Preparation of rare-earth doped NaYF4 luminescent nanoparticles by a high-energy ball milling process

A possible fabrication of photoluminescent fluoride nanoparticles NaYF4 doped with rare-earth ions employing high-energy ball milling is demonstrated. The melt-grown bulk cubic crystal Na0.40Y0.480Yb0.108Er0.012F2.2 and composite Na0.55Y0.360Yb0.081Er0.009F2.10 comprising hexagonal & beta;-NaYF4 modification were used as raw materials. The samples were analyzed using X-ray diffraction analysis, differential scanning calorimetry, transmission electron microscopy, and optical spectroscopy after the milling. Pure single-phase NaYF4 was not obtained even after 14 hours of mechanochemical treatment of individual components. However, high-energy ball milling of the crystals in the presence of oleic acid as a surfactant for 16 hours allowed production of single-phase & beta;-NaYF4:Yb,Er nanoparticles with an average size of 20-30 nm. The milled & beta;-NaYF4:Yb,Er nanoparticles demonstrate intense green fluorescence upon excitation at & lambda; = 980 nm and can potentially be used in biomedicine and nanophotonics. The optimized methodology of high-energy milling has great potential for scalable and reproducible production of & beta;-NaYF4 nanoparticles for industrial and scientific applications. High-temperature X-ray diffraction confirmed the phase transformation of melt-grown & alpha;-NaYF4 crystals into the hexagonal & beta;-modification both during mechanical (4 hours) and thermal treatment in the 653-823 K range. The long duration of the process precludes the possibilities of large scale preparation of the cubic & alpha;-NaYF4 by mechanical milling.