Myriam Moreau

The Organization Pattern of Root Border-Like Cells of Arabidopsis Is Dependent on Cell Wall Homogalacturonan

Border-like cells are released by Arabidopsis (Arabidopsis thaliana) root tips as organized layers of several cells that remain attached to each other rather than completely detached from each other, as is usually observed in border cells of many species. Unlike border cells, cell attachment between border-like cells is maintained after their release into the external environment. To investigate the role of cell wall polysaccharides in the attachment and organization of border-like cells, we have examined their release in several well-characterized mutants defective in the biosynthesis of xyloglucan, cellulose, or pectin. Our data show that among all mutants examined, only quasimodo mutants (qua1-1 and qua2-1), which have been characterized as producing less homogalacturonan, had an altered border-like cell phenotype as compared with the wild type. Border-like cells in both lines were released as isolated cells separated from each other, with the phenotype being much more pronounced in qua1-1 than in qua2-1. Further analysis of border-like cells in the qua1-1 mutant using immunocytochemistry and a set of anti-cell wall polysaccharide antibodies showed that the loss of the wild-type phenotype was accompanied by (1) a reduction in homogalacturonan-JIM5 epitope in the cell wall of border-like cells, confirmed by Fourier transform infrared microspectrometry, and (2) the secretion of an abundant mucilage that is enriched in xylogalacturonan and arabinogalactan-protein epitopes, in which the cells are trapped in the vicinity of the root tip.

Pushing back the limits of Raman imaging by coupling super-resolution and chemometrics for aerosols characterization

The increasing interest in nanoscience in many research fields like physics, chemistry, and biology, including the environmental fate of the produced nano-objects, requires instrumental improvements to address the sub-micrometric analysis challenges. The originality of our approach is to use both the super-resolution concept and multivariate curve resolution (MCR-ALS) algorithm in confocal Raman imaging to surmount its instrumental limits and to characterize chemical components of atmospheric aerosols at the level of the individual particles. We demonstrate the possibility to go beyond the diffraction limit with this algorithmic approach. Indeed, the spatial resolution is improved by 65% to achieve 200 nm for the considered far-field spectrophotometer. A multivariate curve resolution method is then coupled with super-resolution in order to explore the heterogeneous structure of submicron particles for describing physical and chemical processes that may occur in the atmosphere. The proposed methodology provides new tools for sub-micron characterization of heterogeneous samples using far-field (i.e. conventional) Raman imaging spectrometer.

Design, fabrication and physical analysis of TiN/AlN deep UV photodiodes

Deep-ultraviolet solar-blind photodiodes based on high-quality AlN films grown on sapphire substrates with a metal–semiconductor–metal configuration were simulated and fabricated. The Schottky contact is based on TiN metallization. The material is characterized by the micro-Raman spectroscopy and x-ray diffraction technique. The detector presents an extremely low dark current of 100 fA at −100 V dc bias for large device area as high as 3.1 mm2. It also exhibits a rejection ratio between 180 and 300 nm of three orders of magnitude with a very sharp cut-off wavelength at 203 nm (~6.1 eV). The simulation to optimize the photodiode topology is based on a 2D energy-balance model using the COMSOL® software. Simulation performed for different spacing for a given bias between electrodes show that a compromise must be found between the dark current and the responsivity for the optimization of the device performance. The measurement results are in good agreement with the model predictions.

Hydrothermal synthesis of ZTO/graphene nanocomposite with excellent photocatalytic activity under visible light irradiation.

A facile and efficient one-step hydrothermal approach for the synthesis of Zn2SnO4 nanoparticles/reduced graphene oxide (ZTO/rGO) nanocomposites using zinc acetate, tin chloride and graphene oxide (GO) as precursors, and sodium hydroxide as reducing agent has been developed. This approach allows simultaneous reduction of GO and growth of spinel ZTO nanoparticles (NPs) on the rGO sheets. The morphology and microstructure characterizations of ZTO/rGO nanocomposites revealed that this method leads to close interfacial contact of ZTO NPs and rGO and efficient dispersion of ZTO NPs on the surface of rGO sheets. The photocatalytic activity of the ZTO/rGO nanocomposite was investigated for the reduction of rhodamine B under visible light irradiation. Compared to pure ZTO NPs, ZTO/rGO nanocomposite exhibited superior photocatalytic activity with a full degradation of rhodamine B within 15min. The enhanced photocatalytic performance of ZTO/rGO was mainly attributed to excellent electron trapping and effective adsorption properties of rGO.

Thermal Characterization Using Optical Methods of AlGaN/GaN HEMTs on SiC Substrate in RF Operating Conditions

Performance and reliability of wide bandgap high-power amplifiers are correlated with their thermal behavior. Thermal model development and suitable temperature measurement systems are necessary to quantify the channel temperature of devices in real operating conditions. As a direct temperature measurement within a channel is most of the time not achievable, the common approach is to measure the device temperature at different locations close to the hotspot and then to use simulations to estimate the channel temperature. This paper describes a complete thermal characterization of AlGaN/gallium nitride (GaN) on silicon carbide high electron-mobility transistors (HEMTs) when devices are operating in dc bias, pulsed, and continuous wave. Infrared thermography, charge-coupled device-based thermoreflectance microscopy, and micro-Raman spectroscopy have been performed to extract the thermal resistance of the components. Results have been compared with simulations using a 3-D finite-element model to estimate the operating channel temperature. Measurements have shown that the RF-biased thermal resistance and the dc-biased thermal resistance of GaN HEMTs are similar.

