About the step-flow mechanism at the origin of graphene crystallisation at the surface of catalysts

The nucleation and growth of multiwall carbon nanotubes (MWCNTs) at the surface of crystalline iron-based catalysts are studied by in situ annealing and high-resolution transmission electron microscopy. Graphene planes, parallel to the catalyst surface, appear by a mechanism of step flow, where the atomic layers of catalyst are "replaced" by graphene layers. More interestingly, as the catalyst particles have curved or poly-faceted surfaces, those catalyst atomic layers correspond to no definite atomic plane. The step height may thus vary along a given step flow process. Step bunching due to impeded step migration, in certain growth conditions, yields characteristic catalyst nail-head shapes. Mastering this mechanism opens up the way to tailor the structure of MWCNTs, e.g. with highly parallel carbon walls

Graphene-based resistive humidity sensor for in-situ monitoring of drying shrinkage and intrinsic permeability in concrete

Nanosensors dedicated to the structural health monitoring of concrete structures have been only marginally studied. They would however be particularly well-suited to monitor durability-related processes, as these phenomena involve transport of gas and liquids through micro and nano-porosity. In this paper we discuss the relevance and feasibility of embedding rela-tive humidity nanosensors within concrete. It appears that the localized, continuous knowledge of relative humidity within a concrete structure could provide a useful insight into drying shrinkage; it could also contribute to improved intrinsic permeability measurements, leading to improved assessment of structural durability. For the task, we propose a low-cost, downscalable resistive device made of a 10 nm graphene sheet grown directly on glass and atop which are ink-jet printed silver electrodes. The device resistance increases significantly with relative humidity (RH), especially above 40% RH. Relative amplitude of variations are only of about 3% for the two tested devices, but absolute variations (80 Ohms/sq and 480 Ohms/sq) appear measurable by a low-cost and robust signal conditioning electronics. Thus, the idea of using our graphene-based resistive device for embedded humidity monitoring in concrete ap-pears quite promising

Growth mechanisms of carbon nanotrees with branched carbon nanofibers synthesized by plasma-enhanced chemical vapour deposition

International audienceY- and comb-type carbon nanotrees formed from branched carbon nanofibres grown by plasma-enhanced chemical vapour deposition were studied by transmission electron microscopy. Different growth mechanisms are proposed for the two types of nanotrees based on the observed and reconstituted dynamic transformations of the catalyst particles during synthesis. However, the splitting of the larger catalyst particles is required for both kinds of nanotrees, whatever the involved growth mechanism. The carbon nanotrees are well crystallized and connections of the branches are continuous, which may be interesting for future applications in nanoelectronic devices and also composite materials

Vertically oriented nickel nanorod/carbon nanofiber core/shell structures synthesized by plasma-enhanced chemical vapor deposition

International audiencePlasma-enhanced chemical vapor deposition, without a nickel-containing gaseous precur- sor, was used to synthesize continuous nickel (Ni) nanorods inside the hollow cavity of car- bon nanofibers (CNFs), thus forming vertically aligned Ni/CNF core/shell structures. Scanning and transmission electron microscopic images indicate that the elongated Ni nanorods originate from the catalyst particles at the tips of the CNFs and that their forma- tion is due to the effect of extrusion induced by the compressive force of the graphene lay- ers during growth. Different from previous work, each vertically-aligned core/shell structure reported is totally isolated from its neighbors. Continuous Ni nanorods are found to separate into smaller ones with increasing growth time, which was ascribed to (i) the limited amount of Ni available in the tip of the CNF, (ii) the polycrystalline nature of the Ni nanorods and (iii) the combined effects of the compressive stresses on the side of the Ni nanorods and of the tensile stress along their axis

Conjugated Polymer and Hybrid Polymer-Metal Single Nanowires: Correlated Characterization and Device Integration

This book describes nanowires fabrication and their potential applications, both as standing alone or complementing carbon nanotubes and polymers. Understanding the design and working principles of nanowires described here, requires a multidisciplinary background of physics, chemistry, materials science, electrical and optoelectronics engineering, bioengineering, etc. This book is organized in eighteen chapters. In the first chapters, some considerations concerning the preparation of metallic and semiconductor nanowires are presented. Then, combinations of nanowires and carbon nanotubes are described and their properties connected with possible applications. After that, some polymer nanowires single or complementing metallic nanowires are reported. A new family of nanowires, the photoferroelectric ones, is presented in connection with their possible applications in non-volatile memory devices. Finally, some applications of nanowires in Magnetic Resonance Imaging, photoluminescence, light sensing and field-effect transistors are described. The book offers new insights, solutions and ideas for the design of efficient nanowires and applications. While not pretending to be comprehensive, its wide coverage might be appropriate not only for researchers but also for experienced technical professionals

