C. H. Olk

Hydrogen storage capacity of carbon nanotubes, filaments, and vapor-grown fibers

Abstract We have studied the sorption of hydrogen by nine different carbon materials at pressures up to 11 MPa (1600 psi) and temperatures from −80 to +500°C. Our samples include graphite particles, activated carbon, graphitized PYROGRAF vapor-grown carbon fibers (VGCF), CO2 and air-etched PYROGRAF fibers, Showa-Denko VGCF, carbon filaments grown from a FeNiCu alloy, and nanotubes from MER Corp. and Rice University. We have measured hydrogen sorption in two pieces of equipment, one up to 3.5 MPa, and one to 11 MPa. The results so far have been remarkably similar: very little hydrogen sorption. In fact, the sorption is so small that we must pay careful attention to calibration to get reliable answers. The largest sorption observed is less than 0.1 wt.% hydrogen at room temperature and 3.5 MPa. Furthermore, our efforts to activate these materials by reduction at high temperatures and pressures were also futile. These results cast serious doubts on any claims so far for room temperature hydrogen sorption in carbon materials larger than a 1 wt.%.

Electronic-properties of Carbon Nanotubes - Experimental Results

Band structure calculations show that carbon nanotubes exist as either metals or semiconductors, depending on diameter and degree of helicity. When the diameters of the nanotubes become comparable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show evidence of one-dimensional behavior: the current-voltage characteristics are consistent with the functional energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly with the inverse nanotube diameter. Electrical resistivity acid magnetoresistance measurements have been reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the formation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly diamagnetic behavior. The susceptibility al fields above the value at which the magnetic length equals the tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sample the effect of the finite tube diameter, the susceptibility has a much more pronounced temperature dependence.

Scanning tunneling spectroscopy of carbon nanotubes

Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 A show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.

Raman spectra during the electropolymerization of polypyrrole

In situ Raman spectra were taken, between 800 and 1200 cm −1 , on films of polypyrrole during both the electrochemical growth and the reduction reactions. During the reduction reaction we observed a frequency shift of approximately 10 cm −1 of two in-plane vibrational modes. We attribute these shifts to the bond conjugational defects induced by doping and undoping of the polypyrrole films. A quantitative model for this phenomenon based on normal mode calculations is presented. The electrical conductivity and thermopower of polypyrrole doped with tetrafluoroborate and iron chloride ions is also reported for temperatures between 10 and 350K.

IgG antibody and antibody–antigen complex imaging by scanning tunneling microscopy

We have examined two preparations of biomolecules labeled with colloidal gold markers using scanning tunneling microscopy. In the first preparation, we studied gold labeled IgG antibodies. In the second preparation, we studied unlabeled IgG antibodies which were complexed with gold labeled antigen (bovine serum albumin). Images of these noncrystalline biological specimens, obtained under atmospheric conditions, revealed features on a nanometer scale. The images show details of molecular organization in addition to the classical antibody configuration.

Effects of nanoscale morphology on the abrasion of steel by diamondlike carbon

Abstract Previously we showed that the abrasiveness of a coating is closely related to its hardness. In the present work we use a ball-on-disk apparatus to examine the abrasive characteristics of a tungsten-containing diamondlike carbon coating and a metal-free diamondlike carbon coating deposited on steel coupons. We find that roughness with horizontal dimensions on the nanometer-scale is strongly correlated with abrasiveness, while roughness with horizontal dimensions on the micron-scale is not correlated with abrasiveness. The nanometer-scale—but not the micrometer—scale-structure is quickly polished smooth by sliding against steel, explaining the drastic reduction in the abrasiveness of the coating that we have observed.

Growth and characterization of epitaxial bismuth films

Epitaxial bismuth thin films have been grown by molecular‐beam epitaxy for the first time. These films were grown on 〈111〉 oriented single‐crystal barium fluoride. Films grown at low temperatures (<150 °C) had microscopically rough surface morphologies. At higher temperatures, no film would nucleate. This led us to use a two‐step growth process in which films were nucleated at 100 °C followed by growth of a thicker film near the melting point of bismuth. Electron microscope observations show the films to be featureless and defect‐free on the scale of 0.1 μm. The films grow with their trigonal axis parallel to the 〈111〉 axis of the substrate, and Laue backscattering pictures show they are epitaxial. Mobilities at room temperature are as high as 2.5 m2 V−1 s−1, and increase to over 10 at 20 K and 100 at liquid helium temperatures. These values are far superior to those of other bismuth films grown to date, and approach mobilities observed in single‐crystal bismuth. Preliminary results of a study of n‐type T...

Surface structures of stage‐1 CuCl2 graphite intercalation compound observed by scanning tunneling microscopy

We report a scanning tunneling microscope (STM) study of a stage‐1 graphite intercalation compound (GIC) of approximate stoichiometry C6CuCl2. When the sample bias is positive with respect to the tip, we observe a hexagonal symmetry in which all the atoms of the graphite surface plane are imaged. This is in contrast to the threefold symmetry usually seen in atomic resolution STM images of highly oriented pyrolytic graphite (HOPG), which we also observe on a reference sample of HOPG. The threefold symmetry is attributed to the ABAB stacking of the atomic layers in HOPG. In GICs, this stacking sequence is interrupted by the layer of intercalant, so that for the stage‐1 compound all carbon atoms in the plane become equivalent, and sixfold symmetry develops. When the sample is biased negatively with respect to the tip, we observe a strikingly different STM image for the GIC; we believe that the pattern in this case is mainly due to the intercalate layer and the change in the electronic structure of graphite r...

The influence of heat-treatment of graphite fibers on their intercalation with sulfuric acid

Abstract We have intercalated heat-treated vapor-deposited fibers with H 2 SO 4 , and during the process continuously monitored the fiber using an optical microscope. The electrochemical reaction was driven by a constant current source, so that the time elapsed represents the molar fraction of H 2 SO 4 intercalating. The fiber diameter was measured simultaneously, as well as the potential between the graphite and a reference electrode. Host fibers heat-treated at different temperatures between 1400 and 3000°C were studied. In carbons never heated above 2000°C, the cell potential increases with charge uptake and even sometimes shows steps reminiscent of staging; the fiber diameter also increases, but continuously. In fibers heat-treated above 2200°C, both cell potential and diameter increase show staging. In fibers heat-treated above 2800°C inhomogeneities along the fiber axis are identified and the behavior is probably dictated by the density of defects present in the fiber.

ELECTRONIC PROPERTIES OF CARBON NANOTUBES: EXPERIMENTAL RESULTS

Band structure calculations show that carbon nanotubes exist as either metals or semiconductors, depending on diameter and degree of helicity. When the diameters of the nanotubes become comparable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show evidence of one-dimensional behavior: the current-voltage characteristics are consistent with the functional energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly with the inverse nanotube diameter. Electrical resistivity and magnetoresistance measurements have been reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the formation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly diamagnetic behavior. The susceptibility at fields above the value at which the magnetic length equals the tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sample the effect of the finite tube diameter, the susceptibility has a much more pronounced temperature dependence.