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Excimer laser cleans up nanotubes

09 Jan 2007

Laser cleaning could improve the commercial viability of carbon nanotubes. optics.org speaks with John Lehman of NIST to find out more.

A "point and shoot" laser cleaning method could help to speed up the commercialization of carbon nanotubes (CNTs) say researchers in the US. CNTs have a range of unique properties that suit applications such as optical sources, detectors and displays, but the cost-effective removal of performance degrading contaminants remains an issue. (Chem. Phys. Lett. 433 301)

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 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," John Lehman of NIST's Sources, Detectors and Displays Group told optics.org. "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 is using 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. CNTs are placed 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 up to a fluence of 700 mW/cm2.

Pleased with their result, Lehman and his colleagues have turned their attention to finding out more about the cleaning mechanism. "Although we can't say for sure, we believe that it is the combination of exciting the pi plasmons in the presence of ozone that facilitates oxidation of carbon impurities," Lehman said.

He 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.


James Tyrrell is News Editor of Optics & Laser Europe magazine and a contributor to optics.org.

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