Microscopic growth mechanisms for carbon and boron-nitride nanotubes

The growth of carbon (C) and boron nitride (BN) nanotubes cannot be directly observed and the underlying microscopic mechanism is a controversial subject. Transition metal catalysts are necessary to produce single-walled nanotubes (SWNT) of carbon, but they are not needed in the multi-walled (MWNT) case, suggesting different growth mechanisms. Here we report on the results of first-principles dynamical simulations of both single- and double-walled carbon nanotube edges. We find that the open end of carbon SWNTs spontaneously closes by forming a graphitic dome in the 2500-3000 K temperature range of synthesis experiments. On the other hand, "lip-lip" interactions, 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. The resulting end geometry is highly chemically active, and can easily accommodate incoming carbon fragments, thus allowing growth by chemisorption from the vapour phase. The growth mechanisms of boron nitride SWNTs is studied as well, and is compared to the case of pure carbon tubes. In the experimental temperature conditions, the behavior of growing BN nanotubes strongly depends on the nanotube network helicity. In particular, we find that open-ended "zigzag" SWNTs close rapidly into an amorphous like tip, preventing further growth. In the case of "armchair" SWNTs, the formation of squares traps the tip into a flat cap presenting a large central even-member ring. This structure is metastable and able to revert to a growing hexagonal framework by incorporation of incoming atoms. These findings are directly related to frustration effects, namely that B-N bonds are energetically favored over B-B and N-N bonds.