Nature and Decay Pathways of Photoexcited States in
CdSe and CdSe/CdS Nanoplatelets
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Abstract
The nature and decay dynamics of
photoexcited states in CdSe core-only
and CdSe/CdS core/shell nanoplatelets was studied. The photophysical
species produced after ultrafast photoexcitation are studied using
a combination of time-resolved photoluminescence (PL), transient absorption
(TA), and terahertz (THz) conductivity measurements. The PL, TA, and
THz exhibit very different decay kinetics, which leads to the immediate
conclusion that photoexcitation produces different photophysical species.
It is inferred from the data that photoexcitation initially leads
to formation of bound electron–hole pairs in the form of neutral
excitons. The decay dynamics of these excitons can be understood by
distinguishing nanoplatelets with and without exciton quenching site,
which are present in the sample with close to equal amounts. In absence
of a quenching site, the excitons undergo PL decay to the ground state.
In nanoplatelets with a quenching site, part of the initially produced
excitons decays by hole trapping at a defect site. The electron that
remains in the nanoplatelet moves in the Coulomb potential provided
by the trapped hole