Synthesis and Control of ZnS Nanodots and Nanorods with Different Crystalline Structure from an Identical Raw Material S
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Synthesis and Control of ZnS Nanodots and Nanorods with Different Crystalline Structure from an Identical Raw Material Solution and the Excitonic UV Emission Masato Uehara1, Satoshi Sasaki2, Yusuke Nakamura2, Changi Lee1, Hiroyuki Nakamura1 and Hideaki Maeda1,2,3 1 National Institute of Advanced Industrial Science and Technology (AIST), 807-1, Shuku-machi, Tosu, Saga 841-0052, Japan 2 Kyusyu University, 6-1, Kasuga-kouen, Kasuga, Fukuoka 816-8580, Japan 3 CREST, Japan Science and Technology Agency, 4-1-8, Hon-cho, Kawaguchi 332-0012, Japan. ABSTRACT We synthesized ZnS nanocrystals from identical raw material solution by the thermal decomposition of an amine complex. The shapes of products were changed by simply varying heating rate. At higher heating rate, we obtained the isotropic zincblende nanocrystals. At the lower heating rate, the nanorods were formed and the length was increased with the decrease of heating rate. The nanorods had wurtzite structure below 175 °C, and consequently transformed to zincblende phase during a temperature rise to 200 °C. These particle shapes and phases were related to the adsorption properties of amine ligands. Additionally, the synthesized ZnS nanodots and nanorods exhibited predominantly band-edge emission in fluorescence spectra. INTRODUCTION Nanocrystals (NCs) have received much attention because of their attractive properties.[1, 2, 3] The materials properties are strongly influenced by the size and shape. As a result, control of these factors is essential for development and production of NC materials. High-temperature thermal synthesis in organic solvent is a useful technique for the control of NC structure. Surfactant molecules have generally been used. The adsorption properties of surfactants on the particle surface differ with the atomic arrangements of the surface. Consequently, we can control the particle shape by adequate selection of surfactant type and concentration. Many papers describe shape control of NCs using adequate surfactants.[ 1, 2, 3] However, those surfactant types and quantities have been selected and optimized to obtain one target shape. ZnS has direct wide band gap energy of ca. 3.7 eV at ambient temperature.[4, 5] This is one of the attractive materials and recently promising as a large variety of devices[6, 7], besides the phosphors.[8] These attractive optical and piezoelectric properties of ZnS are governed by the size and shape as well as other materials. While many papers have described isotropic ZnS NC synthesis,[9] some papers reported that the nanorods were formed in the synthesis using an amine molecule as a surfactant.[10] In generally, the surfactants adsorb to the material source species and particle surface at low temperature, and desorb from them at high temperature. Therefore, using an adequate heat treatment, we might well be able to control the variable shape from one identical starting material solution. In this study, we synthesized the well-defined ZnS NCs from one raw material with various heating rates in thermal synthesis, and in
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