Abstract Submitted to the    NANOTUBE'04 Conference:

Molecular dynamics simulations based on an empirical potential energy surface were used to study iron catalyzed nucleation and growth of single-walled carbon nanotubes (SWNTs). The simulations show that SWNTs grow from the iron-carbide (FeC) particle at temperatures between 800 and 1400 K, whereas graphene sheets encapsulate the particle at temperatures below 600 K and a three-dimensional (3D) soot-like structure is formed above 1600 K. Nucleation of these carbon (C) structures can be divided into three stages: i) at short times all C atoms dissolve in the FeC particle, ii) at intermediate times the FeC cluster is highly supersaturated in C and carbon strings, polygons and small graphitic islands nucleate on the cluster surface, iii) at longer times the FeC cluster is supersaturated in C and a graphene sheet, SWNT or soot-like structure is grown. At low temperatures the kinetic energy is not sufficient to overcome the attractive forces between the graphitic islands (that ar e formed in stage ii) and the particle, and the island grows into a graphene sheet encapsulates the cluster. At temperatures above 800 K the kinetic energy is sufficiently high to overcome these attractive forces, and the graphitic island lifts off the particle to form a cap. Between 800 and 1400 K these caps grow into SWNTs, and at temperatures higher than 1600 K the large number of defects in the growing carbon structure produces a 3D soot-like structure. The calculations also reveal that the growing SWNT maintains an open end on the FeC cluster due to the strong bonding between the nanotube end atoms and the cluster. The number of defects in the SWNT structure can be reduced by lowering the rate of carbon addition to the FeC cluster.