Particle size effects on the structure and emission of Eu³⁺:LaPO₄ and EuPO₄ phosphors

This paper provides the detailed study of (nano)particle's size effect on structural and luminescent properties of LaPO₄:Eu³⁺ synthesized by four different methods: high temperature solid-state, co-precipitation, reverse micelle and colloidal. These methods delivered monoclinic monazite-phase submicron particles (> 100 nm), 4 × 20 nm nanorods and 5 nm spheres (depending on the annealing temperature), 2 × 15 nm nanorods, and ultra-small spheres (2 nm), respectively. The analysis of emission intensity dependence on Eu³⁺ concentration showed that quenching concentration increases with a decrease of the particle size. The critical distance for energy transfer between Eu³⁺ ions is found to be 18.2 Å, and the dipole-dipole interaction is the dominant mechanism responsible for the concentration quenching of emission. With the increase in Eu³⁺ concentration, the unit-cell parameter slightly increases to accommodate larger Eu³⁺ ions at sites of smaller La³⁺ ions. Photoluminescent emission spectra presented four characteristic bands in the red spectral region: at 592 nm (⁵D₀→⁷F₁), at 612 nm (⁵D₀→⁷F₂), at 652 nm (⁵D₀→⁷F₃) and at 684 nm (⁵D₀→⁷F₄), while in small colloidal nanoparticles additional emission bands from host defects appear at shorter wavelengths. Intensities of f-f electronic transitions change with particles size due to small changes in symmetry around europium sites, while emission bandwidths increase with the reduction of particle size due to increased structural disorder. Judd-Ofelt analysis showed that internal quantum yield of Eu³⁺ emission is strongly influenced by particle's morphology.