Web Release Date: September 23,
Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction
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Contribution from the Department of Chemistry & Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, Joint MRSEC at the University of Oklahoma and the University of Arkansas, Nanomaterials and Nanofabrication Laboratories (NN-Labs), Fayetteville, Arkansas 72703, and Department of Physics and Astronomy, University of Oklahoma, Norman, Oklahoma 73019
Received May 27, 2003
Abstract:
Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of
thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality
core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one
monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors
into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO
and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240
C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram
scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of
CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto
the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy,
TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained
between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by
about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed,
some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell
nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core
nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal
semiconductor nanostructures, such as quantum shells and colloidal quantum wells.
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