EXCITONS IN NANOCRYSTALS

The excitation energies can be described in the frame of the effective mass approximation when the crystals an not too small. The lowest ene rgy exciton state is split in several sublevels by the electron-hole e xchange interaction and by the Crystal field in wurtzite-type semicond uctors. We determine the oscillator strength of the transitions. The o ptical transition to the lowest energy sublevel is forbidden by spin c onservation law. The intensity of the transition to the upper sublevel in cubic nanocrystals is proportional to the number of nanocrystals p er unit volume in the strong confinement regime and to the volume frac tion occupied by the nanocrystals in the weak confinement regime. We s how that, in a cubic or a wurtzite-type nanocrystal, all the sublevels are shifted to higher energy by the polarization of the medium and th at the intensity of the transition to the lowest energy optically allo wed sublevel in a wurtzite-type nanocrystal is very much decreased by the polarization of the medium. The energy relaxation of excited elect ron-hole pairs is made possible by the coupling to acoustical and opti cal phonons. We conclude that no ''phonon bottleneck'' exists in the r elaxation. We show that Auger effect is responsible for ''photodarkeni ng'' of the samples under illumination. It is the source of the random telegraph signal observed in the luminescence of a single nanocrystal . (C) 1998 Elsevier Science B.V. All rights reserved.