Proceeding of 25th International Conference on the Physics of Semiconductors,
Osaka International Convention Center, RIHGA Royal Hotel, Osaka, Japan, September 17-22, 2000, in press.

Optical properties of fullerenes encapsulated insingle-wall nanotubes
H. Kataura1 , Y. Maniwa1 , T. Kodama2 , K. Kikuchi2 , K.Hirahara3 , K. Suenaga3 , S. Iijima3 ,S. Suzuki2 , W. Krätschmer4, Y. Achiba2
1

Department of Physics, Faculty of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji, Tokyo 192-0397,
Japan e-mail: kataura@phys.metro-u.ac.jp
2
Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachiohji, Tokyo 1920397, Japan
3
Japan Science and Technology Corporation, International Cooperative Research Project, Department of Physics, Meijo
University, Nagoya 468-8502, Japan
4
Max-Planck-Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany

Abstract Single-wall carbon nanotubes (SWNTs) encapsulating speciﬁed fullerenes, so called fullerene-peapods, weresynthesized in high yield using a sublimation method. Highresolution transmission electron microscopy revealed that
almost all nanotubes are ﬁlled with high-density fullerene
chains. We measured Raman spectra and optical absorption spectra of C60 - and C70 -peapods to see vibrational
properties and electronic structures of peapods. In the caseof C60 -peapods, Raman intensity of C60 modes decreased
rapidly by laser irradiation. The ﬁnal spectrum at room
temperature is similar to that of the orthorhombic polymer
phase. One-dimensional photopolymerization was strongly
suggested. Optical absorption spectrum of fullerene-peas issimilar to that of fullerene thin ﬁlm. A charge transfer exciton band at 2.6 eV was observed in relatively large intensity
even in the one-dimensionally aligned fullerenes though eachconstituent has only two neighbors.

1 Introduction
Single-wall carbon nanotubes (SWNTs) encapsulating
fullerenes, what is called fullerene-peapods, are considered to be a new solid phase of carbon constructed byzero- and one-dimensional sp2 network systems. This
new solid should have interesting electronic properties
because of the mixed dimensionality. Saito and Okada[1] have calculated the band structure of (10,10) nanotube encapsulating C60 , and found some anti-crossingsin the ﬂat conduction band, which indicate the considerable interactions between conduction electrons of
C60 and nanotube. They claimed that fullerene-peapods
could be a good candidate for a possible superconductivity by alkaline metal doping. Peapods were ﬁrst revealed
by Smith et al. [2] and then were synthesized in highyield by Burteaux et al.[3]. However, no one succeeded
to measure optical properties of peapods to date. Then
we tried to make peapods in suﬃcient yield to measure
the optical properties by using a gas phase reaction.
2 Experimental
Carbon soot containing SWNTs were produced by laser
furnace technique using Ni/Co catalyst [4]. By considering diameters of fullerenes and van der Waals spacing,
the diameter of nanotube should be larger than that of(10,10) to get higher yield of encapsulations. Since thediameter distribution of SWNTs can be controlled by
the furnace temperature [5], we have got thick SWNT
samples by setting the furnace temperature for 1250◦
C. To get a high-purity peapod sample, fullerenes coexisting with nanotubes should be removed completely.
Fullerene-free SWNTs could be obtained by heating carbon soot at 1250 ◦ C in vacuum without exposure to theair. The soot was reﬂuxed in 15 % H2 O2 to remove amorphous carbon and then washed in HCl solution to remove
metal particles [5,6]. The purity of resulting SWNT material was estimated to be higher than 90 %. Since the
puriﬁcation process removes caps of SWNTs [8], the puriﬁed SWNTs are ready to encapsulation of fullerenes.
A sheet of SWNT paper was put in a quartz ampoulewith fullerene powder. The ampoule was evacuated tohigh vacuum. Then the sheet was heated up to 600 ◦ C
to remove all adsorbed molecules. The sheet and thefullerenes were sealed in the ampoule and were heatedin a furnace up to 650 ◦ C. After keeping the temperature for two hours, the ampoule was cooled down toroom temperature. The SWNTs paper was sonicatedin toluene for 1 hour to remove fullerenes coated on
SWNTs surface. The sample was then ﬁltered to get asheet of peapod paper. In the above procedure, a speciﬁcfullerene can be selected. In this work, we used C60 andC70 in 99 % purity.
3 Results and Discussion
In both cases, C60 - and C70 -peapod, we have succeeded
to synthesize high purity peapod samples. Figure 1
shows typical high-resolution transmission electron microscope (HRTEM) images and an electron diﬀraction
pattern of C60 -peapods. Although HRTEM photographs
obtained are similar to those shown in the previous work[3], a yield of fullerene encapsulation of our sample ismuch higher. It is diﬃcult to ﬁnd empty SWNTs in
HRTEM photographs. Furthermore, a cross section image evidently indicates that every nanotube is encapsulating C60 molecules even in the case of a thick bundle. By attentive observations, it was found that C60
molecules in a large diameter nanotube tends to form
zigzag chains. This suggests a formation of dimers. We

