced by the asymmetric Jahn–Teller distortions of the two neighboring Cr-Br6 units.

4.2. Interlayer Translation

The interlayer translation could change the interlayer potential and drive the flow of electrons, which is based on bilayer or multilayer 2D materials. In 2017, Li et al. reported that 2D ver-tical ferroelectricity existing in the graphitic bilayer of BN, AlN, ZnO, MoS2, GaSe, etc. is induced by the interlayer translation, where a closer interlayer distance may lead to a stronger charge transfer from the upper layer to the down layer.[49] As shown in Figure 13c, the polarization could be switched through an interlayer transition by one bond length, indicating a transla-tion pathway between the upper and down layers. The origin of 2D ferroelectricity in this case is similar to the analysis in WTe2, which gives more possibilities for discovering and designing 2D ferroelectrics.[174] Interestingly, in the experimental report published in 2018, the origin of the vertical polarization in metal ferroelectric WTe2 is attributed to the electron-hole cor-relation rather than a lattice instability. They discussed three reasons that make electron-hole correlation effects more likely to be the mechanism of the polarization in WTe2: monolayer conductance at 4 K is almost independent of the vertical elec-tric field; and the polarization flips when the applied potential difference exceeds the spontaneous polarization induced poten-tial and the compensation point at which the electron and hole densities are exactly equal.[115] In addition to these, Ding et al. also demonstrated that the intrinsic 2D ferroelectricity in In2Se3 is induced by different interlayer spacing between the central Se and two In layers (Figure 2b).[48] Later in 2018, the origin of 2H In2Se3 was also attributed to the inequivalent interlayer spacing between the Se atom layer and the adjacent two In atom layers.[106] In addition, the polarization and the interlayer voltage could be enhanced if the interlayer distance is further decreased upon vertical pressure. In the same year, Wang et al. predicted a new kind of 2D ferroelectricity in elemental tellurium multilayers, which exhibits spontaneous IP polari-zation due to the interlayer interaction between lone pairs.[56]

Additionally, the ferroelectric Moire superlattice could also be obtained by twisting a small angle between the layers, where a slight difference in strain indicates the generation of different domains.[174] From these excellent works, we can obtain a new vision of studying 2D ferroelectrics: focusing instead on mon-olayer, bilayer, or multilayer 2D materials, which could be ideal candidates for applications in spintronics and electronics at the nanoscale. Meanwhile, the effect of the even-odd number of layers should also be taken into the consideration since the breaking of inversion symmetry can sometimes only exist in odd-numbered layers.[170,175]

In conclusion, the key to the ferroelectricity in 2D mate-rials is the absence of inversion symmetry, regardless of what mechanism causes it. There are three methods to classifying 2D ferroelectric mechanisms: 1) according to the position where the dipole moment is formed, it could be divided into intralayer and interlayer interactions; 2) if different sorts of par-ticles, including ions and electrons, are taken into considera-tion, intrinsic structural distortion and asymmetric electronic redistribution (electron polarization) could be the intrinsic mechanisms of 2D ferroelectricity; and 3) when the bonds con-figuration are different, ion displacement and covalent bonding might be the reason for the occurrence of ferroelectricity in 2D materials. On the basis of these mechanisms, a few other explanations are also discussed in some materials, including hyperferroelectric metals,[114] 2D perovskite oxide thin films,[122] etc. Additionally, many methods, including importing vacan-cies,[171] applying strain,[75] changing displacement of adjacent layers or atomics,[66,109] etc., have also been used to fabricate or discover the 2D ferroelectrics. At the same time, organic mate-rials are playing an important role in 2D ferroelectricity as well, where multimode proton transfer,[67] molecule rotation,[176] etc., could be the intrinsic mechanisms within them. Although fer-roelectric behaviors and the corresponding mechanisms have been studied in 2D materials and proven by some methods, including SHG, TEM, PFM, etc., it is still controversial and ambiguous in different material systems, which still has a large space for further exploration.

5. Applications and Challenges of 2D Ferroelectric Materials

Due to the limitations of intrinsic ferroelectric mechanisms and the few experimental observations in 2D ferroelectric mate-rials, extensive applications are naturally rare. Therefore, based on the conventional development of ferroelectric materials, applications and outlooks of 2D ferroelectrics and the related work on 2D materials and traditional ferroelectrics are briefly introduced here.

