Boosting narrow-band near-infrared-emitting efficiency of thulium by lattice modulation for reflective absorption bioimaging

Near-infrared (NIR) luminescence materials with narrow-band emissions are essential for brain and muscle activity imaging based on the absorption difference of oxygenated proteins. However, most known NIR-emitting materials are limited by low external quantum efficiency (EQE) and broadband properties. This work presents the careful design of Tm, Na-doped strontium sulfide (SrS: Tm³⁺, Na⁺) phosphor for NIR light-emitting diode (LED), which shows a narrow emitting band of 27 nm. The successful incorporation of Na⁺ into SrS: Tm³⁺ contributes to the suppression of lattice phonons, resulting in significant improvement in EQE from 33.6% to 53.7% and an increase in thermal stability. The efficient host absorption and energy transfer are facilitated by the crystallographic Sr defects and the distortion in the symmetric crystal, disclosed by solid-state NMR, electron paramagnetic resonance (EPR), transient spectra, and X-ray total scattering analysis. Subsequently, efficient identification of vascular patterns based on the differential absorption of hemoglobin enables the potential application of rare-earth luminescent materials in NIR phosphor-converted light-emitting diodes (pc-LEDs) and bioimaging.