An isotope-doped carbon nanotube (40) includes a plurality of first carbon nanotube segments (402) having carbon-12 isotopes and a plurality of second carbon nanotube segments (404) having carbon-13 isotopes. The first and second carbon nanotube segments are alternately arranged along a longitudinal direction of the carbon nanotube. Three preferred methods employ different isotope sources to form isotope-doped carbon nanotubes. In a chemical vapor deposition method, different isotope source gases are alternately introduced. In an arc discharge method, a power source is alternately switched between different isotope anodes. In a laser ablation method, a laser is alternately focused on different isotope targets. In addition, an apparatus for implementing the preferred methods is provided.