Heterovalent Substitution of K2SrP2O7:Cr3+ to Achieve Anti-Thermal-Quenching Broadband Near-Infrared Luminescence

Abstract

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes are highly desirable for biochemical analysis and medical diagnosis applications. However, thermal quenching remains a demanding challenge for developing efficient NIR phosphors. Herein, we report the enhancement of both quantum efficiency and thermal stability in Cr³⁺-activated K₂SrP₂O₇ phosphors through a heterovalent substitution strategy by replacing Sr²⁺ with Al³⁺ in K₂Sr_1–xAl_xP₂O₇ (0.05 ≤ x ≤ 0.2) to obtain optimized broadband NIR emission. Structural modulation via Al³⁺ substitution leads to the optimized composition, K₂Sr_0.88Al_0.1P₂O₇:0.02Cr³⁺, which emits across a broad NIR range of 650–1100 nm peaking at 807 nm with a full width at half-maximum of ∼130 nm under 448 nm excitation. Remarkably, its emission intensity at 150 °C remains 120% of the initial value at room temperature, demonstrating a rare antithermal-quenching behavior. Temperature-dependent XRD studies further reveal that Al³⁺ substitution effectively suppresses lattice expansion at elevated temperatures, indicating enhanced lattice stability under thermal excitation. Detailed structural and spectral analyses show that the substitution enhances local site symmetry, reduces electron–phonon coupling, increases thermally induced absorption probability, and fortifies energetic barriers against nonradiative transitions. These synergistic effects collectively endow this NIR phosphor with a superior thermal stability. Furthermore, NIR light-emitting diodes fabricated with this phosphor exhibit strong potential for applications in information identification, nondestructive detection, and night vision technologies. This study demonstrates a local structure engineering strategy for designing thermally robust Cr³⁺-activated NIR phosphors, offering valuable insights into material discovery and NIR spectroscopy device development.