Synthesis and upconversion luminescence properties of Yb3+, Er3+ co-doped NaYF4 phosphors

Abstract

Recently, the upconversion (UC) phosphors have attracted much attention due to their potential in various fields, such as solid-state lasers, solar energy converter, biodetection. Among various UC materials, the NaYF4 host lattice is known as one of the most efficiency host lattices. It has two structures. One is cubic structure (or α-NaYF4), the other is hexagonal structure (or β-NaYF4). The hexagonal structure is previous reported that is higher about 10 times of UC efficiency relative to its cubic structure. In the thesis, the hexagonal structure NaYF4 crystals were synthesized using a hydro (and solvo)-thermal reaction method in the presence of Na2-ethylenediaminetetraacetic acid (EDTA). The NaYF4 crystals transformation from cubic to hexagonal structure by adjusting synthesis conditions, and an investigation into the UC luminescence properties. Structural and upconversion (UC) luminescence properties of these phosphors were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), luminescence spectra measurements. We confirmed that the UC luminescence efficiency is better, while changing the hexagonal structure with cubic structure. Emission spectra of all phosphors reveal mainly two colors, correspond to green (2H11/2 and 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) transition of Er3+. We confirmed that various synthesis conditions can be cause the transformation from the cubic to hexagonal structure and can adjust the direction of crystal growth.