Study On The Structure And Electronic Properties Of Several Novel Transition Metal Dichalcogenides

The unique electronic properties of low-dimensional materials make them as the hot topics in the field of materials science.Compared with their bulk phases,low-dimensional materials are special in the following aspects:(?)quantum confining effect,(?)good flexibility,(?)high specific surface area,which are good for their applications optoelectronic devices,field effect transistors(FET)and photocatalysis.Two-dimensional(2D)transition metal chalcogenides(TMDCs)have attracted fascinating attention because of their unique properties and potential applications in several technological fields.In this thesis,the atomic and properties of low-dimensional materials including one-dimensional(1D)TM₆X₆ nanowires,two-dimensional(2D)TM₈X₁₂ monolayers are systematically investigated by using the first-principles calculations.The research content is mainly divided into the following sections:(1)We investigate the effect of axial strain on the structures and electronic properties of TM₆X₆(TM=Mo,W;X=S,Se,Te)nanowires.Our results show that the TM₆X₆nanowires recently discovered by experiments(defined as?phase)will be transformed into a new structure(defined as?phase)under the axial tensile strain along the nanowire direction.The critical strain values for the structural phase transition are strongly dependent on the metal atoms and chalcogenide element atoms.The phonon spectra and ab initio molecular dynamics(AIMD)simulations of TM₆X₆ nanowires reveal that both the?-and?-phases are dynamically and thermodynamically stable.The calculated band structures of?-TM₆X₆ nanowires show that the?-TM₆X₆(TM=Mo,W;X=S,Se)nanowires are metallic,while the tellurides are semiconductors with an indirect band gap.We find that the structural phase transition is accompanied by an electronic transition from metal to semiconductor.Among them,other?-TM₆X₆ nanowires exhibit direct bandgap semiconducting characteristics whereas the?-TM₆Te₆ nanowires are indirect bandgap semiconductors.These results suggest that these TM₆X₆ nanowires may have applications in next generation phase-controlled catalytic,electronic and optoelectronic devices.(2)We investigate the structural stability,electronic band structure,carrier mobility and optical properties of monolayer TM₈X₁₂(TM=Mo,W;X=S,Se).The phonon dispersions and AIMD simulations demonstrate that they are both dynamically and thermodynamically stable.All are dynamically stable except for monolayer W₈Te₁₂.Our results show that the monolayer tellurides(Mo₈Te₁₂ and W₈Te₁₂)are metallic,whereas the sulfides(Mo₈S₁₂ and W₈S₁₂)and selenides(Mo₈Se₁₂ and W₈Se₁₂)are semiconductors with direct bandgaps of about 1.32 e V,0.93 e V,1.04 e V and 0.95 e V,respectively.Uniaxial strain drives W₈Se₁₂ from a direct to indirect band gap transition and even a semiconductor-metal transition.We predict that these 2D semiconductors materials possess extremely high in-plane carrier mobilities(?10⁴cm²V^-1s^-1)and significant anisotropy of carrier mobility along different directions,which may be promising in future flexible microelectronic and optoelectronic devices.(3)The effects of biaxial strain on the electronic and magnetic properties of vacancy defects(V_W-1,V_W-2,V_1Se and V_2Se)W₈Se₁₂ structures systematically investigated based on the first-principles calculations.Under the absence of biaxial strain,the calculated results show that the V_W-2vacancy defect structure exhibits half-metal electronic band structure and the ground state is magnetic.However,the band structure of V_W-1 and V_1Se vacancy defect structures become indirect bandgap semiconductors,while the electronic structure of V_2Se vacancy defect structures is a direct bandgap semiconductor,and the ground states of the above three defect structures are all non-magnetic.Under the regulation of biaxial strain,the magnetic state of V_2Sedefect structure changes from non-magnetic to magnetic,and its electronic properties change from direct bandgap semiconductor to metal to half-metal to indirect bandgap semiconductor.While the magnetic state of V_W-2 defect structure has always been magnetic,and its electronic properties change from half-metal to direct bandgap semiconductor to indirect bandgap semiconductor.The results show that the defective W₈Se₁₂ material has a broad application prospect in the field of spintronic devices under the regulation of biaxial strain.