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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.

The cooling and Auger recombination of electron–hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump–probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30–50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.