Atomic transformations and quantum transport in carbon nanotubes

High strain conditions can lead to a variety of atomic transformations in nanotubes, which usually occur via successive bond rotations. The energetic barrier for the rotation is dramatically lowered by strain, and ab initio results for its strain dependence are presented. While very high strain rates must lead to tube 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 tube changes between the 5-7 pairs, potentially leading to metal-semiconductor junctions. The high strain conditions can be imposed on the tube via, e.g., AFM tip manipulations, and we show that such procedures can lead to intratube device formation. The defects and the index changes occurring during the mechanical transformations also affect the electrical properties of nanotubes. We have computed the quantum conductances of strained defective and deformed tubes using the tight binding method. The results show that the defect density and the contacts play key roles in reducing the conductance at the Fermi energy. We also explored the role of bending in changing the electrical properties and found that mechanical deformations affect differently the transport properties of achiral and chiral nanotubes. Our results are in good agreement with recent experimental data.