Morphology of CdS nanocrystals synthesized in a mixed surfactant system

High aspect ratio cadmium sulfide (CdS) quantum rods were synthesized at room temperature in the environment of water-in-oil microemulsions using a combination of two surfactants: the anionic bis(2-ethylhexyl) sulfosuccinate (AOT) and the zwitterionic phospholipid L-alpha-phosphatidylcholine (lecithin). These highly acicular particles, obtained from a water-in-oil microemulsion containing an equimolar mixture of AOT and lecithin, possess an average width of 4.1 nm +/- 0.6 nm, with lengths ranging from 50 to 150 nm. In contrast, conventional spherical US quantum dots are obtained from the AOT water-in-oil microemulsion system, with an average particle diameter of 5.0 nm +/- 0.6 nm. X-ray and electron diffraction analyses reveal that the quantum dots have the face centered cubic structure of zinc blende, while the quantum rods predominantly have the hexagonal structure of wurtzite. Luminescence spectrophotometry of the samples indicates a blue-shift in the emission spectra when quantum rods are synthesized. It is hypothesized that this change in particle morphology is directly related to the shape of the reversed micelle in which it was synthesized and is evidence of surfactant templating.