Carbon nanotube contaminants mess up conductivity

For carbon nanotubes to be used in next-generation nanoscale electronic devices., they need to be as clean as possible, and scientists at Rice and Swansea universities have now found a highly effective way to remove contaminants from carbon nanotubes.

Rice chemist Andrew Barron, also a professor at Swansea in the UK, and his team have figured out how to get nanotubes clean, and in the process have discovered why the electrical properties of nanotubes have historically been so difficult to measure.

Like any normal wire, semiconducting nanotubes are progressively more resistant to current along their length. But over the years, conductivity measurements of nanotubes have been anything but consistent. The Rice-Swansea team wanted to know why.

"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," Barron said. "We wanted to determine the basic science behind the variability observed by other researchers."

They discovered that hard-to-remove contaminants – leftover iron catalyst, carbon and water – could easily skew the results of conductivity tests. Burning those contaminants away, Barron said, creates new possibilities for carbon nanotubes in nanoscale electronics. They report their findings in a paper in Nano Letters.

The researchers first made multiwalled carbon nanotubes between 40nm and 200nm in diameter and up to 30µm long. They then either heated the nanotubes in a vacuum or bombarded them with argon ions to clean their surfaces.

They tested individual nanotubes the same way one would test any electrical conductor: by touching them with two probes to see how much current passes through the material from one tip to the other. In this case, they utilized tungsten probes attached to a scanning tunneling microscope.

In clean nanotubes, the resistance got progressively stronger with increasing distance, as it should. But the results were skewed when the probes encountered surface contaminants, which increased the electric field strength at the tip. And when measurements were taken within 4µm of each other, regions of depleted conductivity caused by contaminants overlapped, which further scrambled the results.

"We think this is why there's such inconsistency in the literature," Barron said. "If nanotubes are to be the next-generation lightweight conductor, then consistent results, batch-to-batch and sample-to-sample, are needed for devices such as motors and generators as well as power systems."

Heating the nanotubes in a vacuum above 200°C (392°F) reduced surface contamination, but not enough to eliminate the inconsistent results, they found. Argon ion bombardment also cleaned the tubes but led to an increase in defects that degrade conductivity.

Ultimately, the researchers discovered that vacuum annealing the nanotubes at 500°C (932°F) reduced contamination enough to measure resistance accurately.

Barron said that engineers who use nanotube fibers or films in devices currently modify the material through doping or other means to get the conductive properties they require. But if the source nanotubes are sufficiently decontaminated, they should be able to get the desired conductivity by simply putting their contacts in the right spot.

"A key result of our work is that if contacts on a nanotube are less than 1µm apart, the electronic properties of the nanotube change from conductor to semiconductor, due to the presence of overlapping depletion zones, which shrink but are still present even in clean nanotubes," Barron said.

"This has a potential limiting factor on the size of nanotube-based electronic devices," he said. "Carbon nanotube devices would be limited in how small they could become, so Moore's Law would only apply to a point."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.