Acetate‐Doping Enables Multimodal Emission and Robust Scintillation in Cs ₃ Cu ₂ I ₅

Low‐dimensional Cu(I)‐based metal halides are attractive luminescent materials for optoelectronics and radiation detection, but achieving precise control over their multimodal emission and scintillation performance remains challenging. Here, the acetate anion is introduced into zero‐dimensional Cs 3 Cu 2 I 5 to engineer excitation‐wavelength‐dependent emission and scintillation. The acetate‐doped Cs 3 Cu 2 I 5 crystals show dual‐band photoluminescence, switching from blue to green emission with increasing excitation wavelength. Spectroscopic analyses and theoretical calculations suggest that this behavior is most plausibly associated with the coexistence of host self‐trapped excitons (STEs) and dopant‐induced extrinsic STEs. Acetate doping also increases the radioluminescence output and improves the scintillation response of Cs 3 Cu 2 I 5 crystals, affording a high light yield of 57 600 photons MeV −1 , a low detection limit of 166.6 nGy air s −1 , and a high spatial resolution of 18 lp mm −1 in flexible scintillator films. This work establishes organic‐anion doping as an effective route to modulate the excited‐state landscape and scintillation behavior of full inorganic Cu(I)‐based halides and expands their opportunities in light‐emitting devices and high‐energy radiation detection.