Study On The Physical Properties Of Two-Dimensional Van Der Waals Ferroelectric Semiconductors And Heterostructures

Ferroelectrics have electric polarizations that are spontaneously aligned in the same direction and the polarization direction can be controlled by an external electric field.It has been extensively studied in non-volatile memory devices such as ferroelectric tunneling junctions and ferroelectric field-effect transistors.With the advent of the post-Moore era,the demand for miniaturization of ferroelectric device in modern electronics is incresing.Therefore,the exploration of out-of-plane ferroelectric properties in the two-dimensional atomic thickness limit has become a hot research topic.For conventional three-dimensional ferroelectric crystals,as the thickness decreases to the nanometer scale,the surface area increases and the interfacial effect is significantly enhanced.At the same time,the depolarization field strongly suppresses the ferroelectric polarization.In contrast,two-dimensional layered materials with saturated interfaces and weak interactions with the substrate can be prepared as heterostructures without the limitation of substrate lattice matching.These proterties make the two-dimensional materials an excellent platform for out-of-plane ferroelectric exploration in the two-dimensional limit.At present,the exploration of out-of-plane two-dimensional ferroelectrics is mainly based on two ideas:on the one hand,we search for intrinsic two-dimensional monolayer crystals satisfying the spatial inversion symmetry breaking in the out-of-plane direction.On the other hand,based on the interlayer degrees of freedom of two-dimensional van der Waals materials,we explore slidding ferrorelctric system induced by interlayer compensation charges.In view of this,in this thesis we focus on representative layered ferroelectric semiconductors copper indium thiophosphate(CuInP2S6)and rhenium disulfide(1T’-ReS2).Using research methods such as piezoelectric force microscopy,second harmonic measurements and transport measurements,we investigate the intrinsic ferroelectric properties and demonstrate prototype devices based on a two-dimensional ferroelectric heterostructures,from which we realized:(1)Verification of intrinsic room-temperature ferroelectric polarization and bulk photovoltaic effect in two-dimensional CuInP2S6,as well as the non-volatile regulation of the electronic structure of two-dimensional semiconductors;(2)Theoretical design,experimental verification,and demonstration of ferroelectric devices with out-of-plane slip ferroelectricity in few-layer 1T’-ReS2.The main contents of this thesis mainly include:In Chapter 1,we review the background of the exploration of two-dimensional ferroelectric properties.Then we introduce the recent research progress of twodimensional ferroelectric materials,as well as the device studies based on twodimensional van der Waals ferroelectric materials.In Chapter 2,we introduce the sample preparation methods and characterization tools commonly used in the experiment,including various mechanical exfoliation methods and two-dimensional heterostructure preparation processes.Then we introduce the common sample characterization methods including photoluminescence and Raman spectrum,second harmonic generation and piezoresponse force microscopy measurements.In Chapter 3,we explore sample preparation,physical characterization,and phase transition temperature(TC)modulation for CuInP2S6.We prepared large-area uniform CuInP2S6 films on the substrate surface by the gold-assisted exfoliation method.We verified the out-of-plane ferroelectricity of CuInP2S6 by second harmonic generation test,observation of spontaneous ferroelectric domains and flipping of ferroelectric domains.By graphene interfacial modification,we achieved the modulation of phase transition temperature of CuInP2S6 nanoflakesIn Chapter 4,we explore the sliding ferroelectricity of 1T’-ReS2.Using piezoresponse force microscopy,we give conclusive experimental evidence of out-ofplane sliding ferroelectricity in few-layer(2 to 5 layers)1T’-ReS2.We also give the phase transition temperature of bilayer 1T’-ReS2 by temperature dependent second harmonic measurement.In Chapter 5,we study the modulation ability and device applications of CuInP2S6 and 1T’-ReS2 in heterostructures.Based on ultrathin CuInP2S6 nanoflakes,we prepared CuInP2S6/MoSe2 heterostructure devices to achieve nonvolatile modulation of exciton complexes in monolayer MoSe2.We found that the nonvolatile behavior of the device is up to 3 months.We also characterized the carrier density tunability in the heterostructure.Based on the ferroelectric semiconductor properties of CuInP2S6,we prepared heterostructure devices based on graphene and ultrathin CuInP2S6 and observed the bulk photovoltaic effect in CuInP2S6.Based on bilayer 1T’-ReS2,we prepared ferroelectric tunneling junction prototype devices and observed resistive state switching caused by the fliping of ferroelectric polarization direction.In Chapter 6,we summarize and prospect the work of this thesis.