Page 392 - Fiber Fracture

374 J. Bernholc et al.
large-diameter nanotubes the key quantity in determining the electrical response is the
density of defects per unit length. This is also in agreement with recent measurements of
Paulson et al. (1999) of the electrical properties of carbon nanotubes under strain applied
with an AFM probe. As the AFM tip pushes the tube, the strain increases without any
change in the measured resistance until the onset of a structural transition is reached.
This corresponds to the beginning of a plastic/brittle transformation that releases the
tension in the NT and coincides with a sharp yet finite increase in resistance. Since the
onset of the plastic/brittle transformation that precedes the breakage is associated with
the formation of a region of high defect density (Buongiorno Nardelli et al., 1998a,b),
the conductance at the Fermi energy is drastically reduced.
In the experiments of Paulson et al. (1999), a clamped multi-walled nanotube was
stretched until breakage with an AFM tip, but after the breakage the ends were manip-
ulated back into contact and a finite resistance was established. As a partial simulation
of this process, we have considered the tube-tube junction depicted in Fig. 14a. Two
open-ended (53) tubes have been put in contact with a small overlap region. The system
was then annealed via a molecular dynamics simulation at a high temperature (3000 K)
for -30 ps, after which the atoms were quenched to their ground state configuration. In

4

Fig. 14. The geometry (a) and the conductance (b) of an annealed contact between two open-ended (5,5)
nanotubes. See text. The Fermi energy is taken as reference.