Low-frequency Raman study of hollow multiwalled nanotubes grown by Fe-catalyzed chemical vapor deposition

In this work, it is shown that some Raman-active modes may be detected, below 500 cm-1, in the spectrum of nanotubes synthesized by iron catalyzed chemical vapor deposition. By comparatively discussing results of Raman, high-resolution transmission electron microscopy, and thermogravimetric analyses, demonstration is given that these spectral features originate from scattering by nanoparticles of iron catalyst encapsulated within the tubes under nonstationary growth regime. Their intensity progressively weakens with increasing carbon supply rate until bands disappear as stationary conditions are reached.