Synthesis of Ultrahigh-Purity\n(6,5) Carbon Nanotubes\nUsing a Trimetallic Catalyst

Chirality-controlled synthesis of carbon nanotubes (CNTs)\nis one\nof the ultimate goals in the field of nanotube synthesis. At present,\ndirect synthesis achieving a purity of over 90%, which can be called\nsingle-chirality synthesis, has been achieved for only two types of\nchiralities: (14,4) and (12,6) CNTs. Here, we realized an ultrahigh-purity\n(∼95.8%) synthesis of (6,5) CNTs with a trimetallic catalyst\nNiSnFe. Partial formation of Ni<sub>3</sub>Sn crystals was found within\nthe NiSnFe nanoparticles. The activation energy for the selective\ngrowth of (6,5) CNTs decreased owing to the formation of Ni<sub>3</sub>Sn crystals, resulting in the high-purity synthesis of (6,5) CNTs.\nTransmission electron microscopy (TEM) reveals that one-dimensional\n(1D) crystals of periodic strip lines with 8.8 Å spacing are\nformed within the as-grown ultrahigh-purity (6,5) CNTs, which are\nwell-matched with the simulated TEM image of closely packed 37 (6,5)\nCNTs with 2.8 Å intertube distance, indicating the direct formation\nof chirality-pure (6,5)-CNT bundle structures. The photoluminescence\n(PL) lifetime increases more than 20 times by the formation of chirality-pure\nbundle structures of (6,5) CNTs compared to that of isolated (6,5)\nCNTs. This can be explained by exciton delocalization or intertube\nexcitons within bundle structures of chirality-pure (6,5) CNTs.