  As the fourth most abundant element in the universe, carbon plays an
important role in the physical and chemical evolution of the interstellar
medium. Due to its unique property to form three different types of chemical
bonds through sp^1, sp^2, and sp^3 hybridizations, carbon can be stabilized in
various allotropes, including amorphous carbon, graphite, diamond, polycyclic
aromatic hydrocarbon, fullerenes, graphene, and carbon nanotubes.
