Controversial new theory for nanotube growth

10 February 2009

US scientists have proposed a new theory for how carbon nanotubes grow. If their

predictions are borne out experimentally, the theory could have practical implications

for researchers trying to control nanotube growth in the lab. But experts say the theory

may be unrealistic.

Carbon nanotubes are essentially rolls of graphene - hollow cylinders of carbon in

which the atoms are arranged in a hexagonal lattice. But they don't roll up like sheets

of paper; they self-assemble or 'grow' in the direction of a tube's length, prompting

scientists to wonder how exactly each new layer of carbon is formed.

Houston, and the Honda Research Institute in Ohio have now put forward a formula

that they say provides a model for the extension process. Yakobson likens it to weaving

a rug - the more atomic kinks or 'threads' are exposed at the growing end, the

faster growth proceeds.

'The kinks are an extension of the spiral lines of atoms that make up the tube,' explains

Yakobson. 'You can visualise these kinks as the ends of threads, so the more

thread ends you have, the faster the tube will grow.'

The number of kinks at the growing edge is ultimately dependent on the tube's chirality,

or the angle at which it is 'rolled'. Chiral tubes expose many kinks and so form

quickly. A non-chiral tube, by contrast, is not formed by adding to a spiral 'thread' but

by the addition of complete rings of carbon atoms. Therefore, explains Yakobson, an

energy barrier has to be overcome each and every time a new ring is initiated.

Nicole Grobert, a nanotubes expert based at the University of Oxford, UK, warns that

the team's work is purely theoretical and unlikely to explain growth processes in real

life systems. 'It has nothing to do with reality, I think, because the conditions in

which the tubes grow are very chaotic,' she says. 'You have to look at the different

methods that are used to grow nanotubes and I should think all of these have different

growth scenarios, so you can't come up with one theory and explain all of them.'

David Tománek, who studies nanostructured materials

at Michigan State University, East Lansing, US, says the model contradicts everything

that is known about the formation process of nanotubes in the presence of catalytic

particles. 'It also contradicts common sense in claiming that a couple of yarns,

representing monoatomic carbon chains, should nicely attach to each other to form a

hollow tube,' he says.

'The jury is still out,' admits Yakobson. 'We're going to have to go through neverending

verification processes.' But he argues that data from previous studies has so

far supported his team's findings - for example, data taken from different growth

methods shows an abundance of nanotubes with large chiral angles, as predicted by

the formula.

Understanding how nanotubes grow would help scientists gain control over their

structure, potentially leading to tubes with predefined properties and applications,

says Grobert. But she thinks Yakobson's theory of nanotube growth is too far fetched.

Hayley Birch

Date: 2015-12-17; view: 849