Origin of Water-Stable CsPbX(3) Quantum Dots Assisted by Zwitterionic Ligands and Sequential Strategies for Enhanced Luminescence Based on Crystal Evolution.

Water stability is a crucial issue always addressed for commercial practical application of perovskite quantum dots (QDs). Recent advances in ligand engineering for in situ synthesis of water-stable perovskite QDs have attracted growing interest. However, the exact mechanism remains unclear. Here, the function of 4-bromobutyric acid (BBA) and oleylamine (OLA) is systematically studied in water-stable CsPbX X = Br and I QDs and confirms that the zwitterionic ligands generated in situ by BBA and OLA are anchored on the QDs surface, thus preventing water from penetrating into the QDs. CsPbBr intermediate crystal found in the crystal structure evolution process of CsPbX QD further reveals a complete crystallization process: PbX + CsX + Br → CsPbBr crystals + X→ CsPbX QDs + Br. Furthermore, it is found that the solvent coordination of the precursor solution has a significant effect on the crystallinity of CsPbBr intermediate crystal, while the Rb doping can effectively passivate the surface defects of CsPbX QDs, thereby jointly achieving photoluminescence quantum yields (PLQY) of 94.6% for CsPbBr QDs (88.2% for CsPbI QDs). This work provides new insights and guiding ideas for the green synthesis of high-quality and water-stable perovskite QDs.