The Investigation On Two-dimensional Group-11-chalcogenide Properties From The Perspective Of System Analysis

In recent years,two-dimensional materials have attracted much attention in the fields of energy,catalysis,electronic devices and optoelectronic devices due to their high specif-ic surface volume,high carrier mobility and excellent optical and mechanical properties.These researches have not been guided by the universal laws,the systematic and com-pleted planning for a long time.And researchers are faced with difficulties,such as long experiment cycle,low output efficiency and lack of mature data and methods.It is diffi-cult to find new structures and build deep relationships between external environment and properties based on the accumulated experience and the characterization techniques.In this thesis,the research of two-dimensional materials are overall considered and planned from the perspective of system science.And the characteristics of two-dimensional group-11 chalcogenides are taken as an example to show the feasibility of this research idea.The main results are as follows:（1）The research system of two-dimensional materials is reviewed from the perspec-tive of system science.In the study of two-dimensional monolayer materials,new physi-cal properties are generated based on the anisotropy and out-of-plane spontaneous polar-ization and the various combinations of different components and structures.The material properties can be further adjusted and optimized by using multi-layer material technol-ogy,electric field and strain.With the rapid development of experimental techniques,it is possible to study the two-dimensional group-11 chalcogenides with non-layered bulk phases.Therefore,the study of two-dimensional material properties follows the research path of different structures and different levels.Putting the specific technical research in this system is beneficial for analyzing and perfecting the research system,which is a feasible way to solve the current problems.（2）Two-dimensional MX monolayers are constructed in the different space groups and elements of three orthogonal（α,β,γ）and six hexagonal（δ,ε,ζ,η,θ,ι）lattices based on the covalently bonding interactions between M（Cu,Ag,Au）and X（S,Se,Te）atoms.ι-Cu S,η-Cu Se,η-Cu Te,ι-Ag S,β-Ag Se,ι-Ag Te,α-Au S,α-Au Se andα-Au Te are determined to be stable.The structures of the materials determines their properties,ιandηphase materials possess metallic properties,whileαandβphase materials exhibit semiconductor properties.Interestingly,theι-phase materials possess spontaneous polar-ization.Using these metallic MX monolayers to form graphene-based heterostructures,the work function of graphene can be modulated from 4.35 e V to the range of 3.87～5.04 e V.αandβ-phase semiconductors have potential applications for hydrogen or oxy-gen evolution reaction.Further,It was found thatα-Au Se/α-Au Te was an ideal Z-scheme photocatalyst of water splitting,which is caused by built-in electric field in the interface.（3）Mechanical properties and negative Poisson’s ratio（NPR）ofα-Cu₂S,α-Cu₂Se,α-Cu₂Te,β-Ag₂S,β-Ag₂Se,α-Ag₂Te,β-Au₂S,β-Au₂Se andα-Au₂Te monolayers are theoretically predicted by using first-principles calculations.These M₂X monolayers possess isotropic positive Poisson’s ratios（PPR）ranging from 0.09 to 0.52,as well as Young’s moduli ranging from 19.92 to 35.42 N/m in x and y directions.Theβ-phase monolayers exhibit PPR with a linear change,when tensile strains are continuous-ly increased.Interestingly,the evolution from PPR to NPR occurs in theα-Cu₂S andα-Ag₂Te monolayers at strains greater than+3%and+4%,respectively,while theα-Cu₂Se,α-Cu₂Te andα-Au₂Te monolayers maintain PPR under tensile strains.In further calculations,NPR（PPR）is demonstrated to involve increased（decreased）bond angles,decreased thickness,and weakened（enhanced）d（M）-p（X）orbital coupling.（4）The tunable Schottky contacts were investigated by the metal-semiconductor heterostructures with graphene and two-dimensional group-11-chalcogenide semiconduc-tors.The barrier heights and contact types of heterostructures differ greatly at equilibrium distances due to the different interfacial dipole moments between layers.Inspired by the influence of interfacial dipole moment on Schottky contact,the regulation of Schottky barrier and the transformation of contact type are realized,which originate from interfa-cial charge transfer and the doping of graphene with holes（electrons）induced by the strain and external electric field,respectively.