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H. Kataura et al.

Fig. 1 High-resolution transmission electron microscope images of C60 -peapods. The left ﬁgure shows a side view of thinbundles and the upper right ﬁgure shows cross section image of the bundle. Evidently, C60 molecules are observed inside
SWNTs in high yield. The right lower ﬁgure shows a typical electron diﬀraction pattern of the peapod bundle. Inter C60
distance estimated from streaks is 0.95 nm.

have estimated a ﬁlling rate of C60 - and C70 -peas to behigher than 70 % by HRTEM observations.
By an electron diﬀraction pattern for a thick bundle,
we found new streak lines inside the ﬁrst arc corresponding to (100) reﬂection by the graphite layer of SWNT.
These streak lines mean one-dimensionally aligned dots;
i.e. fullerenes in nanotubes. From the spacing of streaks,
we can estimate the inter-ball distances. In the case of
C60 -peapods, C60 -C60 distance is estimated to be 0.95
nm. This result is very interesting because the distanceobtained is just the intermediate value. In a fcc crystal, C60 -C60 distance is 1.00 nm while it is 0.92 nm ina polymer crystal [9]. The intermediate distance suggests two possible structures. One of them is a mixtureof oligomers and monomers, which may indicate intermediate inter-ball distance as an average. In this case,however, streaks should be broad, which is inconsistent
with the observed sharp diﬀraction streaks. The otherpossible structure is loosely bonded C60 chain due tovan der Waals interaction between C60 and SWNT. In
the case of C70 -peapods, we observed double streak lines.
The narrower spacing corresponds to 1.10 nm and thewider spacing corresponds 1.00 nm. These values correspond to C70 -C70 distance of lying C70 chain and thatof standing C70 chain, respectively [9]. This result indicates that C70 molecules in SWNT are not rotating

freely. Their long axis is ﬁxed at room temperature similarly to the spinning nature of hcp C70 crystal.
Even after washing in toluene, peapod samples gavestrong Raman signals from the encapsulated fullerenes.
Figure 2 shows typical Raman spectra of C60 -, C70 peapods and a C70 ﬁlm. The basic spectrum structure of
C70 -peapods can be explained by Raman active modesof C70 molecules superposed on those of SWNTs. At
sight, however, it is found that 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, an intensity ratio between peaks
at 1229 and 702 cm −1 in C70 ﬁlm is 5 times larger thanthat in C70 -peapods. Similarly, the intensity ratio between peaks at 1469 and 1445 cm−1 is reversed. Further, most of peaks show appreciable broadening. These
anomalies are probably caused by van der Waals interaction between C70 molecules and SWNTs.
In the case of C70 -peapod we did not observe any
phenomenon concerning with a photopolymerization.
Thus we can try to estimate a ﬁlling rate of C70
molecules in SWNTs. If we use a peak at 702 cm−1 , theintensity in C70 -peapod is 1/2 of that in C 70 ﬁlm. We
have to consider the diﬀerence in molecular density between hcp C70 crystal and full ﬁlled C70 -peapods, as well
as a strong optical absorption by surrounding SWNTs.

