First-Principles Study On Electronic Structures And Magnetic Regulation Of Low-Dimensional Materials

Since the successful preparation of graphene,two-dimensional materials represented by it have attracted the attention of a large number of researchers.These atomic-thick materials are considered to have significant application prospects in electronic devices and spintronic devices.After 2017,a series of two-dimensional intrinsic magnetic materials,such as VS₂,Cr₂Ge₂Te₆,CrI₃,etc.,have been successfully prepared.These works have significantly increased the research interest in two-dimensional materials in spintronics applications.However,almost all of the two-dimensional magnetic materials are subject to their low Curie temperature and are difficult to apply.Therefore,the engineering of magnetism and Curie temperature of two-dimensional materials and their electronic transport properties have become one of the frontier topics in condensed matter physics.This paper studies the magnetic and Curie temperature engineering of two-dimensional monolayer materials CrI₃,RhI₃,the spin injection between 1T-VS₂ andβ₁₂-borophene,the electronic transport properties of the nanotubes rolled fromβ₁₂-borophene,and the electronic structures and magnetic regulations of boron nanotubes by magnetic atoms filling are investigated using the first principles method based on density functional theory,which can provide basic theoretical guidance for future research on related materials and electronic devices.It includes the following four parts:1.The adsorption of boron atoms on monolayer CrI₃ and its effects on the electronic structure,magnetism and Curie temperature have been studied theoretically.According to the adsorption energy,the ab initio molecular dynamic simulations as well as the elastic constants,it is verified that boron atoms can be stably adsorbed on the surface of monolayer CrI₃.The calculated electronic structure shows that the adsorption of boron atoms can enhance the magnetic moment of the monolayer CrI₃.By increasing the adsorption concentration from1B@CrI₃ to 4B@CrI₃,the monolayer CrI₃is transferred from semiconductor to half metallicity and then to semiconductor.In the Monte Carlo simulations,the Curie temperature of monolayer CrI₃ under four adsorption concentrations was calculated to be 81.5 K,117.7 K,129.4 K and 143.9 K,which is greatly improved in comparison with the Curie temperature of monolayer CrI₃ without adsorption.These results show that the adsorption of boron atoms can not only significantly enhance the magnetism of monolayer CrI₃,tune its electronic structures,but also significantly increase its Curie temperature.2.The electronic structures and magnetism of monolayer RhI₃ doped by 3d transition metal atoms（Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn）and VIA group atoms（S,Se,Te）have been studied theoretically.It is shown that the magnetic moment of monolayer RhI₃ is closely related to the type of doping 3d transition metal atoms.The Sc and Co doped systems are non-magnetic semiconductors,the Ni doped system is half metallicity,and the other 3d transition metal doped systems are magnetic semiconductors.Through the analysis within crystal field theory,it is found that the magnetism difference between different doped systems is caused by the difference in the number of valence electrons of impurity atoms.The VIA group atom doped monolayer RhI₃ is a magnetic semiconductor with magnetic moment of 1.Therefore,doping can effectively induce magnetism in monolayer RhI₃,which is a potential way to design two-dimensional spintronic materials.3.The spin injection of 1T-VS₂/borophene heterostructure has been studied theoretically.Based on the symmetry of the crystal structure,the heterostructures with two different stacking orders were constructed withβ₁₂-borophene and monolayer 1T-VS₂.The structural stability of such two heterostructures has been verified theoretically.It was found that the spin polarization rate of borophene in the two stacking structures was 64.57%and 29.93%,respectively.Based on the analysis of the spin injection mechanism,the difference in spin polarization rate can be contributed to the change of the charge transfer and interlayer coupling that are stack dependent.In addition,we found that the interlayer distance has different effects on the spin polarization rate in the two stacking structures.This suggests that the spin injection efficiency from two-dimensional magnetic metals toβ₁₂-borophene depends on both stacking order and stress from the vertical direction.4.The electronic structure and transport properties of nanotubes rolled fromβ₁₂-borophene have been studied theoretically.Through molecular dynamics simulation,it was found thatβ₁₂-borophene can be rolled into structurally stable nanotubes with two kinds of edge chirality:armchair and zigzag edges.Although both chiralβ₁₂-borophene nanotubes with metal characteristic by band structures,the transport calculations have shown that the transmission conductance and voltage-current characteristics of them are significantly different and dependent on the tube diameter.Except for（2,0）-BNT,the voltage-current characteristics of the other BNTs exhibit negative differential resistance,and our analysis shows that the negative differential resistance is caused by the different symmetry of wave functions in the energy bands between the left and right electrodes involved in the bias window.Thus,it is shown that rollingβ₁₂-borophene into nanotubes in different directions can achieve metallic one-dimensional nanowires with different electrical conductivities.The electronic structures and magnetic properties of（7,7）-BNT and（4,0）-BNT with transition metal atom（Fe,Co,Ni）filling were investigated.The results show that the transport properties of boron nanotubes depended on the edge configuration and the diameter,and that magnetic properties can be introduced to boron nanotubes by transition metal atom filling.