proceeding of XVth International Winterschool on Electronic Properties of Novel Materials, Kirchberg, Austria, 2001
to be published in AIP Conference Proceedings.

Fullerene-peapods: synthesis, structure, andRaman spectroscopy
H. Kataura1, Y. Maniwa1, T. Kodama2, K. Kikuchi2,
K. Hirahara3, S. Iijima3,4,
S. Suzuki2, W. Krätschmer5, and Y. Achiba2
1

Department of Physics, Tokyo Metropolitan University, Japan
Department of Chemistry, Tokyo Metropolitan University, Japan3
Japan Science and Technology Corporation, ICORP, Japan
4
Department of Physics, Meijo University, Japan and NEC corporation, Japan
5
Max-Planck-Institut für Kernphysik, Germany
2Abstract. Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, so-called fullerenepeapods, were synthesized in high yield by using diameter-selected nanotubes as pods. Highresolution transmission electron microscopy revealed high-density fullerene chains insidenanotubes. X-ray diffraction measurements indicate 60% filling of C60 molecules in amacroscopic average. Room temperature Raman spectra show one-dimensional
photopolymerization of C60 inside nanotubes by blue laser irradiation, indicating molecularrotation inside. In C70-peapod case, Raman active modes peculiar to C70 were strongly
suppressed by encapsulation. The suppression is probably caused by deformation of moleculedue to the unisotropic interaction between C70 and SWNT. Anomalous resonance effects in C70peas suggest hybridization of electronic states of C70 and nanotubes. High-symmetry mode
Raman intensity suggests the filling factor to be higher than 50%, which is consistent with the X
ray diffraction analysis.

INTRODUCTION
Single-wall carbon nanotubes (SWNTs) have large internal space where molecules
can be introduced by de-capping. Fullerenes are most favorable molecules forencapsulation because of their fit diameters. Generally, physical properties of solids
are strongly depending on a network dimensionality. Since fullerene-peapods havemixed network dimensionality, it is expected they should have very interesting
physical properties. Indeed, Okada (1) has indicated the very attractive band structure
in C60@(10,10), where the LUMO band of C60 goes down to Fermi level of (10,10)linear band and thus the system has two kinds of one-dimensional carriers fromnanotube and C60. Okada suggested that the Piels instability might induce a kind ofdimerization which was often observed in TEM pictures.
Previously, Smith et al. (2) have shown very beautiful TEM photographs offullerene-peapods, but no one could show its electronic properties to date. Difficulties
in experiments were probably due to a low yield of encapsulation. At least, we knowthat commercial NT@rice and HiPco materials are high-purity but are too thin toencapsulate fullerenes. In this work, we used diameter-selected SWNTs and fullereneFIGURE 1. Typical HRTEM image of C60-peapods (left photograph) and electron diffraction
patterns of C60- (upper right) and C70-peapods (lower right). Inter fullerene distances were indicatedin figure estimated from streaks.

vapor to get high-yield fullerene-peapods. We will show Raman spectra of fullerenepeapods.

EXPERIMENTAL
By considering van der Waals spacing, the best diameter of nanotube for fullerene
encapsulation is that of (10,10), 1.36 nm. To get higher yield of fullerene
encapsulation, we prepared empty SWNTs applying diameter control technique in thelaser ablation method (3). In this method, diameter distribution of SWNTs can becontrolled by a sort of catalysts and a furnace temperature. When we choose NiCoalloy (4) as a catalyst and set the furnace temperature to 1250 °C, we can get highpurity SWNTs having mean diameter at 1.36 nm. The diameter distribution wasconfirmed by the X-ray diffraction (XRD) analysis.
NiCo-carbon composite target can co-produce fullerenes in 10% fraction of rawsoot. For the peapods production, unknown fullerenes are the most undesirable
impurities. To get fullerene-free SWNTs, soot was heated in vacuum up to 1250 °C
and all fullerenes were removed. Then the fullerene-free soot was refluxed in 15 %H2O2 water solution at 100 °C for three hours to remove amorphous carbon particles(5,6). Since the soot still contains metal particles, it was washed in HCl solution toremove them. Finally, the purified SWNTs were formed to thin black paper and thendried in vacuum at 650 °C. The purity of resulting SWNT material was estimated to behigher than 90 % by TEM observation. Since the oxidation treatment destroys caps ofSWNTs (7) and HCl treatment increases defects on the wall, the purified SWNTs
already have sufficient number of entrances for fullerenes.
A SWNT paper was put in a quartz ampoule with fullerene powder and theampoule was evacuated to 1×10-6 Torr. After drying process, fullerene powder was

1248 Hg(6)1090 Hg(5)?

710 Hg(3)771 Hg(4)431 Hg(2)* 492 Ag(1)~270 Hg(1)1600 sec

11200 sec

He Temp.
81 - 100

400 sec

Intensity (arb. units)

60

200 sec

1424 Hg(7)C -Peapods40 sec
Intensity (arb. unit)* 1467 Ag(2)A g(2)

Room Temp.

140 014 20

14 40

14 60-1

R am an shift (cm )148 00400

800

12001600

Raman shift (cm -1)

FIGURE 2. Raman spectra of C60-peapods. The left figure shows time resolved Ag(2) mode
spectra of C60-peapods at room temperature measured in air. The right figure shows Raman
spectrum at liquid-helium (upper) and room temperature (lower). Laser wavelength used is 488 nm.

evaporated and made a film on the SWNT paper. The ampoule was sealed and heatedin a furnace up to 650 °C. After keeping the temperature for two hours, the ampoule
was cooled down to room temperature. The SWNT paper was sonicated in toluene for
1 hour to remove fullerenes coated on SWNTs surface. After filtration, we got a sheetof peapod paper. Before the optical measurements, the peapod paper was heated invacuum up to 450 °C to remove toluene. In this work, we used C60 and C70 powder in
99 % purity as fullerene sources.

