Positron annihilation in latex templated macroporous silica films: pore size and ortho-positronium escape

International audienceDepth profling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that o-Ps escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps reemission yield decreases from ~ 0:25 to ~ 0:11 as the pore size increases from 32 to 75 nm. The o-Ps reemission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9:1±0:4 g cm-1. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2)ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1:4 ± 4 nm) and from macropores with large effective size (~ 32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19±2 ns. Our results imply that o-Ps effciently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores become small), this capture into the micropores assists the capture into the macropores

Study of interactions between vacancies and Yttrium,Oxygen in iron for the understanding of oxide particles formation in ODS

International audienc

Soft laser sputtering of GaAs semiconductor surfaces

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Experimental study of the diffusion of Xe and Kr implanted at low concentrations in UO2 and determination of their trapping mechanisms

The fission of uranium dioxide produces gaseous elements degrading nuclear fuel properties. A thorough understanding of the transport and release of gaseous products is thus essential. The present work focuses on xenon and krypton migration mechanism in uranium dioxide. Desorption experiments on ion implanted UO2 were performed at 1300°C. Xe and Kr releases were simulated using a mesoscale model that was developed taking into account single gas atom diffusion and defect traps. We showed that the defects have a high influence on Xe and Kr migration mechanisms and therefore have to be considered to accurately determine diffusion coefficients. We evaluated the diffusion coefficient of Xe and Kr at (1.73 ± 0.15)x10−20 m2/s at 1300°C and we showed that the diffusion of rare gases is subjected to two trapping mechanisms. The first occurs during the ion implantation and the second during high-temperature annealings. The nature of the trapping sites is discussed in the light of the literature on radiation induced defects. This study also consolidates the use of non activated UO2 implanted with heavy ions as a less-hazardeous substitute for irradiated UO2

MATERIAL ANALYSIS AND SURFACE INTERACTION STUDIES USING LASER RESONANT MULTIPHOTON IONIZATION

Resonance Ionization Spectroscopy (RIS) technique is an ultrasensitive method of detection of atoms and molecules. Application of RIS to material analysis requires an atomization step which is based in our set-up on ion beam sputtering. We present a review of the analytical studies we have performed demonstrating the very attractive potentialities of this method. RIS can be also used to study the surface sputtering process itself. Basic mechanisms of low energy laser desorption and vaporisation of metal samples have been investigated in this way

Vacancy−solutes interactions and their role in the formation of oxide nanoparticles in ODS steels

International audienc

Spatial effects in the 800keV 3He implantation in W followed by isochronal annealing at 900K

International audienc

Vacancy defects identification in Au 4 MeV irradiated UO2 using Positron Annihilation Spectroscopy

International audienc

Laser-Induced Reactivity of NH3\mathsf{_3} on GaAs Surface

We have studied the reactivity of NH$_3$ on GaAs (100) surface irradiated by 280 nm laser beam at low fluence ($<$ 50 mJ cm$^{-2}$ per pulse) in an experimental set-up which allows a very sensitive characterization of the surface state using mass spectrometry. Thanks to this analysis technique we could follow desorbed NH$_x$ $(x = 2,~3)$ species during the laser treatment; this phase achieved, GaN$^+$ could be identified as an evidence of N fixation using laser desorption mass spectrometry of the treated surface. This characterization mode demonstrates the fluence and laser shot number dependencies on laser treatment efficiency. At 280 nm the N fixation is maximum for laser fluence of about 25 mJ cm$^{-2}$ (per pulse). Still even in the best nitridation conditions, the process appears to be of little efficiency, since it leads to the formation of less than one monolayer of GaN. These results combined with a numerical model of GaAs laser heating give evidence for the laser-induced nitridation to be thermally assisted.Nous avons étudié la réactivité de NH$_3$ sur la surface d'un échantillon de GaAs irradié par un faisceau laser UV (280 nm) de faible fluence ($<$ 50 mJ cm$^{-2}$). Les expériences ont été menées dans un dispositif expérimental utilisant la spectrométrie de masse et permettant une caractérisation présise de l'état de la surface. Grâce à cette technique d'analyse nous avons pu suivre les espèces NH$_x^+$ $(x = 2,~3)$ désorbées pendant le traitement laser, cette étape terminée, nous avons détecté, par la spectrométrie de masse associée à la désorption laser, les ions GaN$^+$ émis depuis la surface traitée, ils indiquent que la nitruration de GaAs s'est bien produite pendant le traitement. Cette technique de caractérisation nous a permis d'étudier l'influence de la fluence et du nombre de tirs laser sur l'efficacité du traitement. À 280 nm l'efficacité de la nitruration assistée par laser du GaAs est maximum pour une fluence laser proche de 25 mJ cm$^{-2}$ (par tir). Cependant même dans les conditions optimales de nitruration, le processus apparaît peu efficace puisqu'il conduit à la formation de moins d'une monocouche de GaN. Ces résultats expérimentaux couplés à un modèle numérique du chauffage laser de GaAs ont montré que le laser produit, pour les réactions de surface, des effets essentiellement thermiques proches de ceux obtenus par un chauffage conventionnel, lent, du substrat de GaAs

Influence of ion energy on damage induced by Au-ion implantation in silicon carbide single crystals

This article reports on the influence of the ion energy on the damage induced by Au-ion implantation in silicon carbide single crystals. 6H-SiC samples were implanted with Au ions at room temperature at two different energies: 4 and 20 MeV. Both Rutherford Back-scattering spectrometry in channelling geometry (RBS/C) and Raman spectroscopy were used to probe the ion implantation-induced damage. Results show that the accumulated damage increases with the fluence up to the amorphization state. RBS/C data indicate that 4-MeV implantation induces more damage than 20-MeV implantation at a given fluence. This effect is attributed to nuclear collisions since the amount of damage is identical at 4 or 20 MeV when the fluence is rescaled in dpa. Surprisingly, Raman data detect more damage for 20-MeV implantation than for 4-MeV implantation at low fluence (below 10(13) cm(-2)) where point defects are likely formed