Solution-Processed Hybrid Europium (II) Iodide Scintillator for Sensitive X-Ray Detection

Lead halide perovskite nanocrystals have recently demonstrated great potential as x-ray scintillators, yet they still suffer toxicity issues, inferior light yield (LY) caused by severe self-absorption. Nontoxic bivalent europium ions (Eu²⁺) with intrinsically efficient and self-absorption-free d-f transition are a prospective replacement for the toxic Pb²⁺. Here, we demonstrated solution-processed organic-inorganic hybrid halide BA₁₀EuI₁₂ (BA denotes C₄H₉NH₄⁺) single crystals for the first time. BA₁₀EuI₁₂ was crystallized in a monoclinic space group of P2₁/c, with photoactive sites of [EuI₆]^4- octahedra isolated by BA⁺ cations, which exhibited high photoluminescence quantum yield of 72.5% and large Stokes shift of 97 nm. These properties enable an appreciable LY value of 79.6% of LYSO (equivalent to ~27,000 photons per MeV) for BA₁₀EuI₁₂. Moreover, BA₁₀EuI₁₂ shows a short excited-state lifetime (151 ns) due to the parity-allowed d-f transition, which boosts the potential of BA₁₀EuI₁₂ for use in real-time dynamic imaging and computer tomography applications. In addition, BA₁₀EuI₁₂ demonstrates a decent linear scintillation response ranging from 9.21 μGy_air s^-1 to 145 μGy_air s^-1 and a detection limit as low as 5.83 nGy_air s^-1. The x-ray imaging measurement was performed using BA₁₀EuI₁₂ polystyrene (PS) composite film as a scintillation screen, which exhibited clear images of objects under x-ray irradiation. The spatial resolution was determined to be 8.95 lp mm^-1 at modulation transfer function = 0.2 for BA₁₀EuI₁₂/PS composite scintillation screen. We anticipate that this work will stimulate the exploration of d-f transition lanthanide metal halides for sensitive x-ray scintillators.