Ahmed Addad

Gold-free growth of GaAs nanowires on silicon: arrays and polytypism

We report growth by molecular beam epitaxy and structural characterization of gallium-nucleated GaAs nanowires on silicon. The influences of growth temperature and V/III ratio are investigated and compared in the case of oxide-covered and oxide-free substrates. We demonstrate a precise positioning process for Ga-nucleated GaAs nanowires using a hole array in a dielectric layer thermally grown on silicon. Crystal quality is analyzed by high resolution transmission electron microscopy. Crystal structure evolves from pure zinc blende to pure wurtzite along a single nanowire, with a transition region.

Analytical transmission electron microscopy study of a natural MORB sample assemblage transformed at high pressure and high temperature

Abstract Natural mid-ocean ridge basalt (MORB) samples recovered from diamond-anvil cell (DAC) experiments performed between 33 to 89 GPa and 1700 to 2600 K were studied with a transmission electron microscope (TEM). We used the focused ion beam (FIB) lift-out technique to prepare the recovered high-pressure, laser-heated samples for TEM study. Observations of TEM sections show the presence of five phases for samples transformed at pressures ranging from 33 to 45 GPa: Al-bearing Mg-perovskite, Ca-silicate perovskite, stishovite, and two Al-rich phases. The Al-rich phases were identified by selected area electron diffraction (SAED) patterns and chemical composition analysis, and include the new aluminous (NAL) phase with hexagonal structure and the calcium ferrite (CF) type phase. Chemical analyses obtained by analytical transmission electron microscopy (ATEM) show that Mg-silicate perovskite is the major host for Al, with significant amounts also distributed between the CF-type and NAL phases, and less than 1 wt% in stishovite. Beyond pressures of ~40 GPa (~1100 km depth), the Al content of Mg-perovskite and CF-type phase increases. Between 45 and 50 GPa, the NAL phase disappears. This mineralogical change may explain reported seismic anomalies in subduction zones at mid-mantle depths.

Highly controlled synthesis of nanometric gold particles by citrate reduction using the short mixing, heating and quenching times achievable in a microfluidic device

Homodispersed 1.8 nm gold nanoparticles were obtained reproducibly in high yields using the classical Turkevich protocol at a high concentration in a continuous flow capillary reactor. The microfluidic reactor made from commercially available items permitted short mixing, heating and quenching times which are the key parameters of this synthesis.

Hydrothermal synthesis, phase structure, optical and photocatalytic properties of Zn2SnO4 nanoparticles.

Zinc stannate (Zn2SnO4 or ZTO) nanoparticles were synthesized via hydrothermal method using NaOH as a mineralizer. X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) of the synthesized ZTO nanoparticles revealed the formation of highly pure ZTO phase with the spinel-like structure. The nanoparticles have spherical shape with an average size of about 25 nm. The Raman spectrum of the sample was dominated by the A(1g) vibration mode of pure ZTO phase. From UV-Vis measurement, a band gap E(g) of 3.465 eV was determined. The photocatalytic activity of the ZTO nanoparticles was evaluated for the photodegradation of rhodamine B (RhB) under visible light irradiation. The influence of catalyst concentration and irradiation time on the photocatalytic process was investigated. The ZTO catalyst showed the best photocatalytic performance at a concentration of 0.2 g/L, and the photodecomposition of RhB followed first-order kinetics with a rate constant k=0.0249 min(-1). The ZTO-assisted photocatalytic degradation of RhB occurred via two competitive processes: a photocatalytic process and a photosensitized process. The detection of hydroxyl radicals by fluorescence measurements suggests that these species play an important role in the photocatalytic process.

