Preparation And Optical Properties Of Cs₃MnBr₅ And Cs₃MnBr₅:Pb@SiO₂

In the past few decades,the rapid development of electronic media has brought great changes to the development of the world.With the continuous emergence of new applications,people’s daily life has been greatly simplified and convenient,easy access to data has become easier and easier,and the issue of information security has attracted more and more attention.Data disclosure not only brings serious challenges to personal privacy,but also has a far-reaching impact on global development.Therefore,the maintenance of information security has become an urgent problem.The visible pattern of optical recognition has the advantages of easy processing,high amount of information and simple design,which makes the technology widely used in the field of anti-counterfeiting.In the past decade,all-inorganic metal halide perovskite nanocrystals have attracted wide attention because of their excellent optoelectronic properties.Because some perovskite composites have photochromism and can return to their initial state in dark state,it brings broad development prospects in the field of anti-counterfeiting.However,most of the cations in B site contain lead,which is not friendly to the environment.And the reversible photochromism is longer and the cycle stability is poor.Therefore,the development of all-inorganic perovskite with excellent optical properties and high stability is of great significance in the field of anti-counterfeiting.In order to solve the above problems,Cs₃MnBr₅ nanocrystals with excellent optical properties were prepared by modified hot-injection method.On this basis,Cs₃MnBr₅:Pb@Si O₂nanocrystals with core-shell structure were prepared by Pb ion doping,which achieved excellent optical properties,high stability and reversible photochromism,and the coating of Si O₂ core-shell structure on nanocrystals also inhibited the harm of lead to human beings and the environment.The research work includes the following contents:1.Cs₃MnBr₅ nanocrystals were prepared by modified hot-injection method and their optical properties were studied.Cs₃MnBr₅ nanocrystals belong to the tetragonal system of I4/mcm space group.The grain size is uniform and the average particle size is about26.86 nm.The elements mapping and FTIR proved that hydrolysis occurred during the thermal implantation of trimethylbromosilane,resulting in the gradual nucleation of nanocrystals accompanied by the formation of layered Si-O skeleton,and Cs₃MnBr₅nanocrystals and Si O₂ were formed at the same time.Under the excitation of 370 nm,the emission position of Cs₃MnBr₅ nanocrystals is 520 nm,FWHM=54 nm,τ_avg=1.339 ns.XPS also confirmed the element composition and chemical valence.In the 12-month stability test,Cs₃MnBr₅ nanocrystals showed good stability.Through the first-principles calculation,it is found that the conduction band bottom and valence band top of Cs₃MnBr₅nanocrystals are located atΓpoint,which proves that the direct band gap semiconductor is 1.48 e V.2.Through the work in chapter 2,it is found that the preparation of Cs₃MnBr₅nanocrystals is accompanied by the formation of Si O₂.On this basis,through Pb doping and optimizing experimental conditions,core-shell Cs₃MnBr₅:Pb@Si O₂ nanocrystals（Mn:Pb=1:0.1）were successfully prepared by modified hot-injection method.The grain size and core-shell thickness are uniform.Cs₃MnBr₅:Pb@Si O₂ nanocrystals have fast photochromism（about 2 s）and anti-counterfeiting properties of returning to their initial state（about 40 s）in the dark state.The green light emission is attributed to the metal center dmurd transition of Mn²⁺ions in the d5 configuration.The cleavage of Mn-Br bond is accompanied by the migration of Mnions to defect vacancies,which gradually forms Mn-Mncoupling and emits orange-red fluorescence.The doping of Pb makes the synergistic action of[Pb Br₄]^2-unit and[MnBr₄]^2-unit to form a quasi-continuous conduction band,which makes the electron transition easier.The core-shell structure also fundamentally solves the harm to the environment caused by the leakage of Pb,and makes Cs₃MnBr₅:Pb@Si O₂ nanocrystals have high photochromic cycle stability,which provides a feasible strategy for perovskite research in the field of information encryption and anti-counterfeiting.