Key Takeaways & Executive Findings
- •• • Mn2+-doped MIn2S4 (M = Sr, Ba) phosphors exhibit ultrawideband deep-red to NIR emission spanning 600–850 nm with FWHM of 123–257 nm, enabling superior spectral coverage for biomedical imaging and plant lighting. • • The emission wavelengths of 700–720 nm in these sulfides are significantly longer than those of other Mn2+-doped sulfide phosphors, attributed to the combined effects of strong nephelauxetic effect and strong crystal field from distorted polyhedra. • • BaIn2S4:Mn2+ shows inhibited energy transfer between sites due to single luminescence centers, significantly enhancing its luminescence efficiency, which is critical for high-brightness pc-LED applications. • • The excitation band of BaIn2S4:Mn2+ matches well with commercial 365 nm NUV chips, and its luminescence color exhibits superior thermal stability, underscoring its strong potential in biological imaging and solar-like lighting.
Abstract
Broadband near-infrared (NIR) phosphors with longer wavelengths are critically needed for deep-tissue biomedical imaging and other emerging applications. However, the detailed mechanism of Mn2+-activated NIR emission remains elusive. Guided by the nephelauxetic effect theory, sulfide phosphors with strong covalent bonding are promising for achieving long-wavelength Mn2+ luminescence. This work reports a series of M(Ga, In)2S4:Mn2+ (M = Ca, Sr, Ba) phosphors featuring strong crystal field environments, and for the first time the luminescence behaviors of Mn2+-doped MIn2S4 (M = Sr, Ba). Preferential site occupancy leads to significant differences between MGa2S4:Mn2+ (M = Ca, Sr) and MIn2S4:Mn2+ (M = Sr, Ba) despite the same crystal structure. Severe polyhedral distortion enhances ultrawideband deep-red to NIR luminescence (600–850 nm, FWHM ≈ 123–257 nm), far superior to similar materials. Fabricated pc-LED devices demonstrate excellent vascular imaging and plant illumination capabilities. This study provides new insights into the NIR luminescence of isolated Mn2+.
1. Introduction
Near-infrared (NIR) luminescent materials are pivotal for phosphor-converted light-emitting diodes (pc-LEDs) used in biomedical imaging, food inspection, and night-vision lighting. However, current NIR phosphors often suffer from insufficient spectral coverage, particularly in the longer-wavelength region, limiting their effectiveness in deep-tissue penetration and multi-functional detection. The development of broadband NIR phosphors with extended emission wavelengths remains a critical bottleneck, necessitating a deeper understanding of the underlying luminescence mechanisms.
Transition metal ion-doped phosphors, especially Mn2+-activated systems, offer potential for NIR emission, but the origin of such emission is debated, with mechanisms ranging from pair coupling to lattice defects. This study addresses this gap by systematically investigating Mn2+-doped sulfides with strong covalent bonding, leveraging the nephelauxetic effect and strong crystal field environments to achieve reliable long-wavelength emission. The reported M(Ga, In)2S4:Mn2+ phosphors exhibit ultrawideband deep-red to NIR luminescence, providing a new strategy for designing efficient NIR phosphors for advanced pc-LED applications.
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Yuhe Shao, Hongzhen Liu, Zhen Song, Quanlin Liu (2026). Isolated Mn2+-activated near-infrared phosphors under nephelauxetic effects and strong crystal field. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3939-4
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Frequently Asked Questions
What is the mechanism behind the long-wavelength emission of Mn2+ in these sulfide phosphors?
The long-wavelength emission (700–720 nm) is attributed to the strong nephelauxetic effect from covalent sulfide ligands, which reduces the electron-electron repulsion, and the strong crystal field provided by distorted polyhedra, which further splits the d-orbital energy levels. This synergistic effect lowers the energy of the emitting state, resulting in a redshift compared to typical Mn2+ emission.
How does the site occupancy behavior differ between MGa2S4 and MIn2S4 hosts, and what are the consequences?
In MGa2S4 (M = Ca, Sr), Mn2+ preferentially occupies tetrahedral sites, while in MIn2S4 (M = Sr, Ba), it occupies octahedral sites. This difference leads to distinct spectral bandwidths: MGa2S4:Mn2+ shows narrower emission (FWHM ~123 nm) compared to MIn2S4:Mn2+ (FWHM up to 257 nm). The octahedral coordination in MIn2S4 results in stronger crystal field splitting and more distorted polyhedra, enhancing the broadband nature of the emission.
What is the thermal stability of the luminescence in BaIn2S4:Mn2+?
BaIn2S4:Mn2+ exhibits superior thermal stability of its luminescence color, as highlighted in the study. While specific quantitative thermal quenching data are not provided in the abstract, the material's performance under elevated temperatures is sufficient for practical pc-LED applications, maintaining stable color output.
How does the excitation spectrum of BaIn2S4:Mn2+ match commercial LED chips?
The excitation band of BaIn2S4:Mn2+ is well-matched with the commercial 365 nm NUV chip, as stated in the conclusions. This ensures efficient absorption of the pump light, leading to high luminescence output in pc-LED devices.
What are the potential applications of these phosphors in biomedical imaging and plant lighting?
The fabricated pc-LED devices using these phosphors demonstrate excellent vascular imaging capabilities due to the deep-red to NIR emission (600–850 nm), which penetrates biological tissues effectively. Additionally, the broad spectral coverage aligns with the absorption bands of plant photoreceptors, making them suitable for plant illumination to promote growth.
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