Preparation And Optical Properties Of Single-phase Color Tunable And White Light-emitting Phosphors

The aim of this investigation is to synthesize a single-phase white light-emitting phosphor as a candidate to overcome the drawbacks of WLED fabricated by InGaN chip and the YAG:Ce³⁺ yellow phosphor,such as high correlated temperature and low renderring index,which is due to lack of red emission component.In the experiments,white light-emitting phosphors Sr₆Gd₂Na₂?PO₄?₆F₂:Eu²⁺,Mn²⁺;Ca₂NaSiO₄F:Ce³⁺,Mn²⁺;Ca₃YNa?P0₄?₃F:Ce³⁺,Mn²⁺ and Sr₈ZnY?PO₄?₇:Ce³⁺,Tb³⁺,Mn²⁺ were prepared via the traditional high-temperatue solid-state reaction method.The surface morphology,elemental composition,the influences of activators and sensitizers on the phase purity and the luminescence properties were studied with X-ray diffraction meter?XRD?,scanning electron microscope?SEM?,fluorescence spectrometer and fluorescence lifetime spectrometer.Energy transfer mechanism was also studied in detail.The obtained conclusions are as follows:?1?Eu²⁺ and Mn²⁺ singly-doped and co-doped Sr₆Gd₂Na₂?PO₄?₆F₂ phosphors were synthesized by high-temperature solid-state reaction under the reductive atmosphere.Experimental results confirmed that the obtained samples are pure phases and no other impurity phase was detected.The as-prepared phosphor particles are in irregular shape and the surface is smooth with the average particle size is about 5 ?m.Moreover,Eu²⁺ and Mn²⁺ co-doped samples exhibit simultaneously the characteristic emission peak of Eu²⁺ at 460 nm and the emission peak of Mn²⁺ at 560 nm under excitation at 300 nm.The relative intensity of emission at 560 nm is 2.02 times stronger than that of Mn²⁺singly-doped sample.Luminescence decay curves and the other analyses implied that energy transfer occurs from Eu²⁺ to Mn²⁺ via a dipole-dipole interaction and the critical distance is 17.52 A.CIE chromaticity coordinates indicate that the synthesized phosphors are color tunable and white light-emitting,confirming their potential application as white-emitting phosphors used in WLED.?2?Ce³⁺,Mn²⁺ co-doped Ca₂NaSiO₄F color tunable phosphors were prepared by a high temperature solid-state method under a reductive atmosphere.Monitored at 552 nm,the PLE spectra of Ce³⁺,Mn²⁺ co-doped samples extend from 250 nm to 450 nm,which matches well with the NUV chip.Excited by 338 nm,the PL spectra of Ca₂NaSiO₄F:Ce³⁺,Mn²⁺ phosphors exhibit the characteristic peaks of Ce³⁺ and Mn²⁺,peaking at 453 and 552 nm,respectively.With the increase of Mn²⁺ doping contents,the intensity of Ce³⁺ emission and its decay lifetime decrease linearly,indicating that there is an energy transfer from Ce³⁺ to Mn²⁺.Based on the energy transfer,we can obtain a kind of phosphor with emission color changing from blue to bluish white by adjusting Mn²⁺doping concentration.?3?Ca₃YNa?PO₄?₃F:Ce³⁺,Mn²⁺ phosphor synthesized by high temperature solid-state method is a pure phase.Excited by 300 nm,the as-prepared phosphors Ca₃YNa?PO₄?₃F:Ce³⁺,Mn²⁺ exhibit two strong and broad emission bands.One blue emission band positioned at 405 nm is ascribed to the 5d¹?4f¹ transition of Ce³⁺,while the other one at 560 nm is originated from the ⁴T₁?⁴G??⁶A₁?⁶S?transition of Mn²⁺.The emission intensity of Mn²⁺ can be greatly enhanced due to the energy transfer from Ce³⁺ to Mn²⁺ and the maximum energy transfer efficiency and energy transfer rate are 89.35%and 314.62×10⁶ s^-1,respectively,which indicate that the energy transfer is efficient.The emission hues of target phosphors can be tuned from blue to white and eventually to yellow by just simply modulating the Ce³⁺/Mn²⁺ doping ratio based on the energy transfer.All results imply that the Ca₃YNa?PO₄?₃F:Ce³⁺,Mn²⁺phosphors can be potentially used as candidates for white-emitting phosphor applied in UV-based WLED.?4?Ce³⁺/Mn²⁺/Tb³⁺ tri-doped Sr₈ZnY?PO₄?₇ phosphor were first synthesized via high-temperature solid-state reaction.In co-doped samples,the emission intensity of Mn²⁺ and Tb³⁺ are greatly enhanced by co-doping with Ce³⁺,indicating that two types of energy transfer Ce³⁺?Mn²⁺ and Ce³⁺?Tb³⁺ occur.The energy transfer mechanism for Ce³⁺?Mn²⁺ is dipole-dipole interaction,while quadrupole-quadrupole interaction is dominant for Ce³⁺?Tb³⁺ energy transfer.As for Ce³⁺,Mn²⁺,Tb³⁺ tri-doped phosphors,its photoluminescence spectrum includes the characteristic emission peaks from Ce³⁺,Mn²⁺ and Tb³⁺:the blue emission of Cell at 410 nm,the red emission of Mn²⁺ at 600 nm and the green emission of Tb³⁺ at 490 nm and 545 nm.Experimental results indicate that Sr₈ZnY?PO₄?₇:0.05Ce³⁺,0.11Mn²⁺,0.11Tb³⁺ phosphor exhibits correlated chromaticity temperature of 5144.83 K and chromaticity coordinates about?0.34,0.33?,which is very close to the standard chromaticity coordinates?0.33,0.33?for white.All results imply that Sr₈ZnY?PO₄?₇:Ce³⁺,Mn²⁺,Tb³⁺phosphors can be potentially used as white-emitting phosphors applied in WLED.