**3. Surface-enhanced Raman spectroscopy**

The surface-enhanced Raman scattering (SERS), discovered three decades ago, is used to provide a drastic amplification of the Raman signal [4, 5]. The Raman signal amplification is observed when the molecule to be studied is adsorbed on a nanostructured metallic surface and the frequency of incident radiation overlaps the resonant frequency of conduction electrons in the metals. In this case, the incident electric field produces collective oscillations of metal electrons, which in turn generate a large electromagnetic field. This field is superimposed to the incoming field giving rise to its enhancement. Theoretical studies demonstrate that the induced field is particularly intense near sharp tips, interstitial crevices, and more generally, in between adjacent metal nanostructure if the distance is of few nanometers [4, 5, 19].

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

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

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

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

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 experi-

The surface-enhanced Raman scattering (SERS), discovered three decades ago, is used to provide a drastic amplification of the Raman signal [4, 5]. The Raman signal amplification is observed when the molecule to be studied is adsorbed on a nanostructured metallic surface and the frequency of incident radiation overlaps the resonant frequency of conduction electrons in the metals. In this case, the incident electric field produces collective oscillations of metal electrons, which in turn generate a large electromagnetic field. This field is superimposed to the incoming field giving rise to its enhancement. Theoretical studies demonstrate that the induced field is particularly intense near sharp tips, interstitial crevices, and more generally, in between adjacent metal nanostructure if the distance is of few nanometers [4, 5, 19].

[13–17]. Differently, in the Raman spectra of MWNTs,

quantum-confinement in small diameter SWNTs.

intensity in intercalated graphite compounds [18].

**3. Surface-enhanced Raman spectroscopy**

mentally their linkage to the CNT walls.

and G<sup>−</sup>

G peak in two components, G+

206 Raman Spectroscopy

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

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 Raman signal enhancement occurs twice.
