New trends in femtosecond Pulsed Laser Deposition and femtosecond produced plasma diagnostics

International audienceThe availability of compact table top amplified femtosecond lasers and the technical simplicity of experimental design have opened the way to many recent and fast developments towards thin film elaboration by Pulsed Laser Deposition (PLD) with ultra short laser pulses, with the aim of producing materials of high quality previously unattainable or attainable only through more complex means. The first developments of PLD using femtosecond lasers were made on Diamond-Like Carbon thin films elaboration, with the attempt to reach high sp3 content. PLD with ultra short pulses was used recently to deposit several systems such as quasicrystals or oxides with a transfer of the target composition to the deposited films, even for compounds with complex stoechiometry. Femtosecond laser ablation from solid targets has shown its capability in producing nanoparticles of different materials, even in high vacuum conditions. Nanostructured films of doped Diamond-Like Carbon were obtained recently, opening the way to large applications towards functional materials. The characteristics of the plasma are a well-suited signature of the physics of laser-matter interaction and plasma plume creation and expansion. Recent studies on the control of the film growth and femtosecond PLD processes will be reported. Emphasis on actual capability of the existing sources to elaborate high quality materials will be questioned in terms of energy per pulse, time width, repetition rates but also in the need for further source development and beam shaping improvement

Theory of magnetic domains in uniaxial thin films

For uniaxial easy axis films, properties of magnetic domains are usually described within the Kittel model, which assumes that domain walls are much thinner than the domains. In this work we present a simple model that includes a proper description of the magnetostatic energy of domains and domain walls and also takes into account the interaction between both surfaces of the film. Our model describes the behavior of domain and wall widths as a function of film thickness, and is especially well suited for the strong stripe phase. We prove the existence of a critical value of magneto-crystalline anisotropy above which stripe domains exist for any film thickness and justify our model by comparison with exact results. The model is in good agreement with experimental data for hcp cobalt.Comment: 15 pages, 7 figure

Unconventional magnetism in all-carbon nanofoam

We report production of nanostructured carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp2 and sp3 bonded carbon atoms. The material is a novel form of carbon in which graphite-like sheets fill space at very low density due to strong hyperbolic curvature, as proposed for ?schwarzite?. The foam exhibits ferromagnetic-like behaviour up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.Comment: 14 pages, including 2 tables and 7 figs. Submitted to Phys Rev B 10 September 200

Graphene-based textured surface by pulsed laser deposition as a robust platform for surface enhanced Raman scattering applications

International audienceWe have developed a surface enhanced Raman scattering (SERS)-active substrate based on gold nanoparticles-decorated few-layer (fl) graphene grown by pulsed laser deposition. Diamond-Like Carbon film has been converted to fl-graphene after thermal annealing at low temperature. The formation of fl-graphene was confirmed by Raman spectroscopy, and surface morphology was highlighted by scanning electron microscopy. We found that textured fl-graphene film with nanoscale roughness was highly beneficial for SERS detection. Rhodamine 6G and p-aminothiophenol proposed as test molecules were detected with high sensitivity. The detection at low concentration of deltamethrin, an active molecule of a commercial pesticide was further demonstrated

Characterization of different DLC and DLN electrodes for biosensor design

International audienceDiamond-Like Carbon and Carbon-Like Nanocomposite electrodes, novel materials in the field of biosensors, made with different ratio of sp3/sp2 carbon hybridization or doped with elements such as Ni, Si and W, were characterized electrochemically by cyclic voltammetry and by amperometric measurements towards hydrogen peroxide. SiCAr1 and SiCNi5% were chosen as sensitive transducers for elaboration of amperometric glucose biosensors. Immobilization of glucose oxidase was carried out by cross-linking with glutareldehyde. Measurements were made at a fixed potential + 1.0 V in 40 mM phosphate buffer pH 7.4. SiCAr1 seems to be more sensitive for glucose (0.6875 µA/mM) then SiCNi5% (0.3654 µA/mM). Detections limits were respectively 20 µM and 30 µM. Michaelis-Menten constants for the two electrodes were found around 3 mM. 48% and 79% of the original response for 0.5 mM glucose remained respectively for both electrodes after 10 days

Router-level community structure of the Internet Autonomous Systems

The Internet is composed of routing devices connected between them and organized into independent administrative entities: the Autonomous Systems. The existence of different types of Autonomous Systems (like large connectivity providers, Internet Service Providers or universities) together with geographical and economical constraints, turns the Internet into a complex modular and hierarchical network. This organization is reflected in many properties of the Internet topology, like its high degree of clustering and its robustness. In this work, we study the modular structure of the Internet router-level graph in order to assess to what extent the Autonomous Systems satisfy some of the known notions of community structure. We show that the modular structure of the Internet is much richer than what can be captured by the current community detection methods, which are severely affected by resolution limits and by the heterogeneity of the Autonomous Systems. Here we overcome this issue by using a multiresolution detection algorithm combined with a small sample of nodes. We also discuss recent work on community structure in the light of our results

Electrochemical Boron-Doped Diamond Film Microcells Micromachined with Femtosecond Laser: Application to the Determination of Water Framework Directive Metals

Planar electrochemical microcells were micromachined in a microcrystalline boron-doped diamond (BDD) thin layer using a femtosecond laser (Photo 1). The electrochemical performances of the new laser-machined BDD microcell were assessed by differential pulse anodic stripping voltammetry (DPASV) determinations, at nM level, of the four heavy metal ions of the European Water Framework Directive (WFD): Cd(II), Ni(II), Pb(II), Hg(II). The results are compared with those of previously published BDD electrodes [1]. The calculated detection limits are 0.4 nM, 6.8 nM and 5.5 nm 2.3 nM, and the linearities go up to 35nM, 97nM, 48nM and 5nM for respectively Cd(II), Ni(II) Pb(II) and Hg(II). The detection limits meet with the environmental quality standard of the WFD for three of the four metals. It was shown that the four heavy metals could be detected simultaneously, in the concentration ratio usually measured in sewage or runoff waters

A Carbon Nanofilament-Bead Necklace

Carbon nanofilaments with carbon beads grown on their surfaces were successfully synthesized reproducibly by a floating catalyst CVD method. The nanofilaments hosting the pearl-like structures typically show an average diameter of about 60 nm, which mostly consists of low-ordered graphite layers. The beads with diameter range 150−450 nm are composed of hundreds of crumpled and random graphite layers. The mechanism for the formation of these beaded nanofilaments is ascribed to two nucleation processes of the pyrolytic carbon deposition, arising from a temperature gradient between different parts of the reaction chamber. Furthermore, the Raman scattering properties of the beaded nanofilaments have been measured, as well as their confocal Raman G-line images. The Raman spectra reveal that that the trunks of the nanofilaments have better graphitic properties than the beads, which is consistent with the HRTEM analysis. The beaded nanofilaments are expected to have high potential applications in composites, which should exhibit both particle- and fiber-reinforcing functions for the host matrixes