Up-conversion luminescence of Yb3+ and Er3+ doped YPO4, LaPO4 and GdPO4 nanocrystals

In this article we report a comparison of structural and up-conversion (UC) luminescence properties of three compounds used as hosts for Yb3+ and Er3+ ions: YPO4, LaPO4 and GdPO4 phosphates. A co-precipitation method connected with annealing at 900 °C was used in order to synthesize nanocrystalline REPO4:Yb3+,Er3+ materials (RE=Y, La, Gd). The X-ray diffraction patterns collected for the prepared materials indicated their high crystallinity and phase purity for the wide range of used dopants concentrations (0.25-7.5% of Er3+ and 5-30% of Yb3+ ions). Transmission electron microscopy images showed small nanocrystal sizes: around 20 nm for YPO4-based products, 30-50 nm for LaPO4 or 40-150 nm for GdPO4-based materials. The nanomaterials prepared showed yellow UC under 980 nm laser irradiation, which was assigned to Er3+ electronic transitions. The optimal dopants concentration were determined, giving the highest UC intensity: 25% of Yb3+ ions (or 20% in the case of LaPO4) and 1% of Er3+ ions. Also the mechanism of observed UC was analyzed. Two- and three-photon excitation and energy transfer between Yb3+ and Er3+ were confirmed. The effect of crystal cell volume on the green to red ratio of Er3+ UC and the presence of Er3+-Er3+ energy transfer were found.