Research on Luminescence Properties of Rare Earth Ions Doped SrZrO3 and Y3Al2Ga3O12 Phosphors
Author:Li Zhen Zhang
Supervisor:hu yi hua
The rare earth ions have the characteristics of partially filled 4f shell,with completed 5s and 5p shells.The larger radial range of n=5 shell means that the partially filled 4f shell is not affected by local electric field.This results in the transition of a given rare earth element(in a given charge state)within the same 4f state in a variety of different hosts.Of course,not all hosts respond well to the application of rare earth ions in practice.The luminescence field of rare earth ions has been growing steadily over the past decade,mainly because of the increasing demand for light sources.Among the doped materials,the research of rare earth luminescence has been paid more and more attention:display,laser material,data storage,radiation detection and medical application are more and more important fields of rare earth luminescence technology.White LED is widely used as a new all-solid-state lighting source with the advantages of high efficiency,environmental protection,low energy consumption and long life.As an optical probe in medical imaging,the near infrared afterglow phosphor has the advantages of stable luminescence,high signal-to-noise ratio and high sensitivity.However,as the carrier of rare earth ion doping,hosts play an important role in the performance of phosphors.Perovskite host materials have many interesting and interesting properties.Large magnetoresistance,ferroelectric,superconductivity,charge ordering,spin-dependent transport,high temperature difference and structure are common characteristics of perovskite.Garnet host materials are widely used in solid lighting,scintillator and fluorescence display due to their high thermal conductivity and excellent physical and chemical properties.In order to expand the generality of the study and analyze the luminescence principle of rare earth doped materials,the perovskite SrZrO3 and garnet Y3Al2Ga3O12 host materials were analyzed by First-principles calculations and experimental method in this paper,and further analyzed the optical properties after doping rare earth ions Yb3+.Tb3+,Eu3+and Sm3+,contents and related results described as follows:(1)First-principles calculations and experiment analysis were performed to study the internal relation between seven types of intrinsic defects and the persistent luminescence in SrZrO3 host material.The calculation shows that rich zirconium defects have the low energy cost and thus are easy to form.Zr vacancies are too high energy to play any role in defect which is related luminescence phenomenon of SrZrO3 phosphor.However,oxygen vacancies stand out as a likely candidate,because it can yield two carrier reservoirs:a fully-occupied singlet electron’s reservoir which lies above the valence band maximum,and an empty triply degenerate hole’s reservoir which is just below the conduction band minimum.Sr vacancies are not directly relevant to the persistent luminescence due to its too shallow electron trap level.The characteristics of these defects are fully explained by the equilibrium properties of SrZrO3.An experimental study of the thermoluminescence glow for these defects is conducted and the calculation is consistent with the experimental results.A mechanism of the persistent luminescence for SrZrO3:Pr3+ is explained according to oxygen vacancies trap center.Findings of this study may serve as theoretical references for controlling intrinsic traps by more refined experiments.(2)A dual-emission long persistent luminescence phosphor SrZrO3:Yb3+is synthesized through a conventional high-temperature solid-state reaction method.A combined experiment and first-principles methods are carried out to study the inner link between the electronic structures and spectroscopic characteristics of SrZrO3:Yb3+phosphor.The calculation shows that the incorporation of Yb3+ions into SrZrO3 lattice can be done through substituting Yb3+ions for Sr site.It is found that vacancy defects of SrZrO3:Yb3+system easily produce around the doping positions.Especially,the oxygen vacancies,which are deduced as the intrinsic vacancy defect,are responsible for the persistent luminescence in SrZrO3:Yb3+phosphor.The persistent luminescence of SrZrO3:Yb3+consists of two parts:near-ultraviolet persistence originate from self-trapped excitons of host,and near-infrared one stems from 2F5/2→2F7/2 transitions of Yb3+.The photoluminescence and persistent performance are studied as function of the concentration of Yb3+ions.The optimal concentration of Yb3+ions for the photoluminescence emission and long persistent luminescence are both approximately 2.5%.The long persistent luminescence mechanism for SrZrO3:Yb3+is illuminated on the basis of photoluminescence and first-principle calculation results.(3)Demonstrates that a highly thermal stable and tunable-luminescence phosphors Y3Al2Ga3O12:Tb3+,Eu3+with efficient energy transfer,which is prepared by the high-temperature solid-state reaction.First-principles method is performed to analyze the electronic structures,band structure,density of states and formation energy of Y3Al2Ga3O12:Tb3+,Eu3+,For high luminescence output,Y3-xAl2Ga3O12:xTb3+and Y3-Al2Ga3O12:yEu3+with high concentration quenching(x=0.5 and y=0.7)are achieved.The effect of energy transfer from Tb3+to Eu3+on emission color tuning and luminescence thermal stability is studied.The energy transfer efficiency from Tb3+to Eu3+reaches as high as 74.2%and emission color can be tuned from green,yellow and red by adjusting the ratio of Tb3+/Eu3+.What’s more,Y3Al2Ga3O12:Tb3+ Eu3+phosphor exhibits a good thermal stability,of which emission intensity still keeps 60%at 150℃.(4)A highly thermal stable and tunable-luminescence phosphors Y3Al2Ga3O12:Tb3+,Sm3+ is synthesized by a conventional high-temperature solid-state reaction method.The characteristics of energy transfer mechanism,multicolor emission and thermal stability for Y3Al2Ga3O12:Tb3+,Sm3+are discussed in detail.The energy transfer efficiency from Tb3+to Sm3+reaches as high as 62.5%and emission color can be tuned from green to yellow by adjusting the ratio of Tb3+/Sm3+.Furthermore,Y3Al2Ga3O12:Tb3+,Sm3+phosphor shows a better thermal stability,the quenching temperature of Y3Al2Ga3O12:0.5Tb3+,0.1 Sm3+is～155 and～210℃ for 614 and 542 nm,respectively.