Surface Plasmon Enhancement of Eu³⁺ Emission Intensity in LaPO₄/Ag Nanoparticles

A promising way to improve the performance of luminescent materials is to combine them with noble metal nanoparticles. Herein, a set of silver/europium-doped lanthanum orthophosphate (Ag/La_0.95Eu_0.05PO₄) nanostructures with different concentrations of silver nanoparticles were prepared and investigated. The presented overlap between the strongest europium (Eu³⁺) excitation line and the broad silver nanoparticle surface plasmon resonance makes the combination prospective for coupling. X-ray powder diffraction confirmed the monoclinic monazite structure. The transmission electron microscopy revealed particles with a rod-like shape and ~4 aspect ratio. Photoluminescence spectra show characteristic Eu³⁺ ion red emission. One of the requirements for an enhanced luminescence effect is the precise control of the distance between the noble metal nanoparticles and the emitter ion. The distance is indirectly varied throughout the change of Ag nanoparticle concentration in the La_0.95Eu_0.05PO₄ host. The emission intensity increases with the increase in Ag nanoparticles up to 0.6 mol %, after which the luminescence decreases due to the nanoparticles' close packing and aggregation leading to the displacement of La_0.95Eu_0.05PO₄ from the vicinity of the metal particles and reabsorption of the emitted light. The emission intensity of La_0.95Eu_0.05PO₄ increases more than three times when the Eu³⁺ excitation is supported by the localized surface plasmon resonance in the Ag/La_0.95Eu_0.05PO₄ nanostructures.