**1. Introduction**

Carbon nanotubes (CNTs) and graphene are at the center of a significant research effort due to their unique physical and chemical properties, which promise high technological impact. For the future development of all the foreseen applications, it is of particular interest the study of binding interactions between carbon nanostructures and functional groups [1–3]. Indeed,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

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 the carbon lattice.

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 recorded in the SERS spectra.

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].

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

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

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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

amount of defects in graphitic structure, being inversely proportional to the size of ordered

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 propor-

*ω* = *α*/*d* (1)

tional to the value of tube diameter *d* following the Bandow relation: [13, 14]

domain size through the relation: *L* = (2.4 × 10−10)·λ<sup>L</sup>

where the parameter α is experimentally determined.

laser that excites the Raman spectra [9, 11, 12].

structures. The ratio of D and G band intensities provides a way to estimate the

4

·(ID/IG)−1, where λL is the wavelength of

of all the sp2

excited at 785 nm.

sp2

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 groups.
