**2.1 Resonance Raman of modified single- and double-walled carbon nanotubes**

The resonance Raman spectroscopy technique is sensitive to the electronic, structural, and vibrational properties of CNTs. Our group is using resonance Raman spectroscopy to characterize the interactions between nanotubes and different kinds of molecules [25–39], such as the conducting polyaniline (PANI), molecular magnets such as (NBu4)2[Cu(opba)], [MnCu(opba)]n chains, where opba = *ortho*-phenylenebis(oxamate), dyes such as phenosafranine (PS) and Nile Blue (NB), and CNTs doped with bromine or iodine.

For all samples investigated, the resonance Raman spectra are dominated by SWCNT or DWCNT bands at different laser excitation energies (*E*laser) [37–39]. The Raman spectra obtained with laser lines from 790 to 514.5 nm (*E*laser from 1.57 to 2.41 eV) are dominated by the characteristic bands from the SWCNTs or DWCNTs. This behavior is associated with the strong resonance Raman with the van Hove singularities of the single-walled nanotubes [6, 7]. The spectra can be divided into four groups of bands: (1) sharp bands from 120 to 350 cm<sup>−</sup><sup>1</sup> are assigned to the radial breathing modes (RBMs); (2) strong band in the frequency range from 1500 to 1650 cm<sup>−</sup><sup>1</sup> is attributed to stretching modes of carbon atoms (tangential *G* band); (3) the mode at ca. 1300–1350 cm<sup>−</sup><sup>1</sup> is forbidden for symmetry reasons and is related to the disorder-induced *D*-band feature; and (4) finally, the second-order

**99**

**Figure 6.**

*materials).*

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

), from 266 to 270 cm<sup>−</sup><sup>1</sup>

(Δ = +3 cm<sup>−</sup><sup>1</sup>

(Δ = +5 cm<sup>−</sup><sup>1</sup>

assigned to *<sup>E</sup>*<sup>33</sup>

1430 and 1474 cm<sup>−</sup><sup>1</sup>

160 cm<sup>−</sup><sup>1</sup>

*S*

(Δ = +5 cm<sup>−</sup><sup>1</sup>

the heterobimetallic complex.

*G'* band (or 2D band), which is the highly dispersive harmonic of the *D-*band

As a consequence of the intense CNT bands, the analysis remains in the comparison of standard CNTs before and after any chemical change. For instance, **Figure 6** exhibits the Raman spectra, in the RBM region, obtained for the composite between the SWNTs and [MnCu(opba)]n chains. At higher *E*laser (2.33 eV) or 532.0 nm, it was observed many changes in the RBM bands, shifts from 254 to 257 cm<sup>−</sup><sup>1</sup>

(Δ = +4 cm<sup>−</sup><sup>1</sup>

At *E*laser = 2.33 eV also, semiconducting tubes interact with the heterobimetallic polymer, contrarily to that observed for lower *E*laser energies. Hence, the RBM data suggest that the heterobimetallic polymer interacts mainly with metallic tubes independently of the diameter of the tube and excitation energy, however, for semiconducting tubes, solely for tubes with diameter higher than ca. 1.47 nm (**Figure 5**). In certain circumstances, it is possible to use UV laser line, and some bands from the metallic complex emerge in the spectra. For instance, the resonance Raman spectra, at higher *E*laser (3.82 eV), of SWCNTs-(NBu4)2[Cu(opba)] samples show bands at

observed for the other laser lines. In **Figure 6**, it is also observed that RBM bands

the benzene-like ring in the molecular structure of the [Cu(opba)]2<sup>−</sup> anion [34]. The CNT families are assigned by using the Kataura plot (**Figure 7a**). The Kataura plot is the result of calculated energy separations between the van Hove singularities [*E*ii(*d*t)] for SWCNTs obtained from tight-binding calculations [15, 45]. For instance, according to the Kataura plot, at 514.5 nm (*E*laser = 2.41 eV) or 532.0 nm (*E*laser = 2.33 eV), the DWCNTs in resonance have inner tube metallic and outer tube semiconducting. The Br2 doping was reported for SWCNTs [47, 48] and for DWCNTs [49, 50]. A very large Raman bands upshifted at 514.5 nm (2.41 eV)

