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 CsPbBr₃ 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.