Tunable Electronic Structure Of Two-dimensional Group-? Monochalcogenides

Two-dimensional(2D)monolayer group-? monochalcogenides MX(M=Ge,Sn;X=S,Se)have attracted much attention due to its unique electronic structure and physical properties.Group-? monochalcogenides are semiconductors with the strong in-plane anisotropy and tunable band gap,which have the potential applications in solar energy conversion and optoelectronic devices.The electronic structure and physical properties of 2D MX materials can be modulated by the interfacial recombination and anion substitution,which provides the theoretical foundation for the practical applications in the electronic and photonics devices.In this dissertation,the electronic structure and physical properties of 2D GeS/FeCl₂ van der Waals(vd W)heterostructures,2D SiP/GeS and SiAs/GeS vd W heterostructures and the electronic structure and optical properties of the 2D M₂SD(M=Ge,Sn;D=Se,Te)monolayers are investigated systematically by using the density functional theory.The electronic structures of 2D multiferroic GeS/FeCl₂ vd W heterostructure are calculated.The band structure of monolayer GeS and Fe Cl₂ is preserved in the GeS/FeCl₂ vd W heterostructures,where the vd W heterostructure is half-metallic.The spin-dependent electronic structure and magnetism of GeS are induced by the magnetic proximity effects.Spin splitting appears at the conduction band minimum(CBM)and valence band maximum(VBM)of the vd W heterostrucuture,where the splitting can be tailored by the in-plane and out-of-plane strains.The spin splitting direction can be inversed at a compressive strain of-2%and a tensile strain of 4%.The tensile strain makes the system transit from Ohmic contact to Schottky one.The spatial spin polarization and magnetic anisotropy can be modulated by the external strain.The electronic structure of the SiP/GeS and SiAs/GeS vd W heterostrctures has been calculated.The typical type-II band alignment of the heterostructures is beneficial to the separation of the photogenerated electron and hole.The SiAs/GeS vd W heterostrcture has a direct band gap of 0.953 e V,which is beneficial to the optical absorption.The binding energy,charge transfer,band gap and band offset can be tailored by the external strain,which provides the theoretical basis for its applications in the nano-optoelectronic devices.The electronic structure of 2D monolayer M₂SD(M=Ge,Sn;D=Se,Te)is investigated.The 2D M₂SD monolayers are semiconductor.The phonon spectra confirm that 2D M₂SD monolayers are stable.The binding energy of M₂SD monolayers is-5.46,-4.88,-5.07 and-4.76 e V,which is related to the doped atomic radius and eletronegativity.The large optical absorption coefficient of 2D M₂SD monolayers(>10⁵ cm^-1)is beneficial to the applications in the solar cells.