Enhanced red upconversion emission and its mechanism in Yb3+–Er3+ codoped α-NaLuF4 nanoparticles

Abstract

A series of Yb³⁺–Er³⁺ codoped α-NaLuF₄ nanoparticles were prepared via a hydrothermal process. Enhanced red upconversion (UC) luminescence can be achieved by adjusting the initial solution pH, and the intensity of red emission (660 nm) is enhanced 23 times. By establishing rate equations, a theoretical model about the red UC mechanism is presented. It is proposed that the highly efficient red UC emission is mainly due to the UC process from the saturated ⁴I_13/2 (Er³⁺) state, which is induced by the effective energy-back-transfer process (primary): ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) and the cross-relaxation effect among Er³⁺ (minor): ⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2 at a high pH value of the initial solution. Moreover, the influence of solution pH on the red UC luminescence is systematically studied under different hydrothermal conditions. This paper provides an effective route to obtain nanoparticles with bright red UC luminescence, which may be suitable for biological imaging and display devices.