ZnO Coated Nanoparticle Phosphors
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ZnO Coated Nanoparticle Phosphors Masakazu Kobayashi 1,2 1
Department of Electrical Engineering and Bioscience, Waseda University, 3-4-1 Okubo Shinjuku Tokyo, 169-8555 Japan 2 Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-826 Nishiwaseda, Shinjuku Tokyo, 169-0051 Japan ABSTRACT Conventional phosphor materials are doped ternary or quaternary compounds; hence it would be difficult to prepare nanoparticles of those materials by build up methods. Ba2ZnS3:Mn (BZS), SrGa2S4:Eu, and BaAl2S4:Eu nanoparticles were prepared by a break down method, namely the ball-milling method. Transmission electron microscopy (TEM) and TEM- energydispersive X-ray spectroscopy (EDX) measurements showed several-nanometer-size stoichiometric and dispersed nanoparticles were achieved. ZnO-coating was performed and the uniform coating layers were formed on the phosphor nanoparticles. The ZnO-coated nanoparticles exhibited an improved stability in Photoluminescence. Red color phosphor material, namely BZS, was ball-milled and sprayed on the glass substrate. Mn doped BZS absorbs ultra violet light and emits red light peaking at around 640nm. When the single crystal Si solar cell was placed under the transparent nanoparticle layer, short wavelength light was absorbed and converted to long wavelength light. INTRODUCTION Electroluminescence devices are widely expected to have the high brightness and high color purity because of the self light-emission. II-VI compounds such as ZnS and SrS, and thiogalate sulfides such as (Ca,Sr)Ga2S4 have been studied as qualified phosphors, where some rare-earth-ions are introduced as emission centers. There are two types of the light emission mechanism in nanoparticle phosphor materials. In the case of CdSe/ZnS [1-3] and CdSe/CdS [4,5] nanoparticles, the recombination of electron and hole are used for the light emission. On the other hand, ZnS:Mn2+ nanoparticles [6], use an inner shell transition to obtain the light emission. We have prepared CdSe and ZnS:Mn2+ nanoparticles by a break down method, namely the ball-milling method. Nanoparticles with good optical properties were achieved [7,8]. The ball milling method is a simple method and would be suited to mass production. Since Cd and Se are harmful elements to the human body and CdSe nanoparticles require rigorous control of particle size under 10nm to tune the fluorescence color and the peak wavelength [9], ternary sulfide based phosphors such as Ba2ZnS3:Mn (BZS), SrGa2S4:Eu (SGS), and BaAl2S4:Eu (BAS) have been focused on as the alternative phosphor nanoparticle materials which do not include harmful elements. Emission wavelength of these phosphor nanoparticles would be independent of the particle size since the doped impurities would be the origin of the fluorescence. In this paper, these nanoparticle phosphors were coated by ZnO. The bandgap of ZnO is wider than CdSe and CdS, but smaller than ZnS and other phosphor materials. Coating of the
Figure 1 TEM image of BZS nanoparticles
nanoparticle is generally used to for
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