Structural, optical and thermoluminescence properties of Eu³⁺ and Cr³⁺ doped ZnAl₂O₄ phosphors

Zinc aluminate (ZnAl₂O₄), a wide-bandgap spinel oxide, is an attractive host for photonic and dosimetric applications due to its excellent structural and optical stability. In this study, Eu³⁺ and Cr³⁺ doped ZnAl₂O₄ nanophosphors were synthesized via a facile combustion method and characterized using XRD, TEM, UV–Vis, FTIR, photoluminescence (PL), and thermoluminescence (TL) techniques. XRD confirmed single-phase cubic spinel formation, with doped samples exhibiting enhanced crystallinity and larger crystallite sizes (23–42 nm). Optical studies revealed a slight bandgap shift (3.18–3.22 eV) influenced by dopant incorporation . The results also demonstrate that Eu³⁺ and Cr³⁺ incorporation induces dopant dependent defect states in ZnAl₂O₄ nanophosphors, leading to distinct photoluminescence and thermoluminescence responses. Eu³⁺-doped samples show strong ⁵D₀→⁷F₂ red emission indicative of defect-associated low-symmetry environments, whereas Cr³⁺-doped ZnAl₂O₄ exhibits characteristic R-line emission and multiple TL glow peaks arising from dopant-related traps. The extracted trap depth (~0.9–1.0 eV) and frequency factor (~10⁸-10 9 s⁻¹) confirm this. The combined PL–TL analysis establishes a clear correlation between dopant chemistry, defect formation, and optical behavior. • Generalized exponential conduction model : We introduce and experimentally validate a transport equation that includes voltage and current offsets , quantitatively capturing the asymmetry in I–V characteristics. This scaled model describes trap-assisted emission beyond standard Poole scaling. • Crossover from Poole to Frenkel regimes : Through gate-modulated measurements, we demonstrate a crossover in scaling behaviour —from linear (Poole) to square-root (Frenkel) field dependence—across device configurations and bias quadrants. This crossover is rarely resolved in a unified experimental framework. • Electrostatic control of trap emission barrier : Gate voltage modulation in n-SnS/p-Si devices reveal field-tunable barrier height , confirmed by shifts in the scaling parameter (Vo), distinguishing electrostatic barrier tuning from carrier depletion effects.