L-Type Ligand-Assisted Acid-Free Synthesis of CsPbBr3 Nanocrystals with Near-Unity Photoluminescence Quantum Yield and High Stability

X-type ligands, for example, the pair of oleylamine (OAm) and oleic acid (OA), have been widely used to prepare CsPbX3 nanocrystals (NCs). However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in poor performance after cleaning or long-time storage. Herein, density functional theory calculations predict that primary amines (L-type ligands) can stabilize a PbBrx-rich surface and yield a trap-free material with fully delocalized valence band maximum and conduction band minimum states, which can significantly improve the photophysical properties and stability of CsPbBr3 NCs. Along this prediction, a room-temperature reprecipitation method using L-type ligands (OAm, n-octylamine, or undecylamine) as the sole capping ligand has been developed to synthesize high-quality CsPbBr3 NCs with near-unity photoluminescence quantum yield and dramatically improved stability against purification and water treatment. The enhancement can be attributed to the strong binding of unprotonated amines to lead atoms and the effective surface passivation provided by the resulted PbBrx-rich surface, which are highly consistent with the theoretical predictions. This work not only offers an approach to synthesize high-quality perovskite NCs but also provides an in-depth understanding of the surface modification of CsPbX3 NCs for practical applications.