Theoretical Study Of Electronic And Magnetic Properties And Strain Modulation Effect On Some 2D Semiconductor Materials

In recent years,two-dimensional layered materials have attracted much attention due to their low-dimensional characteristics and wide application in optoelectronic devices,spin electronics,catalysts,supercapacitors,solar cells,lithium-ion batteries and chemical and biological sensors.It is found that the impurity content,ambient temperature,light,electric field,magnetic field and pressure will affect the properties of two-dimensional layered materials.The Young's modulus of two-dimensional materials is so large that it can withstand strain easily.And the strain is ubiquitous in practical application of the materials.Therefore,the study and modulation of the performance of two-dimensional materials under stress is of great significance to the application and design of materials.In this paper,we used the density functional theory?DFT?method to calculate the structural stability,the electronic properties,the optical properties and the magnetic properties of the monolayer SnS and MnO₂,with the help of VASP package.The and the coupling effect were studied.The conclusions as follow:Firstly,based on the first-principles study of monolayer SnS,we designed the SIPES?strain-induced-potential energy surface?method and the SIBP?strain-induced band profiles?method.So it expanded from uniaxial,biaxial strain to globally coupled strain.It revealed the effect of stress release on the structure under strain.Because the symmetry of SnS changed in the system,it appeared two new phases?Pmmn and Pm?and the negative Poisson ratio.For the electronic properties,the strain can effectively control the band gap of the system and induce the indirect to direct band gap,and change the semiconductor into semi-metallic.As for the optical properties,the strain can improve the optical absorption characteristics.These results provide theoretical guidance for the application of monolayer SnS in optoelectronic devices.Secondly,the modulation of the electronic,magnetic and surface chemical properties of monolayer MnO₂ are studied with biaxial strain by the first-principles calculation.When applied the biaxial tensile strain,the conduction band is continuously moved down,the material has the tendency to change from semiconductor to semi-metal,and the octahedral coordination field is not distorted with keeping the orbital degeneracy.When the biaxial compressive strain is applied,the energy band is not changed in the whole,except the conduction band at the high symmetrical M point which is mainly composed by Mn dz2 orbit.Here,the bandwidth becomes larger and the energy decreases obviously.On the magnetic properties,The magnetic moment of Mn and O are increasing.With the increase of the strain,the magnetic moment of the Mn atom decreases continuously and the O atom drops to zero with increasing in the opposite direction.The magnetic moments are reversed.In short,mechanism of the study found: the spin polarization effect and the crystal field splitting will affect the band gap and the spin electron dominance at the same time.The effect of tensile strain on the system is mainly through the spin polarization.And the crystal field splitting will decide the effect of the biaxial compressive strain.Thirdly,based on the above results,applied biaxial strain to monolayer MnO₂ adsorbed by CO,it leads to the destruction of p-d interaction between Mn-O and affects the spin electron occupancy and frontline orbital movement.Adsorption of CO checked the results in small tensile strain.The larger tensile strain will restrain the adsorption on the surface.However,the larger compressive strain will promote the catalytic oxidation of CO on its surface.In summary,on the one hand,the strain and its coupling effect can effectively control the electrical and optical properties of the flexible SnS by the SIPES method and the SIBP method.On the other hand,monolayer MnO₂ modulated its basic properties through the spin polarization effect under the tensile strain.Under the compressive strain,the monolayer MnO₂ improved the properties by crystal field division and frontier molecular orbital movement.And the adsorption of CO can essentially destroy the p-d strong interaction between Mn-O and the strain still affect the process of chemical reaction on the surface of MnO₂.