Carbon dots light up the blues

Zheng-Hong Lu, Edward Sargent, and Fanglong Yuan at the University of Toronto, Canada, and collaborators, including Osman Bakr at King Abdullah University of Science and Technology in Saudi Arabia, have developed bright, pure colour deep-blue LEDs based on carbon dots.

In the past, scientists have built deep-blue LEDs using quantum dots – tiny semiconducting particles that fluoresce under ultraviolet light to emit different wavelengths of light depending on their size. However, these LEDs have poor efficiency and require quantum dots containing toxic heavy metals, limiting their use in consumer products. 

“Carbon dots, which are made from abundant, non-toxic organic compounds, have been trialled in LEDs before,” says Yuan. But they were limited in the deep-blue wavelength region because the luminescence was spread across a wide range of wavelengths, he explains. 

Lu and co-workers explored the mechanisms underpinning this spectral broadening. They discovered that, when excited, the oxygen-containing functional groups on the edges of the dots created disruption in the resulting luminescence. 

The researchers used an ‘amine passivation’ technique, treating the dots in high-temperature ammonia liquor to remove oxygen-containing functional groups. “Using these carbon dots, we created narrowband deep-blue LEDs with a maximum luminescence one order of magnitude higher than their quantum dot predecessors,” says Sargent. 

“This is indeed a seminal study for the carbon dots field,” says Andrey Rogach, director of the Centre for Functional Photonics at the City University of Hong Kong. “There have been recent reports by us and others emphasizing the importance of amine passivation for controlling emission colour. It is fascinating to see proof that this can dramatically improve emissions in the deep-blue spectral region.”

“We plan to further improve the optical properties of the materials. There remains further potential to improve the efficiency and stability of LEDs as well,” says Lu.

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