Interface-Dependent Radiative Lifetimes of Yb^3+, Er^3+ codoped Single NaYF_4 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 codoped 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 (PL) lifetime of the 4S3/2 --> 4I15/2 transition from Er3+ ions was observed to change by >60% depending on the ions' separation distance from a planar (water/glass) dielectric interface. The 4S3/2 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 of between 25°C 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 polydimenthylsiloxane composite matrix.