Studies On Energy Transfer / Energy Back Transfer In Rare Earth Ions Doped Materials

Rare earth?RE?ions possess abundant energy levels allowing the emission band to vary from ultraviolet to infrared,and they have potential applications including three-dimensional display,biological tissues,biomedical imaging,laser,solar cell and solid-state lighting.Energy transfer is an important role in rare earth ion doped luminescent materials,and even dominates the luminescent properties of the materials.Therefore,the study of the forward backward energy transfer is of great significance for understanding the luminescent mechanism and exploring new luminescent materials for application.This thesis focuses on the RE ions activated Lu₃Al₅O₁2 and Y₂O₃ oxide materials.We have studied the luminescence properties and the energy transfer processes in Pr³⁺/Ce³⁺ and Tm³⁺/Yb³⁺ codoped materials,respectively.The forward backward energy transfer between the dopant ions is an effective path that in favor of the the population of the lower energy level from the upper energy level.We also present a method to calculate the backward energy transfer?BET?efficiency.This thesis also focuses on the luminescence properties and thermal stability of Ce³⁺ and Tb³⁺ in Ba₂Y₃?SiO₄?₃F material.The major results obtained are as following:1.The series of Lu₃Al₅O₁₂:Ce³⁺,Pr³⁺ samples were synthesized by the high temperature solid state reaction.The emission spectra and decay curves were measured to study the energy transfer process.Upon Pr³⁺ 4f5 d state excitation,the step energy transfers from Pr³⁺ 4f5 d state to Ce³⁺ 5d state followed by energy back transfer from Ce³⁺ 5d state to Pr^{3+ 1}D₂ level are studied in Lu₃Al₅O₁₂.The back transfer results in strongly enhanced red emission line of Pr³⁺ in the codoped sample compared with Pr³⁺ singly doped sample.The efficiencies of Ce³⁺?Pr³⁺ energy back transfer are evaluated.It is evidently found that the Ce³⁺?Pr³⁺ energy back transfer upon Pr³⁺ 4f5 d state excitation is more efficient than the normal Ce³⁺?Pr³⁺ energy transfer upon Ce³⁺ 5d state excitation.The efficient energy back transfer is attributed to preferential excitation of the Ce³⁺ ion with an adjacent Pr³⁺ surrounding in Pr³⁺?Ce³⁺ energy transfer of the first step,whereas Ce³⁺ is excited randomly in the normal energy transfer.2.The samples series of Y₂O₃:0.1%Tm³⁺,2xYb³⁺?x=0-0.1?powders were synthesized via the normal firing precursor method.Upon 782 nm excitation to Tm³⁺:³H₄ level,with introducing Yb³⁺ into Tm³⁺ doped Y₂O₃,one can find not only the emission band of Yb³⁺:²F_5/2level,but also the increased emission intensity of Tm³⁺:³F₄ level.The 2 ?m emission intensity of Tm³⁺:³F₄?³H₆ transition is enhanced by 1.8 times at x=0.1,in which the population of ³F₄ level is increased by backward energy transfer from Yb³⁺ following the forward energy transfer from the upper level Tm³⁺:³H₄ to an intermediate level Yb³⁺:²F_5/2.The forward backward energy transfer process in Y₂?0.9995-x?Tm₀.001Yb_{2 x} O₃ is investigated in detail as a function of Yb³⁺ concentration.The efficiency of Yb³⁺?Tm³⁺ upon 782 nm excitation is calculated to be as high as 88% for codoping with 10 mol% Yb³⁺ while that only 57% upon direct excitation to Yb³⁺ at 980 nm,which is explained by the preferential excitation of Yb³⁺ with a nearby Tm³⁺ in the forward energy transfer process upon 782 nm excitation.3.The samples series of Lu₂O₃: Tm³⁺,Yb³⁺ powders were synthesized via the normal firing precursor method.The two steps energy transfer upconversion processes in Lu₂O₃:Tm³⁺,Yb³⁺ are investigated in detail upon 980 nm excitation.From the dependence of the emissions intensities of Tm³⁺:³H₄ levell on Tm³⁺ and/or Yb³⁺ concentration,the optimal doping concentration for the upconverted NIR emission around 811 nm is determined to be 0.1 mol% Tm³⁺ and 4 mol% Yb³⁺.Based on the steady state equations,it is concluded that the total coefficient of the first step energy transfer keeps unchange with Tm³⁺ concentration and quadratically with Yb³⁺ concentration.Notably,it increases by 500 times when Yb³⁺ concentration increases from y=0.001 to y=0.1.Moreover,the total coefficient of the second step energy transfer increases with Tm³⁺ concentration and reaches the saturation at x>0.005,while it increases with Yb³⁺ concentration and reaches the saturation at y>0.08 due to the rapid transfer rate limit.The results indicate that the excitation diffusions among Yb³⁺ ions and among Tm³⁺ ions substantially enhance the rate of energy transfer from Yb³⁺ to Tm³⁺ and thus dominate the energy transfer upconversion process.4.The Ba₂Y_2.94-3xCe₀.06Tb_3x?SiO₄?₃F phosphors with apatite structure were obtained by solid state reaction.The luminescence properties and energy transfer process between Ce³⁺ and Tb³⁺ under 365 nm excitation were investigated in detail as a function of Tb³⁺ concentration.With the increase of Tb³⁺ concentration,the blue emission intensity of Ce³⁺ decreases rapidly while the green emission intensity of Tb³⁺ increases considerably,which implies the enhanced energy transfer from Ce³⁺ to Tb³⁺.The emission intensity of Tb³⁺ reaches a maximum value at x=0.4.One can find that the emission color can be tunable from blue to yellow-green,and the CIE can be tuned from?0.1754,0.0993?to?0.3540,0.5394?with increasing the concentration of Tb³⁺.The decay curves of blue emission were also monitored under 365 nm excitation to improve the energy transfer process,and the energy transfer efficiency is calculated to be 93% when x=0.5.We also measured the internal quantum efficiency of the BYSF phosphor with fixed Ce³⁺ and various Tb³⁺ concentrations under 350 nm excitation,and there is nearly no decline of internal quantum efficiency with increasing concentration from x=0 to 0.4,indicating no quantum loss in energy transfer from Ce³⁺ to Tb³⁺.In addition,thermal stability of Ba₂Y_2.94-3xCe₀.06Tb_3x?SiO₄?₃F phosphors were also investigated to evaluate the properties.One can find that the intensity of Ba₂Y₁.74Ce₀.06Tb₁.2?SiO₄?₃F at 150 ?C could remain 84% of its initial intensity at room temperatue,which indicates a good thermal stability.