Bi-Assisted Double-Sided ZnO Nanorod Architectures for Fast and Geometry-Enhanced X‑Ray Scintillation

Bi-assisted ZnO nanorod (NR) arrays are investigated as thin, transparent, and fast nanostructured scintillator architectures for X-ray detection. Vertically aligned ZnO:Bi NR arrays were grown hydrothermally on fused-silica substrates in single-sided, refresh-grown, and double-sided configurations to investigate the effects of Bi-assisted growth, effective scintillating volume, and collection geometry on the measured luminescence response. Structural and morphological analyses show that moderate Bi-assisted growth preserves the preferential c-axis-oriented wurtzite ZnO structure while producing dense, high-aspect-ratio NR arrays. Optical spectroscopy reveals a visible-emission-dominant response, with broad yellow-orange photoluminescence centered at ∼580-600 nm and weak near-band-edge emission. Cathodoluminescence imaging and spectral mapping further show that the visible emission is spatially enhanced at NR edge/sidewall regions, supporting a surface/defect-mediated recombination mechanism. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analyses of representative ZnO:Bi samples support the presence of Bi species and surface/defect-related oxygen contributions. Time-resolved PL measurements yield a fast amplitude-weighted average lifetime of ∼5.3 ns, indicating rapid defect-mediated visible recombination. Under 45 kV X-ray tube voltage, double-sided ZnO:Bi NR arrays exhibit broad radioluminescence centered at ∼600 nm in both reflection and transmission geometries. Under the investigated measurement conditions, the integrated detected RL intensity in transmission geometry is approximately 6-8 times that measured in reflection geometry. Among the double-sided samples, ZnO:Bi (5%) provides the highest measured transmission-mode RL intensity, whereas higher nominal Bi loading reduces the response, consistent with increased disorder and nonradiative loss pathways. The spectral separation between strong near-band-edge absorption and yellow-orange RL emission may reduce reabsorption losses and facilitate visible-light transport through the NR architecture. These results demonstrate that Bi-assisted double-sided ZnO NR architectures provide a promising platform for fast, transparent, and geometry-enhanced X-ray scintillators.