For nanotechnology to thrive, there needs to be a way to make carbon nanotubes and their ilk in as pristine a condition as possible to avoid impurities and particles reducing or inhibiting entirely the phenomena we hope to exploit. Now, a putative lower limit has been put on carbon nanotubes because of contact effects, according to the latest research from Swansea and Rice Universities.

Andrew Barron who both institutions and his colleagues have devised a way to make nanotubes clean so that they can test the reproducibility of electronic measurements and processes in these materials. Until now, it has been difficult to obtain consistent measurements, but their findings come at a pricing, suggesting a lower limit below which future nano-electronic devices may not be able to progress.

"We are interested in the creation of nanotube based conductors, and while people have been able to make wires their conduction has not met expectations," explains Barron. "We were interested in determining the basic sconce behind the variability observed by other researchers."As with conventional wires, semiconducting nanotubes show increased resistance to current along their length, but conductivity measurements have not been entirely meaningful. Barron and his colleagues have now demonstrated that contaminants that are difficult to remove, such as residual iron catalyst particles, excess carbon, and even water, have probably been distorting the data for years. He and his team have now shown how such contaminants might be burned away to make more pristine carbon nanotubes. [Barron et al., Nano Lett (2018); DOI: 10.1021/acs.nanolett.7b03390]

In their tests, the team synthesized multiwalled carbon nanotubes of between 40 and 200 nanometers diameter and up to 30 micrometers in length. They heated some of these nanotubes in a vacuum and bombarded others with argon ions to give them a thorough cleaning of their surfaces. They then probed the conductivity of individual nanotubes using tungsten probes attached to a scanning tunneling microscope.

With these clean nanotubes, they observed increase resistance over the length of the nanotubes as one would expect. However, if there were residual surface contaminants the STM's electrical field would be distorted and so skew the measurements. When the team tested conductivity spanning4 micrometer regions, they saw lower conductivity caused by overlapping contaminants which further distorted the results.

"We think this is why there's such inconsistency in the literature," Barron explains. "If nanotubes are to be the next generation lightweight conductor, then consistent results, batch-to-batch, and sample-to-sample, is needed for devices such as motors and generators as well as power systems," he adds. Critically, if even after cleaning, if residual contaminants are less than 1 micrometer apart, then the electronic properties of the nanotube are changed. "This is a potential limiting factor on the size of nanotube based electronic devices," Barron suggests. "This would limit the application of Moore’s law to nanotube devices."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".