  We report pure carbon-based superlattices that exhibit direct band gaps and
excellent optical absorption and emission properties at the threshold energy.
The structures are nearly identical to that of cubic diamond except that
defective layers characterized by five- and seven-membered rings are
intercalated in the diamond lattice. The direct band gaps lie in the range of
5.6~5.9 eV, corresponding to wavelengths of 210~221 nm. The dipole matrix
elements of direct optical transition are comparable to that of GaN, suggesting
that the superlattices are promising materials as an efficient deep ultraviolet
light emitter. Molecular dynamics simulations show that the superlattices are
thermally stable even at a high temperature of 2000 K. We provide a possible
route to the synthesis of superlattices through wafer bonding of diamond (100)
surfaces.
