Showing papers on "Graphene published in 1999"

Aharonov–Bohm oscillations in carbon nanotubes

Abstract: When electrons pass through a cylindrical electrical conductor aligned in a magnetic field, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic flux1. This phenomenon reflects the dependence of the phase of the electron wave on the magnetic field, known as the Aharonov–Bohm effect2, which causes a phase difference, and hence interference, between partial waves encircling the conductor in opposite directions. Such oscillations have been observed in micrometre-sized thin-walled metallic cylinders3,4,5 and lithographically fabricated rings6,7,8. Carbon nanotubes9,10 are composed of individual graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 20 nm. They are able to act as conducting molecular wires11,12,13,14,15,16,17,18, making them ideally suited for the investigation of quantum interference at the single-molecule level caused by the Aharonov–Bohm effect. Here we report magnetoresistance measurements on individual multi-walled nanotubes, which display pronounced resistance oscillations as a function of magnetic flux.We find that the oscillations are in good agreement with theoretical predictions for the Aharonov–Bohm effect in a hollow conductor with a diameter equal to that of the outermost shell of the nanotubes. In some nanotubes we also observe shorter-period oscillations, which might result from anisotropic electron currents caused by defects in the nanotube lattice.

Linear specific heat of carbon nanotubes

Abstract: The specific heat and thermal conductivity of millimeter-long aligned carbon multiwall nanotubes (MWNT's) have been measured. As a rolled-up version of graphene sheets, a MWNT of a few tens nm diameter is found to demonstrate a strikingly linear temperature-dependent specific heat over the entire temperature range measured (10--300 K). The results indicate that interwall coupling in MWNT's is rather weak compared with its parent form, graphite, so that one can treat a MWNT as a few decoupled two-dimensional single wall tubules. The thermal conductivity is found to be low, indicating the existence of substantial amounts of defects in the MWNT's prepared by a chemical-vapor-deposition method.

Tailoring graphite with the goal of achieving single sheets

Abstract: We demonstrate the tailoring of highly oriented pyrolytic graphite (HOPG) to obtain uniformly sized islands of up to several microns in size. There has already been some research on manipulating individual sheets on HOPG surfaces with scanning probe microscope tips; such sheets were obtained either accidentally or with a less controllable indenting technique. Here we present a different approach, which is more reliable and controllable. The HOPG surface was first patterned to create an array of small graphite islands by reactive ion etching of the HOPG surface with an oxygen plasma. These islands were then manipulated with an atomic force microscope tip. Carbon nanotubes represent a promising material for nanotechnology and can be considered as a graphene sheet rolled into a seamless cylinder. While carbon nanotubes are synthesized successfully with laser ablation, carbon arc, or chemical vapour deposition techniques, we speculate that it might be possible, by the controlled fabrication of graphene sheets, to form nanotubes or other novel motifs of use for nanotechnology.

Interference and Interaction in multi-wall carbon nanotubes

Abstract: We report equilibrium electric resistance R and tunneling spectroscopy (dI/dV)measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence (lφ)and elastic-scattering lengths (le)are deduced. Because le is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈1–10 Kfor all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

Further Studies of the Interaction of Hydrogen with Graphite Nanofibers

Abstract: Catalytically grown graphite nanofibers (GNF) are molecularly engineered structures that are produced by the interaction of carbon-containing gases with small metal particles at temperatures around 600 °C. The fibrous solids consist of minuscule graphene sheets stacked at various angles with respect to the fiber axis. This arrangement generates a material possessing unique chemical properties because unlike conventional graphite crystals, only edges are exposed. Such a conformation produces a material composed entirely of nanopores that can accommodate small-sized adsorbate molecules, such as hydrogen, in the most efficient manner. In addition, the nonrigid pore walls can expand to accommodate the gas in a multilayer conformation. GNF exhibit extraordinary behavior toward the sorption and retention of hydrogen at high pressures and abnormally high temperatures. In this paper we discuss some of the critical factors involved in the adsorption of molecular hydrogen and the influence that this process exerts ...

