Dual-Defect Engineering in Halide Nanocrystals Enables Synergistic Photochromism and Persistent Luminescence for X-Ray Colorimetric Imaging and Dynamic Anti-Counterfeiting
The integration of photochromism (PhCh) and persistent luminescence (PersL) into a single material remains a formidable challenge due to the complex role of defects in modulating optical properties. Here, we employ structurally simple CsX (X = Cl, Br) nanocrystals (NCs) as a model system to elucidate the relationship between defects and optical behaviors. We demonstrate that CsX NCs accommodate two distinct types of chlorine vacancy defects upon X-ray irradiation: intrinsic vacancies from synthesis and X-ray-induced vacancies. This dual-defect engineering enables reversible blue coloration under X-ray irradiation (20–70 kV), attributed to recoverable chlorine vacancies that are rapidly eliminated by visible light within 30 s. The photochromic behavior exhibits excellent cycling stability with a color difference (ΔRL1) of 56.9% and a recovery rate (ΔRL2) of 98.1%. Furthermore, Br− incorporation deepens the energy level of intrinsic chlorine vacancies from 0.47–0.71 eV to 0.83 eV, resulting in intense persistent luminescence lasting over 30 minutes. These dual-mode PhCh–PersL characteristics position CsX NCs as promising candidates for X-ray colorimetric imaging and dynamic anti-counterfeiting applications. Our findings establish a defect-oriented design principle extendable to other halide systems, advancing the development of multifunctional photonic materials.