**2. Raman spectroscopy**

In **Figure 1**, the Raman spectra of graphite (HOPG, Cree Corporation, USA), multilayer graphene, single-walled carbon nanotubes (SWNTs), double-walled nanotubes (DWNTs), and MWNTs in the range 100–2100 cm−1 are reported. All the samples that we used were commercially available.

The pristine SWNTs, DWNTs, and MWNTs were grown by catalytic carbon vapor deposition process, with the following characteristic: SWNTs (Nanointegris, USA, selected semiconductor) average diameter: 1.2–1.7 nm, length: 300 nm to 5 μm; DWCNTs (Nanocyl, Belgium) average outer diameter of 3.5 nm, length between 1 and 10 μm, and specific surface area of 500 m2 /g; MWNTs (Nanocyl, Belgium) average diameter: 9.5 nm, average length: 1.5 μm. We used graphene powders with particles consisting of aggregates of platelets with diameter of about 2 μm, thickness less than 5 nm, and average surface area 750 m2 /g (grade C particles, XG Sciences Inc., USA).

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon… http://dx.doi.org/10.5772/intechopen.74065 205

the ability to engineer the electrical, optical, sensing, and dispersion properties of graphene and CNTs by chemical functionalization is widening considerably their potential applications generating a vast and yet largely unexplored family of carbon nanostructures for the realization of devices with novel functionalities. This task is highly challenging and the achievement of a procedure that enables a high density of functional groups, with little or no damage to the carbon structure, is of prime importance as the development of a nondestructive technique

Since long time, Raman spectroscopy has been considered as one of the most powerful tool for the characterization of carbon-based materials (see Section 2); however, Raman features associated with functional groups are usually often not observed in the Raman spectra of nanotubes and graphene due to the small quantity of the molecules attached to

In our studies, we have used the surface-enhanced Raman spectroscopy (SERS), which provides a large amplification of Raman signal when the probed molecule is adsorbed on nanosized metallic surface [4, 5]. Since the SERS effect increases by decreasing the distance between the nanostructure and the adsorbed molecule, by depositing CNTs/graphene as dilute dispersions on SERS active substrates, the Raman signal from the molecular groups deriving from functionalization/synthesis process and bound to the carbon surface, can be amplified. As a consequence, the spectral features of functional groups, otherwise very difficult to see, were

Here, we present a review of principal results obtained by applying SERS for the characterization of pristine and functionalized graphene and multiwalled nanotubes (MWNTs). The obtained results encourage us to consider SERS as a powerful method to obtain a rapid monitor of the procedures used to interface graphene and nanotubes with functionalizing

In **Figure 1**, the Raman spectra of graphite (HOPG, Cree Corporation, USA), multilayer graphene, single-walled carbon nanotubes (SWNTs), double-walled nanotubes (DWNTs), and MWNTs in the range 100–2100 cm−1 are reported. All the samples that we used were com-

The pristine SWNTs, DWNTs, and MWNTs were grown by catalytic carbon vapor deposition process, with the following characteristic: SWNTs (Nanointegris, USA, selected semiconductor) average diameter: 1.2–1.7 nm, length: 300 nm to 5 μm; DWCNTs (Nanocyl, Belgium) average outer diameter of 3.5 nm, length between 1 and 10 μm, and specific surface area of

about 2 μm, thickness less than 5 nm, and average surface area 750 m2

/g; MWNTs (Nanocyl, Belgium) average diameter: 9.5 nm, average length: 1.5 μm. We used graphene powders with particles consisting of aggregates of platelets with diameter of

/g (grade C particles,

that can fully characterize the structure of functionalized graphene and CNTs.

the carbon lattice.

204 Raman Spectroscopy

groups.

500 m2

recorded in the SERS spectra.

**2. Raman spectroscopy**

mercially available.

XG Sciences Inc., USA).

**Figure 1.** Raman spectra of different carbon forms: graphite, graphene, SWNTs, DWNTs, and MWNTs. The spectra were excited at 785 nm.

The Raman spectrum of graphite and its derivatives has two main peaks assigned to the first order D (1320 cm−1) and G modes (1580 cm−1) [6–12]. Second-order Raman spectra of carbon-based materials, and particularly of graphene, are dominated by 2D (G′) band at ~2700 cm−1, caused by the existence of double electron-phonon resonance mechanism, thus making it possible to study the structure of electron bands from the analysis of resonance Raman spectra. The band has a double peak in graphite and a single peak in graphene. Both the intensity ratio I2D/IG and the shape analysis of 2D peak have been used to distinguish mono from bi or multilayer graphene [12, 13].

The D mode (A1g symmetry) can be described as an in-plane breathing vibration of aromatic ring in which the six atoms move away from the center of the Brillouin zone. This band is forbidden in perfect graphitic lattice and it becomes active in the presence of disorder and deviations from an ideal structure. The G mode (E2g symmetry) is due to the in-plane stretching vibration of carbon atoms pairs in which each atom moves vibrating tangentially against the other. This mode is always allowed and it is characteristic not only of graphitic rings but also of all the sp2 structures. The ratio of D and G band intensities provides a way to estimate the amount of defects in graphitic structure, being inversely proportional to the size of ordered sp2 domain size through the relation: *L* = (2.4 × 10−10)·λ<sup>L</sup> 4 ·(ID/IG)−1, where λL is the wavelength of laser that excites the Raman spectra [9, 11, 12].

