THEORY OF CARBON NANOTUBE GROWTH

The kinetics of carbon nanotube growth under are discharge conditions were investigated over different length and time scales using complementary numerical techniques. Relaxation by ab initio molecular dynamics (Car-Parrinello method) shows that large electric fields present at the tube tips are not the critical factor responsible for the open-ended growth observed experimentally. Classical molecular-dynamics simulations using realistic many-body carbon-carbon potentials show that wide tubes that are initially open can continue to grow straight and maintain an all-hexagonal structure. However, tubes narrower than a critical diameter, estimated to be about similar to 3 nm, readily nucleate curved, pentagonal structures that lead to tube closure with further addition of atoms. Very narrow tubes can be grown, however, if a small metal particle prevents tube closure. This effect was simulated explicitly by kinetic Monte Carlo methods. Monte Carlo simulations were also used to study nanotube growth over longer time scales. The resulting structures are in agreement with the above growth scenario, and provide an estimate for the lowest tube tip temperature necessary for the growth of nanotubes.