*Schematic representation of the selective interaction scheme between CNTs and [MnCu(opba)]n chains formed from the reaction of Cu(opba)2– and Mn2+ ions. The wrapping is selective for metallic tubes and for semiconducting with diameter higher than 1.47 nm (the HR-TEM, high resolution transmission microscopy, image of the nanocomposite is also given in the figure). Resonance Raman spectra (RBM band region) excited by laser line at 532.0 nm (2.33 eV) for powder samples 1 (DWCNTs) and 2 ([MnCu(opba)]n-SWCNT* 

) and from 143 to 149 cm<sup>−</sup><sup>1</sup>

*M*

family also have changed their intensities and frequencies from 155 to

. These bands can be assigned to the vibrational modes related to

(Δ = +6 cm<sup>−</sup><sup>1</sup>

.

), and from 276 to 281 cm<sup>−</sup><sup>1</sup>

family with minor diameter than

) in the presence of

frequency, and it is observed here from ca. 2600 to 2700 cm<sup>−</sup><sup>1</sup>

) were seen for tubes assigned to *<sup>E</sup>*<sup>11</sup>

*Modern Spectroscopic Techniques and Applications*

sample (1 μm approximately or smaller). The high lateral resolution and depth of field (the order of a few micrometers) are very useful for the study of multilayered

**2.1 Resonance Raman of modified single- and double-walled carbon nanotubes**

The resonance Raman spectroscopy technique is sensitive to the electronic, structural, and vibrational properties of CNTs. Our group is using resonance Raman spectroscopy to characterize the interactions between nanotubes and different kinds of molecules [25–39], such as the conducting polyaniline (PANI), molecular magnets such as (NBu4)2[Cu(opba)], [MnCu(opba)]n chains, where opba = *ortho*-phenylenebis(oxamate), dyes such as phenosafranine (PS) and Nile

For all samples investigated, the resonance Raman spectra are dominated by SWCNT or DWCNT bands at different laser excitation energies (*E*laser) [37–39]. The Raman spectra obtained with laser lines from 790 to 514.5 nm (*E*laser from 1.57 to 2.41 eV) are dominated by the characteristic bands from the SWCNTs or DWCNTs. This behavior is associated with the strong resonance Raman with the van Hove singularities of the single-walled nanotubes [6, 7]. The spectra can be divided into

radial breathing modes (RBMs); (2) strong band in the frequency range from 1500

related to the disorder-induced *D*-band feature; and (4) finally, the second-order

is attributed to stretching modes of carbon atoms (tangential *G* band);

is forbidden for symmetry reasons and is

are assigned to the

polymeric thin films or other complex materials [42–44].

Blue (NB), and CNTs doped with bromine or iodine.

four groups of bands: (1) sharp bands from 120 to 350 cm<sup>−</sup><sup>1</sup>

**2. Results and discussion**

*Conventional Raman microscope.*

**Figure 5.**

**98**

to 1650 cm<sup>−</sup><sup>1</sup>

(3) the mode at ca. 1300–1350 cm<sup>−</sup><sup>1</sup>

*G'* band (or 2D band), which is the highly dispersive harmonic of the *D-*band frequency, and it is observed here from ca. 2600 to 2700 cm<sup>−</sup><sup>1</sup> .

