Facile synthesis of multicolor tunable ultrasmall LiYF₄: Yb,Tm,Er/LiGdF₄ core/shell upconversion nanophosphors with sub-10 nm size

Sub-10 nm-sized LiYF₄:Yb,Tm/Er upconversion nanophosphors (UCNPs) and multicolor-emitting LiYF₄:Yb,Tm,Er/LiGdF₄ core/shell (C/S) UCNPs were facilely synthesized by using rare earth-oleate (RE(oleate)₃) precursors. The size of LiYF₄:Yb,Tm and LiYF₄:Yb,Er UCNPs with single tetragonal phase was controlled in the range of 3.2–9.6 nm and 3.0–9.0 nm, respectivley, by varying reaction temperature from 230 to 320 °C under the use of RE(oleate)₃ precursors. The sub-10 nm-sized LiYF₄:Yb(25%),Tm(0.5%)/LiGdF₄ (9.7 nm) and LiYF₄:Yb(18%),Er(2%)/LiGdF₄ (9.5 nm) C/S UCNPs were successfully synthesized from ultrasmall LiYF₄-based UCNP cores smaller than 5 nm in size. The blue- and green-emitting sub-10 nm-sized LiYF₄:Yb,Tm/LiGdF₄ and LiYF₄:Yb,Er/LiGdF₄ C/S UCNPs exhibited 566 and 48,245 times higher upconversion luminescence intensities than their core counterparts, respectivley, under excitation with 980 nm near infrared light. In addition, multicolor tunable sub-10 nm-sized LiYF₄:Yb,Tm,Er/LiGdF₄ C/S UCNPs were synthesized and they emitted bright blue, sky blue, aquamarine, and green light by varying dopant composition (Tm and Er) and concentration.