Sodium yttrium fluoride (NaYF 4) is an important upconverting material with many potential uses in chemistry, materials science, and biology, which can be synthesized hydrothermally in both cubic (α) and hexagonal (β) crystallographic polymorphs. Understanding the mechanisms underlying the phase conversion between the cubic and hexagonal polymorphs is of great interest to help inform future efforts to synthesize atomically-precise quantum materials with well-defined sizes and morphologies. In this work, we use a combination of analytical transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), powder X-ray diffraction (XRD), in situ liquid cell TEM, atom probe tomography (APT), and extended x-ray absorption fine structure (EXAFS) measurements to show that the hexagonal NaYF 4 nanowires form through a non-classical crystal growth mechanism involving the formation and subsequent oriented attachment of mesocrystals consisting of cubic (α) phase units. EXAFS spectroscopy also suggests that substitutional Yb 3+ point defects within NaYF 4 are distributed evenly throughout the crystal lattice without clustering, and also that they may exhbit selective substitution into one of the two possible trivalent yttrium sites in the unit cell for hydrothermally synthesized β-NaYF 4 .

The development of upconversion nanomaterials for many photonic applications requires a detailed understanding of their radiative lifetimes that in turn depend critically on local environmental conditions. In this work, hexagonal (β-phase) sodium-yttrium-fluoride (NaYF4) nanowires (NWs) were synthesized and substitutionally co-doped with a luminescent solid solution of trivalent erbium and ytterbium ions. A single-beam laser trapping instrument was used in tandem with a piezo-controlled, variable-temperature stage to precisely vary the nanowire’s distance from the substrate. The spontaneous photoluminescence lifetime of the 4S3/2 → 4I15/2 transition from Er3+ ions was observed to change by >60% depending on the ions’ separation distance from a planar (water/glass) dielectric interface. The 4S3/2 state lifetime is observed to increase by a factor of 1.62 ± 0.01 as the distance from the quartz coverslip increases from ∼0 nm to ∼40 μm. Less significant changes in the luminescence lifetime (≤10%) were observed over a temperature range between 25 and 50 °C. The distance dependence of the lifetime is interpreted quantitatively in the context of classical electromagnetic coupling between Er3+ ions within the nanowire and the adjacent dielectric interface. We also demonstrate potential applications of the NaYF4 NWs for both controlling and probing temperatures at nanometer scales by integrating them within a poly­(dimethylsiloxane) composite matrix.

Fluoride crystals, due to their low phonon energies, are attractive hosts of trivalent lanthanide ions for applications in upconverting phosphors, quantum information science, and solid-state laser refrigeration. In this article, we report the rapid, lowcost hydrothermal synthesis of potassium lutetium fluoride (KLF) microcrystals for applications in solid-state laser refrigeration. Four crystalline phases were synthesized, namely orthorhombic K 2 LuF 5 (Pnma), trigonal KLuF 4 (P3 1 21), orthorhombic 1 KLu 2 F 7 (Pna2 1 ), and cubic KLu 3 F 10 (Fm3m), with each phase exhibiting unique microcrystalline morphologies. Luminescence spectra and emission lifetimes of the four crystalline phases were characterized based on the point-group symmetry of trivalent cations. Laser refrigeration was measured by observing both the optomechanical eigenfrequencies of microcrystals on cantilevers in vacuum, and also the Brownian dynamics of optically trapped microcrystals in water. Among all four crystalline phases, the most significant cooling was observed for 10%Yb:KLuF 4 with cooling of 8.6 ± 2.1 K below room temperature. Reduced heating was observed with 10%Yb:K 2 LuF 5

Hexagonal sodium yttrium fluoride with Na 3x Y 2−x F 6 stoichiometry (β-NaYF) is a promising material for luminescence upconversion applications due to the narrow crystal field splitting of the Yb(III) ion's lower 2 F 7/2 manifold. However, growing single crystals of β-NaYF remains an outstanding challenge due to thermal expansion stresses that cause cracking during melt growth. In this paper, we demonstrate a novel hydrothermal synthesis of β-NaYF with the ability to tune the aspect ratio from microplatelets to microrods with aspect ratios that match computationally predicted cavity (Mie) resonances. These crystals have a rootmean-square roughness below 1 nm after calcination, which makes them ideal for optical cavities. The β-NaYF microcrystals are doped with 10% Yb(III) cations and are used to build optomechanical laser-refrigeration devices consisting of a hexagonal β-NaYF crystal located at the end of a cantilever. Laser refrigeration of these devices by >12.5 °C is observed using calibrated measurements of both the cantilever's fundamental eigenfrequency and a Boltzmann fit to crystal field luminescence from the Yb(III) ions.

Figure 1. Value of x in the formula Na 0.5−x Y 0.5+x F 2+2x as a function of reaction time. The line of best fit corresponds to the equation x = −2.32 × 10 −4 t + 7.64 × 10 −2 . Comments pubs.acs.org/cm