Modern advances in x-ray imaging have greatly improved the quality of medical care. The ability to detect low doses of x-rays is critical to the development of safe radiological tools, but available absorber materials have their limitations. Reducing the x-ray dose would not only minimize harm to patients, but also enable innovative applications such as x-ray video techniques. The ideal material for x-ray absorption should have a high effective atomic mass (Z) and mass density, a long charge-carrier drift length, and a low and stable dark-current density. Recently, metal-halide perovskites have shown promising properties for x-ray detection. However, lead-halide perovskites suffer from ion migration and contain toxic lead. Bismuth-based double perovskites, on the other hand, suffer from low charge-carrier drift lengths due to an exciton self-trapping effect. In this work, we demonstrated the enormous potential of bismuth oxyiodide (BiOI) for x-ray detection. Bismuth oxyiodides are twodimensional layered crystals in which slabs of [I–Bi–O–Bi–I] are connected by van der Waals forces (Figure 1a). This material has a high effective Z number and density, resulting in strong x-ray attenuation. Extensive spectroscopic and magneto-optical measurements, as well as first-principles calculations, allow us to elucidate why this material also exhibits a significant drift length, which is essential for x-ray detectors. While photoexcited charge carriers structurally deform the lattice, they form delocalized large polarons instead of self-capturing excitons or small polarons common in other halide compounds. To study the radial expansion of the exciton, we performed transmission experiments in the presence of strong magnetic fields of up to 65 T. By analyzing the shift of the absorption edge as a function of the magnetic field, we determined the coefficient for the diamagnetic shift and derived the radial expansion of the 1-s exciton (Figure 1b). This gave a diamagnetic shift coefficient of 0.43 μeVT-2, resulting in an r.m.s. radius of the 1-s exciton of 15.3 Å. These values are comparable to those of other layered materials (e.g., WS2) and support the two-dimensional Wannier exciton nature, which span multiple unit cells within the plane. The photophysical principles discussed in this study provide novel design opportunities for materials containing heavy elements and low-dimensional electronic structures for x-ray detectors.

Layered BiOI single crystals capable of detecting low dose rates of X-rays, R.A. Jagt, I. Bravić, L. Eyre, K. Gałkowski, J. Borowiec, K. Reddy Dudipala, M. Baranowski, M. Dyksik, T. W. J. van de Goor, T. Kreouzis, M. Xiao, A.Bevan, P. Płochocka, S. D. Stranks, F. Deschler, B. Monserrat, J. L. MacManus-Driscoll, and R. L. Z. Hoye, Nat. Commun. 14, 2452 (2023).

https://www.nature.com/articles/s41467-023-38008-4