Helicity in ropes of chiral nanotubes: calculations and observation.

Even though isolated defect-free single-wall carbon nanotubes are straight, bundles of chiral single-wall carbon nanotubes are often helical according to our observations using high-resolution electron microscopy. The driving force for the formation of such helices is the energy gain associated with the optimum orientational alignment of neighboring nanotubes. Our total energy calculations allow us to analyze the torsional and bending stress components in helical nanotube ropes and specify under which conditions straight nanotube bundles gain energy upon forming a helix.