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Heat-Transfer Fluids Cut Cost of Quantum Dots

Photonics Spectra
Nov 2005
Anne L. Fischer

Quantum dots may be tiny, but they are hugely important for their optoelectronic properties. A drawback, however, is that they typically cost thousands of dollars per gram.

Approximately 90 percent of the expense of quantum dots lies in the cost of octadecene, an organic solvent used in the manufacturing process. Now researchers at Rice University in Houston have developed an alternative method that employs heat-transfer fluids to yield dots at a fraction of the cost.

According to chemical engineering professor Michael S. Wong, the scientists were working to scale up the production of quantum dots but found that they were spending their research funds faster than they were getting results. They questioned why octadecene, the most expensive ingredient in synthesizing CdSe dots, was presumed to be the only solvent suitable for this purpose.

The recipe for making quantum dots involves heating the octadecene and injecting it with a solution containing cadmium and selenium. The materials decompose and recombine as pure CdSe nanoparticles, with the length of heating time determining the size of the crystals.

The researchers explored the use of heat-transfer fluids to test the hypothesis that any organic solvent would work if it had a sufficiently high boiling point so that it would not decompose at approximately 500 °F, the temperature required for producing quantum dots. They tested two kinds of heat-transfer fluid, Dowtherm A from the Dow Chemical Co. and Therminol 66 from Solutia Inc.

Through careful synthesis studies, they produced quantum dots as good as those formed in octadecene. Because the heat-transfer fluids cost as little as one-seventh the price of octadecene, the material cost of dot production drops by about 80 percent, according to the researchers.

Using population balance modeling, they found that the synthesis chemistry is highly dependent on three properties: the solvent’s viscosity and surface free energy, and the solubility of CdSe in the solvent. The next step is to refine the model with realistic values for these properties so that it can be used to predict particle sizes and growth behavior of the quantum dots.

The researchers found that, because the dots grew more slowly in the heat-transfer fluids than in octadecene, they now can make small quantum dots more easily, which means that a wider range of colors can be accessed.

Wong predicted that using less expensive, alternative solvents will enable labs to forge ahead with the development of quantum dots. Lower synthesis costs may lead to more rapid development in applications such as fluorescence imaging, LEDs and electronics.

Nanotechnology, October 2005, pp. 2000-2011.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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