The Raman spectra of SWNTs display peculiar features as the radial breathing mode (RBM), and the double peak splitting of the G band. The frequency ω of RBMs is inversely proportional to the value of tube diameter *d* following the Bandow relation: [13, 14]

$$
\omega = \alpha/d\tag{1}
$$

where the parameter α is experimentally determined.

The optical transitions of SWNTs can be identified as inter-band transitions between the Van Hove singularities in the 1D electronic density of states (DOS) [14, 15]. The singularities are spikes in the DOS, which occur at specific energies, depending on the nanotube diameter and chirality. From band structure calculations, approximate relations were deduced between the energy *E* of inter-band transitions between DOS singularities and the nanotube diameter, suggesting that the optical absorption peaks should move to higher energy at decreasing nanotube diameter. As the nanotube diameter increases, the singularities move closer together, whereas for small diameter nanotubes, the "spikes" in the DOS are well-separated, especially near the Fermi energy and can provide initial or final states or both for a highly resonant Raman scattering process. The energy of these optical transitions are in the visible range, near to the wavelength of the most commonly used laser for excite Raman scattering. When the laser energy matches the energy difference between spikes for a particular nanotube diameter, the corresponding RBM will dominate the Raman trace. Therefore, the Raman scattering in small diameter SWNTs is diameter selective as direct consequence of the 1D electronic quantum-confinement in small diameter SWNTs.

However, the electromagnetic mechanism does not explain why the enhancement factor of a surface depends on the chemical nature of the adsorbed molecule [19, 20], therefore, it was hypothesized that an additional enhancement is provided by an increase of molecule polarizability due to a deformation of the distribution of the electron cloud or to the formation of

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207

We can then summarize the SERS process steps, depicted in **Figure 2**: (1) the incident laser impinges on nanostructured metallic surface, (2) plasmons excitation with electric field amplification, (3) Raman scattered light emission, and (4) Raman scattered light transferred back to plasmons and scattered in air. The surface enhancement effect is so pronounced because the

To be used in a sensor system, a SERS substrate should enhance the Raman effect sufficiently to enable consistent and uniform chemical detection sensitivity across the surface, maintaining its properties as long as possible in the time and to provide a high number of sites for

In principle, all systems possessing free carriers show the surface enhancement effect. However, the plasmon properties—such a wavelength and width of its resonance—depend on the nature of the metal surface and on its geometry and affect the enhancement factor (EF) of the surface [4, 5, 19]. The width of plasmon resonance resulted to be: w = γ(ε<sup>b</sup> + 3),

to the dielectric constant. Smaller conductivity and a large number of inter-band transitions in the region of plasmon resonance give resonance peak with large width and hence smaller amplification of electric field [19]. To this respect, the coin metals (Ag, Au, and Cu) resulted to be the most appropriate to be used for SERS with their amplification factors much larger than unity. Differently, the enhancement factor of good conductors as Al, Pt, and In is larger but not much larger than unity and is only slightly greater than unity for most other metals. The other advantage of using coin metals is that their plasmon resonance wavelength is in the

Early SERS experiments used gold colloids in solution. Nowadays, by exploiting semiconductor lithographic fabrication technology, periodic patterns on Si surface can be reproducibly fabricated over large areas. Ordered geometry provides uniform SERS signals from anywhere on the active surface, avoiding that only small uncontrolled areas of the total metal surface

is the contribution to the inter-band transitions

resonant charge transfer complex between the metal and the adsorbed molecule.

Raman signal enhancement occurs twice.

**Figure 2.** Scheme of SERS process steps.

where γ is the electron scattering rate and ε<sup>b</sup>

**4. SERS substrates**

molecular adsorption.

visible–near infrared.

The nanotube curvature results in different force constants for atomic displacements along the nanotube axis compared to those in the circumferential direction, causing the separation of G peak in two components, G+ and G<sup>−</sup> [13–17]. Differently, in the Raman spectra of MWNTs, which can be considered as an ensemble of concentric SWNTs with increasing diameters, the G peak splitting is both small and smeared out, leading to a broad G band line shape. The higher energy mode that appears as a shoulder in MWNT Raman trace is the D′ band, which is due to the same tangential vibrations of the G mode, but involving external layers that are not sandwiched between two other layers. As the D band, D′ is a double resonance Raman mode, induced by disorder and/or defects on the side walls of CNTs or ion intercalation between graphitic sheets. Indeed, this band is not observed in graphite, but it has a high intensity in intercalated graphite compounds [18].

As it will be discussed in Section 6, since less information is available on the structure of functional groups attached to the tube walls or basal plane of graphene, in our studies, we have used the SERS technique to record the Raman signal of functional groups and prove experimentally their linkage to the CNT walls.
