Effect Of Electronic Structure And Magnetic Properties Of Monolayer WSe₂ By Strain And Doping

In the last few years,investigation on two-dimensional transition-metal dichalcogenides（TMDs）has been undergoing a rapid development due to their unique optical and electronic properties,such as MoS₂,MoSe₂,WS₂,WSe₂ and others.Layered semiconductor TMDs have been proven to be important candidates for using as an absorber layer in lowcost thin film solar cells.The layered nature of TMDs allows for easy cleavage and formation of ultrathin layers,which are considered as suitable semiconducting counterparts to graphene and may lead to flexible electronics and optoelectronics applications.Belonging to the family of layered TMDs,WSe₂ has been studied in many sides,including transistors,lubricants,electrochemical solar cells and others.WSe₂ has been established themselves as strong candidates for future electronic and optoelectronic applications.WSe₂ will be an ideal platform for studying spin and valley dependent properties as well as for spintronic applications.In order to better promote the application of WSe₂ in the field of spin electronics and electronic,the electronic structure and magnetic properties of two-dimensional materials need to be modulated.Therefore,the paper systematically investigated the electronic structure and magnetic behavior of with strain and group V and VII atoms doped in WSe₂ monolayer based on the first principles calculation method of the density functional theory（DFT）.The results are as follows:Firstly,electronic and magnetic properties of n-and p-type impurities are investigated by means of group V and VII atoms substituting selenium in WSe₂ monolayer.Our results show that for the N,P and As,the systems indicate p-type doping,the fermi level move down and close to the valance bands indicating p-type doping.For the F,Cl,Br and I substitutions,the fermi level move up and close to the conduction bands,which indicates n-type doping occurs in these cases.Moreover,N,P,F,Br and I atom can induce magnetic moment,and the magnetic moment mainly originates from p orbital of the dopant and d orbital of the neighbor W atoms.The biggest magnetic moment is 0.846μ_B of F-doped system.N-,P-,F-（or Br-doped）WSe₂ exhibit magnetic semicondutor（or magnetic metallic）features,Cl-and As-doped systems show no-magnetic metal and I-doped system exhibits half-metallic（HM）properties with 32 meV the half-metallic gap.The formation energy calculations also indicate that it is energetically favorable and relatively easier to incorporate group V and VII atoms into WSe₂ monolayer under W-rich experimental conditions.The formation energy of the F-doped system is the lowest and the next lowest formation energy is obtained in the N-doped system.These results provide important theoretical foundation and reference for their practical application.Secondly,we study the electronic properties of WSe₂ monolayer with biaxial tensile strain and compressive strain.Results show that under the biaxial tensile strain,WSe₂ monolayer retains direct band gap with increasing strain（1%¹3%）and the band gap of WSe₂ continuously decreases with increasing strain,eventually turn to metal when strain is equal to or more than 13%.Under the biaxial compressive strain,WSe₂ monolayer turns to indirect gap and the band gap continuously decreases with increasing strain,finally turn to metal when strain is up to-7%.The strain can reduce the band gap of the WSe₂monolayer regardless of the strain direction.From the results,the band gap of WSe₂ decreases with biaxial tensile strain or compressive strain.By comparison,we can see that the tensile strain appears to be more effective in modulating the band gap of pristine WSe₂ monolayer than the compressive strain from-5%to5%.But the band gap turns to zero quickly from-6%to-7%under compressive strain.However for tensile strain from 5%to 13%,the band gap decreases slowly.Based on of the projected charge density and the distance between W layer and Se layer for WSe₂ monolayer,the W atom and the d orbital and the Se atom p orbital,which indicates theπbond-like is formed.The results show that the high sensitivity of theπbond to strain leads to the effect of strain on band structure.