Fabrication of ZnO Coated ZnS:Mn Nanoparticles
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Fabrication of ZnO Coated ZnS:Mn2+ Nanoparticles Shinji Ishizaki, Yusuke Kusakari, and Masakazu Kobayashi Laboratory for Material Science and Technology, Waseda University, 2-8-26, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan ABSTRACT The fabrication of ZnO coated ZnS:Mn2+ nanoparticles were achieved using simple methods. ZnS:Mn2+ nanoparticles were prepared by a mechanical milling method. Coating of ZnO was then performed using a simple chemical method. Structural properties were evaluated by the X-ray powder diffraction (XRD) and the transmission electron microscope (TEM). Optical properties of the ZnO coated ZnS:Mn2+ nanoparticle were characterized by exciting the particle included solvent using an UV-LED (400 nm). Bright orange color fluorescence was observed, and the fluorescence intensity of ZnO coated ZnS:Mn2+ nanoparticles was enhanced as compared to uncoated ZnS:Mn2+ nanoparticles. The capping of the core surface has probably terminated surface defects of ZnS:Mn2+ nanoparticles, and resulted in the improved fluorescence intensity. INTRODUCTION In recent years, semiconductor nanoparticles are paid attention because of their various properties differing from bulk materials and their considerable promise for numerous applications in various fields. The reduction in the particle size of materials down to nanometers, however, increases the amount of surface defects. The ratio of atoms at the particle surface is increased significantly as compared to that in bulk. Since the surface defect usually act as luminescence killers, the surface modification of nanoparticles is crucial to achieve better optical properties. The coating of nanoparticles (core) by another material (shell) was a widely studied method to terminate those defects and to improve the optical properties [1-8]; Core/Shell nanoparticles could exhibit enhanced photoluminescence properties [1,3-8]. Serious disadvantages of conventional methods to prepare core/shell structures include complicated processes and use of poisonous gasses such as H2S [6]. In this study, a simple method to prepare the Core/Shell structure was demonstrated. The fabrication of ZnS:Mn2+ nanoparticles were performed using a mechanical milling method. This method would own a great advantage in the simplicity of the fabrication process and the capability for the mass production. ZnS:Mn2+ is a widely studied phosphor material for the orange color [5-12], and generally synthesized using chemical methods such as precipitation methods [7-11] and a reverse micelle method [6,12]. ZnO was focused in this study for the shell material. It had been reported that the cathodoluminescent lifetime of the ZnO coated phosphor was dramatically improved [13]. The particle size and distribution of ZnS:Mn2+ nanoparticles were studied using the transmission electron microscope (TEM) as well as the X-ray powder diffraction (XRD) method. The structural properties of the core/shell nanoparticle were investigated by the 2 dimensional mapping of the transmission electron microscope energy dispe
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