1. Introduction

The first class of materials whose properties and characterization via Raman spectroscopy we discuss here are the graphitic allotropes—single and multi-walled carbon nanotubes (SWNT, MWNT), followed by graphene and graphene nano flakes, specifically plasma functionalized graphene nanoplatelets. The chapter will begin with some discussion of the rich Raman spectral features of sp2 carbon allotropes, which will be necessary since there will be an emphasis on

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 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.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

those particular Raman bands and features that provide useful structural/thermal data about carbon nanotube samples. It will be followed by an overview of graphene and graphene nanoplatelets and their usefulness for gas-sensing applications utilizing Raman spectroscopy.

2. Structure of sp<sup>2</sup> nanocarbons

unit cell shown in Figure 1.

intra-planar interactions [5].

–105

Figure 1. Graphene unit cell.

1/3\* a<sup>1</sup>

ratio (~104

sional objects [5].

! <sup>þ</sup> <sup>a</sup><sup>2</sup>

This introductory section presents a cursory discussion of SWNT and MWNT structures. This will be done by first looking at the unit cell of planar graphene and also graphite since the former material is considered to be the conceptual parent material of all sp2 graphitic materials, including SWNTs through the application of a simple rolling up operation. Since the molecular/electronic and geometric structures are highly dependent on graphene, the majority

The two unique Carbon atoms A and B in each unit cell are located respectively at (0, 0) and at

related graphitic material, three dimensional graphite, is accomplished through the stacking of several layers of 2-dimensional graphene layers, where in the A-B Bernal stacking structure, there are 2\*N atoms per unit cell, N being the number of layers [4]. A major structural factor of graphite that results in the electronic structure of 2-dimensional graphene being a reasonable first order approximation of the former is the average inter-layer spacing of 3.35 Angstroms. This distance is much larger than the nearest neighbor Carbon–Carbon distance of 1.42 Å, hence resulting in much weaker overall attractive interaction between layers compared to

Moving now to one of major foci of the chapter single-walled carbon nantoubes (SWNTs), the conceptual operation performed on the single 2-dimensional graphene sheet is "rolling" it up into a cylinder. The diameter distribution of most SWNTs produced by various techniques is dominated by tubes with diameters less than 2 nm, although diameters in the range of 0.7– 10.0 nm are possible [5]. Ignoring the two ends and exploiting the very large length to diameter

The concept of chirality is essential in the description of SWNT structure. It is defined by the chiral vector, denoted by Ch in Figure 1, and several equivalent interpretations of this structural quantity are usually given. For example one may consider the fact that the chiral vector determines the arrangement of the six-sided carbon hexagons in the curved planar wall of the SWNT [5]. Alternatively, one may also view the chirality of a SWNT in terms of the overall symmetry of the constructed SWNT, specifically whether or not the SWNT has vertical mirror plane reflection

) of SWNTs allows one to safely view these sp2 nanocarbons as quasi 1-dimen-

! (Figure 1 adapted from Wong and Akinwande [3]). The progression to the first

!, and a<sup>2</sup>

Raman Spectroscopy of Graphitic Nanomaterials http://dx.doi.org/10.5772/intechopen.72769

! of the graphene

157

of a SWNT's structural features are expressed via the lattice vectors a<sup>1</sup>

The one-dimensional graphite allotrope, carbon nanotube, is conceptually described as being a rolled-up graphene sheet, yielding the cylindrical nanomaterials that have diameters of a few nanometers. The multi-walled varieties contain several concentric cylindrical shells. The Raman bands and the variations under various external perturbations of the sp2 graphitic materials cited above include the graphite G-band common to all sp<sup>2</sup> carbons at around 1580 cm�<sup>1</sup> due to the intraplanar bond stretching of the two carbons in the hexagonal lattice unit cell, and the carbon nanotube specific radial breathing mode (RBM)—which arises as a consequence of their cylindrical geometry. Lastly, there is the defect D-Raman band, which arises due to defects, finite size effects, or any other cause of departure from perfect crystalline regularity, and the 2-D band. The characterization topics discussed in connection with the graphitic materials will be the identification of the chirality types present in carbon nanotube samples using the resonant RBM mode of carbon nanotubes and its connection to the interesting quasi 1-dimensional character of their electronic structure. The other properties obtained via Raman spectroscopy discussed will also be the anomalous thermal expansion and thermal conductivity of the sp<sup>2</sup> graphitic materials investigated.

We have also utilized Raman spectroscopy to understand the behavior of vibrational modes associated with graphene following gas exposure. Specifically, we have studied the effects of water vapor and toxic gases (SO2, NO2, NO), via variable humidity levels, gas concentrations, exposure times, and thermal loading, on the Raman spectra of graphene.

Functionalized graphene nanoplatelets are comprised of an amorphous mixture of graphene sheets. Their thicknesses range from 6 to 8 nm, and the overall density usually lies between 0.03 and 0.1 g/cc. The oxygen content of the majority of samples normally are <1%, with the remaining carbon content exceeding 99.5 wt % (STREM). The morphology of this amorphous material plays a large and significant role in its enhanced mechanical properties, such as stiffness, strength, and surface hardness. By incorporating a small number of certain atoms that differ in the number of valence electrons into the pure crystal, the doping of graphene nanoparticles can lead to an enhancement in conductivity.

For the thermal conductivity measurements of the carbon allotropes, we have used the G-Raman band and its variation with increased sample temperature through laser heating. The method and the useful information it provides is due to Terekhov et al. [1]. Also, edge defect characterization of graphene nanoplatelets based on Eq. (1) due to Cancado et al. [2] has also been included, where L is the characteristic in-plane crystallite size of the graphene nano flake, λ is the laser wavelength, and ID and IG are the intensities of the Raman D-band and G-band, respectively.

$$L = \left(2.4 \times 10^{-10}\right) \times \lambda^4 \left(I\_D / I\_G\right)^{-1} \tag{1}$$

Our research presented here is aimed at extending the knowledge regarding the nature of graphitic nanomaterial-gas sensing interactions and help develop better models for their enhanced understanding, which in turn would make the development and production of more effective in situ gas sensors feasible.
