Fig. 4. Efficient interfacial energy transfer in trilayered nanoparticles.

a Simplified energy-level diagram shows enhanced multiphoton upconversion from Yb³⁺ to Er³⁺ in the C_Y:10% Er@S_Yb@S_Y core-shell-shell nanoparticles (d_SYb ≥ 3.2 nm, d_SY ≥ 2.0 nm). b Luminescence decay curves of Er³⁺ emission at 407 nm for C_Y:10% Er@S_Yb@S_Y nanoparticles with d_SYb increasing from 1.3 to 13.1 nm (d_SY = 2.0–2.7 nm). Inset shows the decay curves of Yb³⁺ emission for the as-synthesized C_Y:10% Er@S_Yb@S_Y core-shell-shell (d_SYb = 8.3 ± 0.7 nm) and C_Y:20% Yb, 2% Er@S_Y core-shell nanoparticles (d_SY = 10.5 ± 0.8 nm). c Relative integrated UCL intensity of 2% Er³⁺-doped UCNPs (for trilayered structure, d_SYb = 8.3 ± 0.7 nm, d_SY = 2.3 ± 0.1 nm; for core-shell structure, d_SY = 10.5 ± 0.8 nm). d NIR downshifting luminescence spectra and the relative integrated intensities (inset) for the C_Y:10% Er@S_Yb@S_Y nanoparticles with varying NaYbF₄ shell thickness (d_SY = 2.0–2.7 nm). The error bars in (c) and (d) (inset) represent the standard deviation of three trials. Source data are provided as a Source Data file.