The relationship between the microstructure and the electrical behavior of nickel/carbon nanocomposite (nc-Ni/C) thin films is reported. A particular attention was accorded to the role of the chemical composition and the nature of the amorphous carbon matrix on the electrical behavior of the material. The nc-Ni/C thin films were synthesized using two different cold plasma processes both allowing to finely control the chemical composition, structure and morphology of the films. The first process combines magnetron sputtering of a nickel target and the deposition of hydrogenated carbon by plasma enhanced chemical vapor deposition using methane as a precursor. The second process consists in the co-sputtering of a nickel and a graphite target in pure argon plasma. For the two deposition processes, a similar increase in the electrical conductivity with the Ni content was observed and attributed to the percolation of the Ni nanograins through the carbon matrix. The percolation threshold was evaluated around 41 at.% of Ni for the two deposition processes. For lower Ni concentrations, the microstructural study indicated that the electrical conductivity is governed by the nature of the amorphous carbon phase which is found to be dependent on the deposition process as revealed by mu-Raman spectroscopy. (C) 2016 Elsevier Ltd. All rights reserved.

We report on a novel fabrication approach of metal nanowires with complex surface. Taking advantage of nodular growth triggered by the presence of surface defects created intentionally on the substrate as well as the high tilt angle between the magnetron source axis and the normal to the substrate, metal nanowires containing hillocks emerging out of the surface can be created. The approach is demonstrated for several metals and alloys including gold, copper, silver, gold–copper and gold–silver. We demonstrate that applying an electrochemical dealloying process to the gold–copper alloy nanowire arrays allows for transforming the hillocks into ring-like shaped nanopores. The resulting porous gold nanowires exhibit a very high roughness and high specific surface making of them a promising candidate for the development of SERS-based sensors.

Recent papers have demonstrated that the growth of carbon nanotubes (CNTs) by plasma enhanced chemical vapor deposition (PECVD) was possible using nanocomposite nickel/carbon (nc-Ni/C) thin films as catalysts. In this study, the growth of CNTs by PECVD in H2/C2H4 atmosphere was achieved using nc-Ni/C thin films deposited by a hybrid plasma process combining the sputtering of a nickel target and the deposition of hydrocarbon by PECVD using Ar/CH4 atmosphere. In order to identify the most favorable conditions to obtain dense CNTs arrays using nc-Ni/C thin films, the Ni content in the catalyst as well as the growth conditions of the CNTs were varied. Films containing 40, 55 and 65at.% of Ni were selected for this study. The growth temperature of the CNTs was varied between 500 and 700°C whereas the electrical power applied to the PECVD source was tuned from 30 to 50W. Scanning electron microscopy and Raman spectroscopy were employed to probe the morphology and the structure of the CNT's. Depending on the chemical composition of the nc-Ni/C thin films, different trends were observed. No CNTs were obtained neither for the highest nickel content (i.e. %Ni=65at.%) nor for the lowest growth temperature (i.e. 500°C). On the other hand, for temperatures exceeding 500°C, while a high power on the PECVD source (i.e. 50W) was found to be necessary to obtain CNTs in the case of films with a moderate Ni content (i.e. %Ni=55at.%), a lower power (i.e. 30W) was sufficient for the film with the lowest Ni content (i.e. %Ni=40at.%). This difference in behavior was attributed to the differences in microstructure of nc-Ni/C thin films which is directly related to their chemical composition.

The chalcogenide compound GaV4S8 is promising for applications as active materials in non-volatile memory applications. We report here a comprehensive study on the thin-film deposition of this compound using a stoichiometric GaV4S8 target and H2S/Ar reactive sputtering. We show that a fraction of 0.5% to 1% of H2S in the reactive plasma is sufficient to compensate the sulfur deficiency that appears in films deposited in pure Ar plasma. This reactive plasma method allows avoiding the addition of elemental sulfur during the annealing treatment required to obtain a crystallized and stoichiometric GaV4S8 layer. A simple Au/GaV4S8/Au structure presents a resistive switching behaviour well suited for non-volatile memory applications.

Titanium carbide/carbon nanocomposite hard coatings, which consist of TiC nanoparticles buried in amorphous carbon matrix, are prepared by a hybrid plasma process combining magnetron sputtering of a titanium target and plasma enhanced chemical vapor deposition using methane as a source for the growth of amorphous carbon. A particular attention was paid to the comparison of the surface and the bulk chemical compositions of the films determined by different analysis techniques: X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), wavelength dispersive X-ray spectroscopy (WDS) and energy dispersive X-ray spectroscopy (EDS). XPS and EDS show an underestimation in carbon concentration compared to RBS and WDS which provide similar values. This underestimation is more important in case of XPS and it is mainly attributed to the presence of oxygen contamination on the surface of the films strongly bonded to titanium. In case of EDS, the underestimation is related to the poor sensitivity of the used probe in regard to light elements such as carbon. Moreover, the structure of the coatings has been extensively probed by transmission electron microscopy, which has shown that the size of the TiC nanoparticles shrinks when increasing the carbon content within the films. We further demonstrate that this structural evolution has a direct impact on the hardness of the coatings which gets reduced when the grain size decreases.

