First Ultraviolet Quantum Dots Shine Brightly

Researchers in South Korea have made the first quantum dots that emit ultraviolet light and used them to make a flexible, light-emitting diode (Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b00392). If their lifetimes can be improved, these potentially low-cost UV LEDs could find uses in counterfeit currency detection, water sterilization, and industrial applications.

UV light is typically produced by mercury lamps or LEDs made from inorganic materials such as gallium nitride. But mercury lamps tend to emit a broad swath of visible wavelengths in addition to UV, while high-performance gallium nitride LEDs are expensive to make. Quantum dots are an attractive alternative, says Changhee Lee, an electrical engineer at Seoul National University. By controlling the size and shape of these nanocrystals, made out of a semiconductor material, researchers can precisely tailor which wavelength the quantum dots emit. Quantum dot LEDs can also be made using potentially less expensive solution-based processes.

Until now, no one had succeeded in making quantum dots that emit wavelengths shorter than about 400 nm, which marks the high end of the UV spectrum. The smaller the crystal, the shorter the wavelength of the light it emits. To get UV nanocrystals, Lee and his colleagues Seonghoon Lee and Kookheon Char had to figure out how make quantum dots with light-emitting cores smaller than 3 nm in diameter.

To make the dots, the group chose cadmium zinc sulfide, a compound that emits high-frequency light. They started by growing 3-nm crystals of CdZnS in solution. Lee and his colleagues then coated the nanocrystals with a zinc sulfide shell, which not only protects them from surface impurities but also shrinks them. During the coating process, cadmium migrates into the shell, contracting the core diameter to about 2.5 nm and shifting the emitted light into the blue. Lee’s quantum dots emit true UV radiation, at about 377 nm. “We can go to much shorter wavelengths than people generally expected from quantum dots,” Lee says.

Lee’s group then made a flexible LED with the quantum dots, using a design for a high-efficiency device they developed in 2012 (Nano Lett., DOI: 10.1021/nl3003254). The researchers showed that the UV LED could illuminate an anticounterfeiting mark in a bill of paper currency.

Franky So, a materials scientist at the University of Florida, says making a device out of the quantum dots that shines brightly enough to reveal the currency mark is a remarkable accomplishment. The quantum dots are more stable than So would have expected.

These first-of-their-kind UV quantum dots, however, are currently not stable enough for commercial use, Lee says. His group is now working on further improving the quantum dots to make them brighter. They’re also updating the LED design to increase the lifetime of the materials.