Quantum Dots Shine Brighter with Acid

The fluorescence efficiency of white-light quantum dots was successfully boosted to 45 percent, a more than tenfold increase from the initial level of 3 percent.

Vanderbilt University chemists accidentally discovered white-light quantum dots in their lab seven years ago when they found that ultrasmall quantum dots, containing only 60 to 70 atoms, could emit white instead of monochromatic light.

“These quantum dots are so small that almost all of the atoms are on the surface, so the white-light emission is intrinsically a surface phenomena,” said Sandra Rosenthal, the Jack and Pamela Egan Chair of Chemistry at Vanderbilt and the leader of the research.

A vial holding original white-light quantum dots is on the left; the enhanced quantum dots are on the right. (Image: Rosenthal Lab)

Unfortunately, the quantum dot’s efficiency was too low for commercial application. While several experts predicted that raising it to practical levels would be impossible, Rosenthal’s team has proved the prediction wrong by reporting a boost in the nanocrystals’ fluorescence efficiency from 3 to 45 percent.

“Forty-five percent is as high as the efficiency of some commercial phosphors, which suggests that white-light quantum dots can now be used in some special lighting applications,” Rosenthal said. “The fact that we have successfully boosted their efficiency by more than 10 times also means that it should be possible to improve their efficiency even further.”

The Vanderbilt team, inspired by a study performed at the University of North Carolina, decided to see whether treating the quantum dots with metal salts would make them shine brighter. They observed a 10 to 20 percent improvement.

“They were acetate salts, and they smelled a bit like acetic acid,” said James McBride, a research assistant professor of chemistry at Vanderbilt. “We knew that acetic acid binds to the quantum dots, so we decided to give it a try.”

By following their noses, the investigators bumped up the quantum dots’ fluorescence efficiency from 8 to 20 percent.

The researchers tested other members of the carbocyclic acid family and discovered that the simplest, most acidic member — formic acid, the chemical that ants use to mark their paths — worked the best, pushing the efficiency as high as 45 percent.

The boost in brightness had an unexpected side effect. It shifted the peak of the quantum dots’ color spectrum slightly into the blue. However, the research team said that it could fine-tune the color balance to make the dots fluoresce white.

Next, the team plans to test different methods for encapsulating the enhanced quantum dots.

The research was described online in the Journal of the American Chemical Society.

For more information, visit: www.vanderbilt.edu