Luminescence Saturation via Mn²⁺–Exciton Cross Relaxation in Colloidal Doped Semiconductor Nanocrystals

Colloidal Mn²⁺-doped semiconductor nanocrystals such as Mn²⁺:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn²⁺ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn²⁺ ion and an exciton within the same nanocrystal. A lower limit of <i>k</i> = 2 × 10¹⁰ s^–1 is established for the fundamental rate constant of the Mn²⁺(⁴T₁)-exciton cross relaxation