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

## TL;DR

This study uses advanced electron microscopy to observe how perovskite quantum dots change under cryogenic cooling, revealing structural transitions and defect dynamics.

## Contribution

The paper introduces in situ STEM techniques to directly visualize strain-coupled phase transitions and defect evolution in perovskite quantum dots under thermal stress.

## Key findings

- Cryogenic cooling induces a reversible orthorhombic-to-monoclinic phase transition in CsPbBr3 quantum dots.
- Strain localization exceeding 20% occurs at surfaces and grain boundaries during phase transitions.
- Controlled cryogenic treatment heals defects but prolonged exposure causes irreversible degradation.

## Abstract

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.

Cryogenic cooling induces an orthorhombic‐to‐monoclinic phase transition in CsPbBr3 quantum dots, accompanied by pronounced strain localization at surfaces and interfaces. Multimodal in situ STEM directly visualize reversible defect healing during moderate cryogenic treatment and irreversible degradation upon prolonged exposure, revealing the intrinsic coupling between structural dynamics, strain evolution, and optical performance in halide perovskites.