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

Current syntheses of CsPbBr halide perovskite nanocrystals (NCs) rely on overstoichiometric amounts of Pb precursors, resulting in unreacted lead ions at the end of the process. In our synthesis scheme of CsPbBr NCs, we replaced excess Pb 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 CsPbBr cubes capped with oleate species (as for the case when Pb is used in excess) and with a photoluminescence quantum yield (PLQY) as high as 90% with some cations (for example with M = In); group 2 yields irregularly shaped CsPbBr 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.