Fig. 2: NIR long-lived luminescent UCNPs.

a Schematic illustration of energy transfer processes in a core/shell/shell UCNP. EM, energy migration; ETU, energy transfer upconversion; CR, cross-relaxation. Representative TEM images of (b) NaYbF₄@NaYF₄:10%Yb,1%Tm@NaYF₄ and (c) NaYF₄@NaYbF₄:2%Tm@NaYF₄. TEM image (top left), size distribution histogram (bottom left), HR-TEM image (top right), and STEM image (bottom right) of representative core/shell/shell NPs. The (100) plane with 0.52 nm interplanar spacing of (b) β-NaYbF₄ and (c) β-NaYF₄ is identified. Data in b, c are representative of n = 3 independent experiments. d Representative emission spectra and lifetime at 800 nm of NaYF₄:x%Yb@NaYF₄:98%Yb,2%Tm@NaYF₄ UCNPs with different Yb³⁺ doping concentration x in EM core under 980 nm excitation. e Representative emission spectra and lifetime at 800 nm of NaYF₄:x%Yb@NaYF₄:y%Yb,z%Tm@NaYF₄ UCNPs with different Yb³⁺ and Tm³⁺ doping concentration in EM core and ETU shell under 980 nm excitation. The lifetime values of a series of nanoparticles are presented in Supplementary Tables 1 and 2. Source data are provided as a Source data file.