Visualizing Strain‐Coupled Cryogenic Phase Transitions and Defect Dynamics in Perovskite Quantum Dots Using In Situ STEM

Perovskite quantum dots (PeQDs) offer high photoluminescence quantum efficiencies, precise spectral tunability, and solution‐processability, making them promising for advanced optoelectronics. However, their structural and defect evolution under thermal stress remains poorly understood. Here, direct nanoscale insights are provided into temperature‐driven phase transition and defect dynamics in CsPbBr 3 PeQDs using high‐resolution, high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) images, 4D STEM, and photoluminescence spectroscopy. Sub‐ångström imaging at room temperature reveals inherent atomic features and octahedral tilting of the lead halide perovskite lattice in PeQDs, suggesting a pre‐tilted, low‐symmetry state before thermal perturbation. The cryogenic cooling induces a reversible orthorhombic‐to‐monoclinic phase transition, distinct from bulk perovskite behavior and accompanied by severe strain localization exceeding 20% at surfaces and grain boundaries. A controlled cryogenic post‐synthesis treatment can effectively heal defects and improve radiative recombination, whereas prolonged cryo‐treatment introduces irreversible structural degradation. These findings highlight the intrinsic structural flexibility of PeQDs and provide a scalable post‐synthesis treatment method to optimize the stability and efficiency of QDs for various optoelectronic applications.