Functionalised nanotube. Computer artwork of a carbon nanotube (tube, centre) that has had different functional groups (groups of atoms which are responsible for a molecule's chemical behaviour) grafted on to its surface. Nanotubes are a type of fullerene molecule, a structural form (allotrope) of carbon whose atoms are arranged into interlinking hexagonal and pentagonal rings. Theoretically, a wide range of shapes can be engineered at the molecular level using fullerenes. Such structures could have a wide range of technological and medical uses.