Synthesis of Organic Capped Colloidal Zinc Oxide Quantum Dots and Their UV Dominant Emission Property
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1207-N10-49
Synthesis of Organic Capped Colloidal Zinc Oxide Quantum Dots and Their UV Dominant Emission Property Takahisa Omata, Kazuyuki Takahashi, Shinichi Hashimoto, Yasuhiro Maeda, Katsuhiro Nose and Shinya Otsuka-Yao-Matsuo Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan ABSTRACT A novel synthesis route to organic-capped and colloidal ZnO quantum dots (QDs) has been developed. Specifically, zinc-di-butoxide was hydrolyzed with very dilute water (100~600 mass ppm) dissolved in hydrophilic benzylamine and polymerized to ZnO by dehydration condensation. After formation of ZnO QDs with 2~3 nm in diameter, growth of the QDs and exchange the surface capping ligand from hydroxyl groups and/or benzylamine to oleylamine were developed by heating the colloidal solution with oleylamine. The size of the ZnO QDs finally obtained was in the range 3~5 nm in diameter. The QDs show high dispersibility in various organic solvents. Clear UV emission due to exciton recombination was observed; and its energy was varied according to the quantum size effect from 3.39 to 3.54 eV. By using lithiumfree zinc-di-butoxide as a starting material, the defect-related VIS emission was successfully decreased and the UV emission becomes dominant. The influence of water concentration in benzylamine and oleylamine on UV emission intensity was also investigated. INTRODUCTION Zinc oxide (ZnO) is a wurtzite-type direct-semiconductor possessing 3.37 eV of energy band gap; its exciton binding energy is 60 meV that is larger than the thermal energy at room temperature. ZnO is attracted much attention as applicable to UV emission LEDs and lasers. Because the simple oxides possessing wurtzite-type structure are limited to the ZnO and the cancer-causing BeO, ZnO is generally alloyed with rock-salt-type MgO in order to adjust its energy band gap [1]. However, the alloying region of ZnO with MgO is limited because of their different crystal structure [2]; therefore, the emission energy of the devices based on ZnO is difficult to tune. Meanwhile, quantum size effect appearing for several nanometer sized semiconductor crystals, i.e., quantum dots (QDs), is very attractive to adjust emission energy without alloying. Several light emitting devices, in which the emission energy is adjusted by the quantum size effect, have been fabricated by using QDs [3-5]. Among the synthesis method, high quality QDs, whose surfaces are capped with the organic surfactant, are obtained by the hot organic solution synthesis originally developed by Murray et al.[6] For the case of ZnO QDs, many researchers have been attempted to develop the synthesis route to ZnO QDs showing size dependent UV emission by several methods such as the oxidation of metallic nanoparticles, thermolysis of zinc organometallics and the amide elimination reaction [7-10]. However, those synthesis methods have a serious disadvantage that the resulting ZnO QDs exhibit intense defectrelated emission in the visible (VIS) region. In the wo
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