Carbon exists in several allotropes including diamond, graphite, fullerenes, and carbon nanotubes. Fullerenes are hollow spherical or cylindrical molecules made entirely of carbon atoms arranged in hexagonal and pentagonal rings. Carbon nanotubes are cylindrical tubes composed of graphene sheets that are rolled up, and can be single-walled or multi-walled depending on the number of concentric tubes. Carbon nanotubes have extraordinary mechanical, electrical, and thermal properties that make them useful for applications such as conductive composites, energy storage, field emission displays, and reinforced materials many times stronger than steel.