**7. Results and discussion**

Controlled volumes of pristine MWNT solutions were dropped on SERS active substrates, the SERS spectra acquired in different spatial positions, maintaining all the experimental parameters unchanged are reported in **Figure 5**.

The diameter of SWNTs can be calculated from the frequency of corresponding RBM, through the Bandow relation [13–15]. Since the RBM intensity strongly decreases when the tube diameter

**Figure 6.** (a) HR-SEM image of pristine MWNT film deposited on SERS substrate. (b) Magnified view of region with larger thickness layer (c) Magnified view of region with thinner layer. The leaning effect of nanopillars is evidenced.

**Figure 5.** SERS spectra of pristine MWNT dried film. The MWNT layer thickness is decreasing from spectrum (a) to (c).

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon…

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The SERS trace (a) has no significant differences in comparison of that reported in **Figure 1**, if one excepts the amplification of the signal whereas curves (b) and (c) show variations in some Raman line intensities, line shape and appearance of several sharp peaks, in the frequency range 400–1100 cm−1, characteristics of C60-like molecules [25].

We observed a gradual increase of the D band, which indicates an enhanced degree of disorder, with a concomitant modification of the G band profile, consisting in a peak splitting due to the increase of D' band at its high energy side. The peak at about 250 cm−1 in the SERS curves (b) and (c) is similar to that at 171 cm−1 in Raman spectrum of SWNTs, reported in **Figure 1**.

The RBMs are not often observed in MWNTs, its observation in SERS trace is consistent with the fact that for a molecule adsorbed on a nano-structured surface, the modes which involve atoms vibrating perpendicularly to the surface are more enhanced than the others [26]. However, the observed RBM width (45 cm−1) is larger than that measured in the SWNT Raman spectra [16, 17].

This finding can be explained considering that in a MWNT also the radial vibrations of outer shells contribute to this mode, producing a number of overlapping RBM peaks larger than those expected for SWNTs with the same distribution of diameter [18].

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon… http://dx.doi.org/10.5772/intechopen.74065 211

**6. Raman spectrometers**

210 Raman Spectroscopy

400–3000 cm−1, acquisition time 10 s).

eters unchanged are reported in **Figure 5**.

range 400–1100 cm−1, characteristics of C60-like molecules [25].

**7. Results and discussion**

Raman spectra [16, 17].

In the first steps of our works, we acquired SERS and Raman spectra using a Raman spectrometer BwTek (i-Raman 785.), equipped with a micro-positioning system for fine xyz adjustments and a video camera for sampling viewing. The system uses an air-cooled charge coupled device (CCD) detector. The 785 nm laser light, to guarantee the coupling with gold plasmons, was focused onto the sample using a 20× objective (corresponding to a laser beam diameter of 90 μm, as already mentioned). Samples were moved into position using the xyz translational stage. The Raman spectra were acquired in the wavelength range 789–1048 nm

Raman mapping was performed with a high-resolution micro-Raman spectrometer (Horiba Xplora) with 785 nm excitation wavelength. The Raman signal was collected through a 100× objective in the range 100–2000 cm−1 with accumulation time of 10 s per spectrum. Areas of the substrates up to 10 × 10 μm were scanned with an acquisition grid of 0.6 and 0.8 μm step size in x and y, respectively. All the spectra were obtained with 100 μW laser power, to avoid sample heating and damaging. The SERS maps were obtained analyzing the peak intensity of every spectrum. Raman spectra were also acquired with 532 nm excitation (range

Controlled volumes of pristine MWNT solutions were dropped on SERS active substrates, the SERS spectra acquired in different spatial positions, maintaining all the experimental param-

The SERS trace (a) has no significant differences in comparison of that reported in **Figure 1**, if one excepts the amplification of the signal whereas curves (b) and (c) show variations in some Raman line intensities, line shape and appearance of several sharp peaks, in the frequency

We observed a gradual increase of the D band, which indicates an enhanced degree of disorder, with a concomitant modification of the G band profile, consisting in a peak splitting due to the increase of D' band at its high energy side. The peak at about 250 cm−1 in the SERS curves (b) and (c) is similar to that at 171 cm−1 in Raman spectrum of SWNTs, reported in **Figure 1**.

