Controllable and white upconversion luminescence in BaYF5:Ln3+ (Ln = Yb, Er, Tm) nanocrystals

BaYF5:Yb3+, Er3+, Tm3+ nanocrystals have been successfully synthesized via a hydrothermal process. SEM and TEM images indicate that the size of the nanocrystals is about 20 nm. Under the excitation of a 980 nm single-wavelength diode laser, BaYF5:Er3+ nanocrystals give strong green upconversion (UC) emission centered at 523 and 541 nm, which is assigned to H-2(11/2) -> I-4(15/2) and S-4(3/2) -> I-4(15/2) transitions of Er3+, respectively. When Yb3+ is codoped in BaYF5 Er3+, red emission around 652 nm can be observed in the emission spectrum. This emission can be assigned to the F-4(9/2) -> I-4(15/2) transition of Er3+. The intensity ratio of the green luminescence to the red luminescence decreases with increasing Yb3+ concentration in BaYF5:Yb3+, Er3+ nanocrystals. The blue UC emissions of BaYF5:Yb3+, Tm3+ nanocrystals appeared near 464 and 480 nm and are assigned to the D-1(2) -> F-3(4) and (1)G(4) -> H-3(6) transitions of Tm3+, respectively. Based on the generation of red, green, and blue emissions, the BaYF5:Yb3+, Er3+, Tm3+ nanocrystals can produce different colors, including white, by controlling the doping concentration of Yb3+ in the BaYF5 nanocrystals. The BaYF5:40%Yb3+, 0.5%Er3+, 0.5%Tm3+ nanocrystals show proper intensities of blue, green, and red emissions, which result in the production of white light (CIE: x = 0.346, y = 0.336).