Anomalous Temperature-Dependent Upconversion Luminescence of Small-Sized NaYF₄:Yb³⁺, Er³⁺ Nanoparticles

Size-dependent quantum confinement has important effects on the energy transfer and radiative and nonradiative transitions in nanophosphors. For lanthanide-doped nanoparticles, the confinement effect is induced mostly via electron–phonon interaction, and analysis of temperature-dependent spectroscopic properties provides an effective method for disclosing its underlying mechanism. Herein, an intriguing and unprecedented enhancement of the upconversion luminescence (UCL) at higher temperatures in hexagonal-phase NaYF 4:Yb 3+, Er 3+ upconversion nanoparticles (UCNPs) is reported. Moreover, this anomalous UCL enhancement shows a strong dependence on the particle size and becomes more significant for UCNPs with a smaller size. This anomalous thermal behavior is interpreted on the basis of phonon-assisted energy transfer and phonon confinement effect. The findings are relevant to the engineering of the nanostructures of UCNPs and to the further understanding of the UCL mechanism.