Light-Induced Ordered Pattern Formation in 2D Copper Halide Perovskites

We report the formation of highly ordered photoinduced patterns in two-dimensional layered copper halide perovskites under ultraviolet illumination in an ultrahigh vacuum. In contrast to conventional random degradation observed in metal halide perovskites, these perovskites form directional nanoscale grooves with organic cation-dependent symmetries: (PEA)₂CuCl₄ exhibits 4-fold symmetry, and (iBuA)₂CuCl₄ shows 8-fold symmetry, while (MA)₂CuCl₄ displays no ordered patterning. Through photoemission electron microscopy, atomic force microscopy, and scanning transmission electron microscopy with spectroscopic analysis, we demonstrate that these patterns result from directional Cu-Cl bond breaking, forming metallic Cu nanoparticles, while organic components are lost to the environment. Nonequilibrium statistical mechanical calculations reveal that anisotropic electron-phonon coupling, modulated by the organic cations, drives this preferential bond breaking. These findings elucidate degradation mechanisms in Cu-based halide perovskites and pathways for using directional electron-phonon coupling on the atomic scale to control mesoscale material patterns.