Fabrication of Ordered Magnetite-Doped Rare Earth Fluoride Nanotube Arrays by Nanocrystal Self-Assembly

We describe a nanocrystal self-assembly method for the preparation of rare earth fluoride nanotube (ReF-NT) arrays and magnetite-doped rare earth fluoride nanotubes (Fe₃O₄–ReF-NTs) by using porous anodic aluminum oxide (AAO) as a hard template. The ReF-NTs can be simply prepared by the impregnation of α-NaYF₄ nanocrystals doped with Yb and Er into the channels of the porous AAO and show a highly ordered nanotube array and excellent upconversion (UC) fluorescence properties. Similarly, the Fe₃O₄–ReF-NTs are obtained by the self-assembly of a mixture of Fe₃O₄ and Yb/Er doped α-NaYF₄ nanocrystals in the AAO pore channels and have a uniform dispersion of magnetite nanocrystals on the rare earth fluoride tube matrix and possess multifunctional magnetic/UC properties. The diameter of these nanotubes can be varied from 60 nm to several micrometers depending on the pore size of the AAO template. The wall thickness can be increased from 10 to 35 nm by increasing the concentration of nanocrystals from 0.02 to 0.4 mmol/L, while the morphology of the nanotubes can be varied from small isolated domain structures to percolating domains and eventually to compact domains. A template-directed formation mechanism is proposed and the quantitative predictions of the model for such self-assembled nanocrystal spreading processes are demonstrated. Strong UC fluorescent emissions are realized for the nanotube arrays and multifunctional nanotubes with UC excitation in the near-infrared (NIR) region. A strong magnetic response of the multifunctional nanotubes is observed, which facilitates their easy separation from solution by magnetic decantation using a permanent magnet.