Exogenous Metal Cations in the Synthesis of CsPbBr₃ Nanocrystals and Their Interplay with Tertiary Amines

Current syntheses of CsPbBr₃ halide perovskite nanocrystals (NCs) rely on overstoichiometric amounts of Pb²⁺ precursors, resulting in unreacted lead ions at the end of the process. In our synthesis scheme of CsPbBr₃ NCs, we replaced excess Pb²⁺ with different exogenous metal cations (M) and investigated their effect on the synthesis products. These cations can be divided into two groups: group 1 delivers monodisperse CsPbBr₃ cubes capped with oleate species (as for the case when Pb²⁺ is used in excess) and with a photoluminescence quantum yield (PLQY) as high as 90% with some cations (for example with M = In³⁺); group 2 yields irregularly shaped CsPbBr₃ NCs with broad size distributions. In both cases, the addition of a tertiary ammonium cation (didodecylmethylammonium, DDMA⁺) during the synthesis, after the nucleation of the NCs, reshapes the NCs to monodisperse truncated cubes. Such NCs feature a mixed oleate/DDMA⁺ surface termination with PLQY values of up to 97%. For group 1 cations this happens only if the ammonium cation is directly added as a salt (DDMA-Br), while for group 2 cations this happens even if the corresponding tertiary amine (DDMA) is added, instead of DDMA-Br. This is attributed to the fact that only group 2 cations can facilitate the protonation of DDMA by the excess oleic acid present in the reaction environment. In all cases studied, the incorporation of M cations is marginal, and the reshaping of the NCs is only transient: if the reactions are run for a long time, the truncated cubes evolve to cubes.