First-principles Study On The Electronic Structure Of Antimony/bismuth Single-layer Two-dimensional Topological Materials

Exploring the effects of chemical functional group modification and substrate on the electronic band gap,spin-orbit coupling and spin-topological properties of twodimensional topological insulators through first-principles calculations of quantum Hall effect two-dimensional topological insulators.Study the applicable temperature of two-dimensional topological insulators from the electronic band gap and topological properties,explore the topological materials that can be used at room temperature,and develop some low-energy quantum devices.The electronic structure of the monolayer structure of the two-dimensional topological material Bi X/Sb X(X = H,F,Cl and Br)has been calculated according to the first-principles broadened plane wave pseudopotential method.The strong spinorbit coupling(SOC)of the px and py orbitals of the Bi/Sb atom is found to lead to the formation of a large band gap in the electron energy band in the range of 0.32-1.08 e V.Calculations of the energy band structure according to the low-energy effective Hamiltonian(LEEH)method are compared with the first-principles calculations and the results for the SOC energy band structure are in good agreement with each other.The effect of the vertical electric field on the electron band gap is examined and it is found that the calculated electron band gap value decreases until it finally disappears after the electric field strength exceeds a certain range.Accordingly,suitable TI materials can be selected for spin field effect transistor(SFET)devices through the modulating effect of the electric field band gap.The structural,electronic and topological properties of Sb C2 X and Bi C2X(X =H,F,Cl and Br)systems were calculated using a first-principles approach by functionalising Sb/Bi monolayers with acetylene-based derivatives.Large and stable electronic band gaps are achieved in both Sb and Bi monolayers functionalised with acetylene-based derivatives,allowing the quantum spin Hall effect to be realised at room and high temperatures.By calculating and analysing the non-trivial topological order and the conductive edge states of the nanoribbons,the acetylene-based derivative-functionalised Sb/Bi monolayers are predicted to be new two-dimensional topological insulating materials with promising practical applications in spintronic devices.The electronic properties and energy band structure of the Sb/Bi@Cl-Ge(111)heterojunction 2D material were investigated by first-principles calculations,which determined that both systems are stable hexagonal honeycomb structures,and obtained an electronic energy gap of 0.54 e V for Sb@Cl-Ge(111)and 0.78 e V for Bi@ClGe(111).The calculation of the edge state and Z2 invariance demonstrates that both Sb with Bi atoms and Ge(111)substrates covered with 1/3 Cl atoms can constitute a wide band gap 2D topological insulator with observable QSH effect at room temperature.The formation mechanism of the topological properties is investigated by analysing the density of states and charge density of Sb@Cl-Ge(111).The substrate Ge(111)removes the Sb-pz orbital component at the Fermi energy level,thus allowing the system to exhibit topological properties.The proposed overlay and substrate scheme gives new ideas for the development and preparation of new topological materials.