Interference and Interaction in Multiwall Carbon Nanotubes

Abstract: We report equilibrium electric resistance R and tunneling spectroscopy dI/dV measurements obtained on single multiwall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence and elastic-scattering lengths are deduced. Because the latter is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density-of-states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at approx. 1-10 K for all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of order 10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-Liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-Liquid as well as Luttinger-Liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

The Effects of Finite Length on the Electronic Structure of Carbon Nanotubes

Abstract: The electronic structure of finite-length armchair carbon nanotubes has been studied using several ab-initio and semiempirical quantum computational techniques. The additional confinement of the electrons along the tube axis leads to the opening of a band-gap in short armchair tubes. The value of the band-gap decreases with increasing tube length; however, the decrease is not monotonic but shows a well-defined oscillation in short tubes. This oscillation can be explained in terms of periodic changes in the bonding characteristics of the HOMO and LUMO orbitals of the tubes. Finite-size graphene sheets are also found to have a finite band-gap, but no clear oscillation is observed. As the length of the tube increases the density of states (DOS) spectrum evolves from that characteristic of a zero-dimensional (0-D) system to that characteristic of a delocalized one-dimensional (1-D) system. This transformation appears to be complete already for tubes 10 nm long. The chemical stability of the nanotubes, express...

Spectral properties of carbon black

Abstract: The internal structure of carbon black particles considerably influences the optical behavior of the material, apart from the shape and agglomeration state of the primary particles. In this paper the correlation between internal structure and spectral behavior of carbon black is investigated experimentally. The carbon blacks were produced by resistive heating of graphite electrodes and condensation in a cooling gas atmosphere. The internal structure of the primary carbon black particles was investigated by high-resolution transmission electron microscopy, electron energy loss spectroscopy, 13 C NMR spectroscopy, and Raman spectroscopy. The primary particles were found to consist of bent or plane structural subunits. The UV π−π* absorption feature of the produced carbon blacks varies in position between 196 and 265 nm depending on the state of bending of the graphene layers in the subunits of particles and/or the dimensions of the plane graphitic microcrystallites and the incorporation of hydrogen. The different curvature radii of the graphene layers or the sizes of microcrystallites can be summarized by an integral dimension like the ratio of sp2/sp3 hybridized carbon atoms. In the mid-infrared spectral region, the absolute value of the absorption coefficient κ is dominated by a continuous absorption due to free charge carriers which are also influenced by the ratio of sp2/sp3 hybridized carbon in the primary particles. The appearance of prominent bands is related to the existence of functional groups, like C–Hn, CO and/or C–O–C.

Carbon films with an sp 2 network structure

Abstract: Laser-arc evaporation of a graphite target has been used to deposit carbon films that exhibit high hardness (45 GPa) and elastic recovery (85%). High-resolution electron microscopy (HREM) and electron energy loss spectroscopy (EELS) were subsequently used to study the microstructure and bonding of the resultant layers. The structure of the films from HREM is seen to consist of a dense array of parallel curved graphene sheet segments packed in various orientations. EELS reveals that the films are comprised of mainly ${\mathrm{sp}}^{2}$-bonded carbon. The results suggest that a form of carbon thin film with fullerenelike structure can be realized. In order to explain how a predominantly ${\mathrm{sp}}^{2}$-bonded material can exhibit such a high hardness, a simple model is proposed to correlate the excellent mechanical properties with the observed structure.

Raman spectroscopy on single- and multi-walled nanotubes under high pressure

Abstract: The pressure dependence of the high-energy Raman modes in single- and multi-walled carbon nanotubes was measured in the range 0–10 GPa. We found the pressure coefficient to be linear in both materials but 25% smaller in MWNT. Given that the curvature effects on vibrational properties of the rolled-up graphene sheets are small, we can explain this difference simply with elasticity theory.

Raman spectroscopy on single- and multi-walled nanotubes under high pressure : Carbon nanotubes

Abstract: The pressure dependence of the high-energy Raman modes in single- and multi-walled carbon nanotubes was measured in the range 0-10 GPa. We found the pressure coefficient to be linear in both materials but 25% smaller in MWNT. Given that the curvature effects on vibrational properties of the rolled-up graphene sheets are small, we can explain this difference simply with elasticity theory.

