Carbon nanotubes consist many carbon atoms arranged in a tube. The carbon atoms are arrayed in a hexagonal lattice similar to the atomic sheets (graphene) in the carbon allotrope graphite. Nanotubes are capped by a hemispherical array of carbon atoms (as seen in Buckminsterfullerene). Nanotubes can form nested structures with greater diameter tubes containing lesser diameter tubes. Nanotubes have great strength and can be excellent electrical conductors or semi-conductors.