Performance Changes of Surface Coated Red Phosphors with Silica

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Performance Changes of Surface Coated Red Phosphors with Silica Nanoparticles and Silica Nanocomposites Y. S. Chung, M. Y. Jeon, and C. K. Kim* School of Chemical Engineering and Materials Science, Chung-Ang UniVersity, 221 Huksuk-dong. Dongjak-gu, Seoul, 156-756, Korea

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 (Sto¨ber 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 PMMAsilica 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. Introduction Phosphors have been widely used in various applications, including plasma display panels (PDP), cathode-ray tubes (CRT), field emission displays (FED), and light emitting diodes (LED).1-18 In particular, white LEDs fabricated from phosphors are gaining significant attention because they have many advantages over the existing incandescent and halogen lamps such as reliability, energy savings, lower maintenance, and increased safety.8-18 A blue light GaN (gallium nitride) LED chip loaded with a yellow phosphor YAG (Y3Al5O12:Ce) is now widely used as a conventional white LED.8-12 However, the combination of a blue LED with yellow phosphor appears to have serious drawbacks such as color shift and a low color rendering index (CRI ≈ 60-70 range) due to two-color mixing. To overcome these disadvantages, the white LEDs based on red, green, and blue primary colors have been studied.12-18 White LEDs can be achieved by combining red, green, and blue LEDs, precoating those three phosphor colors onto a UV LED, or precoating green and red phosphors onto a blue LED. In the current tricolor phosphors of the near-UV InGaN-LED chips, Y2O2S:Eu3+ for red, ZnS for green, and BaMgAl10O17Eu2+ for blue are generally used. Unfortunately, the use of a phosphor mixture containing a large amount of red phosphors is required for sufficient color rendering because the photoluminescence (PL) intensity of the Y2O2S:Eu3+ red phosphor is lower than that of the green and blue phosphors.13,14 Furthermore, the lifetime of the Y2O2S: Eu3+ red phosphor is inadequate under near-UV irradiation due to its instability.14-18 Consequently, the problems with red phosphors should be solved for the advancement of white LEDs. To overcome the drawbacks of the Y2O2S:Eu3+ red phosphor, numerous studies have been performed.14-28 Various alkaline earth sulfides with rare earth ion activators exhibiting performance advantages over the Y2O2S:Eu3+ red phosphors have been * To whom correspondence should be addressed. E-mail: ckkim@ cau.ac.kr.

introduced. However, the application of alkaline earth sulfides has been limited by their instability when exposed to moisture and other atmospheric components. Even though numerous studies have been performed to enhance the performance and stability of alkaline earth sulfides by changing materials and synthesis methods, better results are still being pursued.21-27 Phosphors are often coated with oxides such as silica, Y2O3, ZnO, MgO, and organic polymers to meet the stringent requirements of display devices and LEDs.19-28 The use of coatings is known to improve the adhesion strength between phosphors and glass substrates, to yield excellent stability of the phosphors when exposed to moisture and other atmospheric components, and to protect phosphors from irradiation damage.

Figure 1. Changes in particle size with reaction temperature. Note that the reaction mixture is composed of TEOS (0.28 mol), water (10 mol), NH4OH (2 mol), and methanol (1 L).

10.1021/ie8007488 CCC: $40.75  2009 American Chemical Society Published on Web 12/01/2008

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Figure 2. HR-TEM microphotographs of silica nanoparticles synthesized at various reaction temperatures: (a) 30 °C (average diameter: 50 nm), (b) 60 °C (average diameter: 30 nm), (c) 70 °C (average diameter: 20 nm), and (d) 80 °C (average diameter: 10 nm).

Figure 3. Size change of the silica nanoparticles as a function of water content at two different reaction temperatures.

