Differential regulation of red-emission intensity and red-to-green emission ratio in multilayer Er-rich core–shell nanoparticles via Yb3+ or Tm3+ doping

Balancing red-emission intensity and the red-to-green (R/G) emission ratio remains an important challenge in Er 3+ -based upconversion nanoparticles. Herein, a multilayer core–shell architecture was developed to achieve differential regulation of red-emission intensity and the red-to-green (R/G) emission ratio in Er 3+ -based upconversion nanoparticles. After screening NaYbF 4 : x %Er 3+ @NaYF 4 core–shell nanoparticles, the sample containing 2% Er 3+ exhibited a favorable balance between overall emission intensity and the R/G ratio and was therefore selected as the precursor for constructing NaYbF 4 :2%Er 3+ @NaYF 4 @NaErF 4 :y%M 3+ @NaYF 4 (M = Yb or Tm) multilayer nanoparticles. In this architecture, the Yb-rich core, intermediate NaYF 4 layer, compositionally tunable Er-rich shell, and outer NaYF 4 shell were designed to provide spatially differentiated excitation harvesting, interfacial modulation, spectral regulation, and surface passivation, respectively. Under identical 980 nm excitation and collection conditions, incorporation of 15%Yb 3+ into the Er-rich shell increased the integrated red-emission intensity to 2.40 times that of the undoped multilayer counterpart, whereas incorporation of 0.5%Tm 3+ produced the highest R/G ratio of 17.1 while retaining comparatively strong emission. Power-dependent and time-resolved luminescence measurements, together with the proposed energy-transfer model, indicate that Yb 3+ incorporation is predominantly associated with enhanced local excitation sensitization and energy utilization within the Er-rich region, whereas trace Tm 3+ incorporation is more closely associated with excited-state population redistribution toward the red-emitting pathway. These results demonstrate that red-emission intensity and the R/G ratio can be differentially regulated within the same multilayer platform through Yb 3+ or Tm 3+ incorporation into the Er-rich shell. This work provides a spatially organized and compositionally tunable strategy for tailoring red upconversion emission in Er 3+ -based multilayer nanoparticles.