Cr3+–Cr3+ pairing and multi-site engineering in Sr2ZnAl22O36 phosphors for broadband near-infrared emissions

Abstract

Cr³⁺-activated phosphors are advancing near-infrared (NIR) technology, offering broadband emissions crucial for bioimaging, night vision, and optical communication. In the present study, we introduce a novel Sr₂ZnAl₂₂O₃₆:xCr³⁺ (SZAO) phosphor, designed for enhanced luminescence and thermal stability. X-ray powder diffraction (XRD) and Rietveld refinement confirmed its phase purity, while bond valence sum and bond energy calculations provided insights into Cr³⁺ ion incorporation. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies revealed site-dependent emissions, with sharp ²E → ⁴A₂ transitions from isolated Cr³⁺ ions and broad ⁴T₂ → ⁴A₂ transitions from Cr³⁺–Cr³⁺ pairs, generating broadband NIR emission (650–900 nm). Notably, the phosphor retained 69.9% of its emission intensity at 423 K compared to that at room temperature, demonstrating strong thermal stability. Fluorine doping further improved the performance of the phosphor, reducing thermal quenching from 32% to 19% and increasing the activation energy barrier to 0.27 eV. To demonstrate its practical application, we fabricated an NIR phosphor-converted LED (pc-LED), verifying its high performance. These findings establish SZAO:Cr³⁺ as a high-performance NIR phosphor with excellent emission properties and thermal resilience. With its optimized design, this material lays the foundation for next-generation NIR pc-LEDs.