S. D. M. Brown

Bulk morphology and diameter distribution of single-walled carbon nanotubes synthesized by catalytic decomposition of hydrocarbons

Long and wide ropes/ribbons of single-walled carbon nanotube (SWNT) bundles with rope diameters of 100 mu m and lengths to 3 cm were synthesized by the catalytic decomposition of hydrocarbons. These ropes/ribbons consist of roughly-aligned bundles of aligned SWNTs. The SWNT diameters, as determined from HRTEM images, are 1.69 +/- 0.34 nm, which, in combination with resonant Raman scattering measurements, indicates that our SWNTs have larger diameters than those synthesized by other techniques. The larger SWNTs are promising for gas-storage applications. The easy manipulation of the ropes offer special opportunities for their characterization and applications

Resonant Raman scattering characterization of carbon nanotubes grown with different catalysts

Single-walled carbon nanotube samples produced in the presence of different combinations of metal catalysts have been studied by resonant Raman spectroscopy. The diameter distribution of different samples has been determined by analysis of the laser excitation energy dependence of the tangential modes associated with metallic nanotubes. These modes are resonantly enhanced over a narrow range of the exciting energies, which shifts for different samples. The Raman cross-section expression has been used to fit the experimental Raman excitation profiles. This procedure was used to determine the mean value and the width of the distribution of diameters within each sample.

Reversible Hydrogen Uptake in Carbon-Based Materials

Several approaches for achieving reversible hydrogen uptake by carbon are considered, including intercalation, adsorption by a graphite surface, hydrogenation of fullerenes, and the filling of carbon nanotubes. Most scenarios suggest that it is difficult to achieve an atomic uptake [H/C] ratio exceeding unity. Evidence for H 2 uptake by various carbon materials is reviewed.

Characterization of single-walled carbon nanotubes through Raman Spectroscopy

The importance of Raman Spectroscopy in the characterization of single-walled carbon nanotubes (SWNTs) is emphasized. In particular, resonant Raman spectroscopy, performed using several laser excitation energy lines, explores both the vibrational and the electronic properties of the carbon nanotubes giving a broad picture of their characteristics. This work summarizes the main results obtained in the first-order and anti-Stokes Raman spectral regions.

First and Second-Order Resonant Raman Spectra of Single-Walled Carbon Nanotubes

In summary, overtones and combination modes have been identified in the second-order spectra for the two dominant features in the first-order spectra (the radial breathing mode and the tangential mode) that are associated with the resonant Raman enhancement process arising from the 1D electronic density of states. Just as for the case of the first-order spectra, the resonant contributions to the second-order spectra also involve a different set of (n, m) nanotubes at each laser excitation energy Elaser. A second-order analog is observed for the broad spectral band identified with contributions from metallic nanotubes to the first-order tangential mode spectra. The unique feature of the second-order tangential overtone band shows a larger Elaser range over which the metallic nanotubes contribute, and this effect is attributed to the large (ħωphononc04eV) energy of these phonons. Combination modes associated with (ωtang+ωRBM) and (ωtang+2ωRBM) have been identified. These combination modes show behaviors as a function ofElaser that are consistent with the behavior of their first-order constituents, namely that different nanotubes contribute to the spectra at each value of Elaser. The behavior of the ‘D-band’ and G-band features show a very large phonon frequency dependence on Elaser and show a resonant 2D behavior when the electron and phonon wave vectors coincide, as also occurs in other sp2 carbons.

Breit-Wigner-Fano Lineshape Analysis of the angential G -band of Metallic Nanotubes

We present a resonant Raman study of the tangential G -band in metallic SWNTs. By measuring the Raman spectra for isolated SWNTs, we show that the two different lineshapes observed for semiconducting and metallic SWNTs in bundles also occur for isolated SWNTs. A lineshape analysis of the tangential G -band feature for metallic SWNT bundles is presented, showing that only two components are present, the higher frequency component having a Lorentzian lineshape, and the lower one having a Breit–Wigner–Fano (BWF) line-shape. Through comparisons of the Raman tangential G -band spectra from three different diameter distributions of carbon nanotubes, we find that both the frequency and linewidth of the BWF component are diameter dependent.

Resonant Raman Characterization of Polyparaphenylene Based Carbon Materials

The structure of the polymer polyparaphenylene (PPP), heat-treated at temperatures (T HT ) between 650°C≤ T HT ≤750°C and prepared according to either the Kovacic or the Yamamoto methods, is studied by the resonant Raman technique using five laser excitation energies E laer in the range 1.92–3.05 eV. The resonant Raman analysis allows identification of the benzenoid and quinoid forms of PPP as well as the amorphous material associated with the onset of the carbonization process. The results enable us to follow the evolution of the PPP polymer with heat-treatment temperature and to determine the differences between the materials prepared using the two different methods.

Characterization of polyparaphenylene subjected to different heat treatment temperatures

The authors investigated the structural and electronic properties of samples of polyparaphenylene (PPP), derived from two synthesis methods (the Kovacic and Yamamoto methods). These samples have been subjected to different heat-treatment temperatures (650 C {le} T{sub HT} {le} 2,000 C) and their properties are compared to the polymer prior to heat-treatment (T{sub HT} = 0 C). The photoluminescence (PL) spectra of heat-treated PPP based on the two synthesis methods reflects the differences in electronic structure of the starting polymers. The PL emission from the heat-treated Yamamoto polymer is quenched at much lower T{sub HT} than from the Kovacic material. However, Raman spectra taken of the material resulting from heat-treatment of the polymer (using both preparation methods) indicate the presence of phonon modes for PPP in samples at T{sub HT} up to 650 C.