萬 逸，闞二軍

南京理工大學(xué)理學(xué)院應(yīng)用物理系，南京210094

CONTENTS

I.Introduction 107

II.Structures and ferroelectric nature of CuInP2S6108

A.Single-crystal CuInP2S6growth 108 B.Piezoresponse force microscopy measurement 109

C.Possible in-plane ferroelectricity 112

D.Optical methods to detect ferroelectric order 114

E.Giant negative piezoelectricity 115

F.Tunable quadruple-well ferroelectricity 120

III.CuInP2S6-based heterostructures and applications 121

A.CuInP2S6/Si diode 121

B.CuInP2S6-based pyroelectric devices 122

C.CuInP2S6/MoS2Fe-FET 123

IV.Family of 2D MIMIIIP2(S/Se)6compounds 124

V.Summary and outlook 126

Acknowledgments 126

References 126

I.INTRODUCTION

Ferroelectricity,which describes a physical property associated with spontaneous polarization,has abundant technological applications,such as non-volatile memorizer,ferroelectric field effect transistor(Fe-FET),and energy harvesting[1,2].Spontaneous po-larization commonly stems from an asymmetric distribution of atoms in crystal structure,requiring the structure with broken inversion symmetry to possess two energetically degenerate ground states,corresponding to two stable spontaneous polarization states[3].Based on Landau-Ginzburg-Devonshire(LGD)theory,a ferroelectric material below its Curie transition temperature can be simpli fied and described by a doublewell free energy landscape as a function of electric polarization[4].Previous researches upon ferroelectrics were mostly concerned with the oxide ceramics,such as BaTiO3[5,6]and Pb(ZrxTi1?x)O3(PZT)[7,8].Driven by the technological requirement for device miniaturization,investigation of ferroelectricity in ultrathin perovskite-type compound films with the general formula of ABO3,has been performed intensively[9,10].Recently,a flourishing number of 2D materials with abundant unprecedent properties have been explored[11?15],the developments of which may provide alternative opportunities for the realization of stable ferroelectricity at the quasi single-layer level[16,17],particularly regarding the technologically more practicable ferroelectric out-of-plane polarization.

An idealized Ising model has been proposed for the existence of 2D ferroelectricity long time ago[18],however,the overwhelming majority of realistic materials are restricted by the fact that ferroelectricity would vanish when the film thickness is reduced below a critical value,su ff ering from the effects including surface energy,electron screening,and depolarizing field[19?21].Generally,the emergence of ferroelectric polarization requires the structural centrosymmetric breaking along the polarization direction.In the intrinsic structure of the currently known 2D materials including the most studied graphene and transition metal dichalcogenides,the projection of their atomic positions along the perpendicular direction possesses the inversion symmetry,which seems that any possible out-of-plane polarization has been excluded.However,due to the rapid development of miniaturized devices and instruments,electronic devices with higher densities are urgently required.In the post-Moore’s Law era,the scaling law continues and further condenses the materials geometrical dimensions.2D ferroelectricity,is not only essential for understanding the thickness-dependent size effect,but also bene ficial for promoting the electronic or optoelectronic applications at nanoscale.Thus,even though the dilemma has raised trailblazing challenges and difficulties to the entire research field,abundant efforts have been put forward to surmount these problems existing in the research upon 2D ferroelectricity.

In this review,we present a survey of a roomtemperature ferroelectric material,copper indium thiophosphate(CuInP2S6)and CuInP2S6-based heterostructures,with emphasis on the property and applicability. We first discuss the physics of roomtemperature stable 2D ferroelectricity and giant negative piezoelectricity of CuInP2S6,and also highlight the relevant synthetic routes.Then,we demonstrate the fabrication and functionalization of CuInP2S6-based solid-state devices,and finally establish the outlook on their future development trend and integration into mature mainstream technologies.

II.STRUCTURES AND FERROELECTRIC NATURE OF CuInP2S6

A.Single-crystal CuInP2S6growth

First of all,we’d like to describe the crystal structure to help understand the ferroelectric nature of CuInS2P6,which mainly contains a framework formed by S atoms,inside which the voids are octahedrally filled with the Cu,In and P–P.As shown in Figure 1a and 1b,bulk CuInP2S6are stacked vertically via weaker van der Waals(vdWs)interactions.It has been recognized that,if the temperature drops below the Curie transition temperature Tc,along with the symmetry transition from the high-T phase(C2/c)to the low-T phase(Cc),spontaneous polarization will emerge with a polar axis perpendicular to the layer z-plane,in existence of the o ff-center ordering in the Cu sublattice and the displacement of cations relative to the centrosymmetric positions in the In sublattice.More accurately,CuInP2S6should be regarded as a collinear twosublattice ferrielectric system[22]in which the copper and indium atoms move towards the opposite directions within each layer.

FIG.1.CuInP2S6crystals.The(a)side and(b)top views of CuInS2P6crystal[23].The polarization direction P is parallel to the z-axis,as indicated by the arrow.Copyright?2016,Springer Nature.(c)CVT synthetic route.(d)XRD spectrum for CuInS2P6.Inset:a platelet-like crystal sample placed on a millimeter grid[24].Copyright?2019,John Wiley and Sons.(e)Polarization Psand sublattice contributions from CuI,InIII,and P2S6calculated from crystallographic results on CuInP2S6[22].(f)Room-temperature hysteresis for bulk CuInP2S6[22].Copyright?1997,American Physical Society.

Later in this review article,we will refer CuInP2S6as ferroelectric for simplicity,since the ferrielectric materials usually display the similar macroscopic property as the ferroelectrics,with a switchable spontaneous polarization.

CuInP2S6single-crystal bulk samples are commonly produced by chemical vapor transport(CVT)reaction using the copper,indium,phosphorus and sulfur elements in the stoichiometric proportions of 1:1:2:6 mixed thoroughly as precursors in an evacuated silica tube(Figure 1c)[25,26].The reaction product normally possesses the form of thin platelets parallel to the CuInP2S6z-plane,as indicated by the X-ray di ff raction(XRD)pattern(Figure 1d).The crystals usually form as beige translucent platelets,with irregular shapes(inset of Figure 1d).To examine the ferroelectricity of bulk samples,the dependence of the electric polarization P on the applied electric field E along the out-of-plane direction was investigated on a CuInP2S6crystal,measured by a commercial ferroelectric analyzer[22].As presented in Figure 1e and 1f,the typical hysteresis loop with Ps=2.55μC/cm2and Ec=77 kV/cm is a direct evidence for room-temperature stable ferroelectricity.

