Raman scattering study for heat-treated carbon nanotubes: The origin of ≈1855 cm−1 Raman band

Densely packed multiwall carbon nanotubes, with the innermost tube diameter of ≈0.6 nm, frequently give an unassigned Raman band at ≈1855 cm−1. High temperature heat treatment in anaerobic condition at ≈1600 °C decreases the intensity for the ≈1855 cm−1 band. In addition, after heat treatment at 1550 °C, the ≈1855 cm−1 band appeared for the double-wall carbon nanotubes (≈0.6 nm for inner tube diameter), but not for the single-wall carbon nanotubes (≈1.3 nm diameter). We suggest the existence of chain-like carbon materials inside the original nanotube with ≈0.6 nm diameter. Moreover, these materials are synthesized at ≈1550 °C but decomposed at ≈1600 °C.