Quantum dots emit white light

David Shenkenberg

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 uniform elements.

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.