A First-principles Study On The Control Of The Structure And Electronic Properties Of Two-dimensional Nanomaterials Cu₂Se And AgX

Due to excellent physical and chemical properties,rich properties,and wide application prospects,two-dimensional nanomaterials have attracted the research and attention of many researchers.Among them,two-dimensional transition metal chalcogenides,as a type of two-dimensional composite materials,have become a research hotspot because of their strong topological properties,optoelectronic properties,and superconductivity.However,natural materials with excellent electronic properties are relatively scarce,and it is very necessary to control the electronic properties of two-dimensional materials.First-principles calculation plays an extremely important role in the prediction of two-dimensional nanomaterials and related experimental researches.In this paper,the main content is to use first-principles calculation based on density functional theory to study the geometric structures,electronic properties and energy band regulation of several two-dimensional transition metal chalcogenides.The main research content is:the intrinsic electronic properties and energy band regulation of the experimentally synthesized Cu₂Se monolayer;the electronic properties of the honeycomb-structured AgX monolayer and the regulation of its electronic properties by atomic modification.The specific research results are as follows:1.The effects of strain,defects,and atomic substitution on the electronic properties of the experimentally synthesized Cu₂Se monolayer are predicted.External strain can continuously adjust the band gap of Cu₂Se monolayer.Because the near-side states near the band Fermi level have different responses to strain,after applying appropriate uniaxial or biaxial strains,the band gap of Cu₂Se monolayer is converted from a indirect band gap to a direct band gap.In addition,whether it is a single-vacancy defect or a double-vacancy defect,the Cu₂Se monolayer changes from semiconductor properties to metal properties.And the substitutions of As,Ni,and Zn atoms also have the same effects.Only the substitution of S keeps the Cu₂Se monolayer still semiconducting.These results provide broad application prospects for materials in the field of controllable electronic devices.2.Based on first-principles calculation,a synthesized two-dimensional metal material-honeycomb Ag Te monolayer is reported.The metal-semiconductor transition is achieved by the modification of alkali metal atoms H/Li,Na.And all show thermodynamic stability after being modified by atoms.The newly formed single-layer Ag HTe and Ag YTe（Y=Li,Na）have different atomic and electronic structures.In single-layer Ag HTe,H atom and Te atom are bonded by polar covalent bonds.But the Dirac nodal line fermions（DNLFs）dominated by out-of-plane orbitals Te-p_z/Ag-d_xz,yzdisappear.And because of Li/Na atoms tend to be adsorbed in the hexagon center of Ag Te monolayer,its DNLFs still exist.These results provide an important reference for the application of Ag Te single layer in semiconductor electronic devices.3.The theory predicts that the single layer of AgX（X=S,Se）with honeycomb structure have DNLFs and exhibit metallic properties.After modifications with H/Li,Na,AgX changes from metallic properties to semiconductor properties.In single layer of Ag HX,H atom tends to be adsorbed on top of X atoms and the two are bonded by polar covalent bond,but DNLFs disappear.And because Li/Na atoms are more likely to be adsorbed on the hexagon centers of AgX single layer,its DNLFs still exist.In addition,biaxial strain also has a certain regulatory effect on the band gap of Ag Na X monolayer.This method provides a reference for the application of AgX single layer in semiconductor devices.