Electron Transport Properties And Its Manipulation Of The Transition Metal Dichalcogenides Composite Nanostructures

Thanks to their unique electrical,optical and mechanical properties,transition metal dichalcogenides（TMDs）have become hot topics in the fields of condensed matter physics and materials science in recent years.Strain and partial substitution doping of atoms can be used to change the band structure of transition metal dichalcogenides,enabling transitions between semiconductors,semimetals,metals,insulators,and superconductors.In this paper,based on a first-principles calculation method combining Density functional theory（DFT）and Non-equilibrium Green’s function（NEGF）,it is shown that transition metal dichalcogenides composite nanostructures can be realized between semiconductors,p-type semiconductors,n-type semiconductors and even metals by means of uniaxial strain,biaxial strain and partial substitution doping of sulfur group atoms,and present interesting negative differential resistance effects,etc.The main research contents are as follows:In the first chapter,based on a brief introduction of the TMDs,we summarize the research progress on the transport properties of the TMDs and their regulation,and the main research content and scientific significance are given.In the second chapter,the theories and computational methods related to the transport of the TMDs composite nanostructures are introduced,mainly including Density function theory,Equilibrium and Non-equilibrium Green’s function method,Landauer-Büttiker transport theory and computational methods of transport properties of equilibrium composite nanostructures.Finally,the modeling and computational software involved in this paper and its computational flow are briefly explained.In the third chapter,based on a first-principles calculation method combining DFT and NEGF,the electronic transport properties of the TMDs WX₂-Mo X₂（X=S,Se,Te）composite nanostructures under the biaxial and uniaxial strain are systematically investigated.Under non-strain action,the WS₂-Mo S₂ composite nanostructure is a typical semiconductor,and the device shows a significant negative differential resistance effect.The WSe₂-Mo Se₂ composite nanostructure is metallic,the device current oscillates in[±0.2 V,±1 V],but the WTe₂-Mo Te₂ composite nanostructure is a p-type semiconductor with a conductionvoltage of±0.8 V.In the uniaxial strain range of-7%to 7%,the electronic structure and transport properties of the WS₂-Mo S₂ composite nanostructure has been demonstrated to be sensitive,showing typical semiconductor characteristics.The compressive increases its conductionvoltage,and the tensile strain decreases its conductionvoltage while the DNR effects is also observed at lower biasvoltages.When the tensile strain is increased to 7%,the conventional semiconductor to n-type semiconductor transition can be achieved.The compressed strain may significantly enchance the DNR effect of the WSe₂-Mo Se₂ composite nanostructure,while the tensile strain should induce the interesting DNR in the WTe₂-Mo Te₂ composite nanostructure.In the biaxial strain range of-7%to 7%,the electronic transport properties of the WSe₂-Mo Se₂ composite nanostructure have been demonstrated to be more sensitive when compared to that of the WS₂/Te₂-Mo S₂/Te₂composite nanostructures.The biaxial strain should significantly enchance the the electrical conductivity of the WSe₂-Mo Se₂ composite nanostructures,especially in the-3%～-7%compressed strain.The WS₂-Mo S₂ and WTe₂-Mo Te₂ composite nanostructures behaves typical semiconducting feature under the biaxial strain,the biaxial tensile strain should induce the interesting DNR in the WS₂-Mo S₂ composite nanostructure,while the biaxial compression strain can enhance the conductivity of WTe₂-Mo Te₂ composite nanostructure.In the fourth chapter,based on the first-principles calculation method combining the DFT and NEGF method,the influence of partial substitution doping of sulfur atoms on the electronic transport properties of the TMDs WX₂-Mo X₂（X=S,Se,Te）composite nanostructures.The research results show that after the partial substitution doping of the transition metal sulfides nanostructures by the S atoms,the system not only presents an interesting negative differential resistance effect,but also uses Se atoms to partially substitute dop the S atoms in the WS₂-Mo S₂ composite nanostructure,and the device can be converted into p-type semiconductors.Partial substitute doping the S atoms in the device with a Te atom can not only enhance the conductivity of the device,but also induce the normal semiconductor to p-type semiconductor or even metallic transition.The S atom partially substitutes the Se atom in the WSe₂-Mo Se₂ composite nanostructure,which has little impact on the electronic conductivity of the nanostructure.The Te atoms partially substitute doped the Se atoms in WSe₂-Mo Se₂ composite nanostructure,which can effectively adjust the metalliability of the device.In addition,the S atom partially doped the Te atom in the WTe₂-Mo Te₂ composite nanostructure,and the conductivityvoltage is reduced to±0.3 V,and the conductivity of the device is greatly improved.The Te atoms in WTe₂-Mo Te₂ composite nanostructures are partially doped with Se atoms.The electronic conductivity is enhanced and even be converted into normal semiconductors.In the fifth chapter,summary and outlook.This chapter summarizes the full-text research content,points out the innovation and shortcomings of this paper,as well as the preliminary speculation and prospect of the next work.