Si-based materials and devices for light emission in silicon

We report on the fabrication and performances of extremely efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1.54 μm electroluminescence (EL) at 300 K with a 10% external quantum efficiency, comparable to that of standard light-emitting diodes using III–V semiconductors. Er excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light-emitting MOS devices have been fabricated using Er-doped silicon rich oxide (SRO) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 1%. In these devices, Er pumping occurs by energy transfer from the Si nanostructures to the rare-earth ions. Finally, we have also fabricated MOS structures with Tb- and Yb-doped SiO₂ which show room temperature EL at 540 nm (Tb) and 980 nm (Yb) with an external quantum efficiency of a 10% and 0.1%, respectively.     

STAR CLUSTER: a tracking array for gamma-ray astronomy

Summary A new type of extensive air shower array-telescope is being constructed, close to the GREX scintillator array at Haverah Park. The design of the new instrument is based on identification and measurement of individual shower particles by means of pattern recognition techniques applied to data from LST chambers above and below a thin sheet of high-Z material. Air showers will be detected in the range from 10¹⁴ eV to 10¹⁸ eV. The directions of the shower primaries will be reconstructed with an accuracy of a few tenths of a degree using triangulation between tracks of detected shower particles: >1 GeV electrons, photons and muons. Particle densities measured at the detector locations will be used to calculate shower sizes and core locations. Information about the height of production of the high-energy secondaries will be obtained for each shower. This information will be taken into account in calculating the primary energies. It will be used together with the observed muon content (ratio of muons to all particles), to draw conclusions about the nature of the primary particles. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Calix[n]arenes as components for the construction of micellar systems: synthesis and self-assembly properties of 5,11,17-Tris[(dimethylamino)methyl]-25-monoalkoxy-26,27,28-trihydroxycalix[4]arene derivatives

A series of mono-O-alkylated calix [4] arenes derivatives, with alkyl chain lengths of between 1 and 12 carbon atoms are reported. Monoalkylation is best achieved using potassium carbonate as the weak base and the respective alkyl iodide for chain lengths of one to three carbon atoms and using caesium fluoride as the base and the respective alkyl iodide for longer chain lengths. The mono-alkylated derivatives were converted into the tri-para-dimethylaminomethylene derivatives by the para-quinonemethide reaction in good yields. Surface tension measurements showed that at pH 2, 4, 6 and 8 all the tri-dimethylaminomethylene derivatives showed surfactant behaviour, and at pH 2 all show a Critical Micellar Concentration values. No correlation between Critical Micellar Concentration values and chain length is observed. Dynamic Light Scattering measurements showed that the CMC behaviour may be correlated with the observed aggregate sizes. The solid state structure of mono-O-ethoxy-calix[4]arene is described, in this structure a 1-D inclusion polymer is observed.     

Net,excess and absolute adsorption and adsorption of helium

The definitions of absolute, excess and net adsorption in microporous materials are used to identify the correct limits at zero and infinite pressure. Absolute adsorption is shown to be the fundamental thermodynamic property and methods to determine the solid density that includes the micropore volume are discussed. A simple means to define when it is necessary to distinguish between the three definitions at low pressure is presented. To highlight the practical implications of the analysis the case of adsorption of helium is considered in detail and a combination of experiments and molecular simulations is used to clarify how to interpret adsorption measurements for weakly adsorbed components.     

Complete assignments of 1H and 13C NMR data for trypanocidal eremantholide C oxide derivatives

The chemical transformations of eremantholide C (1), a trypanocidal sesquiterpene lactone isolated from Lychnophora trichocarpha Spreng., gave five new oxide derivatives: 3'-hydroxyeremantholide C (2), 1'-formyleremantholide C (3), 1'-carboxyeremantholide C (4), 1'-carbomethoxyeremantholide C (5) and sodium 1'-carboxylate of eremantholide C (6). The (1)H and (13)C NMR data of all these derivatives were assigned based on 1D and 2D techniques. The derivatives were evaluated against Y and CL strains of Trypanosoma cruzi. All of them were inactive against the Y strain. Compounds 2 and 5 displayed 100% activity on the CL strain while compounds 4 and 6 were partially active on the CL strain.     

Solid-state NMR study of the post-fluorination of (C2.5F)n fluorine-GIC

The conversion of (C2.5F)n fluorine-graphite intercalation compounds (GIC) into covalent graphite fluoride during a post-treatment in pure F2 gas is studied by solid-state NMR. First, a careful characterization of the starting material is performed; in particular, for the first time for fluorinated carbons, two-dimensional 19F--> 13C cross-polarization wide-line separation (CP-WISE) experiments were carried out. This completes the classical NMR data such as 19F and 13C chemical shifts, quantitative 13C solid echo, and C-F bond length measurements, which were estimated by dipolar recoupling using inverse CP MAS. The data of the raw (C2.5F)n and of the samples post-fluorinated at 350, 450, and 550 degrees C were compared to investigate the C-F bonding as a function of the treatment temperature. The C-F bonding is discussed taking into account a hyperconjugation of the C-F bonds with neighboring unfluorinated carbon atoms.     