The coating has to be transparent, and a precise amount of coating must cover the surface of the individual phosphor particle homogeneously to minimize the decrease in PL intensity. From a stability point of view, a layer-like homogeneous coating would be also preferred over an island-like inhomogeneous coating. Since a layer-like homogeneous coating could be

Figure 4. Effects of reaction temperature on silica particle size when fixed amounts of VP monomer (0.2 mol) and AIBN (0.5 wt% of VP monomer) were added to the reaction mixture used for silica nanoparticles synthesis in Figure 1.

expected when smaller particles having even size were used, monodispersed nanoparticles without forming aggregates should be prepared for the phosphor coating.26-29 In this study, the coating of phosphors was performed to fabricate phosphors using four different techniques, P1, P2, P3,

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Figure 5. HR-TEM micrographs of silica nanoparticles synthesized at various reaction temperatures by adding fixed amounts of VP monomer and AIBN to the reaction mixture used for silica nanoparticle synthesis in Figure 1: (a) 60 °C (average diameter: 25 nm), (b) 70 °C (average diameter: 15 nm), (c) 80 °C (average diameter: 10 nm), and (d) 90 °C (average diameter: 5 nm).

and P4, which have a performance advantage over uncoated phosphors. Silica nanoparticles were coated on the phosphor surface via a dip-coating process (P1) and a sol-gel process (P2). The phosphor surface was also coated with a poly(methylmethacrylate) (PMMA)-silica nanocomposite using two different techniques. A reaction mixture containing silica nanoparticles, MMA monomer, initiator, and phosphor formed a PMMA-silica nanocomposite on the phosphor surface (P3). PMMA-silica nanocomposite was also formed on the phosphor surface by reacting methylmethacrylate (MMA) monomer and tetraethyl orthosilicate (TEOS), simultaneously (P4). Changes in the morphology, PL intensity, and stability of the coated phosphors were also discussed. Materials and Procedures High purity Y2O2S:Eu3+ red phosphor was supplied by Samsung SDI (Suwon, Korea). A sol-gel reaction was facilitated by hydrolysis and condensation of tetraethylorthosilicate (TEOS, 98+%, Aldrich Chemical Co., USA) in absolute methanol (MeOH, Aldrich Chemical Co., USA) with a base catalyst. Reagent grade ammonium hydroxide (30% NH3, Samchun Pure Chemical Co., Korea) was used as the base catalyst. 1-Vinyl-2-pyrrolidone (VP 99+%, Aldrich Chemical Co., USA) and 2,2′-Azobis(2-methylpropionitrile) (AIBN, Junsei Chemical Co., Japan) were used as the monomer and initiator for the synthesis of poly(1-vinyl-2-pyrrolidone) (PVP), respectively. Methyl methacrylate (MMA, 99+%, Aldrich Chemical Co., USA) was also used as a monomer for the synthesis of PMMA. Nearly monodispersed and spherical silica nanoparticles were synthesized by a sol-gel process, i.e., the Sto¨ber method.30,31

The size of silica nanoparticles was controlled by varying the amounts of reactants (TEOS and water) and the catalyst (ammonium hydroxide), and by the changing reaction temperature and medium. The reaction was performed at various temperatures for 1 h. The effects of varying reactants, catalyst, and solvent amounts on particle size had been studied previously in our laboratory.31 Since aggregates with primary silica nanoparticles were formed when their size was below about 25 nm, the reaction temperature and medium were varied to prevent aggregate formation. The details of the reaction procedures and related results are discussed in the following section. Phosphor coating with silica nanoparticles (or PMMA-silica nanocomposite) was performed via four different methods, P1, P2, P3, and P4. The details of each are described below. P1: The proper amount of phosphor (10 g) was dispersed in a methanol (1 L) solution containing monodispersed silica nanoparticles (sample 3, 5 nm, 5 g) for 2 h. The resulting phosphor powders were separated via centrifugation at a rotation speed of 3000 rpm for 20 min. Then, the phosphor powders were washed with deionized water several times and dried at 80 °C for 1 day. P2: Methanol (1 L) solution containing TEOS (0.28 mol), water (20 mol), NH4OH (2 mol), VP (0.4 mol), and phosphor powders (10 g) was reacted at 80 °C for 1 h. Note that the composition of the reaction mixture and reaction conditions were identical to those for the synthesis of monodispersed silica nanoparticles (sample 3, 5 nm), with the exception of the addition of phosphor. The resulting phosphor powders were recovered with the same procedures described for P1. P3: The proper amount of phosphor (10 g) was dispersed in methanol (1 L) solution containing silica nanoparticles (sample 3, 5 nm, 5 g), MMA monomer (0.2 mol), and AIBN (0.5 wt % of MMA), and then reacted at 80 °C for

