Quantum dots emit white light
Synthesis of quantum dots using
colloidal methods must occur at a slow rate or the photoluminescence of the particles
will be compromised. Increasing the concentration of coordinating solvents slows
the production rate but creates difficulties with purification and surface control.
Another strategy is to perform a metathesis reaction,
in which two compounds interchange to form two different compounds. However, because
this method yields quantum dots of varying size, shape and photoluminescence, researchers
at the National Institute of Advanced Industrial Science and Technology in Takamatsu
and in Tosu, Japan, developed a colloidal scheme that creates quantum dots with
In a chamber filled with argon at room
temperature, the scientists prepared cadmium selenide quantum dots from cadmium
acetate, tri-n-octylphosphine and tri-n-octylphosphine selenide. They
also added mercaptosuccinic acid to make the particles water-soluble. According
to Jose Rajan of the institute’s Health Technology Research Center in Takamatsu,
using more than 90 percent pure tri-n-octylphosphine caused the reaction
to fail and adding more tri-n-octylphosphine selenide increased photoluminescence.
The investigators reported in the July
3 issue of Applied Physics Letters that these quantum dots emitted white
light. The particles exhibited a spectral broadening of 150 nm FWHM and a photoluminescence
with a Stokes shift of more than 100 nm. Their fluorescence lifetime was 31 ns and
their quantum yield, 0.4.
The particles also had a uniform size
and shape, as revealed by UV-VIS spectroscopy, transmission electron microscopy
and x-ray diffraction measurements.
The researchers designed the quantum
dots for biological applications. The particles had a quantum yield of 0.6 in water
and were stable in high-salt buffers. They also were <10 nm in size, making them
small enough to pass more easily through a cell membrane.
“Our bioimaging experiments were
successful using these dots,” Rajan said.
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