Investigation On Electronic Structure And Optical Properties Of Two-dimensional XY₂/MgPSe₃（X=Mo,W;Y=S,Se）van Der Waals Heterojunction

Papers from the two-dimensional semiconductor materials is becoming more and more deep and widely applied,and the performance of a single material does not meet the requirement of people,choose to have broadband gap of monolayer three phosphorus magnesium and selenium transition metal with high electron mobility double halogen compounds constitute a 2D van der Waals heterojunction,and based on first principles calculations,systemic analyzed theoretically their electronic structure and optical properties and potential applications in optical catalyst,and the influence of defects and vacancies on the electronic structure,for the material provides theory basis for the practical application.The main work content of this paper is divided into two parts:1.To study the electronic structure and optical properties of a two-dimensional XY₂/Mg PSe₃（X=Mo,W;Y=S,Se）heterojunction,the work is divided into four parts:（1）The XY₂/Mg PSe₃model is constructed by stacking 2×2 XY₂supercells and1×1 Mg PSe₃unit cells;considering the different docking modes of the interlayer interface,the most stable structure is selected by calculating the formation energy.（2）The projected energy band and density of states are calculated for the eight heterostructures contained in Two-dimensional XY₂/Mg PSe_3.The size of band gap and the type of band structure are mainly analyzed.（3）For the heterojunction with type-Ⅱenergy band structure,the more accurate HSE06 algorithm is used to further determine its energy band edge position,and then the heterojunction that meets the energy band requirements of photohydrolysis water catalyst is screened out.Finally,the free energy of the heterojunction before and after the reaction is analyzed to determine whether it can drive the reaction process.（4）The optical properties of 2D XY₂/Mg PSe₃and independent monolayers XY₂and Mg PSe₃,including light absorption coefficient,reflectivity and refractive index,were studied.Through the above research,it is found that the Two-dimensional Mo SSe/Mg PSe₃＿Ⅱand WSSe/Mg PSe₃＿Ⅱheterojunctions have the band structure and edge position consistent with the photocatalyst.Further analysis found that the reduction driving force of the WSSe/Mg PSe₃＿Ⅱheterojunction is greater than the hydrogen evolution reaction barrier,which can drive the hydrogen evolution reaction.In addition,the light absorption properties of XY₂/Mg PSe₃are also stronger than the independent single-layer XY₂and Mg PSe₃.2.To study the regulation of stress and defect on the electronic properties of XY₂/Mg PSe₃heterojunction.This work is mainly divided into the following two parts:（1）Uniform biaxial stresses from-3%to 3%were applied to XY₂/Mg PSe₃to study the influence of strain on its electronic structure,and the changes in the size and type of band gaps were mainly analyzed.Then the effects of strain on the light absorption of Mo SSe/Mg PSe₃＿Ⅱand WSSe/Mg PSe₃＿Ⅱare calculated.（2）The Se vacancies and Y vacancies were introduced on the surface of Mg PSe₃and Y vacancies on the surface of XY₂,respectively,to study the effect of vacancy defects on the electronic structure of XY₂/Mg PSe₃.Through the above research,it is found that the biaxial strain has a linear control effect on the band gap of XY₂/Mg PSe₃,and in the process of strain,there are direct band gaps to indirect band gaps and type-Ⅱband structure to type-I.change.This work provides an important theoretical basis for the application of XY₂/Mg PSe₃in photocatalysis and photoelectric devices in the future.