Regularly coiled carbon nanotubes

Regularly coiled carbon nanotubes, their structure and formation mechanism are puzzling questions since many years. The first models were based on the very regular incorporation of a small fraction (of the order of 10%) of nonhexagonal (n-Hx) rings: (pentagons and heptagons) in a perfect hexagonal (Hx) lattice. It is difficult to understand by which mechanism takes place such a regular incorporation of isolated n-Hx rings. In the present work a new family of Haeckelite nanotubes is generated in a systematic way by rolling up a two-dimensional three-fold coordinated carbon network composed of pentagon-heptagon pairs and hexagons in proportion 2:3. In this model the n-Hx rings are treated like regular building blocks of the structure. Cohesion energy calculation shows that the stability of the generated 3D Haeckelite structures falls between that of straight carbon nanotubes and that of C-60. Electronic density of states of the Haeckelite computed with a tight-binding Hamiltonian that includes the G-mu orbitals only shows that the structures are semiconductor. The relation of the structures with experimental observations is discussed.