Figure 3.

Energy-cascaded upconversion in dye-sensitized fluoride core/shell nanocrystals. (a) Scheme of the proposed nanostructure. (b) UCL intensities from the ¹I₆, ¹D₂, and ¹G₄ states of Tm³⁺ as a function of the number ratio of IR-808 dye: NP for the (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% UCNPs. Excitation at 800 nm, 10 W/cm². (c) Dependencies of UCL enhancement in the (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% UCNPs on the number ratio of IR 808 dye: NP for the (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% UCNPs. The enhancement time is defined compared to the core/shell UCNPs without dye. Excitation at 800 nm, 10 W/cm². (d) Proposed energy transfer pathways in the dye-sensitized core/shell structure. The higher energy levels of Nd³⁺ are omitted, as no UCL from the Nd³⁺ ions is observed. The dye molecules on the core/shell nanoparticle surface absorb photons and transfer excitation energy to the Nd³⁺ ions in the shell, which in turn sensitize Yb³⁺ ions in the core. Subsequently, intracore energy transfer from Yb³⁺ to Tm³⁺ excites the Tm³⁺ ions from the ground state to the ¹G₄, ¹D₂, and the ¹I₆ states. Radiative decays from these states produce five-photon induced UCL at 340 nm (¹I₆ → ³F₄), four-photon induced UCL at 350, 450, and 510 nm (from ¹D₂ state to ³H₆, ³F₄, and ³H₅ states), as well as three-photon luminescence at 480 and 650 nm (from ¹G₄ to ³H₆ and ³F₄). (e) UCL excitation spectra (for the integrated UCL intensity from the ¹D₂ state) for: IR-808 dye-sensitized (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% (blue), (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% (red), and canonical (NaYF₄:Yb³⁺ 30%,Tm³⁺ 0.5%)/NaYF₄ core/shell UCNPs (black). The broad and intense UCL excitation band peaked at ~800 nm proves the sensitization of the UCL by the IR-808 dye. (f) Fluorescence decay of the IR-808 dye from the IR-808 sensitized “blank” NaYF₄ nanoparticles as well as the (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄, (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 10%, and the (NaYbF₄:Tm³⁺ 0.5%)@NaYF₄:Nd³⁺ 30% core/shell nanoparticles.