Performance Changes of Surface Coated Red Phosphors with Silica Nanoparticles and Silica Nanocomposites

Abstract

To improve the photoluminescence and long-term stability of the Y2O2S:Eu3+ phosphor, surface coatings with silica nanoparticles and poly(methyl methacrylate) (PMMA)-silica nanocomposites were performed via four different techniques. Phosphors were coated with nearly monodispersed silica nanoparticles (5 nm) by a dip-coating method and a sol−gel method (Stöber method). To fabricate the silica nanopariticles used for the phosphor coating, hydrolysis and condensation reactions for the formation of silica nanoparticles, and radical polymerization for the formation of poly(1-vinyl-2-pyrrolidone) were performed simultaneously. Phosphors were coated with PMMA-silica nanocomposites by using two different methods: by reacting silica nanoparticles and methyl methacrylate (MMA) monomer and by reacting mixtures containing MMA and tetraethylorthosilicate. Between these methods, the latter method exhibited the greatest enhancement of photoluminescence and long-term stability of the phosphors. When phosphors were coated with PMMA-silica nanocomposite by the second method, the PL intensity of Y2O2S:Eu3+ was enhanced approximately 5% over that of the uncoated phosphors. In contrast to a decrease in cathode luminescence (CL) intensity with increasing bombardment time for uncoated phosphor, a nearly constant CL intensity was observed for the phosphors coated with PMMA-silica nanocomposite by the latter method.

Changes in the moisture stability of CaS:Eu2+ phosphors with surface coating methods

“…[19][20][21] The detailed synthetic procedure for silica nanoparticles was described previously. 10,22 Phosphor coating with silica nanoparticles was performed via five different methods, P 1 , P 2 , P 3 , P 4 , and P 5 . The details of each are described below.…”

“…[1][2][3][4][5][6][7][8][9][10] In particular, white LEDs fabricated from phosphors are gaining significant attention because they have many advantages over the existing incandescent lamps. [6][7][8][9][10][11][12] Even though a blue light GaN (gallium nitride) LED chip loaded with a yellow phosphor is now widely used as a conventional white LED, this combination have serious drawbacks in the color rendering index. Because of this, the white LEDs based on red, green, and blue primary colors have been explored.…”

“…[3][4][5][6][7][8][9][10][11] Since the photoluminescence (PL) intensity of the red phosphor is lower than that of the green and blue phosphors, the use of a phosphor mixture containing a large amount of red phosphors is required for sufficient color rendering. [8][9][10] The alkaline earth sulfides are widely used as the red emitting phosphors. Among the red phosphors, CaS:Eu 2+ is considered as the most promising red emitting phosphor because of its a deep red emission with nice color purity.…”

“…Among the red phosphors, CaS:Eu 2+ is considered as the most promising red emitting phosphor because of its a deep red emission with nice color purity. [9][10][11][12][13] However, the application of CaS:Eu 2+ has been limited by their instability when exposed to moisture. The problems with red phosphors should be solved for the advancement of white LEDs.…”

“…[10][11][12][13][14][15][16] Various alkaline earth sulfides with rare earth ion activators exhibiting performance advantages over the CaS:Eu 2+ red phosphors have been introduced. However, the application of alkaline earth sulfides has been limited by their instability.…”