Synthesis And Optical Properties Study Of Narrow Emitting Phosphors And Solid State Carbon Dots

White LEDs are widely used in the backlight of liquid crystal displays.In this technology,the phosphor powder as the key material determines the color gamut range,luminous efficiency and reliability of the backlight unit.Therefore,in addition to a suitable emission wavelength,the phosphors used are required to have narrow emission.However,there are few narrow-band emitting luminescent materials reported so far,and the development of new narrow-band emitting luminescent materials still faces great challenges.Carbon dots have the advantages of low preparation cost,good photostability,non-toxic environmental protection,and easy adjustment of light color.The development of new carbon material phosphors can not only meet the urgent requirements of cost reduction and environmental friendliness,but also an effective strategy for avoiding rare resources and sustainable development,but carbon dots are prone to solid-state fluorescence quenching due to agglomeration in the solid state,which greatly limits the application of carbon dots in white LEDs.Aiming at these two key scientific issues,this dissertation mainly carries out two aspects of work:1)the synthesis and luminescence properties of narrow-band light-emitting bulk luminescent materials;2)the exploration and synthesis of narrow-band emitting solid fluorescent carbon dots.The details are as follows:1.Fast microwave synthesis and luminescence properties of narrow-band green-emitting Cs₃MnBr₅luminescent materials.Cs₃MnBr₅has strong absorption in the 370-400 nm and 430-470 nm regions,which are attributed to the spin-forbidden d-d transition emitted by Mn²⁺,indicating that it can be combined with near-ultraviolet chips and blue-light chips.The Cs₃MnBr₅luminescent material emits bright green light under excitation at 460 nm,the maximum emission peak is located at 520 nm,the full width at half maximum is 42 nm,and the quantum efficiency is41%.In addition,we study and analyze the effect of anionic（Cl^-and I^-）substitution on the optical properties of Mn²⁺.We found that the substitution of anions（Cl^-and I^-）led to a decrease in the luminescence intensity of Cs₃MnBr₅,and other different luminescence properties changed:（1）When Br^-was replaced by Cl^-,No net emission shift was observed in the emission spectrum of Cs₃Mn（Br,Cl）₅due to the dual effects of reduced covalency and increased crystal field strength resulting from the substitution of Cl^-for Br^-.（2）When I^-is used to replace Br^-,and the amount of I^-substitution exceeds 6%,an impurity phase will appear in the XRD of Cs₃Mn（Br,I）₅.At the same time,the emission spectrum has a significant red shift.2.Synthesis,modification and luminescent properties of narrow-band blue-green emitting Li₂CaSiO₄:Eu²⁺luminescent materials.The excitation spectrum of Li₂CaSiO₄:Eu²⁺is located in the range of 250-470 nm,which belongs to the crystal field splitting of Eu²⁺5d energy level,and emits blue-green light of 480 nm under excitation at 375 nm,the full width at half maximum is 31 nm,and the quantum efficiency was 30.88%..In addition,in order to reduce the sample size,we used the citric acid sol-gel method to synthesize Li₂CaSiO₄:Eu²⁺.From the SEM image,we can see that the sample size of the Li₂CaSiO₄:Eu²⁺luminescent material was reduced to about 2μm,and the luminous intensity decreased at the same time.In order to enhance the luminescence intensity of Li₂CaSiO₄:Eu²⁺,the measure of rare earth ion Gd³⁺substitution was taken.When 1%Gd³⁺was introduced into Li₂CaSiO₄:Eu²⁺,its luminescence intensity could be enhanced by 1.8 times.3.Exploratory synthesis of narrow-band emitting carbon dots.First,three broadband emission carbon dots,OAP-e,PAP-e and MAP-e,were synthesized by solvothermal method using aminophenols（o-aminophenol,p-aminophenol and3-aminophenol）as carbon source and ethanol as solvent.OAP-e emits white light,PAP-e and MAP-e emits red ligh under 365 nm UV lamp;secondly,o-phenylenediamine is used as carbon source,nitric acid as catalyst,liquid carbon dots was obtained by the hydrothermal method and dispersing into starch,the luminous intensity decreased due to the narrowing of the distance between the carbon dots.In addition,using o-phenylenediamine as the carbon source,both the hydrothermally synthesized nitrogen-doped carbon dots and the solvothermally synthesized carbon dots（with ethanol as the solvent）emit bright yellow light at 365nm..Finally,using catechol and hydroquinone as the carbon source,the carbon dots obtained by the hydrothermal method and the solvothermal method（ethanol as the solvent）have broadband emission and low luminescence intensity.4.Solid-state fluorescent carbon dots with aggregation-induced fluorescence enhancement properties were synthesized by a one-step hydrothermal method using citric acid and dithiourea as raw materials.The carbon dots exhibit blue light emission in the liquid state（the emission peak is at 450 nm,the quantum efficiency is 7.6%）,while in the solid state it exhibits bright blue-green emission（the emission peak is at490 nm,the quantum efficiency is 29.2%）.This phenomenon is caused by aggregation induction,that is,in the liquid state,the distance between the carbon dots is large and the interaction is weak;in the solid state,the interaction between the carbon dots becomes stronger due to the sharp reduction of the agglomerated carbon dots.,causing charge redistribution between carbon dots and generating new surface states and luminescence energy levels.By comparing the fluorescence decay curves of carbon dots with different concentrations and the fluorescence lifetimes of solid carbon dots at different emission wavelengths,the origin of aggregation-induced luminescence in this system was further explained and demonstrated.Taking advantage of the different luminescence properties of carbon dots in liquid state and solid state,we use carbon dots for fluorescent printing and fluorescent anti-counterfeiting.