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We demonstrated the synthesis of LaPO 4:Er:Yb-doped nanoparticles/nanorods and LaPO 4:Er@Yb core−shell nanoparticles/nanorods by a solution-based technique. The mechanism related to morphology control of LaPO 4:Er:Yb nanorods/nanoparticles is proposed and discussed. Bright-green (550 nm) and red (670 nm) emission were observed due to the transitions 2 H 11/2 + 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 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 LaPO 4:Er:Yb-doped nanoparticles and LaPO 4: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.
Ghosh et al. (Sat,) studied this question.