B.Piezoresponse force microscopy measurement

A.Belianinov et al. first revealed the roomtemperature stable ferroelectric polarization in filmformed CuInP2S6,directly re flected by apparent domain distributions,convincing switchable polarization curves and hysteresis loops[27].With means of the room-temperature ultrahigh-vacuum scanning probe microscopy,the domain patterns obtained from the CuInP2S6films with the thickness>100 nm,are very similar to those obtained from the freshly-cleaved bulk crystals.But in their experiments,as the thickness reduces below 50 nm,observable spontaneous polarization disappears.The researchers ascribed this trend to a well-known polarization instability su ff ering from the depolarization field.However,the critical thickness for ferroelectricity reported by them(≈50 nm)is relatively large,far from the ultrathin limit for 2D materials.In the same year,M.A.Susner et al.then announced the enhanced ferroelectricity in a 20 nm CuInP2S6,based on the composition periodic modification by tuning the Cu/In ratio(Figure 2).The Curie transition temperature Tcof CuInP2S6increases in this composite material,enabled by unique spinodal decomposition which maintains the overall crystal morphology but chemically separates the polar CuInP2S6and non-polar In4/3P2S6phases inside each layer in the vdWs structure.This modi fication method offers a new large-volume synthetic routine for 2D in-plane heterostructures by alternating polar and non-polar regions periodically[28].

FIG.2.Periodic modulation of PFM(upper panels)and AFM(lower panels)images obtained from Cu1?xIn1+x/3P2S6 compounds with the Cu/In ratio of(a)0.94,(b)0.67,(c)0.22,and(d)0.15[28].Copyright?2015,American Chemical Society.

FIG.3.PFM characterizations of CuInP2S6[23].(a,d)AFM topography images,(b,e)PFM amplitude images,and(c,f)PFM phase images for CuInP2S6 flakes.(g)Height and PFM amplitude pro file along the dashed lines as shown in(d)and(e).Copyright?2016,Springer Nature.

The next year,F.Liu et al. reported the more systematic experimental investigation upon roomtemperature switchable polarization in CuInP2S6flakes reducing down to the thickness of the 2D limit[23].To examine the 2D ferroelectricity,measurements based on piezoresponse force microscopy(PFM)technique were performed on a series of CuInP2S6flakes with various thicknesses.In a word,the amplitude signal in PFM images mirrors the absolute magnitude of the piezoelectric response locally,and the phase signal in PFM images signi fies the polarization direction for each of these individual ferroelectric domains.The representative ferroelectric domain distributions of CuInP2S6flakes with the thickness from<10 nm to 100 nm based on the technique of atomic force microscopy(AFM)equipped with the PFM module,are provided in Figure 3a-3c.As clearly seen in Figure 3a and 3b,the intensity of PFM amplitude signals decreases with the reduced sample thickness.In conventional ferroelectric ceramic films,such trend is commonly observed,which is usually attributed to the more obvious depolarization effect in thin- films,or the non-uniform electric field distribution between the sample and the scanning conductive probe.

FIG.4.Ferroelectric polarization switching for CuInP2S6 flakes[23].(a-c)The PFM phase images for CuInP2S6 flakes with di ff erent thicknesses,along with the written box-in-box patterns obtained by reverse DC bias.(d-f)The corresponding butter fly-shaped PFM amplitude curves and PFM phase hysteresis loops measured from CuInP2S6 flakes as shown in(a-c)during the polarization switching process.Copyright?2016,Springer Nature.

In F.Liu’s experiments,the PFM images,including the signals of amplitude and phase,distinguishable from the background persist until the lowest thickness of about 7 nm.Expect the background noise,the entire PFM phase image(Figure 3c)can be characterized by two-color tones with a distinct contrast of 180?,both of which correspond to the two polarization directions parallel and anti-parallel to the CuInP2S6z-axis.In CuInP2S6,the domain radial length increases with the increasing sample thickness,and there also exists an evolution trend in the domain patterns,that is the domain shape evolves from the fractal pro files in thinner flakes to the quasi-dendrite pro files in thicker ones.L.Chen et al.later tried to demonstrate the quantitative relationship between the domain characteristic size W and the sample thickness d.This dependence can be well explained by a scaling law proposed by Landau,and optimized by Lifshitz[29]and Kittel[30](viz,the LLK scaling law),which is initially established to describe the size effect in three-dimensional(3D)ferroelectrics.

The fitted power exponent is m≈0.65,marginally higher than 0.5,which is commonly observed in 3D system[31,32].Relatively weak piezoelectric response was captured from a 2L CuInP2S6flake(Figure 3d-3g),which bene fits a lot from the non-existent surface or interface reconstruction in 2D system.These experimental results indicate that ultrathin CuInP2S6flakes can maintain ferroelectricity even down to a few nanometers.But so far,it is controversial whether a unit cell must contain two adjacent layers rather than monolayer,in consideration of the intralayer site exchange.Therefore,it is still doubtful whether it exists stable ferroelectricity in CuInP2S6monolayers,which demands prompt experimental veri fication.

In physics,ferroelectrics represent the materials that exhibit switchable spontaneous polarization.The hysteresis loops have been obtained with convincing results in CuInP2S6bulk crystals,but it doesn’t guarantee the existence of switchable polarization in ultrathin films,especially considering that the conventional ferroelectric thin- films are with a tarnished reputation for their deteriorated performance related to ferroelectricity,due to the inevitable existence of depolarization effect and surface or interface pinning effect.And,given that,F.Liu et al.performed polarization switching spectroscopy measurements by locally applying an electric voltage through the conductive probe[23].A series of PFM phase images captured from 400,30 and 4 nm CuInP2S6flakes,immediately after writing a box-in-box pattern with reversed DC voltage biases,are provided in Figure 4a-4c.An apparent phase reversal of 180?contrast veri fies the existence of switchable polarization in CuInP2S6thinned down to only about 4 nm.This DC bias writing process didn’t induce damages to the surface morphologies of the scanned sample region,meanwhile,the written domain patterns didn’t vanish after several weeks without protection or encapsulation[23].The well-conditioned ferroelectric polarization in ultrathin CuInP2S6samples is further con firmed by the typical butter fly-shaped PFM amplitude curves and the distinctive 180?reversal in the PFM phase hysteresis loops(Figure 4d-4f).

FIG.5.PFM images and polarization switching spectra of CuInP2S6with di ff erent thicknesses[24].(a-h)Out-of-plane(OP)and in-plane(IP)PFM phase imaging overlaid on sample surface topography.(i-l)PFM vertical and lateral phase-voltage bias hysteresis loops obtained from CuInP2S6.Copyright?2019,John Wiley and Sons.

