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Open Access Publications from the University of California

Synthesis and luminescence properties of rare earth activated phosphors for near UV-emitting LEDs for efficacious generation of white light

Abstract

Solid state white-emitting lighting devices based on LEDs outperform conventional light sources in terms of lifetime, durability, and luminous efficiency. Near UV-LEDs in combination with blue-, green-, and red-emitting phosphors show superior luminescence properties over the commercialized blue-emitting LED with yellow-emitting phosphors. However, phosphor development for near UV LEDs is a challenging problem and a vibrant area of research. In addition, using the proper synthesis technique is an important consideration in the development of phosphors. In this research, efficient blue-, green-yellow, red- emitting, and color tunable phosphors for near UV LEDs based white light are identified and prepared by various synthetic methods such as solid state reaction, sol-gel/ Pechini, co-precipitation, hydrothermal, combustion and spray-pyrolysis. Blue-emittingLiCaPO₄:Eu²⁺, Green/yellow- emitting (Ba,Sr)₂SiO₄:Eu²⁺, color tunable solid solutions of KSrPO₄₋(Ba,Ca)₂SiO₄:Eu²⁺, and red-emitting (Ba,Sr, Ca)₃MgSi₂O₈:Eu²⁺,Mn²⁺ show excellent excitation profile in the near UV region, high quantum efficiency, and good thermal stability for use in solid state lighting applications. In addition, different synthesis methods are analyzed and compared, with the goal of obtaining ideal phosphors, which should have not only have high luminous output but also optimal particle size (~150 - 400 nm) and spherical morphology. For Sr₂SiO₄:Eu²⁺, the sol-gel method appears to be the best method. For Ba₂SiO₄:Eu²⁺, the co- precipitation method is be the best. Lastly, the fabrication of core/SiO₂ shell particles alleviate surface defects and improve luminescence output and moisture stability of nano and micron sized phosphors. For nano- sized Y₂O₃:Eu³⁺, Y₂SiO₅:Ce³⁺,Tb³⁺, and (Ba,Sr)₂SiO₄, the luminescence emission intensity of the core/shell particles were significantly higher than that of bare cores. Additionally, the moisture stability is also improved by SiO₂ shells, the luminescence output of SiO₂ coated green emitting Ca₃SiO₄Cl₂:Eu²⁺ and blue emitting Ca₂PO₄Cl:Eu²⁺ phosphors is comparable to that of fresh phosphors although bare phosphors shows significant luminescence quenching after water exposure

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