Detection of a single light nano-source by photoluminescence microscopy reveals many properties which would be hidden by ensemble averaging. However many applications use light sources packed in a dense layer, for instance light-emission diodes (LEDs) using quantum dots or nanoplatelets. Increasing attention has been brought to such dense systems, showing that they behave differently from single isolated emitters because of interactions, charge transport and energy diffusion between neighboring particles.
Semiconductor nanoplatelets present especially large interactions as they tend to self-assemble with very large areas facing each other and very small separation distances. In particular, by Förster-resonant energy transfer (FRET), an exciton in a given platelet can recombine and create another exciton in the neighboring platelet with a theoretical rate of (3 ps)-1, much larger than radiative recombination. By using adequate solvent and ligands, we have assembled linear chains of hundreds of CdSe 7x21 nm² nanoplatelets, constituting an exceptional model system for analyzing the interaction and collective behavior of stacked nanoparticles.
We focused a laser beam onto one spot of the nanoplatelets chain. Photoluminescence imaging showed light emission from a much larger portion of the chain, with energy propagation over typically 150 nm. A diffusion equation model of this system shows that such propagation distances correspond to a FRET time of a few picoseconds, in good agreement with theoretical estimates. Ongoing experiments are considering how such fast transfer affects the photophysics of nanoplatelets (blinking, multi-exciton recombination) and induces collective effects.
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