Investigation of Carrier Dynamics in CdSe/ZnS Core–Shell Quantum Dots via Scanning Fluorescence Correlation Spectroscopy

Photoluminescence (PL) studies at the single-molecule level are widely used to realize carrier dynamics in nanocrystals, including quantum dots (QDs). Among the single-molecule-based PL techniques, single-particle PL intermittency or blinking studies on surface-immobilized fluorescent particles are very effective for probing slower processes (>10 ms). In contrast, fluorescence correlation spectroscopy (FCS) study in solution phase is used for studying faster processes (<1 ms). Although the recorded FCS curves involve a small ensemble of fluorescent particles, they can still reveal the distribution of a characteristic time constant and indicate sample heterogeneity. The combination of these two techniques, while providing an opportunity to study blinking dynamics of QDs across different time scales, leaves a milliseconds (ms) gap, resulting in an incomplete analysis of blinking. This study employs scanning fluorescence correlation spectroscopy (sFCS) to tackle this issue. This technique enables the tracking of blinking dynamics across a wide range of time scales without a gap, from microseconds to several hundred ms. We performed a comparative analysis between the results of sFCS conducted on surface-immobilized QDs and FCS conducted in the solution phase. This comparison revealed similarities in a qualitative sense but also highlighted significant quantitative differences, particularly in the blinking time scales. These discrepancies stemmed from the different sizes of temporal windows available for blinking study in these two techniques. Interestingly, the characteristic blinking time constant (τR) of our CdSe/ZnS core–shell QDs did not converge even in an extended window of a few hundred ms provided by sFCS. This behavior may be attributed to the power-law statistics of PL intermittency of QDs, where one key consequence is that blinking occurs across all time scales.