5.1. Applications of 2D Ferroelectric Materials

5.1.1. Devices Based on 2D Ferroelectric Materials

Compared with conventional FeRAM, where reading is destruc-tive and unstable at room temperature when the thickness is down to a few unit cells, the memory based on the ultrathin






planar morphology of 2D ferroelectric materials may be non-destructive and more reproducible, which allows for the pos-sibility of electrical modulation of functional properties of the hybrid ferroelectric memory[2,177] and logic device applications, including FeFET,[3,178,179] ferroelectric junctions, and multifer-roic junctions (FTJs),[17,60,180] ferroelectric capacitor,[5,181] switch-able diode,[182] etc. At the same time, 2D ferroelectric materials also have additional advantages over traditional nonvolatile memory, such as a band tuning ability, the miniature size in the OOP direction and the moderated substrate requirement condition.[42]

FTJs consist of two normal metal electrodes with a ferroelec-tric tunnel barrier, which is popular as the sandwich structure. It is known that the reversal of the polarization would change the potential barrier, which is denoted as the TER effect. The performance of FTJs is sensitive to the electrical boundary con-ditions and could be controlled by electrode materials and inter-face engineering. Taking some theoretical works for example, as shown in Figure 14a, ferroelectric junctions of germanene pas-sivated by –CH2F and –CH3, IP heterostructure of germanene

passivated by –CH2F and –I and ferroelectric PN junction based on a hydroxylized Si (111) surface and MoS2 on a hydroxylized silica surface were designed.[60] All of these devices take advan-tage of the combination of a high-mobility semiconductor with fast writing and nondestructive reading in nonvolatile memo-ries. In 2018, a traditional ferroelectric material, tetragonal BFO ultrathin film, in the 2D limit was demonstrated at room temperature. By utilizing PFM, Kelvin probe force microscopy, aberration-corrected STEM, and energy dispersive X-ray, stable ferroelectricity was observed in few unit cells, including 1 u.c. thick BFO films. When the 1 u.c. BFO barrier layer was used in a FTJ device, a large ON/OFF ratio of 370% was obtained, while a much larger TER ratio of 2700% was achieved in a 2 u.c. thick BFO FTJ, which make ultrathin BFO films in the 2D limit promising candidates for high-density data storage.[18]

Additionally, the devices consisting of 2D materials, such as a graphitic bilayer of BN and some 2D ferroelectrics, could be utilized as nanogenerators, piezotronics, and pressurizing FET.[49,66] The observed tunable band alignments with the fer-roelectric layer make them promising devices as nonvolatile

Figure 14. a) Ferroelectric junction, IP heterostructure of germanene, and ferroelectric PN junction on hydroxylized silica surface. Reproduced with permission.[60] Copyright 2016, American Chemical Society. b) In2Se3/graphene and In2Se3/WSe3 heterostructure. Reproduced with permission.[48] Copyright 2016, Nature Publishing Group. c) Schematic diagram of 2D-FTJ (p-type doping MX)/MX/(n-type doping MX) and the k||-resolved transmis-sions of the monolayer In:SnSe/SnSe/Sb:SnSe homostructure at the Fermi energy in 2D Brillouin zone for the P+x and P–x states. Reproduced with permission.[183] Copyright 2016, American Chemical Society.





memory- and graphene-based electronics. Additionally, bilayer heterostructure side views and band structures formed by stacking ferroelectric In2Se3 on nonferroelectric semiconductor WSe2 and graphene are shown in Figure 14b. The switching of the electric dipole of the In2Se3 layer caused an alternated Schottky barrier across the interface and the significant band gap reduction makes the In2Se3-based heterostructure a prom-ising device.[48] Most recently, Shen et al. proposed a planar p-type semiconductor/ferroelectric/n-type semiconductor 2D FTJs via doping engineering. According to the k||-resolved trans-missions calculations shown in Figure 14c, the giant TER effect

of 1460% could be obtained by integrating the transmission probability for states at a Fermi energy over the 2D Brillouin zone, which is significantly comparable to the traditional FTJs and is claimed as the result of the accumulation/depletion of major carriers near the semiconductor in response to polariza-tion switching. This is the first demonstration of the 2D FTJ based on homostructures that promise future ultralow-power, high-speed, and nonvolatile nanoscale applications.[183]

