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 time-dependent 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.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.