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Dive into the research topics where Christelle Varenne is active.

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Featured researches published by Christelle Varenne.


Nanotechnology | 2012

Catalyst-assisted hydride vapor phase epitaxy of GaN nanowires: exceptional length and constant rod-like shape capability

Kaddour Lekhal; Geoffrey Avit; Yamina André; A. Trassoudaine; Evelyne Gil; Christelle Varenne; Catherine Bougerol; G. Monier; Dominique Castelluci

The hydride vapor phase epitaxy (HVPE) process exhibits unexpected properties when growing GaN semiconductor nanowires (NWs). With respect to the classical well-known methods such as metal organic vapor phase epitaxy and molecular beam epitaxy, this near-equilibrium process based on hot wall reactor technology enables the synthesis of nanowires with a constant cylinder shape over unusual length. Catalyst-assisted HVPE shows a record short time process (less than 20 min) coupled to very low precursor consumption. NWs are grown at a fast solidification rate (50 μm h(-1)), facilitated by the high decomposition frequency of the chloride molecules involved in the HVPE process as element III precursors. In this work growth temperature and V/III ratio were investigated to determine the growth mechanism which led to such long NWs. Analysis based on the Ni-Ga phase diagram and the growth kinetics of near-equilibrium HVPE is proposed.


Sensors | 2017

Nanomaterials for the Selective Detection of Hydrogen Sulfide in Air

E. Llobet; J. Brunet; A. Pauly; Amadou Ndiaye; Christelle Varenne

This paper presents a focused review on the nanomaterials and associated transduction schemes that have been developed for the selective detection of hydrogen sulfide. It presents a quite comprehensive overview of the latest developments, briefly discusses the hydrogen sulfide detection mechanisms, identifying the reasons for the selectivity (or lack of) observed experimentally. It critically reviews performance, shortcomings, and identifies missing or overlooked important aspects. It identifies the most mature/promising materials and approaches for achieving inexpensive hydrogen sulfide sensors that could be employed in widespread, miniaturized, and inexpensive detectors and, suggests what research should be undertaken for ensuring that requirements are met.


Biosensors | 2016

Electrochemical Sensors Based on Screen-Printed Electrodes: The Use of Phthalocyanine Derivatives for Application in VFA Detection.

Amadou Ndiaye; Sébastien Delile; J. Brunet; Christelle Varenne; A. Pauly

Here, we report on the use of electrochemical methods for the detection of volatiles fatty acids (VFAs), namely acetic acid. We used tetra-tert-butyl phthalocyanine (PcH2-tBu) as the sensing material and investigated its electroanalytical properties by means of cyclic voltammetry (CV) and square wave voltammetry (SWV). To realize the electrochemical sensing system, the PcH2-tBu has been dropcast-deposited on carbon (C) orgold (Au)screen-printed electrodes (SPEs) and characterized by cyclic voltammetry and scanning electron microscopy (SEM). The SEM analysis reveals that the PcH2-tBu forms mainly aggregates on the SPEs. The modified electrodes are used for the detection of acetic acid and present a linear current increase when the acetic acid concentration increases. The Cmodified electrode presents a limit of detection (LOD) of 25.77 mM in the range of 100 mM–400 mM, while the Aumodified electrode presents an LOD averaging 40.89 mM in the range of 50 mM–300 mM. When the experiment is realized in a buffered condition, theCmodified electrode presents a lower LOD, which averagesthe 7.76 mM. A pronounced signal decay attributed to an electrode alteration is observed in the case of the gold electrode. This electrode alteration severely affects the coating stability. This alteration is less perceptible in the case of the carbon electrode.


Talanta | 2014

Improved selectivity towards NO2 of phthalocyanine-based chemosensors by means of original indigo/nanocarbons hybrid material

