The photoluminescence (PL) spectra of CdSe-core CdS/CdZnS/ZnS-multishell quantum dots (QDs) were studied to understand the radiative and nonradiative relaxation processes in the temperature range from 80 to 360 K. The mechanism of temperature-dependent nonradiative relaxation processes in the CdSe QDs with changing the shell structures was found to evolve from thermal activation of carrier trapping by surface defects/traps in CdSe core QDs to the multiple longitudinal-optical (LO) phonon-assisted thermal escape of carriers in the core/shell QDs. An increase in PL intensity with increasing temperature was clearly observed in the core/shell QDs with a thick CdS monoshell and a CdS/ZnCdS/ZnS multishell. The PL enhancement was considered to come from delocalization of charge carriers localized at the CdSe/CdS interface with the potential depth of ∼30 meV. The experimental results indicated that the improvement of PL quantum efficiency in CdSe-core CdS/CdZnS/ZnS-multishell QDs could be understood in terms of the reduction of nonradiative recombination centers at the interfaces and on the surface of the multishell, as well as the confinement of electrons and holes into the QDs by an outer ZnS shell.

We have studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell quantum dots (QDs) synthesized via hot solution phase chemistry using a nucleation-doping strategy. Efficient PL of the Mn 2+ ions with a quantum yield (QY) of over 35% is demonstrated in the resulting QDs coated with a thick ZnS shell on the MnS core. The MnS/ZnS core/shell QDs with a thick shell exhibit PL enhancement with increasing temperature in the range between 140 and 300 K, resulting from the thermal activation of charge carriers localized at the interface between the MnS core and the ZnS shell. The PL decays of the Mn 2+ ions in the core/shell QDs consist of three exponential components with time constants on the scales of 1-2 ms, hundreds of µs, and tens of µs. Surprisingly, the PL lifetimes of Mn 2+ ions show a very weak dependence on the shell thickness, which is clearly different from that of the PL QY of the QDs. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs can be understood in terms of a significantly enhanced energy transfer from the ZnS shell to Mn 2+ ions and a slightly decreased nonradiative relaxation rate from Mn 2+ ions to surface states/traps of the ZnS shell by the surface passivation of the QDs with a thick ZnS shell.