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 1P1/2 SO, 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.

Ultrafast Photoluminescence Quenching of Initially Excited States in CdSe Quantum Dots Functionalized with a Charge Acceptor Dye

“…The dynamics follow the same general trend previously reported by our group for the BEE and higher excitons of 4.4 and 5.4 nm QDs. 19 a blue circle in Figure 4A. As mentioned in our previous contribution, this ultrafast feature is fully reproducible and has been observed in other studies.…”

“…As shown in our previous study, 19 the spontaneous emission signals can also be resolved for several transiently populated intermediary excitonic states at shorter wavelengths. These signals correspond to transient luminescence produced by the higher excitons that are undergoing accumulation and relaxation in the fs and ps time scales.…”

“…As mentioned in our previous contribution, this ultrafast feature is fully reproducible and has been observed in other studies. 19,54 The 1.3 ps component is well resolved and corresponds to approximately 60% of the total signal amplitude at the maximum, which is reached at 3 ps. These time constants measure the rate of formation of the BEE state that's population gathers from the relaxation processes of the higher and intermediate excitons.…”

“…After the early signal appearance, these intermediary states decay in time scales that are related to the accumulation of the BEE and the formation of trapped states. 19 Specifically, the 550 nm decay corresponds to 5.4 ps, and the one at 500 nm to 2.9 ps (see Table S6).…”

“…PL lifetimes were acquired in a custom-built confocal microscope upgraded with a TCSPC system. ,, A 405 nm (LDH-D-C-405, PicoQuant) and a 485 nm (LDH-D-C-485, PicoQuant) picosecond laser were focused into a 1 cm quartz cell with a 0.25 NA objective ( f = 6.1 mm, Melles Griot). The collected PL was passed through a 510 nm long-pass dichroic mirror (Chroma T510lpxrxt), a 405 nm Notch Filter (Chroma ZET405nf), a 425 nm long-pass emission filter (Chroma ET425lp), and a 700 nm short-pass filter (Thorlabs FESH0700).…”