Physical Mechanism Of Energy Conversion In Two-dimensional Piezoelectric And Ferroelectric Materials

Unique structure and excellent physicochemical properties of two-dimensional（2D）materials facilitate the development of miniaturized high-performance electronic devices.It has broad application prospect in many fields such as energy conversion and non-volatile information storage,etc.Piezoelectric materials,for example,enable the interconversion between mechanical and electrical energy when subjected to external electric field or external stress,thus are widely used in electromechanical systems.And ferroelectric materials with two or more stable polarized states can realize reversible conversion between different steady states under an external electric field,which can be used in the design of nonvolatile memories（NVMs）.In addition,2D materials often exhibit other excellent multifunctional physical properties（e.g.,flexibility,anisotropic electrical and optical properties,and the combination of intrinsic ferroelasticity）,making them promising for applications in flexible electronics,controllable electronics,etc.In this paper,we systematically carry out a careful study of a series of new stable2D materials with good mechanical and electrical properties.We mainly focus on the piezoelectric and ferroelectric properties of these materials,further reveal their internal physical mechanisms and explore their potential applications,providing favorable theoretical guidances for the design of 2D materials with multifunctionality.Our paper consists of six parts.The first chapter briefly summarizes the research status and application of 2D piezoelectric materials and ferroelectric materials.The second chapter introduces the theoretical basis of the first principles calculations and relevant computational software package we used.In the third chapter,analogs of transition metal sulfides（Mo TO monolayers and bilayers）are taken as the research objects,and the internal physical mechanism and influencing factors of the strong piezoelectricity in 2D materials are systematically analyzed.In the fourth chapter,the design and research of directional multifunctional 2D materials are introduced in detail.In the fifth chapter,a group of 2D ferroelectric heterostructures are designed,providing potential guidance for the design of intriguing ferroelectric materials.In the last chapter,we summarized the primary conclusions and innovations of this thesis,and the theoretical research of piezoelectric and ferroelectric materials is outlooked.The main research contents and key conclusions of our paper are as follows:（1）Predicted a stable group of Janus Mo TO（T=S,Se and Te）monolayers and bilayers with structural asymmetry,and systematically investigated their piezoelectric properties for converting mechanical and electrical energy into each other.The“active asymmetric electron-transfer”strategy mainly contributes to the spontaneous remarkable piezoelectricity of 2D materials.When two elements are given but another element can be changed,the larger the electronegativity difference ratio is,the stronger the piezoelectricity of Mo TO will be.Mo Te O bilayers contain remarkable vertical piezoelectricity,and its d₃₃ of Mo Te O bilayer reaches 38.907 pm/V.Importantly,we proposed the new method for calculating the piezoelectric coefficients of 2D materials,which corresponds to the fact that 2D materials have certain layer thickness,and facilitates the comparison of piezoelectric coefficients of 2D and 3D materials.（2）Reported the novel tetragonal Zr₂P₂XY（X/Y=I,Br,Cl or F;X≠Y）monolayers with excellent intrinsic ferroelasticity and remarkable piezoelectricity.The promising anisotropic properties of both mechanics and electron can be perpendicularly controlled based on the intrinsic ferroelasticity.The inhomogeneous charge distribution caused by mirror asymmetry endows these monolayers with strong out-of-plane piezoelectricity.Zr₂P₂Br Cl monolayer possess the largest piezoelectric strain coefficient d₃₃=129.705 pm/V,which is two orders of magnitude higher than that of Mo STe multilayer.Perfect coupling of excellent physical properties makes Zr₂P₂XY monolayers potential candidate material for the design of multifunctional electronic devices.（3）Designed the stable Sn S/Sn SSe heterostructure with excellent sliding ferroelectricity.Sn S/Sn SSe with the stacking state II has the largest e₃₁=0.182×10^-10C/m.Importantly,the direction of vertical polarization is determined by different couplings of neighboring-layers,helping to achieve polarization switching via sliding the top Sn S monolayer with very low energy barrier of 0.106 e V.Besides,the heterostructure inversion method can also realizes the polarization switching of Sn S/Sn SSe.In ground state,the direction of vertical polarization can be switched by the heterostructure inversion from Sn S/Sn SSe to Sn SSe/Sn S through overcoming remarkable lower energy barrier of 0.043 e V.Excelent ferroelectricity makes Sn S/Sn SSe heterostructure a great potential material in controllable nanoelectronics,such as seismic detectors,energy storage devices and nonvolatile memories（NVMs）.In summary,we have obtained a series of 2D materials with excellent piezoelectric or ferroelectric properties through theoretical prediction methods which not only enriches the potential candidates for the deisign of piezoelectric sensors,non-volatile memories and multifunctional device designs,but also further reveal the internal physical mechanism of piezoelectricity and ferroelectricity to promote human understanding of piezoelectric and ferroelectric materials.