The Research Of Surface Functionalized Two-dimensional Materials On Topological Properties

Here,using the Perdew-Burke-Ernzerhof?PBE?exchange correlation functional in the general gradient approximation?GGA?method,we investigate geometric and electronic properties of three types of surface-functionalized two-dimensional?2D?nanomaterials,in which the nontrivial topological states are demonstrated,and manipulate the transformation of quantum state under strain engineering by first-principles calculation.We believe that these materials with excellent properties would promote the development of topological insulators and accelerate the application of spin nanoelectronic devices.Firstly,asymmetric functionalization is an effective avenue to realize the chemical modulation in two-dimensional materials,thus we construct N/O and H asymmetrically functionalized stanene.It is find that the band structure obtains considerable Rashba spin split and shows two types of p-p-s and p_xy-p_z-p_xy band inversion mechanisms.The nontrivial topology of the material can be confirmed by Chern number,Z₂ index and helical edge states.By Bader analysis,we find that the magnetism of the material mainly originates from functionalized N/O atoms,and the Curie temperature of SnOH reaches up to 266 K predicted by Monte Carlo simulation,comparable to the room temperature.More importantly,the application of in-plane biaxial strains can induce the transition of different quantum states for material.These excellent properties provide theoretical support for the future design of field-effect transistors which have a high"on-off"ratio and are stable at room temperature.Secondly,2D cyanided-functionalized SbAs and BiAs monolayers are designed,namely SbAs?CN?₂ and BiAs?CN?₂.Under the strong spin-orbit coupling?SOC?effect,the band structure shows great spin splitting.Further research indicates that the spin-valley Dirac point state would occur in 2D SbAs?CN?₂ and BiAs?CN?₂ monolayers when reach 6.8%and 4.7%biaxial in-plane strain are respectively applied.The transition from topologically trival state to nontrival state,which is verified by the calculation of Z₂ index and helical edge states,is derived from the s-p_xy band inversion under external strain engineering.Besides,we also construct an effective tight-binding model to explore the origination of unequal valley changes in the BiAs?CN?₂ structure.This material,which can realize topological phase transition and spin-valley coupling under external strain engeering,would provide new ideas for design multipurpose and controllable devices in valleytronics in the future.Finally,we construct 2D CH₂OCH₃-functionalized SbAs and BiAs films,and explore the coexistence of ferroelectric and nontrivial topology states.Under the control of the external electric field,the rotation of ligand molecule CH₂OCH₃ is available,and the ferroelectric polarization would be introduced,in which the maximum polarization intensity can reach5.2×10^-10C/m.Since the centeral inversion symmetry of the material is broken,the spin splitting can be observed in band structure,and relatively clean Dirac point near the Fermi surface is formed.More importantly,when the external strain reaches 6%and 7%,the Dirac fermions in the inequivalent Dirac valleys have opposite Berry curvature and spin moment,resulting in the spin valley Hall effect,and this spin-valley-coupled Dirac semimetal at the boundary between trivial and non-trivial topologies.Considering its practical application,the BiAs?CH₂OCH₃?₂/h-BN heterostructure is constructed,in which the h-BN semiconductor is an ideal substrate for BiAs?CH₂OCH₃?₂ without destroying its non-trivial topology.In summary,this material can realize transition between three quantum states under external electric field,which would provide theoretical support and design possibilities for realizing ultra fast,low dissipation and controllable nano memory devices.