Inflection in size-dependence of thermally enhanced up-conversion luminescence of UCNPs

Abstract

The size-dependence of recently discovered thermally enhanced UCL for the UCNPs has been previously regarded as monotonous. However, all the published results for UCL thermo-enhancement to date only cover UCNPs with a size larger than ∼8 nm. In order to find out whether such monotonicity may remain for smaller sizes, herein, we investigate the size-dependence of the UCL thermal behavior of UCNPs with the size down to 3 nm for the first time. The results show that the monotonicity in relationship between the UCL thermo-enhancement ratio and particle size is broken as the size goes down to below ∼7 nm, which is opposite to the inferences drawn from either the surface-photon-assistance mechanism or the surface water molecule absorption/desorption mechanism proposed by some researchers for explaining the UCL thermal enhancement. In particular, we also find that the breaking of monotonicity in the size-dependence of luminescence properties is accompanied by the synchronous advent of inflection in the size-dependence of the lattice thermal expansion coefficient. This result strongly supports the mechanism of the lattice thermal expansion induced alleviation of surface quenching in explaining the UCL thermo-enhancement of UCNPs.