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Center for Nonlinear Studies |
"Single molecule" spectroscopic techniques, such as confocal fluorescence microscopy, provide information about the photophysical properties of individual fluorophores that are difficult to obtain from ensemble-based measurements, even when the ensembles consist of nominally identical fluorophores. A particular area in which these techniques have been used is mapping out the properties of individual colloidal semiconductor nanocrystals, such as quantum dots (QDs) and rods. These measurements reveal that almost all nanocrystals undergo blinking, or fluorescence intermittency, due to abrupt transitions between fluorescent and non-fluorescent states of the nanocrystals. We have applied these techniques to the study of small clusters of CdSe/ZnS core-shell QDs, from which fluorescence intensity trajectories, autocorrelation functions, decay histograms, and lifetime-intensity distributions have been constructed. These measurements confirm that QD clusters exhibit unique fluoresce nce behavior not observed in isolated QDs. In particular, the QD cluster exhibit a short-lifetime component in their fluorescence decay that is correlated with low fluorescence intensity of the cluster. A model based on nonradiative energy transfer to QDs within a cluster that have smaller energy gaps, combined with independent blinking for the QDs in a cluster, accounts for the main experimental features.
Host: Jim Werner, jwerner@lanl.gov, 7-8842