Solid state NMR study of nanodiamond surface chemistry

Solid state NMR measurements using 13C, 1H and 19F nuclei (MAS, CP-MAS) underline the surface chemistry of nanodiamonds from different synthesis (detonation, high pressure high temperature and shock compression). The comparison of the spin-lattice relaxation times T1 and physicochemical characterization (spin densities of dangling bonds, specific surface area and Raman and infrared spectroscopies) for the various samples, as synthesized, chemically purified and fluorinated allows the nature and the location of the various groups, mainly C-OH, C-H and C-F to be investigated. C-OH groups are located only on the surface whereas C-H and dangling bonds seem to be distributed in the whole volume. Fluorination was studied as a chemical treatment for purification and change of the hydrophobicity through the conversion of the C-OH groups into covalent C-F bonds.     

Crystal to crystal transformations and polymorphism in anionic hydrogen bonding networks stabilized by crown ether metal complexes

15-crown[5] or 18-crown[6] complexes of alkali, transition metal and ammonium cations together with polyprotic inorganic and organic anions have been used to construct crystalline molecular salts based on hydrogen bonded anionic networks. This new class of organic-inorganic complexes displays a variety of crystal-to-crystal transformations, mainly associated to the loss/uptake of water molecules and/or to the ionic reorganization accompanying phase transitions on varying the temperature. The dehydration and phase transition processes have been investigated by DSC, TGA and variable temperature X-ray powder diffraction. Most of the complexes described herein have been prepared by solid state mixing of the solid reactants.     

Lysozyme Immobilized on Micro-Sized Magnetic Particles: Kinetic Parameters at Wine pH

In order to use lysozyme as an anti-microbial agent during the winemaking process, hen egg-white lysozyme (LYZ) was covalently immobilized on two different micro-size magnetic particles (tosyl-activated and carboxylated, TSA and CA, respectively). A cell suspension of Oenococcus oeni, an oenological strain involved in the winemaking process, was utilized as LYZ substrate. Both a kinetic study and a study of the stability of free and immobilized LYZ were performed in McIlvane buffer at pH 3.2, that is the average minimum pH value in wine. The activity and kinetic parameters measured for the free LYZ at pH 3.2 are lower than those reported at the optimum pH (4.5); however the residual activity at pH 3.2 is sufficient to be of interest for further immobilization and applications in winemaking. All kinetic parameters of both biocatalysts (LYZ-CA and LYZ-TSA) are altered after immobilization, probably due to the structural modifications in the active site caused by covalent attachment to the supports. The half-life calculated at 25 °C was 39 h for free LYZ, while it increased to 280 and 134 h for LYZ-TSA and LYZ-CA, respectively. This result indicates that immobilization improves the enzyme stability and that LYZ can be utilized in wine applications in its immobilized forms. In addition, LYZ-TSA seems to be the best biocatalyst for further applications in winemaking.     

<Emphasis Type="Italic">In vitro</Emphasis> bioactivity of 70 wt.% SiO<Subscript>2</Subscript> — 30 wt.% CaO sol-gel glass,doped with 1, 3 and 5 wt.% NbF<Subscript>5</Subscript>

The 70SiO₂-30CaO (wt.%) sol-gel glasses doped with 1, 3 and 5 NbF₅ (wt.%) were prepared via polystep sol-gel route. The synthesized glasses were characterized by XRD, FTIR and SEM. Changes in 1.5 SBF solutions were measured by ICP-AES. XRD of the glasses stabilized at 700°C for 6 hours proved the presence of niocalite. FTIR was consistent with XRD data. The in vitro bioactivity study of all glasses prepared were carried out by soaking in 1.5 simulated body fluid (1.5 SBF) at 37°C for 6 and 12 days in static conditions. The FTIR reveals the formation of A-type and B-type carbonate containing hydroxyapatite (CO₃HA) layer. Changes in 1.5 SBF solutions, after 6 days of soaking, show that the Ca concentration increased significantly, compared to the initial Ca content in the 1.5 SBF solution before in vitro test. After 12 days of immersion, the Ca concentration decreased, i.e., the formation of HA phase consumed Ca from 1.5 SBF solution. For all soaking times, the concentration of P is much lower than that the used 1.5 SBF. Based on these results we suggest that Ca and P play an active role in the future of the glasses. SEM depicts that the different morphology of hydroxyapatite can be formed as a function of soaking time.