Electronic Structure Of Two-Dimensional Metal Dichalcogenides And Heterostructures

The family of transition metal dichalcogenides?TMDs?is a kind of layered materials,which are formed by the covalent bonding and van der Waals force.All TMDs have a hexagonal structure with each monolayer composed of three stacked layers.Two-dimensional TMDs could be mechanically and chemically exfoliated into few-and single-atomic layers.TMDs have been attracted more attention owing to their novel physical and chemical properties.TMDs have potential applications in the fields of optoelectronics,spintronics and valleytronics.In this dissertation,the electronic structures of transition metal?Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Y,Zr,Nb,Mo,Tc,Ru,Rh,Pd,Ag,Cd,Hf,W,Re,Os,Lr,Pt,Au,Hg?doped monolayer 1T-TaS₂,TaS₂/TaSe₂ and TaSe₂/TaTe₂ heterostructures at different electric fields?-0.5,-0.3,-0.1,0,0.1,0.3,0.5 V/??,and TX₂/MnO?T:Mo,W;X:S,Se?heterostructures are investigated by using the density functional theory?DFT?.Firstly,the electronic structure of transition metal?TM?in fourth,fifth,and sixth periods doped monolayer 1T-TaS₂ is investigated.The spin-orbit coupling and the van der Waals force are taken into consideration due to the existence of heavy metal atoms of Ta and TM.The binding energy of TM-doped TaS₂ changes from 1.75 to 15.49 eV,where W-doped system is the most stable one.The distortion rate of bond length varies from 0.77%to 9.35%for S-TM,and 0.90%to 8.87%for Ta-TM,respectively.Fermi level goes across the conduction bands whether the TM doped 1T-TaS₂ or not.However,a gap appears between the conduction and valence bands when Mo,Tc,Ru,Rh,Pd,Ag and Cd are doped.The band gaps are 0.34,0.53,0.45 and 0.31 eV for Mo,Ru,Rh and Ag doped systems,respectively.Secondly,the electric field effects on electronic structure of the TaS₂/TaSe₂ and TaSe₂/TaTe₂ heterostructures are studied.TaS₂/TaSe₂ and TaSe₂/TaTe₂ heterostructures are metallic at a zero electric field,where Fermi level goes across the conductance band.The TaS₂/TaSe₂ heterostructure shows spin splitting at an electric field of-0.1 to-0.5 V/?,which gradually decreases with the increase of electric field.On the contrary,when the electric field increases from 0.1 to 0.3 V/?,the spin splitting gradually increases.However,at an electric field of 0.3 V/?,the TaSe₂/TaTe₂ vdW heterostructure is an indirect-band-gap semiconductor with a gap of 0.690 eV,suggesting that the electric field has a great influence on the TaSe₂/TaTe₂ heterostructure.Finally,the electronic structure of the TX₂/MnO?T:Mo,W;X:S,Se?heterostructures is calculated,where the corrections of DFT-D2 and DFT+U are used.It is observed that the spin splitting of MoS₂/MnO heterostructure with different configurations at K point varies from 24to 291 meV,and 18 to 253 meV at K'point.The stacking pattern also induces an analogous p-or n-type doping of MoS₂,revealing that the conductivity of the heterostructures could be tuned by stacking on antiferromagnetic MnO.Besides,the electronic structure of WS₂/MnO,MoSe₂/MnO and WSe₂/MnO heterostructures in the configuration of Mn-terminated?III?is investigated by first-principles calculations,and the spin splitting at K point is 553,324 and481 meV,and 215,9 and 284 meV for K'point,respectively.The valley splitting of MoS₂/MnO,WS₂/MnO,MoSe₂/MnO and WSe₂/MnO is 161,193,171 and 125 meV,respectively.In addition,the significant charge accumulation appears at the interface,indicating that the interlayer coupling exists between MnO and TX₂.