Synthesis of well oil-dispersible BaYF5:Yb3+/Er3+ nanocrystals with green upconversion fluorescence

Nearly monodisperse, regular-shaped and well oil-dispersible tetragonal BaYF5:0.2Yb3+/0.02Er3+ nanocrystals (NCs) were synthesized in water-ethanol-oleic acid-sodium oleate system. The as-obtained NCs exhibited bright upconversion (UC) fluorescence under the 980 nm excitation. Blue (2H9/2-4I15/2), green ((2H11/2, 4S3/2)-4I15/2) and red (4F9/2-4I15/2) transitions were observed. The results indicated that the relative intensity of green to red increased gradually with increasing power density, which were seldom in the previous work. Therefore, the UC properties and mechanism were studied in detail.