Single-walled carbon nanotube is a tubular nanomaterial having atomically smooth surface, and potentially serves as a highly efficient molecular conduit. Recent studies on utilizing carbon nanotube as a molecular conduit have largely focused on the transport through the interior of the material. The exterior of carbon nanotubes, however, has been overlooked by the carbon nanotube community. Mapping the distribution of charges and elements along individual nanotubes, we found that the exterior of single-walled carbon nanotubes is an ion channel that preferentially carries cations under an electric field. Repeating the measurements on a covalently functionalized nanotube suggests that cation-π interaction plays an important role in achieving such cation-preference during transport. The exterior transport of ions leads to formation of salt crystals that amplify Raman scattering of the nanotube itself and molecular species within the crystals.