Face-centered cubic carbon as a fourth basic carbon allotrope with properties of intrinsic semiconductors and ultra-wide bandgap

Carbon is considered to exist in three basic forms: diamond, graphite/graphene/fullerenes, and carbyne, which differ in a type of atomic orbitals hybridization. Since several decades the existence of the fourth basic carbon allotropic form with the face-centered cubic (fcc) crystal lattice has been a matter of discussion despite clear evidence for its laboratory synthesis and presence in nature. Here, we obtain this carbon allotrope in form of epitaxial films on diamond in a quantity sufficient to perform their comprehensive studies. The carbon material has an fcc crystal structure, shows a negative electron affinity, and is characterized by a peculiar hybridization of the valence atomic orbitals. Its bandgap (~6 eV) is typical for insulators, whereas the noticeable electrical conductivity (~0.1 S m⁻¹) increases with temperature, which is typical for semiconductors. Ab initio calculations explain this apparent contradiction by noncovalent sharing p-electrons present in the uncommon valence band structure comprising an intraband gap. This carbon allotrope can create a new pathway to ‘carbon electronics’ as the first intrinsic semiconductor with an ultra-wide bandgap.