Experimentally, although only few 2D materials have been proven to be ferroelectrics, they perform well in electric trans-formation and could potentially be used as light sensors, energy

Figure 15. a) Ferroelectric field-effect transistors based on MoS2 and CuInP2S6 2D van der Waals heterostructure. Reproduced with permission.[179] Copyright 2018, American Chemical Society. b) Electrically controlled planar In2Se3 device. Reproduced with permission.[97] Copyright 2018, American Chemical Society. c) Nonvolatile memory device based on FTJ with IP polarization. Reproduced with permission.[108] Copyright 2016, The American Association for the Advancement of Science.





conversion devices, and for nonvolatile memory. Figure 15a shows the proposed CuInP2S6 transistors composed of an MoS2 flake and CuInP2S6.[179] The dual-gate structure with a proper back-gate voltage enables an electrically controllable device and the metal-insulator-metal capacitor structure shows resistive switching with more than four orders of ON/OFF ratios. Com-pared with CuInP2S6, the electric transport of the In2Se3 device could be determined by both OOP and IP polarization through the diode effect, as shown in Figure 15b.[97] In addition, the SnTe memory was also fabricated based on a transistor struc-ture where the voltage between the source and drain is used to modulate the polarization direction, and thus changes the tunneling current between the ferroelectric film and the source electrode (Figure 15c).[108]

5.1.2. Combination of 2D Materials and 3D Ferroelectric Materials

Ferroelectric materials with electric modulated spontaneous polarization have potential prospects in sensors,[184] photonics and energy-efficient memories,[185] solar cells,[186] and photo-electrochemical applications.[5,187] In 2017, Wang and Hu pub-lished a review about the integration between 2D materials and ferroelectrics, describing the logical and optoelectrical applica-tions.[188] In the same year, Zhou and Chai in detail reviewed studies on 2D electron devices, such as low-power FET, opto-thermal device, and photoelectronic device.[189] These articles not only describe the progress in integrated 2D devices but also give us a clear skeleton to learn what is happening in this field. Here, we just give a brief introduction of 2D integrated devices.

In one type of these devices, for example, FTJs, 2D materials, such as graphene[190] and MoS2,[191] have been used as the top electrode to apply a polarization switching bias, as shown in Figure 16a,b. In the graphene/BTO interface, molecular layers could be easily introduced to the interface, which efficiently stabilizes the polarization of BTO and significantly enhances the TER effect. For the graphene/BTO FTJ with the interfa-cial ammonia layer, a TER effect of 6% × 105% was obtained, indicating the important implications in nonvolatile memories and logic devices.[190] Compared with graphene as the top elec-trode, 2H-MoS2 is less conductive. The asymmetric switching

behavior indicates the dependence of the n/p-type MoS2 on the polarization direction, and the TER effect with an ON/OFF ratio as high as 104 makes the 2D/3D structure a novel pathway for electronic devices with an enhanced performance.[191] By controlling the OOP tunneling conductance across the ferro-electric layer, unique polarization switching has been obtained. The extraordinary properties and characteristics at the interface could be explored with hybrid structures composed of ferroelec-tric thin films and functional 2D materials, providing a simple and straightforward method for interface engineering and resistive switching manipulation in FTJs.[192]

In another type of 2D ferroelectric devices, FeFETs, 2D materials on a ferroelectric substrate were not only used as a gate dielectric to alter the IP conductivity of the adjacent 2D channel,[193] but also used as the channel gated with a ferroe-lectric film to obtain a high ON/OFF ratio, a long time reten-tion and a high endurance.[194] Among these applications, for example, optoelectrical ferroelectric memory was obtained by tuning the ON/OFF ratio of the light.[195] As shown in Figure 17a, the MoS2-PZT FeFET exhibits a large hysteresis of electronic transport and the possibility to write and erase both electrically and optically. In addition to this, polarization-dependent hysteresis electronic transport in graphene-PZT FeFET is also demonstrated by Lipatov et al.[196] Additionally, 2D ferroelectric/nonferroelectric junction based on functional-ized 2D materials or semiconductor surfaces with proper sub-stitution has been designed to obtain high TER.[48,60] What is shown in Figure 17b is ferroelectric FET with 2D MoSe2 chan-nels. The device consisted of 5 nm MoSe2 and could yield a write/erase ratio of more than 103 with a good retention and endurance performance.[178]