C.Possible in-plane ferroelectricity

It is widely recognized that film-formed CuInP2S6possesses out-of-plane spontaneous polarization[23,27].Interestingly,recent experimental results show there also possibly exists in-plane spontaneous polarization in CuInP2S6[24].As shown in Figure 5,J.Deng et al.investigated and validated the existence of the in-plane ferroelectricity intrinsically in CuInP2S6[24].To figure out the critical thickness at which the in-plane polarization disappears,out-of-plane and in-plane PFM scanning measurements were both performed to reveal the domain distribution in CuInP2S6flakes with di ff erent thicknesses.Figure 5a-5h show both out-of-plane and in-plane PFM phase signals overlaid on the corresponding 3D-formed sample surface topography.For the CuInP2S6flakes with the thickness of 320 nm(Figure 5a,5b)and 100 nm(Figure 5c,5d),out-of-plane(Figure 5a,5c)and in-plane(Figure 5b,5d)PFM images both display distinct domain distribution,as a direct evidence for the co-existence of out-of-plane and in-plane polarization components.As the sample thickness reduces down to 80 nm,fragmented,amorphous or smeared out-of-plane ferroelectric domains can still be distinguished(Figure 5e),in contrast,there exists no obvious indication of domain distribution in this in-plane PFM image(Figure 5f).The clear comparison between Figure 5e and 5f substantiates the existent out-of-plane signal and the non-existent in-plane signal.

FIG.6.Schematics for co-existence of out-of-plane and in-plane polarizations in bulk-formed CuInP2S6and non-existence of in-plane signal in flake-formed CuInP2S6[24].The red arrows represent the polarization component,attributed from Cu displacements.(a)Monoclinic structure of CuInP2S6bulk.(b)Trigonal structure of CuInP2S6 flakes below the critical thickness.Copyright?2019,John Wiley and Sons.

Combining the experimental results obtained from AFM and PFM techniques,J.Deng et al.concluded that the critical thickness under which the in-plane ferroelectric component vanishes locates at the thickness range from 90 to 100 nm.As the sample mechanically exfoliated and further thinned down to 30 nm,no recognizable contrast di ff erences could be observed in two PFM phase imaging channels(Figure 5g,5h),corresponding to the out-of-plane and in-plane polarization orientations,respectively.To identify the evolution behavior of in-plane ferroelectric polarization,the vertical(left column,Figure 5i-5l)and lateral(right column,Figure 5i-5l)PFM phase hysteresis loops as a function of DC bias were measured and obtained from CuInP2S6flakes with di ff erent thicknesses.The distinctive 180?switching occurring in the PFM phase signal validates the co-existence of out-of-plane and in-plane switchable ferroelectric polarizations for the sample with the thickness>90 nm.As the thickness reduces down to 30 nm,even though there doesn’t exist clear domain distribution,the vertical piezoresponse signal displays in a hysteresis form,suggesting the existence of out-of-plane ferroelectric polarization.As we can see from Figure 5i,5j and Figure 5k,5l,the distinct 180?switching in the PFM phase loops in the former and no lateral switchable piezoresponse signs in the latter,con firm the disappearance of in-plane ferroelectricity for the sample with the thickness<90 nm.More importantly,after writing box-in-box patterns,the in-plane signal changes simultaneously with the out-of-plane signal,indicating that the in-plane polarization may be intercorrelated with the out-of-plane polarization,similar to another roomtemperature stable 2D ferroelectric,In2Se3[33?36].The elastic property also exhibits a sudden sti ff ness at the corresponding critical thickness(90～100 nm),consistent with the scanning probe microscopy measurement results.On the basis of the density functional theory(DFT)calculations,the researchers from Beijing Institute of Technology,attributed these behaviors to a structural transition from Ccto P31cspace groups in CuInP2S6.

FIG.7.Nonlinear optical detection for ferroelectric order[23].(a)Polar plots of SHG intensity in horizontal(H)and vertical(V)directions versus the excitation polarization for a 100 nm CuInP2S6 flake,under normal incidence excitation.(b)Polar plot of SHG intensity parallel to the excitation polarization as a function of the crystal orientation.(c)Temperaturedependent SHG intensity obtained from CuInP2S6 flakes with di ff erent thicknesses.Copyright?2016,Springer Nature.

As illustrated schematically in Figure 6a,in the monoclinic structure with Ccspace group,the Cu atoms move towards the sulfur plane,which is the ferroelectric phase.The Cu o ff-centering displacement generates not only an out-of-plane displacement of 1.32 ?A,corresponding to a vertical polarization of 3.14μC/cm2,but also an in-plane displacement of 0.18?A,corresponding to a lateral polarization of 4.48μC/cm2.The amplitude value of the lateral polarization is comparable,or even higher than that of the vertical polarization.The researchers traversed all subgroups among the possible space groups and came to a conclusion that only the structure with P31cspace group meets the requirements that out-of-plane polarization exists while in-plane polarization disappears[24].Consequently,it’s highly possible that the CuInS2P6structure belongs to P31cspace group when it’s below the critical thickness.

To verify this conjecture,two structural models including bilayer CuInS2P6with Ccand P31cspace groups were established to describe the thinnest samples.According to the theoretical calculation results,the structure of P31cbilayers is in a lower energy state by 10 meV,compared with that of Ccbilayers,indicating that the P31cstructure is more stable for CuInP2S6films below the critical thickness.Additionally,in the P31cstructural model,the calculated 1.31 ?A o ff-centering displacement of Cu atoms generates a 4.22μC/cm2out-of-plane polarization,and more importantly,excluding the possibility of lateral piezoresponse,in accordance with the experimental observations.Compared to the Ccmonoclinic structure,the P31ctrigonal structure has a shorter crystal lattice along the z-axis direction(12.995?A in P31ctrigonal state vs.13.149?A in Ccmonoclinic state),together with a shorter vdWs interlayer distance(3.116?A in P31ctrigonal state vs.3.244?A in Ccmonoclinic state),resulting in the enhanced sti ff ness of P31ctrigonal structure than that of Ccmonoclinic structure.The structural transition from Ccmonoclinic to P31ctrigonal phase because of the reduction in thickness was experimentally con firmed with the means of electron di ff raction technique and atomic-resolution scanning transmission electron microscopy.The research upon the unexcavated co-existence of vertical and lateral polarization components,replenishes the study of the thicknessdependent ferroelectric property in CuInP2S6,shedding light on the design and optimization of 2D laminar ferroelectric materials.

D.Optical methods to detect ferroelectric order

Ultrathin ferroelectric films usually exhibit favorable ferroelectricity,piezoelectricity,and pyroelectricity,extensively used in microelectronics,optoelectronics,integrated optics and electro-mechanical systems,which have become a hot issue in the research of advanced functional materials.Under the powerful optical-frequency electric field or low-frequency DC electric field,a series of fascinating phenomena may occur in ferroelectrics,such as nonlinear optical effect,electro-optical effect,anomalous photovoltaic effect and photo-induced refractive effect.The research of these effects not only deepens our understanding of the polarization mechanism and the electron motion behavior in ferroelectrics,but also makes ferroelectrics more applicable in scienti fic fields,such as nonlinear optics and electro-optical effects.Under these conditions,optical characterization techniques offer more effective and non-destructive methods to probe crystal structure and ferric order parameters in ferroelectrics.

