Synthesis And Properties Of Single-phased White-emitting Phosphors For Uv-based White-LEDs

White light-emitting-diodes(LEDs) have gained much attentions due to their superior properties such as high energy efficiency, long-lifetime, environment-friendly and safety features. At present, phosphor-converted white LEDs is the most common way to achieve white light. Especially, white LEDs based on ultraviolet(UV) chip have got a giant development, because UV light is insensitive to human eyes. Therefore, white light achieving in this way is only determined by the phosphors. Hence, it is of great importance to synthesis high quality phosphors for fabricating high efficiency and stability white LEDs. In this paper, we have prepared a series of rare earth ions activated oxy-compound white light single-phased phosphors for UV-based whith-LEDs, and the luminescence properties of the obtained phosphors have been investigated in more detail. The results are as follows:1. Single-phased white light and blue light emitting phosphors have been obtained by doping Dy3+ and Tm3+ in GdNbO4 host via a Sol-Gel synthesis method. Under the excitation of UV light, the GdNbO4:Dy3+ sample emits white light and the CIE chromaticity coordinate of GdNbO4:0.02Dy3+ are determined to be x = 0.3214, y = 0.3427. Under the excitation of low voltage electron beams, the luminescence properties of GdNbO4:Dy3+ is similar to its PL. For GdNbO4:Tm3+ phosphor, under the excitation of UV light, it shows the characteristic emission of Tm3+, such as, 1D2-3F4(457 nm) and 1D2- 3H6(361 nm) transitions. While the CL spectrum is almost only consist of the transition of 1D2- 3F4(457 nm) of Tm3+. The CL intensity and color purity of GdNbO4:Tm3+ is higher than that of the commercial FED phosphor Y2SiO5:Ce3+(blue, Product No.1047, Nichia Kagaku Kogyo Kabushiki, Japan).2. YNbO4:RE3+(RE = Eu/Tb/Dy) phosphors with high purity white color were also prepared via a Sol-Gel process. The results indicate that the YNbO4:RE3+(RE = Eu/Tb/Dy) show not only the emission of host lattice, but also the characteristic emission of activators under the excitation of UV light. Energy can transfer from YNbO4 host lattice to activators and the energy transfer efficiency for each activator is diverse. The energy transfer efficiency of YNbO4:RE3+(RE = Eu/Tb/Dy) are determined to be 83.7%, 91.5% and 46.0% for YNbO4:Eu3+, YNbO4:Tb3+ and YNbO4:Dy3+ phosphors respectively. Under the excitation of UV light, the emission color of YNbO4:Eu3+,Tb3+ contain blue(from YNbO4 host lattice), red(from Eu3+) and green(from Tb3+) and the emission color could be easily adjusted in the region of blue, blue-green, green, purple, red and white light by adjusting the concentration and relative content of Eu3+ and Tb3+. Under the excitation of 259 nm, the YNbO4:0.015Eu3+,0.02Tb3+ emits white light and the CIE chromaticity coordinates are determined to be x = 0.3275, y= 0.3382, which are closed to the ideal white light(x=0.333, y= 0.333). The similar situation holds for Eu3+ and Dy3+ co-doped YNbO4 phosphors, white light with high purity can also be obtained in YNbO4:Dy3+,Eu3+. The emission color could be adjusted in the region of blue, blue-white, purple, red and white light by properly adjusting the concentration and relative content of Eu3+ and Dy3+. Under the excitation of 255 nm UV light, the YNbO4:0.02Dy3+,0.04Eu3+ emits white light and the corresponding CIE chromaticity coordinates are x = 0.3445, y = 0.2592. The CL spectra of YNbO4:Eu3+,Tb3+ and YNbO4:Dy3+,Eu3+ phosphors were similar to their PL emission spectra, and the CL intensity increases with raising the exciting voltage and filament current.3. Dy3+ and Eu3+-doped(LiGd)0.5TiO3 phosphors were prepared by a Sol-Gel method. However, white light cannot be observed when Dy3+ single-doped in the host due to that the blue emission(487 nm, 4F9/2- 6H15/2) intensity is higher than yellow emission(580 nm, 4F9/2- 6H13/2). The optimal doping concentration for Dy3+ in(LiGd)0.5TiO3 host is 4 % of Gd. While a single-phased white-emitting phosphor could be obtained by doubly doping the Eu3+ and Dy3+ in(Li Gd)0.5TiO3 host. Such as the(Li Gd)0.5TiO3:0.04Dy3+,0.10Eu3+ emits white light under the excitation of 391 nm and the corresponding CIE chromaticity coordinates are determined to be x = 0.3691, y = 0.3352. Moreover, energy can be transferred from Dy3+ to Eu3+ in(Li Gd)0.5TiO3 host, and the energy transfer efficiency can reach up to 42.0% when Eu3+ doping concentration is 0.30.4. Eu2+, Eu3+ and B3+-doped SrAl4O7 phosphors were prepared via a Sol-Gel process. The results indicate that the emission wavelength of Eu2+ are obviously different when B3+ ions exist in the SrAl4O7 host. The emission peaks and intensities of Eu2+ could be adjusted by doping B3+ in SrAl4O7 host lattice. The emission spectra of SrAl4O7:0.25B3+,Eu2+,Eu3+ phosphors under the excitation of UV, consist of two broad bands of Eu2+ ions peaking at 410 and 474 nm, and two characteristic line emissions of Eu3+ ions peaking at 596 and 617 nm. The relative emission intensities of Eu2+ and Eu3+ could be adjusted by optimizing the doping concentration and reduction conditions. A single-phased white-emitting has been realized in Eu2+, Eu3+ and B3+-doped SrAl4O7 phosphors. For example, the SrAl4O7:0.25B3+,0.03Eu2+/3+ phosphor exhibits white light emission under the excitation of 383 nm and the CIE chromaticity coordinates are determined to be x = 0.3251, y= 0.2823. The color purity further could be improved by co-doping Tb3+ in SrAl4O7:B3+,Eu2+,Eu3+ phosphors.