  We show that photoluminescence properties of PbS nanocrystal thin films are
directly related to film morphology and nanocrystal density. In densely packed
PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is
mainly responsible for the photoluminescence spectra narrowing and shift toward
longer wavelengths. At elevated temperatures, back energy transfer is proposed
to be responsible for an unusual photoluminescence intensity temperature
dependence. In thin films with a low PbS nanocrystal density, the energy
transfer is suppressed, and the effect is dramatically reduced.
