High-Temperature Structural Modification of Bundled Singled-Walled Carbon Nanotubes

Abstract

A systematic high-resolution transmission electron microscopy study of the thermal evolution of bundled single-walled carbon nanotubes (SWNTs) subjected to ~8 h high temperature heat treatment (HTT) in vacuum at successively higher temperatures up to 2200 °C is presented. We have examined purified SWNT material derived from the HiPCO and ARC processes (see Figure 1). These samples were found to thermally evolve along very different pathways that we propose depends on three factors: (1) initial diameter distribution, (2) concomitant tightness of the packing of the tubes in a bundle, and (3) the bundle size. Graphitic Nanoribbons (GNR) were found to be the dominant high temperature filament in ARC material after HTT=2000 °C; they were not observed in any heat-treated HiPCO material. The first two major steps in the thermal evolution of HiPCO and ARC material agree with the literature [1-3], i.e., coalescence, followed by the formation of multiwall carbon nanotubes (MWNTs). However, Arc material evolves to bundled MWNTs, while HiPCO evolves to isolated MWNTs. In ARC material, we find that the MWNTs collapse into multishell GNRs. The thermal evolution of these carbon systems is discussed in terms of the diameter distribution, nanotube coalescence pathways, CC bond rearrangement, diffusion of carbon and subsequent island formation, as well as the nanotube collapse driven by van der Waals forces. The HRTEM results were correlated with micro-Raman Scattering experiments. The thermal transformations produce changes in the radial (R) SWNT band, the G-band (and its substructure), and the relative intensity of the disorder-induced D-band scattering. The Raman spectrum of GNRs is also discussed in detail. Three relatively broad GNR Raman bands are observed: 1320, 1580, 1620, 2702 and 3250cm-1. The large GNR D-band scattering intensity is tentatively assigned to K-point modes activated by the small lateral width of the GNRs that is estimated from TEM and Raman to be ~ 6-8 nm.