J. Brunet; A. Pauly; Marc Dubois; M.L. Rodriguez-Mendez; Amadou Ndiaye; Christelle Varenne; Katia Guérin

A new and original gas sensor-system dedicated to the selective monitoring of nitrogen dioxide in air and in the presence of ozone, has been successfully achieved. Because of its high sensitivity and its partial selectivity towards oxidizing pollutants (nitrogen dioxide and ozone), copper phthalocyanine-based chemoresistors are relevant. The selectivity towards nitrogen dioxide results from the implementation of a high efficient and selective ozone filter upstream the sensing device. Thus, a powdered indigo/nanocarbons hybrid material has been developed and investigated for such an application. If nanocarbonaceous material acts as a highly permeable matrix with a high specific surface area, immobilized indigo nanoparticles are involved into an ozonolysis reaction with ozone leading to the selective removal of this analytes from air sample. The filtering yields towards each gas have been experimentally quantified and establish the complete removal of ozone while having the concentration of nitrogen dioxide unchanged. Long-term gas exposures reveal the higher durability of hybrid material as compared to nanocarbons and indigo separately. Synthesis, characterizations by many complementary techniques and tests of hybrid filters are detailed. Results on sensor-system including CuPc-based chemoresistors and indigo/carbon nanotubes hybrid material as in-line filter are illustrated. Sensing performances will be especially discussed.


Journal of Colloid and Interface Science | 2013

Indigo molecules adsorbed on carbonaceous nanomaterials as chemical filter for the selective detection of NO2 in the environment

Marc Dubois; J. Brunet; A. Pauly; Wikenson Simond; Laurent Spinelle; Amadou Ndiaye; Katia Guérin; Christelle Varenne

In order to enhance the durability of chemical filters for ozone molecules, devoted to microsystem for the selective detection of NO2 in the environment, the adsorption of indigo molecules onto the surface of carbonaceous nanomaterials (multi-walled carbon nanotubes, a mixture of nanodisks/nanocones, nanofibres) was investigated. The surface of the multi-walled carbon nanotubes was coated by π-stacking with adsorbed indigo molecules. An excess of indigo has resulted in a biphasic sample where nanotubes covered with indigo coexist with free indigo particles. Although similar filtering yields toward O3 (close to 100%) and NO2 (around 0%) were obtained as compared to individual materials, the indigo/MWCNTs samples exhibit enhanced durability as chemical filter at high ozone concentration (1 ppm).


Journal of Sensors | 2015

Electrodes Modification Based on Metal-Free Phthalocyanine: Example of Electrochemical Sensors for the Detection of Acetic Acid

Amadou Ndiaye; A. Pauly; Sébastien Delile; J. Brunet; Christelle Varenne; Abhishek Kumar

Electroanalytical properties of tetra-tert-butyl phthalocyanine (PcH 2-tBu) modified electrodes are studied by cyclic voltammetry (CV). The modified electrodes are obtained by CV deposition techniques on gold (Au) and glassy carbon (C) screen-printed electrodes (SPEs) and used for the electrochemical detection of acetic acid (AA). Based on the CV experiments, the electrodeposition mechanism is detailed. The modified PcH 2-tBu electrodes reveal one oxidation and one reduction peak within the potential window of the working electrodes. In the presence of the analyte (acetic acid), the modified electrodes show sensitivity in the range of 10 mM to 400 mM. For the PcH 2-tBu modified Au electrode, a limit of detection (LOD) of 5.89 mM (based on the +0.06 V peak) was obtained while for the PcH 2-tBu modified C electrode a LOD of 17.76 mM (based on the +0.07 V peak) was achieved. A signal decay of 17%, based on 20 experiments, is obtained when gold is used as working electrode. If carbon is used as working electrode a value of 7% is attained. A signal decay is observed after more than 50 cycles of experiments and is more pronounced when higher concentrations of acetic acid are used. A mechanism of sensing is proposed at the end.


Key Engineering Materials | 2014

Functionalized Carbon Nanotubes-Based Gas Sensors for Pollutants Detection: Investigation on the Use of a Double Transduction Mode

Amadou Ndiaye; J. Brunet; Christelle Varenne; Pierre Bonnet; A. Pauly; Marc Dubois; Katia Guérin; Bernard Lauron

With an objective to fabricate Carbon nanotubes (CNTs) based sensors, the solution route is investigated. The dispersion routes are chosen here to avoid the CNTs to form bundles which can reduce their surface area. The results show that SWNTs-based gas sensors made by the surfactant method is possible if the annealing temperature is correctly chosen. The use of a surfactant allows preparing sensing layers which present responses to NO2 exposure in the 50-200 ppb Range. In a second procedure the CNTs are noncovalently functionalized and used as sensing material for BTX (Benzene, Toluene and Xylenes) detection. The noncovalent functionalisation occurs through p-p stacking between the SWNTs framework and the highly delocalized π-system of the macrocycle which are phthalocyanines and porphyrines derivatives. The SWNTs materials are characterized by standard techniques (UV-Vis spectroscopy, TGA, TEM, Raman analysis). For BTX detection, we used a double transduction mode: IDEs (Interdigitated electrodes) and QCM (Quartz Crystal Microbalance) in order to get insight into the sensing mechanism.


