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 RE2+/3+ energy level positions in these fluorides sheds new light on the design of efficient upconversion materials.