MICROSCOPIC GROWTH MECHANISMS FOR CARBON NANOTUBES

The uncatalyzed edge growth of carbon nanotubes was investigated by fi rst-principles molecular dynamics simulations. At experimental tempera tures the open end of single-walled nanotubes closed spontaneously int o a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts, On the other hand, chemi cal bonding between the edges of adjacent coaxial tubes (''lip-lip'' i nteractions) trapped the end oi a double-walled nanotube in a metastab le energy minimum, thus preventing dome closure. These calculations sh ow that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase.