Synthesis And Luminescence Properties Of Several Ce³⁺-Activated Or Sensitized Luminescent Materials

Recently,thanks to their significant advantages such as long application life,high efficiency,high brightness and low energy consumption,white light-emitting diodes?WLED?have become the most widely used lighting and display technology.Among the several combination modes,WLED based on near-ultraviolet LED and RGB?red,green and blue?phosphors has attracted much attention on account of their unique advantages in achieving high color rendering of white light.At present,the thermal stability of these fluorescent materials needs to be further improved and their synthesis process needs to be simplified.In this paper,Ce³⁺was selected as the luminescent center or sensitizer to prepare several luminescent materials by solid state reaction with simple synthesis conditions,aiming at the current problems of poor thermal stability and harsh synthesis conditions of luminescent materials for near-ultraviolet LED,and the relationship between their structures and luminescent properties was explored.The main works are as follows:1.Blueluminescentmaterialwithnearultravioletexcitation(K₇Ca₉[Si₂O₇]₄F:Ce³⁺)was synthesized.The crystal structure and luminescence properties were studied by means of structural characterization and spectral analysis.Under near-ultraviolet excitation,the emission of K₇Ca₉[Si₂O₇]₄F:Ce³⁺shows the blue light with the peak value at 445 nm.And more remarkable,K₇Ca₉[Si₂O₇]₄F:Ce³⁺exhibits an excellent thermal stability,when the temperature reaches 200 ^oC,its luminescent intensity can still keep the room temperature more than 90%of the luminescent intensity,and the color coordinates deviation less than 1.5%.With the increase of the temperature,the luminescence intensity of K₇Ca₉[Si₂O₇]₄F:Ce³⁺increases first and then decreases.By means of thermoluminescence analysis and other means,this anomalous phenomenon may be caused by the defects resulted from the unequal substitution of Ca²⁺by Ce³⁺.The results show that the excellent thermal stability of K₇Ca₉[Si₂O₇]₄F:Ce³⁺is related to the dense structure of matrix material K₇Ca₉[Si₂O₇]₄F,which provides a certain reference value for the future search for ideal luminescent materials.2.A green luminescent material NaBaY?BO₃?₂:Ce³⁺,Tb³⁺was prepared.The crystal structure and luminescence properties were studied by means of XRD and spectral analysis.Dexter energy transfer theory and Inokuti-Hirayama model were used to study the energy transfer phenomenon dominated by dipole-dipole interaction between Ce³⁺and Tb³⁺in NaBaY?BO₃?₂:Ce³⁺,Tb³⁺.Based on this high-efficiency energy transfer,NaBaY?BO₃?₂:Ce³⁺,Tb³⁺can be effectively excited by the near-ultraviolet chip,and the emission is green light with a peak value of 542 nm.The thermostability of NaBaY?BO₃?₂:Ce³⁺,Tb³⁺is superior to the green commercial phosphor?Sr,Ba?₂SiO₄:Eu²⁺,the luminescent intensity under 200 ^oC can keep the room temperature is more than 70%.Meanwhile,the CL spectra of NaBaY?BO₃?₂:Ce³⁺,Tb³⁺were also measured under the various conditions of probe currents and accelerating voltages.Through successive excitation of low-voltage electron-beam,NaBaY?BO₃?₂:Ce³⁺,Tb³⁺displays wonderful degradation property and favorable color stability.When the bombardment time reaches 80 min,the luminescence intensity can maintain more than 99%of the initial intensity.It turns out that NaBaY?BO₃?₂:Ce³⁺,Tb³⁺,on the one hand,can be used as an ideal green luminescent material for near-ultraviolet LED chips;on the other hand,considering their excellent cathode luminescence performance,NaBaY?BO₃?₂:Ce³⁺,Tb³⁺also have great application potential in FED?Field Emission Display?.3.Using Ce³⁺and Mn²⁺as sensitizer and activator,a near-uv excited red luminescent material NaBaSc?BO₃?₂:Ce³⁺,Mn²⁺was prepared.Combined with first-principles calculation,XRD structure refinement and time resolved fluorescence spectra,it is shown that there are three kinds of Ce³⁺luminescent centers in the crystal lattice.Based on the energy transfer from Ce³⁺to Mn²⁺dominated by dipole-dipole interaction,the absorption of Mn²⁺in the near-ultraviolet region was improved,and the luminescence intensity of Mn²⁺was significantly increased?about 160 times?.Under the excitation of 370 nm,the luminescence intensity ratios of Ce³⁺and Mn²⁺show a linear change with the temperature rise within the temperature range of 298 K to 473 K,and the relative sensitivity can reach 3.16%k^-1 at 473 K,indicating that NaBaSc?BO₃?₂:Ce³⁺,Mn²⁺can be used as excellent fluorescent temperature sensor materials.The results indicate that the sensitizer can effectively improve the luminescence performance of Mn²⁺,which can provide novel reference for the development of luminescent materials.4.A single substrate white luminescent material(Ca_0.33Sr_0.67)₇?SiO₃?₆Cl₂:Ce³⁺,Eu²⁺was prepared.By means of XRD test and gaussian fitting,it is shown that Ce³⁺only occupies Ca²⁺site with six coordination in the lattice.According to the energy transfer theory of Dexter and Reisfeld approximation,the energy transfer effect from Ce³⁺to Eu²⁺in(Ca_0.33Sr_0.67)₇?SiO₃?₆Cl₂:Ce³⁺,Eu²⁺is dipole-dipole interaction.By adjusting the concentration of Eu²⁺,the warm white light emission with color coordinate?0.3773,0.396?and color temperature of 4195 K was obtained in(Ca_0.33Sr_0.67)₇?SiO₃?₆Cl₂:Ce³⁺,Eu²⁺.The results suggest that in the same matrix,white light could be produced by adjusting the proportion of Ce³⁺and Eu²⁺.