Christine Leroux

Record Pure Zincblende Phase in GaAs Nanowires down to 5 nm in Radius

We report the Au catalyst-assisted synthesis of 20 μm long GaAs nanowires by the vapor-liquid-solid hydride vapor phase epitaxy (HVPE) exhibiting a polytypism-free zincblende phase for record radii lower than 15 nm down to 5 nm. HVPE makes use of GaCl gaseous growth precursors at high mass input of which fast dechlorination at the usual process temperature of 715 °C results in high planar growth rate (standard 30-40 μm/h). When it comes to the vapor-liquid-solid growth of nanowires, fast solidification at a rate higher than 100 μm/h is observed. Nanowire growth by HVPE only proceeds by introduction of precursors in the catalyst droplets from the vapor phase. This promotes almost pure axial growth leading to nanowires with a constant cylinder shape over unusual length. The question of the cubic zincblende structure observed in HVPE-grown GaAs nanowires regardless of their radius is at the heart of the paper. We demonstrate that the vapor-liquid-solid growth in our conditions takes place at high liquid chemical potential that originates from very high influxes of both As and Ga. This yields a Ga concentration systematically higher than 0.62 in the Au-Ga-As droplets. The high Ga concentration decreases the surface energy of the droplets, which disables nucleation at the triple phase line thus preventing the formation of wurtzite structure whatever the nanowire radius is.

The expression pattern of alfalfa flavanone 3-hydroxylase promoter-gus fusion in Nicotiana benthamiana correlates with the presence of flavonoids detected in situ

Flavanone 3-hydroxylase is an enzyme acting in the central part of the flavonoid biosynthesis pathway. It is generally encoded by a single gene and seems to have a key position for the regulation in this pathway. These two features make a single f3h promoter-gus fusion a suitable tool to study both the f3h expression and the regulation of this pathway. We present here the spatial and temporal analysis of the expression of an alfalfa flavanone 3-hydroxylase (f3h) promoter-gus fusion introduced into Nicotiana benthamiana. The Medicago sativa (alfalfa) f3h promoter directed gus expression in flowers, stems, leaves and roots. In flowers, GUS activity was observed in pollen grains, in ovules, in ovary placenta and in the epidermis, medullary parenchyma, trichomes and second cortical cellular layer surrounding the vascular bundles of the peduncle. In stems, GUS activity was detected at the same places as in the peduncle except for the medullary parenchyma. In roots, we found GUS staining in root hairs, epidermis and in the vascular bundles of the elongated zone. Finally, in leaves, the f3h promoter expressed essentially in the stalk cells of the multicellular trichomes. The expression pattern of the f3h-gus fusion was correlated to the presence of flavonoids in situ. These data indicate that this construct can be very useful to study factors controlling the production of flavonoids.

Growth of cobalt ultra-thin films deposited on Pt(100) surfaces: An Auger electron spectroscopy study

Abstract To investigate the possibility of building Co/Pt modulated multilayers, attention has been paid to the early stages of interface formation between cobalt and platinum. The growth of cobalt layers less than 10 monolayers thick on Pt(100) surfaces has been studied by Auger electron spectroscopy. Growth kinetics obtained by AES show that two different models of interface formation can be possible: the Volmer-Weber case (cobalt islands) or an interdiffusion process between the two metals (similar to suicide formation). Looking at the fine structure of the low-energy platinum Auger transitions suggests that there is a strong interaction between cobalt and platinum as soon as the equivalent of a cobalt 2 monolayer coverage is deposited. This would favor the interdiffusion process model.

Nanoparticles and Thin Films of Cerium Dioxides: Relations between Elaboration Process and Microstructure

Nano-powders and thin films of cerium dioxides for gas sensor applica tions have been elaborated using various specific routes. Nano-powders of CeO 2 were obtained from (i) chemical precipitation followed by high-energy mechanical milling, (ii) soft chemical preparation and (iii) an unusual method of pulverization of liquid solution. These materials were char a terized using X-Ray diffraction and Transmission Electron Microscopy ; depending on the pre paration method, the particle sizes varied from 2 to 10 nanometers. Thin films of CeO 2 were prepared, using a pulsed laser deposition method. According to the preparation parameters, fil m thickness varied from 180 to 750 nm, with diffracting domain sizes of about 8 nm. A qualitative a pproach of the nonstoichiometry of powder cerium dioxides is developed using high temperature elect ica impedance spectroscopy.

Structural and Catalytic Properties of Thin Films of CuOx-CeO2-x Deposited by Laser Ablation

The structural analysis of thin cerium dioxide films doped with Cu, produced by laser ablation for applications in catalytic and gas sensors, was the general aim of the study. The thin films deposited on a (100) silicon substrate were nanocrystalline structure with a well-developed texture. The morphology, as well as the preferred films orientation, is changed with the volume fraction of Cu. The observed changes affect the catalytic properties of the materials obtained which was confirmed by the catalytic tests undertaken with CH4.

