A solvothermal route to size- and phase-controlled highly luminescent NaYF4:Yb,Er up-conversion nanocrystals

Size-controlled hexagonal- and cubic-phase NaYF4:Yb,Er nanocrystals with bright fluorescent emission were successfully synthesized via a solvothermal approach at a relatively low temperature (< 300 degrees C). A mixture of ethanol and ethylene glycol or pure ethylene glycol was used as a solvent, whereby changing the ethanol concentration. It is found that, besides reaction temperature and time, the reactant concentration is an important factor to control crystal phase. High reactant concentration, high reaction temperature, high concentration of ethanol and long reaction durations are favorable to the formation of brightly emitting hexagonal-phase nanocrystals. The effects of these reaction conditions on the size and the luminescent properties of the as-prepared nanocrystals are discussed. It is found that 30-120 nm hexagonal NaYF4:Yb,Er nanoparticles can be prepared using 0.04 M reactant concentration in 0-60% ethanol solution at 220 degrees C for 24 h.