Achieving broadband near-infrared luminescence in Cr3+-Activated Y2Mg2Al2Si2O12 phosphors via multi-site occupancy

Abstract

Cr³⁺-activated near-infrared (NIR) phosphors are key for NIR phosphor-converted light emitting diodes (NIR pc-LED). While, the site occupancy of Cr³⁺ is one of the debates that have plagued researchers. Herein, Y₂Mg₂Al₂Si₂O₁₂ (YMAS) with multiple cationic sites is chosen as host of Cr³⁺ to synthesize YMAS: xCr³⁺ phosphors. In YMAS, Cr³⁺ ions occupy simultaneously Al/SiO₄ tetrahedral, Mg/AlO₆ octahedral, and Y/MgO₈ dodecahedral sites which form three luminescent centers named as Cr1, Cr2, and Cr3, respectively. Cr1 and Cr2 relate to an intermediate crystal field, with transitions of ²E→⁴A₂ and ⁴T₂→⁴A₂ occurring simultaneously. As Cr³⁺ concentration increases, the ⁴T₂→⁴A₂ transition becomes more pronounced in Cr1 and Cr2, resulting in a red-shift and broadband emission. Cr3 consistently behaves a weak crystal field and exhibits the broad and long-wavelength emission. Wide-range NIR emission centering at 745 ​nm is realized in YMAS: 0.03Cr³⁺ phosphor. This phosphor has high internal quantum efficiency (IQE ​= ​86%) and satisfying luminescence thermal stability (I_{423 K} ​= ​70.2%). Using this phosphor, NIR pc-LEDs with 56.6 ​mW@320 ​mA optical output power is packaged and applied. Present study not only demonstrates the Cr³⁺ multi-site occupancy in a certain oxide but also provides a reliable approach via choosing a host with diverse cationic sites and local environments for Cr³⁺ to achieve broadband NIR phosphors.