Quantifying the Influence of Inert Shell Coating on Luminescence Brightness of Lanthanide Upconversion Nanoparticles

Lanthanide upconversion nanoparticles are often coated with inert shells to reduce surface quenching and thereby enhance luminescence brightness. However, the enhancement factors reported by different studies vary widely depending on nanoparticle size, doping concentrations, and excitation irradiance. Reported here are results of a comprehensive empirical and modeling study of upconversion emission from highly (100%) doped nanoparticles of a range of sizes and inert shell thicknesses conducted at the individual particle level. The effect of inert shell coating on upconversion luminescence intensity is shown to depend heavily on the excitation irradiance and the nanoparticle size. For 45 nm NaYbF4:Tm nanoparticles with an optimum NaYF4 shell thickness of 5 nm, the excitation requirement to achieve the same brightness is reduced by 2.8-fold for the 455 nm emission and 6.2-fold for the 800 nm emission, while the superlinearity and the saturation emission intensity are slightly increased. Excellent correlation is obtained between numerical simulation and experimental results, indicating that inert shell coating reduces surface quenching not only for the Yb3+ sensitizers but substantially, also the Tm3+ emitters. The comprehensive understanding of excitation/emission characteristics of core-shell upconversion nanoparticles will help lead to optimal designs for practical applications.