Synthesis And Luminescent Properties Of Silicate/Phosphate Phosphors

White light-emitting diode is a new generation of lighting source with advantages of small size,vibration resistance,high luminous efficiency,energy saving and environmental friendly,long life,and so on.Phosphor-converted white light-emitting diode?pc-WLED?is the main form of white LEDs.At present,the commercial pc-WLED is mainly composed of a blue chip and Y₃Al₅O₁₂:Ce³⁺.However,its application in lighting is limited due to the lack of red light component,which results in the fact that the color rendering index is often lower than 80.There are many ways to solve the above problem,such as the addition of red component on the above combination,the combination of a blue chip and green,red phosphors,the combination of a near ultraviolet chip and blue,green,and red phosphors,and the combination of a near ultraviolet chip and a single phase white phosphor,etc.No mater what kind of combination is used,the phosphors should have advantages such as high quantum efficiency,high thermal stability,suitable spectra,and so on,in order to meet the requirements of white LED.On the other hand,for agricultural applications,phosphors should also have appropriate spectra and high quantum efficiency in order to convert near-ultraviolet and green light efficiently to red light that is needed by crops.In addition,this type of phosphors should also have high stability to withstand different natural conditions.Based on the requirements of phosphors for pc-WLEDs and agricultural light-conversion materials,Li₄M_1.97?SiO₄?2:0.03Ce³⁺ and Ca₂Sr?PO₄?₂:Ce³⁺,Mn²⁺,Na⁺ are taken as the research objects in the thesis.Two kinds of phosphors with excellent performance were obtained based on the adjustment of excitation and emission wavelengths,quantum efficiency,and thermal stability according to the host lattice modification and energy transfer.1.Photoluminescence quantum efficiency?QE?and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes?pc-LEDs?.Li₄Sr_1+xCa_0.97-x?SiO₄?₂:0.03Ce³⁺?-0.7?x? 1.0? phosphors were designed from the initial model of Li₄SrCa?SiO₄?₂:Ce³⁺,and their single-phased crystal structures were found to be located in the composition range of-0.4 ? x ? 0.7.Depending on the substitution of Sr²⁺ for Ca²⁺ ions,the absolute QE value of blue-emitting composition-optimized Li₄Sr_1.4Ca_0.57?SiO₄?₂:0.03Ce³⁺ reaches?94%,and the emission intensity at 200 ? remains 95%of that at room temperature.Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity,increase of force constant in CeO₆,and decrease of vibrational frequency by increasing Sr²⁺ content,which are responsible for the enhanced quantum efficiency and thermal stability.The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.2.A series of Ca₂Sr?PO₄?2:Ce³⁺,Mn²⁺,Na⁺ phosphors were synthesized by a high-temperature solid-state reaction.Under?320 nm excitation,the Ce³⁺ and Mn²⁺ co-doped phosphors exhibit two emission bands peaking at 370 and 645 nm,which originate from 5d-4f and 3d-3d transitions of Ce³⁺ and Mn²⁺,respectively.Luminescence properties,diffuse reflectance spectra,and decay curves indicate that energy transfer?ET?occurs from Ce³⁺ to Mn²⁺,and the ET efficiency reaches its maximum?91%?at an Mn²⁺ content of 0.35.The diffuse reflectance spectrum shows that the co-doped phosphors have strong absorption around 320 nm,which is good for the anti-aging ability of an agricultural film.The absolute quantum efficiency of the Ca_1.6Sr?PO₄?2:0.15Ce³⁺,0.10Mn²⁺,0.15Na⁺ phosphor is?94%.And the co-doped phosphors exhibit good stability in water.Therefore,Ca₂Sr?PO₄?₂:Ce³⁺,Mn²⁺,Na+ phosphors have potential application as a light conversion material in agricultural films.