in press (J. Phys. Soc. Jpn.)

C70 Molecular Stumbling inside Single-Walled Carbon
Nanotubes
Yutaka MANIWA1,6,*, Hiromichi KATAURA1, Masatoshi ABE1, Akihiko
FUJIWARA2,6, Ryuji FUJIWARA1, Hiroshi KIRA1, Hideki TOU1, Shinzo
SUZUKI3, Yohji ACHIBA3, Eiji NISHIBORI4, Masaki TAKATA4, Makoto
SAKATA4 and Hiroyoshi SUEMATSU5
1

Department of Physics, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397
2
JAIST, Tatsunokuchi, Ishikawa 923-1292
3
Department of Chemistry, Tokyo Metropolitan University, 1-1
Minami-osawa, Hachioji, Tokyo 192-0397
4
Department of Applied Physics, Nagoya University, Nagoya 464-8603
5
SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198
6
CREST, JST (Japan Science and Technology Corporation).
(Received August 27, 2002)

We report the structural study of C70-one-dimensional (1D)
crystal formed inside single-walled carbon nanotubes (SWNTs). Xray diffraction measurements were performed between 100 K and 999
K on C70-encapsulated SWNTs with an average diameter of 1.37 nm.
Two different domains, where the dominant alignment of C70
molecular long axis is standing or lying with respect to the tube
axis, were observed; the ratio of the standing to the lying C70domains is roughly 7:3. Thermal expansion of interfullerene
distance gave no evidence for orientational phase transitions, notas in the 3D crystals. Instead, the long-range order of thestanding C70-alignment was thermally destroyed with an activationenergy of 39±4 meV. The results imply the importance of either the
1D fluctuation or the tube-C70 interaction at finite temperatures.

KEYWORDS: thermal expansion, carbon nanotube, SWNT, peapod, C70,
phase transition, structure, XRD, fullerene* E-mail address: maniwa@phys.metro-u.ac.jp

Existence of nanometer-size inner hollow cavities1, 2 in singlewalled carbon nanotubes (SWNTs) encourages us to form onedimensional (1D) system. Recent efforts actually led to successful
synthesis of high-quality fullerene crystals3, 4 inside SWNTs, theso called “peapods”. Although several interesting properties5-10
have been predicted or observed in this system, order-disordertransition related to the orientation of fullerene molecule inside
SWNTs has not yet been clarified. In the case of C60-peapods, thetheoretical calculation predicted that the electronic states near
the Fermi level are substantially modified by the C60-SWNT
interaction5. Thus, in general, the issue of how the electronic
state of peapods is affected by the fullerene orientation is aninteresting one, and information on this is important for basic
understanding of the physical properties of peapods. In thepresent paper, we report the structural study of C70-peapods bymeans of X-ray diffraction (XRD) in a temperature range between
100 K and 999 K.
The peapods were prepared by a vapor reaction method4,6 using
purified SWNTs. The starting raw materials were generated by apulsed laser vaporization method using a carbon rod with Ni and Co
catalysts11. Transmission electron microscopy confirmed that theparallel-aligned carbon nanotubes were packed into bundles with atriangular lattice (Fig.1a). We also compared the results of C70peapods with those of double-walled carbon nanotubes (DWNTs) andC60-peapods. Here, the DWNTs were converted from C60-peapods byheating at ~1470 K (ref. 12). The C70 peapods and DWNTs areschematically illustrated in Figs. 1(b)-1(d).
Figure 2(a) shows the typical powder XRD patterns of C60- andC70–peapods and the pristine SWNTs at RT. The data were collectedusing synchrotron radiation with a wavelength of 0.1000 nm at beamlines BL02B2 of SPring-8 and BL1B of KEK PF. Samples were sealed
in a quartz capillary after being evacuated at ~800 K to remove
adsorbed gases13-15. The observed peaks are basically indexed on thebasis of the 2D triangular lattice11 formed by 1D SWNTs. In Fig.
2(a), we observed marked changes in the XRD profiles on2

