Enhancement of upconversion emission of LaPO₄: Er@Yb core−shell nanoparticles/nanorods

Abstract

We demonstrated the synthesis of LaPO₄:Er:Yb-doped nanoparticles/nanorods and LaPO₄:Er@Yb core−shell nanoparticles/nanorods by a solution-based technique. The mechanism related to morphology control of LaPO₄:Er:Yb nanorods/nanoparticles is proposed and discussed. Bright-green (550 nm) and red (670 nm) emission were observed due to the transitions ²H_11/2 + ⁴S_3/2 → ⁴I _15/2 and ⁴F_9/2 → ⁴I_15/2, respectively. The experimental data for 550- and 670-nm emission bands of doped nanoparticle/nanorod and core−shell nanoparticles/nanorods have been fit with a straight line with a slope of ∼2, which confirms the two-photon absorption process. The enhancement of upconversion emission of LaPO₄:Er:Yb-doped nanoparticles and LaPO4:Er@Yb core−shell nanoparticles/nanorods are mainly due to modifications of surface-related effects. It is found that the tensile strain increases from +1.0% to +1.9% with changing the shape from nanoparticle to nanorod and reversal of the lattice strain (compressive) is obtained for coated nanoparticle/ nanorod. It is worth mentioning that the lattice strain varies with changing the shape and surface coating on nanocrystals and the upconversion emission intensity increases with decreasing the tensile lattice strain and it increases with increasing compressive strain. Analysis suggests that the lattice strain plays an important role in modification of the upconversion properties of the rare-earth-doped nanocrystals.