Cecilia Goyenola

Mechanical properties of graphene nanoribbons

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons in the presence of stress by applying density functional theory within the GGA-PBE (generalized gradient approximation-Perdew-Burke-Ernzerhof) approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of ± 0.02%. The mechanical properties show a linear response within that range while a nonlinear dependence is found for higher strain. The most relevant results indicate that Youngs modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardens the nanostructure. The features of the electronic structure are not sensitive to strain in this linear elastic regime, suggesting the potential for using carbon nanostructures in nano-electronic devices in the near future.

SiNx Coatings Deposited by Reactive High Power Impulse Magnetron Sputtering: Process Parameters Influencing the Nitrogen Content

Reactive high power impulse magnetron sputtering (rHiPIMS) was used to deposit silicon nitride (SiNx) coatings for biomedical applications. The SiNx growth and plasma characterization were conducted in an industrial coater, using Si targets and N2 as reactive gas. The effects of different N2-to-Ar flow ratios between 0 and 0.3, pulse frequencies, target power settings, and substrate temperatures on the discharge and the N content of SiNx coatings were investigated. Plasma ion mass spectrometry shows high amounts of ionized isotopes during the initial part of the pulse for discharges with low N2-to-Ar flow ratios of <0.16, while signals from ionized molecules rise with the N2-to-Ar flow ratio at the pulse end and during pulse-off times. Langmuir probe measurements show electron temperatures of 2-3 eV for nonreactive discharges and 5.0-6.6 eV for discharges in transition mode. The SiNx coatings were characterized with respect to their composition, chemical bond structure, density, and mechanical properties by X-ray photoelectron spectroscopy, X-ray reflectivity, X-ray diffraction, and nanoindentation, respectively. The SiNx deposition processes and coating properties are mainly influenced by the N2-to-Ar flow ratio and thus by the N content in the SiNx films and to a lower extent by the HiPIMS frequencies and power settings as well as substrate temperatures. Increasing N2-to-Ar flow ratios lead to decreasing growth rates, while the N content, coating densities, residual stresses, and the hardness increase. These experimental findings were corroborated by density functional theory calculations of precursor species present during rHiPIMS.

Tetra­kis[μ-2-(3-phenoxy­phen­yl)propionato-κ2O:O′]bis­[(dimethyl­formamide-κO)copper(II)]

The title compound, [Cu2(C15H13O3)4(C3H7NO)2], is formed by the chelate coordination of four racemic fenoprofenate (fenoprofenate is 2,3-phenoxyphenyl propionate) anions and two dimethylformamide molecules to two copper(II) ions, building a paddle-wheel dinuclear molecule. The distorted square-pyramidal coordination of each CuII atom is made up of four O atoms of the four fenoprofenate units and another O atom from a dimethylformamide molecule. The two enantiomeric forms of the fenoprofenate anions are present in the complex, in an optically inactive centrosymmetric arrangement.

Mechanical and Electronic Properties of Graphene Nanostructures

Quite recently a new carbon nanostructure, called graphite nanoribbon (GNR), has emerged, taking the attention of the scientific community because of its promising use in spintronics. It is manly attributed to the work of Son et al. (Son et al. 2006 a; Son et al. 2006 b), who predicted that in-plane electric field, perpendicular to the periodic axis, induces a half-metal state in zigzag nanoribbons (ZGNR). This state corresponds to a one spin flavour with metallic behaviour, while the opposite spin flavour experiences an increase in the energy gap. Apart from the interesting dependence of the electronic structure upon an electric field, this is a promising material for future spintronic devices, since it could work as a perfect spin filter. Very recently Campos-Delgado et al. (Campos-Delgado et al. 2008) reported a chemical vapour deposition route (CVD) for the bulk production of long, thin, and highly crystalline graphene ribbons (less than 20-30 μm in length), with widths from 20 to 300 nm and small thicknesses (2 to 40 layers). In addition, the bottom up synthesis of these nanostructures may be feasible as noted by Hoheisel and collaborators (Hoheisel et al. 2010). This experimental advance further increases the expectations for the use of these materials in high-tech devices. In parallel there is an increased interest in the physical properties of carbon nanostructures in general, due to their outstanding mechanical and electronic properties. Recently, Lee et al. (Lee et al. 2008) measured the mechanical properties of a single graphene layer, demonstrating that graphene is the hardest material known, since the elastic modulus reaches a value of 1.0 TPa. Besides, many efforts have been dedicated to study the electronic properties of graphene, because creating a gap could allow the use of graphene in field effect transistors. Many mechanisms have been proposed with that purpose: nano-pattering, creating quantum dots, using multilayer, doping with heteroatoms such as sulphur (Denis et al. 2009), covalent functionalization (Bekyarova et al. 2009) and applying mechanical stress (Pereira et al. 2009; Gui et al. 2008). Recently Gui (Gui et al. 2008) proposed that graphene under a symmetrical strain distribution always leads to a zero band-gap semiconductor, and the pseudogap decreases linearly with the strain strength in the elastic linear regime. However, asymmetrical strain induces an opening of

Synthesis and properties of CS x F y thin films deposited by reactive magnetron sputtering in an Ar/SF6 discharge

A theoretical and experimental study on the growth and properties of a ternary carbon-based material, CS x F y , synthesized from SF6 and C as primary precursors is reported. The synthetic growth concept was applied to model the possible species resulting from the fragmentation of SF6 molecules and the recombination of S-F fragments with atomic C. The possible species were further evaluated for their contribution to the film growth. Corresponding solid CS x F y thin films were deposited by reactive direct current magnetron sputtering from a C target in a mixed Ar/SF6 discharge with different SF6 partial pressures ([Formula: see text]). Properties of the films were determined by x-ray photoelectron spectroscopy, x-ray reflectivity, and nanoindentation. A reduced mass density in the CS x F y films is predicted due to incorporation of precursor species with a more pronounced steric effect, which also agrees with the low density values observed for the films. Increased [Formula: see text] leads to decreasing deposition rate and increasing density, as explained by enhanced fluorination and etching on the deposited surface by a larger concentration of F/F2 species during the growth, as supported by an increment of the F relative content in the films. Mechanical properties indicating superelasticity were obtained from the film with lowest F content, implying a fullerene-like structure in CS x F y compounds.