First-principles Studies Of Several Two-dimensional Ferroelectric Materials And Related Heterostructures

Ferroelectrics（FEs）are materials possessing spontaneous electric polarization which can be reversed by external electric field,showing broad applications in memory devices.Call for the miniaturization of electronic devices,enormous efforts have been put into the fabrication of few nanometers thick FE thin films,as well as two-dimensional（2D）FE materials that are several atomic layers thick.Recently several kinds of 2D FE materials,as well as 2D FE functional devices with excellent performance,such as ferroelectric tunnel junction,ferroelectric semiconductor field-effect transistor,etc.,have been fabricated in experiments.However,until now,the amounts of the fabricated two-dimensional FE materials are still limited.The design and fabrication of 2D FE functional devices are also in the initial stage.Based on such backgrounds,in the thesis,I investigate the structure and properties of several 2D FE materials and related heterostructures by using first-principles calculations based on density functional theory.The main content includes:prediction of a family of new 2D FEs M₂X₂Y₆ with intrinsic scale-free ferroelectricity;design of FE functional devices based on the heterostructure which could be obtained through intercalating monolayer QL-M₂O₃,which is stable monolayer FEs as we predicted,into graphene/metal interface;design of a 2D multiferroic heterostructure with strong magnetoelectric coupling.Moreover,the formation mechanism and diffusion process of oxygen vacancy in two oxides are investigated.The main results are shown as follows:1.A family of 2D FE materials with new structure prototype,M₂X₂Y₆（M=metal,X=Si,Ge,Sn,Y=S,Se,Te）has been predicted,by using high-throughput calculations.The ferroelectricity of M₂X₂Y₆ originates from the vertical displacement of X-dimer driven by the soft phonon modes.By analyzing the phonon vibration modes,it is found that the FE M₂X₂Y₆ possesses two possible centrosymmetric configurations,which causes two possible FE switching paths of M₂X₂Y₆.Moreover,we find 2D FE M₂X₂Y₆exhibits intrinsic scale-free ferroelectricity,i.e.,the polarization in each unit cell can be reversed independently,and maintains ferroelectricity after contacting with metal electrodes,indicating its potential applications in high-density storage.This work provides a novel family of the 2D FE materials and guidelines for the experimental design of related functional devices.2.The feasibility of constructing FE functional devices via intercalating monolayer FEs into graphene/metal interface has been investigated,by using first-principles calculations based on density functional theory.Calculation results show that the M₂O₃（M=Al,Y）monolayer in the structure prototype of quintuple layer In₂Se₃（QL-In₂Se₃）is stable 2D FE materials.The graphene/QL-M₂O₃/Ru heterostructure,which could be achieved by intercalating QL-M₂O₃ into graphene/Ru interface,shows resistive switching behavior when the polarization of QL-M₂O₃ is reversed,whereby the heterostructure can function as a prototype ferroelectric tunnel junction.Moreover,the doping type of graphene is modulated by the polarization direction of QL-M₂O₃,enabling the fabrication of graphene p-n junction if the QL-M₂O₃ is periodically poled.It is proposed that intercalation is likely to be an effective method to construct the heterostructure.The integration of graphene and monolayer FEs,as well as the construction of related functional devices,can be achieved naturally if the proposals in this work are realized in experiments.3.A 2D multiferroic heterostructure composed of MnSeTe and In₂Se₃ has been theoretically proposed.Calculation results show that the MnSeTe/In₂Se₃ heterostructure exhibits strong interlayer magnetoelectric coupling,which may originate from the polarization-polarization interaction between MnSeTe and In₂Se₃.Moreover,in such a heterostructure,the state of magnetic skyrmions in MnSeTe is likely to be modulated by reversing the polarization of In₂Se₃.Based on the MnSeTe/In₂Se₃ heterostructure,an In₂Se₃/MnSeTe/In₂Se₃ heterostructure in which MnSeTe is sandwiched between two In₂Se₃ layers were proposed,control of the magnetic skyrmions evolution in MnSeTe can be achieved via multiple polarization states in such a heterostructure.The calculated band structures show that the ferroelectric modulation of DMI in MnSeTe may be related with the modulation of Rashba spin-orbit coupling in MnSeTe through polarization reversal.This work provides a method to achieve 2D multiferroics with strong magnetoelectric coupling,as well as a new perspective for the design of related spintronics.4.The（scanning）transmission electron microscopy observed formation and diffusion of oxygen vacancy（VO）in two oxides,CeO₂ and SrAl₂O₄,have been investigated by first-principles calculations.The diffusion of oxygen atoms during the CeO₂ reduction process has been revealed.The simulation results show that there are three possible diffusion paths of O atoms.All the diffusion paths are along the （001） direction,indicating the O atoms diffusion is anisotropic.The Coulomb interaction between the diffusing O atoms and Ce atoms leads to the splitting of the Ce sublattice,facilitating the anisotropic behavior of the O atoms diffusion.For the SrAl₂O₄ system,the mechanism of the formation of the ordered-VO structure in SrAl₂O₄ after electric field-high temperature treatment has been proposed.Calculation results show that the applied electric field leads to the formation of VO at specific O site in SrAl₂O₄,and facilitates the transformation from disordered-VO structure to ordered-VO structure by lowering the transformation barrier.The high-temperature provides the thermal energy for SrAl₂O₄ to overcome the transformation barrier.The calculation results are consistent with the experimental observations and provide perspective for the design of related functional oxides.