Theoretical Study On Electronic And Optical Properties Of Modified Doped Ag₂S And Carbon-based Nanomaterials

Ag₂S,a typical narrow band gap semiconductor exhibiting smaller band gap,nontoxic as well as cytocompatibility,is considered to be used in biomedical imaging,near-infrared emission,high-efficiency solar energy conversion and photocatalytic applications.In order to improve photocatalytic activity and optical properties of Ag₂S,we have studied the electronic and optical properties of Ag₂S by intrinsic defects,doping concentration and metal doping,respectively.On the other hand,carbon based nanomaterials,especially zero dimensional fullerene and two-dimensional graphene,have unique structure and electronic properties.In order to enrich their physical and chemical properties,we have investigated C₆₀ fullerene and graphene,respectively,by using foreign elements modification and further studied their physical properties.The main research results of this paper are as follows:First,by using first-principles calculations,we study the formation energy and electronic properties of Ag₂S with Ag＿I vacancy defect(V_AgI),Ag＿II vacancy defect(V_AgII)and S vacancy defect（V_S）,respectively.We simulated the structures model of non-stoichiometric Ag₂S measured from experiment,the Ag-Ag bond length from our calculation exhibit similar results as observed in experiment.V_Ag vacancy defects in Ag₂S are intrinsic vacancy defects,introduction of V_Ag into crystalline Ag₂S results in the increase of conductivity.High conductivity p-type materials can be obtained by control concentration of intrinsic defects into Ag₂S crystals.Second,using first-principles calculations based on density functional theory（DFT）,electronic structure properties of Cu substitution of Ag＿I(Cu _AgI),Cu substitution of Ag＿II(Cu _AgII),and interstitial Cu（Cu_i）doped Ag₂S with different doping concentrations were investigated.With the increase of doping concentration,the band gap of Cu _AgI and Cu _AgIIsystem is almost unchanged,while the band gap reduction for Cu_i system can mainly be attributed to the downward shift of the d orbitals of Cu atom.The consistency between band gap reduction from first-principles calculations and the red-shift from experimental spectra suggests the doped Cu atom is located at interstitial site in Ag₂S.And than,we studied electronic properties of different kinds of metal doping Ag₂S crystal by using first-principle calculations.Results shows than the binding energy of substitution doping is significantly lower than that of interstitial doping.Interstitial doping results in obvious deformation of Ag₂S lattice.In addition to the metal substitution doping with s¹ electron configuration,other metal doping and interstitial doping with s¹ electron configuration have a great influence on the band structure of Ag₂S crystal.The gap states in forbidden area of Ag₂S crystal is mainly originated from doping atoms.d-electron metal doping induces magnetism in Ag₂S crystal.Finally,we have studied the C₆₀ fullerene and graphene by using foreign elements modification,respectively.Three types of geometries are used to study the properties of semiconductor decorated graphene:partially decorated graphene with one element;fully decorated graphene with different elements;partially decorated graphene with different elements.The calculation results show that the decorated patches donate electrons to graphene layer,causes the electrostatic potential lower in the decorated graphene areas,thus induced an electric field across the boundary between the decorated and non-decorated domains.Therefore,we propose a new mechanism of mass transport on graphene surface,which plays an important role in understanding the selective diffusion and nucleation of adsorbed atoms on graphene surface.Besides,we calculated the electronic and optical properties of endohedral Au clusters fullerenes by using hybrid density functional theory.The energy stability of ground state Au clusters,non ground state Au clusters and（Au）_1～6@C₆₀ is compared.Binding energy of（Au）_1～6@C₆₀ complex increases with the increase of the number of Au atom.Interaction energy and charge transfer analysis show strong covalent interaction between Au clusters and C₆₀ cage.Magnetic moment exhibits even-odd oscillatory for（Au）_1～6@C₆₀ complexes.Au clusters breaks the macroscopic symmetry of C₆₀ cage and the optical properties of（Au）_1～6@C₆₀complex are anisotropic.Spectral intensity of C₆₀ in the ultraviolet region decreases and spectral amplitude of Au clusters in the infrared region increases along with the increase of the number of Au atoms.There are new peaks in the UV region,which mainly attributed to the interaction between Au clusters and C₆₀.Intensity of these peaks increases with the number of Au atoms increase.