Modulating lifetime distribution through variable excitation pulse length and power: a case study on colloidal PbS quantum dots

We investigated the laser power-dependent photoluminescence (PL) decay profiles of oleic-acid capped PbS quantum dots (QDs) in toluene, with a mean size of approximately 2.6 nm, under both long duration (LD) and short duration (SD) excitation pulse limits. The absorption cross-section (ACS) of PbS QDs was assessed and compared using two approaches: power-modulated PL kinetics and fitting of PL amplitude saturation curves. Traditionally, the moderate excitation power interval was overlooked in the former approach due to the power dependence of average ACS values within this range. This study elucidates the reasons behind this power dependence and confirms the validity of ACS values at these excitation powers. We demonstrate that experimental conditions, specifically excitation pulse duration and power, can alter the distribution of decay parameters, leading to the power dependence of various average values, including decay lifetime and ACS. To illustrate this concept, we present a simple model with two contributing components, highlighting the major ideas. The key finding of this study is that both the low-power SD excitation and the moderate-power LD excitation form almost identical distributions of lifetime constants in an ensemble. These distributions exhibit a log-normal shape profile, unlike the Gaussian distributions observed with LD excitation at low excitation powers.