Second harmonic generation(SHG)performs as a sensitive nonlinear optical detection technique for broken inversion symmetry in crystal and provides as a powerful tool for studying ferroelectric order[23,37].The polarization-resolved SHG measurement under normal incidence excitation was first performed by F.Liu et al.from Nanyang Technological University to examine the structural symmetry of CuInP2S6[23].Figure 7a visualizes the excitation polarization angle-dependent intensity of SHG signals for the detection direction along the horizontal()and vertical()direction,which can be effectively fitted by the in-plane second-order susceptibility elements for the non-centrosymmetric point group. The non-zero value of SHG signal directly shows that there exists the broken inversion symmetry in crystal that generates the ferroelectricity.According to the co-linearly polarized crystal orientationdependent SHG spectrum in polar coordinates(Figure 7b),there exists an approximate six-fold rotational symmetry,which mirrors the hexagonal arrangement of the copper and indium sublattices in the ferroelectric phase.Besides,SHG technique enables the detection for the ferroelectric-to-paraelectric transition.Figure 7c plots the normalized intensities of the temperaturedependent SHG signals for CuInP2S6flakes with the thicknesses ranging from 10 nm to 100 nm.All the curves follow the similar temperature evolution:below Curie transition temperature Tc～320 K,the SHG signal is clear,as the temperature increases,the signal gradually decreases and finally disappears.The temperature evolution trend can be explained by the transition from the ferroelectric phase to the paraelectric phase with the increasing temperature,during which the CuInP2S6crystal structure also transits from the non-centrosymmetric phase to the centrosymmetric phase.

Raman scattering measures light inelastically scattered from collective quasiparticle excitations,offering as an efficient technique for the observation of phonons and determination of Curie transition temperature(Tc)in ferroelectrics at nanoscale. When T

E.Giant negative piezoelectricity

FIG.8.Crystal dimensionality-related piezoelectricity[42].(a)Positive or(b)negative longitudinal piezoelectric effects,describing that the crystal lattice elongates or contracts when the external electric field is applied along the direction of the polarization,respectively.(c)Three representative ferroelectric solids:1D chain PVDF,2D sheet CuInP2S6,and 3D network Pb(Zr0.4Ti0.6)O3.Copyright?2019,American Association for the Advancement of Science.

In physics,piezoelectric effect is the phenomenon that depicts a fascinating analogy between stress(or mechanical strain)and applied electrical voltage,which is attributed to the contribution from Jacques Curie and Pierre Curie in 1880,that the concept of piezoelectricity came into existence.To be speci fic,if the mechanical force was applied to some certain crystals,such as quartz,topaz,tourmaline,cane sugar and Rochelle salt,it would generate charges on their opposite faces and an electrical voltage would be subsequently induced.Even though the generated voltage is insufficiently large in most cases,it can be ampli fied clearly via ampli fiers.Accordingly,the inverse piezoelectric effect describes the phenomenon that when voltage is externally applied to crystals with piezoelectricity,distortion in shape and size will occur,albeit by only a tiny amount.Recently,piezoelectric phenomena have been extensively used in many areas[38?41],resulting in widespread applications such as actuators,sonars,sensors and piezoelectric transformers[1,2].

For most ferroelectric materials,there normally exist positive longitudinal piezoelectric coefficients,which means that the crystal lattice will expand when the external electric field is applied along the direction of the polarization(Figure 8a). On basis of a simpli fied rigid ion model,the positive longitudinal piezoelectricity originates from the bond energy anharmonicity induced by the spontaneous ionic displacement,making expansion easier compared with compression.The scarce negative longitudinal piezoelectricity has been previously predicted in some certain compounds theoretically[43,44],however,until now the only known example con firmed experimentally is polymer poly(vinylidene fluoride)(PVDF)and its copolymers,in which the crystal lattice will contract if the electric field is externally applied along the polarization direction(Figure 8b).The origin of negative longitudinal piezoelectricity in PVDF has been a controversial research issue with numerous models proposed[45?48].Most of the proposed models ascribe the negative effect to the amorphous regions in samples.In semi-crystalline PVDF,the crystalline phase contains one-dimensional(1D)molecular chains with stronger internal C–C covalent bonds,packed by weaker vdWs interactions.In stark contrast,most conventional ferroelectric ceramics,such as BaTiO3and Pb(Zr0.4Ti0.6)O3(Figure 8c),are crystallized with stronger ionic or covalent bonds to form 3D continuous networks.Thus,it is very natural to establish the association relationship between the atomic arrangement(or bonding energies)and electro-mechanical coupling behaviors.However,the complexity in PVDF micro-structure makes it difficult to excavate the underlying atomistic origin of the negative longitudinal piezoelectricity.To our relief,CuInP2S6provides as an alternative system to investigate[42].

FIG.9.Characterizations of three representative ferroelectric solids with di ff erent lattice dimensions[42].Polarizationelectric field(P?E)curves,corresponding strain-electric field(S?E)curves,and effective d333(amplitude)and phase signals obtained from(a)1D PVDF,(b)2D CuInP2S6(CIPS)and(c)3D Pb(Zr0.4Ti0.6)O3(PZT).(d)In situ XRD patterns of the CIPS(008)peak under the applied electric field.(e)Strain versus the electric field data from(d).Copyright?2019,American Association for the Advancement of Science.

The inverse piezoelectric effects in three archetypical ferroelectric solids,namely,PVDF,CuInP2S6,and Pb(Zr0.4Ti0.6)O3(PZT),are all investigated via AFM-based techniques,for convenient comparison. This derivative PFM technique not only enables the direct acquisition of longitudinal piezoelectric coefficient d333,but also obtains the sign of piezoelectricity based on the phase signals.The 1D chain PVDF and 2D sheet CuInP2S6behave nearly in the same way,in good agreement with the negative piezoelectricity,in contrast,3D network PZT with positive piezoelectric coefficient shows an opposite butter fly-shaped S?E curve(Figure 9a-9c).The effective piezoelectric coefficient d333under zero field of PVDF,CuInP2S6,and PZT are about?25,?95,and+48 pm/V,respectively,attained from the piezoelectric dynamic measurements.Further studies of in situ micro-zone X-ray di ff raction(XRD)upon the crystal lattice deformation under an electric field validates the existence of negative piezoelectricity intrinsically in CuInP2S6(Figure 9d).The slope(?S/?E)of the correlation between the strain and the applied electric field offers the d333value for CuInP2S6,?106±15 pm/V(Figure 9e),consistent with the piezoelectric measurements,which is the reported highest one with a negative sign among single-phase uniaxial ferroelectrics.

