Synthesis of Hexagonal‐Phase Eu3+‐Doped GdF3 Nanocrystals above Room Temperature by Controlling the Viscosity of the Solvents

The formation of orthorhombic‐phase GdF3 nanocrystals (thermodynamically controlled product) requires higher activation energy than that necessary for the formation of hexagonal‐phase GdF3 nanocrystals (kinetically controlled product). As a result, whenever a reaction is carried out at higher temperatures, formation of the orthorhombic‐phase GdF3 is generally preferred. However, the thermodynamic product is always more stable than the kinetic one at lower temperatures. Here, we report a simple microwave‐assisted method to synthesize stable, hexagonal‐phase, polyvinylpyrrolidone (PVP) functionalized Eu3+‐doped GdF3 nanocrystals at higher temperatures (up to 220 °C). This is achieved by tuning the viscosity of the solvents as well as using KF as fluoride source. Both the morphology and the size of the GdF3 nanocrystals can also be varied by tuning the reaction conditions. Functionalization of the nanocrystals with PVP makes them water‐dispersible. The Eu3+‐doped GdF3 nanocrystals show intense red emission and longer luminescence lifetimes. The high luminescence efficiency of the Eu3+‐doped GdF3 nanocrystals together with their high dispersibility in water can make them suitable for use in bioimaging and MRI imaging applications.