Deciphering the emission dynamics of colloidal PbS quantum dots: a spectroscopic exploration under diverse environmental and experimental conditions

Abstract

This study employs a combination of spectroscopic techniques, including absorption, photoluminescence (PL), time-resolved PL kinetics, and transient absorption measurements, to elucidate the dynamics of excited states of oleic acid (OA)-capped colloidal PbS quantum dots (QDs) in toluene with a mean size of approximately 3 nm. PL decay profiles were properly treated employing either fitting or fitting-free models to extract average decay lifetimes and lifetime distributions. Our findings highlight the profound impact of factors such as particle concentration, size, surface treatment, and the surrounding environment on the optical properties of PbS QDs. We observed a notable blue shift in both absorption and emission spectra, along with shorter decay lifetimes, when the concentration of the particles was reduced. This effect was attributed to the dynamics of concentration-selective ligand desorption on the surface of PbS QDs. An observed distinct peak in the spectral dispersion of average lifetimes was linked to an external origin, which was proved by the addition of varying OA volumes, evidently affecting these features. This study also explores how the PL emission of PbS QD suspensions varies with experimental parameters, such as excitation pulse duration and laser power. A gradual reduction in average lifetimes was demonstrated under the modulation of pulse duration from 15 μs to 150 ns, illustrating a transition from continuous-wave (cw) excitation to quasi-pulsed excitation. This transition was shown to be accompanied by a noticeable shift in the lifetime distributions towards shorter durations, as a consequence of various onset components in the QD ensemble. Comparing with truly pulsed excitation, we experimentally confirmed that the amplitude-average lifetime under cw excitation matches the intensity-average lifetime under pulsed excitation. Lastly, we noted linear and nonlinear behaviors in the PL amplitude as a function of excitation power, with the average lifetime remaining nearly constant under pulsed excitation conditions.