Shielding Upconversion by Surface Coating: A Study of the Emission Enhancement Factor

Surface coating is a commonly used strategy to enhance up conversion emissions by shielding the luminescent core from surface quenching. In this work, we provide insights into the effect of surface coating on upconversion by investigating NaYF₄:Yb/Er nanoparticles and the corresponding NaYF₄:Yb/Er@NaYF₄ core–shell nanoparticles, as a function of dopant concentration of Yb³⁺and excitation power. We observe declining emission enhancement factors with decreasing Yb³⁺concentration and increasing excitation power. Our mechanistic investigations suggest that the phenomenon originates from stepwise excitation in the upconversion process, as well as energy hopping among the Yb³⁺ dopants. This increased understanding of the effect of surface coating on upconversion should be important towards the rational design of lanthanide-doped core–shell nanoparticles for various applications.