Halogen (Cl, Br, I) tuning of 3-methylpiperidine bismuth halides for highly sensitive, low-dose, and stable X-ray detection

X-ray detection has extensive applications in medical imaging, scientific research, and security inspection. Despite the superior radiation detection properties of lead-based perovskites, including high carrier mobility and superior attenuation efficiency, their toxicity and instability arising from ion migration hinders further development. To address these challenges, a series of halogen-tuned lead-free 3-methylpiperidine bismuth halides, (C 6 H 14 N) 3 Bi 2 I 9 , (C 6 H 14 N) 2 BiBr 5 and (C 6 H 14 N) 2 BiCl 5 (C 6 H 14 N = 3-methylpiperidine) single crystals were grown and systematically characterized. 3-methylpiperidine was selected for its steric effect stabilizing the lattice, and the synergistic tuning through halogen variation enabled a structural transition along with modulation of their properties. Notably, the lateral-structured (C 6 H 14 N) 3 Bi 2 I 9 single-crystal X-ray detector exhibits a sensitivity of 388.05 μC Gy −1 cm −2 , a low detection limit of 71.05 nGy s −1 , and a small dark current drift of 2.16 × 10 –8 nA cm −1  s −1  V −1 . Importantly, this material retains its original morphology after 8 months of storage in ambient conditions. Furthermore, flexible X-ray devices fabricated via spin-coating (C 6 H 14 N) 3 Bi 2 I 9 exhibited a favorable X-ray response with a sensitivity of 121.48 μC Gy −1 cm −2 , as well as excellent mechanical stability. This work reveals that halogen tuning effectively optimizes the structure and performance of Bi-based perovskites, providing valuable insights for advancing them in X-ray detection applications.