Synthesis of Ultrahigh-Purity (6,5) Carbon Nanotubes Using a Trimetallic Catalyst.

Chirality-controlled synthesis of carbon nanotubes (CNTs) is one of the ultimate goals in the field of nanotube synthesis. At present, direct synthesis achieving a purity of over 90%, which can be called single-chirality synthesis, has been achieved for only two types of chiralities: (14,4) and (12,6) CNTs. Here, we realized an ultrahigh-purity (∼95.8%) synthesis of (6,5) CNTs with a trimetallic catalyst NiSnFe. Partial formation of NiSn crystals was found within the NiSnFe nanoparticles. The activation energy for the selective growth of (6,5) CNTs decreased owing to the formation of NiSn crystals, resulting in the high-purity synthesis of (6,5) CNTs. Transmission electron microscopy (TEM) reveals that one-dimensional (1D) crystals of periodic strip lines with 8.8 Å spacing are formed within the as-grown ultrahigh-purity (6,5) CNTs, which are well-matched with the simulated TEM image of closely packed 37 (6,5) CNTs with 2.8 Å intertube distance, indicating the direct formation of chirality-pure (6,5)-CNT bundle structures. The photoluminescence (PL) lifetime increases more than 20 times by the formation of chirality-pure bundle structures of (6,5) CNTs compared to that of isolated (6,5) CNTs. This can be explained by exciton delocalization or intertube excitons within bundle structures of chirality-pure (6,5) CNTs.