Photoluminescence Quenching and Intensity Fluctuations of CdSe−ZnS Quantum Dots on an Ag Nanoparticle Film

Intermittent “on” and “off” (blinking) photoluminescence (PL) of single-CdSe/ZnS quantum dots (QDs) is modified when placed on an Ag nanoparticle (NP) film into stochastic fluctuations with nonzero intensity “off” (pseudo off) periods. Also, the PL quantum efficiency (from 0.42 to 0.22) and lifetime (from 5.2 to 1.5 ns) of QDs are considerably decreased at ensemble level in the presence of Ag NPs, and a histogram of the PL lifetime of single-QDs is shifted (from 4.2 to 1.7 ns) and tapered (full width at half-maximum from 3.3 to 1.1 ns) when placed on an Ag NP film. The quenching of the PL quantum efficiency and decrease of the PL lifetime are attributed to ultrafast energy transfer from photoexcited QDs to Ag NPs. The energy-transfer process competes with exciton relaxations and influences carrier trapping in surface defect-states (band gap defects) and Auger relaxation, which are considered to be the origins of blinking. The contribution of surface-states on the modified PL was identified from decreased contribution of a slow component to the PL decays of ensemble- and single-QDs in the presence of Ag NPs. On the basis of ensemble averaged PL intensity and lifetime and single-QD lifetime and intensity trajectory analyses, we propose that the energy-transfer process from photoexcited QDs to Ag NPs result a redistribution of relaxation processes and provide fluctuating trajectories with nonzero intensities to single-QDs. Apart from the observations of modified blinking and narrow lifetime distribution of QDs, the current work partially supports a model proposed by Markus and co-workers that blinking is related to localization of charge carriers in defects.