Time resolved photoluminescence study of CdSe/CdMnS/CdS core/shell/shell nanoplatelets heterostructures*

We have recorded the time evolution of the photoluminescence (PL) for CdSe/CdMnS/CdS core/shell/shell solution-processed nanoplatelets (NP) using ultrafast pulses at 400 nm and 514 nm. Our NPs consist of a core with 5 monolayers (1.5 nm) of CdSe and average lateral dimensions of 55 x 10 nm$^{\mathrm{2}}$. Using 400 nm pulses we excite electron-hole pairs above the CdS shell bandgap; with 514 nm pulses we excite only in the CdSe core. The holes are primarily localized in the CdSe core, while the electrons are delocalized. Our measurements show that at $\Delta t=$0, the peak PL energy for both kind of excitations is the same. ~As a function of time, both types of excitations result in a red-shift. The red shift of with 400 nm excitation is 60 meV and is described by two time scales: ~$\tau _{\mathrm{1}}=$270 ps and $\tau_{\mathrm{2}}=$2.5 ns. ~The red shift with the 514 nm excitation is 30 meV and is described by a single time scale: ~$\tau_{\mathrm{2}}^{\mathrm{\mbox{'}}}=$2.5 ns. ~These results are discussed in terms of dipole layer formation[1]. ~[1] Gu, Y et. al. Phys. Rev. B 71 045340 (2005).