Vibrational modes and thermal transformation of purified single walled carbon nanotubes

Vibrational modes of single-walled carbon nanotubes (SWNTs) and graphitic nanoribbons (GNRs) were studied using Raman scattering and/or Fourier Transform Infrared Spectroscopies, Variations in a three-step purification scheme to remove amorphous carbon and residual catalyst were studied: (step 1) Oxidation, (step 2) Acid Reflux, and (step 3) Thermal Annealing were found to remove most amorphous carbon (oxidation step) and residual metal catalyst (acid reflux step) which were the major impurity phases. By combining IR and Raman, we found considerable wall damage and functional groups (e.g.-COOH and-OH) could be introduced via H2O2 and HNO3 reflux. Surprisingly, vacuum annealing at ∼1100°C for a few hours was found to remove most wall damage and functional groups.;Methods to break up large (purified) bundles of single-walled carbon nanotubes (SWNTs) to individual tubes were also investigated. Amide solvents with ultrasound were found to be very effective in debundling; initial purification treatment strongly impacted the outcome. SWNT material decorated with functional groups (e.g., -COOH) tended to produce higher yields of single tubes. Length and diameter distributions of individual tubes were measured using Atomic Force Microscopy. Aggressive chemical debundling processes were found to lead to more functionalization, higher degree of debundling and shorter tubes.;The IR-active modes of SWNTs was observed for the first time by transmission method, some ten years after the discovery of the Raman-active modes. In concert with theoretical calculations, we were able to assign much of the sharp structure in the IR with anticipated one- and two-phonon lattice mode bands.;Thermal evolution of bundled SWNT materials produced in the electric arc (ARC) and by CVD in CO gas (HiPCO) was also investigated. Although both ARC and HiPCO evolved thermally to multi-walled tubes (MWNTs), we found using electron microscopy that for T>2000°C ARC SWNTs (with significantly narrower diameter distribution) evolved beyond MWNTs to a new dominant form of carbon termed "graphitic nanoribbons" (GNRs). A strong increase in D-band Raman intensity in ARC SWNTs was observed after the formation of GNRs and identified with lateral phonon confinement.