Hydrothermal Synthesis of Ultra-high Aspect Ratio $\beta$-NaYF Disks via Methyliminodiacetic Acid (MIDA)

Abstract

The hexagonal $\beta$-phase of sodium yttrium fluoride (NaYF) is a leading host material for lanthanide upconversion and anti-Stokes fluorescence laser refrigeration based on its low phonon energies and high upconversion efficiency. Recently experiments have been proposed to use this material as an optically-levitated sensor of high-frequency gravitational waves. In order to maximize signal-to-noise in this experiment, the NaYF sensor must have both a two-dimensional, disk-like morphology and also a large mass. Here we report a novel hydrothermal process based on the chelation ligand methylimidodiacetic acid (MIDA) to realize hexagonal $\beta$-NaYF prisms with corner-to-corner diameters up to 44 $\mathrm{\mu m}$ while keeping the height around 1 $\mathrm{\mu m}$. The surface quality is comparable to particles synthesized with EDTA based on atomic force microscopy (AFM) measurements. Unlike particles synthesized with EDTA the $\beta$-NaYF particles show no lensing based on curvature of the hexagonal basal plane. Single crystal X-ray diffraction data were refined to the P-62c (#190) space group which to the best of our knowledge has not been reported in the literature. One of six 44 $\mathrm{\mu m}$ $\beta$-NaYF disks doped with 10% ytterbium showed laser refrigeration of ($-4.9 \pm 1.0$) K suggesting future applications in both levitated optomechanics and microoptics.