Picosecond Energy Transfer in Quantum Dot Langmuir−Blodgett Nanoassemblies

We study spectrally resolved dynamics of Förster energy transfer in single monolayers and bilayers of semiconductor nanocrystal quantum dots assembled using Langmuir−Blodgett (LB) techniques. For a single monolayer, we observe a distribution of transfer times from ∼50 ps to ∼10 ns, which can be quantitatively modeled assuming that the energy transfer is dominated by interactions of a donor nanocrystal with acceptor nanocrystals from the first three “shells” surrounding the donor. We also detect an effective enhancement of the absorption cross section (up to a factor of 4) for larger nanocrystals on the “red” side of the size distribution, which results from strong, interdot electrostatic coupling in the LB film (the light-harvesting antenna effect). By assembling bilayers of nanocrystals of two different sizes, we are able to improve the donor−acceptor spectral overlap for engineered transfer in a specific (“vertical”) direction. These bilayers show a fast, unidirectional energy flow with a time constant of ∼120 ps.