Proceeding of ISNM2001, to be published in AIP proceedings.

Optical Properties of Fullerene-Peapods
H. Kataura1, 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, Japan
3
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
2

Abstract. Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, so-called fullerenepeapods, were synthesized in high yield. High-resolution transmission electron microscopy
revealed that almost all nanotubes are filled with high-density fullerene chains. We measured
Raman spectra of C60- and C70-peapods. In the case of C60-peapods, C60 Raman active mode
intensity decreased rapidly by laser irradiation at room temperature. The final spectrum is similar
to that of orthorhombic polymer phase, which indicates one-dimensional photopolymerization.
At liquid helium temperature, no photopolymerization was observed, and the Raman spectra
obtained indicates a feature of C60-dimer phase. Furthermore, C60-C60 distance estimated fromelectron diffraction pattern measured at room temperature is consistent with C60-dimer phase.
The spontaneous dimerization should be explained in part by internal high-pressure effect.

INTRODUCTION
Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, what is calledfullerene-peapods, are considered to be a new solid phase of carbon constructed byzero- and one-dimensional sp2 network systems (1,2). Since, in general, a physicalproperty of solid is strongly depending on its network dimensionality, fullerenepeapods should have interesting properties due to the mixed dimensionality. Saito andOkada (3) have calculated the band structure of (10,10) nanotube encapsulating C60,and found an anti-crossing between a flat conduction band of C60 and a linear band of
nanotube, which indicates a considerable interaction between conduction electrons
with different network dimensionality. In this work, we successfully synthesized highyield peapod samples and measured Raman spectra of C60- and C70-peapods.

EXPERIMENTAL
By considering a diameter of C60 and van der Waals spacing, the best diameter ofnanotube for fullerene encapsulation is that of (10,10). To get higher yield ofencapsulations, it is desired to control diameter distribution of SWNTs to be thicker
than (10,10). Laser furnace technique realizes a diameter control by changing catalysts
and a furnace temperature (4). We chose NiCo (5) as a catalyst and we got thickenough SWNT samples by setting the furnace temperature to 1250 °C. Typical

Proceeding of ISNM2001, to be published in AIP proceedings.

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 estimatedfrom streaks originating from fullerenes one-dimensionally aligned inside nanotubes.

diameter of the sample is estimated to be 1.36 nm by X-ray diffraction. To get a highpurity peapod sample, unknown fullerenes co-produced with nanotubes are the mostundesirable impurities. Fullerene-free SWNTs were obtained by in-situ heating soot at
1250 °C in vacuum. The soot was refluxed in 15 % H2O2 water solution at 100 °C toremove amorphous carbon and then washed in HCl solution to remove metal particles(6,7). The purity of resulting SWNT material was estimated to be higher than 90 %.
Since purification processes remove caps of SWNTs (8), the purified SWNTs areready to encapsulation of fullerenes. A sheet of SWNT paper was put in a quartz
ampoule with fullerene powder and the ampoule was evacuated to 1×10-6 Torr. Afterdrying process, the sheet and the fullerenes were sealed in the ampoule and were
heated in a furnace up to 650 °C. After keeping the temperature for two hours, theampoule was cooled down to room temperature. The SWNTs paper was sonicated intoluene for 1 hour to remove fullerenes coated on SWNTs surface. In this work, weused C60 and C70 in 99 % purity as fullerene sources.

RESULTS AND DISCUSSIONHigh purity peapod samples were successfully synthesized in both cases, C60 andC70. Figure 1 shows typical high-resolution transmission electron microscope(HRTEM) images and electron diffraction patterns of C60- and C70-peapods. Although
HRTEM photographs obtained are similar to those shown in the previous work (3), ayield of fullerene encapsulation of our sample is much higher. It is difficult to findempty SWNTs in HRTEM photographs. Furthermore, a cross section image evidently
indicates that every nanotube is encapsulating C60 molecules even in case of thickbundles. We have estimated a filling rate of C60- and C70-peas to be higher than 70 %by HRTEM observations.

1248 Hg(6)1090 Hg(5)?

710 Hg(3)771 Hg(4)431 Hg(2)* 492 Ag(1)~270 Hg(1)81 - 100

He Temp.

2nd order of RBM

Intensity (arb. units)

60

Intensity (arb. units)

C -Peapods

1424 Hg(7)* 1467 Ag(2)Proceeding of ISNM2001, to be published in AIP proceedings.

**C70-peapods
****
*

*

*

C70

*film
x 1/4

Room Temp.

0400

800

1200-1

Raman shift (cm )

1600

0400

800

1200

1600

-1

Raman shift (cm )FIGURE 2. Raman spectra of C60- and C70-peapods. The left figure shows Raman spectra of C60peapods at liquid-helium (upper) and room temperature (lower). The right figure shows the Raman
spectra of C70-peapods and C70 thin film measured under the same condition at room temperature.
Laser wavelength used is 488 nm.

