Graphene
Graphene is a single planar sheet of <math>sp^2<math> bonded carbon atoms. It is not an allotrope of carbon because the sheet is of finite size and other elements appear at the edge; a typical graphene would have the chemical formula <math>\rm C_{62}H_{20}<math>. Graphenes are aromatic.
Graphenes may consist of only hexagonal cells but if a pentagonal cell is present the plane warps into a cone shape; insertion of 12 pentagons would create a fullerene. Insertion of a heptagon causes the sheet to become saddle shaped; controlled addition of pentagons and heptagons allows a wide variety of shapes to be made.
Graphenes are interesting because carbon nanotubes may be considered to be graphene cylinders with a graphene cap (that includes a pentagon) at each end.
The IUPAC compendium of technology states: "previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene...it is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed".
