Pr3+:BaY2F8 Crystal Nanoparticles (24 nm) Produced by High-Energy Ball Milling: Spectroscopic Characterization and Comparison with Bulk Properties

Nanocrystals (NC) of Pr3+:BaY2F8 with average diameter of 24 nm have been successfully prepared by high-energy ball milling. The method is versatile, easily scalable, and does not require the use of surfactants or catalysts. NC were prepared starting from high quality single crystal pieces, and their spectroscopic features are analyzed and compared with those of a single bulk crystal. Under 445 nm excitation, we recorded the 10 K and room temperature emission spectra of the two samples. The spectra show the same peak positions and width, and this means that the milling process does not introduce substantial modifications to the crystal structure. Besides, there are strong differences in the relative intensity of the lines emitted toward different lower lying levels in the two samples. In particular, the high-energy transitions seem to be hyper intense in the NC with respect to the bulk sample. On the contrary, the emission lines that end at excited levels are less intense in the NC. In addition, the time evolution of the P-3(0) decay shows striking differences between the nanosized materials and the bulk sample. Despite the exponential decay of the latter luminescence (tau = 43 mu s), NC possess a strong nonlinear component with a lifetime much shorter than in the bulk. Calculations show that nearly 89% of the excited ions contribute to the short-time decay, which is attributed to ions residing near the NC surface.