First-principles Study Of The Effect Of Ag/Ag-Co Doping And Point Defect On The Magnetic And Optical Properties Of ZnO

As a group ?-? oxide semiconductor,zinc oxide?ZnO?has high chemical stability and excellent piezoelectric and photoelectromagnetic characteristics compared to other semiconductors.Therefore,ZnO has broad application prospects in the fields of spin electronics,photovoltaic cells and photocatalysis.Transition metal doping in ZnO is an effective means to adjust the photoelectromagnetic properties of ZnO,and a series of results have been achieved at home and abroad.However,the origin of magnetism of Ag-doped ZnO has been controversial.The absorption spectrum distribution of Ag-doped ZnO has two contradictory results:red shift and blue shift.In addition,the existence of room temperature ferromagnetism of Ag-Co co-doped ZnO remains to be theoretically proven,and the optical properties of Ag-Co co-doped ZnO are rarely studied.In order to solve problems above,this article uses the CASTEP module of Materials Studio 8.0 to investigate the effects of Ag doping and point defect,Ag/Co single-and Ag-Co co-doping on the magnetic and optical properties of ZnO based density functional theory.To resolve the controversy over the magnetic mechanism and absorption spectrum of the Ag-doped ZnO,models of pure Zn₁₅Ag O₁₆,Zn₁₄Ag O₁₆,Zn₁₅Ag O₁₅,Zn₁₆Ag_iO₁₆,Zn₁₅Ag_iO₁₆ and Zn₁₄Ag Ag_iO₁₆ with different point defects were constructed.In order to obtain the Curie temperature of Zn₁₄Ag O₁₆,a model of Zn₁₄Ag Ag_iO₁₆ was constructed.Results revealed that the system which Ag doping and Zn vacancy coexists in ZnO could achieve room-temperature ferromagnetism.This system had an extremely high spin polarization,which was advantageous for the spin-electron injection sources applied in dilute magnetic semiconductors.Result also revealed that the magnetism of Zn₁₄Ag O₁₆was caused by the electron hybrid coupling effects among the O-2p,Ag-4d,Ag-5s,and Zn-4s orbits.And these effects were produced by the hole carriers after complexes were formed by the Ag doping and Zn vacancy.With regard to the most stable structure of the ground state,all doping systems which Ag replacing Zn and O vacancy coexisted in ZnO,and those which Ag replacing Zn,interstitial Ag and O vacancy coexisted in ZnO were all non-magnetic.Thus,these doping system were unsuitable for dilute magnetic semiconductors.By contrast,the doping systems of interstitial Ag and Zn vacancy were magnetic,although the magnetism was relatively weak,thus rendering such systems inapplicable as well.Optical properties research found that compared with undoped ZnO,Ag replacing Zn and Zn vacancy,interstitial Ag and Zn vacancy,interstitial Ag and Ag replacing Zn and Zn vacancy,and Ag replacing Zn and O vacancy.ZnO coexists in ZnO,except for Ag replacing Zn and oxygen vacancies,the band gap is widened and the absorption spectrum distribution is blue shifted.The band gaps of other doped systems are narrowed and the absorption spectrum distribution is red shifted.Effective mass analysis found that Ag interstitial and Zn vacancies co-existed in ZnO,and electrons and holes separated relatively well and were not easy to recombine.This is relatively advantageous for designing and preparing novel photocatalysts.Compared with undoped ZnO,ZnO supercell with Ag replacing Zn and Zn vacancy,interstitial Ag and Zn vacancy,and ZnO supercell with Ag replacing Zn,interstitial Ag,and Zn vacancy have a narrower band gap and a red-shifted absorption spectrum distribution.By contrast,ZnO supercell with Ag replacing Zn and O vacancy has a wider band gap and a blue-shifted absorption spectrum distribution.The effective mass calculations show that the ZnO supercell with interstitial Ag and Zn vacancy has the smallest effective mass of carriers and can thus accelerate the separation of electron–hole pairs,thereby improving photocatalytic activity.This condition is advantageous for designing and preparing novel photocatalysts.In order to systematically study the magnetic and optical properties of Ag and Co doped ZnO,a 72-atom pure ZnO and Ag/Co single-doped and Ag-Co co-doped ZnO supercell models with different Co concentrations were constructed.Magnetic studies show that both ZnO single-doped Ag and co-doped Ag-Co exhibit room-temperature ferromagnetism,whereas ZnO single-doped Co shows anti-ferromagnetism.Ag-Co co-doped ZnO has high stability,high Curie temperature,and high saturation magnetization,which makes it a promising room temperature ferromagnetic material.The magnetic moments of Ag-Co co-doped ZnO are mainly derived from the double-exchange effect between Co-3d state and O-2p state,which is consistent with Ruderman–Kittel–Kasuya–Yosida double exchange mechanism theory.In addition,Ag-doped ZnO undergoes a transition from magnetic to non-magnetic with the Ag doping concentration increasing from 2.77 mol%to 5.55 mol%.This is very beneficial for making magnetic switches.Optical properties studies show that Ag/Co single-and Ag-Co co-doping can remarkly enhance absorption in the visible range.However,Ag single-doped ZnO has a long carrier lifetime and is conducive to the experimental design and preparation of new photocatalysts.