Effects of Er3+ spatial distribution on luminescence properties and temperature sensing of upconverting core–shell nanocrystals with high Er3+ content

Abstract

In upconversion nanocrystals where Er³⁺ acts as the activator, concentration quenching will easily occur when the content of Er³⁺ is high (generally >5mol%), and the Er³⁺ spatial distribution which is a key factor affecting the concentration quenching is an important issue that must be considered. Herein, we selected Yb:NaErF₄ as a light-emitting layer and investigated its upconversion performance and temperature sensing behaviors in two kinds of core–shell nanoarchitectures. Yb³⁺ and Er³⁺ activators were distributed in a three-dimensional sphere and two-dimensional thin layer in Yb:NaErF₄@Yb/Nd:NaYF₄@NaGdF₄ and NaGdF₄@Yb:NaErF₄@Yb/Nd:NaYF₄@NaGdF₄ core–shell nanocrystals, respectively. The difference in Er³⁺ 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 Er³⁺ was demonstrated to have no obvious effect on the transitions of Er³⁺ thermally coupled ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 and the relative sensitivity for temperature determination under 808 nm laser excitation.