Multicolor Tunable Upconversion Luminescence from Sensitized Seed-Mediated Grown LiGdF₄:Yb,Tm-Based Core/Triple-Shell Nanophosphors for Transparent Displays

Red, green, blue, and natural white upconversion (UC) luminescence colors are realized from the tetragonal-structured LiGdF4-based core/triple-shell (C/T-S) upconversion nanophosphors (UCNPs) and the C/T-S UCNP-incorporated polymer composites. The LiYF4:Yb cores are used as sensitized seeds for the formation of LiGdF4:Yb,Tm UC shell followed by the growth of LiGdF4:Tb,Eu color tuning shell. Finally, LiYF4 inert shell is grown on the core/shell/shell UCNPs, and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs exhibit enhanced UC luminescence. The single tetragonal-phased C/T-S UCNPs exhibit blue, green, and red UC luminescence, which is attributed to the electronic transitions in Tm3+ via energy transfer UC process and Tb3+ and Eu3+ via energy migration UC process, respectively. The multicolor UC emissions, including natural white, medium aquamarine, purple, and thistle color, are created by fine-tuning of the ratio of Tb3+ and Eu3+ in the color tuning shell. The transparent polymer composites are prepared by incorporating the C/T-S UCNPs into polydimethylsiloxane, and the polymer composites also exhibit red, green, blue, and natural white light UC emissions, indicating that these multicolor tunable LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs have potential to be applied to transparent volumetric displays.