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The singular properties of double-walled carbon nanotubes

17 April 2015

Rice University researchers have determined that two walls are better than one when turning carbon nanotubes into materials like strong, conductive fibres or transistors.

Individual nanotubes have band gaps and are semiconductors, but when combined, the band gaps overlap and make the double-walled a semi-metal (illustration: Matías Soto/Rice University)

Rice materials scientist, Enrique Barrera and his colleagues have used atomic-level models of double-walled nanotubes to see how they might be tuned for applications that require particular properties.

They knew from others' work that double-walled nanotubes are stronger and stiffer than their single-walled cousins, but they reasoned that it might be possible to tune double-walled tubes for specific electronic properties by controlling their configuration, chiral angles and the distance between the walls.

Carbon nanotubes, grown by various methods, come in two basic varieties: single-walled and multi-walled. But double-walled tubes hold a special place in the hierarchy because they behave somewhat like single-walled tubes but are stronger and better able to survive extreme conditions.

The Rice team found there's even more to them when they started looking at how the inner and outer walls match up using tubes with zigzag chirality (chirality being the angles of their hexagonal arrangement of atoms). Because the electrical properties of single-walled tubes depend on their chirality, the researchers determined to study those properties in double-walled tubes.

"We saw that the inter-wall interaction could affect the electronic properties of double-walled carbon nanotubes and decided to study this effect in a more systematic way using computational simulations," says Rice graduate student, Matías Soto.

It turned out that both the distance between the walls (as small as a fraction of a nanometre) and the individual chirality of the tubes impact the double-walls' electrical properties. In addition, the researchers found the diameter of the tube, especially the inner one, with its more pronounced curvature, has a small but significant impact on the structure's semiconducting properties.

Breaking it down further, they determined that semiconducting nanotubes wrapped around metallic, highly conductive nanotubes could be the best candidates for tuning the band gap, the property that defines the value of a semiconductor.

"The most interesting thing we found was that when you combine a metallic with a semiconductor, the band gap depends on the distance between them," says Soto, adding that while it is not yet possible to do so, the ability to adjust the distance between walls might lead to nanotube transistors.

The research is reported in the journal, Nanotechnology.


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