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

Abstract

AbstractThe 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 3s * 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 3d 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. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)