15 Sep 2005
Rice University researchers find a cheaper way to mass-produce quantum dots.
Researchers at Rice University, US, have come up with a cheaper process for mass-producing cadmium selenide (CdSe) fluorescent quantum dots, or semiconductor nanocrystals. The technique replaces pricey solvents with cheaper oils, reducing raw material costs by as much as 80%.
"CBEN [the Center for Biological and Environmental Nanotechnology] started to undertake some exploratory work more than a year ago on the scale-up issues of quantum-dot manufacture, but the solvents turned out to be so expensive that we just couldn't afford to run more than a few large-reactor experiments," said Michael Wong of Rice University. "That was a great reality check and it made us look at the problem of solvent cost sooner rather than later."
The CdSe quantum-dot manufacture process typically involves dissolving a cadmium precursor and a selenium precursor in an organic solvent at about 250-350 °C. Wong and colleagues replaced expensive solvents such as trioctylphosphine oxide (TOPO) or octadecene with heat transfer fluids. These are heat-resistant oils used to move heat between processing units at chemical plants.
The team successfully made CdSe quantum dots of uniform size using two different heat transfer fluids - DTA, a mixture of biphenyl and diphenyl ether, and T66, a mixture of terphenyl, partially hydrogenated terphenyls and higher polyphenyls/partially hydrogenated polyphenyls. In fact, using either of these two fluids enabled the preparation of smaller-diameter quantum dots in a more controlled fashion than with TOPO or octadecene.
The team was also able to make CdSe/CdS core/shell quantum dots and CdSe quantum rods using heat-transfer fluids.
According to the scientists, they had some initial doubts about whether heat-transfer fluids could be substituted for octadecene. "They were cheap and they didn't break down at high temperatures, but no one uses these compounds for chemical reactions," said Wong.
The team also created a mathematical model to predict the growth kinetics of quantum dots according to the properties of the solvent used. The model looks at the solvent's viscosity, surface free energy and how much bulk CdSe powder it can dissolve.
Fluorescent quantum dots have applications in areas such as bioimaging and displays. They are relatively easy to tune since the wavelength at which they fluoresce depends on the size of the crystal.
The researchers reported their work in Nanotechnology.
Liz Kalaugher is editor of NanoTechWeb.