Tuning the morphology, luminescence and magnetic properties of hexagonal-phase NaGdF: Yb, Er nanocrystals via altering the addition sequence of the precursors.

Hexagonal-phase NaGdF: Yb, Er upconversion nanocrystals (UCNCs) with tunable morphology and properties were successfully prepared via a thermal decomposition method. The influences of the adding sequence of the precursors on the morphology, chemical composition, luminescence and magnetic properties were investigated by transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES), upconversion (UC) spectroscopy, and a vibrating sample magnetometer (VSM). It was found that the resulting nanocrystals, with different sizes ranging from 24 to 224 nm, are in the shape of spheres,  hexagonal plates and flakes; moreover, the composition percentage of Yb-Er and Gd ions was found to vary in a regular pattern with the adding sequence. Furthermore, the intensity ratios of emission colors (f , f ), and the magnetic mass susceptibility of hexagonal-phase NaGdF: Yb, Er nanocrystals change along with the composition of the nanocrystals. A positive correlation between the susceptibility and f of NaGdF: Yb, Er was proposed. The decomposition processes of the precursors were investigated by a thermogravimetric (TG) analyzer. The result indicated that the decomposition of the resolved lanthanide trifluoroacetate is greatly different from lanthanide trifluoroacetate powder. It is of tremendous help to recognize the decomposition process of the precursors and to understand the related reaction mechanism.