Key Engineering Materials | 2014

New Indigo/Nanocarbons Hybrid Material as Chemical Filter for the Enhancement of Gas Sensor Selectivity towards Nitrogen Dioxide

J. Brunet; A. Pauly; Marc Dubois; Christelle Varenne; Katia Guérin; Amadou Ndiaye

A new hybrid material performed by the immobilization of indigo particles on nanocarbonaceous matrix has been developed, characterized and implemented with chemoresistors. If phthalocyanine-based chemoresistors provide a high sensitivity, a low threshold of detection and a partial selectivity towards oxidizing pollutants, indigo/nanocarbons hybrid material acts as a selective ozone filter from air sample and so strongly enhances the sensor selectivity towards nitrogen dioxide. The functionalization, highlighted by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis, occurs in a non-covalent way and proceeds through pi-stacking interactions. With filtering yields higher than 99.5% for ozone and less than 5% for nitrogen dioxide, indigo adsorbed onto multi-wall carbon nanotubes has been identified as the most selective filtering material while exhibiting a much greater durability than indigo or nanotubes separately. Associated to phthalocyanine-based gas sensor, the sustainable, selective and real-time monitoring of NO2 in ppb range has been successfully achieved.


SPIE Micro+Nano Materials, Devices, and Applications | 2013

Hydride VPE: the unexpected process for the fast growth of GaAs and GaN nanowires with record aspect ratio and polytypism-free crystalline structure

Yamina André; A. Trassoudaine; Geoffrey Avit; Kaddour Lekhal; Mohammed R. Ramdani; Christine Leroux; G. Monier; Christelle Varenne; Philip E. Hoggan; Dominique Castelluci; Catherine Bougerol; F. Réveret; J. Leymarie; Elodie Petit; V. G. Dubrovskii; Evelyne Gil

Hydride Vapor Phase Epitaxy (HVPE) makes use of chloride III-Cl and hydride V-H3 gaseous growth precursors. It is known as a near-equilibrium process, providing the widest range of growth rates from 1 to more than 100 μm/h. When it comes to metal catalyst-assisted VLS (vapor-liquid-solid) growth, the physics of HVPE growth is maintained: high dechlorination frequency, high axial growth rate of nanowires (NWs) up to 170 μm/h. The remarkable features of NWs grown by HVPE are the untapered morphology with constant diameter and the stacking fault-free crystalline phase. Record pure zinc blende cubic phase for 20 μm long GaAs NWs with radii of 10 and 5 nm is shown. The absence of wurtzite phase in GaAs NWs grown by HVPE whatever the diameter is discussed with respect to surface energetic grounds and kinetics. Ni assisted, Ni-Au assisted and catalyst-free HVPE growth of wurtzite GaN NWs is also addressed. Micro-photoluminescence spectroscopy analysis revealed GaN nanowires of great optical quality, with a FWHM of 1 meV at 10 K for the neutral donor bound exciton transition.


Archive | 2001

NO 2 and ozone monitoring in an urban atmosphere using Pd/Ge/Pd/InP(p) pseudo Schottky junctions

Laurent Talazac; François Barbarin; Christelle Varenne; Lionel Mazet; Serge Pellier; Christophe Soulier

The gas sensing properties of new Schottky diodes-type gas sensors are presented Pseudo Schottky junctions on p-type InP substrates have a thin n-type InP layer inserted between the Schottky electrode and the bulk p-InP substrate. Its main role is to improve electrical rectifying properties of the diodes as well as to provide a sensing layer for oxidizing gases. These gas sensors are highly sensitive to strong oxidizing gases molecules (NO2, O3) that make the barrier height decrease by realizing charge transfer with the n-type InP layer. These devices have revealed interesting potentialities in gaseous pollutants monitoring in urban atmospheres.

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A. Pauly

Blaise Pascal University

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J. Brunet

Blaise Pascal University

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Amadou Ndiaye

Blaise Pascal University

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Katia Guérin

Centre national de la recherche scientifique

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Bernard Lauron

Centre national de la recherche scientifique

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Marc Dubois

Centre national de la recherche scientifique

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Lionel Mazet

Blaise Pascal University

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B. Lauron

Blaise Pascal University

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André Hamwi

Blaise Pascal University

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