Contributions to the work function: A density-functional study of adsorbates at graphene ribbon edges

Abstract: In the present computational study, we focus on graphene ribbons with zigzag edge atoms with their unsaturated bonds either dangling or terminated by various adsorbates ~H, O, or Cs!. Using this system as a test case, we discuss the two important contributions to the work function—the first one being an anisotropic bulk property related to the electron affinity of the material, and the second one being directly related to the surface dipole moment caused by the spill over of electronic charge into the vacuum. The latter contribution, which tends to increase the work function, can to a large extent be minimized by a judicious choice of adsorbates ~typically, adsorbates that are more electropositive than the surface !. The former face-dependent contribution turns out to be the minimum possible work function achievable for a given surface. Our calculations are based on density-functional theory within the local density approximation using nonlocal pseudopotentials and a plane wave basis set. @S0163-1829~99!03632-2# tion of species at a surface alters the work function of the surface in an understandable manner, with adsorbates having higher electronegativities than the surface increasing the work function while those with lower electronegativities having the opposite effect. 5-7 In the present study, we quantify the above anticipated trends for the test case of graphene ribbon edges with various adsorbates. We also address an appealing—although not unanticipated—correlation between the edge dipole moment and the work function perpendicular to the edge ~and along the ribbon plane!. This correlation points to an interesting way of viewing the work function, viz., by partitioning it into an anisotropic ~face-dependent! bulk cohesive ~electron af- finitylike! part and a part entirely due to the surface or edge dipole moment. The latter, which is a positive contribution to the work function, is due to the spill over of the electron gas into the vacuum region, and can to a large extent be reduced by a judicious choice of adsorbates. In fact, this analysis indicates that there is a minimum possible work function associated with a particular surface or edge that can be at- tained when the surface or edge dipole moment can be made to vanish. Our choice of an allotrope of carbon as a test case in the present study is motivated by the fact that recent attention has focused on carbon-based materials due to their promise as potential candidates for cold-cathode field emission applications. 8 Among the allotropes of carbon, nanotubes seem to be active field emitters, although other forms—like fragments of graphene and diamond-like carbon—are also known to be active. 9 One possible reason the nanotubes are active may be because they display special electronic states localized at the tip atoms. 10,11 It has also been pointed out that the nanotubes may be quite defective, with one such defect being similar to a graphene edge. 12,13 While the car- bon atoms in a cylindrical defect-free nanotube are all three- fold coordinated ~just as in graphite!, those in defected tu- bules may be two-fold coordinated, with an entirely different p-electron network in its vicinity ~as in fragments of graphene!. A recent tight-binding study of graphene ribbons 14 with the edge carbon atoms passivated with H has demonstrated that graphene ribbons with zigzag edges dis- play special localized states near the Fermi level arising pri- marily due to the topology of the p electron networks at the edges. Here, we use density-functional methods and consider zigzag graphene edges with unsaturated bonds either dan- gling or passivated with H, O, or Cs, and assess the impor- tance of such terminations on the edge dipole moment and the work function perpendicular to the edge—quantities which are key to the electron emission properties of these confined systems. In the next section, we give details of the method and models used in this study. We comment about the specifics of the work function and dipole moment calculations and formally relate the former to the latter in Sec. III. In Sec. IV, we present electronic and geometric structure results, calcu- lated work functions and dipole moments for the unpassi- vated and H-, O-, and Cs-terminated zigzag ribbons. We fi- nally conclude with Sec. V.

In situ afm study of interlayer spacing during anion intercalation into hopg in aqueous electrolyte

Abstract: In the context of ion transfer batteries, highly oriented pyrolytic graphite (HOPG) was studied as a model in aqueous electrolytes to elucidate the mechanism of electrochemical intercalation into graphite. The local and time-dependent dimensional changes of the host material occurring during the electrochemical intercalation processes were investigated on the nanometer scale. Atomic force microscopy (AFM), combined with cyclic voltammetry, was used as an in situ analytical tool during the intercalation of perchlorate and hydrogen sulfate ions into and their expulsion from the HOPG electrodes. For the first time, a reproducible, quantitative estimate of the interlayer spacing in HOPG with intercalated perchlorate and hydrogen sulfate ions could be obtained by in situ AFM measurements. The experimental values are in agreement with theoretical expectations, only for relatively low stacks of graphene layers. After formation of stage IV, HOPG expansion upon intercalation typically amounts to 32% when tens of l...

Structural studies of multiwall carbon nanotubes by neutron diffraction

Abstract: We report on structural studies of multiwall carbon nanotubes by wide-angle neutron scattering up to a maximum scattering vector ${Q}_{\mathrm{max}}=166 {\mathrm{nm}}^{\ensuremath{-}1}.$ The derived reduced radial distribution functions of the nanotubes are compared to those determined for graphite and turbostratic carbon, providing evidence that the stacking pattern of graphene tubules in multiwall carbon nanotubes is intermediate between those of the other two carbon forms. The (002) and (004) peaks of the nanotubes appear at smaller angles than graphite, yielding the intertubule spacing of 0.341 nm. At small length scales $(\ensuremath{\lesssim}0.5 \mathrm{nm})$ the nanotube structure resembles that of graphite, including graphitelike interlayer correlations for at least a few adjacent layers. Beyond this range, a systematic decrease in peak amplitudes and deviation from the graphite structure is observed.

