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

Abstract

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 ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_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 ⁴F_9/2 → ⁴I_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 ¹D₂ → ³F₄ and ¹G₄ → ³H₆ 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).