Mn2+–Mn2+ Dimers Induced Robust Light Absorption in Heavy Mn2+ Doped ZnAl2O4 Near-Infrared Phosphor with an Excellent Photoluminescence Quantum Yield and Thermal Stability

Abstract

Transition metal ions, such as Cr³⁺, Fe³⁺, and Ni²⁺, are widely recognized activators for efficient broadband near-infrared (NIR) phosphors. However, the potential of Mn²⁺ ions as NIR-emitting activators is relatively overlooked due to their typically narrowband emission in the visible spectral region and relatively weak absorption. Herein, a heavy Mn²⁺-doped Zn_1-xAl₂O₄: xMn²⁺ (ZAO: xMn²⁺) phosphor is presented that exhibits a single NIR emission band peaked at 830 nm with a bandwidth of 135 nm under excitation at 450 nm. Through comprehensive structural and spectral analysis, this NIR band is attributed to the emission originating from Mn²⁺ ions within the MnO₆ octahedra. Importantly, the formation of Mn²⁺–Mn²⁺ dimers breaks the spin-forbidden rule and significantly enhances the transition probability, as supported by the excited state dynamic analysis. Consequently, the optimal ZAO: 0.70Mn²⁺ sample shows high internal/external photoluminescence quantum yields of 85.8%/36.9%, along with good thermal stability demonstrated by the emission intensity at 423 K retains 60% of that at 298 K. Finally, a prototype NIR pc-LED device is fabricated by combining ZAO: 0.70Mn²⁺ phosphor with a 450 nm blue diode chip, generating an NIR output power of 28.84 mW at 100 mA. This study provides novel insights into high-performance Mn²⁺-activated NIR phosphors.