Page 393 - Fiber Fracture

ATOMIC TRANSFORMATIONS 375

the resulting geometry the two ends bind together to form a small channel between the
tubes, while the tips close in a partial hemisphere (Buongiorno Nardelli et a]., 1998~).
The conductance of the final structure is shown in Fig. 14b. The small contact channel
between the nanotubes enables electron transmission, although at a low level of conduc-
tance (G(EF) % 0.6(2e2/h)). This result does not change significantly if a larger overlap
region is considered, provided that a transmission channel is formed in the process. This
observation is consistent with the experimental findings of Paulson et al. (1999).

SUMMARY

In summary, we have shown that in carbon nanotubes high-strain conditions can lead
to a variety of atomic transformations, often occumng via successive bond rotations.
The bamer for the rotation is dramatically lowered by strain, and ab initio results for its
strain dependence were presented. While very high strain rates must lead to breakage,
(n,m) nanotubes with n,m < 14 can display plastic flow under suitable conditions. This
occurs through the formation of a 5-7-7-5 defect, which then splits into two 5-7 pairs.
The index of the nanotube changes between the 5-7 pairs, potentially leading to metal-
semiconductor junctions. Such transformations can be realized via manipulations of the
nanotube using an AFM tip. Carbon addimers can also induce structural transformations
in strained tubes, potentially leading to the formation of quantum dots in otherwise
brittle tubes.
Defects and strain can obviously affect the electrical properties of nanotubes. We
have computed quantum conductances of strained, defective and deformed nanotubes.
The results show that bent armchair nanotubes keep their metallic character for most
practical purposes, even though an opening of a small symmetry-related pseudo-gap
is predicted in small diameter (d < 0.7 nm) nanotubes. Metallic chiral nanotubes
undergo a bending-induced metal-semiconductor transition that manifests itself in the
occurrence of effective barriers for transmission, while bent zigzag nanotubes are always
semiconducting for the diameters considered in this study (up to 1.5 nm). Topological
defects increase the resistance of metallic nanotubes to an extent that is strongly
dependent on their density per unit length.

ACKNOWLEDGEMENTS

This work was supported in part by grants from ONR and NASA. The computations
were carried out at DoD, NSF and NC Supercomputing Centers.

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