Fine structure of the band-edge excitons and trions in CdSe/CdS core/shell nanocrystals

We present a theoretical description of excitons and positively and negatively charged trions in ``giant'' CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a degenerate valence band. The calculations take into account the complex structure of the CdSe valence band and interparticle Coulomb and exchange interaction. Presented in this paper are the CdSe core size and CdSe/CdS conduction band offset dependencies (i) of the positively charged trion fine structure, (ii) of the binding energy of the negatively charged trion, and (iii) of the radiative decay time for excitons and trions. The results of theoretical calculations are in qualitative agreement with available experimental data.