First-principles Study On Structure And Physical Property Of Two-dimensional Materials Based On Group ? And ? Elements

Two-dimensional?2D?material have been attracted a lot of attention due to their unique properties.Graphene is the first synthesized single layer material,different from the allotropes of carbon element,such as fullerene and carbon nanoribbon,graphene exhibits very robust mechanical strength and high mobility of charge carriers.The unique properties of graphene motivate researchers to search for the other 2D materials with unusual physical properties.A large number of 2D materials are predicted to be stable,and many have been successfully prepared in experiment.The new allotrope of 2D materials provides a new direction for searching new 2D materials.In addition,the substitutional doping and defect engineering have been proved to be one of the most effective approaches to induce the magnetic properties in2D semiconducting materials.As a novel class of quantum materials,topological metals and semimetals,have been extensively studied in recent year.Up to now,only a few 2D node line topological semimetal and topological metal have been theoretically predicted and experimentallt demonstrated.Therefore it is neccessary to find new 2D node line topological semimetal and topological metal.In this thesis,first-principles calculations are performed to investigate the influence of vacancy and nonmetal dopants on the electronic and magnetic properties of monolayer?-arsenene.Furthermore we propose a new 2D tetragonal allotrope of single layer IVBi?IV=Si,Ge,Sn?as well as group IV elements Ge and Sn,which consists of repeated square and octagon rings,and investigate their stability,structure and physical properties by using first-principle calculations.The obtained results are as follow1.We systematically explore the influence of vacancies and a series of substitutional nonmetallic atoms,such as H,F,B,N,P,C,Si,O and S,on the geometrical structure,electronic structure and magnetic properties of?-arsenene.The calculations show that vacancy doped?-arsenene is thermo-dynamically stable at room temperatures.Moreover,all the substitutionally doped?-arsenene samples with nonmetallic atoms are stable.Remarkably,due to the formation of one nonbonding p electron of dopant C and Si or a neighboring As atom around O,Si and vacancies,a doping C,Si,O and S atom and a vacancy induce a magnetic moment of 1.0?_B in?-arsenene.Furthermore,it is found that the magnetic coupling between the moments induced by two C,Si,O and S are long-range anti-ferromagnetic,and the calculated DOS and the spin density distribution show that the p–p hybridization interaction involving polarized electrons is responsible for the magnetic coupling.Our results demonstrate that the magnetism of?-arsenene can be effectively engineered by vacancies and the substitutional doping of some nonmetallic atoms.2.We propose a new 2D tetragonal allotrope of single layer IV-V semiconductor tetragonal IVBi?IV=Si,Ge,Sn?,monolayer T-IVBi?IV=Si,Ge,Sn?,which consists of repeated square and octagon rings.The calculated cohesive energy,phonon dispersion and ab-initio molecular dynamics simulations indicate that monolayer T-IVBi?IV=Si,Ge,Sn?are stable at room temperature and have potential to be synthesized in the future.Furthermore,due to the strong spin–orbit coupling?SOC?of Bi atom,the inclusion of SOC alters character of the fundamental band gaps of monolayer T-IVBi?IV=Si,Ge,Sn?and decreases their band gaps.When SOC is include,the electronic band structures show that monolayer T-SiBi is a direct band gap semiconductor,while monolayer T-GeBi and SnBi are indirect band gap semiconductors.Interestingly,the strain can induce monolayer T-SiBi,T-GeBi and T-SnBi to undergo a direct to indirect band gap transition or an indirect band gap semiconductor to metal transformation.Our results suggest that monolayer T-SiBi,T-GeBi and T-SnBi are potential candidate for nano-electronic applications.3.We propose a new 2D tetragonal allotrope of group IV elements Ge and Sn,monolayer T-Ge and T-Sn,which consists of repeated square and octagon rings.The calculated cohesive energy,ab-initio molecular dynamics simulations and phonon dispersion indicate that monolayer T-Ge and T-Sn are stable at room temperature and have potential to be synthesized in the future.In absence of SOC,monolayer T-Ge and T-Sn are node-line topological semimetal,which is protected by the combination of spatial inversion and time-reversal symmetries.Also the nodal loop in monolayer T-Ge and T-Sn are located near the Fermi level and very flat in energy and momentum space.In the presence of SOC,SOC opens gaps along the nodal loop in monolayer T-Ge and T-Sn and monolayer T-Ge and T-Sn transform into Z₂topological metal,which are identified by Z₂ topological invariant and topological edge state.Furthermore,under the external biaxial strain from-3%to 3%,monolayer T-Ge and T-Sn respectively remain their node-line topological semimetal properties and Z₂ topological metal properties without and with SOC.