Excitonic recombination and relaxation in CdS quantum dots

Size-selective spectroscopic techniques and a tight-binding calculation with restricted configuration interaction are used to study the recombination and the relaxation of excitons in CdS quantum dots. The optical absorption spectrum is calculated and is found to be close to the experiments. The analysis of the luminescence shows that an emission of light corresponding to transitions between quantum confined states is obtained only in QDs larger than 3 nm. The exciton levels are compared with those given by simple effective mass approximation. The photoluminescence excitation spectra suggest that for upper exciton levels non-radiative captures of the carrier may become competitive compared to relaxation processes.