First-principles Calculation Of Optical And Electronic Properties Of Transition Metal Doped ZnO Monolayer

ZnO has the advantages of high chemical stability,high carrier mobility,high exciton binding energy,sterilization,et al,and is widely used in photoelectric detection,transparent electronics,biomedical and other fields.Two-dimensional graphene-like ZnO monolayer materials have attracted extensive attention due to the advantages such as larger band gap width,larger specific surface area,more photocatalytic reaction sites,and shorter photoinduced carrier transport routes.The 3d transition metal（TM）plays a very important role in improving the physical properties of ZnO monolayer,the introduction of TM atom can further expand the application range of ZnO.The electronic and optical properties of ZnO monolayer can be effectively improved by doping and adsorption of TM element on ZnO monolayer,and the application range of ZnO monolayer as photoelectric device material can be expanded.In addition,ZnO monolayer has great application potential in the field of photocatalysis,and intrinsic defects are associated with the preparation of ZnO monolayer,so it is necessary to study the effect of intrinsic defect zinc vacancy(V_Zn)or oxygen vacancy（V_O）co-doping with TM atom on the photocatalytic properties of ZnO monolayer.Based on the GGA+U method,the first-principles calculations were carried out on the ZnO monolayer system doped with one transition metal atom and simultaneously adsorbed with the same TM atom,vacancy(V_Zn or V_O)and TM atom co-doped ZnO monolayer systems.The details are as follows:1.Based on the first-principles calculation,the material models of TM atom single doped ZnO monolayer,ZnO monolayer doped with one TM atom and simultaneously adsorbed with the same TM atom were constructed.In the calculation,four TM elements of chromium（Cr）,manganese（Mn）,iron（Fe）and cobalt（Co）and five adsorption sites of the bridge of O-O bond（B_O）,the bridge of Zn-Znbond(B_Zn),the top of O atom（T_O）,the top of TM atom(T_TM),the center of the hexagonal ring of Zn-O（H）were selected.Firstly,the crystal structures of the above models were optimized,the formation energy and adsorption energy were calculated,and the most stable adsorption sites for TM elements were selected.The most stable adsorption sites for Cr,Mn,Fe and Co were H,To,To and Bo,respectively.After the adsorption of Cr,Mn and Co,the valence band maximum（VBM）and the conduction band minimum（CBM）of the system moved slightly to the lower energy level,while the CBM moved slightly to the higher energy level after the adsorption of Fe at the T_O position.The magnetic moment of the adsorption system mainly comes from TM atom.The calculation results show that the adsorption of Cr,Mn and Fe on the ZnO monolayer doped with the same TM atom has a positive effect on improving the optical absorption coefficient of the system in the visible light（VIS）region.Moreover,the absorption edge of the system adsorbed with TM atom has a different degree of red shift,which increases the optical absorption range.Therefore,ZnO monolayer has a good application prospect in the field of preparing optoelectronic device fabrication.2.Based on the first-principles calculation,three TM elements（Cr,Mn and Fe）were selected to construct the material models of TM atom and vacancy defect(V_Zn,V_O)co-doped ZnO monolayer.In addition,the material models of TM atom single doped ZnO monolayer were also constructed for comparing.The geometric optimization of each system was carried out firstly.The calculation result of formation energy showed that the system of TM single doped ZnO monolayer could exist stably,but the stability of the system was weakened after vacancy doping.Only the system of Cr and V_Zn co-doped ZnO monolayer could still exist stably with the formation energy of-1.662 e V.Compared with TM atom single doped ZnO monolayer systems,the band gap widths of the system are reduced to varying degrees due to the doping of V_Zn,while the band gap widths are increased with the introduction of V_O.In addition,TM-3d state and vacancy introduce magnetic moment into the system,and also introduce more impurity levels into the band gap,which is conducive to the electron transition from VB to CB.The results show that the optical absorption coefficient of Fe and V_Zn co-doped ZnO monolayer system increases obviously in the VIS region,which improves the photocatalytic rate in the visible light region,because the reduction of the band gap width is more conducive to the electron transition.The research could provide some theoretical references for the application of ZnO monolayer in the field of photoelectric device preparation and photocatalysis.