5.1.3. 2D Ferroelectricity-Induced Unique Properties

After discussing the applications of 2D ferroelectricity, ferroe-lectricity-induced novel properties are listed here. As shown in Figure 18a, the occurrence of a reduced symmetry of the valley shown in the band structure of the ferroelectric phase GeSe monolayer implies that the spontaneous valley polari-zation is induced by 2D IP ferroelectricity.[197] Additionally,

Figure 16. Switching of ferroelectric polarization in a) graphene-BTO-SRO (Reproduced with permission.[190] Copyright 2014, Nature Publishing Group) and b) MoS2-BTO-SRO tunnel junction (Reproduced with permission.[191] Copyright 2017, American Chemical Society).





the investigations into the Rashba spin orbit coupling at the interface of a GeTe(111)/InP(111) superlattice indicate that the reversal of the spin texture could be tuned by the ferroe-lectric polarization, indicating a great potential in ferroelectric Rashba semiconductors spin FET. It can be clearly seen from Figure 18b that with different ferroelectric displacements, the energy levels of the Rashba splitting bands gradually shift up or down.[198] Different from the previous works, the monolayer MoS2 growth and transport properties could also be controlled by the ferroelectric substrate.[199] While maintaining the sub-strate LiNbO3 polarization pattern, the monolayer MoS2 growth exhibits a preference for the ferroelectric up-domains in the periodic domain stripes, as shown in Figure 18c, indicating the possibility for ferroelectric nonvolatile gating of TMDs in scal-able devices without the traditional exfoliation and transferring.

6. Opportunities and Outlook

In the field of 2D ferroelectric materials, despite many 2D materials having been predicted and studied, the exploration of

intrinsic 2D ferroelectricity within a plane is still in an early stage, especially experimentally. During the process of developing 2D ferroelectrics, theoretical calculations usually provide directional suggestions to explore and prove 2D ferroelectricity. Although it is still very challenging to observe stable and strong spontaneous polarization in 2D materials with a few unit cells, we will have an increasing amount of approaches to discover novel 2D ferro-electric materials, including the underlying physical properties and intrinsic mechanisms, with the rapid development of charac-terization and fabrication methods. Since the exploration of new 2D ferroelectric materials is always an attractive topic, new funda-mental studies could possibly address the effects of strain, func-tional dopant, interlayer interactions (bilayers and/or multilayers), intralayer bonding, edge effects in transport, collective defects, oxygen vacancies, multiferroic orders coupling, etc. Additionally, IP and OOP correlation could be efficiently used to explore 2D ferroelectricity, that is, inducing vertical polarization via IP dis-placement. Meanwhile, the exploration of 2D ferroelectricity will also contribute to new scientific frontiers, such as spintronics and valleytronics, leading to exciting technological advances. Additionally, some freestanding traditional ferroelectrics, such

Figure 17. a) Effect of visible light illumination of MoS2-PZT FeFET and optical switching of MoS2-PZT memory. Reproduced with permission.[195] Copyright 2015, American Chemical Society. b) Device structure, retention and endurance performances of MoSe2 FeFET. Reproduced with permis-sion.[178] Copyright 2017, IOP Publishing Ltd.





as BiFeO3, down to a single unit cell with robust ferroelectricity could also be fabricated through the unique methods, indicating that the chances to break the limitation of the size effect for tra-ditional ferroelectric materials still exist and the development of traditional ferroelectric materials down to the nanoscale cannot be ignored.[18,200] In addition, the change in topography is an una-voidable problem during 2D ferroelectricity studies, so to over-come and break through this is another key point to obtaining further improvement of this field. Finally, with the increasing number of discoveries of 2D ferroelectric materials to date, mini-mized and integrated functional electronics are expected soon, which will open a new scientific era.

AcknowledgementsThis work was supported by the National Key Research and Development Program of China (Grant No. 2017YFA0303403), National Natural Science Foundation of China (Grant Nos. 11304097, 11874149, 11774092, and 51572085), Shanghai Science and Technology Innovation Action Plan (Grant No. 19JC1416700), ECNU (East China Normal University) Multifunctional Platform for Innovation (006), and Fundamental Research Funds for the Central Universities.

Conflict of InterestThe authors declare no conflict of interest.

Keywords2D ferroelectric mechanisms, 2D ferroelectrics, 2D materials

Received: August 2, 2019Revised: October 23, 2019

Published online:

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