Methylammonium lead bromide perovskite magic-sized clusters and quantum dots were synthesized using a new heated ligand assisted reprecipitation (HLARP) technique using organic amines and acids as capping ligands. The optical properties of these nanoparticles were analyzed using UV−vis electronic absorption and photoluminescent spectroscopy. Varying the temperature of the precursor solution while keeping the antisolvent temperature consistent allows for tuning between perovskite magic-sized clusters (MSCs) and quantum dots (PQDs) without the need to use excessive concentrations of capping ligand. Higher precursor solution temperatures favor MSCs, while lower temperatures favor PQDs. Furthermore, increasing the temperature of the system shifts the original emission band from 436 to 453 nm, by increasing the size and potentially through the introduction of surface defects. Low frequency Raman spectroscopy reveals that MSCs have vibrational frequencies that are similar to those of bulk perovskite. Electrospray mass spectrometry and infrared spectroscopy were used to probe the ligands on the surface of the MSCs, indicating that amine is the primary capping ligand and the surface is presumably cation rich.

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.

BackgroundResearch into perovskite nanocrystals (PNCs) has uncovered interesting properties compared to their bulk counterparts, including tunable optical properties due to size‐dependent quantum confinement effect (QCE). More recently, smaller PNCs with even stronger QCE have been discovered, such as perovskite magic sized clusters (PMSCs) and ligand passivated PbX2 metal halide molecular clusters (MHMCs) analogous to perovskites.ObjectiveThis review aims to present recent data comparing and contrasting the optical and structural properties of PQDs, PMSCs, and MHMCs, where CsPbBr3 PQDs have first excitonic absorption around 520 nm, the corresponding PMSCS have absorption around 420 nm, and ligand passivated MHMCs absorb around 400 nm.ResultsCompared to normal perovskite quantum dots (PQDs), these clusters exhibit both a much bluer optical absorption and emission and larger surface‐to‐volume (S/V) ratio. Due to their larger S/V ratio, the clusters tend to have more surface defects that require more effective passivation for stability.ConclusionRecent study of novel clusters has led to better understanding of their properties. The sharper optical bands of clusters indicate relatively narrow or single size distribution, which, in conjunction with their blue absorption and emission, makes them potentially attractive for applications in fields such as blue single photon emission.