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Figure 6. Size and morphology changes of the formed silica nanoparticles when the VP content in the reaction mixture was changed from 0.2 to 0.4 mol: (a) sample 1 in Figure 4, (b) silica nanoparticles (5 nm) formed by changing the VP content in the reaction mixture for the synthesis of sample 1 from 0.2 to 0.4 mol, (c) sample 2 in Figure 4, and (d) silica nanoparticles (3 nm) formed by changing the VP content in the reaction mixture for the synthesis of sample 2 from 0.2 to 0.4 mol.

1 h. The resulting phosphor powders were recovered with the same procedures as described for P1. P4: Methanol solution containing TEOS (0.28 mol), water (20 mol), and NH4OH (2 mol) was prepared. MMA (0.4 mol), AIBN (0.5 mol% of MMA), and phosphor powders (10 g) were added to the methanol solution, and then the solution was reacted at 80 °C for 1 h. The resulting phosphor powders were recovered with the same procedures as described for P1. The morphologies of silica nanoparticles and those of Y2O2S: Eu3+ before and after coating were investigated with a high resolution transmission electron microscope (HR-TEM, model: JEM-3010, Japan). To obtain an average particle size, 10 TEM images were obtained from ten different specimens for each sample, and then image analysis software (model: Series ver. 4.0, BMI, Korea) was used to calculate the average particle size. Phosphor coating with silica (or PMMA-silica nanocomposite) was also confirmed with FT-IR (model: Magna 750, Nicolet, USA) and energy dispersive spectroscopy (EDS, model: ED2000, Oxford, England). The photoluminescence (PL) was measured with a monochromatic and photomultiplier detector (model: LH1751300, ORC, Korea) at 30 °C using a particular wave (254 nm) from a xenon lamp (150 W) as an excitation source. All of the PL spectra exhibited here were recorded using band widths of 1 nm at emission slit. Five specimens of each phosphor were prepared for the PL examination. All phosphors were tested in duplicate, and their results were averaged. For the long-term stability test of the phosphors, changes in the cathodeluminescence (CL) with aging time was measured under a bombardment 10 kV electron beam with an average current density of 45 µA/ cm2 for 30 min. To examine the thermal stability and the moisture resistance of the phosphors before and after coating, samples were aged in a temperature-controlled humidity cham-

ber (model: TH-G-180, Jeiotech, Korea) at 100 °C and 80% relative humidity and then changes in the PL intensity with aging time were tested. Result and Discussion Changes in the Size of Silica Nanoparticles. Nearly monodisperse and spherical silica particles were synthesized by the Sto¨ber method. In a previous study, silica nanoparticles ranging from 10 to 450 nm were synthesized by changing the solvent (methanol or ethanol), amount of reactants (water and TEOS), and catalyst (ammonium hydroxide).31 The decline in the particle size was observed by decreasing the TEOS and NH4OH concentrations, and by increasing the water concentration. Nearly monodispersed silica nanoparticles having various sizes from 50 to 450 nm could be synthesized when ethanol was used as a solvent, while silica nanoparticles having various sizes from 10 to 50 nm could be synthesized when methanol was used as a solvent. It is known that particle size varies with the solvent used due to the differences in the size of nuclei formed in each solvent.32 Nearly monodisperse and spherical nanoparticles were formed when particles larger than 25 nm were formed. However, aggregates formed with primary silica nanoparticles (network structure) below about 25 nm. To prevent the formation of aggregates and to further reduce the size of the silica nanoparticles, the reaction temperature and reaction medium were changed in this study. Figures 1 and 2 denote changes in particle size with reaction temperature when concentrations of TEOS (0.28 mol), water (10 mol), and NH4OH (2 mol) were fixed and methanol (1 L) was used as a solvent. The particle size decreased with increasing reaction temperature and then remained constant. The

