A novel one-pot strategy to rapidly synthesize bright red emitting upconversion nanocrystals with core–shell–shell structure

Abstract

β-NaErF₄:0.005Tm upconversion nanocrystals (UCNCs) with core–shell structure are quite popular due to efficient red upconversion luminescence. However, the traditional seed-mediated growth method is not continuous and quite time consuming, while the successive layer-by-layer strategy is unfriendly because of the usage of toxic sodium trifluoroacetate (Na-TFA). In this work, we utilized a novel one-pot strategy to synthesize β-NaErF₄:0.005Tm with core–shell–shell structure. The morphology of the core could be significantly regulated. NaYbF₄ and NaYF₄ shells could be epitaxially grown on the core by simply adding fresh raw materials and heating to 300 °C, successively. Thanks to the suppression of surface quenching and energy back transfer from Yb³⁺ to Er³⁺, the population density of excited states of Er³⁺ (⁴I_13/2, ⁴I_11/2 and ⁴I_9/2) was much improved, resulting in the tremendously enhanced upconversion intensity of β-NaErF₄:0.005Tm@NaYbF₄@NaYF₄. This one-pot protocol could supply core–shell–shell red UCNCs with regular morphology, enhanced intensity and long-lasting decay lifetime, offering promising opportunities for applications such as bioprobing, anti-counterfeiting, photodynamic therapy, etc.