Role of Size and Shape in Photoluminescence and Ultra-Low-Frequency Raman of Methylammonium Lead Iodide Perovskite Quantum Dots

The photophysical properties of methylammonium lead iodide (MAPbI₃) quantum dots (QDs) have not been systematically studied for size and shape dependence. Here, we synthesize MAPbI₃ QDs using ligand-assisted reprecipitation, controlling the injection speed and reaction times to produce QDs with different sizes and shapes. Dropwise injection yields ∼5 nm spherical QDs, emitting photoluminescence (PL) at 2.06 eV. In contrast, swift injections yield larger (>10 nm) rectangular QDs with varying aspect ratios, supported by an infinite quantum well model. The PL lifetime of QDs increases with their size, and the size variation significantly influences the ultra-low-frequency Raman modes at 81, 107, and 127 cm^-1, in contrast to what is observed in polymorphic MAPbI₃ thin films. Our findings, supported by first-principles density functional theory, show that key PL and Raman properties are governed by the sizes and shapes of MAPbI₃ QDs. This study contributes to the understanding of the optical behavior of these QDs, which is crucial for their potential applications and environmental implications.