Nanoporous carbon thin films are prepared by means of a selective etching process in nitric acid applied to nanocomposite copper/carbon (nc-Cu/C) thin films grown by magnetron co-sputtering process. Laying on the electrical percolation theory, we demonstrate that to achieve a full etching of the copper phase present within the nc-Cu/C films, the Cu nanoparticles must be percolated. We further show that by adjusting the initial copper content within the nc-Cu/C films (between 61 and 85at.%), the pore size can be tuned accurately between 2 and 11nm. Contrary to what one may expect, increasing the pore size from 2 to 11nm induces an increase in the electrical conductivity of the nanoporous films from 82 to 308Scm−1. This unexpected electrical behavior is attributed to the structural modification of the carbon skeleton forming the porous material during the etching process. We further show that the transparency of such nanoporous films can be also controlled by tuning the pore size. The fact that the films with the highest electrical conductivity show the lowest optical absorption coefficient makes such material a very promising candidate for transparent electrode applications. This low temperature (less than 100°C) synthesis approach will pave the way for the direct integration of conductive nanoporous carbon materials in thin film-based flexible electronic devices.

In this work we probe the structural properties of amorphous hydrogenated carbon thin films prepared by plasma-enhanced chemical vapor deposition in a low pressure inductively coupled plasma using X-ray reflectometry in order to study the effect of varying the ion energy on the density of these films. The ion energy is varied by varying the RF power used to bias the substrate. It is shown that a very low ion energy is already sufficient to obtain a dense diamond-like carbon (DLC) film, in contrast with other deposition techniques where much higher ion energies are required to obtain a dense DLC film. The results of this study are corroborated by Raman spectroscopy and ellipsometry measurements. The X-ray reflectometry data analysis is detailed in order to highlight some methodological problems encountered during the data fitting which could lead to an incorrect interpretation of the measured curves.

Nickel/carbon composite (Ni/C) thin films were used as catalyst supports for the growth of vertically aligned multiwalled carbon nanotubes (MWCNTs) at temperature as low as 420°C. Nickel nanoparticles embedded within the carbon matrix of Ni/C films have served as catalysts for the synthesis of nanotubes by PECVD using acetylene/ammonia plasma. Two different nickel contents (40at.% and 60at.%) in the films were used. Analysis indicated a diffusion of nickel atoms in the form of nanoparticles to the film surface upon annealing. This diffusion depends on both annealing temperature and nickel concentration in the films and affects the MWCNT growth at low temperature. The MWCNT synthesis was tested at growth temperature ranging between 335 and 520°C. The growth of MWCNTs at 420°C was only achieved by using Ni/C films with a high nickel content (60at.%). These MWCNTs did not present considerable loss in their growth rate and structural quality compared to MWCNTs grown on classical substrates (Ni catalysts deposited on TiN), at higher temperature (520–600°C). The results suggest that carbon saturation at the surface and subsurface of nickel catalysts of the Ni/C films is responsible for the improvement of MWCNT growth at low temperature.

An oligothiophene having a donor-acceptor-donor chromophore with hydrogen bonding groups is used as electron donor in planar heterojunction organic photovoltaic cells. We focus on the contact between the anode and the oligothiophene. Different anode buffer layers (ABLs) have been used, MoO3 and CuI, alone or coupled with MoO3. The thicknesses were 4 nm and 3 nm for MoO3 and CuI respectively. It is shown that the ABL improves the cells performances. The best results are achieved with the couple MoO3/CuI through an increase of the open circuit voltage and short circuit current. The optical absorption, the surface roughness and the organic film conductivity depend on the ABL. The conductivity of the oligothiophene film is one order of magnitude higher when the ABL is a CuI film. The influence of the ABL can be explained partly by the fact that it raises the anode work function. Nevertheless, the study of the structures ITO/ABL/oligothiophene shows that each ABL exhibits specific advantages and disadvantages. Therefore the couple MoO3/CuI allows summing up the advantages of both ABLs, MoO3 allows a very good band matching and avoids too high leakage current, while CuI allows achieving high Jsc thanks to its effect on the TTB conductivity.

Titanium/carbon nanocomposite hard coatings, which consist of TiC nanoparticles buried in an amorphous carbon matrix, have been prepared by a hybrid plasma process combining magnetron sputtering of a titanium target and plasma enhanced chemical vapor deposition using methane as a source for amorphous carbon. The chemical composition of the bulk has been determined by Rutherford backscattered spectroscopy, and wavelength dispersive X-ray spectroscopy. The surface composition has been evaluated by X-ray photoelectron spectroscopy before and after argon ion erosion. Bulk and surface analysis techniques did not provide the same results. The origin of these differences is discussed in details. Moreover, the structure of the coatings has been extensively probed by transmission electron microscopy, which has shown that when increasing the carbon content within the coating the size of the TiC nanoparticles shrinks. We further demonstrate that this structural evolution has a direct impact on the hardness of the coatings which gets reduced when the carbon content increases.

