PORTLAND, Ore. Light-emitting nanotubes (LENs) have been demonstrated, but their efficiency has been mysteriously lower than theory would predict. Now researchers claim to have discovered the mechanism limiting efficiency along with a remedy to the problem.
The 40-fold boost in efficiency to 20 percent, enough to perhaps make LENs commercially viable, was reported by University of Connecticut professor Fotios Papadimitrakopoulos working with in collaboration with the university's Institute of Material Science and Nanomaterials Optoelectronics Laboratory.
Nanotubes can be coaxed to emitting light when exitons--high-energy bound electron-hole pairs--decay by producing a photon. Exitons can be introduced inside nanotubes electrically (by injecting holes in one end and electrons in the other) or optically (by shining a laser on them).
However, only about 1 percent of the exitons actually luminesce. The rest "burn" their excess energy, making heat instead of illumination. The cause, according to Papadimitrakopoulos, was oxidation on the surface of the nanotube after exposure to air.
"Everybody knew that oxygen adsorbs carbon nanotubes, but they thought it was innocuous," said Papadimitrakopoulos. "It turns out it is not innocuous when it comes to luminescence because oxygen creates a channel for nonluminescent decay."
One solution discovered by the team was a self-assembling material that displaces oxygen from the surface of the nanotubes, replacing it with a protective riboflavin-like (vitamin B2 derivative) coating that prevents further oxidation.
"We have created a way to effectively take out the oxygen," said Papadimitrakopoulos. "We use a self-assembling molecule that raps itself around the length of the nanotube in a helical manner, knocking off the oxygens whose bond is weaker."
Without oxygen to burn, exitons have no other means of shedding their excess energy beyond emitting a photon, thereby boosting the efficiency of luminescence to 20 percent with the prospect of further improvements.
Next the researchers want to apply the increased luminescence to two application areas: medical electronics and optoelectronics.
For medicine, B2-coated nanotubes would have a third layer of protein deposited, making them stick to specific target cells like tumors. Patients would then swallow nanotubes, which would be flushed through the system except where they stick to tumors. Illuminated by infrared light, the stuck nanotubes could be induced to luminesce, thereby pinpointing tumors.
For optoelectronics, ultra-small light emitters could enable on-chip optical busses. The light wavelengths from luminescent nanotubes are in the near-IR communications bands. Unlike gallium arsenide in the same bands, carbon nanotubes can readily integrate with silicon-based chips. Papadimitrakopoulos' group hopes to craft nanotube-based optoelectronic devices that could be used for ultra-smalll fiber optics components, IR light modulators and sensors.