Exciton storage by Mn2+ in colloidal Mn2+-doped CdSe quantum dots

Colloidal Mn2+-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to τexc = 15 μs at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by ∼103 and are shown to arise from the creation of excitons by back energy transfer from excited Mn2+ dopant ions. A kinetic model describing thermal equilibrium between Mn2+ 4T1 and CdSe excitonic excited states reproduces the experimental observations and reveals that, for some quantum dots, excitons can emit with near unity probability despite being ∼100 meV above the Mn2+ 4T1 state. The effect of Mn2+ doping on CdSe quantum dot luminescence at high temperatures is thus completely opposite from that at low temperatures described previously.