Enhancing red upconversion emission of Ho3+ in a single NaYbF4:Ho3+ microdisk through building different core-shell structures

The construction of different core-shell structures has been widely used to enhance and regulate the luminescent properties of rare-earth-doped micro/nano-materials. This study aimed to enhance the red upconversion emission intensity of Ho3+ in NaYbF4 microdisks through coating with NaYF4 inert and NaYbF4 active shells. The red upconversion luminescence characteristics of a single NaYbF4:2%Ho3+ microdisk with different core-shell structures under near-infrared 980 nm excitation were studied using a confocal microscope. The experimental results showed that the red emission intensity of the NaYbF4:2%Ho3+@NaYbF4 core-shell microdisk was enhanced about 2.4 times compared with the single NaYbF4:2%Ho3+ core microdisk. Meanwhile, the NaYbF4:2%Ho3+@NaYbF4@NaYF4 core-shell-shell microdisk was constructed to further enhance the red emission intensity, which produced much higher red emission intensity and R/G ratio compared with the single NaYbF4:2%Ho3+ core microdisk (about 3.5 times and 1.2 times, respectively). The enhancement mechanisms of red emission were studied based on spectral characteristics and lifetime. The results indicated that the luminescent properties of microcrystals can also further improved through building different core-shell structures, and it is very helpful to expand the application of display and optoelectronic devices.