The effects of down- and up-conversion on dual-mode green luminescence from Yb3+- and Tb3+-doped LaPO4 nanocrystals

Abstract

Monoclinic LaPO₄ nanocrystals doped with Yb³⁺ and Tb³⁺ ions were synthesised by co-precipitation followed by annealing at 900 °C. These materials exhibited intense luminescence under ultraviolet (UV) and near infrared (NIR) irradiation. The structural properties of the products were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The obtained nanomaterials had a single phase, a spherical shape of nanocrystals and an average size of 42 ± 5 nm. The spectroscopic properties of LaPO₄:Yb³⁺,Tb³⁺ nanocrystals were investigated in detail using excitation and emission spectra. Additionally, the luminescence lifetimes for both Tb³⁺ and Yb³⁺ ions were calculated from the measured decays. The optimal dopant concentrations were estimated with respect to the intensity of luminescence. A cooperative energy transfer (CET) mechanism, which causes the observed up-conversion, was proposed because of the observed dependence of integral intensity on the power of the pumping laser. The calculated efficiencies of the CET were much higher than the efficiencies of the opposing quenching processes; these parameters were measured in samples doped with high amounts of Tb³⁺ ions. These materials demonstrated intense green up-conversion in addition to UV-excited emission.