© Springer Science+Business Media Dordrecht 2016 / Reducing AgNO3 by glucose at basic pH coated the surface of silica spheres with a high density of hemispherical silver nanoparticles (average diameter 3.2±1 nm). A much lower silver concentration than is standard favored heterogeneous nucleation of silver on the silica surface at the expense of homogeneous nucleation in solution. The slow growth rate of the nuclei promoted the formation of discrete silver particles rather than a continuous shell. Based on scanning electron microscopy and transmission electron microscopy, the surface coverage of silver seed particles was as high as 25% at 10 °C without prior functionalization of the silica. The particles were composed of metallic silver based on x-ray photoelectron spectroscopy. There was a sharp increase in the silver surface coverage and decrease in the particle size when the temperature was raised from 5 °C to 10 °C and the amount of silica was decreased from 0.2 to 0.025 V/V. The size was controlled by the diffusion barrier through the ion shell surrounding the silica spheres and by maintaining reaction conditions where the particles on the surface compete for silver.

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The Electrochemical Quartz Crystal Microbalance (EQCM) was used as the main investigation tool coupled with other conventional electrochemical methods to study the electrocatalytic properties of polycrystalline Pt electrodes, including two separate projects. The first project studied the early stage of oxide film formation on the Pt surfaces and the inhibition of the catalytic properties by the oxide film. The inhibition of the fast electrode reaction of small molecules by the growth of oxide film allows those molecules to be used as probes for the nature of the oxide film. The hydrogen oxidation current, jox calculated by differencing the cyclic voltammetry currents with and without H₂ present showed a characteristic plateau-to-plateau profile, which implies a transition from the free Pt surface to the Pt surface completely covered by oxide film. This method allows determination of the onset potential for oxide formation and also the critical potential where a full monolayer of oxide is formed. This method applies to other fast surface reactions such as oxygen reduction reaction (ORR), and the results are enhanced by forced convection in the rotating disk electrode (RDE) experiments. The initial oxidation species was identified by charge and EQCM frequency analysis. Our results support the formation of a species with stoichiometry Pt₂O, for example, with an oxygen atom in the bridging position between two adjacent Pt atoms. In the second project, the stability of the Pt electrodes in acid media with Ag⁺ present was investigated. A substantial frequency drift (8.3 Hz cycle⁻¹, or 44 ng cm⁻² cycle⁻¹) was observed during Ag electrodeposition and stripping on the bare polycrystalline Pt surface. Cyclic voltammograms in pure HClO₄ solution showed nearly no frequency drift while the addition of 10⁻³ mol L⁻¹ Ag⁺ resulted in an immediate and characteristic frequency drift. The frequency drift appeared to be consistent with loss of material from the electrode surface and the ICP-MS detected a maximum Pt concentration of 2.3×10⁻⁶ mol L⁻¹ in solution due to Pt dissolution. The Pt concentration calculated from the EQCM frequency drift matched the ICP-MS results. This allowed the EQCM for direct investigation of Pt dissolution at different system temperatures, sweep rates, and potential ranges. The much higher rate of dissolution with Ag present than that in pure HClO₄ solution can be explained by the formation of Pt-Ag alloy during Ag underpotential deposition and the co-dissolution of Pt and Ag.

Ce travail est consacré à l’étude de l’incorporation volontaire des dopants dans des films de carbure de silicium épitaxiés par la technique de dépôt chimique en phase vapeur. Le rôle des principaux paramètres de croissance (température, flux de dopant, vitesse de dépôt, pression dans le réacteur et le rapport C/Si) sur l’incorporation d’azote et d’aluminium a été étudié en détail. Les travaux menés jusqu’ici ont largement exploré les caractéristiques de l’incorporation de dopants, en particulier l’incorporation d’azote et ont montré des résultats parfois très dépendants de l’équipement de croissance utilisé. Afin d’explorer cette influence, une étude expérimentale exhaustive sur l’incorporation de N et Al a été réalisée sur des couches homoépitaxiées 4H-SiC sur la face carbone et sur la face silicium de substrats 4H-SiC dans nos réacteurs CVD. Cette étude a été complétée par une analyse des propriétés structurales, optiques et électriques de couches 4H-SiC dopé Al. Aussi, la fabrication de diodes pn a été expérimentée sur les couches épitaxiées dans nos réacteurs. Nous avons pu observer différentes tendances expérimentales selon la nature du dopant, l’orientation cristalline du substrat et l’environnement chimique durant la croissance. Nous en déduisons que le mécanisme derrière les tendances observées est largement influencé par des facteurs comme les conditions de croissance (c'est-à-dire la température de croissance et/ou la pression) et la couverture de carbone à la surface de la croissance, surtout sur la face C / This work is dedicated to the investigation of intentional dopant incorporation in silicon carbide epilayers grown by chemical vapor deposition technique. The role of main process conditions (growth temperature, dopant supply, deposition rate, growth pressure and C/Si ratio) on both, Nitrogen and Aluminum incorporation was studied in details. Previous works have widely explored the characteristics of dopant incorporation, especially the nitrogen incorporation addressing a potential influence of growth equipment for the observed incorporation trends. An exhaustive experimental study of N and Al incorporation was performed for homoepitaxial 4H-SiC layers grown on Si- and C-faces of 4H-SiC substrates in our CVD setups to explore such influence. It was completed by the assessment of the structural, optical and electrical properties of the Al doped 4H-SiC films. Furthermore, the fabrication of pn diodes was tested on the grown layers. We have observed different experimental tendencies depending on dopant nature, crystal orientation and chemical environment. We conclude from these observations that the mechanism behind the experimentally obtained tendencies is widely influenced by factors such as process conditions (i.e. growth temperature and/or pressure) and the carbon coverage at the grown surface, especially on C-face