Many of the elements used in efficient II-VI quantum-dot LEDs today, such as CdS, CdSe and Pb, are highly toxic. This is a major drawback for many applications and silicon-based LEDs are much better in this respect. Moreover, the colour of light emitted from these devices can be tuned simply by varying the size of the nanocrystals thanks to the quantum size effect. This means that the smaller the particles get, the more blue-shifted the emitted light becomes with larger particles emitting in the red part of the electromagnetic spectrum (680 nm) and smaller particles in the yellow-orange (625 nm).

“Compared with previous reports on SiLEDs that focused on near-infrared and red-emitting devices, our LEDs can emit in the yellow-orange spectral region,” explained team member Florian Maier-Flaig. “Indeed, ours is the first demonstration of yellow-orange-emitting SiLEDs based on size-selected nanocrystals,” he told nanotechweb.org.

Longer lifetimes

Separating out particles of different sizes also significantly increases device operation lifetimes – to as long as at least 40 hours in some cases, he adds. The colour of the light emitted from the new LEDs also appears to be much less sensitive to the applied drive voltage.

And that is not all: as well as emitting light in the yellow-orange range, the nanocrystals might also be employed as so-called down-converter red-emitting “phosphors” to generate white light, says Maier-Flaig. “Such devices are currently fabricated using spectrally broad-emitting phosphors that do not emit strongly in the red part of the spectrum – a region that is important for producing ‘warm’ white light.”

Although the devices made by the Karlsruhe and Toronto researchers have high external quantum efficiencies of up to 1.1%, as well as low turn-on voltages of just 2 V for the red emitters, they would like to improve on these figures. “High quantum yields can be reached for red-emitting silicon nanocrystals, but we need to further increase these values especially at shorter light wavelengths,” said Maier-Flaig. “This might be achieved by functionalizing the nanocrystals with molecules that passivate the surfaces of the nanoparticles and enhance charge transport between them. Such strategies might even allow us to tune the emission wavelength over a broader range.”

The current work is detailed in Nano Letters.