Influence of Ligand Exchange on Single Particle Properties of Cesium Lead Bromide Quantum Dots

Improving the properties of perovskite quantum dots (QDs) for quantum-light applications such as single photon emission requires a systematic understanding of the influence of ligand surface chemistry on single particle properties including line width and photoluminescence (PL) blinking. Here, we investigate the influence of ligand exchange on both the optical and structural properties of the single QDs. We examine PL blinking using a wide-field fluorescence microscope with ligand-exchanged QDs. We find that the zwitterionic ligands lecithin and phosphoethanolamine (PEA-C8C12) reduce blinking compared to dodecylammonium bromide (DDAB) and a bidentate dicationic quaternary ammonium bromide (DC) ligands, which have mono- and bidentate cationic head groups, respectively. The champion PEA-C8C12 shows a nonblinking fraction of 0.21, compared to 0.01 for the cationic-capped QDs. We further investigate the effect of ligand capping on low-temperature line width. We probe single particle line widths and show that zwitterionic PEA-C8C12 and lecithin outperform the cationic surface ligands for ligand-exchanged samples, having a narrower average single particle line width (24 meV for lecithin and 18 meV for PEA-C8C12 compared to 39 meV for DC-capped and 42 meV for DDAB-capped QDs) over long integration times. We rationalize these findings by quantifying ligand surface coverage using nuclear magnetic resonance, observing that QDs capped by zwitterions have higher ligand surface coverage (6.5 ± 1.6 for lecithin and 7.4 ± 1.9 ligands/nm2 for PEA), consistent with their superior performance at the single particle level. These line width and blinking results show that zwitterionic ligands are a preferable design strategy for reducing PL blinking and improving single particle line widths.