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Figure 7. HR-TEM micrographs of the uncoated and coated phosphors with silica nanoparticles (or PMMA-silica nanocomposites): (a) uncoated Y2O2S: Eu3+ phosphor, (b) coated Y2O2S:Eu3+ phosphor with silica by coating method P1, (c) coated Y2O2S:Eu3+ phosphor with silica by coating method P2, (d) coated Y2O2S:Eu3+ phosphor with silica by coating method P3, and (e) coated Y2O2S:Eu3+ phosphor with silica by coating method P4.

size of the nearly monodisperse silica nanoparticles prepared at the reaction temperature of 30 °C (50 nm) decreased to approximately 10 nm when the reaction temperature was greater than or equal to 80 °C. Nearly monodisperse and spherical nanoparticles were formed with particles larger than 30 nm. However, aggregates formed with primary silica nanoparticles below about 20 nm. These results have several possible explanations. For example, when smaller particles are formed, more particles produced. Since small particles have a higher surface tension than large particles, they easily aggregate to make their surface to be more stable. Therefore, primary silica particles, the sizes of which are less than about 20 nm, formed a network structure caused by aggregation, while spherical particles greater than 20 nm were formed without aggregation. The size of silica nanoparticles could be reduced from 50 to 10 nm by increasing the water content at a reaction temperature of 30 °C. As shown in Figure 3, the particle size was also further reduced by increasing the water content and reaction temperature from 30 °C to 80 °C. However, aggregates with primary silica nanoparticles still formed when particle sizes were less than about 20 nm. The results obtained here indicate that the size of silica nanoparticles could be further reduced by increasing

reaction temperature but problems related to the formation of aggregates cannot be solved by changing only the reaction temperature. To fabricate nearly monodisperse silica nanoparticles less than 20 nm without forming aggregates, VP monomer was added to the reaction mixture. When VP monomer was added to the reaction mixture with the proper amount of initiator (AIBN), the formation of silica nanoparticles by hydrolysis and condensation reactions, and the formation of PVP by radical polymerization could simultaneously occur. Figure 4 shows the effects of reaction temperature on the silica particle size when fixed amounts of VP monomer (0.2 mol) and AIBN (0.5 wt% of VP monomer) were added to the reaction mixture as shown in Figure 1. Particle size was further reduced by adding VP monomer to the reaction mixture regardless of the reaction temperatures. As shown in Figure 5, aggregates formed with primary silica nanoparticles were not observed when the particle size was greater than or equal to 10 nm. However, aggregates were still formed when the silica particle size was about 5 nm. To prevent aggregate formation, the content of VP monomer in the reaction mixture was increased from 0.2 to 0.4 mol, while the contents of other components in the reaction mixture were

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Figure 8. HR-TEM micrographs of the coated phosphors observed at high magnification: (a) coated Y2O2S:Eu3+ phosphor with silica by coating method P1, (b) coated Y2O2S:Eu3+ phosphor with silica by coating method P2, (c) coated Y2O2S:Eu3+ phosphor with silica by coating method P3, and (d) coated Y2O2S:Eu3+ phosphor with silica by coating method P4.

Figure 9. EDS pattern of phosphor coated with silica nanoparticles by coating method P2.

fixed. Particle size was further reduced from 10 to 5 nm without the formation of aggregates as shown in Figure 6. The size of particles (5 nm primary particles in Figure 5) was also reduced to about 3 nm without the formation of aggregates. These results indicate that aggregate formation can be prevented by adding the proper amount of VP monomer to the reaction mixture. Since PVP is soluble in water and methanol, the formed silica particles

could be easily separated from the resulting solution via centrifugation. In summary, nearly monodisperse silica nanoparticles, which are smaller than 25 nm, could be fabricated without primary particle aggregation by adding the proper amount of VP monomer. Silica nanoparticles approximately 5 nm in diameter, which do not form aggregates, were used for the phosphor coating.

