20.3 Outline of Experimental Observations
375
to the spectral features [6]. In place of the peak maximum normalization crossing all
the spectra for the same specimen of different sizes, as depicted in Fig. 20.1 insets,
the peak area normalization is physically meaningful as this process minimizes the
experimental artifacts such as scattering due to surface roughness.
The Raman shifts are very sensitive to the feature size of the examined substance
at the nanometer scale because of the raised number ratio of the undercoordinated
atoms in the skin shells [36]. For the layered graphene instance [42], the D and 2D
bands undergo a redshift but the G band shifted from 1582 to 1587 cm
−1 when its
number-of-layer (n) is reduced from 20 to one [21, 43]. When the n is increased
from a few to multiple, the Raman peaks turn from the dominance of the monolayer
component to the dominance of the bulk graphite component [21]. The opposite
number-of-layer trends suggest that different yet unclear mechanisms govern the G
mode and the D/2D modes. One can also estimate the exact number of layers of a
graphene [44, 45] and the diameter of a single-walled CNT [46] as the frequency
of the radial breathing mode ω RBM is inversely and empirically proportional to the
thickness of the graphene and the CNT diameter.
The nature of the G and the D modes in carbon allotropes is conventionally interpreted as the resonant excitation of the π states and the long-range polarizability
of the unpaired π bonding electrons [47, 48]. Visible Raman spectral data on amorphous, disordered, and diamond-like carbon are classified in three stages showing the
factors that control the peak positions, peak intensities, and peak widths of the G and
the D modes. The Raman spectra depend on the configuration of the sp sites in the
sp
2 -bonded clusters. In cases where a fraction of sp
3 bonding is involved in the sp
2
clustering, such as in the as-deposited tetrahedral amorphous carbon (ta-C) or hydrogenated amorphous carbon (a-C:H) films, the visible Raman spectral parameters can
be used to derive the sp
3 fraction. However, the Raman modes and their shifts are
directly related to the oscillation of lattices in different geometries by perturbation
to the Hamiltonian.
From the dispersion of the Raman phonon frequencies and peak intensities with
excitation wavelength, Ferrari et al. [47, 48] derived the local bonding and structural
disorder of graphene. They found three basic features. Under visible light excitation,
graphene shows the D mode around 1350 cm
−1 (1332 in diamond [49]) and the G
mode around 1600 cm
−1 (1580 in graphite [50] and show thermal stiffening that
was attributed to the weakening of the electron-phonon coupling effect). Under UV
excitation, an extra T peak appears, which lies around 1060 cm
−1 for the H-free
carbons and around 980 cm
−1 for the hydrogenated carbons. The G peak shows
structural disorder being attributed to the stretching motion of sp
2 pairs. This G peak
disperses only in amorphous networks, with a dispersion rate proportional to the
degree of disorder. The dispersion of the D peak is strongest in the ordered carbon,
but it is weak for the amorphous carbon.
Figure 20.2 shows the D/2D and the G mode frequency evolution with the GNRs
thickness compared with bulk highly oriented pyrolytic graphite (HOPG) reference
[20, 21]. When the bulk graphite evolves into a monolayer GNR, the 2D peak
shifts downwardly from 2714 to 2678 cm
−1 and the D peak changes from 1368
375
to the spectral features [6]. In place of the peak maximum normalization crossing all
the spectra for the same specimen of different sizes, as depicted in Fig. 20.1 insets,
the peak area normalization is physically meaningful as this process minimizes the
experimental artifacts such as scattering due to surface roughness.
The Raman shifts are very sensitive to the feature size of the examined substance
at the nanometer scale because of the raised number ratio of the undercoordinated
atoms in the skin shells [36]. For the layered graphene instance [42], the D and 2D
bands undergo a redshift but the G band shifted from 1582 to 1587 cm
−1 when its
number-of-layer (n) is reduced from 20 to one [21, 43]. When the n is increased
from a few to multiple, the Raman peaks turn from the dominance of the monolayer
component to the dominance of the bulk graphite component [21]. The opposite
number-of-layer trends suggest that different yet unclear mechanisms govern the G
mode and the D/2D modes. One can also estimate the exact number of layers of a
graphene [44, 45] and the diameter of a single-walled CNT [46] as the frequency
of the radial breathing mode ω RBM is inversely and empirically proportional to the
thickness of the graphene and the CNT diameter.
The nature of the G and the D modes in carbon allotropes is conventionally interpreted as the resonant excitation of the π states and the long-range polarizability
of the unpaired π bonding electrons [47, 48]. Visible Raman spectral data on amorphous, disordered, and diamond-like carbon are classified in three stages showing the
factors that control the peak positions, peak intensities, and peak widths of the G and
the D modes. The Raman spectra depend on the configuration of the sp sites in the
sp
2 -bonded clusters. In cases where a fraction of sp
3 bonding is involved in the sp
2
clustering, such as in the as-deposited tetrahedral amorphous carbon (ta-C) or hydrogenated amorphous carbon (a-C:H) films, the visible Raman spectral parameters can
be used to derive the sp
3 fraction. However, the Raman modes and their shifts are
directly related to the oscillation of lattices in different geometries by perturbation
to the Hamiltonian.
From the dispersion of the Raman phonon frequencies and peak intensities with
excitation wavelength, Ferrari et al. [47, 48] derived the local bonding and structural
disorder of graphene. They found three basic features. Under visible light excitation,
graphene shows the D mode around 1350 cm
−1 (1332 in diamond [49]) and the G
mode around 1600 cm
−1 (1580 in graphite [50] and show thermal stiffening that
was attributed to the weakening of the electron-phonon coupling effect). Under UV
excitation, an extra T peak appears, which lies around 1060 cm
−1 for the H-free
carbons and around 980 cm
−1 for the hydrogenated carbons. The G peak shows
structural disorder being attributed to the stretching motion of sp
2 pairs. This G peak
disperses only in amorphous networks, with a dispersion rate proportional to the
degree of disorder. The dispersion of the D peak is strongest in the ordered carbon,
but it is weak for the amorphous carbon.
Figure 20.2 shows the D/2D and the G mode frequency evolution with the GNRs
thickness compared with bulk highly oriented pyrolytic graphite (HOPG) reference
[20, 21]. When the bulk graphite evolves into a monolayer GNR, the 2D peak
shifts downwardly from 2714 to 2678 cm
−1 and the D peak changes from 1368