Stress Metrology : The challenge for the next generation of engineered wafers

Raman spectroscopy is a powerful and versatile technique for stress measurements in complex stacks of thin crystalline layers at macroscopic and microscopic scales. Using such a technique we show that thick SiGe layers epitaxially grown using graded buffer method are fully relaxed (>95%) at a macroscopic scale but exhibit a small strain modulation at a microscopic scale. For the first time we report the results of Raman micro-mapping of stress distribution in SGOI wafers produced by Smart Cut TM technology. We conclude that Smart Cut TM is a unique method to manufacture the next generation of engineered wafers that can combine strained and/or relaxed SiGe alloys, Si and Ge films, while keeping their initial strain properties at both scales. It is important to develop Raman spectroscopy tool for in-line process control in fabrication of strained Silicon On Insulator (sSOI) wafers.

Structural and Luminescence Properties of Highly Crystalline ZnO Nanoparticles Prepared by Sol--Gel Method

ZnO nanoparticles were synthesized using sol–gel method. The structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), Raman spectroscopy, and photoluminescence (PL). XRD analysis demonstrates that the nanoparticles have the hexagonal wurtzite structure and the particle size is increased with annealing temperature. The average size of the nanoparticles was determined by SEM as well as XRD data and found to be ~50 nm after annealing at 800 °C. A sharp, strong and dominant UV emission with a suppressed green emission has been observed at 300 and 10 K, indicating the good optical properties of ZnO nanoparticles. The 10 K UV band is dominated by a neutral-donor bound exciton, and the surface-related SX emission at 3.31 eV is evidenced.

The interplay between the paracetamol polymorphism and its molecular structures dissolved in supercritical CO2 in contact with the solid phase: In situ vibration spectroscopy and molecular dynamics simulation analysis.

The aim of this paper is to characterize the distribution of paracetamol conformers which are dissolved in a supercritical CO2 phase being in equilibrium with their corresponding crystalline form. The quantum calculations and molecular dynamics simulations were used in order to characterize the structure and analyze the vibration spectra of the paracetamol conformers in vacuum and in a mixture with CO2 at various thermodynamic state parameters (p,T). The metadynamics approach was applied to efficiently sample the various conformers of paracetamol. Furthermore, using in situ IR spectroscopy, the conformers that are dissolved in supercritical CO2 were identified and the evolution of the probability of their presence as a functions of thermodynamic condition was quantified while the change in the crystalline form of paracetamol have been monitored by DSC, micro IR and Raman techniques. The DSC analysis as well as micro IR and Raman spectroscopic studies of the crystalline paracetamol show that the subsequent heating up above the melting temperature of the polymorph I of paracetamol and the cooling down to room temperature in the presence of supercritical CO2 induces the formation of polymorph II. The in situ IR investigation shows that two conformers (Conf. 1 and Conf. 2) are present in the phase of CO2 while conformer 3 (Conf. 3) has a high probability to be present after re-crystallization.

Ferromagnetism induced in ZnO nanorods by morphology changes under a nitrogen–carbon atmosphere

We use thermal carbonization with acetylene and nitrogen to treat hydrothermally grown ZnO nanorods on silicon substrates. The method is found to be strongly temperature dependent. Treatment temperatures below 800 °C do not induce any morphological changes of the rods, while temperatures above 800 °C cause significant erosion of the rods leading to hollow- and tubular-like structures. The temperature dependent weight decrease confirms the observation of erosion and X-ray photoelectron spectroscopy (XPS) measurements indicate significant decrease in Zn content. Raman spectra evidence the presence of a diamond-like carbon layer around the rods. The coupling of resonant and non-resonant Raman results with photoluminescence measurements allow us to suggest that both nitrogen and carbon are present within the ZnO lattice. The samples treated above 800 °C are also found to be ferromagnetic and the magnetization increases as the treatment temperature increases (up to 1.45 emu cm−3 at 5 K). Thermal annealing in nitrogen atmosphere does not cause either morphological changes or ferromagnetism, suggesting that the erosion results from the additional carbon source (acetylene) of the treatment. Complexes formed by carbon and nitrogen atoms at the surface of the treated and eroded samples are believed to be at the origin of the ferromagnetism.

Structural and luminescence properties of highly crystalline ZnO nanoparticles prepared by sol-gel method

ZnO nanoparticles were synthesized using sol–gel method. The structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), Raman spectroscopy, and photoluminescence (PL). XRD analysis demonstrates that the nanoparticles have the hexagonal wurtzite structure and the particle size is increased with annealing temperature. The average size of the nanoparticles was determined by SEM as well as XRD data and found to be 50nm after annealing at 800 C. A sharp, strong and dominant UV emission with a suppressed green emission has been observed at 300 and 10K, indicating the good optical properties of ZnO nanoparticles. The 10K UV band is dominated by a neutral-donor bound exciton, and the surface-related SX emission at 3.31eV is evidenced. # 2012 The Japan Society of Applied Physics