Gas‑phase construction of compact capping layers for high‑performance halide perovskite x‑ray detectors

Medical X-ray imaging demands detectors that are both ultra-sensitive and stable under high bias, yet ion migration and surface defects still plague 3-D perovskites. A Shenzhen Technology University-led team (Prof. Shuang Xiao & Prof. Xingzhu Wang) now reports a one-step gas-phase route that grows nanometre-thin, pin-hole-free 2-D (PA)₂PbBr₄ or (HDA)PbBr₄ capping skins directly onto MAPbBr₃ single crystals in <1 min. The resulting 2-D/3-D heterojunction delivers a record sensitivity of 22 245 µC Gyₐᵢᵣ^-1 cm^-2, 240 µs response time and a dark-current drift only 1.17 × 10^-4 nA cm^-1 s^-1 V^-1—beating the world’s best perovskite X-ray detectors by >10× in signal stability.

Why This Matters

• Ultra-Low Dose Imaging: Detection limit drops to 0.13 µGyₐᵢᵣ s^-1 (≈1/100 of a chest scan), enabling paediatric and dental CT with <50 % exposure.
• Ion-Migration Shut-Down: DFT and temperature-conductivity tests show Br⁻ activation energy doubles (0.23 eV) under the sterically crowded propyl-ammonium layer, suppressing bias-induced drift by two orders of magnitude.
• Photoconductive Gain: Staggered type-II band alignment and trap density reduction (9 → 1.5 × 10¹⁰ cm^-3) extend carrier lifetime to 521 ns, multiplying signal without extra noise.
• Ambient Stability: After 2.15 Gy cumulative dose (≈10⁵ chest X-rays), capped crystals retain >99 % sensitivity and square-wave response in 40 % RH air—no encapsulation needed.

Innovative Design & Features

• Gas-Phase Vapor Exchange: Amine vapour (PA or HDA) reacts with the crystal surface for 40–180 s, converting <300 nm of 3-D lattice into pure n = 1 2-D phase; growth rate scales inversely with molecular size, giving angstrom-level control.
• Steric Hindrance Engineering: Ruddlesden–Popper (PA)₂PbBr₄ exhibits tighter Pb²⁺-NH₃⁺ coordination and higher crystal-space occupancy than Dion–Jacobson (HDA)PbBr₄, yielding larger dangling-bond formation energy and stronger Br⁻ confinement.
• Negligible Parasitic Absorption: 300 nm (PA)₂PbBr₄ attenuates only 0.022 % of 60 keV X-rays; 82.9 % absorption still occurs in the 1.5 mm MAPbBr₃ bulk, preserving high quantum efficiency.
• Scalable & Universal: Process works on MAPbI₃, FAPbI₃ and polycrystalline films; no solvents or high vacuum, compatible with roll-to-roll vapor reactors.

Applications & Future Outlook

• Portable Point-of-Care Scanners: 50 µm-pixel arrays on flexible PI are being fabricated for bedside neonatal imaging, targeting 50 µGy per frame—below the yearly background dose.
• Security & Food Inspection: High-speed chopper (1 kHz) and 240 µs response enable real-time line-scanning of luggage and agricultural products at 5 m s^-1 belt speed.
• Next-Gen Synthesis: Team is extending the chemistry to mixed-halide and Pb-free perovskites, while integrating the process into existing MOCVD lines for 200 mm wafer processing.

By turning a simple vapor cue into an atomically precise ionic barrier, the work delivers the first perovskite X-ray detector that unites hospital-grade sensitivity, microsecond speed and rock-solid baseline—paving the way for safer, sharper and greener medical imaging anywhere the beam shines.