Ultrafast Photoluminescence Kinetics from Hot Excitonic States in CdSe Nanocrystals

Abstract

The evolution of transient emissions from higher energy excitonic states of core CdSe quantum dots of 4.4 and 5.4 nm diameters was investigated via femtosecond up-conversion measurements, with a high level of detail and spectral range. A kinetic model was used to describe the transient luminescence signals in terms of relaxation of both charge carriers giving a complementary view of the dynamics that follow photoexcitation with ultraviolet photons that can be more difficult to visualize with other techniques. Our results are consistent with the existence of two simultaneous processes: an ultrafast hole relaxation pathway from the initially pumped states including 1S1/2 or 1P1/2SO, which occurs on the order of 200 fs, and a slower relaxation pathway, which occurs through a ladder-type mechanism and is completed within a few picoseconds. The slower channel involves an electron relaxation from states 2S and 1P via Auger-type energy transfer to hot hole states present deep in the valence band, followed by two hole relaxation steps associated with phonon “bottleneck”-type processes and/or trapped states. The present is the first detailed study of the evolution of the transient spontaneous emission signals that accompany these relaxation processes. We foresee that these signals will be of significant use in ascertaining the population evolution of the upper excitons in the context of diverse applications that involve charge or energy transfer from the higher states.