Unravelling the Atomic-Scale Surface Chemistry for Intrinsically Bright and Stable Luminescence in CdSe Quantum Platelets.

Two-dimensional colloidal CdSe quantum platelets (QPLs) represent a promising class of next-generation luminescent materials, yet the atomic-level surface chemistry that governs their optoelectronic properties remains poorly understood. In this study, we combine ligand passivation experiments with density functional theory (DFT) to establish a comprehensive atomic-scale surface structure model. The model explicitly identifies four characteristic surface features of pristine QPLs: under-coordinated Cd and Se atoms, along with CdX and SeMX binding modes. Our results demonstrate that native X-type carboxylates provide baseline Cd-site passivation (CdX), yielding an initial photoluminescence quantum yield (PLQY) of approximately 30%. L-type amines or phosphines disrupt native CdX passivation by forming CdXL, CdL, or SeL configurations, which induces more deep traps and quench fluorescence. Z*-type halide-amine ligands initially induce similar disruption, but they ultimately reorganize into a stable SeMXL binding mode that surpasses the original emission intensity. In contrast, Z-type ligands can rapidly achieve complete surface passivation via SeMX coordination by effectively eliminating Se-site deep trap states, boosting their fluorescence. In particular, passivation with Z-type cadmium octanoate equips the QPLs synthesized in ambient air with a record PLQY of 95%, together with exceptional spectral purity and enhanced emission stability.