Theoretical Study On The Design And Electromagnetic Properties Of Two-dimensional Materials

Since the discovery of graphene in 2004,two-dimensional(2D)materials have received extensive attention from researchers,and gradually become an important area of common concern for many disciplines such as physics,chemistry,and materials.Because two-dimensional materials have unique structural characteristics of atomic thickness,and exhibit excellent photoelectric,mechanical and thermal properties,they have important application value in different fields,especially the development of high-performance nanodevices.For example,graphene has a room temperature electron mobility of about 10⁴ cm²V^-1s^-1,enabling the manufacture of fast-running transistors.While molybdenum disulfide changes from the bulk material to the single layer,the electronic structure changes,the indirect band gap becomes the direct band gap,the observed photoluminescence and absorption spectrum also change with the number of layers,and can be used in a variety of low-dimensional optoelectronic devices.However,magnetic material,an important member of condensed matter physics and material physics,has long been absent in the field of two-dimensional materials.Until 2017,two-dimensional magnetic materials were synthesized for the first time in the laboratory,and long-range magnetic order was confirmed by experiments under two-dimensional conditions.Since then,magnetic research has become one of the frontier hotspots in the field of two-dimensional materials.At present,the two-dimensional materials with intrinsic magnetism found in experiments are few,and the Curie temperature is relatively low,which can not meet the actual application needs.In addition,the theoretical mechanism of two-dimensional magnetic generation and regulation has not yet reached a consistent conclusion.Under this research background,we tried to find new two-dimensional materials through theoretical calculations,and conducted in-depth research on their electronic structure and magnetism.Based on first principles calculations,the crystal structure,electronic structure and magnetic properties of the two-dimensional transition-metal phosphorous chalcogenides TMPS₄ are systematically studied in this paper.Corresponding to different transition metal elements,this series of materials exhibits rich electronic structure characteristics,involving ferromagnetic,antiferromagnetic and non-magnetic metal and semiconductor materials.Through an in-depth study of the magnetic mechanism,the article also found the existence of a slant easy axis in two-dimensional magnetic materials.The main contents are as follows:1)The electronic structure and magnetic properties of two-dimensional transition-metal phosphorous chalcogenides TMPS₄.We studied the single-layer structure of TMPS₄ through first principles calculations,where TM involves all nine elements of 3d transition metals,including Sc,Ti,V,Cr,Mn,Fe,Co,Ni,and Cu.This paper first studies the stability of these materials.The calculation results found that the exfoliation energy of these two-dimensional materials is close to graphene and the formation energy is negative,indicating that it is possible to prepare TMPS₄ through mechanical exfoliation and chemical synthesis.The molecular dynamics simulation at room temperature further predicts the thermodynamic stability of the material at room temperature.The paper further studies the electronic structure and magnetic properties of the material,and finds that the 2D TMPS₄ system has rich electrical and magnetic properties and involves a variety of spintronic device materials,among which the important ones are ferromagnetic and antiferromagnetic semiconductors and semi-metallic materials.Using molecular field theory and exchange model,the paper satisfactorily explained the difference of magnetic moment and the law of atoms coupling in the system.For the ferromagnetic semiconductor CrPS₄,the Monte Carlo method based on the Heisenberg model predicts that the Curie temperature is about 50 K,which is higher than the two-dimensional magnetic materials CrI₃(45 K)and Cr₂Ge₂Te₃(30 K)discovered before the experiment.Finally,the paper calculated the carrier mobility of semiconductor Sc PS₄ based on the theory of deformation potential,and found that the hole mobility was as high as 8580 cm²V^-1s^-1.Therefore,this series of TMPS₄monolayers have many different electronic structures and magnetic characteristics,and have broad application prospects in future low-dimensional electronic devices.2)Study on the magnetic anisotropy of the two-dimensional ferromagnetic material FePS₄.We further study the magnetic properties of single-layer FePS₄.The calculated band structure and density of states indicate that the single-layer FePS₄ belongs to ferromagnetic semimetal,that is,it behaves as a semiconductor in one spin channel and as a metal in another spin channel.There is huge application potential in chemical transport devices.The Monte Carlo method based on the Heisenberg model predicts that the Curie temperature of the single-layer FePS₄ is about 60 K.When further studying the nonlinear magnetic properties of FePS₄ under consideration of spin-orbit coupling,we found that FePS₄ exhibits obvious magnetic anisotropy,and the direction of the easy axis is neither in the two-dimensional plane nor along the normal direction of the plane direction,but lies at an angle with the normal.Further theoretical analysis speculates that this phenomenon is due to the competition between the magnetocrystalline anisotropy of the FeS octahedron and the additional shape anisotropy of the planar structure of the material.