Bright type-II photoluminescence from Mn-doped CdS/ZnSe/ZnS quantum dots with Mn ²⁺ ions as exciton couplers

Mn²⁺ ions were introduced as exciton couplers to enhance the quantum yield (QY) of type-II photoluminescence (PL) from CdS/ZnSe/ZnS quantum dots (QDs) with slow hot-exciton cooling and low radiative rate. Transient absorption spectroscopy verifies the faster bleach recovery and faster peak red-shifting at the charge-transfer state. And the transient PL peak of the QDs changes from blue-shifting to red-shifting due to Mn²⁺ doping. The QY of type-II PL can be enhanced from ∼35% to ∼60% by Mn²⁺ doping. As the energy-transfer-stations of hot excitons during rapid ET (∼100 ps), Mn²⁺ ions transform more excitons from hot to cold for emission. As the couplers of cold excitons during long thermal equilibrium (∼100 ns), Mn²⁺ ions further decrease exciton trapping by strong bidirectional coupling. This work provides a unique way of acquiring high QY of type-II PL, and highlights the general law of PL enhancement in Mn-doped QDs.