RESULTS AND DISCUSSION
The most direct technique to see the fullerene encapsulation is high-resolution (HR)TEM. Figure 1 shows a typical example of HRTEM image of C60-peapods. Results forC70-peapods are not indicated but were very similar. In the electron diffraction pattern,
we can observe very clear streaks originating from one-dimensional C60 and C70crystals inside nanotubes. It is very difficult to estimate macroscopic fullerene fillingrate from HRTEM results, but we know (10) peak observed in XRD is very sensitive
to the molecular encapsulation since the additional electron density inside SWNT
changes the form factor (7). We have observed very strong reduction in (10) peakintensity in C60-peapods (8). Careful analysis reveals a filling factor of C60 is higher
than 60%, which is consistent with the filling factor estimated from EELS
measurements. From the streaks in electron diffraction, we can get one-dimensionallattice constant 0.95 nm for C60-peas. On the other hand, XRD indicates 0.97 nm as anaverage value. We know that the C60-C60 distance is 1.00 nm in fcc crystal and 0.92
nm in polymer crystals (9). Intermediate inter-ball distance in one-dimensional C60
crystal inside SWNT strongly suggests one-dimensional dimer or origomer phase. On
the other hand, in C70-peapods case, double streaks indicate that the sample contains

two kinds of one-dimensional crystals with different lattice constants, 1.00 and 1.10
nm. Each of them just corresponds to the standing eggs and lying egg structure of C70.
Interestingly, streaks are very sharp and high-contrast, which means there is nointermediate lattice constants. Indeed, when we used thinner SWNTs as pods, weobserved only one streak corresponding to the long lattice constant 1.10 nm. Thismeans C70 tend to take a standing alignment if the pod is large enough.
Figure 2 shows typical Raman spectra of C60-peapods. At room temperature, Ag(2)mode of C60 molecule was strongly reduced by blue laser irradiation. At heliumtemperature, on the other hand, we can measure the Raman spectra stably. The
temperature dependent photoreaction is very similar to the photopolymerization in fcc
C60 crystal. Since the photopolymerization is concerned with the spinning nature ofC60 molecule, this result suggests that C60 or C60-origomers are spinning in SWNTs atroom temperature. Since the final spectrum is similar to that of orthorhombic polymer(10), laser induced one-dimensional photopolymers were suggested. At heliumtemperature, all the Raman active modes of C60 molecule were observed. Further,additional peaks were observed around 90 cm-1 that is close to an external vibrationalmode of C60 dimer at 96cm-1 (11). Additionally, very weak Ag(1) and Ag(2) modes
and broadening of Hg(1) mode are indicating formation of dimers or origomers. These
results are consistent with electron diffraction and XRD results.
In the case of C70-peapods, all the Raman active modes of C70 were observed evenat room temperature and no photoreaction was observed. Raman spectra measured byusing several laser lines are indicated in Fig. 3. Raman spectra of C70-peas have someanomalies in relative Raman intensity and peak widths. For example, peak intensity at

C -pe apo ds70

(pea pod -K)

70

2.6 0 e V
Inten sity (arb. units)

Inten sity (arb. units)2.62 e V
pe apods

2.60 eV
2.54 e V
2.50 e V
2.47 e V

2.54 eV
2.5 0 eV
2.47 eV
2.41 e V

S W N Ts2.41 e V

2.3 3 eV

2.33 eVC100

C -peas2.62 eV

150

200
-1

R am an shift (cm )

250

70

film

400

1/2

600

800

1000 1200 1400
-1

R am an shift (cm )FIGURE 3. Resonance Raman spectra of C70-peapods. Left figure shows radial breathing modes inSWNTs (solid line) and peapods (dashed line). In the right figure, upper seven curves are spectra ofC70-peas, I(C70-peapods) – I(SWNTs). The bottom one is that of C70 thin film. Excitation laser
energies are indicated in figures.

1060 cm-1 is a half of that in C70 film and independent of excitation energy, showing afilling factor to be higher than 50%. On the other hand, peak intensity at 1184 and
1230 cm-1 is strongly depending on the excitation. Intensity ratio I(1060) / I(1184) isanomalously larger than that in C70 film and is also depending on the excitation energy.
These anomalies should be concerned with the encapsulation. As mentioned above,C70-peapod has two kinds of crystalline structures, standing chain and lying chain. Theformer structure is permitted only in thick SWNTs. The left figure in Fig. 3 shows that
2.50eV laser can resonate thick SWNTs but 2.62 eV excitation cannot. If theinteraction between C70 and SWNT depends on the crystal structure, standing or lying,and if electronic states of C70 and SWNT are hybridised as predicted in the theoretical
calculation (1), resonance effect of C70-peas should reflect electronic structure ofwrapping SWNTs. Anomalous resonance effects on C70-peas observed here suggest aconsiderable hybridisation between C70 and SWNT. This is consistent with the opticalabsorption spectrum of C70-peas that shows large local field effects (12).
If the Dimerization of C60 and high-density packing of C70-peas are caused by aninternal high-pressure state predicted by Tománek (13), SWNT can be applied topressurized nano-cylinders in near the future.

ACKNOWLEDGEMENTS
Authors thank Prof. Susumu Saito, Prof. Okada and Prof. Tománek for fruitful
discussions about peapod materials. Authors thank Mr. Misaki for electron diffraction
measurements. This work was supported in part by Japan Society for Promotion ofScience, Research for the Future Program and supported in part by the Grant-in-Aidfor Scientific Research on the Priority Area “Fullerenes and Nanotubes” by theMinistry of Education, Science, and Culture of Japan.

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