The growth of carbon (C) and boron nitride (BN) nanotubes cannot be directly observed and the underlying microscopic mechanism is a controversial subject. Here we report on the results of first-principles dynamical simulations of both single- and dobule-walled carbon nanotube edges.We find that the open end of carbon single-walled nanotubes (SWNTs) spontaneously closes by forming a graphitic dome in the 2500-3000 K temperature range of synthesis experiments. On the other hand, "lip-lip" in-teractions consisting of chemical bonding between the edges of adjacent coaxial tubes, trap the end of the double-walled carbon nanotube into a metastable energy minimum, preventing dome closure. TShe resulting end geometry is highly chemically active, and can easily accommodate incoming carbon fragments, thus allowing for growth by chemisorption from the vapour phase.
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