Facile synthesis of intense green light emitting LiGdF₄:Yb,Er- based upconversion bipyramidal nanocrystals and their polymer composites

A pathway for achieving intense green light emitting LiGdF ₄:Yb,Er upconversion nanophosphors (UCNPs) via Y³⁺ doping is demonstrated. It was revealed that Y³⁺ doping initiated the formation of a tetragonal phase and affected the particle size. Single tetragonal-phase LiGd_0.4Y_0.4F₄:Yb(18%),Er(2%) (LGY_0.4F:Yb,Er) UCNPs exhibited strong upconversion (UC) green luminescence and tetragonal bipyramidal morphologies. They showed 1325 and 325-fold higher photoluminescence intensity than the 0 and 80 mol% Y ³⁺-doped LiGdF₄:Yb,Er UCNPs, respectively. Additionally the particle size (edge length) of LiGdF₄:Yb,Er-based upconversion tetragonal bipyramids (UCTBs) was controlled from 60.5 nm to an ultrasmall size of 9.3 nm with varying Y³⁺ doping concentration. In an LGY _0.4F:Yb,Er UCTB, uniform distribution of all constituent elements was directly confirmed by using high-angle annular dark-field scanning transmission electron microscopy and energy-filtered transmission electron microscopy (EFTEM) image analyses. In particular, existence of activator Er³⁺ ions with extremely small quantity was clearly seen over a particle on the EFTEM image. Moreover, the LGY_0.4F:Yb,Er UCTBs were successfully incorporated into the polydimethylsiloxane (PDMS) polymer and the highly transparent UCTB-PDMS composites showed bright green light under the excitation of 980 nm infrared light.