Theoretically,longitudinal strain S33,generated from a high-temperature centrosymmetric paraelectric phase,which can be mathematically expressed by a Taylor expansion of electric displacement[49]

FIG.10.Simpli fied rigid ion model of three ferroelectric materials[42].(a-c)Crystal structures and the relevant dipole charges of PVDF,CuInP2S6and PZT.(d,e)Negative(positive)piezoelectric effect in polar solids with discontinuous(continuous)lattice.The potential energy pro files of the corresponding chemical bonds are shown on the bottom.Copyright?2019,American Association for the Advancement of Science.

To estimate the longitudinal electrostriction coefficient Q33for these three samples quantitatively,the data can be fitted based on Equation(2),either from quasi-static measurements byor from dynamic measurements byboth of which yield the commensurate values.The derivative value of coefficient Q33for CuInP2S6is about?3.4 m4/C2,the largest reported value among the currently known inorganic compounds,and excels that for PVDF.According to the first term in Equation(2),negative Q33also implies spontaneous strain,which suggests that there exists expansion in lattice parameter along the polarization direction due to the abrupt drop in spontaneous polarization,when heating the sample across the first-order critical temperature point for the ferroelectric-to-paraelectric transition.

FIG.11.Origin of gigantic negative longitudinal piezoelectricity in CuInP2S6[42].(a)Re fined crystal structure.(b)Cross-sectional electron density map.(c)Calculated vdWs distance d and layer thickness r versus strain along z-axis.(d)Evolution of free energy F and polarization P versus the lattice constant along z-axis.(e)Electron density pro file along the blue dashed line as shown in(b).Copyright?2019,American Association for the Advancement of Science.

In conventional perovskite oxide ferroelectrics,such as 3D network PZT,the formation of macroscopic polarization can be ascribed to the ionic o ff-centering in a continuous lattice connected with powerful covalent or ionic bonds.However,in 1D chain PVDF or 2D sheet CuInP2S6,the situations are not quite the same.The electric dipoles emerge within isolated layers or chains connected with weak vdWs interactions,to form discontinuous lattices,along with the quantization of electric dipoles(Figure 10a-10c).L.You et al.,simpli fied the ball-and-spring model as shown in Figure 10d and 10e,to elucidate the underlying mechanism.In PZT(higher part of Figure 10e),if an electric field is applied along the polarization direction,the ions with positive charges will move along the direction of the electric field,whereas the negative ions will move along the opposite direction.Due to the di ff erent potential energy pro files(shown at the bottom of Figure 10e),the curvatures of which denote the elastic compliances of chemical bonds with k2>k1,thus,piezoelectricity in PZT possesses a positive longitudinal coefficient.Accordingly,in PVDF and CuInP2S6,strong intramolecular bonds are modulated by dipole interactions(Figure 10d).The remarkable di ff erence of around 102in magnitude between inter-and intra-molecular bond energies,will result in a colossal anisotropy and underlie the negative piezoelectricity.When externally applying an electric field,due to the softness of the intermolecular bond,the enhanced dipole interaction will lead the shrink of vdWs gap to exhibit the overwhelming superiority over the expansion of intra-molecular bond with k3>>k4,resulting in a negative longitudinal piezoelectricity.Due to the spatial con finement effect intrinsically,charge redistributions occur within the 1D chain or the 2D sheet in existence of an electric field.The redistribution of charge density may also induce a change in the intra-molecular bond length,contributing to the dominant negative piezoelectric response,as established in PVDF,viz,the negative longitudinal piezoelectricity.

To demonstrate the gigantic negative piezoelectricity at atomistic level, first-principles calculations based on DFT were performed and highly dispersive distribution of Cu atoms is found[22,42].As shown in Figure 11,four Cu sites are assigned to one of these polarization states to explain for this dispersive distribution.The occupancies of Cu sites labelled as 1,2,3,and 4 are calculated to be about 0.32,0.37,0.08,and 0.12,respectively,as mirrored by the atomic radius in Figure 11a.2D cross-sectional electron density map of the foremost atomic plane can be plotted by doing a fast Fourier transformation(Figure 11b).This remarkable asymmetry in the intensity distribution is observed to spread more into the vdWs gap,besides the highly Cu atomic anisotropic dispersion along the out-of-plane direction.As we can see from the line pro file across the Cu atomic electron density(Figure 11e),there additionally exists a characteristic feature besides the thermal dispersion of the major peak,implying a metastable Cu site inside the vdWs gap.

The first-principles calculation results deliver two signi ficant messages.The first point is that,the negative piezoelectricity is the electro-mechanical response intrinsically from CuInP2S6,and the vdWs gap deformation is responsible for the majority of the lattice distortion due to its softness.The second point is that,the huge piezoelectric and electro-strictive coefficients stem from the considerable Cu atomic displacive instability due to the low energy of the metastable interlayer site,about 14 meV per Cu atom shown in Figure 11d,which can be readily surmounted by the 26 meV room-temperature thermal energy. The smearing electron density of Cu sites is found to follow the Maxwell-Boltzmann distribution law,demonstrated by the re fine atomic structure based on single-crystal crystallography.It is possible for the quasi- first-order interlayer-intralayer Cu site transition to smear into the quasi-second-order mode with a flattened energy pro file,suitable for the enhancement of piezoelectricity,under such a strong thermally excited occupancy dispersion[50].As a consequence,the value of roomtemperature d333should lie among the range calculated for the ground and excited states,shown as the lower and upper bounds in Figure 11d,respectively.Additionally,the calculation results verify that,the variation in the vdWs distance d can explain for the majority of lattice constant distortion along the z-axis,while the layer thickness r of CuInP2S6hardly varies(Figure 11c),as an indication of a Poisson’s ratio close to zero.Generally speaking,the displacive instability of Cu atoms is a consequence of the second-order Jahn-Teller distortion,describing the geometry in tetrahedrally coordinated CuIunits that the interlayer sites can be stabilized by the enhancement of the coupling between Cu 4s and S sp orbitals between the adjacent layers,marked by the red dashed line in Figure 11a.The hopping motion,which is thermally active,along with the soft vdWs interaction,induce a gigantic mechanical deformation in existence of an electric field.