As a consequence of the intense CNT bands, the analysis remains in the comparison of standard CNTs before and after any chemical change. For instance, **Figure 6** exhibits the Raman spectra, in the RBM region, obtained for the composite between the SWNTs and [MnCu(opba)]n chains. At higher *E*laser (2.33 eV) or 532.0 nm, it was observed many changes in the RBM bands, shifts from 254 to 257 cm<sup>−</sup><sup>1</sup> (Δ = +3 cm<sup>−</sup><sup>1</sup> ), from 266 to 270 cm<sup>−</sup><sup>1</sup> (Δ = +4 cm<sup>−</sup><sup>1</sup> ), and from 276 to 281 cm<sup>−</sup><sup>1</sup> (Δ = +5 cm<sup>−</sup><sup>1</sup> ) were seen for tubes assigned to *<sup>E</sup>*<sup>11</sup> *M* family with minor diameter than observed for the other laser lines. In **Figure 6**, it is also observed that RBM bands assigned to *<sup>E</sup>*<sup>33</sup> *S* family also have changed their intensities and frequencies from 155 to 160 cm<sup>−</sup><sup>1</sup> (Δ = +5 cm<sup>−</sup><sup>1</sup> ) and from 143 to 149 cm<sup>−</sup><sup>1</sup> (Δ = +6 cm<sup>−</sup><sup>1</sup> ) in the presence of the heterobimetallic complex.

At *E*laser = 2.33 eV also, semiconducting tubes interact with the heterobimetallic polymer, contrarily to that observed for lower *E*laser energies. Hence, the RBM data suggest that the heterobimetallic polymer interacts mainly with metallic tubes independently of the diameter of the tube and excitation energy, however, for semiconducting tubes, solely for tubes with diameter higher than ca. 1.47 nm (**Figure 5**). In certain circumstances, it is possible to use UV laser line, and some bands from the metallic complex emerge in the spectra. For instance, the resonance Raman spectra, at higher *E*laser (3.82 eV), of SWCNTs-(NBu4)2[Cu(opba)] samples show bands at 1430 and 1474 cm<sup>−</sup><sup>1</sup> . These bands can be assigned to the vibrational modes related to the benzene-like ring in the molecular structure of the [Cu(opba)]2<sup>−</sup> anion [34].

The CNT families are assigned by using the Kataura plot (**Figure 7a**). The Kataura plot is the result of calculated energy separations between the van Hove singularities [*E*ii(*d*t)] for SWCNTs obtained from tight-binding calculations [15, 45]. For instance, according to the Kataura plot, at 514.5 nm (*E*laser = 2.41 eV) or 532.0 nm (*E*laser = 2.33 eV), the DWCNTs in resonance have inner tube metallic and outer tube semiconducting. The Br2 doping was reported for SWCNTs [47, 48] and for DWCNTs [49, 50]. A very large Raman bands upshifted at 514.5 nm (2.41 eV)

#### **Figure 6.**

*Schematic representation of the selective interaction scheme between CNTs and [MnCu(opba)]n chains formed from the reaction of Cu(opba)2– and Mn2+ ions. The wrapping is selective for metallic tubes and for semiconducting with diameter higher than 1.47 nm (the HR-TEM, high resolution transmission microscopy, image of the nanocomposite is also given in the figure). Resonance Raman spectra (RBM band region) excited by laser line at 532.0 nm (2.33 eV) for powder samples 1 (DWCNTs) and 2 ([MnCu(opba)]n-SWCNT materials).*

**Figure 7.**

*(a) The Kataura plot at RBM region is based on the extended tight binding model for SWCNTs [15, 45, 46]. The horizontal lines show the laser lines used in the experiments. The numbers indicate the 2n+m families. Red circles correspond to metallic white blue to semiconducting tubes. The diameters of the tubes can be calculated by using the expression νRBM = 233/dt + 14 [46]. (b) Resonance Raman spectra at 532.0 nm (2.33 eV) of DWCNTs before and after doping (black and red lines, respectively) with 31 wt% of Br2. For illustrative purpose, the schematic representation of the exohedral doping is also given in the figure.*

laser line in both the RBM and G<sup>+</sup> band for brominated SWCNTs are observed. Indeed, the doping induces a Fermi level depression of 1.2–1.4 eV in the DWCNTs after bromination, mainly in the outer tubes. The Br–Br molecular vibration is reduced to ca. 233 cm<sup>−</sup><sup>1</sup> in comparison with molecular bromine (323 cm<sup>−</sup><sup>1</sup> ) [37–39], as a consequence to the charge transfer with the carbon tubes.

