Phonon-assisted energy back transfer-induced multicolor upconversion emission of Gd2O3:Yb3+/Er3+ nanoparticles under near-infrared excitation

Abstract

Manipulation of upconversion (UC) emission is of particular importance for multiplexed bioimaging. Here, we precisely manipulate the UC color output by utilizing the phonon-assisted energy back transfer (EBT) process in ultra-small (sub-10 nm) Gd₂O₃:Yb³⁺/Er³⁺ UC nanoparticles (UCNPs). We synthesized the Gd₂O₃:Yb³⁺/Er³⁺ UCNPs by adopting the laser ablation in liquid (LAL) technique. The synthesized Gd₂O₃:Yb³⁺/Er³⁺ UCNPs are small spherical and monoclinic structures. Continuous color-tunable (from green to red) UC fluorescence emission is achieved by increasing the concentration of Yb³⁺ ions from 0 to 15 mol%. A phonon-assisted energy back transfer (EBT) process from Er³⁺ (⁴S_3/2 → ⁴I_13/2) to nearby Yb³⁺ (²F_7/2 → ²F_5/2), which can significantly enhance red emission at 672 nm and decrease green emission, is responsible for the color-tunable UC emission by increasing the Yb³⁺ concentration in Gd₂O₃:Yb³⁺/Er³⁺ UC nanoparticles.