Tuning the emission color of SrLaAlO4:Er,Yb upconversion phosphors by decorating their surface with CsPbBr3-xIx quantum dots

In this work, SrLaAlO₄:Er³⁺ (2 mol%),Yb³⁺ (4 mol%) upconversion phosphor was synthesized by using a combustion method. Later, CsPbBr_3-xI_x (x = 0, 1.5 and 0.75) perovskite quantum dots (QDs) were synthesized by using hot injection method. According the analysis by X-ray diffraction, the SrLaAlO₄:Er,Yb (SL:Er,Yb) phosphors and QDs had tetragonal and cubic phases, respectively. Moreover, the analysis by microscopy revealed that the SL:Er,Yb phosphors are composed of micro-grains with irregular morphology, while the CsPbBr_3-xI_x QDs had a morphology of cubes. The surface of the SL:Er,Yb phosphors was decorated with CsPbBr_3-xI_x QDs and obtained three composite powders: CsPbBr₃+SL:Er,Yb, CsPbBr_1.5I_1.5+SL:Er,Yb and CsPbBr_2.25I_0.75+SL:Er,Yb. Those composites were firstly excited with UV light (380 nm) and produced emissions in the green and orange-red regions by downconversion. Interestingly, the emission intensity of the composite powders was 45–75 % higher than that for the individual CsPbBr_3-xI_x QDs or SL:Er,Yb phosphors. Later, the same composite powders were excited with NIR light (980 nm), in consequence, intense green and yellow emissions were produced by upconversion. In particular, the CsPbBr_1.5I_1.5+ SL:Er,Yb composite produced strong red emission by upconversion because the presence of the QDs on the SL:Er,Yb surface promoted the following cross relaxation process: ⁴I_11/2 (Er³⁺)+ ⁴F_7/2 (Er³⁺)→⁴F_9/2 (Er³⁺)+ ⁴F_9/2 (Er³⁺). In general, the quantum dots deposited on the SL:Er,Yb surface not only enhanced the red upconversion emission, but also provoked a color tuning when they are excited with NIR or UV light. Those last effects have not been reported in the literature previously. Thus, the results of this investigation demonstrate that combining perovskite quantum dots and the SL:Er,Yb forms a luminescent material able to tune its emission by upconversion or downconversion. This last property can be utilized for the design of new LED based lamps, which are employed for general illumination in houses and buildings.