First-principles Studies Of Polarization And Topological Properties In Two-dimensional Systems

The final purpose of materials science is to achieve certain functional materials to satisfy the increasing demands of living and working.The entire history of human civilization is a history of materials science.For instance,the early "Stone Age","Bronze Age" and "Iron Age" are named as the materials of tools used at those times,which implies the importance of materials for the development of human civilization.Nowadays,with the rapid development of science and the increase in living standards,most traditional materials can hardly satisfy people’s needs.Thus the development of new materials has received more and more attention.Nano-material is the symbol of materials science in the twenty-first century and the basis of future nano-devices.The two-dimensional(2D)materials,which have only one or several layers of atoms such as graphene,h-BN,MoS2 monolayer,have received extensive attention because of their unique and outstanding properties.However,up to now,the research on 2D materials are still in the infant stage.Many interesting physical phenomena in 2D systems have not been properly understood.Benefiting from the development of computational materials science,we are able to explain,predict and control the properties of materials and design more ideal new materials on the basis of first-principles calculation and statistic mechanics.These studies will benefit the future theoretical and experimental research,and gradually make the 2D materials enter people’s real lives.Chapter one briefly introduces the development of computational materials science,its position in the modern materials science and its importance in the research field of 2D materials.Then we introduce several specific 2D systems,namely graphene,porous graphene,group Ⅳ Xenes and transition metal halides,and discuss several important physical properties in 2D systems,namely gas separation,magnetism,quantum spin Hall effect,quantum anomalous Hall effect and multiferroics.Chapter two briefly introduces the major methods used in the computational materials science research,i.e.the statistic mechanics methods based on the classical mechanics and the quantum chemistry methods based on the quantum mechanics.Here we mainly introduce two widely used methods,namely density functional theory(DFT)method and the Monte-Carlo(MC)simulation.The DFT method is used to study the ground state properties of materials at absolute zero kelvin,and the MC method is used to study the magnetic behavior of excited state at finite temperature.(?)In chapter three,we study the edge magnetism of zigzag graphene nanoribbon terminated by 3d transition metals.To enhance the edge magnetism of zigzag graphene nanoribbon,we used 3d transition metal atoms to modify its zigzag edge to induce strong and robust magnetism.We find that using different 3d transition metals will result in different edge magnetic order.Interestingly,the zigzag edge modified by Ti or V atoms exhibits novel spiral spin order,which implies the existence of magnetoelectric properties.In chapter four,we propose a new kind of bonded bilayer quasi-2D structure.By applying a vertical pressure to reduce the interlayer distance of two single layers,strong chemical bonding could be formed between these two layers,resulting in a bonded bilayer structure which has very different properties compared to the van der Waals(vdW)bilayer.Through this method,we can construct more quasi-2D materials with new and unique properties.Here we introduce two kinds of such bonded bilayer systems.One is the bonded bilayer porous graphene,which exhibits much better gas separation performance than individual porous graphene.The other one is bonded bilayer group IV Xenes(graphene,silicene,germanene,stanene).Among them,the bonded bilayer germanene and stanene possess non-trivial topological electronic structure and become topological insulators under small in-plane strain.Besides,by applying in-plane strain,the bonded bilayer silicene can also transform from metal to normal insulator and then to a topological insulator.In chapter five,we turn to the family of transition metal halides,which are naturally vdW layered materials and recently received much attention.Recent experimental studies claim that these layered materials can be easily exfoliated down to monolayer,which exhibits unique properties.Based on this,we first study the properties of monolayered RuX3(X = Cl,Br,I),which present novel quantum anomalous Hall effect.Then in the research on CrBr3 system,we find that charge doping can induce anomalous spontaneous symmetry breaking in this system,and result in multiferroicity.Our studies suggest that 2D multiferroics could widely exist in the transition metal halide systems.At last,based on the recent discovery of ferromagnetic semiconductor properties in 2D CrI3,we propose that by doping W atoms in CrI3 to construct alloy transition metal halides CrWI6,the ferromagnetic couplings among this system can be much enhanced.Through this study,we propose that 2D ferromagnetic semiconductors with high Curie temperature can be achieved by constructing transition metal alloy compounds.