Stéphane Esnouf

Silicon threshold displacement energy determined by photoluminescence in electron-irradiated cubic silicon carbide

In view of the potential use of silicon carbide (SiC) in the nuclear industry, it is of major interest to understand point defect formation in this material. This work is a contribution to the determination of the silicon threshold displacement energy in the cubic polytype of SiC using electron irradiations with increasing energies from 275 to 680 keV. The photoluminescence signal of the silicon vacancy was related to the number of displacements per atom in the silicon sublattice. This quantity was calculated taking into account the energy loss and angular dispersion of electrons in the target. A best fit of experimental data was obtained for a displacement cross section using a threshold displacement energy of 25 eV along the [100] lattice direction. We checked the relevance of this result by comparing the experimental concentration of silicon single vacancies measured by electron paramagnetic resonance spectroscopy with the theoretical number of displaced silicon atoms.

XAS investigation of biorelevant cobalt complexes in aqueous media.

Cobalt is an essential element of biological cycles involved in numerous metallobiomolecules, but it becomes a toxic element at high concentration or a radiotoxic element because of its use in the nuclear industry. “Molecular speciation” in biological media is an essential prerequisite to evaluate its chemical behaviour as well as its toxic or beneficial effects. In this scheme, we have focused on the coordination properties of the thiol-containing amino acid cysteine (Cys) and the pseudo-peptide N-(2-mercaptopropionyl)glycine (MPG) towards the Co2+ cation in aqueous media. XAS at the Co K edge and traditional spectroscopic techniques have been coupled in order to structurally characterize the cobalt coordination sphere. Oxidation states and geometries of the bis- and tris-cysteinato Co(III) complexes are in agreement with the literature data. In addition, bond lengths between the metallic centre and the donor atoms have been determined. The structure of a new dimeric N-(2-mercaptopropionyl)glycinato Co(II) complex in solution is also reported. The coordination of MPG to Co(II) through the thiolate and carboxylate functions is ascertained. This work provides fundamental structural information about biorelevant complexes of cobalt, which will contribute to our understanding of the chemical behaviour and the biological role of this radionuclide.

Thermal stability of irradiation-induced point defects in cubic silicon carbide

This work aims specifically at studying the evolution of point defects induced by electron irradiation in the cubic polytype of SiC (3C-SiC) at temperatures ranging from 10 to 1450 K by means of photoluminescence (PL) spectroscopy. We identified a first annealing stage between 200 and 245 K, which probably results from migration of interstitials in the carbon sublattice. Moreover, we confirmed the high thermal stability of defect-related PL signals up to about 1100 K and calculated the activation energies associated with their annihilation. Finally, we studied the effect of a high temperature treatment at 1400 K on the DI center PL intensity in a single-crystal sample irradiated by electrons below the threshold displacement energy of the silicon sublattice. This allows checking the relevance of recent defect models based upon the migration of atoms in the carbon sublattice during the irradiation process. We conclude that the DI center does not involve the silicon vacancy and could be assigned to an isolat...

A combined spectroscopic and theoretical approach to investigate structural properties of Co(II)/Co(III) tris-cysteinato complexes in aqueous medium.

Physiological and toxicological effects of metallic ions depend on their speciation and on the structure of their associated bioligand complexes. In the field of chemical and/or nuclear toxicological studies, we are investigating cobalt complexes with biorelevant ligands such as amino acids or peptides. The aqueous reaction of cobalt dichloride with an excess of cysteine (Cys, C3H5NSO22−) in a basic medium under an anaerobic atmosphere and subsequent oxidation by O2, afforded the mononuclear complexes Co(II):3Cys and Co(III):3Cys, respectively. A combination of X-ray absorption spectroscopy (XAS) measurements and Car-Parrinello molecular dynamics (CPMD) simulations allowed us to assess structural features of the already explored Co(III):3Cys complex. Inclusion of the temperature effects in the CPMD calculations gives an implicit access to disorder effects in the extended X-ray absorption fine structure (EXAFS) equation. The very good agreement between the measured and the simulated data showed the accuracy of these models provided by CPMD. The present investigation is completed by new UV-visible, X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) data of Co(II):3Cys. These data are consistent with a Co(II) high-spin d7 complex in a distorted octahedral geometry. This work contributes to the knowledge of topics such as metal–bioligand interaction which is of major interest in the field of bioinorganic chemistry.

Irradiation/temperature synergy effects on degradation and ageing of chlorosulphonated polyethylene

Chlorosulphonated polyethylene (CSPE) irradiated with 2.5 MeV β particles has been studied by means of FTIR spectroscopy. The effects of dose rate, temperature and atmosphere on molecular changes were investigated. The main modifications were loss of sulphonyl chloride groups, formation of trans-vinylene groups and in the presence of oxygen formation of oxidation products. Results were discussed in terms of interspur and intraspur reactions.

Vacancy Defects Induced by Low Energy Electron Irradiation in 6H and 3C-SiC Monocrystals Characterized by Positron Annihilation Spectroscopy and Electron Paramagnetic Resonance

In this work we used Positron Annihilation Spectroscopy (PAS) and Electron Paramagnetic Resonance (EPR) to investigate the properties of vacancy defects produced by low energy electron irradiation. N-doped 3C-SiC and 6H-SiC monocrystals have been irradiated with electrons at different energies from 240keV to 900keV. EPR measurements show that Frenkel pairs VSi 3-/Si are created in 6H-SiC when electron irradiation is performed at low energy (240-360 keV). EPR also indicates that the silicon displacement threshold energy is higher in 3C-SiC than in 6HSiC. Moreover, PAS results show that the size and concentration of the vacancy defects decrease when the electron energy decreases for both polytypes. PAS detects vacancy defects in 240keV electron irradiated 3C-SiC, and the detection of the carbon vacancy is proposed.