Capped nanotube. Computer artwork of a capped carbon nanotube. It is called capped because the ends of the tubes have been rounded off to form an enclosed structure. The molecular model shows the hexagonal and pentagonal carbon structure found in fullerenes, a structural form (allotrope) of carbon. Theoretically, a wide range of molecular shapes can be engineered at the molecular level using fullerenes. Such structures could have a wide range of technological and medical uses.