First-principles Study On Optical Properties Of New Single-Element And Heterojunction Two-dimensional Materials

Since the discovery of graphene in 2004,two-dimensional nanomaterials have attracted much attention because of their unique physical properties.As a new type of two-dimensional material,two-dimensional transition metal sulfide has a tunable band gap and excellent electronic properties.The single-layer molybdenum diselenide(MoSe2)consists of three atomic layers,including upper and lower selenium atomic layers and the middle molybdenum atomic layer.Compared with traditional silicon-based materials,molybdenum diselenide can provide a higher carrier density,and can adjust the energy band by changing the number of layers.Bismuth is the heaviest element in the VA group,and hexagonal crystals can be formed after bonding between bismuth atoms.The unique quasi-layered crystal structure of bulk bismuth has many unique features,with the highest resistivity and Hall coefficient.The two-dimensional bismuthene exhibits a honeycomb structure or a black phosphorus-like structure.Due to the strong spin-orbit coupling effect(SOC),the bismuthene has Rashba splitting,resulting in a wide range of band gap changes,which in turn leads to bismuth Changes in optical propertiesThis article uses Density Functional Theory to systematically study the electronic and optical properties of the above two two-dimensional materials.The specific work of this paper is as follows1.The structural stability,electronic and optical properties of the interlayer graphene-molybdenum diselenide-graphene(G-MoSe2-G)heterostructure were theoretically studied through density functional theory.In order to minimize the lattice mismatch,we selected a 4×4 graphene supercell and a 3×3 MoSe2 supercell to form a sandwich structure van der Waals heterojunction.In this case,the lattice mismatch is only 1.01%.The results of this study show that the relative motion of the heterojunction layers along the plane hardly affects the electronic and optical properties and structural stability.The vertical and biaxial strains applied to the heterojunction can significantly change the energy band structure,resulting in changes in optical properties.Vertical strain can change the interlayer interaction.When the interlayer distance decreases,the interlayer repulsion increases rapidly,and electrons are transferred from molybdenum diselenide to graphene The absorption peak of the heterojunction obviously redshifts,and the absorption coefficient decreases.When the in-plane biaxial stress is applied,the width of the forbidden band decreases with the increase of stress,and the absorption peak is red-shifted.By studying the charge transfer and projected energy band between graphene and MoSe2 layers,we elucidated the mechanism of stress-tuned optical properties2.Based on the first-principles density functional theory calculations,the structure,electronic and optical properties of single-layer and double-layer buckled bismuthene were systematically studied under the consideration of the spin-orbit coupling effect(SOC).The calculation results show that when biaxial strain is applied,the single-layer bismuth band can realize the transition between metal,indirect semiconductor and direct semiconductor.The optical absorption peak redshifts with increasing stress,and the absorption coefficient increases significantly in the mid-infrared(MIR)range.When the vertical strain is less than 1.3 A,the double-layer bismuth olefin remains semi-metallic,and when the strain is equal to 1.4A,the double-layer bismuth olefin becomes an indirect band gap semiconductor with a band gap of 57meV.When the vertical strain is in the range of+0.6A to-0.4A,the absorption peak can be extended from 1540 nm to 2040 nm,with the increase of the absorption coefficient in the MIR region.In addition,we also explored the effect of point defects on electronic and optical properties by constructing a supercellular structure of 5×5 expanded cells.By using the energy band unfolding technique to calculate the projected energy band and state density at different defects,we found that the band gap of bismuth can be tuned by doping different elements.In the MIR region,both substitution and vacancy can increase the absorption coefficient of buckled bismuth.The above research results show that bismuthene has great application potential as a stress sensor,photodetector and optical modulator in the MIR region.