Ligand-Engineered Methylammonium Lead Bromide Nanoplatelets: Single-Photon Emission and Strong Light-Matter Coupling

Lead halide perovskite nanoplatelets (LHP NPLs) are of immense interest in the materials science and optoelectronics communities owing to their strong quantum confinement leading to narrow emission peaks, thickness-dependent tunable photoluminescence, and large exciton binding energies. Thus far, their further development and photophysical investigations at ensemble and single-particle levels have been impeded by suboptimal ligand passivation, inferior environmental and colloidal stability compared to their 3D nanocrystal counterparts, and limited compositional diversity. Here, we report highly monodisperse methylammonium lead bromide (MAPbBr₃) NPLs (11.3 ± 2.3 × 1.7 ± 0.5 nm) with tunable ligand chemistry and enhanced photoluminescence quantum yields of up to 80%. NPLs capped with zwitterionic ligands exhibit improved stability upon air exposure and sequential purification. Nuclear magnetic resonance spectroscopy on ligand-exchanged NPLs, capped with either phosphocholine- or phosphoethanolamine-type ligands, confirms the partial replacement of the pristine ligands. Owing to the size and shape uniformity of synthesized MAPbBr₃ NPLs, they readily form assemblies of stacked face-to-face NPLs, as observed in both colloidal dispersions and films, giving rise to concentration-dependent multicolor emission. The temperature dependence of the NPL emission exhibits a nonmonotonous trend, attributed to the highly anisotropic confinement and the consequent exciton-phonon coupling. We also observed photoluminescence from single MAPbBr₃ NPLs at both room and cryogenic temperatures, revealing highly polarized fine-structure emission lines with single-photon purity exceeding 80%. The high uniformity and optical transparency of MAPbBr₃ NPL films enabled their integration into optical cavities, where they exhibited strong light-matter coupling with a substantial Rabi splitting of 200 meV.