Low temperature synthesis of ZnS and CdZnS shells on CdSe quantum dots

• Published in: Nanotechnology Vol. 21; no. 25; p. 255604
• Main Authors: Zhu, Huiguang, Prakash, Arjun, Benoit, Denise N, Jones, Christopher J, Colvin, Vicki L
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 25.06.2010
• Subjects: Alkenes - chemistry; Cadmium Compounds - chemistry; Microscopy, Electron, Transmission; Nanoparticles - chemistry; Nanoparticles - ultrastructure; Quantum Dots; Selenium Compounds - chemistry; Sulfides - chemistry; Temperature; Zinc Compounds - chemistry
• ISSN: 0957-4484; 1361-6528
• DOI: 10.1088/0957-4484/21/25/255604

Methods for synthesizing quantum dots generally rely on very high temperatures to both nucleate and grow core and core-shell semiconductor nanocrystals. In this work, we generate highly monodisperse ZnS and CdZnS shells on CdSe semiconductor nanocrystals at temperatures as low as 65 degrees C by enhancing the precursor solubility. Relatively small amounts of trioctylphosphine and trioctylphosphine oxide have marked effects on the solubility of the metal salts used to form shells; their inclusion in the precursor solutions, which use thiourea as a sulfur source, can lead to homogeneous and fully dissolved solutions. Upon addition to suspensions of quantum dot cores, these precursors deposit as uniform shells; the lowest temperature for shell growth (65 degrees C) yields the thinnest shells (d < 1 nm) while the same process at higher temperatures (180 degrees C) forms thicker shells (d approximately 1-2 nm). The growth of the shell structures, average particle size, size distribution, and shape were examined using optical spectroscopy, transmission electron microscopy, x-ray diffraction, and transmittance small angle x-ray scattering. The photoluminescence quantum yield (QY) of the as-prepared CdSe/ZnS quantum dots ranged from 26% to 46% as compared to 10% for the CdSe cores. This method was further generalized to CdZnS shells by mixing cadmium and zinc acetate precursors. The CdSe/CdZnS nanocrystals have a thicker shell and higher QY (40% versus 36%) as compared to the CdSe/ZnS prepared under similar conditions. These low temperature methods for shell growth are readily amenable to scale-up and can provide a route for economical and less energy intensive production of quantum dots.