Page 386 - Fiber Fracture

368 J. Bernholc et al.

ADATOMS-INDUCED TRANSFORMATIONS AND PLASTICITY

Nanotubes are obviously produced at carbon-rich conditions and additional carbon
atoms are likely to be present on nanotube walls. These ‘adatoms’, introduced either
during growth or processing, can facilitate structural transformations in nanotubes, as
described below.
The energetically preferred position for single carbon adatoms is to form handles
between pairs of nearest-neighbor carbon atoms (Maiti et al., 1997). The adsorption
energy for the adatoms varies weakly as a function of the nanotube diameter, ranging
from 5.5 eV on a (53 tube with a 0.78 nm diameter to 4.9 eV for a graphene sheet.
The most important effect of adatoms on strained nanotubes is to reduce the activation
energy for the Stone-Wales transformation. Essentially, the activation energies for the
bond rotation are all uniformly reduced by 1.0-1.2 eV for all strains (Orlikowski et
a]., unpublished). This effect may be understood in terms of an increased flexibility
for rotation of bonds that are next to the adatom handle, and implies that the presence
of adatoms on strained nanotubes significantly enhances the rate of (5-7-7-5) defect
formation.
The adatoms diffuse relatively fast and will eventually condense into ‘addimers’.
When a nanotube is strained, an addimer can induce additional bond rotations. Fig. 10
shows a typical dynamical evolution of a 3 nm long (480 atoms) (10,lO) armchair
tube with a carbon addimer at 2500 K and under a 3% strain. This addimer initially
sits on the surface of the nanotube. Within 4 ps, it is incorporated into the nanotube,
forming a novel defect consisting of back-to-back pentagons plus two heptagons, i.e., a
(7-5-5-7) defect (Fig. 1 Oa). This defect then undergoes substantial further evolution.
After 356 ps, the bond emanating from the vertex of one of the pentagons and pointing
away from the defect rotates to form a defect structure consisting of a single, rotated
hexagon that is separated from the rest of the nanotube through a ‘layer’ of (5-7) pairs
(Fig. lob). Moreover, the creation of rotated hexagons continues; after 421 ps, a defect
with two hexagons forms (Fig. IOc), while a third hexagon is incorporated after 2.35
ns (Fig. 1Od). If this process of adding hexagons were to continue, the defect structurc
would eventually wrap itself completely about the circumference of the tube, forming a
short segment of a nanotube with a different helicity.
In the absence of strain, the formation energy of the (7-5-5-7) defect is lowest for
both the armchair (10,lO) and the zigzag (17,O) tubes, indicating that structures with
rotated hexagons are not to be expected. However, the formation energy is strongly
strain-dependent. For the (10,lO) tube under a 5% strain, the defect with TWO rotated
hexagons has the lowest energy, indicating that structures containing more hexagons
represent transient, metastable configurations. Under a 10% strain, the formation energy
decreases as the number of hexagons in the defect increases, showing that larger strains
lead to the wrapping of the defect about the tube. Furthermore, the formation energy
of the (10,lO) tube oscillates with the number of hexagons it contains. This is simply a
reflection of the geometry of the armchair tubes. For a defect containing an even number
of hexagons, the bonds that need to be rotated in order to incorporate the next hexagon
are all at an angle with respect to the ones already present (Fig. lOd), so that the hexagon
must necessarily be formed next to rather than directly above the existing hexagons.