F.Tunable quadruple-well ferroelectricity

The laminar material family of thio-and selenophosphates has aroused great research enthusiasm as potential dielectrics for the burgeoning 2D-based electronics.Although the polarization values are relatively low[22,51],CuInP2S6has been proven to possess the rare negative longitudinal piezoelectric coefficient[23,42,52],as mentioned in Section II.E.To unravel the novel ferroelectric-related properties of CuInP2S6,J.A.Brehm et al.examined the potential energy surfaces of the polar displacement via the DFT means.

Taking the mid-plane as the zero point,the researchers calculated the crystal energy as a function of the Cu atomic displacement away from the initial equilibrium positions along the z-axis.The calculated results prove that there exist two stable phases in CuInP2S6.The first one corresponds to the state that Cu atoms locate in the layer,assigned as lowpolarization(LP)state,with a relatively large piezoelectric coefficient with a negative sign.The second one corresponds to the state that Cu atoms locate close to the layer,chemically bonded with the S atoms in the adjacent layer,assigned as high-polarization(HP)state,with a relatively small piezoelectric coefficient with a positive sign[22,51].In consideration of its parallel or anti-parallel orientation along the z-axis direction,CuInP2S6can be regarded as ferroelectrics exhibiting a quadruple-well potential energy pro file with respect to the Cu displacement along the z-axis,according to the terminology established by Stengel and Inguez[53].For simpli fication,CuInP2S6will be referred as quadruplewell ferroelectricity,with the plus-minus sign±LP and±HP to distinguish the four distinct polarization states.

The total-energy calculations with respect to Cu displacements with di ff erent lattice constants along the z-axis,viz c lattice parameters,are also performed.The structural,energy and polarization changes are negligible,as estimated by the other layered materials with the Poisson ratios from 0.1 to 0.3[54,55].Take the positive polarization value region for example,the local energy minima corresponding to+LP state disappear with the c-lattice parameters decreasing from 13.63?A to 12.57?A whereas the energy minima corresponding to the+HP state disappear with the increasing c-lattice parameters.These calculated results demonstrate that,quadruple-well ferroelectricity is tunable by lattice parameters,in other words,by applying strain,and can degenerate into a double-well,which also indicates that,although the energy barrier between LP and HP phases is slightly lower than the room-temperature thermal energy,the LP and HP phases can be stabilized by tuning the c-lattice parameter reasonably.

Direct evidence for all the existing four polarization states is provided by PFM in experiments.The piezoelectric constant can be de fined by the strain variance,Sij,as a function of electric field.The PFM technique captures the surface displacement?L in existence of electric voltage?V,applied along the direction perpendicular to the tested sample surface.The effective longitudinal piezoelectric constant d can then be extracted from d=?L/?V.However,it is very challenging to extract the tensor components of piezoelectricity from the obtained PFM signals,limited by the capability to detect feeble signal,which can be a ff ected by many factors,such as strain,mechanical clamping,electrostatic interaction,and the inhomogeneous radial field distribution around the biased tip inevitably[1,56].It has been proven that in some certain systems,longitudinal piezoelectric constant is highly consistent with the component of piezoelectric tensor,which is along the crystallographic axis[57,58].

Herein,PFM probes the effective longitudinal piezoelectric constant,d,consistent with d333.In this case,both the surface displacement and the applied electric field component are mainly parallel to CuInP2S6zaxis.PFM experiments were performed from 23?C to 70?C across the Curie temperature,on phase-separated CuInP2S6and In4/3P2S6[28]. The Cu element-free In4/3P2S6without ferroelectricity is measured as a reference,for showing the contribution from nonferroelectric phases to the measurement of piezoelectric response locally.The PFM image captured at room temperature shows four distinctive ferroelectric domain signals from ferroelectric CuInP2S6and negligible signals from non-ferroelectric In4/3P2S6.Considering that the energy barrier between HP and LP phases is slightly lower than kBT,the spatially separated LP and HP phases in PFM measurements can be well understood by strain stabilization locally,viz the local c-lattice parameter variation,which validates the theoretical prediction.

頭部壓瘡是重度顱腦損傷患者的常見并發(fā)癥之一,軀體的壓瘡已引起重視,但頭部壓瘡重視程度還不夠,一旦發(fā)生將給家庭、醫(yī)院乃至社會醫(yī)療體系帶來巨大的經(jīng)濟(jì)負(fù)擔(dān)[4],更嚴(yán)重者危及生命。醫(yī)務(wù)人員要提高對頭部壓瘡的重視,充分認(rèn)識壓瘡對機(jī)體帶來的不良影響,提高防范意識[5]。我科在常規(guī)護(hù)理基礎(chǔ)上,通過加強(qiáng)護(hù)士對壓瘡知識學(xué)習(xí),病因預(yù)防,患者及家屬的宣教,彈性排班及護(hù)理模式,頭部壓瘡危險因素評估,臥位護(hù)理,病情觀察,發(fā)熱護(hù)理等全面護(hù)理措施,可有效地降低頭部壓瘡的發(fā)生率。

Due to the rapid development of PFM measurement technique,it is feasible to in situ observe the evolution of all polarization states,under di ff erent external factors.The temperature-dependent PFM mapping collections across the Curie temperature Tcare systematically performed by J.A.Brehm et al.,to examine the pyroelectric properties of CuInP2S6and the respective Curie transition temperature for each phase.For CuInP2S6below 55?C,ferroelectric domains corresponding to four polarization states are all clear and distinguishable.When the temperature reaches 60?C,the?LP state vanishes.If further increasing the temperature up to 65?C,along with the disappearance of±HP states,the number of polarization states will reduce to the only one(+LP state).When it’s above the Curie temperature,for example 70?C,the piezoelectric constant of CuInP2S6is no longer distinguishable from the nonpolar regions.The researchers utilized the switching spectroscopy piezoresponse force microscopy(SS-PFM)to investigate the response from each phase in CuInP2S6to the externally applied electric field.The SS-PFM results show that under the external DC voltage,there exist the switching between?LP and+HP and between+LP and?HP,rather than the switching between±LP and±HP,further validating the coexistence of quadruple wells as theoretically predicted.In CuInP2S6,the generation of these four distinctive phases is attributed to the Cu atom displacement along the z-axis,rather than the multiple double-well structure existing in other materials,viz,each ferroelectric domain corresponds to each axial direction,consistent with the molecular dynamics simulations theoretically[59].These observations provide new pathways for fundamental researches and functional devices,such as filters,energy converters,detectors and memorizers.

III.CuInP2S6-BASED HETEROSTRUCTURES AND APPLICATIONS

A.CuInP2S6/Si diode

The evidences mentioned above unambiguously validate the room-temperature ferroelectricity in 2D film-formed CuInP2S6[22?24,27],making it a favorable non-volatile memory element.