The RBMs are not often observed in MWNTs, its observation in SERS trace is consistent with the fact that for a molecule adsorbed on a nano-structured surface, the modes which involve atoms vibrating perpendicularly to the surface are more enhanced than the others [26]. However, the observed RBM width (45 cm−1) is larger than that measured in the SWNT

This finding can be explained considering that in a MWNT also the radial vibrations of outer shells contribute to this mode, producing a number of overlapping RBM peaks larger than

those expected for SWNTs with the same distribution of diameter [18].

corresponding to Raman shifts of 75–3200 cm−1 (resolution better than 3 cm−1).

**Figure 5.** SERS spectra of pristine MWNT dried film. The MWNT layer thickness is decreasing from spectrum (a) to (c).

The diameter of SWNTs can be calculated from the frequency of corresponding RBM, through the Bandow relation [13–15]. Since the RBM intensity strongly decreases when the tube diameter

**Figure 6.** (a) HR-SEM image of pristine MWNT film deposited on SERS substrate. (b) Magnified view of region with larger thickness layer (c) Magnified view of region with thinner layer. The leaning effect of nanopillars is evidenced.

increases, we can roughly estimate the inner diameter of MWNTs from their RBMs in the SERS spectra by using the same rule. The RBM at 250 cm−1 corresponds to an inner tube diameter of 0.8 nm. These values are compatible with those reported in the data sheet of MWNTs from the manufacturer.

**Figure 6** presents a HRSEM view, taken at different magnification of pristine MWNT films. The MWNTs are tightly interlaced forming a web of filaments that are distributed across the SERS active surface, occupying the regions with highest Raman signal amplification as the space between the nano-pillars. However, the layer thickness is not uniform across the deposited patch and this is reflected in the observed differences between the SERS spectra reported in **Figure 5**.

In fact, these variations can be interpreted by considering the breaking of the nanotubes into species such as amorphous carbon, tubular fragments, and closed shell fullerenes. Such reactions are of chemical nature occurring at the nanotube metal substrate interface.

**Figure 7** illustrates the comparison between the Raman spectra of pristine and covalently functionalized MWNTs. The carboxyl functionalization was performed by a reflux in sulfuric/nitric acid (Nanolab, average diameter = 30 nm, length 1–5 μm, from now on denoted as COOH-MWNTs). This process leads to the concentration of COOH groups on the nanotube surface. By reacting the carboxylate CNTs with ethylene diamine, nanotubes with amide linkage and a primary amine group at the end of carbon chain are obtained (Nanolab, average diameter = 15 nm, length 1–5 μm, from now on denoted as NH2 -MWNTs). However, in the Raman spectra of covalently functionalized vibrational modes that can be directly related to molecular groups are not observed.

As a consequence, the use of Raman spectroscopy to study the structure of functionalized CNTs is routinely limited to the evaluation of the ID/IG ratio, since the interaction between carbon nanotube wall and functional groups is expected to produce a higher density of sp3

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon…

However, when a set of spectra is recorded from the same pristine CNTs sample, we frequently observed a variation in the ID/IG ratio, up to the 15% of its average value, which is sometimes apparently of the same order as that recorded after the chemical functionalization. Although a more efficient discrimination can be reached by applying the principal component analysis [30], the observation of Raman spectral changes induced by the chemical func-

In **Figure 8** are illustrated the different morphologies of functionalized MWNT-dried films.

of filaments. Differently, the drying of COOH-MWNT solution forms a very thin layer with

We acquired the SERS spectra from functionalized MWNT films in different spatial positions, maintaining all the experimental parameters unchanged obtaining reproducible spectra,

In addition to the signal enhancement, compared to conventional Raman spectra, the SERS traces show sharp peaks in wavenumber region ascribable to vibrations of molecular groups;



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hybridized carbon sites, with consequent increase of ID/IG ratio [27–29].

view of region (a); (d) magnified view of region (b) evidencing the formation of a dense layer.

tionalization is challenging.

shown in **Figure 9**.

Due to the small outer diameter, the NH2

**Figure 8.** HR-SEM images of (a) COOH-MWNT and (b) NH2

some groups of few isolated nano-tubes.

