**1. Introduction**

Since Kroto *et al.* discovered C60 by laser vaporizing graphite into a helium stream in September 1985, fullerenes are at the heart of nanotechnology [1]. Other fullerenes were discovered shortly afterward with more and fewer carbon atoms. Since the discovery of multiwalled carbon nanotubes and single-walled carbon nanotube (SWCNT) in 1991 [2, 3], fullerenes and

© 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.

carbon nanotube systems have attracted significant attention from the scientific community. A remarkable property of SWCNT is its ability to have been filled with various fullerenes and metallofullerenes, fullerenes adducts, metal complexes, and other small molecules. These fillings are highly dependent on the nanotube diameter and the inserted molecule size, so that even small changes in SWCNT diameter can alter the geometry of fullerene arrays. This class of hybrid materials has been dubbed as "peapods" (C60@SWCNT and C70@SWCNT), reflecting structural similarities to real peapods. After the discovery of C60 peapods by Smith et al. in 1998 [4], many experimental studies clearly evidenced the existence of various fullerenes like C70, C76, and C80 inside SWCNTs [5–7]. However, these materials represent a new class of a hybrid system between fullerenes and SWCNTs where the encapsulated molecules peas and the SWCNT pod are bonded through van der Waals interactions. Using high-resolution transmission electron microscopy (HR-TEM) experiments, the peapods are clearly observed, as seen in **Figure 1**, and organized into bundles [6–8].

C60 and C70 molecules encapsulated inside SWCNTs adopt different configurations according to the nanotube diameter. Then, we report for each structure its associated calculated Raman responses. The dependencies of the Raman spectrum with different structural parameters such as nanotube diameter, fullerenes configuration, and the filling level are discussed. Finally, we evaluate, for each peapod configuration, the reliability and the transferability of the experimental method proposed by Kuzmany [12] to estimate the relative C60 and C70 concentrations

Structural and Vibrational Properties of C60 and C70 Fullerenes Encapsulating Carbon Nanotubes

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A C60 (C70) carbon peapod is considered to be consisting of a number of C60 (C70) molecules inside SWCNT. The configurations of C60 and C70 molecules within nanotube are diameter dependent. Carbon peapods can be produced in a very high yield close to 90% simultaneously by heating opened SWCNTs and fullerenes in a scaled quartz tube [6] and by the vapor phase reaction by using open end SWCNTs [7, 8]. The encapsulated molecules peas and the

Several theoretical and experimental studies have been reported on C60 formed within SWCNTs, and several interesting structural properties have been predicted or observed. In particular, theoretical calculations of C60 peapods suggest that the smallest tube diameter for encasing C60 molecules inside SWCNT is around the diameter corresponding to (10,10) or (9,9) SWCNTs [7]. Hodak and Girifalco showed that the guest molecules structure within the nanotube is diameter dependent [13, 14]. Using a convenient Lennard-Jones expression of the van der Waals intermolecular potential to derive the optimum configurations of C<sup>60</sup> molecules inside single wall carbon nanotubes, Chadli et al. have found that the C60 molecules adopt a linear configuration with SWCNT diameters below 1.45 nm and a zigzag configuration for SWCNT diameters between 1.45 and 2.20 nm [15–17] (see **Table 1**). The optimum C60 packing can be characterized by the angle formed by three consecutive C60 (see **Figure 2a**). This angle θ is found to depend primarily on the nanotube diameter and does not depend significantly on the nanotube chirality. In the following paragraphs, the peapods in which the

C60 molecules adopt a linear (zigzag) configuration are called linear (zigzag) peapods.

The calculations of structural parameters of C60 peapods are extended to a larger range of nanotube diameters in which C60 molecules can adopt a double helix (**Figure 2b**) or a twomolecule layer (**Figure 2c**) configuration. When the tube diameter increases up to 2.28 nm, the energy minimizations show that two other optimal configurations of C60 molecules are possible: a double helix structure (2.15 ≤ D ≤ 2.23 nm) and a two-molecule layer (2.23 ≤ D ≤ 2.28 nm). Optimized structural parameters issued from the energy minimizations are listed in **Table 2**.

**2. Structure and dynamics of C60 and C70 peapods**

**2.1. Configuration of C60 and C70 inside carbon nanotubes**

SWCNT pod are bonded through van der Waals interactions [6].

in peapods.

*2.1.1. C60 peapods*

The physicochemical properties of the fullerene molecules inserted inside carbon nanotubes are generally well known in their stable phase. But what happened when these same molecules are confined inside a carbon nanotube? Furthermore, changes in the electronic and mechanical properties of carbon nanotubes induced by the insertion of these molecules have been demonstrated [9, 10]. Peapods are typically characterized by one or more of the conventional techniques such as transmission electron microscopy (TEM), Raman spectroscopy, electron diffraction, electron energy loss spectroscopy (EELS), and X-ray diffraction. Raman spectroscopy is a useful tool to characterize carbon nanotubes and related nanomaterials and widely used by experimentalists as a fast and nondestructive method to identify the type of nanoparticle and to study their electronic and vibrational properties [11]. In this chapter, the structure and vibrational properties of C60 and C70 peapods are reviewed. We show that the structure of the

**Figure 1.** (a) Electron microscopy image of C60 peapod (from reference [4]). (b) Schematic representation of the molecular structure of an individual C60 peapod.

C60 and C70 molecules encapsulated inside SWCNTs adopt different configurations according to the nanotube diameter. Then, we report for each structure its associated calculated Raman responses. The dependencies of the Raman spectrum with different structural parameters such as nanotube diameter, fullerenes configuration, and the filling level are discussed. Finally, we evaluate, for each peapod configuration, the reliability and the transferability of the experimental method proposed by Kuzmany [12] to estimate the relative C60 and C70 concentrations in peapods.