encapsulation: the 10 peak around Q~4.5 (1/nm) is strongly
depressed, and its onset shifts to the high-Q side, consistent
with earlier works on molecular encapsulation inside SWNTs (refs.
13-15). We also observed new peaks around Q=6 (1/nm), indicated byarrows. The lack of the corresponding peaks in the DWNTs (Fig.
2(c)) shows that they are assigned to diffraction peaks due to the
1D array of fullerene molecules inside. The estimated lattice
constant, a, of the 1D fullerene crystals, which corresponds tothe interfullerene distance, is shown in Table I, along with theinterfullerene gap defined by g=a-M, where M is the mean diameterof the fullerene molecule.
The observed features mentioned above were well reproduced bya simulation (Fig. 2(b)), in which the SWNTs and fullerene
molecules are approximated to homogeneously charged shells11.
Within the framework of this model, we cannot discuss the
structures in detail, such as the carbon coordinates within agraphene sheet. However, we could obtain the average triangularlattice constant L and tube diameters D, and found that L and D
almost retain on encapsulation: L=1.649±0.010 nm and D=1.352±
0.002 nm for the C60-peapods, and L=1.683±0.010 nm and D=1.373±
0.002 nm for the C70-peapods.
In the case of C70 peapods, the two types of interfullerene
distances, corresponding to the two peaks shown by arrows in Fig.
2(a), were observed: 1.00±0.01 nm and 1.10±0.01 nm at RT. These
distances nearly correspond to those for the so-called “standing”
and “lying” alignments4 of the C70 molecules with an elongated“rugby-ball” shape, as schematically shown in Figs. 1(b) and 1(c),respectively. The observed clear double-peaked structure implies
that there are two different domains; in one of them, the longmolecular axes of C70 tend to align perpendicular to the tube axis
at RT, but in the other, most C70 molecules align parallel to thetube axis. The simulation results indicate that the SWNTs for the
standing alignment are dominant compared to those for the lying

3alignment in the present sample, and the ratio of these domains is
7:3.
Because the SWNTs studied have a diameter distribution thatpeaks around ~1.37 nm (the mean diameter estimated from thepresent XRD profile), the mean gaps between the tube wall and C70
molecule is 0.29 nm for the standing alignment and 0.33 nm for the
lying alignment. Interestingly, the former gap is nearly equal to0.292 nm in the bulk crystal of solid C60 and to 0.29-0.31 nm in
SWNT bundles. These findings suggest the importance of tube-C70
interaction in the stabilization of the standing alignment. The
lying alignment may be favorable for thicker SWNTs, because the C70
molecules may tend to lie in order to gain the contact area
between the tube wall and fullerene molecule. In thinner tubes,
where the tube wall must be substantially deformed to insert thestanding C70, accommodation of the standing C70 is also difficult.
Thus, the tube diameter is one of the important factors fordetermining the orientation of fullerene molecules inside the
SWNTs. SWNTs with a different diameter actually gave a different
standing alignment.
Now we discuss the thermal expansion of the C70 crystal.
Figure 3 shows the temperature dependence of the interfullerene
distance normalized at RT, along with that of solid C60 (shown by asolid straight line). We observed that the thermal expansion forthe standing alignment is substantially larger, while that for the
lying alignment is comparable to that of the bulk C60 crystal. Thisis unusual if the thermal expansion coefficient for the lattice
constant, αL, is dominated by that of the intermolecular gap, αg.
Assuming that the thermal expansion of the molecular axis isignored, the αg in the standing alignment is estimated to be ~11 x
10-5 (1/K) from the observed αL as an average value between 300 K
and 999 K, which is much larger than 2.6 x 10-5 (1/K) for thegraphite interlayer gap, ~5.4 x 10-5 (1/K) for the solid C60 (ref.
16) and ~4.2 x 10-5 (1/K) for the SWNT bundles15.