**Figure 7.** Raman spectra of MWNT mat. (a) Raman spectrum acquired from pristine MWNT film, ID/IG = 0.8; (b) Raman spectrum acquired from COOH-functionalized MWNTs, ID/IG = 1.2; (c) Raman spectrum acquired from NH2 functionalized MWNTs, ID/IG = 1.4.

increases, we can roughly estimate the inner diameter of MWNTs from their RBMs in the SERS spectra by using the same rule. The RBM at 250 cm−1 corresponds to an inner tube diameter of 0.8 nm. These values are compatible with those reported in the data sheet of MWNTs

**Figure 6** presents a HRSEM view, taken at different magnification of pristine MWNT films. The MWNTs are tightly interlaced forming a web of filaments that are distributed across the SERS active surface, occupying the regions with highest Raman signal amplification as the space between the nano-pillars. However, the layer thickness is not uniform across the deposited patch and this is reflected in the observed differences between the SERS spectra reported in **Figure 5**. In fact, these variations can be interpreted by considering the breaking of the nanotubes into species such as amorphous carbon, tubular fragments, and closed shell fullerenes. Such reac-

**Figure 7** illustrates the comparison between the Raman spectra of pristine and covalently functionalized MWNTs. The carboxyl functionalization was performed by a reflux in sulfuric/nitric acid (Nanolab, average diameter = 30 nm, length 1–5 μm, from now on denoted as COOH-MWNTs). This process leads to the concentration of COOH groups on the nanotube surface. By reacting the carboxylate CNTs with ethylene diamine, nanotubes with amide linkage and a primary amine group at the end of carbon chain are obtained (Nanolab, average

Raman spectra of covalently functionalized vibrational modes that can be directly related to

**Figure 7.** Raman spectra of MWNT mat. (a) Raman spectrum acquired from pristine MWNT film, ID/IG = 0.8; (b) Raman spectrum acquired from COOH-functionalized MWNTs, ID/IG = 1.2; (c) Raman spectrum acquired from NH2



tions are of chemical nature occurring at the nanotube metal substrate interface.

diameter = 15 nm, length 1–5 μm, from now on denoted as NH2

molecular groups are not observed.

functionalized MWNTs, ID/IG = 1.4.

from the manufacturer.

212 Raman Spectroscopy

**Figure 8.** HR-SEM images of (a) COOH-MWNT and (b) NH2 -MWNT films deposited on SERS substrate; (c) magnified view of region (a); (d) magnified view of region (b) evidencing the formation of a dense layer.

As a consequence, the use of Raman spectroscopy to study the structure of functionalized CNTs is routinely limited to the evaluation of the ID/IG ratio, since the interaction between carbon nanotube wall and functional groups is expected to produce a higher density of sp3 hybridized carbon sites, with consequent increase of ID/IG ratio [27–29].

However, when a set of spectra is recorded from the same pristine CNTs sample, we frequently observed a variation in the ID/IG ratio, up to the 15% of its average value, which is sometimes apparently of the same order as that recorded after the chemical functionalization. Although a more efficient discrimination can be reached by applying the principal component analysis [30], the observation of Raman spectral changes induced by the chemical functionalization is challenging.

In **Figure 8** are illustrated the different morphologies of functionalized MWNT-dried films. Due to the small outer diameter, the NH2 -MWNTs are tightly interlaced forming a dense web of filaments. Differently, the drying of COOH-MWNT solution forms a very thin layer with some groups of few isolated nano-tubes.

We acquired the SERS spectra from functionalized MWNT films in different spatial positions, maintaining all the experimental parameters unchanged obtaining reproducible spectra, shown in **Figure 9**.