The use of a large variety of laser lines [37–39] permits to monitor differences in the charge transfer behavior to the inner and outer tubes from the adsorbed bromine or iodine. **Figure 7b** shows that the Br2 molecules are interacting with the outer tube surface of the DWCNTs and that the adsorbed bromine molecules act as an electron acceptor. In addition, it is observed that metallic tubes are extremely sensitive to doping process. In fact, the presence of Br2 molecules changes the Raman spectra of the metallic tubes even when they are in the inner configuration surrounded by semiconducting outer tubes of DWCNTs.

The differences between metallic and semiconducting tubes can also be analyzed in the other regions of the Raman spectra of CNTs. For instance, the presence of the lower frequency component of the *G-*band spectra is associated with the

**101**

semiconducting *G<sup>−</sup>*

*red and yellow regions.*

**Figure 8.**

sheet of sp2

G band (ca. 1582 cm<sup>−</sup><sup>1</sup>

the tubes and the graphene layers.

**2.2 Raman imaging of commercial sample of graphene**

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

metallic tubes. This is the result of the strong electron-phonon coupling observed in metallic nanotubes, and it gives rise to Kohn anomalies in the phonon dispersions [51, 52]. Hence, the influence of the metallic tubes can easily be distinguished from

*Resonance Raman imaging of commercial samples of graphene. The images on left are from the top: optical image obtained with a 50× lens, the Raman imaging using the G band as intensity marker (green image), and the Raman imaging using the D band as intensity marker (blue image). In addition, the Raman imaging obtained from D/G ratio is also given (red-yellow image) and the corresponding resonance Raman spectra from* 

[53, 54]. The changes in relative intensities, dispersion, and linewidth of the G' (2D) band can also be used as a probe to extract information about interactions between

More recently, our group is dedicated to prepare modified graphene samples with a myriad of molecules. However, due to the limitations of chemical procedures for the preparation of chemically modified graphene, the resulting samples are very inhomogeneous. Graphene consists in a two-dimensional single-layer

and very high carrier mobility and thermal conductivity. However, the interesting properties exhibited by graphene are only observed for graphene films that contain only one or a few graphene layers [3, 4]. It is optically difficult to observe low number of layers, but single-, double-, and multilayer graphenes can be differentiated by their Raman fingerprints. The graphene Raman spectrum is dominated by

) and 2D band (ca. 2685 cm<sup>−</sup><sup>1</sup>

used Raman to characterize samples consisting of varying number of graphene layers and found that the 2D band width and shape modify with increasing the number of layers. Therefore, the spectral shape of the 2D band is representative of the number of graphene layers and can be used to determine that number. By using

feature, which shows a Breit-Wigner-Fano (BWF) line shape

hybridized carbon atoms having excellent electron transport properties

). Ferrari and coworkers [55]

*Raman Spectroscopy and Imaging of Carbon Allotropes DOI: http://dx.doi.org/10.5772/intechopen.90867*

#### **Figure 8.**

*Modern Spectroscopic Techniques and Applications*

**100**

laser line in both the RBM and G<sup>+</sup>

reduced to ca. 233 cm<sup>−</sup><sup>1</sup>

**Figure 7.**

band for brominated SWCNTs are observed.

