Band gap of essentially fourfold-coordinated amorphous diamond synthesized from ${\mathrm{C}}_{60}$ fullerene

The band gap and density of state for a form of amorphous diamond synthesized from ${\mathrm{C}}_{60}$ fullerene by shock compression were measured by electron energy-loss spectroscopy in this study. The dielectric functions of the material were derived from the loss function obtained from its spectrum by Kramers-Kronig transformation. The imaginary part of the dielectric function, ${\varepsilon }_2,$ showed that the magnitude of the gap was up to 4.5 eV, somewhat smaller than that of crystalline diamond, which is 5.5 eV, but larger than those of any other amorphous material reported before. The excitation of interband transition was observed only at the Γ point, not at the X and L points. The density of state around the gap was rather broad. The radial distribution function analysis made previously revealed that the carbon atoms of the material were tetrahedrally coordinated; furthermore, these tetrahedra were arranged in the same manner as that of crystalline diamond within the region of unit cell size. The characteristic electronic properties of the amorphous diamond measured were consistently attributable to the unique atomic configuration of the material. The band gap and density of state were discussed in relation to the fraction of fourfold coordination, the density, and the short-range structure, comparing with other amorphous carbon materials.