Non-oxidizing Purification Method for Large Volumes of Long, Undamaged Single-Walled Carbon Nanotubes
A mild route to the purification of single-walled carbon nanotube (SWNT) soot has been investigated. The removal of all forms of impurities commonly found in bulk SWNT material (metal catalyst nanoparticles, amorphous carbon, and bundles of SWNTs) has been obtained. This technique, which involves purification based on centrifugation at low centrifugal force, results in deposits that are composed of individual SWNTs that do not have the defects that result from harsh oxidative treatments. While ultracentrifugation has been used to separate and purify raw SWNT soot, this method removes all but the shortest nanotubes. Therefore, it is not practical for many electronic applications. The method presented in this manuscript removes carbonaceous and metallic impurities, while leaving long undamaged SWNTs in suspension. A liquid deposition method under development in this group was used to form electrically continuous two-dimensional networks of SWNTs. Atomic force microscopy (AFM) then is used to verify the efficacy of centrifugation cycles for purification of SWNT suspensions. AFM images of the deposits indicate significant improvement of deposit morphology, with long individual SWNTs that are observed without the presence of impurities. SWNT networks formed from these purified deposits behaved as metallic conductors at high density and as semiconductive thin films at low density. This further indicates that bundles of SWNTs were removed, along with other undesirable materials.