Focused Ion Beam Milling Reveals the Role of Crystal Planes in Perovskite Self-Healing

Self-healing is rarely observed in semiconductors, where structural distortions typically result in an irreversible performance loss. Halide perovskites defy this paradigm, exhibiting spontaneous recovery of optoelectronic properties even at room temperature, yet the underlying mechanisms remain poorly understood. Here, we subject CsPbBr₃ single crystals to facet-oriented focused ion beam (FIB) milling to induce localized mechanical damage and directly track the subsequent healing dynamics. By selectively exposing different crystallographic orientations, we correlate structural reconstruction with photoluminescence recovery. Milling aligned with low-index surfaces enables complete recovery, often with enhanced emission compared to that of the pristine surface, whereas milling across facets, along effectively higher-index crystal planes, leads to permanent emission quenching. The differences arise due to the facet-dependent stabilization and higher formation energies of Br interstitials for higher-index surfaces, a hypothesis that is supported by DFT modeling. Our work establishes facet-oriented FIB milling as a versatile approach for systematically probing self-healing processes in functional materials.