4Such a large thermal expansion of the standing alignment must
be due to a reorientation of C70 molecules inside the tubes. In thecase of 3D solid C70, it has been known1,17 that the structuralphase transition successively occurs at ~340 K and ~280 K: the C70
molecules exhibit a quasi-free rotation above ~340 K, below which
the molecular long axis freezes but the short axis is still
dynamically disordered, and finally the complete freezing occurs
below ~280 K. These orientational ordering transitions accompany achange in the lattice constant. In the present standing C70peapods, almost all the molecules are aligned perpendicular to thetube axis at RT, so that when the T is raised from RT a jump of
2~3% in the lattice constant is expected at a transition into the“free-rotational state”. The data, however, did not show such asudden change. Instead, the lattice constant gradually increases
with increasing T, indicating that the C70 molecular stumbling isthermally activated18.
Assuming the simplest model for a quantitative discussion,
the lattice constant is given by
a = M S + (1 + α g T ) g (0) + ( M L − M S )exp(− E0 / k BT )[2 + exp(− E0 / k BT )](1),where MS, ML, g(0), and E0 are the mean lengths of molecular shortand long axes, intermolecular gap at T=0 K, and activation energy
for molecular stumbling, respectively. The second term describes
the thermal expansion of the intermolecular gap and thecoefficient αg is taken to be ~5.4 x 10-5 (1/K), the value for thesolid C60. The third term is due to the C70 stumbling, where E0 is
the energy required to make a C70 molecule lie in the standing“ferromagnetic” array of molecules. (Note that the standing C70
molecule can take two possible orientations perpendicular to thetube axis.) The least-squares fit with this equation leads to agood reproduction of the data, as shown by the thick solid curvein Fig. 3, using the parameters, E0=39±4 meV and g(0)=0.276±0.08
nm.
5Why is the long-range order of standing alignment graduallydestroyed with increasing temperature? One possibility is the casethat the C70-intermolecular interaction is negligibly small and thestanding alignment is stabilized solely by the tube-C70
interaction. In this case, the lying C70 should be thermally
excited as observed. However, the fact that the transition
temperatures (~280 K and ~340 K) in the bulk 3D crystals arecomparable in magnitude to the observed E0/kB~449 K suggests thatthe intermolecular interaction can not be ignored, because thetransition temperature for the bulk materials is taken as atentative magnitude for the intermolecular interaction. Thus,
alternatively, the one dimensionality (1D) of the crystal must be
considered. In 1D system with a short-range interaction19, it is
known that the long-range order is destroyed by 1D fluctuation at
finite temperatures. In 1D ising model with a ferromagnetic groundstate, it is known that the domain-wall excitation is described bya similar formula to eq. (1). In the present case, however, since
the lying and standing domains may have different energies, thesituation is slightly different from the case of the above isingmodel. Here, we encounter a new situation where not only
anisotropic C70-C70 interaction but also anisotropic C70-SWNT
interaction must be taken into account, and we need moresophisticated analysis for the lattice expansion.
Anyway, the present studies clarified that the standingalignment is dominant and stable at the low temperature in C70peapods with typical SWNTs with a mean diameter of 1.37 nm. With
increasing temperature, C70 stumbling is thermally activated with arather small activation energy of 39 meV. Thus, in a broad
temperature region including room temperature, the C70 molecularreorientation cannot be ignored, because it affects many basic
physical properties of C70-peapods. In the future, however, themolecular dynamics should be investigated as a function of the C70filling inside the SWNTs, as well as a function of the diameter of
SWNTs.
Acknowledgements
6

Authors thank Professor Y. Okabe for carefully reading themanuscript.

This work was supported in part by a Grant-in-Aid for

Scientific Research on the Priority Area “Fullerenes andNanotubes” by the Ministry of Education, Culture, Sports, Scienceand Technology of Japan, and by a grant from Japan Society for
Promotion of Science, Research for the Future Program. H. K is
thankful for a Grant-in-Aid for Scientific Research (A), 13304026
by the Ministry of Education, Culture, Sports, Science andTechnology of Japan.

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8

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Suzuki and Y. Achiba: Chem. Phys. Lett. 336(2001)205.
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9

(a)

Figure 1.

(b)

(c)

(d)

(a) An example of transmission electron microscopy (TEM)

image of C70-peapods. (b), (c) and (d) Schematic illustrations of
C70 standing alignment, lying alignment, and double-wall carbon
nanotube (DWNT), respectively. The DWNT has a (10, 10) index forthe outer tube, and (5, 5) for the inner tube. The inset shows across section of a bundle. The scale is 10 nm.

10Intensity [a.u.](a) observed

Empty
C70
C60

(b) simulated

10 peak(c) DWNTs

4

Figure 2.

6

8

10
12
Q (1/nm)

14

16

XRD profiles of pristine (empty) SWNTs, C70-peapods andC60-peapods: (a) observed and (b) simulated. In the simulations, asmall amount of bundles of empty SWNTs was assumed to be included:
15 wt% for the C60-peapods and 30 wt% for the C70-peapods. (c)Observed XRD profile of DWNTs. The arrow shows the lack of thepeak due to 1D C60 crystal observed in C60-peapods.

11

1.03
standing
1.02

C70-peapods
lying

a(T)/a(RT)

1.01
C60
1
0.99
0.98

Figure 3.

200

400

600 800 1000 1200
T [K]Normalized interfullerene distance (1D lattice constant)

as a function of temperature. The solid straight line shows thatof solid C60. The solid curve is the least-squares fit with eq. (1)in the text.

12

Interfullerene distance (lattice constant), interfullerene

Table I.

gap and tube-fullerene gap estimated by XRD analysis.
[nm]

C60-peapod (#1)(#2)
C70-peapod
C60 solid****

InterfullereneInterfullerene

fullerene-distance, agap*, gtube gap**

0.97

0.26

0.321

0.95 (0.95

***

0.24

0.321

1.00 (1.00

***

0.290

0.289

1.10 (1.10

***

0.304

0.332

)
)
)

1.002

0.292

*The molecular shape is assumed to be spherical with a mean diameterof 0.710 nm for C60 and that of a rugby ball with a mean short (a-b)
axis of 0.712 nm and a mean long (c-) axis of 0.796 nm for C70 (ref.
1).
**The gap between the tube wall and fullerene molecules estimatedusing the average tube diameter.
***

From electron diffraction measurements (ref. 4).

****

Reference 1.

13