Theoretical Design And Research Of Two-Dimensional Organic Frameworks

Two-dimensional organic frameworks have many unique merits,such as large specific surface area,adjustable pore size and tunable electronic structures.They have a broad application in the field of chemistry,and become one of the hottest research topics in recent years.However,the fundamental research of two-dimensional organic framework in the field of physics has just started.The well-known topological flat band,quantum anomalous Hall effect and ferromagnetism are seldom studied in this class of material.In this thesis,combing the density functional theory and tight-binding model,the topological flat band,spin polarization and in-plane quantum anomalous Hall effect are systematically studied in two-dimensional organic frameworks.This thesis includes the following six chapters.In chapter 1,we give a brief review on the research background of two-dimensional organic frameworks,including its design principles,synthetic methods,functionalization and applications.Based on the unsolved scientific issues in two-dimensional organic frameworks,we propose the motivation and contents of our research in this thesis.In chapter 2,we introduce the theoretical methods used in this thesis,including density functional theory,tight-binding model,and first-principles software.In chapter 3,we propose a universal scheme to design the topological flat band in two-dimensional covalent organic frameworks.Based on the simplified wave-function interference cancellation pattern in frontier orbital(the highest occupied and lowest unoccupied orbital)of the molecular building blocks,we can identify the presence and absence of topological flat band(valence and conduction band)in the two-dimensional covalent organic frameworks.The reliability of this scheme is confirmed by firstprinciples calculations,demonstrating a new bottom-to-up way for constructing the topological flat band in two-dimensional covalent organic frameworks.Our scheme can unify the well-known topological flat bands in Kagome,Yin-Yang and Lieb lattices in the same framework,further enriching its construction theory and expanding its research in two-dimensional covalent organic frameworks.In chapter 4,we propose a bipolar semiconductor with topological flat bands in two-dimensional covalent organic frameworks.By doping electron and hole in the topological flat bands,a 100%spin polarization with tunable spins can be achieved at the Fermi level.Based on the experimentally synthesized two-dimensional covalent organic framework fluoride-graphdiyne(F-GDY),we confirm the carrier and optical doping induced spin-polarized channels in flat bands by first principles calculations.The energy window of spin polarization is tunable by the concentration of carrier and optical doping.Moreover,under the irradiation of circularly polarized light,a giant anomalous Hall effect is also achieved in fluoride-graphdiyne.Our results not only extend the bipolar semiconductor from magnetic to non-magnetic system,but also introduce the two-dimensional covalent organic frameworks to spintronics field.In chapter 5,we propose an intriguing non-collinear orbital model for realizing the quantum anomalous Hall effect with in-plane magnetization in two-dimensional metal organic frameworks.The typical Dirac,Kagome and Hxy band structures can be obtained by turning the orientation of non-collinear orbit.Using the direction of inplane magnetic moment and the orientation of non-collinear orbit as two independent parameters,a two-dimensional topological phase.diagram with spin-orbit coupling is mapped out,showing an in-plane quantum anomalous Hall effect with a tunable Chen number(+1 or-1).Based on the experimentally synthesized two-dimensional metal organic framework Pr2(C6O4Cl2)3,we show its equivalence to our model and identify the realization of in-plane quantum anomalous Hall effect in this metal organic framework by first-principles calculations.The non-collinear orbital model can be used to design quantum anomalous Hall effect and predict more exotic topological states,further enriching the research of topological physics in two-dimensional metal organic frameworks.In chapter 6,we summarize our works in this thesis,and outlook the future researches of two-dimensional organic frameworks.