Hydrothermal alteration of the Soultz-sous-Forêts granite (Hot Fractured Rock geothermal exchanger) into a tosudite and illite assemblage

Abstract: The Soultz-sous-Forets granitic basement represents the reservoir of an experimental Hot Fractured Rock (HFR) geothermal exchanger presently tested in the northern Rhine Graben (France), referring now to the concept of Enhanced Geothermal System (EGS). The injected fluids circulate in the natural fracture network of the granite and through its hydrothermally altered matrix. One of these fractured and altered zones, located about 800 m below the granite-sediment boundary, contains tosudite, which is a rather rare mixed-layer chlorite/smectite that crystallized here ahead of a fibrous illite/quartz/calcite paragenesis. Tosudite occurs mainly in the relics of plagioclase grains that were progressively altered by interacting with Li-bearing hydrothermal fluids percolating in the granite fractures. The age of the hydrothermal alteration activity is inferred from K-Ar dating of varied particle sizes of the associated illite: two distinct hydrothermal episodes of illite crystallization could be set at about 63 and 18-Ma or less, without further detectable precipitation, especially during the rifting of the Rhine Graben. Precipitation of fibrous illite in the pore space of the altered granite is expected to have reduced its permeability, as frequently observed in sandstone reservoirs. Clay crystallization may, therefore, represent a significant drawback for engineering the geothermal programme, as the chemical composition of the injected fluids shall be designed to reduce and even prevent illite precipitation and promote tosudite precipitation, when mixing with the natural fluids still present in the granite.

Phenomenological analysis of densification mechanism during spark plasma sintering of MgAl2O4

Spark plasma sintering (SPS) of MgAl2O4 powder was investigated at temperatures between 1200 and 1300°C. A significant grain growth was observed during densification. The densification rate always exhibits at least one strong minimum, and resumes after an incubation period. Transmission electron microscopy investigations performed on sintered samples never revealed extensive dislocation activity in the elemental grains. The densification mechanism involved during SPS was determined by anisothermal (investigation of the heating stage of a SPS run) and isothermal methods (investigation at given soak temperatures). Grain-boundary sliding, accommodated by an in-series {interface-reaction/lattice diffusion of the O

Cell micropatterning on superhydrophobic diamond nanowires

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Morphology of magneto-electrodeposited Cu2O microcrystals

-anions} mechanism controlled by the interface-reaction step, governs densification. The zero-densification-rate period, detected for all soak temperatures, arise from the difficulty of annealing vacancies, necessary for the densification to proceed. The detection of atomic ledges at grain boundaries and the modification of the stoichiometry of spinel during SPS could be related to the difficulty to anneal vacancies at temperature soaks.

Cyclodextrins as effective additives in AuNP-catalyzed reduction of nitrobenzene derivatives in a ball-mill

Cell micropatterning was achieved in a spatially controlled manner based on heterogeneously wetted superhydrophilic/superhydrophobic diamond nanowire (NW) surfaces. Diamond NWs were synthesized on boron-doped diamond substrates using reactive ion etching and functionalized with octadecyltrichlorosilane to achieve superhydrophobicity. Superhydrophilic motifs of 400×400 μm(2) and 10×10 μm(2) single cell-sized motifs, surrounded by superhydrophobic regions, were then generated by selectively exposing the substrates to UV light. This design allowed successful patterning of single HeLa and MCF-10A cells within the superhydrophilic regions without additional surface modification. To add a further level of complexity, micropatterned co-cultures were obtained using bovine serum albumin to promote cell adhesion. This method is simple and does not require any complicated processing steps such as mask deposition or template removal. Potential applications are in the development of cell-based biological assays in well-controlled and biologically relevant environments.

Ruthenium-containing β-cyclodextrin polymer globules for the catalytic hydrogenation of biomass-derived furanic compounds

Morphological development during electrocrystallization of cuprous oxide crystals has been reported. Studies have been focused on the effects of magnetohydrodynamic convection generated by superimposition of a high magnetic field up to 12 T. Variation of the crystal morphology from dense to labyrinthine and finally very highly porous structure with magnetic field has been obtained, whereas no change in the crystallization growth envelope was shown. Here crystals grown under microconvection induced by magnetic field are different because of changes in the diffusion process. These results have shown that magnetic field acts principally on the branching growth but does not modify the crystal habit.