Interface-Dependent Radiative Lifetimes of Yb³⁺, Er³⁺ Co-doped Single NaYF₄ Upconversion Nanowires

The development of upconversion nanomaterials for many photonic applications requires a detailed understanding of their radiative lifetimes that in turn depend critically on local environmental conditions. In this work, hexagonal (β-phase) sodium-yttrium-fluoride (NaYF₄) nanowires (NWs) were synthesized and substitutionally co-doped with a luminescent solid solution of trivalent erbium and ytterbium ions. A single-beam laser trapping instrument was used in tandem with a piezo-controlled, variable-temperature stage to precisely vary the nanowire's distance from the substrate. The spontaneous photoluminescence lifetime of the ⁴S_3/2 → ⁴I_15/2 transition from Er³⁺ ions was observed to change by >60% depending on the ions' separation distance from a planar (water/glass) dielectric interface. The ⁴S_3/2 state lifetime is observed to increase by a factor of 1.62 ± 0.01 as the distance from the quartz coverslip increases from ∼0 nm to ∼40 μm. Less significant changes in the luminescence lifetime (≤10%) were observed over a temperature range between 25 and 50 °C. The distance dependence of the lifetime is interpreted quantitatively in the context of classical electromagnetic coupling between Er³⁺ ions within the nanowire and the adjacent dielectric interface. We also demonstrate potential applications of the NaYF₄ NWs for both controlling and probing temperatures at nanometer scales by integrating them within a poly(dimethylsiloxane) composite matrix.