Precision-Guided Formation of Uniform MAPbBr₃ Cuboid Crystals for Stable Light Amplification via Block Copolymer Self-Assembly

Size-uniform and stable halide perovskite crystals are essential for reliable amplified spontaneous emission (ASE) and lasing, yet simultaneous control over crystal morphology, optical quality, and operational stability remains challenging. Here, we demonstrate a block copolymer-assisted strategy for the controlled formation of uniform MAPbBr₃ cuboid crystals using polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP). The P2VP block coordinates with surface Pb²⁺sites through pyridyl nitrogen, while the PS block provides a hydrophobic polymer matrix, enabling spatial confinement, surface passivation, and environmental protection. Under an optimized polymer concentration of 3 wt%, MAPbBr₃ crystals evolve into monodisperse cuboids with lateral dimensions of approximately 400 nm. Structural and spectroscopic analyses confirm the formation of cubic MAPbBr₃ crystals encapsulated by the PS-b-P2VP matrix, with Pb-N coordination contributing to enhanced interfacial stability. The resulting hybrid crystals exhibit sharp photoluminescence at 527 nm with a narrow linewidth of 15 nm, prolonged carrier lifetime, and suppressed nonradiative recombination. Owing to improved optical gain and reduced loss, the PS-b-P2VP/MAPbBr₃ composite shows a distinct ASE peak near 550 nm with a reduced threshold of 57 μJ cm^-2, compared with 81 μJ cm^-2 for pristine MAPbBr₃. Moreover, the composite maintains stable ASE behavior under repeated high-fluence excitation and preserves its crystal structure after 60 days in ambient air. This work highlights block copolymer self-assembly as an effective route to simultaneously regulate perovskite crystal growth, interfacial passivation, and optical amplification stability for solution-processable perovskite laser systems.