Broken symmetries in scanning tunneling images of carbon nanotubes

Abstract: Scanning tunneling images of carbon nanotubes frequently show electron distributions that break the local sixfold symmetry of the graphene sheet. We present a theory of these images that relates these anisotropies to the off-diagonal correlations in the single-particle density matrix, and allows one to extract these correlations from the observed images. The theory is applied to images of the low-energy states reflected at the end of a tube or by point defects, and to states propagating on defect free semiconducting tubes. The latter exhibit a switching of the anisotropy in the tunneling image with the sign of the tunneling bias. @S0163-1829~99!50920-X# Scanning tunneling microscopy and spectroscopy is a powerful tool for studying the structural and electronic properties of carbon nanotubes at the atomic scale. Several experimental groups have reported tunneling images of isolated single wall carbon nanotubes 1,2 and of tubes packed into bundles or ‘‘ropes.’’ 3 In some cases these measurements have allowed a direct determination of the diameters and wrapping vectors for the tubes and these observations, combined with scanning tunneling spectroscopy, have confirmed the idea that the semiconducting or conducting behavior of a tube is controlled by its wrapping vector. 1,2

Electron backscattering on single-wall carbon nanotubes observed by scanning tunneling microscopy

Abstract: Single-wall carbon nanotubes, seamless cylindrical molecules formed from a graphene sheet, are either conducting or semiconducting, depending on the particular wrapping vector that defines the waist of the tube. Scanning tunneling microscopy experiments have tested this idea by simultaneously measuring a tube's lattice structure and electronic properties. Here we present a series of STM images of single-wall carbon nanotubes with a strikingly rich set of superstructures. The observed patterns can be understood as due to interference between propagating electron waves that are reflected from defects on the tube walls and ends, or as intrinsic to states propagating on semiconducting tubes. The measured broken symmetries can be used to directly probe electronic backscattering on the tube and provide a key element in the understanding of low-energy electron transport on these structures.

Chemical Synthesis of Graphite Perfluorooctanesulfonate Using K2MnF6 in Hydrofluoric Acid or Mixed Acid Solutions

Abstract: Graphite intercalation compounds (GICs) of nominal composition CxC8F17SO3·yF are prepared under ambient conditions in 48% hydrofluoric acid, using the oxidant K2MnF6. The product compositions are evaluated by mass uptake, TGA, DSC, and elemental analysis. PXRD analysis indicates that the stable product after 3−4 day reaction (x ≅ 18, y ≅ 4) is a GIC comprised of a solid solution of stage 2 and stage 3. A similar product, although containing a greater graphite fluoride impurity, can be obtained within 1 h using this method at 50 °C. Longer reactions at elevated temperature lead to product degradation and other impurities. The addition of up to 83 vol % concentrated HNO3 or 17% fuming H2SO4 produces a stage 2 intercalation compound within hours. One-dimensional structural modeling for the stage 2 GIC provides refined values for the graphene carbon/intercalant mole ratio, distance between graphene and sulfonate oxygen planes, position of additional intercalated fluoride, and the chain takeoff angle. The prod...

Co-sputtering C-Cu thin film synthesis : microstructural study of copper precipitates encapsulated into a carbon matrix

Abstract: Co-sputtered C-Cu thin film depositions have been performed in the temperature range 80-873 K, the atomic carbon concentration varying from 16% to 96%. To characterize the microstructure of the C-Cu thin films, transmission electron microscopy, extended X-ray absorption fine structure and grazing incidence small angle X-ray scattering experiments have been used. During the deposition process, a demixing occurs of the carbon and copper species due to their very low solubilities that leads to the formation of nanometric copper precipitates homogeneously distributed in a more or less graphitic matrix. These precipitates have an elongated shape in the direction of the thin film growth. When the deposition was performed at 273 K for copper atomic concentrations CCu > 55%, as well as for all thin films synthesized at 573 K whatever the CCu value, the formation of graphene layers parallel to the surface of the copper precipitates was observed so that an encapsulation of the Cu aggregates in carbon cages...