Page 391 - Fiber Fracture

ATOMIC TRANSFORMATIONS 373

Table 1. Conductances of armchair nanotubes with point defects

Pristine (5-7-7-3 (5-7)-(7-5) (5-7-8-7-3
(53 2.00 I .70 1.11 I .26
(10,IO) 2.00 1.85 I .33 I .72
(5-7-7-5) is the bond rotation defect; (5-7H7-5) corresponds to the onset of plastic behavior with the two
(5-7) pairs separated by one row of hexagons; (5-74-7-5) corresponds to the onset of brittle behavior.
with the opening of a higher-order carbon ring, see text. In units of 2e2/h.

breaking of the degeneracy in the band structure opens a gap (Eg x 0.1 eV), clearly
present in Fig. 13. For large deformations (0 = 42"), this gap is widened (E, = 0.2
eV), increasing the semiconducting character of the nanotube. One can then expect
that bending in a large-diameter, metallic chiral nanotube will drive it towards a
semiconducting behavior. This behavior is actually computed for a (1 2,6) chiral tube
(d = 1.2 nm), as shown in the inset of Fig. 13. A bending-induced gap of -60 meV is
opened at a relatively small angle (12"), whereas the NT was perfectly conducting prior
to bending. This result demonstrates that local barriers for electric transport in metallic
chiral NTs can occur with no defect involved and just be due to a deformation in the
tube wall. Given the relatively small values of the energy gaps, the conductance will be
affected only at low temperatures, leading to the assignment of these tubes to the (3)
class of behavior in Bezryadin et al. (1 998).
Although bending by itself can already cause a significant change in the electrical
properties, defects are likely to form in a bent or a deformed nanotube, because of the
strain occurring during the bending process. It is now well established that a carbon
nanotube under tension releases its strain via the formation of topological defects
(Buongiorno Nardelli et al., 1998a,b). We have investigated how these defects affect the
conductance of metallic armchair nanotubes of different diameters. Table 1 summarizes
our results for (53) and (1 0,lO) NTs under 5% strain, both pristine and in the presence
of different topological defects: (1) a (5-7-7-5) defect, obtained via the rotation of
the C-C bond perpendicular to the axis of the tube; (2) a (5-7) pair separated from a
second (7-5) pair by a single hexagon row, as in the onset of the plastic deformation of
the nanotube; and (3) a (5-7-8-7-5) defect, where another bond rotation is added to
the original (5-7-7-5) defect, producing a higher-order carbon ring (onset of the brittle
fracture). While strain alone does not affect the electronic conduction in both tubes, the
effect of defects on conductance is more evident in the small-diameter (53) NT, while
it is less pronounced in the larger (10,lO) NT. Our results for the (10,lO) tube with a
single (5-7-7-5) defect compare very well with a recent ab initio calculation (Choi and
Ihm, 1999). If more than one (5-7-7-5) defect is present on the circumference of the
NT, the conductance at the Fermi level is lowered: for the (10,lO) NT it decreases from
2 (2e2/ h) to I .95, I .70 and 1.46 (2e2/ h) for one, two or three defects, respectively.
The decrease in conductance is accompanied by a small increase in the DOS at the
Fermi energy. This is due to the appearance of defect states associated with the pen-
tagons and heptagons within the metallic plateau near the Fermi level. These localized
states behave as point scatterers in the electronic transmission process and are respon-
sible for the decrease in conductance (Crespi et al., 1997). This result confirms that in