Size dependence of the upconverted luminescence of NaYF4:Er,Yb microspheres for use in ratiometric thermometry

Abstract

The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF₄:Er,Yb microspheres with sizes between 0.7 and 2 μm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 μm, the maximum sensitivity decreases from 36.8 × 10⁻⁴ to 24.7 × 10⁻⁴ K⁻¹. This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the ⁴S_3/2 → ²H_11/2 population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.