Novel applications line up like peas in a pod

Research teams in the UK and the US are building up the clearest picture yet of how fullerenes (C₆₀ buckyballs) pack into carbon nanotubes to produce a range of different ’peapod’ structures. Understanding these structures could lift the lid on a wealth of exciting, novel applications, they say.

Peapods - molecular arrays of fullerenes in single-walled carbon nanotubes (SWNTs) - were first described in 1998 by David Luzzi at the University of Pennsylvania, US. Now, Andrei Khlobystov and colleagues at the University of Oxford, UK, claim to have carried out the first experimental study of molecular packing of fullerenes in single- and double-walled carbon nanotubes with a wide range of internal diameters (A N Khlobystov et al, 2004, Phys. Rev. Lett., 92, 245507).

’As the relative dimensions of the balls and tube change, startlingly different patterns emerge,’ Khlobystov told Chemistry World. ’Linear chains, zigzag arrangements and double-helical phases are all seen with beautiful clarity in transmission electron micrographs’.

The study is welcomed by David Tomanek, professor of physics at Michigan State University, US, whose latest unpublished data represent a theoretical counterpart to Khlobystov’s work.

Tomanek studies the energetics and equilibrium packing geometry of fullerenes in nanotubes. ’Why fullerenes are "sucked in" to nanotubes, why they pack more tightly than when forming a "pile of cannon balls" in a crystal, and how much stress they induce on the tube wall,’ he explained.

His latest findings, some of which were presented in July at the International Conference on the Science and Application of Nanotubes (NT’04) in Mexico, suggest that fullerenes inside nanotubes can be compressed by an effective pressure of gigapascals. ’When other molecules are enclosed in-between, chemical reactions can be induced by these extreme pressures,’ he said, ’which makes the peapods effective autoclaves.’

The potential of nanotubes as vessels for chemical reactions is not lost on Khlobystov. ’We are currently studying what chemical reactions can be done inside nanotubes and how the nanotube affects the reactions’ pathways,’ he said, adding that his group plans to publish on this shortly.

Tomanek’s theoretical predictions are in general agreement with Khlobystov’s experimental observations, but nanotubes with diameters above 1.60nm, which were investigated by Khlobystov, were not included in Tomanek’s study. ’I cannot test [Tomanek’s] model for more complex packing patterns of C ₆₀, such as double-helix or two-molecule layer, observed in nanotubes with diameters above 2.16nm,’ says Khlobystov.

Complex arrangements of fullerenes in wider nanotubes could have additional important practical applications, he says. ’In our research group we consider peapod structures as candidate materials for solid-state quantum computers,’ he explained, ’where each fullerene would act as a quantum bit (or qubit)’. The spacing and interactions between fullerenes in nanotubes are crucial in this case, he says, because they determine the mechanism of communication between qubits. ’By using nanotubes of different diameters we can control the geometry of peapods and, thus, we can control the interactions between qubits.’

Tomanek shows that the geometry of peapods can be well described by a continuum model, where the energy of the encapsulated molecules depends on the nanotube diameter and not the exact atomistic structure of the nanotube sidewalls.

This supports Khlobystov’s hypothesis that the behaviour of C₆₀ in carbon nanotubes can be approximated quite accurately ’by the behaviour of hard macroscopic spheres in a cylinder, where the most important geometrical parameter determining the type of the packing arrangement is a ratio of the diameter of the sphere to the diameter of the cylinder’.

Bea Perks