NIR-II Luminescence With A Recorded 76% EQE Through Energy Extraction From Dark Cr³⁺

NIR-II luminescent materials are the key enablers of smart spectroscopy-based techniques, offering capabilities for nondestructive analysis and bio-imaging. Here, a broadband NIR-II luminescence with a recorded external quantum efficiency (EQE) of 76% in LiGa₅O₈ are demonstrated, using heavily doping Cr³⁺ ion (10%) as a light harvester and a controllable energy transfer (ET) from Cr³⁺ to Ni²⁺ (0.4%) ions. Even though the Cr³⁺ ion produces substantially weak luminescence due to concentration quenching, the introduction of Ni²⁺ ions effectively extract the excitation energy to generate NIR-II luminescence. Complementary investigations using synchrotron-based EXAFS fittings and structural refinement disclose a significant structural distortion in the LiGa₅O₈ compound, which facilitates the relaxation of the parity-selection rules for Cr³⁺ and Ni²⁺ ions. Additionally, DFT calculations identify specific site occupations, which favor unidirectional ET from Cr³⁺ to Ni²⁺ ions. As a result, the significant absorption of excitation light by heavy-doping Cr³⁺ ions and high radiative transition probability in Ni²⁺ ions synergistically result in a record high EQE. These findings provide pioneering insight into rational NIR-II light generation by deliberated control of ET pathway in heavily doped systems, thereby with promising implications for NIR spectroscopy applications. © 2024 Wiley-VCH GmbH.