FIG.12.Electric characterizations of CuInP2S6/Si diode[23].(a)I?V characteristics of CuInP2S6/Si diode.(b)Resistanceswitching pulse hysteresis loop.(c)Out-of-plane ferroelectric switching spectroscopy measurements on the same diode device.Copyright?2016,Springer Nature.

The applicable potentials of CuInP2S6materials are tested in a prototype ferroelectric diode,as schematically fabricated in the inset of Figure 12a[23].The diode was fabricated by exfoliating a 30 nm CuInP2S6flake onto a Si substrate.A considerable I?V characteristics hysteresis window and resistive switching are observed(Figure 12a and 12b).The ON and OFF states as labelled in Figure 12b can be assigned,corresponding to the low-resistance and high-resistance states. This resistance switching behavior and the resultant memory phenomenon are probably attributed to the ferroelectric polarization switching in CuInP2S6,based on the out-of-plane ferroelectric switching spectroscopy measurements on the same device(Figure 12c).The coercive voltage in resistance-switching pulse hysteresis loop coincides with the bias in PFM amplitude loop,where the resistive switching takes place,strongly suggesting that the resistive switching in the vdWs CuInP2S6/Si diode originates from the ferroelectric polarization reversal of CuInP2S6.The ratio of around 102between ON and OFF states is almost equal to that in ferroelectric tunneling junctions based on conventional 3D ceramics.Despite these results are obtained from unoptimized tentative devices,it still provides a promising candidate for novel non-volatile memorizers based on 2D ferroelectrics.

B.CuInP2S6-based pyroelectric devices

As mentioned in Section II,CuInP2S6bilayers still exhibit the pyroelectric properties below the Curie temperature.Given this,2D pyroelectric nano-generator is fabricated to probe the current generated in CuInP2S6by converting the charges resulted from the temperature variation[25].

The current mapping substantiates the existence of pyroelectricity in CuInP2S6,further making it a desirable energy-harvesting element.According to the customs,the pyroelectric current I could be described as a function of the temperature changing rate dT/dt,I=p·A ·(dT/dt),in which p refers the CuInP2S6pyroelectric coefficient and A is the Au electrode area in CuInP2S6-based pyroelectric device.The equation means that the generated pyroelectric currents are in direct proportion to the temperature changing rate dT/dt.When the temperature increases from 298 K to 308 K,a peak of?100 pA current can be observed.The negative sign of pyroelectric current is due to the flow of electron away from the sample by heating.Contrarily,cooling the samples will increase the CuInP2S6polarization intensity and promote the surface charge density,thenceforth the current flows towards the sample with a positive sign,implying that the obtained signals are generated in the CuInP2S6-based device as a pyroelectric nano-generator.If further increasing the temperature ramping rate,the observed output current will increase accordingly.These measured results enormously upgrade the self-powered functionalities for ultrathin materials and create new possibilities for pyroelectric devices based on 2D materials,such as thermal nano-sensors and nano-generators.

FIG.13.Structure of CuInP2S6/MoS2Fe-FET[60].(a)Schematic view of a CuInP2S6/MoS2Fe-FET with few-layer MoS2 as the channel.The back-gate con figuration consists heavily doped n-Si as gate electrode and SiO2as gate dielectric.The top-gate con figuration consists Ni/Au as gate electrode and CuInP2S6as ferroelectric gate insulator.(b)Recolored SEM image of CuInP2S6/MoS2Fe-FETs.(c-f)Fabrication process of Fe-FETs.Copyright?2018,American Chemical Society.

C.CuInP2S6/MoS2Fe-FET

Room-temperature Fe-FETs based on CuInP2S6/MoS2heterostructure are preliminarily demonstrated[60],as shown in Figure 13a.The device based on CuInP2S6/MoS2enables an interface without dangling bonds,ideal for a dual-gate con figuration Fe-FET,by integrating MoS2as the semiconducting channel,with CuInP2S6as the ferroelectric top-gate insulator and SiO2as the back-gate dielectric. As shown in Figure 13b,a recolored image obtained by scanning electron microscopy(SEM)technique is provided to sketch out the structure of the fabricated device with di ff erent channel lengths.The fabrication process of CuInP2S6/MoS2Fe-FETs is systematically presented in Figure 13c-13f.The transfer of CuInP2S6onto MoS2is via a mature dry transfer method.MoS2channel was beforehand transferred onto a 300 nm Si/SiO2by the scotch tape exfoliation method,followed by electron beam evaporation of Ni electrodes and lift-o ffprocess.Thus,a MoS2-based transistor with the back-gate con figuration is successfully fabricated.And then,CuInP2S6was mechanically exfoliated onto the surface of a transfer medium(PDMS),and the PDMS/CuInP2S6film was then stamped onto the targeted MoS2.The transfer medium of PDMS was then mechanically removed without residues due to the stronger adhesive force between CuInP2S6and SiO2.

Figure 14 presents the room-temperature electrical measurement results of a CuInP2S6/MoS2Fe-FET.The MoS2channel material used in this device has the size of 1.0 μm in length and 2.1 μm in width.The thicknesses of the MoS2channel and the CuInP2S6ferroelectric gate insulator are about 7 nm and 0.4μm,respectively.Figure 14a and 14b show the ID?VGScharacterization curves measured based on the top-gate and back-gate con figurations at various drain-to-source voltage(VDS),respectively. The transfer characteristics curves show apparent anticlockwise and clockwise ferroelectric hysteresis loops at all various VDSin the top-gate and back-gate con figurations,respectively.The clockwise hysteresis loop in transfer characteristics curves are common in MoS2-based metal-oxidesemiconductor field-effect transistors(MOSFETs)and can be attributed to charge trap existing in SiO2layer or at SiO2/MoS2interface.In the ID?VGScharacteristics as shown in Figure 14c,the ferroelectric hysteresis loop can be controlled by the back-gate voltage.Under a more negative VBG,we will observe a better and more standard hysteresis loop,as a direct indication that the back-gate voltage can be used to adjust the ON/OFF ratio for excellent memory performance.

FIG.14.Characterization of CuInP2S6/MoS2Fe-FET[60].(a-c)Top-,back-,and dual-gate ID?VGScharacteristics.(d,e)I?V measurement illustration and characteristics at di ff erent sweep ranges of a ferroelectric CuInP2S6MIM capacitor.Copyright?2018,American Chemical Society.

