The work is a collaboration between NIST and the US National Renewable Energy Laboratory (NREL). The cleaning technique came to light when researchers used an excimer laser -- a type of ultraviolet chemical laser routinely used in eye surgery and semiconductor manufacturing -- to calibrate the response of CNT-coated detectors. "We needed a quantitative measure of damage, but soon realized that we were also purifying the CNTs," says John Lehman of NIST's Sources, Detectors and Displays Group. "The detector material was visibly blacker following the measurement, which suggested an increase in porosity."

An electron microscope image taken before and after processing confirmed that exposure to the excimer source had eliminated defective carbon in the sample, but left the CNTs intact. Lasers have been used in the past to selectively destroy CNTs in bulk, but here the NIST/NREL team used much shorter wavelengths.

The group's set-up is based around a 248 nm excimer laser that delivers 20 ns pulses at a frequency of 10 Hz. The researchers place the CNTs in the beam path for 30 seconds by opening and then closing a manual shutter. Data indicates that tube diameters as small as 0.8 nm can withstand the cleaning process.

Pleased with their result, Lehman and colleagues have turned their attention to finding out more about the cleaning mechanism. Lehman thinks that the purification treatment could be taken a step further and used to not only clean, but also fine tune the performance of CNTs. "We plan to scale the treatment down to the individual tube as well as scaling up to manage bulk quantities," Lehman revealed.

The team is currently searching for additional funding to continue its work.

The study was published in Chem. Phys. Lett..