  We study spectrally resolved dynamics of Forster 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, inter-dot
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.
