*2.1.1. C60 peapods*

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,

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

as seen in **Figure 1**, and organized into bundles [6–8].

70 Fullerenes and Relative Materials - Properties and Applications

structure of an individual C60 peapod.

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**.


**Table 1.** Optimized linear and zigzag structural parameters of the C60 molecules inside SCNTs for different nanotubes diameter and chirality.

The optimal interlayer C60-SWCNT distance is calculated around 0.30–0.33 nm, which is close to gaps commonly observed in carbon systems. For all optimized configurations, the interfullerene C60-C60 distance varies from 0.998 to 1.01 nm. This result is in good agreement with the previously reported peapod interball separation of 0.97 nm from electron-diffraction profiles [18] and 0.95 nm from HRTEM data [7]. The predicted phases of C60s inside SWCNTs have been experimentally observed by Kholoystov et al. HRTEM micrographs [19].

*2.1.2. C70 peapods*

between 2.16 and 2.28 nm.

taining C70 in both orientations.

Fullerenes with ellipsoidal shape-like C70 are of particular interest. Unlike the spheroidal molecules such as C60, there are several geometrically distinct orientations possible for the C70 molecule within a nanotube. Experimentally, depending on the nanotube diameter, two different orientations (with regard to the nanotube axis) of a C70 molecule encapsulated into SWCNTs are observed by Chorro et al.: the lying down orientation where the long axis of C<sup>70</sup> molecules is parallel to the nanotube long axis, and the standing up orientation where the C<sup>70</sup> long axis is perpendicular to the nanotube axis [19, 20]. The value of the nanotube diameter beyond which the change from the lying to standing orientation occurs is experimentally estimated to ∼1.42 nm. Besides, HRTEM measurements showed that there is no SWCNT con-

**Table 2.** Packing phases parameters obtained from minimized energy of the C60 molecules inside SCNT with diameters

**C60 phases Tube index (n,m) Tube diameter (nm) C60: tube distance (nm) C60 − C60**

(16,16) 2.171 0.323 1.001 (18,14) 2.176 0.325 0.998 (25,5) 2.181 0.323 1.001 (19,13) 2.183 0.329 0.998 (28,0) 2.193 0.331 1.001 (27,2) 2.197 0.312 1.003 (24,7) 2.206 0.309 1.003 (26,4) 2.210 0.308 1.006 (22,10) 2.221 0.30 1.008

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

(17,16) 2.239 0.297 1.002 (18,15) 2.242 0.302 1.008 (19,14) 2.247 0.308 1.005 (20,13) 2.255 0.306 1.006 (24,8) 2.259 0.307 1.008 (21,12) 2.266 0.311 1.01 (29,0) 2.271 0.303 1.007 (28,2) 2.276 0.308 1.009 (22,11) 2.280 0.310 1.01

Double helix (22,9) 2.164 0.321 1.003

Two molecule layers (28,1) 2.233 0.312 1.01

**Distance (nm)**

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**Figure 2.** (a) Schematical representation of carbon peapods showing some of the parameters used for the geometrical optimization of the C60 molecules inside the nanotube (see text). (b) and (c) Schematic view of ordered phases resulting from C60 packing in SWCNTs: (b) double helix and (c) two-molecule layers.



**Table 2.** Packing phases parameters obtained from minimized energy of the C60 molecules inside SCNT with diameters between 2.16 and 2.28 nm.
