K and C points of the graphite Brillouin zone, respectively [30–33]. Also
second-order Raman spectra have been observed in MWCNTs and SWCNTs
resulting in a G’-band, which is basically the overtone of the D-band. Some other
overtone bands have also been observed in MWCNTs and SWCNTs. The G’-band
appears also in highly ordered graphite without defects. While elastic phonon
scattering results in the D-band, inelastic phonon scattering is responsible for the
G’-band. The metallic or semiconducting nature of CNTs is decided by the chirality
of the individual tube, and the chirality followed by the structure determines the
electron-phonon coupling [34, 35]. Therefore, the Raman spectra help to learn more
about the conductivity of CNTs by analysis of distinguishable G-band spectral
features. Also the diameter influences the Raman spectra of the CNTs. Some studies
[20, 21] have shown that the G-band line shape and the D-band wavenumber
positions depend on the diameter of the tubes.
In a more recent study [36], a relationship was proposed for the calculation of the
apparent Young’s Modulus, which is also called modulus of elasticity defining the
tensile elasticity mathematically. The estimate is based on the crystallinity of the
CNTs as described by the crystallinity parameter I D /I G . The latter can be derived
from the Raman line intensities of the D- and G-bands. The G-band is essentially
nondispersive, while the D-band wavenumber and intensity depend on the excitation wavelengths. Heise et al. presented a series of studies [37, 38] on Raman
investigation of carbonaceous materials, i.e., SWCNTs, MWCNTs, graphitized
porous carbon, and graphite, using different excitation wavelengths. In those
studies, Lorentzian lineshapes were used only for band deconvolution. In their
recent study [38], they found that the I D /I G ratio can be used for the measurement of
degree of alignment for CNTs materials, which was well supported by SEM results.
The authors also performed a dispersion study on both D- and G’-bands of the
mentioned carbonaceous materials taking four different excitation wavelengths,
namely 488, 532, 785, and 1064 nm, into account. The dispersion of D- and G’bands evaluated from the wavenumber positions obtained through line shape
analysis of the experimental profiles turned out to be 52.7 ± 1.3 and
109.9 ± 4.0 cm
−1 /eV, respectively. However, the G-band position was only
slightly affected under the different excitation wavelengths. In CNTs, the
wavenumber features between 1500 and 1600 cm
−1 are associated with the tangential vibrational modes of the CNTs. The multiple nature of G-band is only seen
in CNTs, and it has not been observed in any sp
2 -bonded carbon materials [34]. The
unique optical properties of CNTs are observed due to the quantum confinement of
their electronic states in the two directions perpendicular to the nanotube axis.
When the energy of incident photons matches with the energy of an optical transition, the intensity of the Raman signal gets enhanced, known as resonance Raman
effect [39]. The CNTs with different diameter and chirality have different values of
resonance energy.
In view of the above discussion, we have performed a more extended comparative resonance Raman study of CNT materials such as graphite, SWCNTs, and
MWCNTs with different excitation wavelengths. In the following, the results of our
analysis of the experimental Raman spectral data will be discussed. The spectral
4 Material Analysis Using Raman Spectroscopy
125
second-order Raman spectra have been observed in MWCNTs and SWCNTs
resulting in a G’-band, which is basically the overtone of the D-band. Some other
overtone bands have also been observed in MWCNTs and SWCNTs. The G’-band
appears also in highly ordered graphite without defects. While elastic phonon
scattering results in the D-band, inelastic phonon scattering is responsible for the
G’-band. The metallic or semiconducting nature of CNTs is decided by the chirality
of the individual tube, and the chirality followed by the structure determines the
electron-phonon coupling [34, 35]. Therefore, the Raman spectra help to learn more
about the conductivity of CNTs by analysis of distinguishable G-band spectral
features. Also the diameter influences the Raman spectra of the CNTs. Some studies
[20, 21] have shown that the G-band line shape and the D-band wavenumber
positions depend on the diameter of the tubes.
In a more recent study [36], a relationship was proposed for the calculation of the
apparent Young’s Modulus, which is also called modulus of elasticity defining the
tensile elasticity mathematically. The estimate is based on the crystallinity of the
CNTs as described by the crystallinity parameter I D /I G . The latter can be derived
from the Raman line intensities of the D- and G-bands. The G-band is essentially
nondispersive, while the D-band wavenumber and intensity depend on the excitation wavelengths. Heise et al. presented a series of studies [37, 38] on Raman
investigation of carbonaceous materials, i.e., SWCNTs, MWCNTs, graphitized
porous carbon, and graphite, using different excitation wavelengths. In those
studies, Lorentzian lineshapes were used only for band deconvolution. In their
recent study [38], they found that the I D /I G ratio can be used for the measurement of
degree of alignment for CNTs materials, which was well supported by SEM results.
The authors also performed a dispersion study on both D- and G’-bands of the
mentioned carbonaceous materials taking four different excitation wavelengths,
namely 488, 532, 785, and 1064 nm, into account. The dispersion of D- and G’bands evaluated from the wavenumber positions obtained through line shape
analysis of the experimental profiles turned out to be 52.7 ± 1.3 and
109.9 ± 4.0 cm
−1 /eV, respectively. However, the G-band position was only
slightly affected under the different excitation wavelengths. In CNTs, the
wavenumber features between 1500 and 1600 cm
−1 are associated with the tangential vibrational modes of the CNTs. The multiple nature of G-band is only seen
in CNTs, and it has not been observed in any sp
2 -bonded carbon materials [34]. The
unique optical properties of CNTs are observed due to the quantum confinement of
their electronic states in the two directions perpendicular to the nanotube axis.
When the energy of incident photons matches with the energy of an optical transition, the intensity of the Raman signal gets enhanced, known as resonance Raman
effect [39]. The CNTs with different diameter and chirality have different values of
resonance energy.
In view of the above discussion, we have performed a more extended comparative resonance Raman study of CNT materials such as graphite, SWCNTs, and
MWCNTs with different excitation wavelengths. In the following, the results of our
analysis of the experimental Raman spectral data will be discussed. The spectral
4 Material Analysis Using Raman Spectroscopy
125