Harnessing stereochemically active Sn-chemistry to engineer exciton localization in core-shell halide perovskite nanocrystals.

Three-dimensional (3D) halide perovskite nanocrystals (HPNs) offer high photoluminescence quantum yields with compositionally tunable emission; however, they suffer from the tendency of exciton localization toward the surface, leading to nonradiative losses and thereby necessitating strategies that confine excitations away from surface states. Core-shell architectures provide a route to spatially control exciton localization within the core, thereby suppressing surface-mediated losses. Despite growing experimental efforts, atomistic understanding of emission in core-shell HPNs remains limited. Conventional computational approaches rely on bulk or slab-based models and often interpret emission behaviour solely from the ground-state electronic structure, thereby neglecting excited-state structural and electronic relaxation and failing to capture finite three-dimensional confinement and realistic carrier distributions in nanocrystals (NCs). In this study, using state-of-the-art density functional theory, we have explored the excited-state carrier localization and emission behaviour in all inorganic core-shell [Cs(Pb/Sn)(Cl/Br/I)-CsPbBr and Cs(Pb/Sn)(Cl/Br/I)-CsPbI] NCs. We have demonstrated that excitons can be selectively localized at the core or shell by tuning the intrinsic interfacial electronic structure and inducing local structural asymmetry. Importantly, confinement is not solely governed by ground-state halides or metal energy level positions as conventionally considered; instead, it can be reversed by the stereochemical activity of metal s lone pairs, which stabilize excitons in a self-trapped nature. s lone-pair activation dominates over halide energy-level considerations, enabling emission even from wider-bandgap regions of the core-shell heterostructure. This mechanism remains robust across shell compositions and NC sizes, thereby providing broad tunability of the emission wavelength. Notably, harnessing the advantageous excited-state chemistry of Sn at the core combined with CsPbBr shells shows the strongest stabilization of core-localized excitons, reducing surface-related instability and enabling broadband emission. Overall, our results establish the excited-state s lone-pair activation as a practical design parameter to enforce carrier confinement and suppress surface mediated nonradiative losses, thus providing a general framework for exciton engineering in 3D core-shell HPNs.