Designer Sulfonium-Based Capping Ligands for Lead Halide Perovskite Nanocrystals

The ongoing quest for improved capping ligands for lead halide perovskite nanocrystals (LHP NCs) is fueled by the immense potential of these emitters as classical and quantum light sources. Herein, we introduce a structurally diverse library of long-chain trialkylsulfonium ligands that provide robust surface passivation of CsPbBr3 and MAPbBr3 NCs, achieving photoluminescence quantum yields approaching 90% for cationic and exceeding 90% for zwitterionic ligands. Classical force-field molecular dynamics simulations assess the trialkylsulfonium ligand headgroup as a close analogue of a frequently used quaternary ammonium headgroup in terms of its binding to the perovskite surface. When comparing trialkylsulfonium ligands that differ by the number and positions of long alkyl chains, the conformational flexibility of the ligand was found to play a dominant role in determining the ligand binding strength, surpassing the influence of headgroup geometry or substitution. Guided by this fundamental understanding, we rationally designed sulfonium sulfonate zwitterionic ligands that form robust colloids, also under extreme dilution. The resulting NCs exhibit a suppressed blinking rate (on-time fraction ∼85%) and high single-photon purity (g2 (0) = 0.12). Furthermore, applying "entropic" sulfonium ligands facilitates the stabilization of highly concentrated NC colloids (up to 1.1 g of inorganic mass per mL). These findings benchmark sulfonium-based ligands as practically applicable for diverse applications of LHP NCs, including quantum photonics and downconversion layers for displays.