Space‐Confined Kinetic Growth of Thick 2D Perovskite Single Crystals for High‐Performance X‐Ray Detection

Two‐dimensional (2D) perovskites have emerged as promising scintillators for ultrafast X‐ray imaging due to their unique quantum confinement effects. However, their intrinsic thermodynamic growth habits, driven by highly anisotropic chemical bond strengths, typically restrict crystals to thin, plate‐like morphologies, which limits their X‐ray stopping power and overall light yield. Here, we report a space‐confined kinetic manipulation strategy to circumvent the conventional thermodynamic thickness limitations, enabling the growth of macroscopically thick phenethylammonium lead bromide single crystals (PEA 2 PbBr 4 SCs). By reallocating chemical potential gradients and maintaining a stable metastable window, we successfully accelerated longitudinal growth along the ( 00l ) facets, achieving crystals with thicknesses exceeding 3.2 mm and a low lateral‐to‐longitudinal aspect ratio (<3). The resulting SCs exhibit exceptional structural quality, and the enhanced X‐ray attenuation in these thick SCs leads to a superior light yield of 37 500 photons MeV −1 , doubling that of the thin counterparts (0.6 mm, 15 224 photons MeV −1 ). This work provides a robust pathway for synthesizing thick 2D perovskite SCs for high‐performance radiation detection and imaging without compromising their inherent exciton confinement advantages.