Nature and decay pathways of photoexcited states in CdSe and CdSe/CdS nanoplatelets.

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.