In Situ Optical Spectroscopic Probing of Doping Dynamics in Strongly Quantum-Confined CsPbBr ₃ Perovskite Nanoplatelets

The practical application of halide perovskite nanocrystals as scintillators is significantly limited by self-absorption losses caused by their small intrinsic Stokes shift. While Mn2+ doping is an effective strategy to achieve a large Stokes shift via exciton-to-Mn2+ energy transfer, its efficiency in perovskite nanocrystals often remains low. This work demonstrates that strong quantum confinement can effectively enhance the energy transfer efficiency and pinpoints the critical role of the surface adsorption of the Mn2+ dopant on governing the doping efficiency. In situ optical spectroscopic and compositional analyses reveal that efficient energy transfer (up to ∼75%) occurs only in thinner CsPbBr3 nanoplatelets (2 and 3 monolayers), where a high density of surface defect sites facilitates Mn2+ incorporation into the lattice. In contrast, thicker and well-passivated nanoplatelets (4 monolayers) exhibit exciton quenching without Mn2+ emission, indicating energy transfer to surface-adjacent, nonluminescent Mn2+ ions. We introduce a three-regime doping model that classifies Mn2+ ions based on their position relative to the excitonic wavefunction, thereby unifying the observed thickness-dependent phenomena. This study underscores that surface adsorption dictates doping efficiency, providing a mechanistic roadmap for the design of highly efficient scintillators.