ICMCTF2010 Session H1-2: Nanoparticle, Nanowire, Nanotube, and Graphene Thin-Films and Coatings

Carbon based nanostructures, such as nanotubes or single-wall and onion-like fullerenes, have outstanding mechanical, thermal and electrical properties for which they attract a lot of theoretical as well as experimental attention. One of the key issues for determining the mechanical stability of complex systems such as onion-like carbons and fullerenes is detailed knowledge of the stresses originating from the curved surfaces inherent to these structures.

In this study we present ab initio calculations of the curvature energy of a variety of carbon-based nanostructures: graphene, zig-zag and armchair single-wall nanotubes and C₆₀, C₇₀, C₁₈₀, and C₂₄₀ fullerenes. It turns out that the curvature energy as a function of radius follows a power law in all cases. Moreover, when only single-wall nanotubes are considered, this law is independent on other properties such as the nanotube type (armchair or zig-zag) or electronic properties (conductor or semiconductor).

In order to strengthen the conclusions for fullerenes, we present two approximations of larger buckyballs (i.e. with more than 240 atoms) which are difficult to handle fully ab initio. The first approach uses parts of curved surfaces composed of hexagons having the outer bonds being passivated with H atoms. The second approach uses ab initio data to construct inter-atomic potentials which are subsequently used in Monte Carlo simulations. Both approaches give comparable predictions for the curvature energy power law exponent. Drawbacks and advantages (e.g. decomposing the total energy into stretching, bending and torsion contributions as done in Monte Carlo simulations gives the possibility to compare various contributions to the total surface energy) of both approaches will be discussed in detail.

The calculated properties of curved carbon surfaces may be readily used in large-scale continuum based models of complex structures.

Vertically aligned carbon nanotubes were synthesized using FeNi or Fe sputtered catalyst layers on glass substrates using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) or direct current PECVD (DC-PECVD). This paper will compare the growth mechanisms of CNTs synthesized by RF-PECVD and DC-PECVD, based on gas flow rate, plasma power, and catalysts. Tip growth CNTs were produced at 180^oC, 10 sccm CH₄, and 30 W by RF-PECVD using 8 or 4 nm FeNi or 4nm Fe island films sputtered onto glass substrates. CNTs could not be grown using DC-PECVD under the same deposition conditions (180^oC, 10 sccm CH₄ and 30 W). Tip growth of CNTs occurred at 180^oC, 15 sccm CH_4, and 50 W using DC-PECVD with sputtered FeNi island catalysts on increasing the plasma power and CH₄ flow rate.

RF-PECVD provided more efficient decomposition of gas molecules than DC-PECVD. The major difference between RF-PECVD and DC-PECVD is the higher concentration of reactive radicals in the former. However, in DC-PECVD, the CNT growth was well aligned vertically. FeNi thin film catalysts exhibited higher activity and better wetting ability than the Fe island thin film catalysts.

Nanostructured materials such as thin films, nanotubes, nanowires and nanorods are playing a crucial role in electronics, optoelectronics, magnetic and thermal devices. One-dimensional materials, due to their low dimensions of nm-sized magnitude, exhibit exceptional physical and chemical properties in comparison with their bulk counterparts. Transition metal oxides (TMOS) have received considerable attention both from the fundamental science and technological applications (solid lubricants, catalysis, magnetic media, etc.), point of view. In the current study, β-ΑgVO₃ (silver vanadate) nanorods were synthesized using a room temperature wet chemical approach (hydrothermal method). Structural and morphological characterization using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Raman spectroscopy was performed. In situ Differential Scanning Calorimetry (DSC)/Thermogravimetric Analysis (TGA) and High Temperature X-ray Diffraction (HT-XRD) studies were carried out between 25-1200^oC and 200-650^oC, respectively, to study the thermal properties and monitor the phase evolution as a function of temperature. The length of the β-ΑgVO₃ nanorods, based on the SEM studies was found to be 20-40 μm, whereas the width and the thickness were found to be between 300-600 nm and 50-100 nm, respectively. TEM studies revealed the existence of Ag nanoparticles at the surface of the nanorods. A strong correlation was found between data obtained from HT-XRD and tribological performance. The tribological performance of these materials deposited on H13 steel substrates was evaluated at room temperature and 600^oC in humid air against Si₃N₄ balls of a diameter of 6 mm. The room temperature coefficient of friction (CoF) was found to be 0.85, whereas this value drops to 0.2 at 600^oC tests due to crystallization of silver nanoparticles at high temperatures, acting as a solid lubricant.

We present a new method for the fabrication of nano electronic devices with horizontal silicon nanowires. A web of horizontally connected silicon nanowires is grown on a silicon substrate using thin aluminum film as a catalyst. A thin layer of oxide is thermally grown on a silicon substrate. The oxide layer is then etched at selective places using photolithography. A thin layer of aluminum is thermally evaporated on the substrate with the patterned oxide layer. When the sample is annealed above the eutectic temperature, we show that the silicon gets deposited along the grain boundaries of aluminum in the form of thin nano wires. We show that this phenomenon is due to the high solubility of silicon in aluminum at high temperatures.

The surface morphology was analyzed using Scanning Electron Microscopy (SEM). The compositional analysis was done using Energy Dispersive X-ray spectroscopy (EDX).

Cu₂O is a non-stoichiometric p-type semiconductor which exhibits wide band gap (2.1 eV) and high bonding energy (~150 meV). It has been proposed to be very attractive as a potential photocatalystic and photoelectric material under visible light irradiation. In this work, Cu nanowire arrays have been synthesized via porous alumina membrane (PAM) template with high aspect ratio, uniform pore size (120~140 nm) and ordered pore arrangement . The Cu₂O nanowire arrays are prepared from oxidization of Cu metal nanowire arrays. The electrochemical deposition potential of Cu metal nanowire (0.282V vs. SCE) is determined by cyclic voltammetry. The microstructure and chemical composition of Cu nanowire arrays have been characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and dispersive x-ray spectrometer (EDS). The photoluminescence spectra (PL) is employed to analyze the Cu₂O/PAM nanowire arrays luminescent properties. The results indicate that the Cu nanowire arrays are assembled into the nanochannel of porous alumina template with diameter of 120~140 nm. The valence of copper is controlled by porous alumina template during annealing process. Copper nanowires have been transformed to Cu₂O phase without the space limitation of PAM template, however Cu₂O phase nanowire arrays are obtained under the template embedded.

Vertically aligned ZnO nanorod films with well-defined areas were deposited onto the glass substrate by the ink-jet printing and hydrothermal processes, which allowed the control of the dimensions and positions of the nanorods. At first, the seed solution (zinc acetate in ethanol) was ink-jet printed on the interdigitated electrodes. Then, vertically aligned ZnO nanorods were grown on the patterned seed layer by the hydrothermal approach. The influences of seed-solution properties and the ink-jet printing parameters on the printing performance and the morphology of the nanorods were studied. Unlike screen printing or lithography, inkjet printing does not require stencils or masks, therefore allowing rapid design and prototyping. The effects of nanorods structure, nanorods size and dye sensitization on the gas-sensing capability of ZnO-nanorod gas sensors were demonstrated. Due to the high surface-to-volume ratios and high crystallinity of the nanorod-array structure, devices with promising sensor characteristics were obtained.