The rapid development of halide perovskite synthesis offers the opportunity to fabricate high-quality perovskite nanocrystals (NCs), whose structural uniformity can lead to assembled supra-structures with improved device performance and novel collective properties. Light is known to significantly affect the structure and properties of halide perovskites and plays a crucial role in the growth and assembly of their crystals. Nevertheless, the light-induced growth mechanisms of perovskite NCs are not yet clearly understood. In this work, we performed a systematic study of the visible-light-induced template-free synthesis of CsPbBr₃ nanowires (NWs) generated through self-assembly of cubic (in phase and close to cubic morphology) NCs. Using atomic-resolution electron microscopy, we visualized the cubic-to-orthorhombic phase transition in NCs and the interface between coalesced NCs. Remarkably, the images of the …