Synthesis and Properties of New Multinary Silicides R₅Mg₅Fe₄Al_<i>x</i>Si_{18–<i>x</i>} (R = Gd, Dy, Y, <i>x</i> ≈ 12) Grown in Mg/Al Flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R₅Mg₅Fe₄Al_<i>x</i>Si_{18–<i>x</i>} (<i>x</i> ≈ 12). These phases have a new structure type in tetragonal space group <i>P</i>4<i>/mmm</i> (<i>a</i> = 11.655(2) Å, <i>c</i> = 4.0668(8) Å, <i>Z</i> = 1 and <i>R</i>₁ = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and ²⁷Al and ²⁹Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at <i>T</i>_N = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field

Synthesis and Properties of New Multinary Silicides R₅Mg₅Fe₄Al_<i>x</i>Si_{18–<i>x</i>} (R = Gd, Dy, Y, <i>x</i> ≈ 12) Grown in Mg/Al Flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R₅Mg₅Fe₄Al_<i>x</i>Si_{18–<i>x</i>} (<i>x</i> ≈ 12). These phases have a new structure type in tetragonal space group <i>P</i>4<i>/mmm</i> (<i>a</i> = 11.655(2) Å, <i>c</i> = 4.0668(8) Å, <i>Z</i> = 1 and <i>R</i>₁ = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and ²⁷Al and ²⁹Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at <i>T</i>_N = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field

Ca₅₄In₁₃B_4–<i>x</i>H_23+<i>x</i>: A Complex Metal Subhydride Featuring Ionic and Metallic Regions

Reactions of CaH₂ with group 13 metals in a 1:1 Ca/Li flux mixture produce Ca₅₄In₁₃B_4–<i>x</i>H_23+<i>x</i> (2.4 < <i>x</i> < 4). This compound has a complex new structure [<i>Im</i>3̅, <i>a</i> = 16.3608(6) Å, <i>Z</i> = 2] which can be viewed as a body-centered cubic array of Bergman-related clusters that are composed of a central indium atom surrounded by an icosahedron of 12 calcium atoms; hydride ions cap each face, forming a pentagonal dodecahedron that is further surrounded by a calcium shell. These In@Ca₁₂@H₂₀@Ca₃₀ clusters are surrounded by a disordered calcium indium hydride network. Indium is not completely reduced by the flux; the structure features ionic hydride regions and metallic calcium indium regions, confirmed by electronic structure calculations and ¹H and ¹¹⁵In solid-state NMR spectroscopy. This compound can therefore be viewed as a “subhydride”, akin to the alkali metal suboxides that feature ionic oxide clusters surrounded by metallic regions

High Refractive Index Polymers Based on Thiol–Ene Cross-Linking Using Polarizable Inorganic/Organic Monomers

The self-initiation of the thiol–ene coupling reaction of tetravinyl monomers containing main group elements and trivinyl heterocycles with alkyl and aryl dithiols resulted in the formation of highly cross-linked prepolymer gels which upon final curing at 120 °C yielded hard, monolithic polymeric materials. Because of the presence of highly polarizable main group elements such as Si, Ge, Sn, and S and the relative absence of highly electronegative elements, the resulting polymers exhibited high refractive indices ranging from 1.590 to 1.703 and Abbe numbers between 24.3 and 45.0. All of the polymers were highly transparent over the UV–vis region of the spectrum. Moreover, due to the high cross-linked density achievable in specific compositions, very hard materials capable of being ground and polished could be produced. The range of compositions produced yields important structure–property relationships, indicating the effect of monomer structure on mechanical and optical properties

A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes

R₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ (R = La or Ce; <i>x</i> ≤ 0.9; <i>y</i> ≤ 0.2) and R₃₃Fe_{13–<i>x</i>}Al_<i>x</i>B₁₈C₃₄ (R = Ce or Pr; <i>x</i> < 0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R–T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group <i>Im</i>3̅<i>m</i> (<i>a</i> = 14.617(1) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and <i>a</i> = 14.246(8) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0142 for Ce₃₃Fe₁₃B₁₈C₃₄). Their structures can be described as body-centered cubic arrays of large Fe₁₃ or Fe₁₄ clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe₁₄ clusters in Ce₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce^3+/4+ valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce₃₃Fe₁₃B₁₈C₃₄ indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at <i>T</i>_C = 180 K is due to magnetic moments on the Fe₁₃ clusters

A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes

R₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ (R = La or Ce; <i>x</i> ≤ 0.9; <i>y</i> ≤ 0.2) and R₃₃Fe_{13–<i>x</i>}Al_<i>x</i>B₁₈C₃₄ (R = Ce or Pr; <i>x</i> < 0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R–T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group <i>Im</i>3̅<i>m</i> (<i>a</i> = 14.617(1) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and <i>a</i> = 14.246(8) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0142 for Ce₃₃Fe₁₃B₁₈C₃₄). Their structures can be described as body-centered cubic arrays of large Fe₁₃ or Fe₁₄ clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe₁₄ clusters in Ce₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce^3+/4+ valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce₃₃Fe₁₃B₁₈C₃₄ indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at <i>T</i>_C = 180 K is due to magnetic moments on the Fe₁₃ clusters