Optical properties of fullerenes encapsulated in single-wall nanotubes

Optical density (arb. units)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.

1424 Hg(7)60Intensity (arb. units)

0.2* 1467 Ag(2)C -Peapods

3

Room Temp.

0.15

C -peas
60

C

60

0.05
CT

C

0

0

400

800

1200

film

0.1

1600

60

0

1

2

3

4

in hexane
5

6

Photon energy (eV)-1Raman shift (cm )**C70-peapods
*
*

**
*

*

*

*

C70
film
x 1/4

Optical density (arb. units)

Intensity (arb. units)

2nd order of RBM

0.2

0.15

C -peas
70

0.1

C

70

0.05

0

0

400

800
1200
-1
Raman shift (cm )1600

Fig. 2 Raman spectra of C60 - and C70 -peapods. The upper ﬁgure shows Raman spectra of C60 -peapods at liquidhelium and room temperature. The lower ﬁgure shows theRaman spectra of C70 -peapods and C70 ﬁlm measured under
the same condition at room temperature. Laser wavelengthis 488 nm.

These factors are depending on the morphology of nanotubes that is not well known and also depending on how
many kinds of nanotubes are coexisting in the sample.
In other words, Raman intensity of C70 -peas is depending on a number of nanotubes that are not resonatedwith the excitation laser. It is diﬃcult to estimate thesefactors but a ﬁlling rate of C70 in our sample can be
simply estimated to be 50 % as a minimum guaranteedvalue. This is consistent with the ﬁlling rate estimatedfrom HRTEM observations.
In the case of C60 -peapod, Raman modes corresponding to C60 molecules decreased very quickly by laser irradiation at room temperature. Since the ﬁnal spectrum
is similar to that of orthorhombic polymer crystal synthesized under high pressure and high temperature [10],one-dimensional photopolymerization was suggested. To
see the unpolymerized state, it should be necessary tomeasure the Raman spectrum at low temperature where
the spinning of C60 is stopped. The sample was annealed
and cooled down to liquid helium temperature without
exposure to light. Then the Raman spectrum did notshow any change by the irradiation of laser. All the Raman active modes of C60 molecule were observed. Fur-

film

C

in hexane

4

5

70

0

1

2

3

6

Photon energy (eV)Fig. 3 Optical absorption spectra of C60 - and C70 -peapods.
The upper ﬁgure shows optical absorption spectra of C60 peas, thin ﬁlm and hexane solution measured at room temperature. The lower ﬁgure shows the C70 -peas. Absorptions
by empty SWNTs are subtracted for eyes.

ther, additional peaks were observed around 90 cm−1that is close to an external vibrational mode of C 60
dimer at 96cm−1 [11]. Although the frequency is slightly
lower than the intrinsic dimer mode, the peak shouldbe the external vibrational modes of the dimers or theoligomers inside nanotubes. The most distinctive featureof the spectrum is very weak Ag modes. Peak frequencies
of Ag modes are lower than the intrinsic ones and are
the same value to the orthorhombic polymer phase. The
signiﬁcant reduction of Ag modes intensity and low frequency external modes suggest a formation of the dimers
or oligomers. However, it is well known that Hg(1) andAg(1) modes are sensitive to the formation of the dimer.
In the case of C60 dimer phase, Hg(1) and Ag(1) modes
split into three peaks [11]. In the case of peapods, abroadening of Hg(1) mode was observed but no splittingof the other modes were observed. Further, it is apparent
that C60 molecules are not polymerized at liquid heliumtemperature. These antinomic results lead to the intermediate states of C60 molecules that are loosely bonded
together under van der Waals interactions between C60
and nanotubes because the results cannot be explained
only by interactions between C60 molecules. This condition is consistent with the result of electron diﬀraction.

