Spin Manipulation Within Topological Insulators Bi₂Se₃ And Two Dimensional Nanomaterials TMN_x-G And BeN₂

Nowadays,the human society is in the electronic information age,which mainly relies on the charge properties of electrons.With the development of society,electronic devices have gradually moved towards miniaturization,multi-function,high-speed and low-power consumption.Thus,higher requirements have artisen on the size and performance of the transistors.However,as the sizes of transistors gradually become smaller,the quantum tunneling effect began to become significant,Moore’s law gradually became invalid,and the number of components on an integrated circuit is getting closer to the physical limit that silicon technology can accommodate.Meanwhile,the heating problem of integrated circuits has become more and more serious,and the heat loss has approached half of the total energy consumption.A major problem that needs to be solved urgently arises,that is,how to minimize the collision and scattering of the electrons in the integrated component materials,make the electrons run in an orderly and high-speed manner,and reduce the energy consumption of the device.As reported by the previous studies,effective use of the spin properties of electrons is expected to solve the problems of heat dissipation and leakage,and realize new electronic devices with good stability,fast data processing,low power consumption and non-volatile properties.The main research objects are carbon-doped topological insulator Bi2Se3 and two dimensional nanomaterials—transition metal doped graphene derivatives and fluorinated BeN2,aiming to manipulate their electronic structures and magnetic properties,reveal the magnetic mechanism among them,and tap their potential value in spintronic device applications.The mainly contents are stated as follows:In the first chapter,based on theoretical and experimental approaches,a comprehensive and systematic simulation study on the various doping configurations,stability and magnetic properties of the carbon-doped topological insulator Bi2Se3(C-Bi2Se3)was carried out.It shows that most C dopants energetically tend to replace Se sites in the form of dimers and trimers.Furthermore,we used the climbing elastic band method(CI-NEB)to simulate the migration and diffusion of carbon dopant in C-Bi2Se3.From the simulation results of CI-NEB,it is found that isolated C is difficult to migrate and diffuse in C-Bi2Se3.However,the migration of carbon would become easy,and carbon clusters are formed in the system only if the distance between C dopants is very close.This means that isolated,paired and trimer carbon dopants can coexist in C-Bi2Se3.As for the properties of C-Bi2Se3,it will exhibit spin polarization and charge doping effects when the isolated C doped in the interstitial position between van der Waals(vdW)layers or at Se atomic position.However,C-Bi2Se3 would exhibit neither magnetic nor charge doping effects,when C dopants exist in the form of pairs and trimers.Experimentally,the non-magnetic element C was successfully doped into the topological insulator Bi2Se3 by the high-temperture melting method.The weak ferromagnetism of the material was observed,which verifies the theoretical results to a certain extent.In a word,C-Bi2Se3 is indeed spin-polarized,but the magnetic moment and hole doping effects are very small.These studies suggest that to obtain strong ferromagnetic C-Bi2Se3 experiments need carefully designing to dope single carbon atoms into bulk dispersedly.In the second chapter,using the first-principles calculations based on density functional theory,the structure and magnetic properties of graphene derivatives containing carbon vacancies doped with transition metals(TMNx-G,X=3,4;TM=Sc～Zn)are presented.The calculations found that the TMNx-G system is stable,and most of the systems(except CuN4G,ZnN3-G,ScN4-G,NiN4-G and ZnN4-G)obtain magnetic moments.The structure of TMN4G is more stable than TMN3-G.When a certain degree of external strain is applied,the magnetic moment of TMN3-G will not change significantly(the range of change is less than 1 μB),while TMN4-G exhibits a controllable spin state change,including three different spin state transition:1)With a small tensile strain,TMN4-G(TM=Mn,Fe and Co)can transition from a low spin state to a high spin state;2)With a certain tensile strain,the non-magnetic ScN4-G and NiN4-G can exhibit spin polarization;3)Tensile strain could induce the magnetic moment in CuN4-G disappear.These findings opened up a new way for graphene-based applications in spintronics.In the last chapter,via the first-principles approach based on hybrid functional,the stable configurations,electronic and magnetic properties of the fluorinated two-dimensional BeN2(F-BeN2)have been studied.F atoms are adsorbed on Be ion sites and each fluorine adsorbate induces robust 1μB magnetic moment.As a result,a F-BeN2 sheet becomes a ferromagnetic half-metal from a semiconducting intrinsic BeN2.The semiconducting band gap of the halfmetallic F-BeN2 sheets varies from 2.26 to 2.92 eV with different F coverage.With external strains,the F-BeN2 maintains the ferromagnetic and half-metallic features and the semiconducting band gap changes only a little.It suggests that the F-BeN2 can be a good lightelement magnet with promising potential for spintronics application.