May 9, 2012 -- White-light quantum dots (QDs) are cadmium selenide materials that convert the blue light of light-emitting diodes (LEDs) to a high-quality warm white light, in a similar spectrum to incandescent light. Vanderbilt University researchers have developed white-light quantum dots from discovery in 2005 (with fluorescent efficiency of 3%) to a fluorescent efficiency of 45%.
Quantum dots produce colors based on the size of the quantum dot particles -- shifting from red to blue as the nanocrystal shrinks. Ultra-small quantum dots, containing 60-70 atoms, emit white instead of monochromatic light. Almost all of the atoms of these QDs are on the surface, making white-light emission “intrinsically a surface phenomena,” said Sandra Rosenthal, the Jack and Pamela Egan Chair of Chemistry, who directed the research.
The high efficiency is comparable to some commercial phosphors, said Rosenthal. It could be improved even further. Rosenthal expects white-light quantum dots to be used in some special lighting applications.
Current white-light LEDs offer 28-93 lumens/watt. If the enhanced quantum dots were combined with the most efficient ultraviolet (UV) LEDs, it would register a luminous efficiency of about 40 lumens/watt, said James McBride, research assistant professor of chemistry, who has been involved in the research from its inception. As UV LEDs are improved, the hybrid LED/QD combo would gain higher efficiency.
Instead of creating a material “shell” around the QDs, the researchers treated quantum dots with metal (acetate) salts, following a lead from University of North Carolina research. The salts produced an 8% brightness increase. Once the researchers determined that acetate salts had an effect, they tried acetic acid, which binds to quantum dots. The acetic acid treatment increased the quantum dots fluorescent efficiency to 20%. From there, researchers tried other members of the carbocyclic acid family. Formic acid, the most acidic and simplest, pushed the efficiency up to 45%.
The brightness increase shifted the peak of the color spectrum of the quantum dots slightly into the blue, which is a light spectrum many consumers find unpleasant with today’s LEDs. The researchers will use other methods to correct the color balance.
The researchers’ next step is to test different methods for encapsulating the enhanced quantum dots.
Other contributors to the study include graduate students Teresa E. Rosson, Sarah M. Claiborne and undergraduate research student Benjamin Stratton, who is now at Columbia University. The research is described online in the Journal of the American Chemical Society. The work was supported by a grant from the National Science Foundation.