4

Finally, we think about the spinning nature of C 60 -peas.
Since the 2+2 cycloaddition reaction is depending on
the meeting probability of a pair of double bonds, photopolymerization concerns with the spinning nature of
C60 s. Since the temperature dependence of photopolymerization of C60 -peas is similar to the fcc crystalline
phase, the spinning nature of C60 -peas should be alsosimilar to the crystalline phase.
Optical absorption spectra of peapods are indicatedin Fig. 3. Where, optical absorption by empty SWNTs
was subtracted for convenience. Sharp three peaks at
0.7, 1.2 and 1.8 eV are residuals of absorption peaks of
SWNTs, which are corresponding to the optical transition between spikes due to one-dimensional van Hove
singularities [12]. The lowest two peaks are corresponding to semiconducting nanotubes and the third lowest
peak is corresponding to metallic ones in the sample [13].
Broadening and up-shift of absorption peaks by encapsulation of fullerenes causes these residuals. It is foundthat eﬀects of encapsulation on electronic structure ofnanotubes are similar for both the semiconducting andmetallic nanotubes. On the other hand, structures from
2 to 5 eV are mainly ruled by fullerenes. At sight, it isfound that the absorption spectrum of C60 -peas is similar to that of thin ﬁlm more than isolated molecules inhexane. The bandwidth of each peak is broader thanthose in the thin ﬁlm. We can observe similar bandbroadening in a photopolymerized C60 thin ﬁlm. On theother hand, in both C60 - and C70 -peas cases, absorption
bands show downshifts compared with those in the thinﬁlm. The downshifts of absorption bands between theisolated molecules and thin ﬁlms are thought to be explained by dipole interactions [14]. In other words, diﬀerences in local ﬁeld cause peak shifts. Since the local ﬁeld
correction is strongly depending on the symmetry, thedownshifts observed fullerene-peas are probably causedby diﬀerences in local ﬁeld in SWNT.
Absorption band at 2.6 eV in thin ﬁlm is usually assigned as an absorption by charge transfer (CT) excitons[9,14]. The CT exciton band is observed in C60 -peas withsimilar relative intensity to the thin ﬁlm. If the chargetransfer occurs between C60 molecules, intensity of CT
exciton band should depend on the number of neighbors.
C60 in fcc crystal has twelve neighbors, while C60 -pea has
only two neighbors in the nanotube. By considering thediﬀerence in number of neighbors, the relative absorption intensity of CT excitons in C60 -peapod should beanomalously large. If the anomaly is caused by the interactions between C60 and nanotubes, similar anomaly
should be observed in C70 -peas. Furthermore, if C70 s are
standing in a nanotube that was conﬁrmed by the electron diﬀraction, the interaction between C70 s and thenanotube should be larger than that in the C60 -peapod.
Indeed, in the case of C70 -peapod, very large absorption
band around 2.6 eV is observed.

H. Kataura et al.

4 SummaryIn the case of C60 -peapod, results of electron diﬀraction,
Raman scattering and optical absorption lead to a similar image, a formation of dimers or oligomers as an initialstate. However, detailed structure is not clear. A kindof charge transfer between C60 and SWNT should play
an important role to characterize the vibrational andthe electronic properties of C60 -peapods. In the case of
C70 -peapods, such intermediate polymer phase was notsuggested. However, interactions between C70 peas and
SWNT are stronger than C60 -peapods.
Acknowledgements Authors thank Prof. J.E. Fischer forgiving us a good advice to the Raman measurements. This
work was supported in part by Japan Society for Promotionof Science Research for the Future Program and supported
in part by the Grant-in-Aid for Scientiﬁc Research on the
Priority Area ”Fullerenes and Nanotubes” by the Ministryof Education, Science, and Culture of Japan.

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