First Principles Study Of The Electronic Structure And Optical Properties Of Two-dimensional WSe₂

Since the successful stripping of graphene,two-dimensional materials have been a hot research field.Layered transition metal chalcogenides（TMDCs）have shown great potential in various fields due to their unique electrical and optical properties,including in optoelectronics,lubrication,and catalysis.As a representative member of TMDCs,two-dimensional WSe₂ is considered an excellent material for photodetection due to its direct band gap structure of 1.6～1.8 e V.The electronic structure and optical properties of two-dimensional WSe₂ can be effectively adjusted by introducing defects,doping,adsorption,strain regulation,and external electromagnetic fields,which expands its range of applications.Therefore,this paper deeply explores the impact of introducing defects,doping,and strain regulation on the photoelectric properties of single-layer WSe₂ through first-principles calculations based on density functional theory.The research contents and results are as follows:（1）Based on the structure of 2H-WSe₂,a single-layer WSe₂ model is established and the photoelectric properties of 2H-WSe₂ and single-layer WSe₂ are calculated.The calculation results show that the bandgap values of 2H-WSe₂ and single-layer WSe₂ are1.451e V and 1.548e V,and the bandgap types are indirect bandgap and direct bandgap,respectively.The static permittivity of monolayer WSe₂ is 8.76,and the absorption peaks are located at 6.54e V and 11.4e V,corresponding to absorption wavelengths of190nm and 110nm.（2）The geometric structure and photoelectric properties of monolayer WSe₂ were calculated under five different defects,respectively.The results show that the structural distortion is the most serious under the tungsten and selenium atomic interchange defects,and the formation energy is the largest,which is not easy to form a stable structure.The band gap value at tungsten vacancy defect is the smallest at 0.093 e V,when the defect energy level as the center of the composite.In tungsten vacancy,selenium vacancy and tungsten-selenium interchange defects transformed into indirect band gap.The absorption edge of monolayer WSe₂ under different defects is red-shifted relative to the structure without defects,which has stronger long-wave absorption ability.（3）The crystal structures and optoelectronic properties of the intrinsic and As-and Br-doped monolayers of WSe₂ were analyzed at different strains.n-type and p-type semiconductors with band gap values of 1.469 e V and 1.447 e V,respectively,were doped with As and Br,and the band gap type changed to indirect band gap after Br doping.Under biaxial tensile strain,the intrinsic and doped structures will be adjusted to direct bandgap,and under biaxial compressive strain to indirect bandgap,and the bandgap width shows a decreasing trend with the increase of strain degree,and it is found by data fitting that 0%～5%tensile strain is the best stage to adjust the bandgap.When the strain degree is greater than 10%,it gradually converts to metal.The red-shift phenomenon occurs in all the conditions of the intrinsic structure except for the-5%strain.red-shift phenomenon occurs in the As-doped system after applying strain,and red-shift occurs in the absorption side of the Br-doped system except for the 10%strain.red-shift occurs in the long-wave portion of the Br-doped system after applying strain.The absorption peaks in the Br-doped system after applying strain are mainly generated by the electron leap between the sender energy level and the conduction band bottom.（4）The geometrical structure and optoelectronic properties of transition metal（Tc,Nb,Ta）and rare earth element（La,Eu,Ho）doped monolayer WSe₂ were calculated and analyzed,and the results show that the formation energy results show that the formation energy is lower and the structure is more stable after the transition metal substitution doping.The band gap values after transition metal doping are 1.11e V,1.40e V,1.44e V.Tc doping introduces the sender energy level and the conductivity type changes to n-type.After the doping of rare earth elements,multiple impurity energy levels are introduced near the Fermi energy level,which are contributed by La-d,Eu-f and Ho-f orbitals as known from the density of states,and the band gap types are changed to indirect band gaps with the forbidden band widths of 0.68e V,0.61e V and0.78e V.In comparison with the intrinsic WSe₂ structure,the static dielectric constant increases after Tc doping,and the red-shift phenomenon occurs,increasing the optical absorption The static permittivity of Nb and Ta doping is relatively reduced and blue-shifted.The doping of rare earth elements increases the static permittivity significantly relative to the intrinsic structure,and all of them show significant red-shift phenomenon,which enhances the long-wave absorption ability.In the low-energy region,the light reflectance of the doped structure is significantly higher than that of the intrinsic structure.