Preparation And Luminescence Mechanism Of UpconversionLuminescent Material LiBiF₄:Er³⁺/Yb³⁺ By Low-temperature Self-flux Method

In this disertation,LiBiF₄:Er³⁺/Yb³⁺micro-/nano-materials were prepared via low-temperature self-flux method,and the effects of constant temperature and Li/Ln/F molar ratio on the particle morphology size and upconversion luminescence performance of the product were studied.The fine energy level structure and upconversion luminescence mechanism of Yb³⁺and Er³⁺in LiBiF₄:Er³⁺/Yb³⁺materials were determined experimentally and theoretically.The effects of pump power density and excitation time on upconversion luminescence efficiency were theoretically studied.The main research contents are as follows:（1）Tetragonal LiBiF₄:Er³⁺/Yb³⁺micro-/nano-materials were prepared via low-temperature self-flux method using low fluxing point Li NO₃,Ln（NO₃）₃x H₂O（Ln=Er,Bi,Yb）,and NH₄F as raw materials.The experimental results show that the optimal molar ratio（Li/Ln/F）of Li NO₃,Ln（NO₃）₃x H₂O,and NH₄F for the synthesis of pure LiBiF₄:Er³⁺/Yb³⁺is 1.2:1:5.The morphology of the product gradually evolves from nearly spherical to tetragonal biconical with increasing temperature and time.（2）The ground state and excited state energy level structures of Er³⁺and Yb³⁺were obtained by using the experimental absorption spectrum,fluorescence spectrum and up conversion fluorescence spectrum.On this basis,the group chain method and Wigner-Eckart theorem were used respectively to obtain the matrix element expressions of the Hamiltonian and H_CF of the crystal field（CF）.The steepest descent method and Levenbery Marquardt iteration method were used to fit the crystal field parameters,and the crystal field parameters and fine energy level structures of Er³⁺and Yb³⁺were obtained.Research has shown that the crystal field parameters and energy levels obtained by using the Levenbery Marquardt iteration method are closer to the experimental values,and the fitting error of Er³⁺crystal field energy level splitting is slightly larger than that of Yb³⁺.（3）The upconversion luminescence mechanism and energy transfer process of LiBiF₄:Er³⁺/Yb³⁺were researched by using variable power fluorescence spectroscopy and fluorescence decay curve.On this basis,the Monte Carlo model was used to simulate the upconversion luminescence process of Yb³⁺and Er³⁺in LiBiF₄:Er³⁺/Yb³⁺.The results show that the maximum upconversion luminescence efficiency of this material can reach 1.554%.The influence of excitation pulse width and pump power on the upconversion luminescence efficiency were studied using this model.It was found that after the excitation time exceeded 1 second,the upconversion efficiency of the grains oscillated within the range of 1.55±0.005%.By adjusting the pump energy density calculation in the model,it was found that the luminescence process of LiBiF₄:Er³⁺/Yb³⁺is a two-photon process,which is close to the number of upconversion photons involved in the model,proving the rationality of the model.