On the mechanisms of precipitation of graphene on nickel thin films

Growth on transition metal substrates is becoming a method of choice to prepare large-area graphene foils. In the case of nickel, where carbon has a significant solubility, such a growth process includes at least two elementary steps: (1) carbon dissolution into the metal, and (2) graphene precipitation at the surface. Here, we dissolve calibrated amounts of carbon in nickel films, using carbon ion implantation, and annealing at 725 \circ or 900 \circ. We then use transmission electron microscopy to analyse the precipitation process in detail: the latter appears to imply carbon diffusion over large distances and at least two distinct microscopic mechanisms

New thermodynamic data for CoTiO3, NiTiO3 and CoCO3 based on low-temperature calorimetric measurements

The low-temperature heat capacities of nickel titanate (NiTiO3), cobalt titanate (CoTiO3), and cobalt carbonate (CoCO3) were measured between 2 and 300 K, and thermochemical functions were derived from the results. Our new data show previously unknown low-temperature lambda-shaped heat capacity anomalies peaking at 37 K for CoTiO3 and 26 K for NiTiO3. From our data we calculate standard molar entropies (298.15 K) for NiTiO3 of 90.9 ± 0.7 J mol-1 K-1 and for CoTiO3 of 94.4 ± 0.8 J mol-1 K-1. For CoCO3, we find only a small broad heat capacity anomaly, peaking at about 31 K. From our data, we suggest a new standard entropy (298.15 K) for CoCO3 of 88.9 ± 0.7 J mol-1 K-1

AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL DEVICES

Les films de nitrure de silicium sont réalisés par deux procédés de PECVD différents. Dans le premier, ces films sont formés dans une décharge à courant continu dans un plasma d'argon-silane-azote alors que dans le second dispositif, une espèce active d'azote est créée sélectivement dans une post-décharge d'azote et réagit avec le silane dans une zone de réaction où est positionné le substrat. Dans les deux cas, les films réalisés sont sous-stoechiométriques en azote. Leurs caractéristiques : morphologie, structure et composition sont étudiées et comparées.Silicon nitride thin films are obtained using two different PECVD devices. In the first one, these thin films are created in a DC discharge in an argon-silane-nitrogen gas mixture whereas in the second, nitrogen active species are selectively produced in a flowing nitrogen post-discharge and then react with silane in a reaction region where is positioned the substrate. In both cases, the obtained films are substoichiometric in nitrogen. Their characteristics : morphology, structure and composition are studied and compared