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Figure 10. FT-IR spectrum of the uncoated and coated phosphors with PMMA-silica nanocomposite by coating method P4: (a) uncoated phosphors and (b) coated phosphor with PMMA-silica nanocomposite by coating method P4.

Figure 11. Changes in PL intensities of the Y2O2S:Eu3+ phosphors with different coating methods.

Figure 12. Normalized CL intensity as a function of the cumulated aging time for uncoated and coated phosphors by coating method P4.

Phosphor Coating with Silica Nanoparticles and Silica-PMMA Nanocomposites. Phosphor coating with silica nanoparticles (or silica-PMMA nanocomposite) was performed using four different methods, P1, P2, P3, and P4. To enhance

Figure 13. Normalized PL intensity as a function of the cumulated aging time for uncoated and coated phosphors by coating method P4. Note that phosphors were aged in a temperature-controlled humidity chamber at 100 °C and 80% relative humidity.

stability, an even layer-like coating would be much more preferable than an uneven coating. However, a thicker coating on the phosphor surface might result in decreased luminescence. Because of this, an evenly coated phosphor with proper thickness is desirable. Regardless of the coating processes, the coated phosphor apparently had the same color (red) as the uncoated phosphor. However, changes in the detailed morphology were observed. The uncoated phosphor had a smooth and clean surface, as shown in TEM photographs [Figure 7(a)]. Morphologies of the coated phosphors depended on the coating methods as shown in Figures 7 and 8. The surfaces of the phosphors were rough when they were coated using methods P1, P2, and P3, while the phosphor surface was evenly coated with method P4. When a phosphor was coated using method P1, an islandlike uneven coating was clearly observed. Methods P2 and P3 gave a better result in covering the phosphors surface with coating materials than that of P1. The surface roughness was increased by adding MMA monomer to the reaction mixture (method P3). When phosphor coating was performed with method P4, the phosphor surface was covered continuously and evenly with coating materials. The formation of the coating layer was also confirmed with EDS and FT-IR. Figure 9 shows the EDS pattern of the phosphor coated with silica nanoparticles using coating method P2. The peak resulted from the silica nanoparticles, which was not observed with uncoated phosphor, but was observed in the phosphor-coated particles by methods P1 and P2. Figure 10 shows the FT-IR spectrum of the uncoated phosphors and that of the PMMA-silica nanocomposite-coated phosphor using coating method P4. The characteristic stretching peak associated with carbonyl groups in PMMA (around 1740 cm-1) and that associated with the Si-O-Si linkages (around 1100 cm-1), which were not observed with uncoated phosphors, were observed in the FT-IR spectrum of the coated phosphor. These typical peaks resulting from PMMA-silica nanocomposites were also observed in the FT-IR spectrum of the phosphor coated via method P3. Changes in the PL Intensity of Phosphors with Surface Coating. Changes in the luminous intensities of the coated phosphors were investigated. As shown in Figure 11, the highest luminous intensity of the phosphors before and after surface

