**4.3. Raman experiments on C60 and C70 peapods**

Raman spectroscopy was employed to further characterize the peapod samples and to obtain structural information on the inserted fullerene inside tubes. Many groups have performed the Raman experiments of filling SWCNTs with C60 or C70 molecules [12, 42–46] since the first synthesized by Smith et al. [4]. **Figure 5** presents two examples of peapod experimental Raman spectra, one for C60@SWCNT and the other for C70@SWCNT.

Usually, experimental Raman spectra have been obtained on an ensemble of peapods organized into bundles. Nevertheless, the analysis of these experimental results for bundles of C60 peapods [8, 44] leads to the well-established conclusion: the Peapod Radial Breathing-Like Mode (PRBLM) of the peapod having a tube diameter close to 1.37 nm was split into two components, downshifted and upshifted, respectively, with respect to the position of the RBM in empty SWCNT. The upshift was possibly assigned to the stress feeling by the tubes from the inside C60. For diameters between 1.45 and 1.76 nm, a single PRBLM measured

**Figure 5.** RBM Raman spectra taken for heat-treated C70 (top) and C60 (bottom) peapods (from reference [46]).

and its frequency are downshifted compared to the RBM in the empty SWCNT bundles. The observed downshift of the PRBLM [43, 44] is explained by the hybridization effect between SWCNTs and C60 molecules electronic states, leading to a decreasing of the electron density in the vicinity of the SWCNT, which induces a decreasing of the force constant of the C-C bond. Pfeiffer et al. [45] measured all the fundamental Raman lines of the encaged C60 peas. They observed that both nondegenerate and totally symmetric Ag modes of C60 peas exhibit a splitting into two components. They attributed this splitting to the presence of both moving and static fullerenes inside the tubes.

Kuzmany et al. [12] performed a detailed Raman analysis to evaluate the concentration of C60 molecules inside nanotubes. As expected, the relative concentrations derived from the measurement of normalized intensity ratio for each Raman mode of C60 are close. The C60 filling degree of a reference peapod sample was determined from electron energy loss spectroscopy experiment in order to evaluate the absolute C60 concentration of each peapod sample from the measurements of the Raman spectra (see **Table 1** of [12]).

Concerning the C70 peapods, Hirahara et al. [46] characterized one-dimensional crystals of a variety of larger fullerenes C70 peapods by using high-resolution transmission electron microscopy and electron diffraction. They concluded that the C70 admit two different orientations depending on the nanotube diameter: the lying and standing orientation. The intermolecular distances of various fullerenes in SWCNTs are considerably smaller than those for bulk fullerene crystals, suggesting an effect of confinement in the one-dimensional channels inside SWCNTs. Raman experiments on SWCNTs encasing C70 molecules have been reported [19, 21, 42, 47, 48]. Ryabenko et al. [42] concluded that the PRBLM mode of C70 peapods are downshifted by ∼2–3 cm−1 compared to empty nanotubes. For different orientations of C70s in C70@SWCNT peapods organized into bundles, the measured PRBLM downshift after the C70 encapsulation is ∼2–6 cm−1 [47]. The downshift of the PRBLM suggests a structural relaxation or the tube diameter transformation is expected to occur with the assistance of such added carbon atoms. However, such experimental work should include Raman investigations on samples showing peapods having various structural characteristics: different tube diameters, different fullerene concentration, and bundles with various sizes, for example, various numbers of tubes.