New streak lines inside the first arc reflection were observed in electron diffraction
patterns for peapod-bundles. These streaks mean one-dimensional fullerene crystals in
nanotubes. C60-C60 distance was estimated to be 0.95 nm, which is larger than apolymer’s value, 0.92 nm, and smaller than a monomer’s value, 1.00 nm (9). Theintermediate C60-C60 distance suggests a dimer phase as a possible structure of theone-dimensional C60-crystal in nanotube. In case of C70-peapods, we observed very
sharp double streaks. The narrower streaks indicate C70-C70 distance to be 1.10 nm andthe wider streaks 1.00 nm. The double streaks mean two different crystal structuresinside nanotubes, standing and lying alignment (9). Interestingly, there is nointermediate state. This result indicates C70 prefers standing alignment as high-density
packing, if the diameter of nanotube is sufficiently large. This may cause larger
interaction between C70-peas and nanotubes than C60-peapods.
Figure 2 shows typical Raman spectra of C60-, C70-peapods and a C70 film. In thecase of C60-peapod, Ag(2) Raman modes of C60 molecule decreased very quickly bylaser irradiation at room temperature. Since the final spectrum is similar to that oforthorhombic polymer (10), formations of one-dimensional photopolymers weresuggested. An unpolymerized Raman spectrum was measured at liquid heliumtemperature. Where, 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
vibrational mode of C60 dimer at 96cm-1 (11). Although the frequency is slightly lower
than the intrinsic dimer mode, the peak should be originating from dimers inside
nanotubes because of the following reasons. The most distinctive feature of thespectrum is very weak Ag modes. Peak frequencies of Ag modes are lower than theintrinsic ones and are the same value to the orthorhombic polymer phase. It is wellknown that Hg(1) mode is sensitive to a formation of dimer (11). A broadening ofHg(1) mode observed is consistent with the dimerization. The significant reduction ofAg modes intensity and a appearing of low frequency modes strongly suggest aformation of dimers.

Proceeding of ISNM2001, to be published in AIP proceedings.

In the case of C70-peapods, all the Raman active modes of C70 were observed evenat room temperature and no photoreaction was observed. At sight, however, it is foundthat there are some anomalies in relative Raman intensity and peak widths. Raman
mode intensity lying higher that 1100 cm-1 is strongly reduced. For example, anintensity ratio between peaks at 1229 and 702 cm-1 in C70 film is 5 times larger thanthat in C70-peapods. Further, most of peaks show appreciable broadening. Theseanomalies are probably caused by anisotropic interactions between C70s and SWNTs.
If we use a peak at 702 cm-1 as a standard, however, we can estimate a filling rate ofC70 to be higher than 50% by using mode intensities of the peapod and the thin film.
A spontaneous dimerization and high-density packing of C70 are probably caused inpart by an internal high-pressure state. A fullerene molecule at a tip of nanotube
pushes fullerenes inside driven by a van der Waals interaction. Tománek has
calculated an internal pressure to be 0.1 GPa caused by this effect (12). The highpressure state stabilizes a dimer phase (13) and standing C70s at room temperature.
This effect is very interesting for future applications of peapod materials aspressurized nano-cylinders.

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 the
Ministry of Education, Science, and Culture of Japan.

REFERENCES
1 B. W. Smith, M. Monthioux, D. E. Luzzi, Nature 396 (1998) 323.
2 B. Burteaux, A. Claye, B. W. Smith, M. Monthioux, D. E. Luzzi, J. E. Fischer, Chem. Phys. Lett. 310 (1999) 21.
3 S. Saito and S. Okada, Proc. 3rd Symposium on Atomic-Scale Surface and Interface Dynamics (Fukuoka, 1999) p.
307.
4 H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki and Y. Achiba, Carbon 38 (2000) 1691.
5 A.Thess et al., Science 273 (1996) 483.
6 K. Tohji, private communication.
7 R.Rosen, W. Simendinger, C. Debbault, H. Shimoda, L. Fleming, B. Storner and O. Zhou, Appl. Phys. Lett. 76(2000) 1668.
8 Y. Maniwa, Y. Kumazawa, Y. Saito, H. Tou, H. Kataura, H. Ishii, S. Suzuki, Y. Achiba, A. Fujiwara and H.
Suematsu, Jpn. J. Appl. Phys. 38 (1999) L668.
9 M.S. Dresselhaus, G. Dresselhaus, and P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, AcademicPress, California 1996.
10 A. M. Rao, P. C. Eklund, J-L. Hodeau, L. Marques, M. N. Regueiro, Phys. Rev. B 55 (1997) 4766.
11 S. Lebedkin, A. Gromov, S. Giesa, R. Gleiter, B. Renker, H. Rietschel, W. Krätschmer, Chem. Phys. Lett. 285(1998) 210.
12 D. Tománek, private communication.
13 V. A. Davydov et al., Phys. Rev. B 61 (2000) 11936.