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coating was always approximately 627 nm, which demonstrates that the phosphors emit red visible light. Note that the emission spectra of the uncoated phosphor were in agreement with those of the coated phosphors regardless of the coating methods. A slight decrease in PL intensity was observed when phosphors were coated using methods P1 and P2, while an increase in PL intensity was observed when phosphors were coated using methods P3 and P4. Nonhomogeneous coating of the phosphors (methods P1 and P2) with silica might result in a decrease in PL intensity. A higher PL intensity of the phosphors coated by methods P3 and P4 may arise for the following reasons. Generally, organic-inorganic nanocomposite materials show a higher transparency than inorganic xerogel. Furthermore, due to a good refractive index match between PMMA (1.49) and silica (1.46), coating a surface composed of PMMA-silica composite has better transparency than that composed of silica alone.24,25,33 A decrease in the mismatch between refractive indexes at the coating surface reduces optical scattering. Because of this, the increased transparency might enhance emission intensity. Light diffraction might also enhance the PL intensity of the phosphors coated with PMMA-silica nanocomposites because visible light from the phosphor could be diffracted and multiplied by silica particles dispersed in the PMMA matrix on the phosphor surface.24,33 The most direct evidence of the protective nature of the phosphor coating could be found by CL aging analysis. The accelerated aging test was carried out with an electron beam at an acceleration voltage of 10 kV and an average current density of 45 µA/cm2 for 30 min. Figure 12 shows the normalized CL intensity as a function of the cumulated aging time for the uncoated and coated phosphors. A decreasing intensity ratio for the uncoated phosphor, in contrast to a nearly constant value for coated phosphors (method P4) with increasing bombardment time, was observed. The thermal stability and the moisture resistance of the phosphors before and after coating were also examined by aging them in a temperature-controlled humidity chamber at 100 °C and 80% relative humidity. As shown in Figure 13, a decreasing intensity for the uncoated phosphor, in contrast to a nearly constant value for coated phosphors (method P4) with aging time, was also observed. These results indicate that the PMMA-silica nanocomposite coating on the phosphors served as a protective layer by retarding the surface-related damage caused by irradiation and atmospheric components. Conclusions The surface of the Y2O2S:Eu3+ phosphor was coated using four different techniques for the enhancement of PL intensity and long-term stability. For the surface coating of the phosphors, nearly monodisperse silica nanoparticles having a 5-nm diameter were prepared by the Sto¨ber method. The formation of nanoparticle aggregates was prevented by simultaneously performing the hydrolysis and condensation reactions for the formation of silica nanoparticles using TEOS and radical polymerization for the formation of PVP from VP monomer. When phosphors were coated onto silica nanoparticles (5 nm) by dip coating or Sto¨ber methods, the phosphor’s surface was unevenly coated. Phosphors were also coated with PMMA-silica nanocomposite by two different methods. Even and continuous coating on the phosphors surface was observed when a PMMA-silica nanocomposite was formed from TEOS and MMA. A decrease in PL intensity was observed when phosphors were coated onto silica nanoparticles, while an increase in PL intensity was observed when phosphors were coated onto the PMMA-silica nanocomposite. A comparison of the PL intensity of the

