First-principles Calculations Of Electronic And Optical Properties Of 2D Mg₂Si

Two-dimensional（2D）materials have been the research hotspot for a long time due to their unusual properties compared to those of the parent bulk materials.Three-dimensional（3D）Mg₂Si materials have great potential for preparing optoelectronic devices such as infrared detectors and solar cells,and 2D Mg₂Si materials are expected to have even better properties.A 2D Mg₂Si model（space group Pmmn,No.59）is theoretically predicted using the first-principles calculations method based on density functional theory,and the stability of the 2D Mg₂Si model is theoretically verified,while the electronic and optical properties of the 2D Mg₂Si are theoretically calculated to investigate its superiority over the parent material.The effects of different vacancy defect elements,different vacancy defect locations,and different vacancy defect concentrations on the electronic and optical properties of 2D Mg₂Si materials are investigated to understand the macroscopic properties of the materials from a microscopic perspective,which provide guidance for future experimental studies of 2D Mg₂Si materials and their applications in electronic devices and optoelectronic devices.A supercell model of 2D Mg₂Si was constructed firstly using Materials Studio software,and 16 structural models of 2D Mg₂Si materials with vacancy defects were constructed then in order to investigate the effects of vacancy defects of different elements,locations and concentrations on the 2D Mg₂Si materials.In this work,the stability,electronic and optical properties of these models were calculated using VASP software.The stability,electronic and optical properties of defect-free 2D Mg₂Si materials were calculated and analyzed.Based on the results of the phonon spectrum,it is possible that 2D Mg₂Si could be synthesized experimentally,and two experimental methods applicable to the synthesis of 2D Mg₂Si were conjectured.The results show that the 2D Mg₂Si is a direct bandgap semiconductor with a bandgap of about 1.03 e V calculated by the method of HSE06,which is suitable for application in the absorber layer of solar cells.The absorption coefficient of the 2D Mg₂Si is higher than that of bulk Mg₂Si in the red visible and infrared region.It is exciting that the absorption coefficient of 2D Mg₂Si in the near-infrared region can still reach the order of 10⁵ cm^-1.These calculations provide inspiration for experimental methods of synthesizing 2D Mg₂Si and theoretical references for the preparation of micro-nano optoelectronic devices based on 2D Mg₂Si materials.The stability,electronic and optical properties of the 2D Mg₂Si system with different vacancy defect elements,different vacancy defect locations and different vacancy defect concentrations were calculated and analyzed.The calculated results show that the lower the vacancy defect concentration,the more stable the structure is,and below 2 at%it is more stable than the intrinsically defect-free 2D Mg₂Si,which tends to have a low concentration of vacancy defect structure during the material preparation.2D Mg₂Si with Si vacancy defects are still semiconductors,and the more marginal the location of the Si single vacancy defect,the smaller the band gap of the material.Mg single vacancy defects change the material type from direct band gap semiconductor to metallic.The defect states of 2D Mg₂Si with Si vacancy defects are mainly provided by Mg atoms,and the defect states of 2D Mg₂Si with Mg vacancy defects is mainly provided by Si atoms.The absorption coefficients of 2D Mg₂Si with Si vacancy defects reach the order of 10⁵ cm^-1in the near-infrared optical and the wavelength range of the high absorption coefficient is larger than that of 2D Mg₂Si without defects,so 2D Mg₂Si with Si vacancy defects are more promising for applications in near-infrared optoelectronic devices.The absorption coefficient of the material for photons increases when the location of vacancy defects appears in the center.The lower the concentration of vacancy defects,the smaller the absorption of photons by the material.These calculations provide important reference values for the experimental research of 2D Mg₂Si.