Key Takeaways & Executive Findings
- •• • Record external quantum efficiency (EQE) of 45.3% at 1542 nm emission in Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskite single crystals, surpassing typical NIR-II PLQYs below 20% in prior systems, enabling high-sensitivity bioimaging and low-loss optical communication. • • Sb3+ sensitization provides broadband ultraviolet absorption via spin-allowed s-p transitions, effectively harvesting UV photons and transferring energy to Er3+ through self-trapped exciton emission, overcoming the weak absorption of parity-forbidden 4f-4f transitions. • • High Er3+ doping activates cross-relaxation (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2), which selectively populates the NIR-emitting 4I13/2 state and suppresses competitive visible emission, a mechanism directly evidenced by temperature-dependent spectroscopy and lifetime decay analyses. • • Density functional theory calculations reveal that Sb3+ incorporation introduces in-gap states, reduces local symmetry, and enhances charge localization, facilitating efficient energy transfer and charge redistribution, providing a general design principle for high-efficiency lanthanide-based NIR-II emitters.
Abstract
Lead-free halide double perovskites have attracted significant attention owing to their eco-friendliness, structural tunability, and self-trapped exciton emission. Nevertheless, achieving efficient and stable near-infrared-II (NIR-II) luminescence, especially in materials incorporating lanthanide ions, remains a considerable challenge in photonics research. Herein, we report a notable advance in the design and synthesis of Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskite single crystals, which exhibit an unprecedented external quantum efficiency of 45.3% for emission at 1542 nm. Sb3+ acts as a broadband ultraviolet absorber and transfers energy to Er3+ via self-trapped exciton emission. Moreover, at high concentrations of Er3+, Er3+-Er3+ cross relaxation (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2) selectively populates the NIR-emitting 4I13/2 state, suppressing competitive visible emission pathways. This synergistic host-sensitizer-activator design strategy, supported by density functional theory calculations, addresses long-standing efficiency limitations and opens new avenues for high-performance NIR-II emitters in bioimaging, night vision, and optical communications.
1. Introduction
The second near-infrared biological window (NIR-II, 1000–1700 nm) is critical for advanced biomedical imaging, night vision, and optical communications due to reduced photon scattering, minimal tissue absorption, and enhanced penetration depth. Rare-earth ions, particularly Er3+ with its 4I13/2 → 4I15/2 transition near 1540 nm, are promising NIR-II emitters, yet their practical use is constrained by intrinsically weak absorption from parity-forbidden 4f-4f transitions. Prior sensitization strategies using ns2 ions (e.g., Sb3+, Bi3+) have improved visible and NIR-I emissions, but NIR-II emissions from Er3+ have remained inefficient, typically with PLQYs below 20%.
This work addresses the bottleneck by engineering cross-relaxation in Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskites. The host-sensitizer-activator design leverages Sb3+ broadband absorption and self-trapped exciton emission to transfer energy to Er3+, while high Er3+ concentrations activate a specific cross-relaxation pathway that selectively populates the NIR-emitting 4I13/2 state, suppressing competitive visible emission. This synergistic approach achieves a record EQE of 45.3% at 1542 nm, setting a new benchmark for lead-free NIR-II emitters and establishing a general design principle for high-efficiency luminescent materials.
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Jingheng Nie, Chaohong Liao, Huiwang Lian, Renping Cao, Sijie Liu, Baoxia Liu, Bang Lan, Jing Wang (2026). Cross-relaxation engineering in Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskites enabling a record 45.3% EQE in NIR-II luminescence. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4021-5
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Frequently Asked Questions
What is the exact external quantum efficiency (EQE) achieved and how does it compare to previous NIR-II emitting materials?
The Sb3+-sensitized Cs2NaLuCl6:Er3+ double perovskite single crystals achieve a record EQE of 45.3% at 1542 nm emission. This is a significant improvement over typical NIR-II PLQYs, which are usually below 20% in most reported systems, indicating a major advancement in efficiency.
How does Sb3+ sensitization overcome the weak absorption of Er3+ ions?
Sb3+ ions exhibit strong broadband absorption due to spin-allowed s-p transitions, acting as antennas that harvest ultraviolet photons. They transfer energy to Er3+ via self-trapped exciton emission, effectively bypassing the parity-forbidden 4f-4f transitions that limit direct Er3+ absorption.
What is the role of Er3+-Er3+ cross-relaxation in achieving high NIR-II efficiency?
At high Er3+ concentrations, a specific cross-relaxation process (2H11/2 + 4I15/2 → 4I9/2 + 4I13/2) selectively populates the NIR-emitting 4I13/2 state while suppressing competitive visible emission pathways. This mechanism is confirmed by temperature-dependent spectroscopy and lifetime decay analyses, and it is key to the record efficiency.
What do density functional theory (DFT) calculations reveal about the role of Sb3+ incorporation?
DFT calculations show that Sb3+ incorporation introduces in-gap states, reduces local symmetry, and enhances charge localization. These effects facilitate efficient energy transfer and charge redistribution, contributing to the enhanced luminescence performance.
What are the potential applications of this material and what are the next steps for practical implementation?
The material's high NIR-II efficiency and stability make it suitable for bioimaging, night vision, and optical communications. Future work should focus on scaling up synthesis, improving photostability under prolonged excitation, and integrating the material into device architectures for real-world applications.
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