First-principles Study Of The Effects Of Strain Engineering And Surface Passivation On Two-dimensional Materials

All kinds of miracles created in the information age are inseparable from the vigorous development of the semiconductor industry.Almost every major breakthrough in the semiconductor industry will bring huge upgrades in information technology,bioengineering technology,new material technology,Marine technology,space technology and other fields.Since the discovery of graphene,the special structure and various incredible properties of two-dimensional materials have attracted the attention of numerous researchers.Taking graphene as an example,unlike the traditional crystal structure,two-dimensional materials can be understood as materials with van der Waals layered structure,where electrons can only move freely at the nanoscale of two dimensions(1-100nm)in the unit of single molecular layer.And the properties of twodimensional materials are often completely different from those of bulk materials due to the van der Waals interaction between layers,quantum confined field effect and symmetry changes.The most typical example is graphene,which is different from graphite with high conductivity,high thermal conductivity and high mechanical strength.At present,new two-dimensional materials are also emerging,such as hexagonal boron nitride(h-BN),transition metal chorionic compounds(MX2),black phosphorus and graphene-like layered MXene.In addition to the continuous discovery and preparation of two-dimensional materials,the effective tuning of materials has also become the focus of research.Various methods,including doping,strain engineering,applied electric field,photoexcitation and synthesis of heterostructures,are likely to bring new properties and effects to materials.In addition,the magic-angle heterojunction system has shown abnormal electrical and optical properties,attracting a great deal of attention in recent years.In this paper,the structural,elastic,stability,electronic,optical and transport properties of two-dimensional materials such as BiTeX(X=Br,I),ZnSb and T12S monolayer are studied and discussed using first-principles calculation methods of density functional theory,with emphasis on strain engineering and surface passivating.For spintronic devices,the Rashba effect caused by the combination of strong spin orbit coupling and inversion symmetry breaking is an important feature in the development of next-generation devices because it allows us to generate and manipulate spin polarized charge carriers by non-magnetic methods.In this paper,the stability of two-dimensional BiTeX monolayers and the influence of biaxial tensile strain on their Rashba effect are studied using first-principles density functional theory.First,the results of stress-strain curve simulation show that BiTeBr monolayers can withstand biaxial load stresses up to 4.36 GPa with a critical strain of 17%,but further phonon dispersion calculations show that the structure becomes dynamically unstable for strains exceeding 7%,so the maximum stress is also limited to 2.79 GPa.In addition,the indirect band gap of two-dimensional BiTeX exhibits linear dependence on strain,and decreases with the increase of strain.This can be attributed to the different bonding properties of near-gap states,that is,the minimum conduction band is antibonding,while the maximum valence band is non-bonding.Finally,the Rashba effect under strain from 0%to 7%is calculated.The corresponding Rashba parameters follow the linear dependence of strain and can be adjusted from 1.28 eV·(?) to 1.71 eV·(?).Therefore,Rashba effect can be enhanced by applying strain 33.6%.This enhancement can be understood as the strain-induced the charge transfer and enhanced charge distribution nonuniformity.It also suggests that other methods such as applying electric fields,passivating surfaces with polar molecules,and introducing dopants into the outer layers can be used to control Rashba effects.These methods can broaden the scope of Rashba materials and can be used to effectively exploit their advantages in spintronic applications.On the other hand,recent studies have found that layered ZnSb can be stably isolated from non-layered ZnSb materials.In this paper,our studies have shown that the material can be transformed into a semiconductor by doping halogen atoms.In this paper,we perform the first-principles density functional calculations of two-dimensional ZnSbX(X=Cl，Br and I)monolayers to explore their properties.The structure analysis shows that the doped halogen atom X is more likely to form ionic bonding with Sb atom.In addition,halogen functional groups significantly change the electronic properties of ZnSb monolayers.Two-dimensional ZnSbX monolayers are semiconductors with an indirect band gap of 1 eV,while the intrinsic ZnSb monolayer behaves as a metallic material.Further strength calculations show that ZnSbX monolays are excellent flexible materials with a maximum Young’s modulus of 30.16 N/m.At the same time,the carrier mobility also presents anisotropic characteristics.The chloride atom doped ZnSb has the best transport properties,and the corresponding electron mobility of ZnSbCl can reach 6598 cm2V-1s-1.In addition,based on the analysis of the optical properties of two-dimensional ZnSbX monolayers,we found that the main absorption peaks of these materials are observed near the frequency of the near ultraviolet region.On the whole,our research extends the application of ZnSb from non-lamellar phase materials to layered semiconductors.The novel ZnSbX monolayer has the characteristics of narrow band gap,high carrier mobility and near ultraviolet absorption,which has broad application prospects in the field of new semiconductors and solar energy.Low dimensional materials with high anisotropy performance have unique advantages in various application fields,but most of the research objects are transition metal chalcogenides.In this paper,an anisotropic metal-coated two-dimensional semiconductor 2H-Tl2S with a structure similar to that of transition metal chiogenates has been studied by first-principles density functional calculations.Structural analysis shows that 2H-T12S is stable both in mechanical and dynamic stability.In addition,when calculating the band structure of 2H-Tl2S,we found that 2H-Tl2S is a kind of indirect bandgap semiconductor.And there is strong valley spin splitting in the conduction band,while the valley spin splitting mostly occurs in the valence band of traditional two-dimensional valley electron materials,which indicates that 2H-Tl2S has great application prospect in valley electronics and spintronics.2H-Tl2S has an indirect band gap of 1.52 eV.By applying appropriate strain,the band gap can be changed to change the semiconductor properties.For example,the indirect band gap semiconductor can be changed to direct band gap semiconductor by applying 5%biaxial tension,while the 2H-Tl2S can be changed from semiconductor to metal by applying-14%compression strain.At the same time,this band gap can absorb visible light,so 2H-Tl2S has a great application prospect in optoelectronic devices.The study of two-dimensional T12S shows that metal-coated two-dimensional semiconductors may have different properties from traditional transition metal sulfides,and also opens up the research ideas of related materials.