Increased photo-stability of quantum dots in segregated bilayer films

We report a comparative study of photo-stability of CdSe/ZnS quantum dots (QDs) in a variety of thin film samples. These include electrophoretically deposited single and differently sized segregated bilayer films and self-assembled mixed-sized films. Our studies follow static and dynamic QD photoluminescence over prolonged periods of photo-excitation and find that compared to both single-sized and mixed-sized films, the segregated bilayer samples exhibit highest photo-stability. These films show a QD emission quench rate of ∼2.5 times slower than the others and have almost negligible spectral shifts (<2 nm). Time-resolved measurements indicate very short inter-layer energy transfer (ET) time for the acceptor QDs in the bilayer, coupled with low ET efficiency for the donor dots. Further analysis reveals a complex interplay of intra- and inter-ensemble ET, with ET rates that have disparate spectral dependence between the mixed and bilayer films, and we conclude that this leads to the enhanced photo stabilit...