Supernatant Controlled Synthesis of Monodispersed Zinc Sulfide Spheres and Multimers
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Supernatant Controlled Synthesis of Monodispersed Zinc Sulfide Spheres and Multimers Yanning Song and Chekesha M. Liddell Department of Materials Science and Engineering Cornell University, Ithaca, NY 14853 ABSTRACT Uniform zinc sulfide spheres and multimers ranging in size from ~ 90 nm to 1.0 micron were produced in large quantities by adding varying amount of supernatant in a secondary nucleation process. The particle morphology was investigated using scanning and transmission electron microscopy. Smaller particles exhibited increased surface roughness in the nitrate system. Characterization by x-ray and electron diffraction showed that the particles were built up from nanocrystallites. The relationship between the particle size, porosity, and refractive index was studied by modeling UV-Vis spectra using the Mie scattering method. Monodispersed zinc sulfide spheres and multimers in the size range from 100 nm to 600 nm can be used as high refractive index building blocks for photonic crystals with band gaps covering the entire visible spectrum as well as portions of the near-IR and UV regions. INTRODUCTION Photonic crystals are materials with a periodic refractive index (or dielectric constant) that creates an optical band gap [1]. They have enabled new ways to control light by preventing its propagating through the structure due to Bragg reflection, in the same way as semiconductors manipulate electrons [2]. The self-assembly of monodispersed colloidal particles [3] has been demonstrated as an easy route to the formation of three-dimensionally ordered structures with submicron periodicity. The structures prepared by these methods, have generally led to close packed structures such as facecenter cubic (FCC). Theoretical studies show that due to symmetry-induced degeneracy, only a pseudogap exists no matter how large the refractive index contrast in the crystal [4]. Larger and more stable band gaps can be generated by using non-spherical objects as the building blocks to form lattices with reduced symmetry [5]. However, only a few methods (i.e., ion irradiation [6] and geometrical confinement [7]) are available for the synthesis of non-spherical inorganic colloids with monodispersed size and well-defined shape. Zinc sulfide is a semiconductor with direct band gap of 3.54 eV and refractive index of 2.43 (488 nm), appropriate for photonic applications including photonic crystals. Monodispersed zinc sulfide particles have been prepared by several methods. For example, Wilhelmy and Matijevic [8] used the thermal decomposition of thioacetimide (TAA) to prepare micron sized spherical particles by aging the reaction mixture for several hours using a two step procedure. Celikkaya and Akinc [9] found that the particle size and size distribution is affected considerably by the chemical nature of the supporting anions and by the sulfide ion generation rate. Liddell et al. determined that non-spherical zinc sulfide multimers in the size range 600 nm to 3 microns could be synthesized by precipitation induced clustering
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