In addition to the signal enhancement, compared to conventional Raman spectra, the SERS traces show sharp peaks in wavenumber region ascribable to vibrations of molecular groups;

structural defects. It is worth to note that because there is no shift between SERS bands and the corresponding ones in the Raman spectrum, the interaction between MWNTs and the

We can note also that in the SERS spectra of functionalized MWNTs, shoulders of G band at

the presence of the latter peak could be ascribed to the combination of N-H bend and C-N stretch, since these bands appear in both functionalized samples, they were assigned to disor-

rings and rings with other orders, whereas the higher frequency peak, was often recorded in amorphous hydrogenated carbon, due to the C─C bond strain caused by the formation of

Surprisingly, also in the SERS spectra of pristine graphene, we recorded a large number of sharp peaks belonging to molecular group vibrations, as illustrated in **Figure 10**: namely C─C═O bending (520–570 cm−1), C─O─C deformation, (850 cm−1), O─H (900–960 cm−1) and

Functional groups as ethers, carbonyls, carboxyls, and hydroxyls are likely retained in the graphene structure, as a consequence of chemical exfoliation process and the presence of oxygen and hydrogen atoms in few at percentage was confirmed by XPS measurements of

We can also note that close to the G band are present the peaks of disordered carbon already observed in functionalized MWNTs. The presence of disordered carbon peaks in the SERS

**Figure 10.** SERS spectra of pristine graphene. The spectra (a), (b), and (c) were acquired at different scanning positions

bonds. In fact, the peak at 1510 cm−1 was observed in carbon with sixfold

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon…


215

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1510 and 1544 cm−1 are clearly distinguishable. Although in the SERS trace of NH2

(1040 cm−1) bending, and C─C(O)─C stretching 1180–1270 cm−1) [31, 34].

active surface is physical.

dered carbon with sp3

manufacturer [35].

across the dried film.

CH2

C─H covalent bonds [6, 7, 9, 11, 33].

**Figure 9.** SERS spectra of pristine (a), COOH-MWNTs (b), and NH2 -MWNTs (c).

namely the C─O─C stretching (shoulder at around 1260 cm−1), CH3 deformation (1320 cm−1), C─N stretching (1240 cm−1, 1460 cm−1), and C═O stretching at 1730 cm−1 [31, 32]. The presence of these bands proves experimentally the linkage of desired molecular groups to the nanotube walls. It is worth to note that the C═O stretching appears in both covalently functionalized samples owing to the functionalization procedure, which introduces the linkage of amides after that of carboxylate.

In the SERS spectrum of COOH-MWNTs, the D′ band is not distinguishable as shoulder next to the G band, indicating that this sample has a lower amount of defects, as expected for carbon nanotubes with large diameter, which exhibit reduced tip curvature and in turn fewer structural defects. It is worth to note that because there is no shift between SERS bands and the corresponding ones in the Raman spectrum, the interaction between MWNTs and the active surface is physical.

We can note also that in the SERS spectra of functionalized MWNTs, shoulders of G band at 1510 and 1544 cm−1 are clearly distinguishable. Although in the SERS trace of NH2 -MWNTs, the presence of the latter peak could be ascribed to the combination of N-H bend and C-N stretch, since these bands appear in both functionalized samples, they were assigned to disordered carbon with sp3 bonds. In fact, the peak at 1510 cm−1 was observed in carbon with sixfold rings and rings with other orders, whereas the higher frequency peak, was often recorded in amorphous hydrogenated carbon, due to the C─C bond strain caused by the formation of C─H covalent bonds [6, 7, 9, 11, 33].

Surprisingly, also in the SERS spectra of pristine graphene, we recorded a large number of sharp peaks belonging to molecular group vibrations, as illustrated in **Figure 10**: namely C─C═O bending (520–570 cm−1), C─O─C deformation, (850 cm−1), O─H (900–960 cm−1) and CH2 (1040 cm−1) bending, and C─C(O)─C stretching 1180–1270 cm−1) [31, 34].

Functional groups as ethers, carbonyls, carboxyls, and hydroxyls are likely retained in the graphene structure, as a consequence of chemical exfoliation process and the presence of oxygen and hydrogen atoms in few at percentage was confirmed by XPS measurements of manufacturer [35].

We can also note that close to the G band are present the peaks of disordered carbon already observed in functionalized MWNTs. The presence of disordered carbon peaks in the SERS

**Figure 9.** SERS spectra of pristine (a), COOH-MWNTs (b), and NH2

after that of carboxylate.

