Fluorescence intermittency and energy transfer in small clusters of semiconductor quantum dots

We have studied isolated semiconductor nanocrystal quantum dots (QDs) and small clusters of QDs by singlemolecule time-correlated single-photon counting, from which fluorescence intensity trajectories, autocorrelation functions, decay histograms, and lifetime-intensity distributions have been constructed. These measurements confirm that QD clusters exhibit unique fluorescence behavior not observed in isolated QDs. In particular, the QD clusters 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.