Cation-Size-Mismatch Tuning of Photoluminescence in Oxynitride Phosphors

Red or yellow phosphors excited by a blue light-emitting diode are an efficient source of white light for everyday applications. Many solid oxides and nitrides, particularly silicon nitride-based materials such as M₂Si₅N₈ and MSi₂O₂N₂ (M = Ca, Sr, Ba), CaAlSiN₃, and SiAlON, are useful phosphor hosts with good thermal stabilities. Both oxide/nitride and various cation substitutions are commonly used to shift the emission spectrum and optimize luminescent properties, but the underlying mechanisms are not always clear. Here we show that size-mismatch between host and dopant cations tunes photoluminescence shifts systematically in M_1.95Eu_0.05Si_5–<i>x</i>Al_<i>x</i>N_8–<i>x</i>O_<i>x</i> lattices, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu²⁺ dopant, but a blue shift when the M = Ca host is smaller. Size-mismatch tuning of thermal quenching is also observed. A local anion clustering mechanism in which Eu²⁺ gains excess nitride coordination in the M = Ba and Sr structures, but excess oxide in the Ca analogues, is proposed for these mismatch effects. This mechanism is predicted to be general to oxynitride materials and will be useful in tuning optical and other properties that are sensitive to local coordination environments

Deactivation and Rejuvenation of Pellet MgO/SiO₂ Catalysts for Transesterification of Soybean Oil with Methanol to Biodiesel: Roles of MgO Morphology Change in Catalysis

Pellet MgO/SiO₂ catalysts with pore diameters of 30 and 40 nm were prepared by incorporating Mg­(NO₃)₂ on granular SiO₂ followed with calcination at 500 °C. These pellet catalysts and MgO powder were tested and compared by using transesterification of soybean oil with methanol. Combined results of the test reaction, FT-IR, XRPD, and pore-size distribution suggested that the reaction is catalyzed by both Brønsted and Lewis base sites and is limited by mass transfer. The observation of Mg²⁺ in the reaction products and gum formed on the catalysts further indicated that the catalysts are deactivated by carbonaceous deposition and MgO leaching. Thus, the deactivated catalysts were rejuvenated by an ethanol wash followed with makeup MgO. However, inferred from synchrotron XRPD results, we suggested that the rejuvenation process cannot recover the morphological change of MgO that took place in the reaction and rejuvenation, leading to a decrease in product selectivity for FAME

Uric Acid Spherulites in the Reflector Layer of Firefly Light Organ

<div><p>Background</p><p>In firefly light organs, reflector layer is a specialized tissue which is believed to play a key role for increasing the bioluminescence intensity through reflection. However, the nature of this unique tissue remains elusive. In this report, we investigated the role, fine structure and nature of the reflector layer in the light organ of adult <i>Luciola cerata</i>.</p> <p>Principal Findings</p><p>Our results indicated that the reflector layer is capable of reflecting bioluminescence, and contains abundant uric acid. Electron microscopy (EM) demonstrated that the cytosol of the reflector layer's cells is filled with densely packed spherical granules, which should be the uric acid granules. These granules are highly regular in size (∼700 nm in diameter), and exhibit a radial internal structure. X-ray diffraction (XRD) analyses revealed that an intense single peak pattern with a d-spacing value of 0.320 nm is specifically detected in the light organ, and is highly similar to the diffraction peak pattern and d-spacing value of needle-formed crystals of monosodium urate monohydrate. However, the molar ratio evaluation of uric acid to various cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺) in the light organ deduced that only a few uric acid molecules were in the form of urate salts. Thus, non-salt uric acid should be the source of the diffraction signal detected in the light organ.</p> <p>Conclusions</p><p>In the light organ, the intense single peak diffraction signal might come from a unique needle-like uric acid form, which is different from other known structures of non-salt uric acid form. The finding of a radial structure in the granules of reflector layer implies that the spherical uric acid granules might be formed by the radial arrangement of needle-formed packing matter.</p> </div

SEM micrographs of the reflector layer and the photogenic layer in the light organ.

<p>A) Light organ (LO), located at the abdominal tissue section (the 6^th body segment), is a slab-like tissue with a thickness about 240 µm. B) The photogenic layer (P) located at the ventral light organ is a 40 µm thick tissue, and shows a morphology distinct from that of the reflector layer (R). C) In the reflector layer (R), densely packed spherical granules are found, and some of them had become hollowed (indicated as arrows). DO: Dorsal organ.</p

The X-ray diffraction (XRD) patterns of the light organ and other tissues of <i>L. cerata</i>.

<p>Homogenized tissues of the light organ (LO), the dorsal organ (DO), the thorax (T) and the head (H) were prepared for XRD analyses. The d-spacing value of 0. 320 nm, corresponding to the diffraction peak of light organ, is indicated.</p

Light Intensity measurement of dorsal and ventral light organs.

