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The white LED as an energy-efficient form of electrical lighting that is used widely for general white lighting sources. The development of the blue InGaN LED chip has made it possible to produce a conventional two-band white LED by coating a yellow-emitting Y3Al5O12:Ce phosphor onto a blue LED chip. The combination of blue emission from a blue LED chip and yellow emission form a Y3Al5O12:Ce phosphor is perceived as white light by the human eye. Nevertheless, these two-band white LEDs are unable to produce all the nature-equivalent colors, particularly in the red region. To improve the color rendering index of a white LED, the three-band white LED has been fabricated by coating a mixture of a greenand red-emitting phosphors onto a blue LED chip. CaS:Eu, Ba2Si5N8:Eu , NaY(W,Mo)2O8:Eu , CaMoO4:Eu , and Y2(MoO4)3:Eu 3+ have been used redemitting phosphors for three-band white LED because these phosphors show strong absorptions at 465 nm and strong emissions in the red region. Most of these phosphors can be synthesized by simple solid state reactions, and a few micron-sized phosphors have been obtained. When micronsized inorganic phosphors are coated on a blue LED chip, a large fraction of the blue light is lost by back scattering due to the micron-sized phosphors. Inorganic nanophosphors do not show any scattering effect in the visible region. Therefore, inorganic nanophosphors need to be developed for a highly efficient white LED by reducing back scattering. On the other hand, nanophosphors agglomerate easily in solvents, and little is known regarding the preparation of a suspension of inorganic nanophosphors. This paper reports the first simple method for preparing a transparent suspension of BaMoO4:Eu ,Na nanophosphors and the feasibility of this BaMoO4:Eu ,Na suspension for a threeband white LED. Figure 1 shows the powder X-ray diffraction (XRD) patterns of the BaMoO4:Eu ,Na phosphor prepared by a hydrothermal process at 80 °C. The XRD patterns of the BaMoO4:Eu ,Na phosphor matched the tetragonal BaMoO4 (JCPDS 29-0193, a = 0.5580 nm, c = 1.2821 nm). Since Eu and Na ions substitute for Ba ions in the host BaMoO4 structure, it was confirmed that the host BaMoO4 was synthesized without impurities according to the XRD patterns. Figure 2(a) and 2(b) shows the excitation and emission spectra of the BaMoO4:Eu ,Na phosphor, respectively. BaMoO4:Eu ,Na phosphor shows strong absorption in the ultraviolet region at approximately 300 nm, which is due to the O → Mo charge transfer transition in BaMoO4. 21

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Preparation and Photoluminescence Properties of Transparent Red-Emitting Suspension of BaMoO4:Eu3+,Na+Nanophosphor for a Three-Band White LED