Effects of Er spatial distribution on luminescence properties and temperature sensing of upconverting core-shell nanocrystals with high Er content.

In upconversion nanocrystals where Er acts as the activator, concentration quenching will easily occur when the content of Er is high (generally >5mol%), and the Er spatial distribution which is a key factor affecting the concentration quenching is an important issue that must be considered. Herein, we selected Yb:NaErF as a light-emitting layer and investigated its upconversion performance and temperature sensing behaviors in two kinds of core-shell nanoarchitectures. Yb and Er activators were distributed in a three-dimensional sphere and two-dimensional thin layer in Yb:NaErF@Yb/Nd:NaYF@NaGdF and NaGdF@Yb:NaErF@Yb/Nd:NaYF@NaGdF core-shell nanocrystals, respectively. The difference in Er spatial distribution in the core-shell structure resulted in significant modification of red-to-green ratios and decay behaviors upon excitation at 376 nm, 808 nm, 980 nm and 1532 nm, and the related mechanisms were systematically investigated. In addition, the spatial distribution of Er was demonstrated to have no obvious effect on the transitions of Er thermally coupled H → I and S → I and the relative sensitivity for temperature determination under 808 nm laser excitation.