Theoretical Investigations Of The Ferroics Multifunctional Properties In Novel Two-dimensional Materials

In recent years,the researches on two-dimensional(2D)materials has achieved rapid progress in theory and experiment since the discovery of graphene,and various novel 2D materials have been successfully predicted or synthesized,including h-BN,transition metal chalcogenides,MXenes,CrI3,In2Se3 and so on.2D materials have unique structural characters and excellent physical and chemical properties comparing with their bulk counterparts.This promotes the development of miniaturized and high-performance devices on a large extent,and has broad application prospects in many fields including information and new energy.For example,2D ferroics have two or more stable spin polarization states,spontaneous electrical polarization states or structural strain states,and the reversible conversion between these stable states can be realized under the external factors(magnetic field,electric field and stress).It provides a perfect platform for developing nonvolatile memory.It is worth pointing out that 2D materials simultaneously possessing two or more primary ferroic orders(that is,2D multiferroic materials)can realize more emerging polymorphic properties,which make it has potential for applications in magnetoelectric nanodevices.In addition to the coexistence of different ferroic orders,2D materials often exhibit other multi-functional properties(such as a combination of high carrier mobility,excellent optical properties,and negative Poisson's ratio),making them being with great potential applications in the fields of photocatalysis and nanomachinery.In this dissertation,we have systematically studied the electronic structure,ferroics and mechanical properties of variously new 2D materials as well as their applications in nonvolatile memory,sensors and photocatalys and other fields,revealing their physical mechanisms and providing theoretical guidance for the application of 2D materials in storage devices and multifunctional devices.This dissertation contains six chapters.In the first chapter,we briefly summarize the research progress and applications of 2D materials.In the second chapter,we introduce the theoretical fundamentals of first-principles calculations and relevant software packages.In the third chapter,we investigate the ferroelectric and ferromagnetic properties of 2D systems and its regulation.In the fourth chapter,we introduce the design and research of the 2D intrinsic triferroicity material in detail.In the fifth chapter,we study 2D multifunctional materials as well as their potential applications in multifunctional devices.In the sixth chapter,we summarize the main research contents and innovations of dissertation,and provide prospect for the future of new 2D ferroelectric and multiferroic materials.The main research contents and conclusions of this dissertation are as follows:(1)We explore the electronic structure and ferroelectric properties of monolayer y-SbX(X?As,P).We first prove that both systems are dynamically and thermally stable.The particular crystal structure breaks the inversion symmetry,which is the necessary ingredient for a spontaneous electric polarization.This leads to ferroelectricity.Our results show that they have the large in-plane spontaneous polarization and the suitable energy barriers of ferroelectric switching.In addition,we propose the spontaneous polarization and energy barriers of both systems can be regulated regularly by strain.More importantly,they exhibit Curie temperatures much higher than room temperature.Furthermore,we find that they are semiconductor materials that the band edges straddle the water redox potentials.(2)We investigate the electronic structure and spin polarization properties of the bilayer CrIb interfaced with monolayer MoSeTe.We design a series of composite systems:MoSeTe/CrI3?MoTeSe/CrI3?MoTeSe/CrI3/SeTeMo?MoTeSe/CrI3/TeSeMo and MoSeTe/CrI3/TeSeMo,which confirm that the weak van der Waals interaction between layers can redistribute the charge of the system,resulting in different energy band structures of the above composite systems.Our results show that the interlayer magnetic ground state of pristine bilayer CrI3 is find to be antiferromagnetic(AFM).When bilayer CrI3 and monolayer MoSeTe are stacked to form a longitudinal heterostructure,the magnetic ground state for bilayer CrI3 transforms from AFM to ferromagnetic(FM).In addition,we find that MoSeTe/CrI3,MoTeSe/CrI3/TeSeMo and MoSeTe/CrI3/TeSeMo systems are half-metals,while MoTeSe/CrI3 and MoTeSe/CrI3/SeTeMo systems are semiconductors.Moreover,we also reveal the underlying mechanism of the magnetic transition and half-metal/semiconductor properties in the bilayer CrI3.(3)We investigate an 2D intrinsically triferroic semiconductor—monolayer FeO2H,harboring antiferromagnetism,ferroelasticity and ferroelectricity simultaneously.Our results show that monolayer FeO2H has great stability as well as easy experimental fabrication from its layered bulk.Magnetism originates from transition-metal atom with half filled d shells.we confirm that its magnetic ground state is antiferromagnetic and reveal the underlying physics mechanism of such antiferromagnetic coupling.At the same time,we find that monolayer FeO2H is a promising ferroelastic material with suitable energy barrier for ferroelastic switching and reversible ferroelastic strain.In addition,we also reveal the ferroelectricity and piezoelectricity of monolayer FeO2H.More interestingly,the directional control of its ferroelectric polarization is achievable by 90° reversible ferroelastic switching.(4)We investigate a promising 2D multifctional semiconductor with ferroelectricity,ultrahigh carrier mobility,and negative Poisson's ratio in monolayer BI.Our results show that the system exhibits inherent ferroelectric properties and has a larger in-plane ferroelectric polarization due to the breaking of the spatial symmetry of its crystal structure.In addition,monolayer BI has an extremely high carrier mobility,even comparable to that of graphene,broad-wavelength range and a remarkably high absorbance coefficient.Moreover,we also find that monolayer BI is an auxetic material with a negative Poisson's ratio.These excellent properties make monolayer BI a compelling multifunctional material,offering an ideal platform for diverse low-dimensional devices applications.(5)We investigate the photocatalytic performance and the phenomenon of negative Poisson's ratio in monolayer PtI2 as well as its potential applications in multifunctional devices.Our results show that monolayer PtI2 is a 2D semiconductor that can be peeled off from the layered bulk,and the effect of strain on the band gap is explored.We also find that it is a promising 2D photocatalyst,exhibiting outstanding catalytic performances toward oxygen evolution reactions,with the water oxidation reaction occurring spontaneously under light irradiation.Moreover,monolayer PtI2 features a unique negative Poisson's ratio because of the special wrinkled hinge-like geometric structure.These excellent properties suggest that monolayer PtI2 could be realized in the laboratory and utilized as a promising multiftunctional material in the fields of photocatalysis and nanomachinery.(6)We explore the electronic and optoelectronic properties of 2D X-T12S(X=E,1T,2H)systems.We find that the systems are 2D metal-shrouded semiconductors,and their band structure and band gaps can be regulated regularly by strain.In addition,we also demonstrate that they exhibit excellent flexible mechanical properties,superior light absorption and photocurrent properties.More importantly,monolayer 2H-Tl2S displays strong spin-valley coupling in the conduction band.Finally,we investigate the structural phase transition from 2H phase to 1T phase in monolayer Tl2S.Our results have stimulated the research interest in 2D metal-shrouded semiconductor materials,and the combination of these excellent properties makes them being with great application potential in multifunctional devices.