Carbon nanotubes and carbon onions are two fascinating forms of crystalline nanosize carbons that have received considerable attention in recent years (1,2). The remarkable physical properties of nanotubes make them valuable material for applications ranging from electronic devices to nanoprobes. The dimensions and topology of these structures make them fascinating objects for study. Multiwalled nanotubes and onions are made of concentric graphite layers, with different geometry. Singlewalled nanotubes on the other hand are made of singular graphene cylinders, often self-assembled into larger rope-like structures. Nanotubes can be produced in gram quantities using electric arc or laser ablation. Carbon onions, however, are structures that assemble from carbonaceous soot under intense electron irradiation in an electron microscope. Hence the onions are metastable structures that become unstable and disintegrate in the absence of irradiation.

The multiwalled carbon nanotubes and carbon onions have been shown to encapsulate foreign materials (metals, oxides, carbides), either during growth or by capillarity filling (in the case of nanotubes) (1), or during irradiation of metal containing soot (onions) (3).