Developing low-cost graphene devices

In spite of numerous efforts for developing the applications of graphene, it remains difficult to put the remarkable physical properties of this material into devices. This is mainly due to the fact that large-area (industrial) graphene includes in its structure and on its surfaces a significant density of defects that make as many traps and scattering centres for charge carriers. The idea of the present work, contrary to diminishing the defect density, is to use the defects and the very large surface to volume ratio of that 2D material, to transform it into high sensitivity sensors. When defects are useful, low-temperature growth becomes the method that best satisfies both physical and financial demands. Here, we further decrease preparation costs by performing growth not only at low temperature directly on the final insulating substrate (glass), but also by printing the device contacts by ink-jet printing. Graphene layers actually develop at the interface between a metallic catalytic film and the insulating substrate during plasma-enhanced chemical vapour deposition (PE-CVD).1,2. Resistivity of the graphene foils was measured by the four-point methods using ink-jet printed electrods, and a resistivity as low as 820 ohms/sq were obtained. Moreover, the sensitivity of such graphene foils to water vapour was evaluated, with the prospect to use them in humidity sensors for civil engineering. In this presentation, we explain how graphite may precipitate at the interface in addition to the surface.2,3 Then we show examples of graphene obtained at temperatures in between 450 and 550°C, on glass (Fig.), fused silica, alumina and SiO2//Si. Transmission electron microscopy indicates that the structure is nanocrystalline. We finally show the humidity response of the fabricated device. Results seem to indicate that high-defect density, thin deposits are more sensitive to water vapour than thicker ones

Synthesis of conducting transparent few-layer graphene directly on glass at 450 °C

International audiencePost-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq-1. Finally the samples equipped with printed contacts appear to be efficient humidity sensors

Synthesis of few-layered graphene by ion implantation of carbon in nickel thin films

International audienceThe synthesis of few-layered graphene is performed by ion implantation of carbon species in thin nickel films, followed by high temperature annealing and quenching. Although ion implantation enables a precise control of the carbon content and of the uniformity of the in-plane carbon concentration in the Ni films before annealing, we observe thickness non-uniformities in the synthesized graphene layers after high temperature annealing. These non-uniformities are probably induced by the heterogeneous distribution/topography of the graphene nucleation sites on the Ni surface. Taken altogether, our results indicate that the number of graphene layers on top of Ni films is controlled by the nucleation process on the Ni surface rather than by the carbon content in the Ni film

Growth of individual carbon nanotubes on an array of TiN/Ni nanodots patterned by e-beam lithography and defined by dry etching for field emission application

In this paper, we demonstrate a new technique to realize TiN/Ni nanodots array on silicon substrate using e-beam lithography and dry etching techniques. After patterning the Ni nanodisk (7 nm thick, 150 nm in diameter) at perfectly controlled location, individual vertically aligned carbon nanotubes (VACNTs) were grown using plasma-enhanced chemical-vapor deposition (PECVD). In addition, a field emission cathode (1 mm diameter circular emission area) based on a hexagonal array (20μm spacing) of individual VACNTs delivered a high emission current of 4.23 mA for an applied electric field of 22.5V/μm

Cyclodipeptide synthases, a family of class-I aminoacyl-tRNA synthetase-like enzymes involved in non-ribosomal peptide synthesis

Cyclodipeptide synthases (CDPSs) belong to a newly defined family of enzymes that use aminoacyl-tRNAs (aa-tRNAs) as substrates to synthesize the two peptide bonds of various cyclodipeptides, which are the precursors of many natural products with noteworthy biological activities. Here, we describe the crystal structure of AlbC, a CDPS from Streptomyces noursei. The AlbC structure consists of a monomer containing a Rossmann-fold domain. Strikingly, it is highly similar to the catalytic domain of class-I aminoacyl-tRNA synthetases (aaRSs), especially class-Ic TyrRSs and TrpRSs. AlbC contains a deep pocket, highly conserved among CDPSs. Site-directed mutagenesis studies indicate that this pocket accommodates the aminoacyl moiety of the aa-tRNA substrate in a way similar to that used by TyrRSs to recognize their tyrosine substrates. These studies also suggest that the tRNA moiety of the aa-tRNA interacts with AlbC via at least one patch of basic residues, which is conserved among CDPSs but not present in class-Ic aaRSs. AlbC catalyses its two-substrate reaction via a ping-pong mechanism with a covalent intermediate in which l-Phe is shown to be transferred from Phe-tRNAPhe to an active serine. These findings provide insight into the molecular bases of the interactions between CDPSs and their aa-tRNAs substrates, and the catalytic mechanism used by CDPSs to achieve the non-ribosomal synthesis of cyclodipeptides