Specific Raman Signatures of a Dimetallofullerene Peapod

We report on the Raman spectroscopy of a dimetallofullerene peapod, $({\mathrm{L}\mathrm{a}}_2@{\mathrm{C}}_{80}{)}_m@\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}$. Drastic changes are observed with respect to pristine nanotubes. A sharp intense line at $142{\mathrm{c}\mathrm{m}}^{-1}$ is interpreted as a signature of polymerization of the encapsulated metallofullerenes. Additional strong signatures appear at about 400, 520, and $640{\mathrm{c}\mathrm{m}}^{-1}$, respectively. Their intensity suggests the existence of an enhancement effect. The stiffening and the up-shift of the $G$-band modes appear to imply that a charge transfer process between the nanotube and the peas occurs.

Figure 1

Low-frequency Raman spectra of pristine tubes and of $({\mathrm{L}\mathrm{a}}_2@{\mathrm{C}}_{80}{)}_m@\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}$ peapods. (a) RBMs of pristine tubes; $\lambda =514\mathrm{n}\mathrm{m}$ and $P=10\mathrm{m}\mathrm{W}$. (b) Raman signal of metallofullerenes intercalated between nanotubes. Width of the signal at $142{\mathrm{c}\mathrm{m}}^{-1}$: $7{\mathrm{c}\mathrm{m}}^{-1}$; $\lambda =488\mathrm{n}\mathrm{m}$ and $P=2.5\mathrm{m}\mathrm{W}$. (c) Peapod Raman spectrum at low excitation energy; $\lambda =488\mathrm{n}\mathrm{m}$ and $P=0.1\mathrm{m}\mathrm{W}$, respectively. Inset: magnification of the RBM signal at $153{\mathrm{c}\mathrm{m}}^{-1}$ . (d) Typical peapod spectrum in the case of increased power; $\lambda =514\mathrm{n}\mathrm{m}$ and $P=1.5\mathrm{m}\mathrm{W}$, respectively. Inset: magnification of the intermediate-range spectrum.

Figure 2

Raman spectrum of pristine tubes and $({\mathrm{L}\mathrm{a}}_2@{\mathrm{C}}_{80}{)}_m@\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}$ peapods including the $G$ band. (a) Spectrum of the pristine tubes [see relevant Fig. 1a]. (b) Signal of peapods: $\lambda =476\mathrm{n}\mathrm{m}$ and $P=1.4\mathrm{m}\mathrm{W}$. (c) Signal of peapods at a different sample location: $\lambda =514\mathrm{n}\mathrm{m}$ and $P=1.5\mathrm{m}\mathrm{W}$. A comparison between (b) and (c) demonstrates that the spectral features are similar in both cases. (d) Lorentzian-type fit of the $G$ band corresponding to (a). The main lines of the $G$ band peak at $1584{\mathrm{c}\mathrm{m}}^{-1}$ (width $12{\mathrm{c}\mathrm{m}}^{-1}$) and at $1558{\mathrm{c}\mathrm{m}}^{-1}$. The broad $D$ band is observed at $1339{\mathrm{c}\mathrm{m}}^{-1}$. Two additional lines at 1407 and $1471{\mathrm{c}\mathrm{m}}^{-1}$ can be assigned to metallofullerenes. (e) Corresponding fit of the $G$ band in the case (c) of peapods. The $G$-band lines are up-shifted to $1590{\mathrm{c}\mathrm{m}}^{-1}$ (width $7{\mathrm{c}\mathrm{m}}^{-1}$) and $1565{\mathrm{c}\mathrm{m}}^{-1}$.