A Safer, Easier, Faster Synthesis for CdSe Quantum Dot Nanocrystals

www.JCE.DivCHED.org • Vol. 82 No. 11 November 2005 • Journal of Chemical Education 1697 Properties that vary with particle size are an important feature of nanoscale materials. CdSe quantum dot nanocrystals vary in color from green–yellow to orange–red and luminesce from blue to yellow, where shorter wavelength, higher energy, electronic transitions correspond to smaller crystal sizes. CdSe quantum dot nanocrystals are a visually engaging way to demonstrate quantum effects in chemistry, since their transition energies can be explained as a “particle in a box,” where a delocalized electron is the particle and the nanocrystal is the box. Readers are referred to the article by Kippeny and coworkers in a previous issue of this Journal that contains additional background information and a theoretical discussion (1). CdSe nanocrystal solutions are also an example of colloidal suspensions where nanocrystals do not settle out of solution and cannot be readily filtered. The synthesis provided here has several advantages over the previous method (1). Others note that the starting materials in that preparation, especially dimethyl cadmium, are extremely toxic, expensive, unstable, explosive, or pyrophoric, making the system difficult to control or reproduce (2–4). Peng and others (4–9) have pioneered the kinetic synthesis of CdSe nanocrystals from CdO and elemental Se as an example of green chemistry, where relatively safe materials are used, although the hazards associated with the CdO and Se should not be overlooked. This procedure is suitable for students who can exercise careful handling of reagents and is most applicable in an inorganic or physical chemistry course. Additional advantages of the method reported here are that inert atmosphere techniques are not required and that the resulting crystals are grown at a significantly lower temperature (225 C). Lab preparation and synthesis of a series of CdSe nanocrystals can be easily accomplished within one hour, producing a similar CdSe absorption series as that reported by Peng and Peng (4). The resulting CdSe nanocrystal solutions yield high quality absorbance and emission spectra with particle sizes (10) ranging from approximately 1.8 nm to 4.0 nm and emission wavelengths ranging from 490 nm to 580 nm.