Blinking statistics of small clusters of semiconductor nanocrystals

Individual and small clusters of CdSe/ZnS core/shell semiconductor nanocrystals are studied with single-molecule, time-correlated, single-photon counting confocal fluorescence microscopy to obtain fluorescence intermittency (blinking) statistics and correlations. Clusters were distinguished from individual nanocrystals using intensity autocorrelations and decay histograms. Because clusters do not exhibit on/off blinking, we have constructed objective prescriptions to distinguish between high and low emission intervals using lifetime as well as intensity data. The high/low blinking of clusters is statistically similar to the on/off blinking of individual nanocrystals. These results support a model for the distinctive fluorescence properties of small nanocrystal clusters based on nonradiative energy transfer, in which the high/low blinking of a cluster is a consequence of on/off blinking of the "acceptor" (smallest energy gap) nanocrystal in the cluster.