First-principles Studies Of Two-Dimensional MX₄?M=Ru,Os;X=S,Se,Te? Materials

With the progress of modern science and technology,the research field of new materials expands constantly and has a great impact on people's daily life.With the deepening of research,people found that two-dimensional(2D)materials have unique properties,which are different from bulk materials.It makes the research of 2D materials plays an increasingly important role in scientific research and the future technologic development.Therefore,it is a very important research project for designing 2D materials with new physical properties.In this thesis,by first-principles calculation method,we design a series of two-dimensional MX4(M = Ru,Os;X = S,Se,Te)materials and investigate the physical properties of monolayer MX4(M = Ru,Os;X = S,Se,Te).The main conclusions are summarized below:1)A series of novel two-dimensional MX4 monolayer materials with stable structure are designed and its electronic structure was predicted.Based on chemical intuition,we design an eight-coordinate 2D MX4(M=Ru,Os;X=S,Se,Te)materials,which has a sandwich structure formed by the M atomic layer in the middle and two X atoms.Among these materials,RuS4 and OSS4 monolayer are stable in the structure.In the energy band structures of the monolayer MX4,except for the single layer RuS4(semi metallic),all of them exhibit metallic properties.The band structure of these materials has an energy degeneracy point at/near the Fermi level.Their band structures have an energy degeneracy point with valence band maximum(VBM)and conduction band minimum(CBM)degenerated at the ? point in the Brillouin zone.When the spin-orbit coupling(SOC)is included,the energy degeneracy of energy degeneracy point of monolayer RuS4,monolayer RuTe4,monolayer OSS4 and monolayer OsTe4 are opened.However,the band structures of monolayer RuSe4 and monolayer OsTe4 have almost no change.This difference originates from strong hybridization of X pz orbital and M dyz+dxz orbitals in monolayer RuSe4 and monolayer OsTe4 materials.2)Monolayer RuS4 is found to be a novel two-dimensional topological insulator.By first principles calculation and Wannier function method,the Z2 topological invariants and edge electronic states of monolayer RuS4 are calculated to determine the nontrivial topological electronic structure.In addition,we also determine the origin of topological properties of monolayer RuS4 by means of orbital valence bond analysis and tight binding model.