Excitation energy dependence of Raman bands in multiwalled carbon nanotubes

Vertically aligned multiwalled carbon nanotubes (MWCNTs) were grown on 1- and 3-nm cobalt (Co) films, at various growth times by microwave plasma enhanced chemical vapor deposition technique and their microstructural properties were analyzed with the help of Raman spectra that were obtained from different sources of laser excitation energies (E-L: 2.41, 1.96 and 1.58 eV). The variation of D and G band positions in MWCNTs grown on 1- and 3-nm Co films follows a similar behavior, and an anomalous behavior was observed in the E-L dependence of the D'-band wavenumber. In the second-order spectra, the G' band varied strongly according to structure with the laser excitation energy (E-L). The I-D/I-G ratio decreased with the increase of E-L for all MWCNTs; however, for a fixed E-L, the I-D/I-G dispersion is higher at lower E-L. The crystallite sizes were estimated using IDIG and E-L. We have shown that, for all MWCNTs, I-D/I-G ratio is inversely proportional to E-L(4). Copyright (C) 2010 John Wiley & Sons, Ltd.