Study of purification and growth of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have been intensively studied because of their many potential applications. However, all known processes for formation of SWNTs involve transition-metal catalysts, residues of which are invariably present in the as-produced SWNT material. To truly realize the full potential of SWNTs, a controlled and scalable multi-step purification method has been developed to remove iron impurities and non-nanotube carbon materials from raw SWNTs produced in the high pressure CO (HiPco) process. Here, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C2H2F4 or SF6. The iron fluorides are removed by a soxhlet extraction with a 6M HCl solution. Iron content of approximately 1 wt% with approximately 70% SWNT yield has been achieved by this method. Furthermore, a hot filament chemical vapor deposition (HFCVD) method has been developed to grow vertically aligned single-walled carbon nanotubes (VA-SWNTs). Silicon substrates decorated with islands of iron were rapidly inserted into a preheated furnace in which a hot filament (temperature greater than 2000°C) is activating the gas. Characterization of VA-SWNTs by using Raman spectroscopy, fluorescence spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM) clearly shows that the VA-SWNTs have diameters ranging from 0.78 to 1.6 nm. Finally, HFCVD method has been used to investigate the effect of atomic hydrogen on the growth of VA-SWNTs. It is obvious that rapid heating of the substrate in the presence of atomic hydrogen facilitates the nucleation and growth of SWNTs before the aggregation of catalyst occurs. By comparing the Raman shifts of SWNTs synthesized from a mixture of isotopic methane (13CH 4) and normal acetylene (12C2H2), we found that SWNT growth occurs with addition of acetylene and ethylene that are formed by methane decomposition on the hot filament