Non-Invasive Characterization of the Organic Coating of Biocompatible Quantum Dots Using Nuclear Magnetic Resonance Spectroscopy

Colloidal quantum dots, made of semiconductor cores and surface coated with an organic shell, have generated much interest in areas ranging from spectroscopy to charge and energy transfer interactions to device design, and as probes in biology. Despite the remarkable progress in the growth of these materials, rather limited information about the molecular arrangements of the organic coating is available. Here, several nuclear magnetic resonance (NMR) spectroscopic techniques have been combined to characterize the surface ligand structure(s) on biocompatible CdSe-ZnS quantum dots (QDs). These materials have been prepared via a photoinduced ligand exchange method in which the native hydrophobic coating is substituted, in situ, with a series of polyethylene glycol-modified lipoic acid-based ligands. We first combined diffusion ordered spectroscopy with heteronuclear single-quantum coherence measurements to outline the conditions under which the detected proton signals emanate from only surface-bound ligands and identify changes in the proton shifts between free and QD-bound ligands in the sample. Quantification of the ligand density on different size QDs was implemented by comparing the sharp ¹H signature(s) of lateral groups in the ligands (e.g., the OCH₃ group) to an external standard. We found that both the molecular architecture of the ligand and the surface curvature of the QDs combined play important roles in the surface coverage. Given the non-invasive nature of NMR as an analytical technique, the extracted information about the ligand arrangements on the QD surfaces in hydrophilic media will be highly valuable; it has great implications for the use of QDs in targeting and bioconjugation, cellular imaging, and energy and charge transfer interactions

Characterization of the Ligand Capping of Hydrophobic CdSe–ZnS Quantum Dots Using NMR Spectroscopy

We have combined a few advanced solution phase NMR spectroscopy techniques, namely, ¹H, ³¹P, heteronuclear single quantum coherence (HSQC), and diffusion ordered spectroscopy (DOSY), to probe the composition of the organic capping layer on colloidal CdSe–ZnS core–shell quantum dots grown via the “hot injection” route. Combining solution phase ³¹P and ¹H NMR with DOSY, we are able to distinguish between free ligands and those coordinated on the QD surfaces. Furthermore, when those NMR measurements are complemented with matrix-assisted laser desorption ionization (MALDI) and FTIR data, we find that the organic shell of the as-prepared QDs consists of a mixture of tri-<i>n</i>-octylphosphine oxide (TOPO), tri-<i>n</i>-octylphosphine (TOP), alkyl amine, and alkyl phosphonic acid (L- and X-type ligands); the latter molecules are usually added during growth at a rather small concentration to improve the quality of the prepared nanocrystals. However, NMR data collected from QD dispersions subjected to two or three rounds of purification reveal that the organic shell composition (of purified QDs) is essentially dominated by monomeric or oligomeric <i>n</i>-hexylphosphonic acid, along with small fractions of surface-coordinated or hydrogen-bonded 1-hexadecyl amine and TOP/TOPO. This is true even though large excesses of TOP and TOPO surfactants are used during QD growth. This proves that <i>n</i>-hexylphosphonic acid (HPA) exhibits substantially higher coordinating affinity to the QD surfaces, compared to other phosphorus-containing surfactants such as TOP and TOPO. Finally, we test the utilitys of DOSY NMR to provide accurate data on the translational diffusion coefficient (and hydrodynamic radius) of QDs, as well as freely diffusing ligands in a sample. This proves that DOSY is a highly effective characterization technique for such small colloids and organic surfactants where DLS reaches its limit

Highly Efficient Broadband Yellow Phosphor Based on Zero-Dimensional Tin Mixed-Halide Perovskite

Organic–inorganic hybrid metal halide perovskites have emerged as a highly promising class of light emitters, which can be used as phosphors for optically pumped white light-emitting diodes (WLEDs). By controlling the structural dimensionality, metal halide perovskites can exhibit tunable narrow and broadband emissions from the free-exciton and self-trapped excited states, respectively. Here, we report a highly efficient broadband yellow light emitter based on zero-dimensional tin mixed-halide perovskite (C₄N₂H₁₄Br)₄SnBr_<i>x</i>I_6–<i>x</i> (<i>x</i> = 3). This rare-earth-free ionically bonded crystalline material possesses a perfect host-dopant structure, in which the light-emitting metal halide species (SnBr_<i>x</i>I_6–<i>x</i>^4–, <i>x</i> = 3) are completely isolated from each other and embedded in the wide band gap organic matrix composed of C₄N₂H₁₄Br^–. The strongly Stokes-shifted broadband yellow emission that peaked at 582 nm from this phosphor, which is a result of excited state structural reorganization, has an extremely large full width at half-maximum of 126 nm and a high photoluminescence quantum efficiency of ∼85% at room temperature. UV-pumped WLEDs fabricated using this yellow emitter together with a commercial europium-doped barium magnesium aluminate blue phosphor (BaMgAl₁₀O₁₇:Eu²⁺) can exhibit high color rendering indexes of up to 85