Morphology and phase control of fluorides nanocrystals activated by lanthanides with two-model luminescence properties

Abstract

The morphology, size and phase control of luminescent fluoride nanocrystals through doping has become a new research hotspot due to their improved properties. In this work, Yb³⁺ ions, as one of the most efficient sensitizers for various lanthanide activators, were doped in NaGd(Y)F₄ nanocrystals. The results show that no obvious influence was observed for Yb³⁺-doped NaYF₄ nanocrystals, while the influence of Yb³⁺ doping on NaGdF₄ nanocrystals was remarkable. The NaGd_1−xYb_xF₄ nanocrystals were synthesized by a hydrothermal route and had a morphology of rice-like nanorods. By controlling the synthesis parameters, the average size and slenderness of the nanocrystals increased gradually with addition of Yb³⁺ ions. In contrast, the NaGd_1−xYb_xF₄ nanocrystals maintained a hexagonal phase, which is more beneficial for application as a luminescent host, until the content of Yb³⁺ ions reached x = 0.9. The growth and transformation mechanism of NaGd_1−xYb_xF₄ nanocrystals was proposed to be a result of the competition between ion diffusion and an Oswald ripening process. Photoluminescence (PL) spectra confirm the efficient up-conversion and near-infrared (NIR) two-model luminescence properties of Er³⁺ (Tm³⁺) activated NaGd(Y)_1−xYb_xF₄ nanocrystals. Simulated analysis results indicate that a colloidal solution of mixed luminescent nanocrystals is expected to find application as the activated medium of three dimensional displays and a broadband optical amplifier.