Colloidal Synthesis and Characterization of Optically Active ZnO/ZnS Core/Shell Nanocrystals
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Colloidal Synthesis and Characterization of Optically Active ZnO/ZnS Core/Shell Nanocrystals Krishnaprasad Sankar1, Brian A. Akins1, Tosifa A. Memon1, Nathan J. Withers1, Shin T. Bowers1,2, Tingyi Gu1,3, Jiangjiang Gu1,3, Melisa R. Greenberg1,4, Gennady A. Smolyakov1, and Marek Osinski1 1 Center for High Technology Materials, University of New Mexico, 1313 Goddard SE, Albuquerque, NM, 87106-4343 2 Division of Engineering, Brown University, Box 0424, Providence, RI, 02912 3 School of Electronic, Information, and Electrical Engineering, Shanghai Jiao Tong University, Shanghai, China, People's Republic of 4 CVI Laser, LLC, 200 Dorado Pl. SE, Albuquerque, NM, 87123 ABSTRACT ZnO colloidal nanocrystals have been synthesized using two different approaches and characterized by high-resolution transmission electron microscopy and photoluminescence (PL) spectroscopy. ZnO nanocrystals synthesized from zinc alkoxy alkyl precursors in the MeIm/H2O coordinating solvent showed only visible surface-defect related emission in their PL spectra. No band-to-band UV emission was observed after ZnS coating of those ZnO nanocrystals. In contrast, a strong band-to-band UV emission dominated PL spectra of ZnO nanocrystals synthesized through wet-chemical acid-catalyzed esterification of zinc acetate. INTRODUCTION ZnO nanocrystals (NCs) possess a number of properties making them very attractive for optical applications, such as wide bandgap, large exciton binding energy, radiation hardness, and strong surface-defect-related green emission. As such, they are of interest as potential high-speed nanophosphors for conversion of UV light into visible emission in UV LEDs and laser diodes. Gas and chemical sensors, electro- and photoluminescent devices, solar energy conversion, and transparent UV-protection films can be named as potential applications of ZnO nanocrystals. Being a non-toxic substance, ZnO is a very attractive material for biomedical applications. When doped with various other materials, the wide bandgap of ZnO could be used to get a wide spectral range of narrow emission lines. ZnO nanoparticles have been synthesized in several different ways, namely hydrolysis, high temperature decomposition, and direct injection, each achieving high quality NCs [1-6]. Through hydrolysis, large amounts of ZnO nanocrystalline powder could be obtained [7]. ZnO colloids have been synthesized based on the hydrolysis of zinc salts or zinc alkoxy alkyls in organic solvents [6, 7]. However, these routes provided NCs with hydroxylated surfaces, adsorbed water, and organic molecules that could affect the material and emission properties, as well as cause decay of emission through generation of surface states, unless thermal treatment was applied post synthesis. The development of non-hydrolytic approaches could provide more control over the surface. This could be achieved through the use of a coordinating solvent to organically cap the NC. High temperature decomposition of organometallic precursors in a coordinating solvent has been the most su
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