phosphors coated using method P4 with that of the uncoated phosphors showed that the PL intensity enhancement in the coated phosphors was approximately 5%. Furthermore, the longterm stability of the former was better than that of the latter. Acknowledgment This study was supported by research grants from the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center (ARC). Literature Cited (1) Lehmann, W.; Ryan, F. M. Cathodoluminescence of CaS:Ce3+ and CaS:Eu2+ phosphors. J. Electrochem. Soc. 1971, 118, 477. (2) Chakrabarti, K.; Mathur, V. K.; Rhodes, F. J.; Abbundi, R. J. Stimulated luminescence in rare-earth-doped MgS. J. Appl. Phys. 1988, 64, 1363. (3) Shanker, V.; Tanaka, S.; Shiiki, M.; Deguchi, H.; Kobayashi, H.; Sasakura, H. Electroluminescence in thin-film CaS:Ce. Appl. Phys. Lett. 1984, 45, 960. (4) Pandey, R.; Sivaraman, S. Spectroscopic properties of defects in alkaline-earth sulfides. J. Phys. Chem. Solids 1991, 52, 211. (5) Swiatek, K.; Godlewski, M.; Niinisto, L.; Leskela, M. J. Optical recombination mechanisms in Eu2+-doped CaS and SrS thin films. J. Appl. Phys. 1993, 74, 3442. (6) Jia, D.; Jia, W.; Evans, D. R.; Dennis, W. M.; Liu, H.; Zhu, J.; Yen, W. M. Trapping process in CaSEu+2,Tm+3. J. Appl. Phys. 2000, 88, 3402. (7) Chung, C.; Jean, J. Protective magnesia coating on Y2O2S:Eu3+ phosphor. J. Am. Ceram. Soc. 2006, 89, 2726. (8) Jia, D.; Zhu, J.; Wu, B. Improvement of persistent phosphorescence of Ca0.9Sr0.1S: Bi3+ by codoping Tm3+. J. Lumin. 2000, 91, 59. (9) Ozaki, H.; Iwamoto, S.; Inoue, M. Effects of amount of Si addition and annealing treatment on the photocatalytic activities of N- and Si-codoped titanias under visible-light irradiation. Ind. Eng. Chem. Res. 2008, 47, 2287. (10) Mueller-Mach, R.; Mueller, G. O. Light-emitting diodes: research, manufacturing, and applications IV. Proc. SPIE 2000, 3938, 30. (11) Wu, H.; Zhang, X.; Guo, C.; Xu, J.; Wu, M.; Su, Q. Three-band white light from InGaN-based blue led chip precoated with green/red phosphors IEEE Photo. Tech. Lett. 2005, 17, 1160. (12) Niki, I.; Narukawa, Y.; Morita, D.; Sonobe, S.; Mitani, T.; Tamaki, H.; Mruazaki, Y.; Yamada, M.; Mukai, T. Third International Conference on Solid State Lighting. Proc. SPIE 2004, 5187, 1. (13) Hu, Y.; Zhuang, W.; Ye, H.; Zhang, S.; Fang, Y.; Huang, X. Preparation and luminescence properties of red-emitting phosphor for white led. J. Lumin. 2005, 111, 139. (14) Guo, C.; Huang, D.; Su, Q. Method to improve the fluorescence intensity of CaS red-emitting phosphor for white led. Mater. Sci. Eng., B 2006, 130, 189. (15) Guo, C.; Luan, L.; Chen, C.; Huang, D.; Su, Q. Preparation of Y2O2S:Eu3+ phosphors by a novel decomposition method. Mater. lett. 2008, 62, 600. (16) Pham-Thi, M. Optical studies of Cd1-xMnxTe films grown on (001)InSb by pulsed laser evaporation and epitaxy. J. Appl. Phys. 1992, 71, 2811. (17) Yamashita, N. Coexistence of the Eu2+ and Eu3+ centers in the CaO:Eu powder phosphor. J. Electrochem. Soc. 1993, 140, 840. (18) Wang, Z.; Liang, H.; Zhou, L.; Wu, H.; Gong, M.; Su, Q. Luminescence of (Li0.333Na0.334K0.333)Eu(MoO4) 2 and its application in near UV InGaN-based light-emitting diode. Chem. Phys. Lett. 2005, 412, 313. (19) Park, W.; Yasuda, K.; Wagner, B. K.; Summers, C. J.; Do, Y. R.; Yang, H. G. Uniform and continuous Y2O3 coating on ZnS phosphors. Mater. Sci. Eng., B 2000, 76, 122. (20) Velikov, K. P.; Van Blaaderen, A. Synthesis and characterization of monodisperse core-shell colloidal spheres of zinc sulfide and silica. Langmuir 2001, 17, 4779. (21) Feldmann, C.; Merikhi, J. Adhesion of colloidal ZnO particles on ZnS-type phosphor surfaces. J. Colloid Interface Sci. 2000, 223, 229. (22) Kominami, H.; Nakamura, t.; Sowa, k.; Nakanishi, y.; Hatanaka, Y.; Shimaoka, G. Low voltage cathodeluminescence properties of phosphors coated with In2O3 by sol-gel method. Appl. Surf. Sci. 1997, 113/114, 519. (23) garashi, T.; Kusunoki, T.; Ohno, K.; Isobe, T.; Senna, M. Degradation proof modification of ZnS-based phosphors with ZnO nanoparticles. Mater. Res. Bull. 2001, 36, 1317. (24) Novak, B. M. Hybrid nanocomposite materialssbetween inorganic glasses and organic polymers. AdV. Mater. 1993, 5, 422.

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ReceiVed for reView May 8, 2008 ReVised manuscript receiVed September 24, 2008 Accepted October 26, 2008 IE8007488