Studies On Luminescence Characteristics And Energy Transfer Process In Tm³⁺(Er³⁺)/Yb³⁺ Doped Lu-or Zr-based Oxides

Owing to the special electronic configuration of rare earth ions,rare earth luminescent materials have plenty of advantages,such as high conversion efficiency,high color purity,rich in luminescent colors,etc.Consequently,rare earth luminescent materials have long been widely applicated in lighting,display,medical treatment,energy and other fields.The energy transfer process between rare earth ions is of vital importance for optical properties of rare earth luminescent materials.By studying the energy transfer process between rare earth ions,it is beneficial to understand the luminous mechanism in rare earth materials,improve optical properties of current materials,explore new materials and expand the applications of rare earth luminescent materials.Therefore,research on energy transfer mechanism between rare earth ions has profound theoretical significance and application values.In this paper,we focus on the Tm³⁺/Yb³⁺codoped lutetium-compounds(Lu_1.6Sc_0.4O₃,Lu₂O₃-Zr O₂,Lu₂O₃)and Er³⁺/Yb³⁺codoped Sc₄Zr₃O₁₂.The optical properties and energy transfer mechanism have been systematically analyzed in different rare earth oxides hosts.We also explore the applications in display and imaging,solar cells and optical temperature sensing based on the rare earth luminescent materials.The major research results are as following:?1?The series of Tm³⁺/Yb³⁺codoped Lu_1.6Sc_0.4O₃ samples were synthesized by urea homogeneous precipitation method.Under 808nm excitation into the Tm³⁺:³H₄level,a considerable enhancement in intensity of Tm³⁺:³F₄?³H₆ emission with respect to Tm³⁺:³H₄?³F₄ emission is observed in Tm³⁺/Yb³⁺codoped Lu_1.6Sc_0.4O₃.The Tm³⁺-Yb³⁺-Tm³⁺forward-backward energy transfer is proved to generate an additional route for the ³H₄?³F₄ nonradiative relaxation,that is,energy transfer from Tm³⁺:³H₄ to Yb³⁺:²F_5/2 and the subsequent back transfer from Yb³⁺:²F_5/2 to Tm³⁺:³F₄.The analysis of emission spectra reveals that back transfer from Yb³⁺that excited by the forward energy transfer is more efficient than by absorption of 980 nm infrared light.The efficiency can reach as high as 60%with an extremely low Tm³⁺concentration?0.05%?.We propose that those Yb³⁺ions with nearby Tm³⁺ions in the forward energy transfer are preferentially excited instead of equally excited as by absorption of light.The efficiencies of the energy back transfer at different Yb³⁺concentrations are evaluated,indicating that the forward-backward energy transfer can act as a dominant route for the Tm³⁺:³H₄?³F₄ nonradiative relaxation when Yb³⁺concentration is higher than 5%.A method to determine the radiative rate of Tm³⁺:³H₄ state based on the model of cross relaxation is also demonstrated.?2?Near infrared?980 nm?to near infrared?800 nm?and blue?490 nm?upconversion have been studied in 0.2%Tm³⁺and 10%Yb³⁺co-doped Lu₂O₃-ZrO₂solid solutions as a function of ZrO₂ content in the range of 0-50%,prepared by high temperature solid reaction.The continuous enhancement of upconversion luminescence is observed with increasing Zr O₂ content up to 30%.The analyses of Yb³⁺emission intensity and lifetime indicate enlarged absorption of 980 nm excitation laser and enhanced energy transfer from Yb³⁺to Tm³⁺with adding Zr O₂.The spectrally inhomogeneous broadening of the dopants in this disordered solid solution is considered to play the main role on the enhancement by providing better matches with the excitation laser line and increasing the spectral overlap for efficient energy transfer from Yb³⁺to Tm³⁺.In addition,the inhomogeneous broadening is also validated to improve energy migration among Yb³⁺ions and energy back transfer from Tm³⁺to Yb³⁺.Hence,it is understandable that a drop in upconversion luminescence intensity occurs as the concentration of ZrO₂ is further increased from30%to 50%.This work indicates the possibilities of disordered crystals to achieve intense upconversion luminescence for biological and optoelectronic applications.?3?The series of Lu₂O₃:Tm³⁺,Yb³⁺samples were synthesized via the normal firing precursor method and near-infrared downconversion phenomenon has been demonstrated in Lu₂O₃:Tm³⁺/Yb³⁺phosphor upon direct excitation of Tm³⁺:¹G₄ level at 463 nm.The efficient energy transfer from Tm³⁺:¹G₄?Yb:²F_5/2 has been elucidated by the excitation spectra,the visible and NIR spectra as well as the decay curves of Tm:¹G₄ state.The mechanism of downconversion in Lu₂O₃:Tm³⁺/Yb³⁺has been discussed in detail.According to study of the dependence of the energy transfer rate with the concentration of Yb³⁺,it could be included that energy transfer?ET?from Tm³⁺to Yb³⁺is a single-step ET process instead of a cooperative one.By varying the Yb³⁺concentrations,we obtain the Lu₂O₃:0.2%Tm³⁺/30%Yb³⁺sample with theoretical quantum efficiency as high as 148.2%.Because the excited state of Yb³⁺just above the band edge of crystalline silicon,it suggested that Lu₂O₃:Tm³⁺/Yb³⁺sample will be beneficial to improve the conversion efficiency of c-Si solar cells.?4?The series of Tm³⁺/Yb³⁺codoped single phase?-Sc₄Zr₃O₁₂ samples were synthesized via high temperature solid-state method.Here,intense green upconversion luminescence has been obtained in Er³⁺/Yb³⁺doped?-Sc₄Zr₃O₁₂ for the first time and its temperature sensing performance has been investigated.The structure of?-Sc₄Zr₃O₁₂ is given by Rietveld refinement of XRD data and the site occupancy of Er³⁺ions has been determined.Compared with cubic Sc₂O₃ and ZrO₂,under 972 nm excitation,the green emission from Er³⁺centers in Sc₄Zr₃O₁₂ is increased by 59-fold and 264-fold,respectively.By experimental analysis,this enhancement of upconversion luminescence is attributed to the low-symmetrical environment of Er³⁺,generation of Yb³⁺clusters and high internal efficiency of Yb³⁺emission in Sc₄Zr₃O₁₂.In addition,the fluorescence intensity ratio of two green emission bands(²H_11/2/⁴S_3/2?⁴I_15/2)is studied as a function of temperature ranging from 303-793 K in Sc₄Zr₃O₁₂.The maximum sensitivity by calculation is 0.00634 K^-1at 573 K,and the sensitivity is still as high as 0.00534 K^-1 at 793 K.The stability of Sc₄Zr₃O₁₂ thermometer has also been examined by recycling test.These findings suggest?-Sc₄Zr₃O₁₂ is a promising upconversion host and could achieve high-sensitivity optical temperature sensing with wide measuring range.