Pavel Nikolaev

Crystalline Ropes of Metallic Carbon Nanotubes

Fullerene single-wall nanotubes (SWNTs) were produced in yields of more than 70 percent by condensation of a laser-vaporized carbon-nickel-cobalt mixture at 1200°C. X-ray diffraction and electron microscopy showed that these SWNTs are nearly uniform in diameter and that they self-organize into “ropes,” which consist of 100 to 500 SWNTs in a two-dimensional triangular lattice with a lattice constant of 17 angstroms. The x-ray form factor is consistent with that of uniformly charged cylinders 13.8 ± 0.2 angstroms in diameter. The ropes were metallic, with a single-rope resistivity of <10−4 ohm-centimeters at 300 kelvin. The uniformity of SWNT diameter is attributed to the efficient annealing of an initial fullerene tubelet kept open by a few metal atoms; the optimum diameter is determined by competition between the strain energy of curvature of the graphene sheet and the dangling-bond energy of the open edge, where growth occurs. These factors strongly favor the metallic (10,10) tube with C5v symmetry and an open edge stabilized by triple bonds.

Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide

Abstract Single-walled carbon nanotubes (SWNTs) have been produced in a gas-phase catalytic process. Catalysts for SWNT growth form in situ by thermal decomposition of iron pentacarbonyl in a heated flow of carbon monoxide at pressures of 1–10 atm and temperatures of 800–1200°C. The SWNT yield and diameter distribution can be varied by controlling the process parameters, and SWNTs as small as 0.7 nm in diameter, the same as that of a C60 molecule, have been generated. This process shows great promise for bulk production of carbon nanotubes.

Unraveling Nanotubes: Field Emission from an Atomic Wire

Field emission of electrons from individually mounted carbon nanotubes has been found to be dramatically enhanced when the nanotube tips are opened by laser evaporation or oxidative etching. Emission currents of 0.1 to 1 microampere were readily obtained at room temperature with bias voltages of less than 80 volts. The emitting structures are concluded to be linear chains of carbon atoms, Cn, (n = 10 to 100), pulled out from the open edges of the graphene wall layers of the nanotube by the force of the electric field, in a process that resembles unraveling the sleeve of a sweater.

Growth and Sintering of Fullerene Nanotubes

Carbon nanotubes produced in arcs have been found to have the form of multiwalled fullerenes, at least over short lengths. Sintering of the tubes to each other is the predominant source of defects that limit the utility of these otherwise perfect fullerene structures. The use of a water-cooled copper cathode minimized such defects, permitting nanotubes longer than 40 micrometers to be attached to macroscopic electrodes and extracted from the bulk deposit. A detailed mechanism that features the high electric field at (and field-emission from) open nanotube tips exposed to the arc plasma, and consequent positive feedback effects from the neutral gas and plasma, is proposed for tube growth in such arcs.

Raman scattering test of single-wall carbon nanotube composites

Raman spectroscopy is used to infer elastic properties of single-wall carbon nanotubes (SWNTs) in composites. This letter presents strain-induced frequency shift of tangential Raman active modes of SWNTs embedded in epoxy resin subjected to bending. Epoxy curing and sample extension in the tensile strength test are found to create residual strains on the SWNT ropes. We demonstrate that specimen compression in combination with the Raman microprobe technique provides a means for determining of these strains and hence load transfer effectiveness.

Diameter doubling of single-wall nanotubes

Abstract High-yield single wall nanotube (SWNT) material consisting primarily of zero-helicity armchair tubes was annealed at 1400 or 1500°C under medium-high vacuum, flowing argon and flowing hydrogen atmospheres. As many as 60% of the nanotubes so treated coalesced with neighbors, the proportion depending on the atmosphere, resulting in nanotubes with twice (2.7 nm) and occasionally three times (4.1 nm) the diameter of the (10,10) tube that is the dominant armchair tube in these samples. Since only nanotubes with equal helical pitch angle and chirality can coalesce seamlessly, the high conversion ratio adds support to other recent evidence that no type of SWNT is formed in these samples in greater numbers than the (10,10) tube.

ELECTRON NANO-DIFFRACTION STUDY OF CARBON SINGLE-WALLED NANOTUBE ROPES

Abstract Electron diffraction patterns from regions about 0.7 nm in diameter have been obtained, in conjunction with dark-field imaging, in a scanning transmission electron microscope to study the average helicity and the local variations of helicity of the individual carbon nanotubes within the ropes of single-walled carbon nanotubes recently produced by double laser irradiation. It has been found that the predominant helicity is that predicted theoretically, namely that giving the metallic, C5V symmetry, (10,10) tube.

RAMAN SCATTERING STUDY OF COALESCED SINGLE WALLED CARBON NANOTUBES

High temperature heat treatment of single wall carbon nanotube bundles in flowing H 2 was used to produce a significant fraction (∼40%) of diameter-doubled, or coalesced tubes with a mean diameter corresponding to that of ∼(20, 20) tubes. At three laser excitation wavelengths (514.5, 647, and 1064 nm), a reduction in the Raman scattering intensity of the strong radial and tangential modes was observed in the H 2 -treated sample, consistent with the reduced fraction of tubes in the sample after coalescence. However, using 488 nm excitation, little or no change is observed in the Raman spectrum after the H 2 treatment, suggesting that this excitation wavelength couples only to chiral symmetry tubes. Using the 647 nm excitation, the effect of H 2 treatment on the tangential band is quite unique, and a significant change in the shape of the tangential band was observed. Our lineshape analysis, and other results reported in this issue, suggest that this unique change of shape is due to lost scattering intensity from metallic tubes partially compensated by tangential mode scattering from the coalesced tubes. The normally prominent radial breathing mode band, which would be expected at ∼90 cm −1 for ∼(20, 20) tubes was not observed, indicating that these larger diameter tubes do not exhibit strong resonant scattering, at least at any of the wavelengths used in this study.

Quantifying the semiconducting fraction in single-walled carbon nanotube samples through comparative atomic force and photoluminescence microscopies.

A new method was used to measure the fraction of semiconducting nanotubes in various as-grown or processed single-walled carbon nanotube (SWCNT) samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results show large variations among growth methods and effective sorting by density gradient ultracentrifugation. This counting-based method provides important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques.

Strain Measurements on Individual Single-Walled Carbon Nanotubes in a Polymer Host: Structure-Dependent Spectral Shifts and Load Transfer

The fluorescence spectra of individual semiconducting single-walled carbon nanotubes embedded in polymer films were measured during the application of controlled stretching and compressive strains. Nanotube band gaps were found to shift in systematic patterns that depend on the (n,m) structural type and are in excellent agreement with the predictions of theoretical models. Loss of nanotube-host adhesion was revealed by abrupt irregularities in plots of spectral shift vs strain.