Electronic state calculations of spherical diamond nanocrystals

Electronic-state calculations of diamond nanocrystals simulated by ultrasmall quantum spheres of diamond passivated by hydrogen are performed by the extended Hückel-type nonorthogonal tight-binding method. Two kinds of surface configuration (ideal and dimerized ones) are studied. Special attention has been paid to surface as well as quantum-confinement effects. The calculated results have demonstrated that, while the HOMO (highest occupied molecular orbital) energies are independent of the surface configuration and depend clearly on the size of the diamond spheres, the LUMO (lowest unoccupied molecular orbital) energies of the diamond spheres with one or two dimers on the surface are rather insensitive to the size, in agreement with experiment. The latter is found to be ascribed to the occurrence of surfacelike states associated with the backbonds of the dimer. It is shown that calculated lifetimes across the energy gap are less than 100 microseconds, suggesting that the diamond nanocrystals are promising light-emitting materials.