Green Synthesis of Upconverting NaYF⁴ and NaGdF⁴ Materials and Energy Levels Determination

An alternative green route using a domestic microwave oven to synthesize NaGdF₄:Yb,Tm and NaYF₄:Yb,Er was successfully developed. Decreasing dopants concentration increased the β-NaREF₄/α-NaREF₄ ratio in NaGdF₄. The scanning electron microscopy (SEM) images of NaGdF₄ showed particles ranging from 500 nm to 1 μm with formation of aggregates up to 100 μm, with two different surface morphologies. The NaGdF₄:Yb,Tm emission spectrum showed white-like emission due to Er³⁺ and Eu³⁺ contamination. The bandgaps of undoped NaGdF₄ and NaYF₄ were measured using synchrotron radiation vacuum-UV (VUV) diffuse reflectance, with values of 10.20 eV for the cubic phase and 10.80 eV for the hexagonal phase for both materials. The determination of the RE^2+/3+ energy level positions in these fluorides sheds new light on the design of efficient upconversion materials.

Keywords:
upconversion; microwave assisted solid-state synthesis; bandgap determination