A cousin of the spheroidal fullerene molecules is carbon nanotubes. These are cylindrical fuller- enes with either open or closed ends and consist of carbon atoms arranged in a hexagonal pat- tern. There are two main families of carbon nanotubes that are distinguished by being either single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). SWNTs actually are a graphene sheet that is seamlessly wound into a cylinder. MWNTs consist of multiple SWNTs that are concentrically wound around each other and nested together to create a tubes-within- tubes configuration (Figure 15.11). The type of nanotube also is determined by manner in which the hexagonal pattern of carbon rings is arranged in the cylindrical structure.