Enhanced Core–Valence Luminescence in Ultrafast Cs ₂ ZnCl ₄ Scintillation Crystals via Bandgap Engineering

Core–valence luminescence (CVL) in Cs2ZnCl4, typically characterized by a subnanosecond decay time, holds great value in the field of ultrafast scintillation applications. However, its light yield is comparatively low, and the underlying regulatory mechanism of CVL remains inadequate. In this work, the study revealed the inherent origin and enhancement regulation mechanism of CVL in F–- or Rb+-doped Cs2ZnCl4 single crystals. The doped Cs2ZnCl4 showed red-shifted emission at 223 nm excitation with an enhanced Stokes shift, effectively reducing self-absorption. Rb+ doping introduced deeper Rb 4p orbitals, which hindered hole participation in CVL. In contrast, F– doping introduced F 2p orbitals in the valence band, narrowing the core valence gap while broadening Cs 5p and Zn 3d levels, thereby enhancing the electron–hole recombination efficiency. Consequently, the light yield increased by 24% compared with pure Cs2ZnCl4. Remarkably, the radioluminescence (RL) intensity of Cs2ZnCl4:F was significantly enhanced under X-ray excitation while preserving its ultrafast decay characteristics (1.83 ± 0.006 ns). This work provides new insights into the modulation mechanisms of CVL and advances the rational design of CsCl-based halides for ultrafast scintillators.