Tuning the Red-to-Green-Upconversion Luminescence Intensity Ratio of Na3ScF6: 20% Yb3+, 2% Er3+ Particles by Changes in Size

Na3ScF6: 20% Yb3+, 2% Er3+ samples were synthesized with different reaction times and reaction temperatures using the solvothermal method. We carried out a series of tests on Na3ScF6 crystals. The XRD patterns showed that the monoclinic phases of the Na3ScF6 samples could be synthesized under different reaction conditions, and doping with Yb3+ ions and Er3+ ions did not change the crystal structures. The SEM images showed that the sizes of the samples gradually increased with reaction time and reaction temperature. The fluorescence spectra showed that the emission peaks of the prepared samples under 980 nm near-infrared (NIR) excitation were centered at 520 nm/543 nm and 654 nm, corresponding to the 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 transitions, respectively. With the increasing size of the samples, the emission intensities at 654 nm increased and the luminescence colors changed from green to red; at the same time, the red-to-green luminescence intensity ratios (IR/IG ratios) increased from 0.435 to 15.106—by as much as ~34.7 times. Therefore, this paper provides a scheme for tuning the IR/IG ratios of Na3ScF6: 20% Yb3+, 2% Er3+ samples by changing their sizes, making it possible to enhance the intensity of red upconversion, which has great potential for the study of color displays and lighting.