Vapor liquid solid-hydride vapor phase epitaxy (VLS-HVPE) growth of ultra-long defect-free GaAs nanowires: Ab initio simulations supporting center nucleation

High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy and micro-Raman spectroscopy revealed polytypism-free zinc blende (ZB) NWs over lengths of several tens of micrometers for a mean diameter of 50 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors GaCl of which high decomposition frequency after adsorption onto the liquid droplet catalysts, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the droplets. High influxes of Ga and As species then yield high axial growth rate of more than 100 μm/h. The diffusion of the Ga atoms in the liquid droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3 × 10(-9) m(2)/s. The fast diffusion of Ga in the droplet favors nucleation at the liquid-solid line interface at the center of the NW. This is further evidence, provided by an alternative epitaxial method with respect to metal-organic vapor phase epitaxy and molecular beam epitaxy, of the current assumption which states that this type of nucleation should always lead to the formation of the ZB cubic phase.

Self-catalyzed GaAs nanowires on silicon by hydride vapor phase epitaxy

Gold-free GaAs nanowires on silicon substrates can pave the way for monolithic integration of photonic nanodevices with silicon electronic platforms. It is extensively documented that the self-catalyzed approach works well in molecular beam epitaxy but is much more difficult to implement in vapor phase epitaxies. Here, we report the first gallium-catalyzed hydride vapor phase epitaxy growth of long (more than 10 μm) GaAs nanowires on Si(111) substrates with a high integrated growth rate up to 60 μm h-1 and pure zincblende crystal structure. The growth is achieved by combining a low temperature of 600 °C with high gaseous GaCl/As flow ratios to enable dechlorination and formation of gallium droplets. GaAs nanowires exhibit an interesting bottle-like shape with strongly tapered bases, followed by straight tops with radii as small as 5 nm. We present a model that explains the peculiar growth mechanism in which the gallium droplets nucleate and rapidly swell on the silicon surface but then are gradually consumed to reach a stationary size. Our results unravel the necessary conditions for obtaining gallium-catalyzed GaAs nanowires by vapor phase epitaxy techniques.

Monoclinic superstructure in orthorhombic Ce10W22O81 from transmission electron microscopy.

A complex rare-earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high-resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-ray diffraction experiments were confirmed for Ce10W22O81. A C2/c monoclinic superstructure, with cell parameters a = 7.8, b = 36.1, c = 22.2 Å and β = 100.2°, was shown and attributed to a partial oxidation of Ce(3+) leading to interstitial oxygen ions.

Leaching Process of Cerium Extraction from Mixture of Cerite-Monazite Mineral

Rare earth oxides have been widely investigated in catalysis as structured and electronic promoters to improve the activity and thermal stability of catalysts. Cerium has an important role in three-way catalysis and fluid catalytic cracking, two significant catalytic processes by their economic relevance and tonnage. Cerium and other rare earths have been studied as possible heterogeneous catalysts at selective oxidation of hydrocarbons. Cerite and monazite are minerals with high concentration of cerium element. Extraction of cerium metal using conventional leaching processes has shown low yields or high costs. The main purpose of this research work is to optimize the parameters in cerium purification stage from this mineral using leaching process. To separate particles with different granulometries, the mineral is ground and fractioned with sieves of 80, 200, 250 and 400 mesh. In order to put off organic components and oxidize cerium(III) to cerium(IV), samples were roasted at 1073K by twenty-four hours. The roasted samples were solubilized by acid attack (leaching) for approximately twenty-four hours; according to the acid used hydrochloric or sulfuric), cerium and other trivalent elements are solubilized as chloride or sulfate solution. Cerium was extracted by selective precipitation at pH~3,4 using ammonium or natrium hydroxide as pH changer. After filtration and drying, the precipitated product was characterized by XRD (x-ray diffraction), and then process efficiency was determined (cerium percentage and the different phases in the powder). Particles granulometry, roasting process (time and temperature), as well as leaching parameters (acid used, time, temperature and concentration of reagents) were the main variables studied.

Self-catalyzed growth of GaAs nanowires on silicon by HVPE

We report on the first self-catalyzed growth of GaAs nanowires on patterned and non-patterned silicon (111) wafers by hydride vapor phase epitaxy (HVPE) with a record elongation rate of 30 μm/h. The crystalline structure was analyzed using high resolution transmission electron microscopy (HRTEM). Self-catalyzed growth proceeds under gallium rich conditions at low-temperature (600 °C). Nanowires exhibit cylindrical rod-like shape morphology with a mean diameter of 50 nm and are randomly distributed.