Multiphonon photoluminescence and Raman scattering in semiconductor quantum dots

Multiphonon optical spectra of semiconductor quantum dots, in particular, of nanocrystals with structure imperfections, are studied in the framework of a non-adiabatic approach. The phonon modes and the amplitudes of the electron-phonon interaction are found taking into account both electrostatic and mechanical boundary conditions, as well as the finite number of vibrational degrees of freedom in quantum dots. Selection rules for Raman scattering are deduced for quantum dots of semiconductor materials with a degenerate valence band. The effects of non-adiabaticity of the exciton-phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to a substantial difference of the optical spectra from the Franck-Condon progression. Calculated optical spectra compare well with experimental data on photoluminescence and Raman scattering in CdSe and PbS quantum dots. (C) 1998 Elsevier Science Ltd. All rights reserved.