Exciton dynamics and energy transfer processes in semiconductor nanocrystals

Abstract

Exciton dynamics provide unique information on both the nature of optical transitions and the local environment of an optically active species. However, experimental facilities for measuring (fast) luminescence decay dynamics have been developed long after techniques for time-averaged optical spectroscopy (absorption, excitation and emission spectroscopy). Studies on the dynamics of excited states therefore lag behind of steady state spectroscopy. This situation is also true for research on the optical properties of semiconductor nanocrystals. The first scientific record relating the change in optical properties of semiconductor nanocrystals to the particle size dates back to 1926 when Jaeckel explained the red-shift of the absorption onset in glasses containing CdS particles to a change in the CdS particle size [1]. It took until the 1980’s that a fundamental understanding of the effects underlying the size-dependent optical properties of semiconductor nanocrystals were explained by quantum confinement and the name quantum dots (QDs) was introduced for these nanocrystals [2–4]. Since then the work on semiconductor nanocrystals has developed to an active and still growing field of research. Initially information on the optical properties of QDs was obtained from luminescence (excitation and emission) and absorption spectra. The results from the optical spectra could be related to theoretical calculations on the energy level structure. As for other optically active systems, additional information was later obtained from the dynamics of the excited state. The decay kinetics of the luminescence provide information on for example the nature of excited states, the quality of nanocrystals, competition between radiative and non-radiative recombination processes and interactions between QDs.