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Jean F.,CNRS Neel Institute | Zhou T.,CNRS Physics of Materials and Microstructures | Blanc N.,CNRS Neel Institute | Felici R.,European Synchrotron Radiation Facility | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parameter of the main, nonrotated phase displays a characteristic hysteresis whose lower and upper branches correspond to phases in which integer numbers (m,n)Ir of the Ir lattice parameters match integer numbers (p,q) Gr of the graphene lattice parameters, i.e., commensurate phases, (19,0)Ir×(21,0)Gr and (9,0)Ir×(10, 0)Gr, respectively. With a higher growth temperature, graphene presents, in addition to a main nonrotated, incommensurate phase, domains with a small rotation (2.37) relative to the substrate, corresponding to a (7,2) Ir×(8,2)Gr commensurate phase. Despite the weak interaction between graphene and iridium, the thermal expansion coefficient of graphene is positive at all temperatures, even at liquid helium temperature contrary to free-standing graphene. It is also close to the iridium thermal expansion because of the tendency to form commensurate phases, which creates a strong link between the graphene and substrate lattices and decreases the elastic energy. © 2013 American Physical Society.


Demichel O.,CNRS Physics of Materials and Microstructures | Calvo V.,CNRS Physics of Materials and Microstructures | Noe P.,CNRS Physics of Materials and Microstructures | Salem B.,CNRS Laboratory for Microelectronics Technolgy | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Nanowires are unique objects to explore quasi-one-dimensional electronic systems. In order to obtain quantum-confined excitons in silicon nanowires, we strongly oxidized such nanowires and obtained core-shell silicon-silicon dioxide structures. Low-temperature photoluminescence measurements were performed after each oxidation step to link the band-gap energy to the dimensions of the nanowires. Surprisingly, the electronic band gap first decreases prior to increasing at quantum dimensions (the silicon nanowires, once oxidized, are stressed by the silicon dioxide shell). The controlled nanowire oxidation allowed us to develop an empirical model to estimate the strain-induced gap energy shift. With this model, we extracted the quantum energy as a function of the diameters of smaller nanowires, and its diameter dependence is in agreement with tight-binding calculations. © 2011 American Physical Society.


Lopez-Haro M.,CNRS Physical Eletrochemistry Materials and Interfaces Lab | Lopez-Haro M.,CNRS Physics of Materials and Microstructures | Dubau L.,CNRS Physical Eletrochemistry Materials and Interfaces Lab | Castanheira L.,CNRS Physical Eletrochemistry Materials and Interfaces Lab | And 6 more authors.
ECS Transactions | Year: 2013

This study shows that the modifications in structure and composition of Pt3Co/C nanoparticles during real life PEMFC operation (3422 hours of operation at T = 70 °C under H2/air) is much richer than previously thought from accelerated stress tests. Using aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy, we show that Co atoms are continuously dissolved from the mother electrocatalyst, and that three different metal nanostructures emerge during PEMFC operation: (i) Pt-Co/C core-shell nanoparticles characterized by an alloyed core surrounded by a thick Pt-rich shell, (ii) Pt-Co/C "hollow" nanoparticles and (iii) pure Pt/C "hollow" nanoparticles, from which Co dissolution has been completed. The high surface area carbon support is not stable either, its less organized domains being preferentially corroded, a fast kinetics process that is followed by slower corrosion of its graphitic regions. © The Electrochemical Society.


Demichel O.,CNRS Physics of Materials and Microstructures | Calvo V.,CNRS Physics of Materials and Microstructures | Besson A.,CNRS Physics of Materials and Microstructures | Noe P.,CNRS Physics of Materials and Microstructures | And 5 more authors.
Nano Letters | Year: 2010

The past decade has seen the explosion of experimental results on nanowires grown by catalyzed mechanisms. However, few are known on their electronic properties especially the influence of surfaces and catalysts. We demonstrate by an optical method how a curious electron-hole thermodynamic phase can help to characterize volume and surface recombination rates of silicon nanowires (SiNWs). By studying the electron-hole liquid dynamics as a function of the spatial confinement, we directly measured these two key parameters. We measured a surface recombination velocity of passivated SiNWs of 20 cm s-1, 100 times lower than previous values reported. Furthermore, the volume recombination rate of gold-catalyzed SiNWs is found to be similar to that of a high-quality three-dimensional silicon crystal; the influence of the catalyst is negligible. These results advance the knowledge of SiNW surface passivation and provide essential guidance to the development of efficient nanowire-based devices. © 2010 American Chemical Society.

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