Abstract
The capture of photoexcited hot electrons in semiconductors before they lose
their excess energy to cooling is a long-standing goal in photon energy
conversion. Semiconductor nanocrystals have large electron energy spacings that
are expected to slow down electron relaxation by phonon emission, but hot
electrons in photoexcited nanocrystals nevertheless cool rapidly by energy
transfer to holes. This makes the intrinsic phonon-bottleneck limited electron
lifetime in nanocrystals elusive. We used a combination of theory and
experiments to probe the hot electron dynamics of negatively charged Cadmium
Sulfide (CdS) colloidal quantum dots (QDs) in the absence of holes. Experiments
found that these hot electrons cooled on a 100 ps timescale. Theoretical
simulations predicted that pure phonon-bottleneck limited electron cooling
occurs on a similar timescale. This similarity suggests that the experimental
measurements reflect the upper limit on hot electron lifetimes in these CdS QDs
and the lower limit on the rates of processes that can harvest those hot
electrons.