  Carbon nanotubes are promising building blocks for various nanoelectronic
components. A highly desirable geometry for such applications is a coil.
However, coiled nanotube structures reported so far were inherently defective
or had no free ends accessible for contacting. Here we demonstrate the
spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils
of up to more than 70 turns with identical diameter and chirality, and free
ends. We characterize the structure, formation mechanism, and electrical
properties of these coils by different microscopies, molecular dynamics
simulations, Raman spectroscopy, and electrical and magnetic measurements. The
coils are highly conductive, as expected for defect-free carbon nanotubes, but
adjacent nanotube segments in the coil are more highly coupled than in regular
bundles of single-wall carbon nanotubes, owing to their perfect crystal
momentum matching, which enables tunneling between the turns. Although this
behavior does not yet enable the performance of these nanotube coils as
inductive devices, it does point a clear path for their realization. Hence,
this study represents a major step toward the production of many different
nanotube coil devices, including inductors, electromagnets, transformers, and
dynamos.
