Studies On The Flexoelectric Effect In 2D Ferroelectrics CulnP₂S₆

Ferroelectric materials display a spontaneous,intrinsic polarization that can be switched by external electric fields.The first ferroelectric material,Rochelle salt,was discovered about a century ago.Despite the modern theory of ferroelectricity based on the Berry phase was established,ferroelectrics have also been widely used in the field of functional electronic devices such as ferroelectric capacitors,ferroelectric fieldeffect transistors,and ferroelectric tunneling junctions.However,the conventional bulk oxides ferroelectrics are limited by the critical size effect,and their ferroelectricity is gradually suppressed as the thickness becomes thinner.Meanwhile,the ionic or covalence bonds in conventional bulk oxides ferroelectrics have much greater interaction strength but lack sufficient slip systems for dislocation movement under deformation,leading to a brittle fracture.The above factors limit its development in ultra-thin electronic devices as well as flexible electronics devices.In recent years,the emergence of 2D vdW ferroelectrics,represented by CuInP2S6,has opened up new horizons for this dilemma.Thanks to its interface-saturated chemical environment,structural stability of single atomic layer,and excellent mechanical properties,it is expected to break through the limitations of conventional bulk oxide ferroelectrics in applications.Nevertheless,for vdW ferroelectrics with atomic thickness,the out-ofplane switching electric field at 2D limit inevitably results in large leakage current and even material breakdown.This makes the effective modulation of polarization and the controllable preparation of ferroelectric domains in the 2D ferroelectrics still very challenging,and there is an urgent need to develop new approaches of polarization modulation beyond electrical methods.In view of this,this essay takes an experimental perspective,based on the good flexibility and ultrathin properties of 2D materials,by using methods such as SPM and spectroscopic measurements:(1)The mechanical properties,such as Young’s modulus and fracture strength,of 2D CuInP2S6 were quantitatively investigated;(2)A giant strain gradient(～106 m-1)was introduced in the CuInP2S6 thin flakes,and the mechanical modulation of ferroelectric polarization was achieved by using the giant flexoelectric effect.Thus,the uniform large-area ferroelectric stripe domains are artificially and controllably generated at the macroscopic scale.Moreover,the intrinsic mechanism was investigated by combining with the phenomenological theory and the first-principles calculations.The contents are displayed in the following parts:In Chapter 1,firstly,we introduce the research progress of 2D vdW ferroelectrics and summarize the types of 2D layered ferroelectrics realized experimentally and their related applications.Meanwhile,the main research object of this essay—2D vdW ferroelectrics CuInP2S6 is introduced.Then,we also introduce the mechanical properties of 2D material and outline the experimental research on the mechanical properties of different 2D material at current stage.Finally,the flexoelectric effect in ferroelectrics is introduced,including the basic principle of the flexoelectric effect and the modulation of piezoelectric response,polarization direction and photovoltaic effect of the ferroelectrics by using the flexoelectric effect.In Chapter 2,we introduce the main experimental techniques and the methods of sample preparation involved in this essay.At first,the basic principles of AFM,PFM and KPFM and their applications in measuring force-distance curves and characterizing ferroelectric domains/loops are presented.Then,we introduce the principle of SHG and Raman spectroscopy.At last,we present the methods of sample preparation including chemical vapor transfer method,gold assisted mechanical exfoliation method and preparation of corrugated PDMS substrate.In Chapter 3,we introduce the study of the mechanical properties of 2D CuInP2S6.Firstly,the preparation of large-area freestanding CuInP2S6 thin flakes with high efficiency is described in detail.Then,the mechanical property tests of CuInP2S6 thin flakes based on the nanoindentation method are presented,and the parameters of Young’s modulus and fracture strength are given quantitatively.Moreover,the mechanical properties of 2D CuInP2S6 are analyzed in comparison with those of other two-dimensional materials.Our results provide guidance for the design and application of 2D CuInP2S6-based flexible ferroelectric devices.In Chapter 4,we introduce the large-scale stripe domain engineering in 2D CuInP2S6 via giant flexoelectric effect.Giant strain gradients(～106 m-1 at nanoscale are achieved via substrate engineering.The electric polarizations in ultrathin CuInP2S6 are mechanically switched without an applied electric field.Moreover,the intrinsic mechanism was investigated by combining with the phenomenological theory and the first-principle calculations.Our results highlight the potential of ultrathin 2D ferroelectrics for developing vdWs ferroelectrics based flexible 2D electronics.In Chapter 5,we summarize the research result of this essay and prospect the new directions in the field of two-dimensional ferroelectrics and flexoelectrics.