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

Abstract

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 Na_0.40Y_0.480Yb_0.108Er_0.012F_2.2 and composite Na_0.55Y_0.360Yb_0.081Er_0.009F_2.10 comprising hexagonal β-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 β-NaYF₄:Yb,Er nanoparticles with an average size of 20–30 nm. The milled β-NaYF₄:Yb,Er nanoparticles demonstrate intense green fluorescence upon excitation at λ = 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 β-NaYF₄ nanoparticles for industrial and scientific applications. High-temperature X-ray diffraction confirmed the phase transformation of melt-grown α-NaYF₄ crystals into the hexagonal β-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 α-NaYF₄ by mechanical milling.