Researches On Physical Properties And Devices Of Van Der Waals Layered Ferroelectric Materials CuIn（Cr）P₂S₆

In recent years,copper-containing transition metal thiophosphate has attracted more and more attention due to ferroelectric,antiferroelectric,ferromagnetic,valleys and other properties,CuInP2S6（CIPS）is one of the few vdW room temperature ferroelectrics confirmed by experiments so far.Recent studies have shown that Cu⁺ions play an important role in ionic conductivity and ferroelectric polarization,for example,giant negative piezoelectric coefficient and quadruple-well ferroelectric in CIPS are all due to the vertical migration of Cu⁺ions.However,the microscopic process and physical mechanism of how Cu⁺ions migrate in the lattice and the influence on ferroelectric polarization are still unclear.On the other hand,the strong coupling effect between Cu⁺ions migration and ferroelectric polarization in CIPS has certain drawbacks.When electrically switching ferroelectric domains,it will cause long-range migration of Cu⁺ions,which will cause Cu to precipitate onto the sample surface and even destroy the ferroelectricity.Thereby,it is urgent to find effective means to control ferroelectric polarization and inhibit surface deformation caused by Cu⁺ions conductance.Compared with other vdW room temperature ferroelectric materials,CIPS has the advantages of stable out-of-plane ferroelectric polarization and wider band gap,therefore,CIPS shows great application prospects in ferroelectric field-effect transistors,ferroelectric tunnel junctions,negative capacitance transistors,etc.In conclusion,the rich physical properties of copper-containing transition metal thiophosphate attract us to further study.This article focuses on CIPS and CuCrP2S6（CCPS）with balance of ionic conductivity and polarization as the core,the regulation of ferroelectric polarization and the migration of Cu⁺ions were systematically studied with the help of scanning probe microscope（SPM）,and the potential application of CIPS was preliminarily explored.1.CIPS nanosheets with different thickness obtained by mechanical exfoliation technique and the thickness-dependent raman spectra were studied.The reversal dynamics of ferroelectric domain was systematically studied by SPM,which shows that the stability of ferroelectric domain is related to the applied voltage,pulse time and the thickness of nanosheets.By constructing Mo S2/CIPS heterostructures,Mo S2 can effectively screen the polarized charges of CIPS and improve the stability of ferroelectric domain of thin CIPS flake,providing an important reference for optimizing ferroelectric properties of CIPS.2.The nanoscale mapping of copper ions migration in layered CCPS and CIPS has been systematically demonstrated with SPM and high-resolution transmission electron microscopy（HR-TEM）and Cu⁺ions-diffusion-induced serial structural transformation has been observed directly at a resolution of nanometers.Cu⁺ions migration goes through two-stage process:for Vtip≤-4 V,copper ions can migrate reversibly across the layers to form copper-deficient intermediate phase Cu1-x Cr（In）P2S6;for Vtip>-4 V,irreversible copper ions migration has been observed accompanied by a new phase Cr（In）PS4 and phase decomposition（Cu2S and Cr P）generation.This study elucidated lattice structure transitions caused by ion migration in CCPS and CIPS and deepened the understanding of intrinsic metal-ion transport mechanisms in copper-containing metal thiophosphate system.3.Taking CIPS as a model system,the interplay between ionic kinetics and ferroelectric switching properties is systematically studied.Firstly,in-plane electric field successfully induced unconventional out-of-plane domain inversion.The intralayer hopping motions of Cu⁺ions play a role in ferroelectric domain inversion.These findings not only provide direct evidence for the microscopic origin of ferroelectric polarization in CIPS,but also establish a new mechanism to reverse ferroelectric dipoles simply and effectively.However,the direction of ferroelectric domain reversal caused by in-plane copper ions migration is random.In order to effectively control ferroelectric domain reversal and suppress the polar structure damage caused by Cu⁺ion conductivity,we propose to realize the reversible reversal of the ferroelectric domain by constructing the ferroelectric-liquid interface.At room temperature,[DEME]cations and[DDBS]anions spontaneously adsorbed on the surface of CIPS and an additional electric field consistent with the direction of the depolarization field is provided to induce downward and upward polarization reversal,respectively.Such ion adsorption-induced reversible polarization switching enriches the functionalities of solid-liquid interfaces and provides a new idea for the development of interface-controlled vdW ferroelectric-based devices.4.The photodetector based on multilayered CIPS is fabricated and characterized,showing an ultrafast response to 280 nm laser（response time<0.5 ms）.In addition,it also exhibits ultralow dark current（～100 f A）,high on/off ratio（～10³）,and high specific detectivity of 7.38×10¹⁰ Jones.The photoresponsivity is 10.8 m A W^-1,which is comparable to those of photodetectors based on Mo S2 and graphene.It shows potential application prospects in low power consumption and high-speed photodetection.