Interplay between Perovskite Magic-Sized Clusters and Amino Lead Halide Molecular Clusters.

Recent progress has been made on the synthesis and characterization of metal halide perovskite magic-sized clusters (PMSCs) with ABX 3 composition (A = CH3NH3 + or Cs+, B = Pb2+, and X = Cl-, Br-, or I-). However, their mechanism of growth and structure is still not well understood. In our effort to understand their structure and growth, we discovered that a new species can be formed without the CH3NH3 + component, which we name as molecular clusters (MCs). Specifically, CH3NH3PbBr3 PMSCs, with a characteristic absorption peak at 424 nm, are synthesized using PbBr2 and CH3NH3Br as precursors and butylamine (BTYA) and valeric acid (VA) as ligands, while MCs, with an absorption peak at 402 nm, are synthesized using solely PbBr2 and BTYA, without CH3NH3Br. Interestingly, PMSCs are converted spontaneously overtime into MCs. An isosbestic point in their electronic absorption spectra indicates a direct interplay between the PMSCs and MCs. Therefore, we suggest that the MCs are precursors to the PMSCs. From spectroscopic and extended X-ray absorption fine structure (EXAFS) results, we propose some tentative structural models for the MCs. The discovery of the MCs is critical to understanding the growth of PMSCs as well as larger perovskite quantum dots (PQDs) or nanocrystals (PNCs).