Shape, Size, and Phase-Controlled Rare-Earth Fluoride Nanocrystals with Optical Up-Conversion Properties

High-quality rare-earth fluorides, alpha-NaMF4 (M=Dy, Ho, Er, Tm, Y, Yb, and Lu) nanocrystals and beta-NaMF4 (M=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Y, Yb, and Lu) nanoarrays, have been synthesized by using oleic acid as a stabilizing agent through a facile hydrothermal method at 130-230 degrees C. The phase, shape, and size of the products are varied by careful control of synthetic conditions, including hydrothermal temperature and time, and the amounts of reactants and solvents. Tuning the hydrothermal temperature, time, and the amount of NaOH can cause the transformation from the cubic alpha-NaMF4 to hexagonal phase beta-NaMF4. Upon adjustment of the amount of NaOH, NaF, M3+, and ethanol, the morphologies for the beta-NaMF4 nanoarrays can range from tube, rod, wire, and zigzagged rod, to flower-patterned disk. Simultaneously, the size of the rare-earth fluoride crystals is variable from 5 nm to several micrometers. A combination of "diffusion-controlled growth" and the "organic-inorganic interface effect" is proposed to understand the formation of the nanocrystals. An ideal "1D growth" of rare-earth fluorides is preferred at high temperatures and high ethanol contents, from which the tube- and rodlike nanoarrays with high aspect ratio are obtained. In contrast, the disklike beta-NaMF4 nanoarrays with low aspect ratios are produced by decreasing the ethanol content or prolonging the reaction time, an effect probably caused by "1D/2D ripening". Multicolor up-conversion fluorescence is also successfully realized in the Yb3+/Er3+ (green, red) and Yb3+/Tm3+ (blue) co-doped alpha-NaYF4 nanocrystals and beta-NaYF4 nanoarrays by excitation in the NIR region (980 nm).