Showing papers by "Jannik C. Meyer 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.

The Structure of Suspended Graphene

Abstract: Submitted for the MAR07 Meeting of The American Physical Society The Structure of Suspended Graphene JANNIK MEYER, ANDRE GEIM, MIKHAIL KATSNELSON, KOSTYA NOVOSELOV, TIM BOOTH, SIEGMAR ROTH, University of Manchester — The recent discovery of graphene has sparked significant interest, which has so far been focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles. However, the structure of graphene is also puzzling. On one hand, graphene appears to be a strictly 2D material and exhibits such a high crystal quality that electrons can travel submicron distances without scattering. On the other hand, perfect 2D crystals cannot exist in the free state, according to both theory and experiment. This is often reconciled by the fact that all graphene structures studied so far were an integral part of larger 3D structures, either supported by a bulk substrate or embedded in a 3D matrix. We describe individual graphene sheets freely suspended on a microfabricated scaffold. These membranes are only one atom thick and still display a long-range crystalline order. However, our studies by transmission electron microscopy have revealed that suspended graphene sheets are not perfectly flat but 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 an ample scope for fundamental research and new technologies whereas the observed corrugations in the third dimension may shed light on subtle reasons behind the stability of 2D crystals. Andre Geim University of Manchester Date submitted: 01 Dec 2006 Electronic form version 1.4

E 33 and E 44 optical transitions in semiconducting single-walled carbon nanotubes: Electron diffraction and Raman experiments

Abstract: By combining, on the same freestanding single-walled carbon nanotubes, electron diffraction and Raman experiments, we were able to obtain the resonance energy of unambiguously $(n,m)$-identified single-walled carbon nanotubes. We focus on the analysis of the first optical transition of metallic tubes $({E}_{11}^{M})$ and the third and fourth transitions of semiconducting tubes (${E}_{33}^{S}$ and ${E}_{44}^{S}$, respectively) in comparison with calculated values using a nonorthogonal tight-binding approach. For semiconducting tubes, we find that the calculated energies ${E}_{33}^{S}$ and ${E}_{44}^{S}$ have to be corrected by non-diameter-dependent (rigid) shifts of about $0.43\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $0.44\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively, for tubes in the $1.4--2.4\text{\ensuremath{-}}\mathrm{nm}$-diameter range. For metallic tubes in the $1.2--1.7\text{\ensuremath{-}}\mathrm{nm}$-diameter range, we show that a rigid shift $(0.32\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ of the calculated transition energy also leads to a good estimation of ${E}_{11}^{M}$. The rather large and non-diameter-dependent shifts for the third and fourth transitions in semiconducting tubes question a recent theoretical study, which relates the shifts to electron-electron correlation and exciton binding energy and suggest that the exciton binding is very small or missing for the higher transitions ${E}_{33}^{S}$ and ${E}_{44}^{S}$, contrary to the lower transitions ${E}_{11}^{S}$ and ${E}_{22}^{S}$.

A study of the effect of different catalysts for the efficient CVD growth of carbon nanotubes on silicon substrates

Abstract: We report on the identification of efficient combinations of catalyst, carbon feedstock, and temperature for the ethanol chemical vapour deposition (CVD) growth of single-wall carbon nanotubes (SWCNTs) onto silicon substrates. Different catalyst preparations, based on organometallic salts (Co, Fe, Mo, Ni acetate, and bimetallic mixtures), have been spin coated onto thermally grown silicon dioxide on silicon chips to perform tests in a temperature range between 500 and 900 °C. The samples have been then characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Assuming the growth of high-quality isolated nanotubes as target, the ratio in Raman spectra between the intensity of the G peak and of the D peak has been used as the main parameter to evaluate the performance of the catalytic process. A comparison made for both single metals and bimetallic mixtures points out best conditions to achieve efficient CVD growth of SWCNTs.

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.

Synthesis of individual single-walled carbon nanotube bridges controlled by support micromachining

Abstract: Single-walled carbon nanotubes (SWNTs) were directly grown onto poly-crystalline silicon grids by catalytic thermal chemical vapour deposition. We demonstrate that simple micromachining of the catalyst-covered support can influence the number, location and alignment of suspended SWNTs. Sharp apexes formed by over-etching circular microstructures enable the scalable, cost-efficient formation of mostly individual, straight SWNT bridges, as verified by Raman scattering and electron diffraction.

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.

Raman spectroscopy of (n,m)‐identified individual single‐walled carbon nanotubes

Abstract: The goal of our "complete experimental" approach was to relate the Raman response of an individual single-walled carbon nanotubes (SWNT) to its (n,m) structure determined from an independent way. In this aim, a procedure including transmission electronic microscopy (TEM), Raman spectroscopy, and electron diffraction experiments on the same SWNT has been developed. The independent determinations of both structure and Raman features of semiconducting and metallic nanotubes allows to discuss several questions concerning: (i) the relation between diameter of the tubes and the radial breathing mode (RBM) frequency, (ii) the values and the nature of the E S 33 and E S 44 optical transition energies.