M. Jaouen

Carbon onions formation by high-dose carbon ion implantation into copper and silver

High-fluence (>1016 cm−2) ion implantations of 120-keV carbon ions have been performed at elevated temperature (≥400°C) into metals (Ag, Cu) in which carbon is almost immiscible. The different carbon structures so synthesized have been characterized by transmission electron microscopy and atomic force microscopy. It mainly results in the formation of numerous carbon onions composed of concentric graphitic layers. We will show that pure carbon onion layers can even be obtained by carbon ion implantation into a silver thin film deposited onto silica. Furthermore, we discuss the potential of the technique in adjusting the size and controlling the microstructure of the carbon onions by varying implantation parameters such as temperature and fluence. We also briefly discuss the nucleation and growth mechanisms of the carbon onions during the ion implantation process.

Carbon-onion thin-film synthesis onto silica substrates

Abstract A three-step technique devoted to the synthesis of pure carbon-onion thin films deposited onto silica substrates has been developed. Carbon onions have been synthesized into silver thin films deposited on silica substrates by using 120 keV carbon ion implantations at 500°C. After silver was removed by thermal annealing, pure carbon-onion thin films can be deposited onto silica substrates. The carbon onions thus obtained show an almost perfect spherical shape, their mean size varying with the implanted fluence. The possibility of recovering a carbon onion powder using this new procedure is discussed, thus opening the way to study some carbon-onion properties which are still unknown.

Experimental evidence of Cr magnetic moments at low temperature in Cr2A(A=Al, Ge)C.

From x-ray magnetic circular dichroism experiments performed at low temperature on Cr2AlC and Cr2GeC thin films, it is evidenced that Cr atoms carry a net magnetic moment in these ternary phases. It is shown that the Cr magnetization of the Al-based compound nearly vanished at 100 K in agreement with what has been recently observed on bulk. X-ray linear dichroism measurements performed at various angles of incidence and temperatures clearly demonstrate the existence of a charge ordering along the c axis of the structure of Cr2AlC. All these experimental observations support, in part, theoretical calculations claiming that Cr dd correlations have to be considered to correctly describe the structure and properties of these Cr-based ternary phases.

Nanostructure and magnetic properties of BN-encapsulated Fe(B) and Fe2N nanoparticles prepared by dual ion-beam sputtering

BN–Fe nanocomposite films have been produced by dual ion-beam sputtering codeposition. The effect of using a reactive nitrogen assistance beam during the elaboration process has been studied by transmission electron microscopy, selected area electron diffraction, grazing incidence small-angle x-ray scattering, and magnetic measurements. Experimental results show that without assistance, we produce nearly spherical Fe(B) nanoparticles (2–3 nm in diameter) exhibiting a superparamagnetic behavior. When growing under assistance, we obtain vertically elongated nanoparticles of paramagnetic ζ-Fe2N encapsulated in disordered hexagonal BN. This study demonstrates the possibility of producing encapsulated nanoparticles with high shape anisotropy by activating the surface diffusion of the incoming deposited species.

Steric effect on the M site of nanolaminate compounds M2SnC (M = Ti, Zr, Hf and Nb)

In this paper we report calculations based on an all electron ab initio full-potential linearized augmented plane-wave method using the generalized gradient approximation within the density functional theory to determine the structures of Ti(2)SnC, Zr(2)SnC, Hf(2)SnC and Nb(2)SnC. The lattice constants obtained after geometry optimization are in good agreement with experimental data. It is observed from these results that there exists a steric effect on the M site. For M atoms with atomic radius (Zr, Hf) larger than that of tin, the polyhedra (octahedron and trigonal prism) constituting the unit cell are less distorted compared to those related to M atoms with atomic radius (Ti, Nb) similar to that of tin. The computed values for the bulk modulus of these ternary carbides are predicted to be about 69% of those of the corresponding binary carbides MC. The analysis of the projected local density of states shows that the major hybrids come from M (M = Ti, Zr, Hf, Nb) d and C p states.

Thin film of spherical carbon onions onto silver

Abstract We have performed high dose carbon ion implantation into polycrystalline silver substrates at high temperature (500–600 °C). The formation of a high density of spherical carbon onions on the nearby silver substrate was characterized by atomic force microscopy experiments and transmission electron microscopy. The influence of the implantation temperature as well as that of silver grain sizes and crystallographic orientations are analysed.

Electron microscopy study of carbon onions synthesized by ion implantation

Abstract The structure of concentric-shell carbon onions, which were synthesized by carbon-ion implantation into copper and silver substrates, is investigated by carrying out transmission electron microscopy (TEM) characterizations. TEM observations at high specimen temperatures reveal that carbon onions with coherent shells and a highly symmetric structure can be obtained by ion implantation. The onions show a decreasing intershell spacing towards their centre. Contrast features in bright- and dark-field images of carbon onions are compared with high-resolution lattice images. A structure model is suggested on the basis of local faceting of the onions and stacking order between the shells. The relationship between carbon onions and fullerenes with defects such as pentagonal or heptagonal rings in the shells is discussed.

Grazing incidence small-angle x-ray scattering applied to the characterization of nanocomposite thin films

Grazing incidence small-angle x-ray scattering (GISAXS) allows to investigate precisely the microstructure of nanocomposite thin films containing metal nanocrystals produced using different synthesis techniques. We present results on the size, size distribution, shape, and correlation length of metallic nanoparticles embedded in different matrices fabricated by sequential pulsed laser deposition, magnetron sputtering, and ion-beam sputtering co-deposition. The morphology of the nanoparticles is discussed in terms of the different growth process that takes place in each case.

Bonding mechanisms in γ-TiAl: Comparison between absorption near edge fine structures and theoretical predictions

Abstract The near edge fine structures of the γ-TiAl alloy recorded either in XAS (K edges) or in EELS (L edges) have been compared with calculations based on band structure or full multiple scattering techniques. Good agreement between experiment and theory has been achieved for the Ti-K, Al-K and Al-L edges. The discrepancy observed between experiment and theory at the Ti-L23 edges is interpreted as the failure of the single particle approximation at these edges due to core hole and strong exchange interaction. Strong hybridization between the Ti and Al atoms is demonstrated and supported by analysing charge density maps obtained from the band structure calculations. The existence of Ti-d-Al-p, Ti-d-Al-s and Ti-d-Ti-d hybrids, and therefore covalent character in the bonding, are discussed in terms of the mechanical properties of the γ-TiAl alloy.

Carbon onions produced by ion-implantation

Carbon films were produced by high dose carbon ion-implantation into polycrystalline copper and silver substrates held at high temperature (>400 °C). The carbon layers so obtained were characterized by Transmission Electron Microscopy and Atomic Force Microscopy. Numerous carbon-onions (50–200 nm in diameter), randomly distributed into a graphitic film were obtained after the implantation into copper whereas a continuous film of carbon onions (10–30 nmin diameter) was observed onto the silver substrates. We discuss the growth mechanisms of the carbon clusters and propose that the onion formation is simply based on a carbon precipitation in the substrate volume. We propose that the here presented process is a powerful way to synthesize a high density of carbon onions, especially in silver substrates.