Showing papers by "S. Roth published in 2007"

The structure of suspended graphene sheets

Abstract: Graphene — a recently isolated one-atom-thick layered form of graphite — is a hot topic in the materials science and condensed matter physics communities, where it is proving to be a popular model system for investigation. An experiment involving individual graphene sheets suspended over a microscale scaffold has allowed structure determination using transmission electron microscopy and diffraction, perhaps paving the way towards an answer to the question of why graphene can exist at all. The 'two-dimensional' sheets, it seems, are not flat, but wavy. The undulations are less pronounced in a two-layer system, and disappear in multilayer samples. Learning more about this 'waviness' may reveal what makes these extremely thin carbon membranes so stable. Investigations of individual graphene sheets freely suspended on a microfabricated scaffold in vacuum or in air reveal that the membranes are not perfectly flat, but exhibit an intrinsic waviness, such that the surface normal varies by several degrees, and out-of-plane deformations reach 1 nm. The recent discovery of graphene has sparked much interest, thus far focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles1,2,3. However, the physical structure of graphene—a single layer of carbon atoms densely packed in a honeycomb crystal lattice—is also puzzling. On the one hand, graphene appears to be a strictly two-dimensional material, exhibiting such a high crystal quality that electrons can travel submicrometre distances without scattering. On the other hand, perfect two-dimensional crystals cannot exist in the free state, according to both theory and experiment4,5,6,7,8,9. This incompatibility can be avoided by arguing that all the graphene structures studied so far were an integral part of larger three-dimensional structures, either supported by a bulk substrate or embedded in a three-dimensional matrix1,2,3,9,10,11,12. Here we report on individual graphene sheets freely suspended on a microfabricated scaffold in vacuum or air. These membranes are only one atom thick, yet they still display long-range crystalline order. However, our studies by transmission electron microscopy also reveal that these suspended graphene sheets are not perfectly flat: they exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm. The atomically thin single-crystal membranes offer ample scope for fundamental research and new technologies, whereas the observed corrugations in the third dimension may provide subtle reasons for the stability of two-dimensional crystals13,14,15.

Influence of catalysts' activation on their activity and selectivity in carbon nanotubes synthesis

Abstract: In this paper we have investigated the influence of catalysts' activation conditions on their selectivity in carbon nanotubes CVD synthesis by means of methane decomposition reaction using Fe- and FeCo-catalysts synthesized via polymerized complex rout providing homogeneous distribution of catalytic components. It was shown that variation of catalysts' reduction conditions results in formation of different types of NTs (MWNTs or SWNTs). The most effective catalyst activation leading to formation of SWNTs consists in catalyst reduction by reaction mixture at high temperature. That can be explained in terms of carbon deposits nucleation on metals.

Transport current improvements of in situ MgB2 tapes by the addition of carbon nanotubes, silicon carbide or graphite

Abstract: Various types of carbon nanotube (CNT), as well as SiC and graphite powders, were used at the 5 wt% level as additions to a mixture of commercial Mg and B powder for the fabrication of single-core, in situ tapes using two-axial rolling deformation in an Nb/Fe sheath and final heat treatment at 650 °C/0.5 h in Ar. Transport current measurements showed that well distributed CNT, SiC and graphite additions lead to an improvement of Jc(μ0H) characteristics. The presence of carbon-containing particles causes substitution of boron by carbon, which decreases the critical temperature and increases the upper critical field as well as the current density in high magnetic fields. The uniform distribution of CNTs or other carbon-containing particles is an important factor for effective carbon substitution. This observation may be important for the development of practical MgB2 composite superconducting wires intended for magnets.

Investigation of the shift of Raman modes of graphene flakes

Abstract: In the present work, we use Raman spectroscopy as sensitive tool for the characterization of graphene samples. We observed diverse shifts in the position of the Raman mode close to 2650 cm -1 in various as-prepared graphene flakes. In order to elucidate the reason for this variation, we checked different substrates (Si/SiO 2 and Si/Al 2 O 3 ) and the effect of the annealing of graphene in argon. We find that most of as-prepared graphene flakes were non-intentional doped by holes, i.e. by physisorbed water and/or oxygen.

Thin transparent carbon nanotube networks: effects of ion irradiation

Abstract: We investigate the effect of irradiation by Ar + and N + ions on the properties of thin transparent networks of single-wall carbon nanotubes (SWNTs). The irradiation produces a small reduction in the infra-red transmittance of the networks at a wavelength of 2000 nm that correlates with the much larger reduction in conductance, and the peaks in the infra-red-visible absorbance spectrum are strongly reduced. Ion irradiation greatly increases the localization of the charge carriers, suppressing the conductance at low temperatures more severely than at room temperature. The conductance follows variable-range hopping behaviour for all samples. Nonlinearity in the current-voltage characteristics of the thin SWNT networks is observed only at very low temperatures.

Single-walled carbon nanotube silica composites obtained by an inorganic sol–gel route

Abstract: In the present work we first report on a successful incorporation of single-walled carbon nanotubes (SWCNTs) into silica matrix prepared using an inorganic sol-gel method. Through this route nonaqueous solvents are avoided and the stability of the carbon nanotubes suspensions is not affected. SWCNTs produced by Catalytic Chemical Vapor Deposition (CCVD) were dispersed in deionized water using an amphilic surfactant. As a precursor for the silica matrix an inexpensive silicic acid was used. By this route SWCNTs/silica composites were produced in the form of films and pellets. Microhardness measurements and electron microscopy suggest an important interaction between SWCNTs and the silica matrix what is important from the application point of view.

HRTEM and EELS investigation of functionalized carbon nanotubes

Abstract: We have successfully resolved and visualized the structure of some chemically functionalized carbon nanotubes (CNTs) using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). To represent the chemically modified CNT, we selected three systems. The first system is oxidized and surface thiolated MWCNT, the second system is Dy 3 N @ C 80 peapods prepared by depositing trimetal nitride fullerenes into SWCNT. The formed structure is the Dy 3 N @ C 80 @SWCNT. The third system is the conventional C 60 @SWCNT fullerene peapods, fluorinated by xenon difluoride (XeF2) up to 18% of F. We achieved detection of very low amount (0.6%) of sulfur and proved covalent bonding onto MWCNT surface. We present EELS imaging of the separated metal clusters inside endohedral metallofullerene peapod bundles and in the fluorinated C 60 peapods we show homogeneous fluorination across the whole surface.

Raman spectroelectrochemistry on SWNTs at higher doping levels: Evidence for a transition to intercalative doping

Abstract: We studied the transition from the electrochemical double-layer charging regime to intercalative doping of SWNT buckypaper in KCl aqueous solution. For this purpose we used doping levels by applying constant potentials above 1 V approaching and slightly exceeding the oxidation potential for Cl - ions. At each potential in situ Raman measurements of the radial breathing mode (RBM), the high-energy tangential mode (HEM) and the disorder-induced (D) mode were performed. From a comparative analysis of the Raman spectra we conclude that above 1 V a significant penetration of chlorine species into the interstitial channels of the SWNT bundles and possibly functionalization take place.

Catalytic chemical vapour deposition growth of single wall carbon nanotube films on different substrates for transparent electronic devices

Abstract: In this work we present our results concerning the growth of transparent carbon nanotube networks. The networks have been successfully grown on both oxidized silicon and quartz-glass substrates. Conductive and optical properties of these transparent films are discussed and compared to those obtained with other approaches.