MOCVD growth mechanisms of ZnO nanorods

ZnO is a promising material for the fabrication of light emitting devices. One approach to achieve this goal is to use ZnO nanorods because of their expected high crystalline and optical quality. Catalyst free growth of nanorods by metalorganic chemical vapour deposition (MOCVD) was carried out on (0001) sapphire substrates. Arrays of well-aligned, vertical nanorods were obtained with uniform lengths and diameters. A thin wetting layer in epitaxy with the sapphire substrate is formed first, followed by pyramids and nanorods. The nucleation of nanorods occurs either directly at the interface, or later on top of some of the pyramids, suggesting various nucleation mechanisms. It is shown that crystal polarity plays a critical role in the growth mechanism with nanorods of Zn polarity and their surrounding pyramids with O polarity. A growth mechanism is proposed to explain that most threading dislocations lie in the wetting layer, with only a few in the pyramids and none in the nanorods

Surface pre-coating of talc particles by carboxyl methyl cellulose adsorption : study of adsorption and consequences on surface properties and settling rate

This paper investigates the adsorption of different sized carboxyl methyl cellulose (CMC) onto talc particles (adsorption isotherm, adsorption reversibility), its consequences on the particles properties (electrophoretic mobility and surface wetting) and its effect on their dispersion (settling coefficient). Throughout the paper, the properties of talc particles dispersed in CMC solution are compared to CMC pre-coated talc particles: talc particles dried in a solution of CMC before their redispersion. The adsorption of CMC onto talc was quantified at around 0.4 mg of CMC per m2 of talc and was seen to be irreversible on washing the talc particles with distilled water. When characterising talc surface properties, it was found that CMC adsorption leads to an increase of the negative surface charge and to an increase of the wettability. The settling velocity of CMC pre-coated talc particles in water can be around 50 % lower than that of the initial talc particles. The pre-coating of talc particles by CMC is then assumed to increase their stabilization : particle aggregation is hindered by adsorbed CMC layer inducing electrosteric repulsion between the talc particles. The technique of talc pre-coating with CMC makes the talc dispersion easier and could open interesting perspectives in engineering processes using talc dispersions

Core-shell multi-quantum wells in ZnO / ZnMgO nanowires with high optical efficiency at room temperature

International audienceNanowire-based light-emitting devices require multi-quantum well heterostructures with high room temperature optical efficiencies. We demonstrate that such efficiencies can be attained through the use of ZnO/Zn(1-x)MgxO core shell quantum well heterostructures grown by metal organic vapour phase epitaxy. Varying the barrier Mg concentration from x=0.15 to x=0.3 leads to the formation of misfit induced dislocations in the multi quantum wells. Correlatively, temperature dependant photoluminescence reveals that the radial well luminescence intensity decreases much less rapidly with increasing temperature for the lower Mg concentration. Indeed, about 54% of the 10K intensity is retained at room temperature with x=0.15, against 2% with x=0.30. Those results open the way to the realization of high optical efficiency nanowire-based light emitting diodes

High conductivity in Si-doped GaN wires

4 pagesInternational audienceTemperature-dependent resistivity measurements have been performed on single Si-doped GaN microwires grown by catalyst-free metal-organic vapour phase epitaxy. Metal-like conduction is observed from four-probe measurements without any temperature dependence between 10 K and 300 K. Radius-dependent resistivity measurements yield resistivity values as low as 0.37 mohm.cm. This is in agreement with the full width at half maximum (170 meV) of the near band edge luminescence obtained from low temperature cathodoluminescence study. Higher dopant incorporation during wire growth as compared to conventional epitaxial planar case is suggested to beresponsiblefortheuniqueconductivity

Strain relaxation by dislocation glide in ZnO/ZnMgO core-shell nanowires

4 pagesInternational audiencePlastic relaxation of the misfit stress in core-shell semi-conducting nanowires can lead to structural defects, detrimental to applications. Core-shell Zn{0.7}Mg{0.3}O/ZnO quantum well heterostructures were deposited on ZnO nanowires. Strain along the a and c axes of the wurtzite structure is relaxed through the glide of dislocation half-loops from the free surfaces, within pyramidal and prismatic planes. Some half-loops are closed up in the barriers to accommodate the misfit at two consecutive interfaces of the quantum well stack. Dislocations are also observed within the nanowire core: contrary to two-dimensional structures, both the core and the shell can be plastically relaxed

Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire

Controlling the growth of zinc oxide nanowires is necessary to optimize the performances of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. In this view, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire / ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO

Assessment of targeted non-intentionally added substances in cosmetics in contact with plastic packagings. Analytical and toxicological aspects

