Surface Passivation of Cs₂AgI₃:Cu with AgI for High‐Performance X‐Ray Imaging Scintillators

High‐performance X‐ray scintillators are essential for advanced imaging technologies in various fields, including security, aerospace, high‐energy physics, and health care. However, the existing scintillation materials in the X‐ray community face significant challenges related to low light yield, long decay time, afterglow, and toxicity. This study reports a novel post‐synthesis surface engineering strategy for copper (Cu)‐doped dicesium silver iodide (Cs 2 AgI 3 ) using a precisely controlled silver iodide (AgI) treatment to address surface defects, significantly enhancing radiative channels. Density functional theory calculations indicate that AgI treatment passivates the surface defects introduced by Cu⁺ doping, substantially reducing nonradiative recombination centers. Thus, the proposed scintillator achieves an exceptional light yield of over 55 000 photons MeV −1 and a rapid response time of ≈426.4 ns, significantly outperforming the existing commercial scintillators. Furthermore, the scintillator film exhibits an impressive X‐ray imaging resolution (18.5 lp mm −1 ), enhanced durability, and easy processing, facilitating the scalable production of flexible scintillation screens. These significant advancements underscore the potential of the surface engineering strategy for next‐generation scintillation materials in X‑ray imaging technology.