We report the systematic control of the morphology of β-NaYF4:Yb,Er/Tm upconversion nanophosphors (UCNPs) from large spheres (37.9 nm) to rods (length = 60.1 nm, width = 21.5 nm) and from rods to hexagonal prisms (length = 48.8 nm, width = 44.0 nm) or small spheres (14.0 nm) by the use of a surfactant, an additive, and lanthanide doping. Increasing the ratio of oleic acid (OA) to 1-octadecene (ODE) caused a decrease in the size of the UCNPs, and increasing the OA/ODE ratio above a critical value caused the particle shape to change from a sphere to a rod. The length-to-width aspect ratio (AR) of upconversion nanorods (UCNRs) was finely manipulated from 1.28 to 2.80. The rounded tips of the UCNRs were flattened by adding Cl(-) ions, and the UCNRs changed to hexagonal prisms with a controllable AR depending on the quantity of Cl(-) ions. Additionally, the morphology of the β-NaYF4-based UCNPs was controlled by lanthanide doping. The size and AR of the UCNRs decreased with Gd(3+) doping, and the UCNRs ultimately transformed into small spheres (14.0 nm) with high monodispersity. Doping with Ce(3+) ions also decreased the AR of the UCNRs from 2.80 to 1.27. In addition, highly transparent polymer composites for 3D volumetric displays were fabricated by blending high-AR β-NaYF4:Yb,Er/Tm UCNRs with polydimethylsiloxane. These composites exhibited bright green and blue upconversion light during excitation with 980 nm light.

A pathway for achieving intense green light emitting LiGdF4:Yb,Er upconversion nanophosphors (UCNPs) via Y(3+) doping is demonstrated. It was revealed that Y(3+) doping initiated the formation of a tetragonal phase and affected the particle size. Single tetragonal-phase LiGd0.4Y0.4F4:Yb(18%),Er(2%) (LGY0.4F: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 Y(3+) doping concentration. In an LGY0.4F: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 Er(3+) ions with extremely small quantity was clearly seen over a particle on the EFTEM image. Moreover, the LGY0.4F: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.