Preparation And Properties Of Rare Earth Doped Phosphors With Multiple Luminescent Centers

Rare earth doped luminescent materials are widely applied in many fields,such as daily lighting and display,catalysis,solar cell,biological detection and medical treatment.It has become a hot research field to artificially regulate the luminescent properties of rare earth doped luminescent materials,which can meet the growing demand for new materials.The 4f-4f transitions of rare earth are not sensitive to the crystal field due to the shielding effect of outer electrons,therefore the 4f-4f emission of rare earth is relatively fixed.Although the 4f-5d transitions of rare earth are sensitive to the crystal field due to the exposure of outer 5d electrons,the emission range of 4f-5d transitions is still narrow when compared with the applied requirement.Therefore,it is particularly important to seek rational strategies for effectively regulating the emission of 4f-4f and 4f-5d transitions.In this thesis,for the 4f-4f transitions,multiple luminescent centers were introduced by co-doping different rare earth ions to regulate the luminescence properties,while for the 4f-5d transitions,identical rare earth ions were incorporated in multiple cation sites to generate multiple luminescent centers for the purpose of regulating the luminescence properties.Based on the rare earth doped phosphors with multiple luminescence centers,the energy transfer behavior between co-doped rare earth ions and the distribution and migration behavior of activators among multiple cations are also investigated.The main contents are:1.A series of NaGdF₄:Yb/Eu@NaGdF₄:Ce@NaGdF₄:Yb/Tb@NaYF₄ core-multishell nanoparticles were synthesized by multi-step thermal decomposition method.Tb/Eu are doped into different shell to effectively eliminate deleterious cross-relaxation,which can quench the up-conversion emission.The experimental results show that the up/down conversion luminescence colors of nanoparticles can be adjusted by regulating the thickness of NaGdF₄:Yb/Tb shell.The adjustment range of up conversion luminescence color is larger than that of down conversion one.In the up-conversion process,the molar ratio of NaGdF₄:Yb/Tb to NaGdF₄:Yb/Eu components increase with the thickness of NaGdF₄:Yb/Tb shell.It is the molar ratio of NaGdF₄:Yb/Tb to NaGdF₄:Yb/Eu components resulting in the relative emission intensity of Tb gradually increasing and the relative emission intensity of Eu gradually decreasing,rather than the"filtration effect"reported in previous literature.In the process of the down conversion,the interfacial energy transfer(Ce³⁺?[Gd³⁺]n?Ln³⁺,Ln=Eu/Tb)is confined in an narrow space region?approximately 2 nm?away from the NaGdF₄@NaGdF₄ interface.At the interface,Thetwodown-conversionenergytransferCe³⁺?[Gd³⁺]n?Eu³⁺and Ce³⁺?[Gd³⁺]n?Tb³⁺exist simultaneously,and are competitive against each other for catching the UV excitation energy from NaGdF₄:Ce shell.2.A series of Sr₃Y?PO₄?₃:xCe³⁺,yTb³⁺,zSm³⁺phosphors were synthesized by high temperature solid-state method.There exist metal-metal charge transfer quenching between Ce³⁺and Sm³⁺(Ce³⁺+Eu³⁺?Ce⁴⁺+Eu²⁺),which gives rise to the invalid energy transfer between Ce³⁺and Sm³⁺.Acting as an intermediate bridge for energy transfer,Tb³⁺with high doping concentration can realize the Ce³⁺?(Tb³⁺)_n?Sm³⁺energy transfer process.The experimental results demonstrate that the energy transfer mechanisms of Ce³⁺?Tb³⁺and Tb³⁺?Sm³⁺are dipole-dipole and exchange interactions,respectively.The incorporation of Ce³⁺has little influence on the energy transfer mechanism of Tb³⁺?Sm³⁺.With gradually increasing Tb³⁺/Sm³⁺doping concentration,the color tones can be changed from blue through green and finally to orange.At the same time,the energy transfer efficiency increases continuously,and can reach values above 90%.In addition,the quantum efficiency of Sr₃Y?PO₄?₃:0.02Ce³⁺,0.90Tb³⁺,0.02Sm³⁺sample is calculated to be 60%.The thermal stability of Sr₃Y?PO₄?₃:0.02Ce³⁺,0.90Tb³⁺,0.02Sm³⁺sample is outstanding,and the intensity retention still remained at 85.3%at temperature as high as 493 K.The thermal stability and thermal quenching activation energy of Sr₃Y?PO₄?₃:0.02Ce³⁺,0.90Tb³⁺,0.02Sm³⁺sample are higher than that of similar phosphors reported in the literature.3.A Novel Ba₄Gd₃K₃?PO₄?₆F₂:Eu²⁺phosphor with broadband absorption and emission was prepared by high temperature solid-sate method.The two emission peaks centered at 447,480 nm and the weak trailing band around 580 nm are attributed to the distribution of Eu²⁺ions in M?1?,Gd?2?and K?3?sites,respectively.The strong crystal field splitting of Eu²⁺at the K?3?site result in the broad full width at half-maximum around 580 nm.The Ba₄Gd₃K₃?PO₄?₆F₂:Eu²⁺phosphor shows blue-white emission,and the emission spectrum almost covers the full visible region.Theexperimentalresultsindicatethatthequenchingconcentrationof Ba₄Gd₃K₃?PO₄?₆F₂:Eu²⁺phosphor is 0.06.The critical distance Rc=12.5?,and the quenching mechanism is dipole-dipole interaction.In addition,white LED?Ra=81?can be fabricated by depositing BGKPOF:Eu²⁺and red CaAlSiN₃:Eu²⁺phosphors on395 nm LED chip.4.A series of Mg-doped Ca_10.5-xMg_x?PO₄?₇:Eu²⁺?0?x?1.5?single-phase phosphors were prepared by high-temperature solid-state method.The experimental results demonstrate that the substitution of Mg in the Ca_10.5?PO₄?₇ lattice is preferential.Mg²⁺ions can only occupy Ca?4?and Ca?5?sites.Firstly,smaller Mg²⁺ions preferentially enter Ca?5?site rather than Ca?4?site.When Ca?5?site is almost filled with Mg²⁺,Mg²⁺ions are forced to rapidly occupy Ca?4?site.There are three luminescent centers in Ca_10.5?PO₄?₇:Eu²⁺phosphor.The distribution of Eu²⁺ions at Ca?3?site results in the strong emission at 418 nm,and the distribution of Eu²⁺ions at Ca?1?and Ca?2?sites give rise to the trailing emission at long wavelength.The electron cloud of O in the Ca-O-Mg bond is partial to the side of Mg due to the preferential substitution of smaller Mg²⁺,resulting in a more ionic surrounding around Ca?1?and Ca?2?sites.To compensate the covalence change,more Eu²⁺activators would migrate from Ca?3?sites to Ca?1?and Ca?2?sites.As a result,the relative emission intensity at long wavelength increases gradually.On account of the preferentially substituted Mg²⁺ions,the color tones of Ca_10.5-xMg_x?PO₄?₇:Eu²⁺?0?x?1.5?series phosphors are changed from blue-purple through warm-white and finally to magenta.More importantly,the whiteLEDwhichwasfabricatedbydepositingtherepresentative Ca_10.5-xMg_x?PO₄?₇:Eu²⁺phosphors on 365 nm UV LED chips exhibits quite high color rendering index?CRI?R_a?85?and R₉?91?values.These results indicate that the as-obtained Ca_10.5-xMg_x?PO₄?₇:Eu²⁺single-phase phosphor should be an excellent candidate for high-quality solid state lighting.