Boosting Narrow-Band Near-Infrared-Emitting Efficiency of Thulium by Lattice Modulation for Reflective Absorption Bioimaging
Research Background
Near-infrared (NIR) light holds significant application value in the biomedical field, particularly in non-invasive high-resolution imaging. NIR light can penetrate biological tissues and exhibits notable absorption differences for oxygenated and deoxygenated hemoglobin at specific wavelengths (e.g., 800 nm), making it an ideal light source for bioimaging. However, existing NIR luminescent materials generally suffer from low external quantum efficiency (EQE) and broad emission bandwidth, leading to low signal-to-noise ratios and limiting their application in bioimaging.
To address this issue, researchers have begun exploring the potential of rare-earth ions (such as thulium ions, Tm³⁺) as NIR luminescent materials. Thulium ions exhibit sharp emission peaks, enabling high-resolution NIR imaging. However, due to the parity-forbidden nature of the 4f/4f electronic transitions in thulium ions, their absorption efficiency and quantum yield are low, and they are prone to non-radiative relaxation. Therefore, improving the NIR luminescence efficiency of thulium ions has become a key focus of current research.
Source of the Paper
This paper was co-authored by Kaina Wang, Jipeng Fu, Sibo Zhan, and others, with the research team coming from multiple institutions, including the Institute of Optoelectronic Materials and Devices at China Jiliang University, the Center for High Pressure Science and Technology Advanced Research, and the School of Materials Science and Engineering at the University of Science and Technology Beijing. The paper was published on March 13, 2025, in the journal Chem, titled Boosting Narrow-Band Near-Infrared-Emitting Efficiency of Thulium by Lattice Modulation for Reflective Absorption Bioimaging.
Research Process
1. Material Synthesis and Characterization
The research team first synthesized thulium (Tm³⁺) and sodium (Na⁺) co-doped strontium sulfide (SrS: Tm³⁺, Na⁺) phosphors through high-temperature solid-state reactions. The specific steps included mixing SrCO₃, Tm₂O₃, S, and Na₂CO₃ in stoichiometric ratios, grinding them uniformly, and then sintering them in a muffle furnace at 1100°C for 2 hours. Activated carbon was used as a reducing agent during the synthesis process to prevent sulfide oxidation.
The synthesized samples were analyzed for crystal structure using X-ray diffraction (XRD), confirming their cubic rock-salt structure. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis showed that the sample particles ranged in size from 5 to 15 micrometers, with Sr, S, Tm, and Na elements uniformly distributed. Additionally, the research team measured the actual doping concentrations of Tm³⁺ and Na⁺ using inductively coupled plasma optical emission spectrometry (ICP-OES).
2. Spectral Performance Study
The research team investigated the optical properties of SrS: Tm³⁺, Na⁺ using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. The results showed that under 282 nm excitation, Tm³⁺ exhibited NIR emission peaks at 794 nm (³H₄ → ³H₆) and 1224 nm (³H₅ → ³H₆). By introducing Na⁺, the team successfully suppressed lattice phonons, increasing the external quantum efficiency (EQE) from 33.6% to 53.7% and significantly improving the material’s thermal stability.
3. Lattice Defects and Local Disorder Mechanism
To reveal the impact of Na⁺ doping on material performance, the research team studied lattice defects and local disorder mechanisms using solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and X-ray total scattering analysis. The results indicated that the introduction of Na⁺ not only compensated for the positive charge brought by Tm³⁺ doping but also enhanced energy transfer efficiency through local lattice distortion. Additionally, the team calculated the band structure of SrS using density functional theory (DFT), finding that Sr vacancy defects significantly broadened the material’s bandgap, thereby enhancing host absorption.
4. Fabrication of NIR LEDs and Bioimaging Applications
The research team combined SrS: Tm³⁺, Na⁺ phosphors with 280 nm ultraviolet LED chips to fabricate NIR LEDs. By recording vascular images of human arms and palms using an infrared camera, the team validated the material’s potential in bioimaging. The results demonstrated that the NIR LEDs based on SrS: Tm³⁺, Na⁺ could clearly display vascular distribution, outperforming commercial semiconductor light sources in imaging quality.
Research Results and Conclusions
Development of High-Efficiency NIR Luminescent Materials: Through lattice modulation and Na⁺ doping, the research team successfully developed SrS: Tm³⁺, Na⁺ phosphors with high-efficiency NIR luminescence, achieving an EQE of 53.7% and significantly improved thermal stability.
Lattice Defects and Local Disorder Mechanism: The study revealed the mechanisms by which Sr vacancy defects and Na⁺ doping affect material performance, demonstrating the critical role of local lattice distortion in enhancing energy transfer efficiency.
Bioimaging Applications of NIR LEDs: The NIR LEDs based on SrS: Tm³⁺, Na⁺ exhibited excellent performance in vascular imaging, providing a new solution for non-invasive high-resolution bioimaging.
Research Highlights
High-Efficiency Narrow-Band NIR Luminescence: Through lattice modulation and Na⁺ doping, the research team significantly improved the NIR luminescence efficiency of thulium ions, achieving high-efficiency narrow-band NIR emission.
Revealing the Local Disorder Mechanism: Using solid-state NMR, EPR, and X-ray total scattering analysis, the research team deeply explored the mechanisms by which lattice defects and local disorder affect material performance.
Validation of Bioimaging Applications: The research team successfully applied SrS: Tm³⁺, Na⁺ phosphors in NIR LEDs, validating their potential value in bioimaging.
Research Significance and Value
This study successfully developed high-efficiency narrow-band NIR luminescent materials through lattice modulation and Na⁺ doping, addressing the issues of low efficiency and broad bandwidth in existing NIR luminescent materials. The research not only provides a new solution for high-resolution bioimaging but also offers new insights for optimizing the performance of rare-earth ion luminescent materials. Furthermore, the revealed local disorder mechanism provides theoretical guidance for enhancing the performance of other optoelectronic materials.