Optical fine structure of II-VI and III-V nanocrystals:: the role of d orbitals in a tight-binding approach

The electronic structure of GaAs and CdSe nanocrystals is studied by using a nearest neighbor tight-binding model extended to include d orbitals. Our results show that the inclusion of d orbitals in a minimal basis set is necessary for a proper description of the lowest electron states, especially in the strong confinement regime. We have checked that simpler sp(3)s* models yield an incorrect description of electron states made from bulk band-states away from the Brillouin zone center. For GaAs nanocrystals with sizes below 2.5 nm, the d model predicts that the ground electron state is primarily built of L-point band-states, giving the nanocrystal an indirect-gap character. The sp(3)d(5) model also explains successfully the evolution of the absorption peaks in CdSe nanocrystals with confinement. We provide an interpretation of the fine band-edge structure based on an orbital-forbidden dark exciton mechanism, which is in agreement, for the first time, with both the resonant and non-resonant experimental Stokes shifts. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.