Evolution of fluorescence resonance energy transfer between close-packed CdSeS quantum dots under two-photon excitation

Energy transfer (ET) processes between quantum dots (QDS) were investigated by means of steady-state and time-resolved up-conversion luminescence measurements. Two types of CdSeS QDs with different Se/S molar ratios at the similar sizes of ~4.5 nm emit green and orange up-conversion luminescence at infrared laser excitation, separately. The power dependence and nanosecond luminescent decays of QDs films demonstrated that up-conversion luminescence was attributed to two-photon absorption and ET process occurred from green-emitting QDs to orange-emitting QDs. The ET rate was estimated quantitatively to be 0.03 ns(-1) by Dexter theory. The decrease of ET rate is due to Se doped substituted in the Sulfur sites. The band-edge excitonic state is predominating at the initial time evolution and responsible for peak shift and ET. The surface emission of orange-emitting QDs becomes slower, and is attributed to the trapping of electrons from QDs donors.