Energy transfer from a single semiconductor nanocrystal to dye molecules.

In an energy transfer (ET) process, it is the optical responses of donor and acceptor materials on the single-particle level that ultimately determine its overall performance. Here we conduct time-tagged, time-resolved optical measurements to correlate the photoluminescence (PL) intensities and lifetimes of a donor semiconductor nanocrystal (NC) and acceptor dye molecules linked to its surface. We reveal that the PL intensity of dye molecules follows exactly the blinking behavior of the donor NC and shows a step-like quenching behavior due to the photobleaching effect. The corresponding recovery of the NC PL intensity has allowed us to realize the textbook definition of PL quantum efficiency measurement in dye molecules upon absorbing a single exciton. Our theoretical fitting of the lifetime data demonstrates that the buildup time of acceptor PL could be solely determined by the radiative lifetime of dye molecules when it is any shorter than the NC lifetime, thus confirming the long-existing Förster theory on ET dynamics.