Simultaneous Cationic and Anionic Ligand Exchange For Colloidally Stable CsPbBr3 Nanocrystals

Post-synthesis ligand exchange has been employed extensively on lead halide perovskite (LHP) nanocrystals (NCs), but the complex ligand shell composition of the starting NCs prevented a clear understanding of the exchange process and the surface chemistry of the final NCs remained poorly characterized. Here, we describe a ligand exchange strategy involving the displacement of both cationic and anionic ligands on native model systems of CsPbBr3 NCs, which are exclusively coated with Cs-oleate. These ligands are exchanged with various quaternary ammonium bromides (R4NBr) and complete exchange is confirmed by nuclear magnetic resonance (NMR) spectroscopy analysis. The displacement of the native Cs-oleate ligands with proton-free R4NBr delivers NCs with excellent colloidal stability and near-unity PLQY, which is preserved after washing with polar solvents and over three weeks of storage in air, and after heating a solution of NCs to 80 °C, as confirmed by NMR analysis. The results, together with density functional theory calculations, suggest that the higher stability of quaternary ammonium capped NC is not due to a stronger binding interaction to the surface, but rather to weaker solventligand interactions of R4NBr compared to Cs-oleate, driving the former to the surface of the NC.