Near-infrared light sources play an increasingly important role in night vision, bioimaging, non-destructive inspection, and information security. Lanthanide ions are attractive activators for NIR emission owing to their stable 4f electronic transitions. However, their parity-forbidden nature results in extremely weak absorption, low excitation efficiency, and narrow excitation windows, which severely restrict their practical applications. Although lead-free halide double perovskites provide a structurally stable and environmentally friendly host platform, achieving broadband excitation together with high luminescence efficiency remains a major challenge.
In a new paper published in Light: Science & Applications, a research team led by Jun Lin from the Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, co-workers have reported a synergistic co-sensitization strategy using Mo4+ and Ag+ ions in lanthanide-doped double perovskites Cs2NaMCl6 (M = In, Bi). By sequentially introducing Mo4+ and Ag+ ions, the researchers constructed a hierarchical energy transfer network that dramatically enhances photon absorption and radiative recombination of lanthanide ions.
Mo4+ ions act as both efficient absorbers and energy transfer mediators owing to their d-d transitions, enabling strong absorption across the ultraviolet, visible, and part of the near-infrared region. Meanwhile, Ag+ incorporation induces local lattice distortion and converts dark self-trapped excitons into bright, further increasing light harvesting efficiency. As a result, lanthanide emission can be excited over an ultra-broad wavelength range from 250 to 850 nm.
Remarkably, the photoluminescence quantum efficiency of Er3+-doped samples approaches unity, and the NIR emission intensities of Ho3+, Er3+, Tm3+, and Yb3+ ions are enhanced by tens to thousands of times compared with singly doped counterparts. Temperature-dependent spectroscopy and first-principles calculations reveal multiple energy transfer pathways involving self-trapped excitons and Mo4+ intermediate states, accounting for the high efficiency and excellent thermal stability of the system.
To demonstrate application potential, the researchers fabricated near-infrared phosphor-converted LEDs and successfully showcased night vision imaging, deep penetration visualization, and multi-mode information encryption. The strategy is applicable to both In-based and Bi-based double perovskites, highlighting its generality for designing high-performance NIR luminescent materials.