214 Raman Spectroscopy

namely the C─O─C stretching (shoulder at around 1260 cm−1), CH3


C─N stretching (1240 cm−1, 1460 cm−1), and C═O stretching at 1730 cm−1 [31, 32]. The presence of these bands proves experimentally the linkage of desired molecular groups to the nanotube walls. It is worth to note that the C═O stretching appears in both covalently functionalized samples owing to the functionalization procedure, which introduces the linkage of amides

In the SERS spectrum of COOH-MWNTs, the D′ band is not distinguishable as shoulder next to the G band, indicating that this sample has a lower amount of defects, as expected for carbon nanotubes with large diameter, which exhibit reduced tip curvature and in turn fewer

deformation (1320 cm−1),

**Figure 10.** SERS spectra of pristine graphene. The spectra (a), (b), and (c) were acquired at different scanning positions across the dried film.

spectra of graphene and functionalized MWNTs is a further proof of evidence of the formation of tetrahedrally coordinated bonds between carbon atoms and molecular groups.

**8. Conclusions and outlook**

**Author details**

Sabina Botti<sup>1</sup>

Denmark

**References**

the distribution of functional groups across the scanned area.

\*, Alessandro Rufoloni1

\*Address all correspondence to: sabina.botti@enea.it

1 Fusion and Nuclear Security Department, ENEA, Frascati, Italy

Nanotechnology. 2008;**3**:387-394. DOI: 10.1038/nano.2008.135

Analytical Chemistry. 2005;**77**:338A-346A and references therein

posites. Advanced Materials. 2010;**22**:1672-1688

Electroanalytical Chemistry. 1977;**84**:1-20

izing groups either of synthesis process of graphene as chemical exfoliation.

In summary, we performed SERS measurements on graphene pristine and functionalized MWNTS and graphene nano-structured film deposited on gold-coated Si nano-pillar substrates. The strong surface enhancement effect allowed us to record the Raman signal from functional molecules that are not recorded in conventional Raman spectra proving experimentally their linkages to the CNT walls and graphene edges. By using the relative intensities of specific SERS features as contrast parameters to obtain SERS maps, it is possible to study

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon…

The obtained results encourage us to consider SERS as a powerful method to obtain a rapid monitor either of the procedures used to interface graphene and nanotubes with functional-

, Tomas Rindzevicius2

2 Department of Micro and Nanotechnology, Technical University of Denmark, Lyngby,

[1] Hersam MC. Progress towards monodisperse single-walled carbon nanotubes. Nature

[2] Byrne MT, Gun'ko YK. Recent advances in research on carbon nanotube-polymer com-

[3] Kim SW, Kim T, Kim YS, Choi HS, Lim HJ, Yang SJ, Park CR. Surface modifications for the effective dispersion of carbon nanotubes in solvents and polymers. Carbon. 2012;**50**:3-33

[4] Jeanmaire DL, Van Duyne RP. Surface Raman spectro-electrochemistry. Journal of

[5] Haynes CL, McFarland AD, Van Duyne RP, Surface-enhanced Raman spectroscopy.

[6] Ferrari AC. Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and non-adiabatic effects. Solid State Communications. 2007;**143**:47-57

and Michael Stenbæk Schmidt2

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We acquired SERS spectra for each point of a selected area of pristine graphene films with a high-resolution micro-Raman spectrometer (Horiba Xplora) (see **Figure 11**). The areas were scanned with an acquisition grid of 0.6 and 0.8 μm in x and y, respectively. We selected the wavenumber region 500–1280 cm−1 to represent the functional groups concentration and we constructed the SERS images, illustrated in **Figure 11(b)** and **(d)**, plotting the intensity of the peaks located in this region.

Optical images show significant variation in the dried film thickness along the scanned area, due to a poor dispersibility of pristine graphene in the distilled water. However, the SERS signal is always detectable with good spectral reproducibility and an intensity variation much smaller than that expected, considering that the SERS signal from the second monolayer and beyond is strongly reduced. This finding further confirms that the functional groups are distributed at the edges of graphene structure: the larger thickness and hence the weaker enhancement experienced by the molecular groups, can be compensated by a higher concentration.

**Figure 11.** (a) Image of pristine graphene dried film, acquired with the optical microscope coupled with the Raman spectrometer; the green rectangle indicates the region selected to acquire the map (b). (c) and (d) represent the same for another region of the substrate. The visualization of Raman maps is obtained by using as contrast parameters the intensity of functional group peaks.
