Photoluminescence Enhancement in Erbium Nanoparticles via Controlled Phase Transformation

Erbium-based nanoparticles (Er-NPs) were synthesized by Pulse Laser Ablation in Liquid (PLAL) and then heated at temperatures between 200°C-1000°C. A crystal structure transition from mixed cubic-monoclinic phase to pure cubic Erbium oxide (Er₂O₃) phase is observed at 600°C, accompanied by strong volume compaction of the Er-NPs. Through careful examination of their morphology, crystal structure, and chemical composition, we investigated the effects of post-synthesis thermal annealing on the 4f-4f optical transitions associated with Erbium ions (Er³⁺). Our results indicate that thermal treatment conducted in a N₂ atmosphere at ∼600°C promotes the stabilization of Er-NPs in their favorable oxidation state for optimal red photoluminescence (PL) around 665 nm. This is connected to the thermo-activated elimination of hydroxyl groups, the atomic densification that significantly reduces the Er-NP size and crystal disorder, as well as the stabilization of oxygen ligands leading to cubic crystal symmetry. The contribution of several competing mechanisms to the observed PL is outpaced by energy transfer processes to the 4f emitting levels of Er³⁺, whose efficiency becomes optimal for reduced interatomic distances. The improved properties of Er-NPs demonstrate their potential as next-generation tunable nanomaterials for integration in optical sources, display devices, as well as in high-reliability and temperature-resistant thermal sensors.