Dopant Dependence of Surface Quenching in Lanthanide-Doped Upconversion Nanoparticles

Inert shell coating is one of the most commonly used approaches for enhancing the emission of lanthanide-doped upconversion nanoparticles by rejecting surface quenching. However, an agreement on the critical shell thickness that is required to sufficiently shield the luminescent core has not yet been reached. In this study, the luminescence properties of Yb³⁺-doped NaYF₄ core nanoparticles coated with a NaLuF₄ protection shell were systematically examined. We correlate the shielding effect with the shell thickness and the core nanoparticle composition based on the obtained emission intensity and luminescence lifetime measurements. Our results suggest that heavily doped core nanoparticles typically need a thick protection shell (>10 nm) due to the energy migration that occurs within the core particle, which results in energy dissipation predominantly at the core/shell interface. As this energy migration is interrupted by reducing the dopant concentration or by introducing Tm³⁺ as trapping centers, the excitation energy is localized in the interior of the core nanoparticles. Consequently, a relatively thin shell (∼5 nm) becomes sufficient for protecting the luminescent core. Our findings may provide theoretical guidance for the future design of lanthanide-doped upconversion nanoparticles with efficient luminescence and small feature sizes.