Home > Press > Method Slashes Quantum Dot Costs By 80 Percent
Rice Scientists Replace Pricy Solvents With Cheap Processing Fluids
Method Slashes Quantum Dot Costs By 80 Percent
Houston, TX | September 07, 2005
In an important advance toward the large-scale
manufacture of fluorescent quantum dots, scientists at Rice University have
developed a new method of replacing the pricy solvents used in quantum dot
synthesis with cheaper oils that are commonplace at industrial chemical
Rice's study, which was conducted under the auspices of the Center for
Biological and Environmental Nanotechnology (CBEN), is published online and
slated to appear in the October issue of the journal Nanotechnology.
"CBEN 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 the study's lead author, Michael Wong,
assistant professor of chemical and biomolecular engineering and of
chemistry. "That was a great reality check, and it made us look at the
problem of solvent cost sooner rather than later."
Quantum dots typically cost more than $2,000 per gram from commercial
sources, and pricy solvents like octadecene, or ODE - the least expensive
solvent used in quantum dot preparation today - account for about 90 percent
costs of raw materials.
Heat transfer fluids - stable, heat-resistant oils that are used to move
heat between processing units at chemical plants - can cost up to seven
times less than ODE. Replacing ODE with the heat-transfer fluid Dowtherm A,
for example, reduces the overall materials cost of making quantum dots by
about 80 percent.
Quantum dots are tiny crystals of semiconducting materials - cadmium selenide
or CdSe is the most popular flavor - that measure just a few nanometers in
diameter. Most of the commercial possibilities discussed for quantum dots - bioimaging, color displays, lasers, etc. - relate to their size-controlled
fluorescence. For example, CdSe quantum dots have the ability to absorb
high-energy photons of ultraviolet light and re-emit them as photons of
visible light. They glow different colors depending on the size, shifting
from the red to the blue end of the spectrum as the crystals get smaller.
The reproducible synthesis of high-quality quantum dots became a reality in
the early 1990s when researchers at MIT pioneered a new method of producing
quantum dots with uniform sizes and well-defined optical signatures. The
basic recipe for making quantum dots hasn't changed much since it was first
developed. A solvent is heated to almost 500 degrees Fahrenheit and
solutions containing cadmium and selenium compounds are injected. They
chemically decompose and recombine as pure CdSe nanoparticles. Once these
nanocrystals form, scientists can adjust their optical properties by growing
them to precisely the size they want by adjusting the cooking time.
The solvent originally used for this process was trioctylphosphine oxide, or
TOPO, which costs more than $150 per liter. Later, other scientists
introduced a new recipe by replacing TOPO with a mixture of ODE and oleic
Wong said the CBEN research team, which included CBEN Director Vicki Colvin,
professor of chemistry, and Nikos Mantzaris, assistant professor of chemical
and biomolecular engineering and of bioengineering, had some initial doubts
about whether heat-transfer fluids could be substituted for ODE.
"They were cheap and they didn't break down at high temperatures, but no one
uses these compounds for chemical reactions," said Wong.
In addition to finding that other quantum dot nanostructures could be made
in heat transfer fluids, the team concluded that any solvent could be used
to replace ODE. Thanks to a mathematical modeling approach developed by
Mantzaris, the team now has a method for predicting the particle size and
growth behavior of quantum dots based on only three physical properties of a
given solvent: viscosity, surface free energy, and solubility of bulk
cadmium selenide powder.
The research was funded by the National Science Foundation.
Other co-authors include graduate students Sabashini Asokan, Karl Krueger
and Zuze Mu; post doctoral research associate Ammar Alkhawaldeh; and
undergraduate researcher Alessandra Carreon.
The Center for Biological and Environmental Nanotechnology is a National
Science Foundation Nanoscale Science and Engineering Center dedicated to
developing sustainable nanotechnologies that improve human health and the
environment. Located at Rice University in Houston, CBEN is a leader in
ensuring that nanotechnology develops responsibly and with strong public
For more information, please visit cben.rice.edu
About Rice University:
Rice University is consistently ranked one of America's best teaching and
research universities. It is distinguished by its: size - 2,850 undergraduates
and 1,950 graduate students; selectivity - 10 applicants for each place in the
freshman class; resources - an undergraduate student-to-faculty ratio of
6-to-1, and the fifth largest endowment per student among American
universities; residential college system, which builds communities that are
both close-knit and diverse; and collaborative culture, which crosses
disciplines, integrates teaching and research, and intermingles
undergraduate and graduate work. Rice's wooded campus is located in the
nation's fourth largest city and on America's South Coast.
For more information, please visit www.rice.edu
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