PEG-mediated solvothermal synthesis of NaYF4:Yb/Er superstructures with efficient upconversion luminescence

In this work, we report a facile solvothermal synthesis of NaYF4:Yb/Er nano-structured materials with controlled shapes by tuning the solvents. Ethylene glycol as the solvent results in the preparation of NaYF4:Yb/Er nanoparticles with diameter of about 10 nm, while polyethylene glycol (MW = 400, abbreviated as PEG-400) as the solvent facilitates the formation of NaYF4 superstructures with the particle size of about 160 nm. These NaYF4 superstructures are built from nanoparticles with diameter of about 10 nm. Their compositions have been confirmed by energy-dispersive X-ray analysis, and they have simultaneously cubic and hexagonal phase structures. Fourier transform infrared (FT-IR) spectrum reveals the presence of PEG-400 ligands on their surface, which confers high solubility of NaYF4 superstructures in water. Under continuous-wave excitation of 980 nm laser, NaYF4 superstructure aqueous solution exhibits efficient up-converting luminescence which is almost twice as strong as that of the building blocks (nanoparticles). This improvement may result from the fact that these NaYF4:Yb,Er superstructures can serve as 980 nm laser-cavity mirrors. Therefore, these superstructures have great superiority as luminescent labeling materials for biological applications.