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

TL;DR

The paper reports a MIDA-assisted hydrothermal route to grow hexagonal β-NaYF disks with ultra-high aspect ratio, achieving up to 44 μm in width and ~1 μm in height by replacing EDTA with MIDA to enhance surface coverage. Morphology is tuned by the protonation state of MIDA via NaOH, yielding rods, disks, and semicircular disks, with the best hexagonal disks obtained near the pH inflection point and displaying flat basal planes that suppress lensing. Single-crystal XRD reveals a novel P-6_2c space group for the large disks, with Yb3+ doping around 8.5%, and laser refrigeration to -4.9 K observed in one particle, indicating potential for levitated optomechanics and microoptics. However, synthesis shows sensitivity to initial conditions and may produce phase byproducts, and overall cooling efficiency is modest due to nonradiative losses and nonstoichiometry, suggesting avenues for optimization in dopant distribution and shelling to advance practical applications.

Abstract

The hexagonal $β$-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 $β$-NaYF prisms with corner-to-corner diameters up to 44 $\mathrm{μm}$ while keeping the height around 1 $\mathrm{μm}$. The surface quality is comparable to particles synthesized with EDTA based on atomic force microscopy (AFM) measurements. Unlike particles synthesized with EDTA the $β$-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 $\mathrm{μm}$ $β$-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.

Figures (14)

• Figure 1: Chemical structures of EDTA and MIDA. Illustration of the hypothetical surface coverage increase on a -NaYF nucleus by substituting EDTA with two equivalents of MIDA.
• Figure 2: Calculated relative abundances of the different ionic MIDA species depending on pH (a). Predicted and measured pH value for a MIDA solution after the addition of NaOH. The inflection point, marked as blue cross, is calculated to be at a pH value of 6.1. The p$K_a$ values of MIDA are 2.146 and 10.088 (b) Ockerbloom1956. Hydrothermal synthesis results using MIDA in different protonation states by the addition of 0.75 (c), 1.00 (d) and 1.50 (e) of sodium hydroxide. Optical bright- (f) and darkfield (g) images of the -NaYF disks obtained with Na_1.00MIDA. Crystallographic directions for the left particle are given in the optical brightfield image.
• Figure 3: Size, morphology and surface comparison of Na_2.00EDTA -NaYF ((a), (b), (e)) and Na_1.00MIDA -NaYF ((c), (d), (f)). AFM surface characterization ((a), (d)), SEM side ((b), (c)) and top ((e), (f)) view, the inset illustrates the particle dimensions to scale.
• Figure 4: (hk0) Burger projection calculated from the single crystal X-ray diffraction images of the measured Na_1.00MIDA -NaYF drilling (a) and packing of the resulting refined crystal structure with P$\bar{6}$2c space group at 100K (b). Comparison of experimental and predicted powder X-ray diffraction patterns for published crystal structures in full view (c) and zoomed in for crucial diffractions (d).
• Figure 5: Schematic of the experimental setup used for the mean fluorescence wavelength temperature calibration and laser power dependent laser refrigeration trials (a). Illustration (b) and optical image (c) of a -NaYF disk mounted on a fiber core to minimize substrate interactions. The illumination spot, with a size of 5, is virtually overlaid as a red circle. Measured temperature calibration curve (d) and 1020nm power-dependent temperature change of three different particles (b).