Besides the room-temperature stable 2D ferroelectricity in CuInP2S6and its potential applications in CuInP2S6-based Fe-FETs,the CuInP2S6-based metalinsulator-metal(MIM)capacitor,as shown in Figure 14d and 14e,exhibits switching characteristics available for the application devices,such as non-volatile resistance random access memory(ReRAM)[60].The voltage bias externally applied to the MIM capacitor was initially set from zero to a positive maximum,then swept to a negative maximum and finally returned to the zero point.In this Ni/CIPS/Ni capacitor,I?V characterization curves under various gate voltage sweeping ranges(VMax),nearly all present a ferroelectric resistive switching with the ON/OFF ratio of more than 104between low-resistance and high-resistance states.The polarization switching in ferroelectrics results in the resistive switching between these two states,which also induce a change in the band alignment between metal Ni and CuInP2S6.In CuInP2S6,the resistive switching characteristics are modulated by the extremum value(VMax)in the voltage waveform.To be speci fic,a larger VMaxwill induce a smaller ON resistance,along with a larger voltage bias to switch to the high-resistance state.These electrical characterization of CuInP2S6/MoS2Fe-FET results demonstrate 2D-formed CuInP2S6as a practicable candidate for non-volatile ReRAM devices.

IV.FAMILY OF 2D MIMIIIP2(S/Se)6 COMPOUNDS

The new family of transition metal thiophosphate materials,generally with a chemical formula of MIMIIIP2(S/Se)6,offers abundant bulk crystals that exhibit piezoelectric or ferroelectric properties[61].Given that these are vdWs crystals with ionic bonding within each layer,they can be mechanically exfoliated to few layers even monolayer. Based on firstprinciples calculations,B.Xu et al. and W.Song et al.theoretically predicted the existence of out-ofplane ferroelectric polarization orderings intrinsically in MIMIIIP2(S/Se)6compounds monolayer,AgBiP2Se6[62]and CuInP2Se6[63],respectively.More interestingly,the ground state of AgBiP2Se6monolayer is not purely ferroelectric. In AgBiP2Se6,the vertical ferroelectricity stems from the the o ff-centering displacements of Ag+and Bi3+ions anti-parallelly,the compensated ferrielectric state of which has advantage on degrading the depolarization field to stabilize the ferroelectricity at the ultrathin limit.Additionally,B.Xu et al.predicted the AgBiP2Se6monolayer as a photocatalyst in visible wavelengths for water-splitting since the vertical polarization might promote the separation in electron-hole pairs,attributed to its remarkable adsorption for visible light and practical band-edge alignment for applications.

FIG.15.Optimized structures and characterizations of CuCrP2S6[66].(a,b)Crystal structures of CuCrP2S6.Red arrows refer the spin directions.(c)AFM image of a 13.3 nm CuCrP2S6nanosheet.(d)Ferroelectric switching spectroscopy characterizations of this sample.(e,f)Magnetic characterization of massively stacked CuCrP2S6nanosheets:(e)thermal magnetic variation with zero field cooling,and(f)magnetic hysteresis loop.Copyright?2019,Royal Society of Chemistry.

Multiferroics represent the materials that possess two or more primary ferric order parameters,ideal for novel applications with high integration density,such as the conceptual low-consumption storage devices with electric writing and magnetic read.Researches on ferroelectric materials were previously focused on 3D perovskites,such as BiFeO3[64]and TbMnO3[65].However,along with the miniaturization trend of device applications,these micro-or nano-devices constituted by 3D materials in the thin- film form su ff er from surface or interface dangling bonds and non-negligible leakage current in existence of quantum tunneling,deteriorating the device performance.The emergency and flourishment of 2D materials shed light on these intractable issues and offer new platform for next-generation magnetoelectric devices at the nanoscale,bene fitted from their atomic smoothness and considerable dielectric constants.

On account of first-principles calculations,Y.Lai et al.from Peking University proposed laminar CuCrP2S6as a type-I multiferroic candidate,where magnetism and ferroelectricity originates from Cr3+and Cu+ions,respectively[66],as shown in Figure 15.Convincing experimental results upon ferroelectricity was observed in a～10 nm nano flake(Figure 15c),shown as a typical ferroelectric hysteresis loop and a butter fly curve using PFM at room temperature(Figure 15d).The ferromagnetism is further con firmed by a thermomagnetic curve(Figure 15e)and a hysteresis loop(Figure 15f)obtained from CuCrP2S6nanosheets that are massively stacked.The experimental observation results and the first-principles predictions based on first-principles calculations are in good agreement.Similarly,using first-principles calculations,X.Feng et al.predicted two materials,Sc2P2Se6and ScCrP2Se6monolayers with ferroelectricity and multiferroic property,respectively[67].The Sc2P2Se6monolayer is expected to exhibit the out-of-plane ferroelectric polar-ization due to the P atomic asymmetric arrangement.And the ScCrP2Se6monolayer can be formed by substituting half of the Sc atoms in Sc2P2Se6with magnetic Cr atoms to display the multiferroicity.This theoretical result may provide a new platform for the research of 2D multiferroicity and non-volatile magneto-electric devices.Recently,a quaternary vdWs ferromagnetic semiconductor AgVP2Se6has also been predicted theoretically[68]and realized experimentally[69].Based on the experimental results,the AgVP2Se6material possesses remarkable thickness-dependent magnetic properties,and a<10 nm AgVP2Se6flake displays a 19 K Curie temperature and a typical rectangular hysteresis loop with a 750 Oe coercive field at 2 K.This excavated semiconducting 2D ferromagnet provides an alternative platform for investigating magnetism at ultrathin limit and realizing potential applications in nextgeneration spintronic devices.This series of work further expands the properties and functionalities of the 2D MIMIIIP2(S/Se)6compounds.

V.SUMMARY AND OUTLOOK

2D ferroelectrics have shown many advantages over conventional 3D ferroelectric ceramics,such as dimensional stability,mechanical strength,transparency and flexibility,promising its potentials for information storage, field-effect transistors and sensing devices.To conclude,in this study,we reviewed the researches on a room-temperature stable 2D ferroelectric material,CuInP2S6,including first-principles calculations,piezoelectric characterizations,and piezoelectricity-induced functionality.Despite the number of research reports on 2D ferroelectric materials is incomparable to that on 2D magnetic materials,the potential of which to excavate new science and technology has aroused considerable attention in 2D ferroelectricity.To promote the research upon 2D ferroelectricity in its fancy,further attempts need to be done,mainly featured in the following aspects:(1)seeking 2D piezoelectric or ferroelectric materials having excellent long-term storage stability,especially in the ambient environment;and the mechanism behind low-dimensional ferroelectricity.(2)quantitative measurement and characterization of low-dimensional ferroelectric parameters,including ferroelectric polarization value,piezoelectric coefficient,and critical transition temperature;(3)controlled synthesis of high-quality 2D ferroelectric materials on a large scale with lower cost and enhanced device integration.

ACKNOWLEDGMENTS

This work is supported by the NSFC(51522206,11574151,11774173 and 51790492),and the fundamental research funds for the central universities(30915011203,30918011334,30919011403 and 30920021152).