Lanthanide-doped Sr2ScF7 nanocrystals: controllable hydrothermal synthesis, the growth mechanism and tunable up/down conversion luminescence properties

Abstract

Sr₂ScF₇:Ln³⁺ (Ln = Ce, Tb, Eu, Sm, Dy, Er, Tm, Ho and Yb) nanocrystals were firstly synthesized via a one-step hydrothermal route without employing any surfactants. The shape and size of the Sr₂ScF₇ nanocrystals could be readily tuned from nanorods with 120 nm length and 50 nm width to nanoparticles with a uniform diameter of 15 nm by doping 30% Ln³⁺ with a larger ionic radius. Furthermore, the influence of pH values, F⁻ sources and different surfactants on the sizes and morphologies (including nanorods, quadrangular microplates, cubes and polyhedrons) of the as-prepared products was systematically investigated and the possible formation mechanism for the products has been proposed. The XRD, SEM, EDS, PL analysis and decay lifetimes were used to characterize the products. For DC photoluminescence, the Sr₂ScF₇:Ln³⁺ nanocrystals show the characteristic f–f transitions with emission colors of bluish violet (Ce³⁺, Tm³⁺), green (Tb³⁺, Er³⁺, Ho³⁺), blue (Dy³⁺) and orange (Eu³⁺, Sm³⁺) respectively. Under single wavelength 980 nm excitation, the blue UC emissions of Sr₂ScF₇:Yb³⁺,Tm³⁺ nanocrystals at 474 nm due to the ¹G₄ → ³H₆ transition of Tm³⁺, the green UC emissions of Sr₂ScF₇:Er³⁺ nanocrystals at 522/544 nm assigned to the ²H_11/2 → ⁴I_15/2/⁴S_3/2 → ⁴I_15/2 transitions and the red UC emissions of Sr₂ScF₇:Yb³⁺,Er³⁺ at 660 nm from ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ were observed. Based on the generation of red, green, and blue emissions, the Sr₂ScF₇:Yb³⁺,Er³⁺,Tm³⁺ nanocrystals could produce multicolor, particularly in the white region (0.320, 0.330) by controlling the doping concentration of Tm³⁺ in the Sr₂ScF₇:Yb³⁺,Er³⁺,Tm³⁺ nanocrystals. Controlling the doping concentration of Tm³⁺ is an effective way of modulating the luminescence properties of Sr₂ScF₇:Ln³⁺ by controlling its size and morphology. The as-synthesized phosphors might be potentially applied in the fields of color displays, light, photonics and biological imaging.