Designs And Studies Of Low-Dimensional Ferroelectric And Ferromagnetic Functional Materials Via First-principles Calculations

Since the 21st century,the development of the Internet and 5G technology has brought about the explosive growth of global electronic data.International Data Corporation（IDC）predicts that worldwide data will reach a staggering 163 ZB by 2025,which is more than four times the amount of data in 2018.In order to meet the rapidly growing demand for electronic data storage and processing,how to develop non-volatile information devices with higher density,faster response and lower energy consumption is a big challenge for the electronics manufacturing industry.Due to the critical size effect and interface effect,the traditional ferroic materials are difficult to be applied to the next generation of nanoscale information storage devices.In recent years,a lot of progress has been made in theoretical and experimental researches on low-dimensional ferroelectricity and ferromagnetism,and the coupling between low-dimensional ferroelectricity and magnetism and other physical properties has also produced many novel physical phenomena.Nonetheless,the microscopic mechanism of ferroicity enhancement in low-dimensional systems and the contact property between two-dimensional ferroic materials and other materials have not been clear.How to regulate and use low-dimensional ferroicity to meet the needs of practical applications is one of the focuses of condensed matter theory and experimental research.Based on this,in this dissertation,via density functional theory（DFT）methods,we study the surface/interface effects of low-dimensional ferroic materials and their internal regulation mechanisms,as well as the interaction mechanisms between electric polarization,electronic structure and magnetism,and explore their possible applications in electronic devices and chemical catalysis based on the idea of ferroelectric polarization regulation.The main results are summarized as follows:（1）Dimension effect on ferroelectricity of GeS nanoribbons（GeSNRs）is investigated.The in-plane polarization of the armchair GeSNRs is along the periodic direction,while the one of the zigzag GeSNRs is along the width direction.Due to boundary deformation and depolarization field effect,the in-plane polarization decreases as the width decrease,while the out-of-plane polarization exhibits width-dependent odd-even difference due to the symmetry considerations.For bare armchair GeSNRs,the paraelectric/ferroelectric（PE/FE）phase transition occurs at critical width,and the ferroelectricity disappears when the width is less than the critical width.The boundary hydrogenation effectively removes the effect of the depolarization field and ferroelectricity is preserved.（2）The induction of high-temperature ferromagnetic half-metallicity and out-of-plane piezoelectricity in GaN monolayer by superhalogens is proposed.The idea is to take advantages of superhalogens which have strong electronegativity and are hard to aggregate,to form stable surface adsorption on the two-dimensional GaN,and superhalogens introduce local lone pair holes leading to the p-electron ferromagnetic half-metallicity with high Curie temperature（about 290 K）.At the same time,the charge transfer between the superhalogens and the two-dimensional GaN induces a strong out-of-plane electric polarization,resulting in obvious piezoelectricity.This work provides a new platform for the study of magnetoelectric interaction and a new potential direction for the development of two-dimensional spintronic information devices.（3）Van der Waals contacts between ferroelectric semiconductor In₂Se₃ monolayer and two-dimensional metals are investigated.When a metal contact with a semiconductor,electrons usually flow from the side with a low work function to the side with a high work function,then a built-in electric field is generated in the interface.In₂Se₃ has a spontaneous polarization in the vertical direction.After In₂Se₃ contacts with metal,its ferroelectric field overlaps with the built-in electric field,then we are able to regulate the contact properties by changing the direction of polarization.When the polarization of In₂Se₃ is reversed from downward to upward,the original n-type Schottky contact is turned to ohmic contact in In₂Se₃/M₃C₂（M=Zn,Cd,Hg）;while in In₂Se₃/VS₂ and In₂Se₃/TSe₂（T=V,Nb,Ta）,the p-type Schottky contact is turned to n-type Schottky contact.Therefore,ferroelectricity can effectively affect the contact properties between semiconductors and metals,which provides a possibility for the design of new electronic devices.（4）Based on In₂Se₃ monolayer,the effect of polarization direction on the catalytic performance of CO using Pt/In₂Se₃ is predicted,and a new idea of using ferroelectric polarization to achieve controllable catalysis is proposed.It is found that Pt can adsorb and disperse stably on the surface of In₂Se₃,and by controlling the direction of polarization in In₂Se₃,the change of electrostatic potential caused by the reversal of polarization can lead to the change of adsorption energy of reactants and intermediates,and the free energy barrier in the reaction process also depends on the direction of polarization.Pt/In₂Se₃ with upward polarization is prone to CO poisoning,while Pt/In₂Se₃ with downward polarization has a very low energy barrier in the catalytic cycle,showing a good CO catalytic oxidation activity,so that the reaction rate can be regulated by the external field.