<p>A) Ventral view (left panel) and dorsal view (right panel) of an isolated luminescent light organ. The light organ (including photogenic layer and reflector layer) was dissected from the 6^th and 7^th body segments of a dying <i>L. cerata</i>. B) Evaluation of light intensity from the ventral and dorsal luminescent light organs. The averaged light intensity (optical density, O.D) of ventral or dorsal light organs was evaluated from at least three images using ImageJ, and normalized by the averaged light intensity of ventral light organ.</p

The XRD patterns of the light organ and other artificial uric acid or urate crystals.

<p>Needle-formed crystals of monosodium urate monohydrate (MSUM) and plate-formed crystals of uric acid dihydrate (UAD) were prepared as described in Method and Material. The d-spacing values for the intense peak of MSUM and UAD, similar to that of the light organ of <i>L. cerata</i> (LO), are indicated, respectively.</p

Neighboring-Cation Substitution Tuning of Photoluminescence by Remote-Controlled Activator in Phosphor Lattice

Highly efficient red phosphors with superior intrinsic properties that are excited by ultraviolet or blue light-emitting diodes are important white light sources for our daily life. Nitride-based phosphors, such as Sr₂Si₅N₈:Eu²⁺ and CaAlSiN₃:Eu²⁺, are commonly more red-shifted in photoluminescence and have better thermal/chemical stability than oxides. Cation substitutions are usually performed to optimize photoluminescence and thermal quenching behavior. However, the underlying mechanisms are unclear in most cases. Here we show that neighboring-cation substitution systematically controls temperature-dependent photoluminescence behavior in CaAlSiN₃:Eu²⁺ lattice. Trivalent cation substitution at the Ca²⁺ site degrades the photoluminescence in high-temperature environments but achieves better thermal stability when the substituted cation turns monovalent. The neighboring-cation control of lifetime decay is also observed. A remote control effect that guides Eu²⁺ activators in selective Ca²⁺ sites is proposed for neighboring-cation substitution while the compositional Si⁴⁺/Al³⁺ ratio adjusts to the valence of M^<i>n</i>+ (<i>n</i> = 1–3) cation. In the remote control effect, the Eu²⁺ activators are surrounded with nitride anions which neighbor with M³⁺-dominant and Si⁴⁺/Al³⁺-equivalent coordination when M is trivalent, but shift to the site where surrounded nitride anions neighbor with M⁺-dominant and Si-rich coordination when M is monovalent. This mechanism can efficiently tune optical properties, especially thermal stability, and could be general to luminescent materials, which are sensitive to valence variation in local environments

Multiscale mechanisms of nutritionally induced property variation in spider silks

<div><p>Variability in spider major ampullate (MA) silk properties at different scales has proven difficult to determine and remains an obstacle to the development of synthetic fibers mimicking MA silk performance. A multitude of techniques may be used to measure multiscale aspects of silk properties. Here we fed five species of Araneoid spider solutions that either contained protein or were protein deprived and performed silk tensile tests, small and wide-angle X-ray scattering (SAXS/WAXS), amino acid composition analyses, and silk gene expression analyses, to resolve persistent questions about how nutrient deprivation induces variations in MA silk mechanical properties across scales. Our analyses found that the properties of each spider’s silk varied differently in response to variations in their protein intake. We found changes in the crystalline and non-crystalline nanostructures to play specific roles in inducing the property variations we found. Across treatment <i>MaSp</i> expression patterns differed in each of the five species. We found that in most species <i>MaSp</i> expression and amino acid composition variations did not conform with our predictions based on a traditional <i>MaSp</i> expression model. In general, changes to the silk’s alanine and proline compositions influenced the alignment of the proteins within the silk’s amorphous region, which influenced silk extensibility and toughness. Variations in structural alignment in the crystalline and non-crystalline regions influenced ultimate strength independent of genetic expression. Our study provides the deepest insights thus far into the mechanisms of how MA silk properties vary from gene expression to nanostructure formations to fiber mechanics. Such knowledge is imperative for promoting the production of synthetic silk fibers.</p></div

Comparisons of the expressions of the MaSp1 genes previously isolated from the <i>Argiope trifasciata</i> (MaSp1a) and <i>Latrodectus hesperus</i> (MaSp1b), and the MaSp2 genes from <i>Argiope trifasciata</i> (MaSp2a) and <i>Latrodectus hesperus</i> (MaSp2b), across treatments for each of the five spiders.

<p>Where P = protein fed and N = protein deprived treatments, <i>Ak</i> = Argiope <i>keyserlingi</i>, <i>Et</i> = <i>Eriophora transmarina</i>, <i>Lh</i> = <i>Latrodectus hasselti Np</i> = <i>Nephila plumipes</i>, <i>Pg</i> = <i>Phonognatha graeffei</i>.</p