Carbon nanofibres (CNFs) consist of graphene layers stacked to form quasi-1D filaments. Like carbon nanotubes, they have unique physical and electronic properties, making them attractive for numerous applications, from field emitters to biological probes. Until now, however, it has been difficult to control the internal graphitic structure of these nanomaterials, so limiting their widespread use.

"CNF structure can be likened to a stack of Styrofoam cups with each cup representing a bunch of graphene layers," explained Anatoli Melechko of the Oak Ridge National Lab. "The shape of these cups can vary widely, from a shallow 'plate' to a 'tall glass'."

"Usually, people end up with one of the structures for reasons that are not well understood. However, we found that in the catalytic plasma enhanced chemical vapour deposition process, the structure can be switched from one to another during growth by changing the conditions – namely, the total pressure in the chamber."

Increasing the pressure in the chamber can increase the growth rate of the CNFs 100-fold. The resulting fibres have internal structures approaching those of multiwalled carbon nanotubes.

Recent work by other groups showed that growth along the graphitic plane of the fibres is much faster than in the perpendicular direction – that is the direction in which the graphene layers are stacked. This led researchers to think that the smaller the cone angle (the angle between the graphene layers and the fibre axis), the faster the growth would be.

"The problem was that we had no idea how to make the cone angle smaller," Melechko told "So, we approached the problem from the opposite end: we started changing the growth conditions to maximize the growth rate and then looked to see if the CNF structure had changed."

The team found that when the growth rate had increased 100-fold, the structure of the fibre had changed drastically. The fibres had small cone angles approaching those of nanotubes – which have cone angles equal to zero.

"The chemical, electrical and mechanical properties of CNFs depend on their internal structure," said Melechko. "The ability to control this therefore gives us a handle on how to manipulate these properties. The nanofibres could now be tuned for specific applications."

The team will now try to understand exactly why and how the growth conditions affect CNF structure.

The work, sponsored by the US Department of Energy, was reported in J. Appl. Phys..

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