Inducing octahedral distortion to enhance NIR emission in Cr-doped garnet Ca3(Al, Sc)2Ge3O12

Abstract

An efficient Cr³⁺-activated broadband near-infrared (NIR) phosphor is the key enabler to integrate compact NIR light-emitting diodes (pc-LEDs) for food testing and medical detection applications. High luminescence efficiency is key for the practical applications of phosphors. Herein, we design a series of efficient Cr³⁺-doped Ca₃Al_2−ySc_yGe₃O₁₂ garnet phosphors to induce octahedral distortion for enhancing NIR luminescence efficiency. We adopt the best fitted ideal polyhedron to characterize the octahedral distortion and found that the degree of distortion reaches its maximum at y = 0.2 and decreases with y deviating from 0.2. As a result, the composition with y = 0.2 shows the highest luminescence intensity. The composition dependence of the octahedral distortion coincides with that of the luminescence intensity, indicating that the luminescence properties of Ca₃Al_2−ySc_yGe₃O₁₂:Cr³⁺ are closely related to the octahedral distortion. The luminescence internal/external quantum efficiency (IQE/EQE) is significantly enhanced from 65.4%/25.7% in Ca₃Al₂Ge₃O₁₂:0.04Cr³⁺ to 85.4%/33.5% in Ca₃Al_1.8Sc_0.2Ge₃O₁₂:0.04Cr³⁺. The optimal Ca₃Al_1.8Sc_0.2Ge₃O₁₂:0.04Cr³⁺ phosphor exhibits excellent luminescence thermal stability (∼91% at 423 K) and high NIR output power (38.2 mW at 100 mA) with an electro-optical conversion efficiency of 13.7%. This work provides a strategy for enhancing the NIR luminescence of Cr³⁺.