Carbon is also known in several other allotropic forms, including semiconducting buckminsterfullerene (C). Solutions of pure buckminsterfullerene have a deep purple color. So does the surface of "buckyballs" (buckminsterfullerene). Solutions of pure C (buckminsterfullerene) have a deep purple color. Helium can form intercalation compounds with the fullerites, including buckminsterfullerene C and C. The buckminsterfullerene molecule has two bond lengths. The best known allotropes are graphite, diamond, and buckminsterfullerene. A related fullerene molecule, named buckminsterfullerene (C fullerene), consists of 60 carbon atoms. A common, shortened name for buckminsterfullerene is "buckyballs". Solid buckminsterfullerene has small spaces between the C balls. The van der Waals diameter of a buckminsterfullerene molecule is about 1.1 nanometers (nm). For example, buckminsterfullerene C is systematically named (-"I")[5,6]fullerene. Combined with buckminsterfullerene, pentacene is used in the development of organic photovoltaic prototypes. In solid buckminsterfullerene, the C molecules adopt the fcc (face-centered cubic) motif. Buckminsterfullerene is the most common naturally occurring fullerene.