Fabrication of Upconverting YbPO4:Er Crystals by Naturally-Derived Gel Growth and Subsequent Thermal Treatment

Abstract

YbPO4:Er nanocrystals with upconversion fluorescence were successfully prepared using a two-step process, i.e., gelatin gel growth of YbPO4:Er·nH2O precursors and subsequent annealing. The YbPO4:Er·nH2O precursors grown in gelatin gels were milky white, spherical in shape and uniform in size and appearance. The spherical YbPO4:Er·nH2O precursors were an aggregation of numerous, dense nanospheres (10−50 nm). The average diameters were dependent on growth conditions, such as growth period and concentrations of gel and inner and outer solutions. After the precursor spheres were annealed, their shapes changed dramatically to an aggregation of YbPO4:Er nanocubes (50−300 nm) in three dimensions (900 °C) and large and well-formed crystals (∼500 nm) of YbPO4:Er (1100 °C). The prepared YbPO4:Er crystals showed a bright green upconversion emission under 980 nm laser irradiation by a two-photon upconversion process.