International audienceContainer-content interactions are common in the food and pharmaceutical industries. However, these studies are more complicated in the cosmetic industry, and it is necessary to ensure consumer safety. The objective of this work was to develop a strategy for the toxicological evaluation of leachables for cosmetic packagings. Eleven common plastic packagings were selected to evaluate interactions with 5 simulants (acidic, alkaline and neutral water, 30% and 96% ethanol) chosen to mimic cosmetics behavior. A GC-MS method was developed to screen for 12 non-intentionally added substances of particular concern: 10 phthalates, bisphenol A and distearyl thiodipropionate (European Pharmacopoeia plastic additive 17). Results were analyzed using a toxicological procedure established for this study. Some phthalates and bisphenol A were detected in several samples, but only one contaminant, diisobutyl phthalate (DiBP), was found to be above the set concentration threshold. Using toxicological data, this concentration was found to be safe for users. 96% ethanol appeared to be the strongest simulant in term of extraction, with a maximum concentration of 491 μg/L for DiBP in a 100% styrene-acrylonitrile copolymer packaging. In water simulants, less contaminants were extracted, with concentrations under 20 μg/L

Auto-protective redox buffering systems in stimulated macrophages

BACKGROUND: Macrophages, upon encounter with micro-organisms or stimulated by cytokines, produce various effector molecules aimed at destroying the foreign agents and protecting the organism. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are front line molecules exerting strong cytotoxic activities against micro-organisms and many cells, including macrophages themselves. Using cells of the murine macrophage cell line (RAW 264.7) stimulated in vitro with lipopolysaccharide (LPS) and/or interferon (IFN-γ), which induce strong endogenous NO production, we examined by which mechanisms a fraction of activated macrophages protect themselves from nitrosative stress and manage to escape destruction? RESULTS: We observed that survivors (10–50% depending on the experiments) had acquired a resistant phenotype being capable to survive when further exposed in vitro to an apoptosis inducing dose of the NO donor compound DETA-NO. These cells expressed an increased steady-state levels of Mn SOD, CuZn SOD and catalase mRNA (130–200%), together with an increased activity of the corresponding enzymes. Intracellular concentration of glutathione was also increased (× 3.5 fold at 6 hours, still maintained × 5.2 fold at 48 hours). Neither mRNA for glutathione peroxydase, γ-glutamylcysteine synthase and glutathione reductase, nor thioredoxine and thioredoxine reductase, were significantly modified. Additional experiments in which RAW 264.7 cells were stimulated with LPS and/or IFN-γ in the presence of relatively specific inhibitors of both Mn and Cu/Zn SOD, aminotriazol (ATZ) catalase inhibitor and buthionine sulfoximine (BSO) glutathione inhibitor, showed that inhibiting LPS-induced up-regulation of intracellular redox buffering systems also prevented acquisition of the resistant phenotype. CONCLUSIONS: Our data suggest a direct causal relationship between survival of a fraction of macrophages and a up-regulation of key sets of auto-protective intracellular redox buffering systems, occurring simultaneously with modulation of expression of apoptotic molecules of the Bcl(2)-Bcl-(XL)/Bax-Bad family

Thermoelectric and micro-Raman measurements of carrier density and mobility in heavily Si-doped GaN wires

International audienceCombined thermoelectric-resistivity measurements and micro-Raman experiments have been performed on single heavily Si-doped GaN wires. In both approaches, similar carrier concentration and mobility were determined taking into account the non-parabolicity of the conduction band. The unique high conductivity of Si-doped GaN wires is explained by a mobility µ=56 cm2 /V s at a carrier concentration n = 2.6 10^20 /cm 3. This is attributed to a more efficient dopant incorporation in Si-doped GaN microwires as compared to Si-doped GaN planar layers. (c) 2013 AIP Publishing LLC

Assessment of targeted non-intentionally added substances in cosmetics in contact with plastic packagings. Analytical and toxicological aspects

Container-content interactions are common in the food and pharmaceutical industries. However, these studies are more complicated in the cosmetic industry, and it is necessary to ensure consumer safety. The objective of this work was to develop a strategy for the toxicological evaluation of leachables for cosmetic packagings. Eleven common plastic packagings were selected to evaluate interactions with 5 simulants (acidic, alkaline and neutral water, 30% and 96% ethanol) chosen to mimic cosmetics behavior. A GC-MS method was developed to screen for 12 non-intentionally added substances of particular concern: 10 phthalates, bisphenol A and distearyl thiodipropionate (European Pharmacopoeia plastic additive 17). Results were analyzed using a toxicological procedure established for this study. Some phthalates and bisphenol A were detected in several samples, but only one contaminant, diisobutyl phthalate (DiBP), was found to be above the set concentration threshold. Using toxicological data, this concentration was found to be safe for users. 96% ethanol appeared to be the strongest simulant in term of extraction, with a maximum concentration of 491 μg/L for DiBP in a 100% styrene-acrylonitrile copolymer packaging. In water simulants, less contaminants were extracted, with concentrations under 20 μg/L