Single Molecule Studies of Energy Transfer in Semiconductor Nanocrystal Clusters

Enhanced fluorescence intermittency has been reported in single molecule fluorescence experiments on small clusters of semiconductor nanocrystals$^{1}$ (NCs).~ We report here on studies of small clusters of NCs by single molecule time-correlated single photon counting. According to this analysis, clusters typically blink on a microsecond to millisecond time scale; whereas, isolated NCs blink on a much longer millisecond to second time scale. A fast-decay component in the cluster fluorescence lifetime, not present in single NCs, is correlated with low fluorescence intensity.~ A model based on nonradiative energy transfer to NCs with smaller bandgap, combined with independent blinking for the NCs in the cluster, accounts for the main experimental features.~ In this model the smallest-gap NC dominates the emission properties, in particular the ``off'' time distribution of the cluster, which experimentally resembles that for a single NC. [1] Yu, M. and A. Van Orden, \textit{Enhanced Fluorescence Intermittency of CdSe-ZnS Quantum-Dot Cluster}, Physical Review Letters, 2006 \textbf{237402-4}