Quantitative evaluation of various NIR-to-red upconversion mechanisms in NaYF4:20%Yb3+,2%Er3+ nanoparticles

In the most popular NaYF₄:Yb/Er upconversion nanoparticles (UCNPs), the red emission is attributed to four potential excitation routes encompassing two- and three-photon excitation processes. Consequently, this red emission typically exhibits a super-quadratic dependency on near-infrared (NIR) excitation intensity, with the nonlinear order n being dependent on the individual contributions (C_is) of these four excitation routes. Notably, the C_is values are not constant but significantly impacted by the surface quenching of the UCNPs, leading to a decrease in the n value. However, a quantitative assessment of these variable C_is has not been undertaken, hindering a comprehensive understanding of the quenching effect on the UC mechanisms. In this work, we prepare four NaYF₄:Yb/Er nanocrystal samples with varying degrees of surface quenching, achieving through the modulation of particle size and core-shell structure. We quantitatively evaluate the C_is values and identify the primary excitation route responsible for the red emission. Our results reveal that the contribution of three-photon excitation increases from 7% in the 30 nm bare core to 74% in 90 nm core with shell at an excitation intensity of 200 mW cm⁻². This observation high-lights the impact of surface quenching suppression. Furthermore, we discover that the quenching effect operates by reducing the lifetimes of the Yb³⁺²F_5/2 and Er³⁺⁴S_3/2 levels, while enhancing the NIR emission intensity ratio of the Er³⁺⁴I_13/2 → ⁴I_15/2 transition to the Yb³⁺²F_5/2 → ²F_7/2 transition. Our findings provide physical insights into the excitation mechanisms underlying the red UC emission in NaYF₄:Yb/Er UCNPs.