Apport de la microbalance à quartz dans l'étude de l'influence des ions sulfate, chlorure et magnésium sur la cinétique d'entartrage

Dans le présent travail, nous avons fait appel à la microbalance à quartz électrochimique pour étudier l'influence des ions chlorure, sulfate et magnésium sur la cinétique d'entartrage de l'or au contact d'eaux synthétiques. Les résultats chronoélectrogravimétriques permettent l'évaluation des étapes de germination et de croissance cristalline. Des analyses infrarouge des dépôts formés permettent de lier la nature des espèces identifiées aux paramètres cinétiques, en particulier en présence d'ions SO$_{4}^{2-}$ ou Mg2+. Des effets de seuil sont mis en évidence pour chaque type d'ion étudié. Ils sont essentiellement attribués à des phénomènes d'accumulation d'espèces sur le métal ou sur les produits superficiels, comme l'indique l'étude de l'effet du potentiel d'électrode et de la concentration des espèces en solution

Self-Organization and Ordering in Nanocrystalline Si/SiO2 Superlattices

NRC publication: Ye

SiO2/Si multiple quantum wells emit light

Peer reviewed: YesNRC publication: Ye

Ordering and Self-Organization in Nanocrystalline Silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 \uc5 into brick-shaped crystallites oriented along the left fence111right fence crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.NRC publication: Ye

Characterization of diamond-like carbon by Raman spectroscopy, XPS and optical constants

Crystalline diamond coatings and, increasingly, diamond-like amorphous carbon (DLC) films are used for tribological and protective layers for their hardness and chemical inertness. They are also under investigation for their electron emitting properties, with possible applications in field emission displays. In this study, hydrogen-free DLC films were deposited by laser ablation using a KrF excimer laser and fluences between 0.5 and 2 J/cm2. Information about the disorder and short range order in the films was gained via Raman spectra. As the ratio of graphitic and diamond bonds (sp2 and sp3) is often hidden in these measurements, other analytical methods have to be included. XPS spectra exhibit for certain films the presence of sp3 bonds or graphitic growth. For a better determination of the sp3-content, a correlation with optical properties in the near IR to near UV region was established. These values depended strongly on the substrate temperature and the laser fluence. DLC formation with large sp3 contents could be demonstrated without substrate heating. Vickers hardness values and measurements on the electron emissivity of the films are strongly correlated to the sp3 content and the preparation method of the films.NRC publication: Ye

The prevalence of galaxy overdensities around UV-luminous Lyman ???? emitters in the Epoch of Reionization

Before the end of the Epoch of Reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly alpha lines of z greater than or similar to 6 galaxies in the intergalactic medium. Nevertheless, Ly alpha has been detected up to very high redshifts (z similar to 9) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly alpha transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly alpha emitters: EGS-zs8-1, EGS-zs8-2, and EGSY-z8p7 at z(spec) = 7.73, 7.48, and 8.68, respectively. We combine the multiwavelength HST imaging with Spitzer data to reliably select z similar to 7-9 galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor similar to 3-9x). Our analysis thus reveals ubiquitous overdensities around luminous Ly alpha emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. While three UV luminous galaxies already have spectroscopic redshifts, the majority of the remaining fainter, surrounding sources are yet to be confirmed via spectroscopy. JWST follow-up observations of the neighbouring galaxies identified here will thus be needed to confirm their physical association and to map out the ionized regions produced by these sources