Energy-Transfer Pathways That Sensitize Mn ²⁺ Emissions Using Mn@CsPbCl ₃ Nanocrystals as a Generic Platform

Photoexcitation of various Mn2+-doped semiconductor nanocrystal hosts gives rise to the celebrated Mn2+ emission. Yet, despite extensive research, the mechanism of sensitizing Mn2+ ions remains little understood. We use Mn2+-doped CsPbCl3 nanocrystals as a platform to probe the de-excitation pathways, using thermal-dependencies of steady-state and time-gated excitonic and dopant emissions, establishing some universal behaviors. By employing many-body ion-in-a-crystal-field calculations to obtain the multiplet structure of the dopant, we reveal a near-resonant energy-transfer from the host to the higher-lying excited 4Eg state of Mn2+, rather than the lowest excited 4T1g state, providing an explanation for the long-standing puzzle of ultrafast Mn2+ sensitization despite the pronounced Stokes-shifted emission. Subsequent relaxations from 4Eg state to 4T1g state involving a series of spin-allowed intermediate multiplet states are achieved through multiphonon processes, leading to phonon bottlenecks with a distinctive threshold-like temperature-dependence. We derive the temperature-dependent rate equations that quantitatively explain the observed thermal-dependencies, reinforcing the proposed host-to-dopant energy-transfer mechanism.