) [37–39],

Indeed, the doping induces a Fermi level depression of 1.2–1.4 eV in the DWCNTs after bromination, mainly in the outer tubes. The Br–Br molecular vibration is

*(a) The Kataura plot at RBM region is based on the extended tight binding model for SWCNTs [15, 45, 46]. The horizontal lines show the laser lines used in the experiments. The numbers indicate the 2n+m families. Red circles correspond to metallic white blue to semiconducting tubes. The diameters of the tubes can be calculated by using the expression νRBM = 233/dt + 14 [46]. (b) Resonance Raman spectra at 532.0 nm (2.33 eV) of DWCNTs before and after doping (black and red lines, respectively) with 31 wt% of Br2. For illustrative* 

The use of a large variety of laser lines [37–39] permits to monitor differences in the charge transfer behavior to the inner and outer tubes from the adsorbed bromine or iodine. **Figure 7b** shows that the Br2 molecules are interacting with the outer tube surface of the DWCNTs and that the adsorbed bromine molecules act as an electron acceptor. In addition, it is observed that metallic tubes are extremely sensitive to doping process. In fact, the presence of Br2 molecules changes the Raman spectra of the metallic tubes even when they are in the inner configuration

The differences between metallic and semiconducting tubes can also be analyzed in the other regions of the Raman spectra of CNTs. For instance, the presence of the lower frequency component of the *G-*band spectra is associated with the

as a consequence to the charge transfer with the carbon tubes.

*purpose, the schematic representation of the exohedral doping is also given in the figure.*

surrounded by semiconducting outer tubes of DWCNTs.

in comparison with molecular bromine (323 cm<sup>−</sup><sup>1</sup>

*Resonance Raman imaging of commercial samples of graphene. The images on left are from the top: optical image obtained with a 50× lens, the Raman imaging using the G band as intensity marker (green image), and the Raman imaging using the D band as intensity marker (blue image). In addition, the Raman imaging obtained from D/G ratio is also given (red-yellow image) and the corresponding resonance Raman spectra from red and yellow regions.*

metallic tubes. This is the result of the strong electron-phonon coupling observed in metallic nanotubes, and it gives rise to Kohn anomalies in the phonon dispersions [51, 52]. Hence, the influence of the metallic tubes can easily be distinguished from semiconducting *G<sup>−</sup>* feature, which shows a Breit-Wigner-Fano (BWF) line shape [53, 54]. The changes in relative intensities, dispersion, and linewidth of the G' (2D) band can also be used as a probe to extract information about interactions between the tubes and the graphene layers.

#### **2.2 Raman imaging of commercial sample of graphene**

More recently, our group is dedicated to prepare modified graphene samples with a myriad of molecules. However, due to the limitations of chemical procedures for the preparation of chemically modified graphene, the resulting samples are very inhomogeneous. Graphene consists in a two-dimensional single-layer sheet of sp2 hybridized carbon atoms having excellent electron transport properties and very high carrier mobility and thermal conductivity. However, the interesting properties exhibited by graphene are only observed for graphene films that contain only one or a few graphene layers [3, 4]. It is optically difficult to observe low number of layers, but single-, double-, and multilayer graphenes can be differentiated by their Raman fingerprints. The graphene Raman spectrum is dominated by G band (ca. 1582 cm<sup>−</sup><sup>1</sup> ) and 2D band (ca. 2685 cm<sup>−</sup><sup>1</sup> ). Ferrari and coworkers [55] used Raman to characterize samples consisting of varying number of graphene layers and found that the 2D band width and shape modify with increasing the number of layers. Therefore, the spectral shape of the 2D band is representative of the number of graphene layers and can be used to determine that number. By using

Raman imaging has been possible to analyze a large area of the graphene samples in order to search irregularities. For example, **Figure 8** shows the Raman imaging obtained from a commercial sample of graphene (Sigma-Aldrich). The D- and G-band intensities were used as probe to see the differences in a large area of the sample.

The D and G images can be combined to form an image from the D/G ratio of intensities (red-yellow image). The Raman spectra of two spots of the sample exemplify the point; yellow regions have more intense D bands than red regions.
