Reducing Grain Size and Enhancing Luminescence of NaYF4:Yb3+, Er3+ Upconversion Materials

A variety of NaYF4:Yb/Er upconversion materials were successfully synthesized via a facile hydrothermal method by altering the volume ratios (0:1, 1:1, 2:1, 3:1, 4:1, 5:1) of ethanol to water and various alcohols (methanol, n-propyl alcohol, isopropyl alcohol, ethylene glycol, polyethylene glycol, and phemethylol), and the grain size and luminescence of the materials were effectively tailored. The addition of alcohol effectively reduces the grain size of the materials and simultaneously leads to the surprising enhancement of the UC emission. The increase in the volume ratio of EtOH/H2O causes the transformation of the crystal structure of the materials from hexagonal to cubic phase. The particle size of the material with the EtOH/H2O of 2:1 is almost 1000 times smaller than that of the material without EtOH, and the strongest luminescence is observed at the EtOH/H2O of 2:1. The type of alcohol in the solvent exhibits a significant influence on the luminescent properties of the materials. The NaYF4:Yb/Er nanocrystals obtained using different alcohols as solvent possess much a smaller grain size and stronger upconversion luminescence than that using water as solvent, and the NaYF4:Yb/Er materials with n-propyl alcohol exhibit the strongest luminescence intensity. The mechanism of the reduction of grain size and enhancement in the luminescence of the NaYF4:Yb3+, Er3+ upconversion materials has been proposed. This study may suggest a simple method to significantly reduce the grain size and improve the upconversion luminescence properties of NaYF4-based fluorescence material.