Luminescence Properties Of Single-phased Ca₃Y?PO₄?₃,KBaYSi₂O₇ Phosphors For NUV-pumped White LED

Light emitting diodes?LED?, as promising next generation solid-state sources, have being aroused more and more interest because of their fascinating advantages including energy saving, mercury-free, long life, short response time,small size, and high energy efficiency. Currently, commercial white LEDs are fabricated by assembling blue InGa N LED chip with yellow-emitting YAG:Ce³⁺phosphor via mixing yellow and blue light to generate white light. However, the color-rendering index of white light made by complementary blue and yellow emissions is poor?CRI<75? and high correlated color temperature?CCT>7750K?because of the lack of contribution from red light at long wavelength range and the important role of LED chip and the phosphor to determine the color-rendering index of white light, which limits the further extension of white LEDs in the application of indoor lighting. To overcome these concerns mentioned above, a feasible approach is to use a NUV-LED chip coated with blue, green and red phosphors together. this type of w-LEDs has controlled color temperature and exceptional color rendering index. However, the luminescent efficiency of this kind of white LEDs is relatively low owing to the unavoidable re-absorption of the blue light by the red and green phosphors. Therefore, in order to improve the luminous efficiency of w-LEDs the above scheme isimproved by us. an alternative approach is more promising that warm white light is achieved in a single-component host via designing energy transfer between emitters. However, the program compounded is very short of white LED phosphor. In order to get white light of the high color-rendering index and stability, small color aberration, it is necessary to develop a new type material of host used to white LED.The paper chooses Ca₃Y?PO₄?₃ and KBaYSi₂O₇ as the material of host.Eu²⁺-doped, Eu²⁺/Mn²⁺ co-doped and Ce³⁺/Tb³⁺/Mn²⁺ tri-doped Ca₃Y?PO₄?₃phosphors have been synthesized via a high temperature solid-state reaction method. And preliminary characteristics of luminescence of KBa YSi₂O₇:Eu²⁺phosphor are studied. The crystal characteristics, luminescence properties,thermal stability as well as the energy transfer mechanism are investigated. The main results obtained are listed as follow:1. A series of Eu²⁺ doped and Eu²⁺/Mn²⁺ co-doped Ca₃Y?PO₄?₃ phosphors with tunable emission color have been synthesized by conventional high temperature solid-state route under the reducing atmosphere in CO. The powder X-ray diffraction?XRD?, diffuse reflectance spectra?DRS?, photoluminescence?PL? including temperature-dependent luminescence and fluorescence decay were employed to characterize the as-prepared phosphors. All of the profiles match well with that of the eulytite-structured CYPO. No detectable impurity phase can be observed in the obtained samples even though Eu²⁺ ions were introduced into the host lattice. The CYPO host can provide a suitable band gap for Eu²⁺ ions to act as an emission center by diffuse reflectance spectra?DRS? of the host. The band gap of the CYPO host is Eg=5.05 e V. In the Eu²⁺-doped CYPO samples, an intense and broad green emission band is presented under the excitation of 370 nm UV light, which is attributed to the 4f-5d transition of Eu²⁺ions in the eulytite-structured CYPO with cation and oxygen sublattice disorders.Emission color changes from green to warm white light is observed when different concentrations of Mn²⁺ ions were introduced into the lattice of CYPO:Eu²⁺ phosphor, originating from an efficient energy transfer from Eu²⁺ to Mn²⁺ions, which was confirmed by fluorescent dynamics of Eu²⁺ions as well.The energy transfer mechanism from Eu²⁺ to Mn²⁺ is demonstrated to be a quadrupole-quadrupole interaction based on the concentration quenching as well as the energy transfer efficiency of 90%. The temperature-dependent luminescence of CYPO:Eu²⁺, Mn²⁺ phosphor reveal that 52% of initial intensity can be kept at 150?, suggesting relative good thermal stability of the phosphor.Our results indicate that the CYPO:Eu²⁺,Mn²⁺phosphor may be a potential candidate for use as near ultraviolet?NUV? pumped white light-emitting diodes?LEDs? phosphor.2. A series of Ce³⁺-doped, Ce³⁺/Tb³⁺ co-doped, Ce³⁺/Mn²⁺ co-doped and Ce³⁺/Tb³⁺/Mn²⁺ tri-doped Ca₃Y?PO₄?₃ phosphors have been synthesized by conventional high temperature solid-state route under the reducing atmosphere in CO. The powder X-ray diffraction?XRD?, photoluminescence?PL?including temperature-dependent luminescence were employed to characterize the as-prepared phosphors, which indicates the as-prepared CYPO:Ce³⁺,Tb³⁺,Mn²⁺ phosphor has not only the emission peak of Ce³⁺ ions,which corresponds to the 4f65d1-4f7 transition of Ce³⁺, but also the emission peak of Tb³⁺ ions, which corresponds to the 5D4-7F5 transition of Tb³⁺, as well as the emission peak of Mn²⁺ions. The both of energy transfer of Ce³⁺/Tb³⁺and Ce³⁺/Mn²⁺ co-doped Ca₃Y?PO₄?₃ phosphors are governed by electric dipole-dipole interaction. Temperature-dependent luminescence deduced that CYPO:Ce³⁺,Tb³⁺,Mn²⁺ has good luminescent stability under the high temperature environment. A wide-range-tunable white light emission is obtained by precisely controlling the contents of Tb³⁺ and Mn²⁺, whose chromaticitycoordinate is?0.35, 0.32?. The above results indicate that CYPO:Ce³⁺,Tb³⁺,Mn²⁺phosphor may have potential value for application in the NUV-pumped white LEDs.3. A novel blue-emitting Eu²⁺-doped KBa YSi₂O₇ phosphor has been synthesized by conventional high temperature solid-state reaction. All of the profiles match well with the calculated XRD patterns according to the crystal structure parameters of KBa YSi₂O₇ reported by Uwe Kolitsch et al. It can be found that all XRD patterns have the similar profiles and no detectable impurity phase can be observed in the obtained samples even though Eu²⁺ ions were introduced into the host lattice. It can be seen that the PLE spectra monitored at468 nm show a broad absorption band ranged from 250 nm to 450 nm. It corresponds to the 4f7-4f65d1 transition of Eu²⁺ ions. Under 342 nm excitation, the emission spectra shows a broad emission band with a maximum at about 468 nm which corresponds to the 4f65d1-4f7 transition of Eu²⁺. The band gap energy of KBa YSi₂O₇ through diffuse reflection spectra analysis is about 4.98 e V, which indicates the KBaYSi₂O₇ host can provide a suitable band gap for Eu²⁺ions to act as an emission center. According to fitting result, the nature of multipolar interaction for energy transfer between Eu²⁺ ions in KBaYSi₂O₇:Eu²⁺ is mainly governed by dipole-dipole. Temperature-dependent luminescence deduced that KBa YSi₂O₇:Eu²⁺ has good luminescent stability under the high temperature environment. The results above suggest that KBa YSi₂O₇ host can be used as a potential NUV pumped w-LEDs host.