Fig. 2: Photoluminescence characterization of the Tm-NPs with enhanced NIR-IIb/c luminescence.

a Emission spectra of the core-shell Tm-NPs with variable Tm³⁺ dopant concentrations in the core showing DSL from 1600 to 2100 nm upon excitation at 800 nm. b Corresponding decay times. c Proposed downshifting mechanism for Tm³⁺ in the NIR-II region. d, e LiTmF₄@Y NPs with different shell thicknesses irradiated at 800 nm/1208 nm. f Emission spectra were obtained with various Er³⁺ ions doped into the LiTmF₄: x%Er@ LiYF₄ NPs. g Corresponding decay curves for Tm³⁺ emission at 1680 nm. h Proposed downshifting mechanisms for LiTmF₄:x%Er@LiYF₄ NPs under 800 nm light excitation. i Emission spectra of Tm/Er@Y alloy NPs upon irradiation at 980 nm and 1530 nm.