The growth of nickel/carbon nanocomposite thin films by a hybrid plasma process, which combines magnetron sputtering and plasma enhanced chemical vapor deposition, has been investigated. This study has shown that the films consist of nickel-rich nanostructures embedded in an amorphous carbon matrix. The size, the distribution, the density, and the shape of these nanostructures are directly dependent to the total carbon content within the films. At low carbon content (∼28 at. %), dense nanowire array perpendicularly oriented to the surface of the substrate can be fabricated. For an intermediate carbon concentration (∼35 at. %), the nickel phase was organized into elongated nanoparticles. These nanoparticles became spherical when reaching a higher carbon content (∼54 at. %). The extensive structural study allowed the representation of a structure zone diagram, as well as, the development of a scenario describing the growth mechanisms that take place during the deposition of such nanocomposite material.

We report on the synthesis and magnetic characterization of ultralong (1 cm) arrays of highly ordered coaxial nanowires with nickel cores and graphene stacking shells (also known as metal-filled carbon nanotubes). Carbon-containing nickel nanowires are first grown on a nanograted surface by magnetron sputtering. Then, a post-annealing treatment favors the metal-catalyzed crystallization of carbon into stacked graphene layers rolled around the nickel cores. The observed uniaxial magnetic anisotropy field oriented along the nanowire axis is an indication that the shape anisotropy dominates the dipolar coupling between the wires. We further show that the thermal treatment induces a decrease in the coercivity of the nanowire arrays. This reflects an enhancement of the quality of the nickel nanowires after annealing attributed to a decrease of the roughness of the nickel surface and to a reduction of the defect density. This new type of graphene–ferromagnetic-metal nanowire appears to be an interesting building block for spintronic applications.

Supporting Information for Highly ordered ultralong magnetic nanowires wrapped in stacked graphene layers Abdel-Aziz El Mel, Jean-Luc Duvail, Eric Gautron, Wei Xu, Chang-Hwan Choi, Benoit Angleraud, Agnès Granier and Pierre-Yves Tessier Address: Institut des Matériaux Jean Rouxel, IMN, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France, Telephone: +33 (0) 240 376 434, Fax: +33 (0) 240 373 959 and Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA

We report on an efficient strategy for the fabrication of an ultra-long suspended nanowire mesh suitable for nanodevice architectures on a polymer surface. First, nickel nanowires are synthesized directly on a template substrate by magnetron sputtering. Laser interference lithography followed by deep reactive ion etching is used to create the nanograted template substrate constituted of one-dimensional line pattern arrays of 240 nm in periodicity. Ordered alignment of ultra-long nanowires (∼180 nm in diameter) with high fidelity to the template pattern is observed by scanning electron microscopy. The transfer of the pre-defined parallel nanowire array from the template surface to a target polymer substrate for electrical characterization of the system is demonstrated. The electrical behaviour of the nanowire mesh, suspended between two electrodes, was found to be linear, stable, and reproducible. This result suggests that this nanofabrication process will open an efficient way to the design and construction of novel nanodevices.

Thiw work studies the effect of metal reactivity on the microstructure and the physical properties in metal/carbon nanocomposite coatings.

X-ray Photoelectron Spectroscopy (XPS) is commonly used to study the chemical composition of TiC/C nanocomposite films. Nevertheless, XPS remains a surface analysis technique and the obtained chemical information can be strongly affected by the surface oxidation and carbon contamination of the nanocomposite samples due to their exposure to air. Generally, an erosion stage is performed before XPS analysis using argon ion bombardment to remove the surface contamination. Since ion bombardment is likely to modify the surface chemical composition of the films, the question of whether XPS results are really representative of the bulk nanocomposite material can be addressed. Therefore, this study is devoted to the effect of ion bombardment on the surface chemical composition of nanocomposite films. TiCx and TiCxOy films were grown by a hybrid plasma process combining Physical Vapor Deposition and Plasma Enhanced Chemical Vapor Deposition. Then, the samples were transferred to the XPS system where an in situ study of the modification of the surface chemical composition under argon ion bombardment was performed. XPS results are compared to Energy Dispersive X-ray analysis.

Hierarchical carbon nanostructures based on ultra-long carbon nanofibers (CNF) decorated with carbon nanotubes (CNT) have been prepared using plasma processes. The nickel/carbon composite nanofibers, used as a support for the growth of CNT, were deposited on nanopatterned silicon substrate by a hybrid plasma process, combining magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD). Transmission electron microscopy revealed the presence of spherical nanoparticles randomly dispersed within the carbon nanofibers. The nickel nanoparticles have been used as a catalyst to initiate the growth of CNT by PECVD at 600°C. After the growth of CNT onto the ultra-long CNF, SEM imaging revealed the formation of hierarchical carbon nanostructures which consist of CNF sheathed with CNTs. Furthermore, we demonstrate that reducing the growth temperature of CNT to less than 500°C leads to the formation of carbon nanowalls on the CNF instead of CNT. This simple fabrication method allows an easy preparation of hierarchical carbon nanostructures over a large surface area, as well as a simple manipulation of such material in order to integrate it into nanodevices.

This paper focuses on the study of the microstructure of TiC/C composite coatings deposited by hybrid PVD/PECVD process.