β-NaYF4:Yb,Tm: upconversion properties by controlling the transition probabilities at the same energy level

Abstract

A series of Yb³⁺/Tm³⁺ co-doped β-NaYF₄ (β-NaYF₄:Yb,Tm) crystals have been successfully synthesized via the molten salt synthesis (MSS) method. A unique strong green emission of Tm³⁺ was observed, which cannot be obtained by any other method. The red, green and blue (RGB) light emissions have been obtained simultaneously in the presence of a single activator (Tm³⁺). The influencing factors of crystal growth have been studied in detail, in order to understand the change of the transition probabilities of Tm³⁺. Subsequent studies indicate that the RGB light emissions of Tm³⁺ can be effectively tuned by varying the concentration of sensitizers (Yb³⁺ and Mn²⁺) and the reaction time. Consequently, the effective multicolor upconversion (UC) luminescence was obtained, especially a strong white light, which was not obtained in the β-NaYF₄:Yb,Tm system earlier. To our knowledge, it is the first time that the UC properties in the β-NaYF₄:Yb,Tm system have been investigated by altering the transition probabilities at the same energy level of Tm³⁺.