Metal halide perovskites, such as methylammonium lead bromide, have recently attracted considerable attention due to their interesting and useful photoelectric properties. Here, two types of methylammonium lead bromide magic-sized clusters (MSCs), passivated with oleylamine and oleic acid, were synthesized using ligand-assisted reprecipitation (LARP) and heated LARP (HLARP) methods. The optical properties of these MSCs were characterized using UV−vis electronic absorption and photoluminescence (PL) spectroscopies. The HLARP synthesis resulted in a two-fold increase in the PL quantum yield of the MSCs to 76%. The stability of the MSCs was tested using timedependent PL spectroscopy. LARP MSCs in solution degraded completely after 14 days under ambient conditions, while HLARP MSCs lasted for 26 days. To stabilize them, the MSCs were added to a non-coordinating matrix, paraffin. Both MSCs showed significantly improved resistance to water with the addition of paraffin. Solid LARP MSCs lost all luminescence with and without the addition of paraffin by about 3 h. Solid HLARP MSCs without paraffin started to aggregate after 3 h, but paraffin stabilized HLARP MSC films were stable for 8 days. This improved stability in solid state form allowed for accurate, nonaggregated analysis using Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Raman spectroscopy revealed that the HLARP MSCs show an additional peak at 147 cm −1 compared to LARP MSCs, which is attributed to methylammonium. X-ray diffraction and transmission electron microscopy confirm that MSCs have a quasi-crystalline orthorhombic structure.

In the synthesis of cesium lead bromide (CsPbBr 3 ) perovskite quantum dots, with an electronic absorption and emission band around 510 nm, and perovskite magic-sized clusters (PMSCs), with an electronic absorption and emission band around 430 nm, another distinct absorption and emission around 400 nm is often observed. While many would attribute this band to small perovskite particles, here we show strong evidence that this band is a result of the formation of lead bromide molecular clusters (PbBr 2 MCs) passivated with ligands, which do not contain the A component of the ABX 3 perovskite structure. This evidence comes from a systematic comparative study of the reaction products with and without the A component under otherwise identical experimental conditions. The results support that the near 400 nm band originates from ligand-passivated PbBr 2 MCs. This observation seems to be quite general and is significant in understanding the nature of the reaction products in the synthesis of metal halide perovskite nanostructures.