Ltd, Newnham, England) for NIR excitation (1064 nm) and a GaAs detector. The
following experimental conditions were chosen: Spectral resolution of 16 cm
−1
with 1000 interferogram scans coadded at 100 mW of laser power. Spectra of
reasonable signal-to-noise ratio could only be obtained for graphite and SWCNT
samples. For recording spectra of other materials, a rotating sample holder was
employed to reduce excessive sample heating from laser irradiation.
The nonlinear band fitting was done with either OPUS software from Bruker
Optics, Ettlingen (Germany) (Lorentzian bandshapes only) or ORIGIN version 6.0
from Microcal Software, Inc., Northampton, MA (including BWF functions; for
their definition, see below) using appropriate starting function estimates and the
Levenberg–Marquardt algorithm.
4.3 Results and Discussion
The Raman spectra of graphite, SWCNTs (from CarboLex and Aldrich), and
MWCNT materials between 1800 and 1200 cm
−1 , recorded using the three laser
excitation wavelengths of 532, 785, and 1064 nm, are presented in panels (a), (b),
and (c) of Fig. 4.1, respectively. The Raman spectra along with the fitted peaks are
presented in respective Figs. 4.2, 4.3, and 4.4. The spectral parameters obtained
from the deconvolution of the G-bands for the excitation wavelengths 532, 785, and
1064 nm are listed in Tables 4.1, 4.2, and 4.3, respectively.
The experimentally recorded G-bands of both types of SWCNTs (CarboLex)
and SWCNTs (Aldrich) have a noticeable asymmetry for all three excitation
wavelengths, and, therefore, the G-bands were fitted assuming the contribution of
three or four peaks. In case of the G-band of SWCNT (CarboLex) and SWCNTs
(Aldrich) recorded with 532 nm excitation wavelength, the best fit results were
obtained using two different line shape functions, BWF for the band component on
the low wavenumber side and Lorentzian functions for the other two bands.
The BWF line shape function has been defined as [40–42]:
I x
ð Þ ¼ I 0
1 þ x À x BWF
ð
Þ =qC
½
2
1 þ x À x BWF
ð
Þ =C
½
2
ð4:1Þ
The experimentally observed bands along with the fitted individual band components are presented in Fig. 4.2a, b. However, for the G-band of MWCNTs, the
best fit results were obtained by fitting the band with two Lorentzian bands, and the
experimental as well as fitted spectra are presented in Fig. 4.2c. The low
wavenumber components of the G-bands in the Raman spectra of SWCNTs
(CarboLex) and SWCNTs (Aldrich) recorded using 532 nm excitation wavelength
can be found at approximately 1566 and 1572 cm
−1 , respectively. The value of the
fitting parameter, 1/q, was found to be around −0.12 for both types of SWCNT
G-bands. The wavenumber positions, symmetry, C (half of the full width at half
4 Material Analysis Using Raman Spectroscopy
127
following experimental conditions were chosen: Spectral resolution of 16 cm
−1
with 1000 interferogram scans coadded at 100 mW of laser power. Spectra of
reasonable signal-to-noise ratio could only be obtained for graphite and SWCNT
samples. For recording spectra of other materials, a rotating sample holder was
employed to reduce excessive sample heating from laser irradiation.
The nonlinear band fitting was done with either OPUS software from Bruker
Optics, Ettlingen (Germany) (Lorentzian bandshapes only) or ORIGIN version 6.0
from Microcal Software, Inc., Northampton, MA (including BWF functions; for
their definition, see below) using appropriate starting function estimates and the
Levenberg–Marquardt algorithm.
4.3 Results and Discussion
The Raman spectra of graphite, SWCNTs (from CarboLex and Aldrich), and
MWCNT materials between 1800 and 1200 cm
−1 , recorded using the three laser
excitation wavelengths of 532, 785, and 1064 nm, are presented in panels (a), (b),
and (c) of Fig. 4.1, respectively. The Raman spectra along with the fitted peaks are
presented in respective Figs. 4.2, 4.3, and 4.4. The spectral parameters obtained
from the deconvolution of the G-bands for the excitation wavelengths 532, 785, and
1064 nm are listed in Tables 4.1, 4.2, and 4.3, respectively.
The experimentally recorded G-bands of both types of SWCNTs (CarboLex)
and SWCNTs (Aldrich) have a noticeable asymmetry for all three excitation
wavelengths, and, therefore, the G-bands were fitted assuming the contribution of
three or four peaks. In case of the G-band of SWCNT (CarboLex) and SWCNTs
(Aldrich) recorded with 532 nm excitation wavelength, the best fit results were
obtained using two different line shape functions, BWF for the band component on
the low wavenumber side and Lorentzian functions for the other two bands.
The BWF line shape function has been defined as [40–42]:
I x
ð Þ ¼ I 0
1 þ x À x BWF
ð
Þ =qC
½
2
1 þ x À x BWF
ð
Þ =C
½
2
ð4:1Þ
The experimentally observed bands along with the fitted individual band components are presented in Fig. 4.2a, b. However, for the G-band of MWCNTs, the
best fit results were obtained by fitting the band with two Lorentzian bands, and the
experimental as well as fitted spectra are presented in Fig. 4.2c. The low
wavenumber components of the G-bands in the Raman spectra of SWCNTs
(CarboLex) and SWCNTs (Aldrich) recorded using 532 nm excitation wavelength
can be found at approximately 1566 and 1572 cm
−1 , respectively. The value of the
fitting parameter, 1/q, was found to be around −0.12 for both types of SWCNT
G-bands. The wavenumber positions, symmetry, C (half of the full width at half
4 Material Analysis Using Raman Spectroscopy
127