Engineering Energy Flow in Ho ³⁺ -Doped Upconversion Nanoparticles: Multilayer Design for Overcoming Concentration Quenching and Enabling Switchable Output

Upconversion nanoparticles (UCNPs) have attracted considerable attention for their unique anti-Stokes luminescence and exceptional photostability. However, UCNPs incorporating Ho³⁺ as an activator typically suffer from suboptimal luminescence efficiency owing to complex energy-level configurations and dominant nonradiative decay processes. Herein, we overcome these limitations by implementing a rationally designed multilayer core-shell architecture that achieves spatial separation of sensitizers and activators while precisely controlling the intermediate layer thickness to optimize energy transfer kinetics. The incorporation of Nd³⁺ as an outer-sensitization layer under 808 nm excitation enhances photon harvesting capacity while mitigating thermal accumulation effects. Additionally, the strategic doping of Ce³⁺ within the Ho³⁺-activated core facilitates dynamic spectral modulation between green and red emissions through cross-relaxation mechanisms. This investigation not only realizes substantial enhancement in upconversion quantum yield but also establishes a generalizable design paradigm for developing next-generation luminescent materials with tailorable optoelectronic characteristics.