Structural properties of junctions between two carbon nanotubes

The junction between two different nanotubes can be realized with the insertion of a single pentagon-heptagon pair in the honeycomb network. This defect bends the structure at an angle that depends on the distance between the pentagon and heptagon. The atomic structure of several junctions was optimized with the help of empirical potentials, the nanotubes on both sides of the junctions being considered as infinitely long. Local densities of σ + π electron states were evaluated in the interfacial region with a tight-binding Hamiltonian. From there, the electronic energy of these junctions was calculated and compared to that of the isolated nanotubes. It was established that the energy associated with the pentagon-heptagon pair in a graphitic tubule is of the order of 6 eV. An automatic algorithm for connecting any two tubules was developed. By restricting the Hamiltonian to the sole π electrons, this algorithm made it feasible to study the energetics of the pentagon-heptagon defect in a systematic way.