As exceptional one-dimensional conductors, metallic single-walled carbon nanotubes (SWCNTs) are ideal candidates for a variety of electronic applications such as nanocircuit components and power
transmission cables. In particular, (n,n) chirality, or "armchair," metallic nanotubes are predicted to be truly gapless and intrinsically insensitive to disorder, consistent with experimentally observed ballistic conduction behavior. Unfortunately, progress toward such applications has been slowed by the inherent problem of nanotube synthesis whereby both semiconducting and metallic nanotubes are produced.

In an effort to advance our sample preparation toward the ideal individualized, metallic-only nanotube sample for DC and THz conductivity measurements, we employ the technique of density-gradient ultracentrifugation (DGU). This technique can separate SWCNTs by electronic type, effectively producing samples of only metallic nanotubes and semiconducting nanotubes, respectively. The intrinsic differences in mass density of different-chirality nanotubes are primarily due to varying diameter of different chiralities and hence diameter of the surfactant micelle and binding affinities of different types of nanotubes for the surfactant, both of which contribute to different densities. The separation is accomplished by subjecting an aqueous suspension of surfactant-suspended SWCNTs to travel through a mass density gradient via ultracentrifugation (~200,000g for 18 hours). Due to the variation in density of the gradient, different density nanotubes will migrate towards different regions of the gradient, under the applied force of the centrifuge versus the buoyant density of the nanotube, until an equilibrium is reached. At this point, the nanotubes are now suffciently separated in vertical distance in the gradient so that they may be extracted using simple fractionation techniques.

Through a variety of spectroscopy experiments, including photoluminescence excitation spectroscopy, absorption spectroscopy, and Raman spectroscopy, we have demonstrated that we can prepare aqueous suspensions of nearly all metallic SWCNTs. Furthermore, we have been using the technique of resonance Raman scattering (RRS) spectroscopy, in collaboration with Dr. Stephen Doorn at CINT of Los Alamos National Lab, to quantitatively determine the chirality distribution of these metallic-enriched samples. This has allowed us to identify the chiralities of nanotubes that are present in an ensemble sample by tuning the excitation photon energy through the energies of the E22 semiconducting and E11 metallic interband transitions. There is evidence that we are predominantly enriching armchair SWNTs, i.e., (n,m) nanotubes where n = m. For example, let us take the 2n + m = 21 family, i.e., (10,1), (9,3), (8,5), and (7,7), where the (7,7) tube is the only truly metallic tube among them. Our RRS data suggests that the relative abundance, reflected in the relative peak intensities, increases with increasing chiral angle. This is truly striking considering the fact that the Raman intensity actually tends to decrease with increasing chiral angle due to the decreasing electron-phonon coupling strength. This method will provide a route to a large-scale sample consisting of armchair SWNTs only. We are currently using these samples in DC transport and THz conductivity measurements.

Publications & News:

E. H. Hároz, J. G. Duque, B. Y. Lu, P. Nikolaev, S. Arepalli, R. H. Hauge, S. K. Doorn, and J. Kono, "Unique Origin of Colors of Armchair Carbon Nanotubes," Journal of the American Chemical Society, accepted for publication. (abstract)

"Scientists solve mystery of colorful armchair nanotubes" by Rice News, January 11, 2012

"How nanotubes get color" by Chemical & Engineering News, January 9, 2012

E. H. Hároz, J. G. Duque, W. D. Rice, C. G. Densmore, J. Kono, and S. K. Doorn, "Resonant Raman Spectroscopy of Armchair Carbon Nanotubes: Absence of Broad G^{-} Feature," Physical Review B 84, 121403(R) (2011). (abstract, full text)

E. H. Hároz, W. D. Rice, B. Y. Lu, S. Ghosh, R. H. Hauge, R. B. Weisman, S. K. Doorn, and J. Kono, "Enrichment of Armchair Carbon Nanotubes via Density Gradient Ultracentrifugation: Raman Spectroscopy Evidence," ACS Nano 4, 1955 (2010). (abstract, full text)

"Nano parfait a treat for scientists" by Rice News, May 12, 2010