APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 5
2 FEBRUARY 2004
Controlled orientation of ellipsoidal fullerene C70 in carbon nanotubes
Andrei N. Khlobystov,a) Roberto Scipioni, Duc Nguyen-Manh, David A. Britz,
David G. Pettifor, and G. Andrew D. Briggs
Department of Materials, Oxford University, Parks Road, Oxford, OX1 3PH United Kingdom
Sergey G. Lyapin, Arzhang Ardavan, and Robin J. Nicholas
Department of Physics, Oxford University, Parks Road, Oxford, OX1 3PU United Kingdom
共Received 18 July 2003; accepted 8 December 2003兲
Density functional theory calculations predict two orientations for ellipsoidal C70 fullerenes inside
single-walled carbon nanotubes 共SWNTs兲 of different sizes: transverse orientation for C70 in 共11,11兲
nanotubes (d⫽14.9 Å) and longitudinal orientation for C70 in 共10,10兲 nanotubes (d⫽13.6 Å).
SWNTs with these diameters have been prepared and filled with the C70 fullerenes, and
characterized by Raman spectroscopy and high-resolution transmission electron microscopy,
showing the orientations predicted by theory. © 2004 American Institute of Physics.
关DOI: 10.1063/1.1644614兴
Single-walled carbon nanotubes 共SWNTs兲 offer a template for ionic and molecular nanomaterials.1 They can serve
as one-dimensional molds for a broad range of metal
halides2–5 and transition metals.6 Molecularly self-assembled
carbon structures can be made by allowing fullerenes to enter
nanotubes, forming Cn @SWNT peapods. The fullerene molecules may themselves contain endohedral atoms, thus offering potential for quantum scale electronic materials.7 A key
question is the nature of the interaction between the
sp 2 -hybridized carbon atoms in the tube and the molecules,
especially because of the possibility of altering the electronic
properties of the nanotube by inserting fullerenes of different
sizes and doping.8 –10
Fullerenes with ellipsoidal shape are of particular interest because, unlike the spheroidal molecules such as C60 ,
there are several geometrically distinct orientations possible
for an ellipsoidal molecule within a nanotube. Therefore, the
nanotube electronic properties sensitive to the symmetry of
fullerene arrays11,12 can be tuned by controlling the fullerene
orientation. By far the most abundant ellipsoidal fullerene
available in isomerically pure form is 关5,6兴 fullerene-C70 .
Two different repeat lengths of 11.0 and 10.1 Å, have been
measured by electron diffraction for large bundles of
C70@SWNTs which have been attributed to the presence of
two orientations of C70 in SWNTs.13 However, orientations
of C70 were not directly observed for isolated nanotubes,
which precludes finding out how many different orientations
are possible within a given nanotube or the distribution of
orientations within the sample. The accuracy of the electron
diffraction techniques also suffers significantly from various
factors such as the tilt of the nanotube bundle and the presence of partially filled nanotubes14 which emphasizes the importance of the direct visualization of C70 .
We have synthesized C70@SWNT structures, using
SWNTs of the appropriate diameters, in order to demonstrate
the influence of the nanotube diameter on the orientation of
the ellipsoidal fullerenes. Raman spectroscopy, conventiona兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
ally employed for measurements of nanotube diameters, has
demonstrated the broad diameter distribution of the aspurchased 共Aldrich兲 raw nanotubes 关Fig. 1共a兲兴. Raman analysis performed at different excitation wavelengths has confirmed that the diameter distribution was selectively
narrowed during purification15 to two major diameters 关Fig.
1共b兲兴. The frequency ␻ RBM of the radial breathing vibration
mode 共RBM兲 yields the diameters from the relationship
d NT⫽248/␻ RBM , 16 to indicate the presence of 共10,10兲 and
共11,11兲 armchair nanotubes with diameters of 13.56 and
14.92 Å, respectively. Theoretical17,18 and experimental19
studies on the optimum sizes of SWNTs suitable for encap-
FIG. 1. Raman spectra of the RBM for 共a兲 raw SWNT, 共b兲 purified SWNT,
and 共c兲 C70@SWNT. The spectral line shapes are fitted by a sum of Lorentzians 共dashed lines兲. Laser wavelength⫽476 nm.
0003-6951/2004/84(5)/792/3/$22.00
792
© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 84, No. 5, 2 February 2004
FIG. 2. Relative energy of C70@SWNT system plotted as a function of
angle ␪ calculated for 共10,10兲 SWNT 共triangles兲 and 共11,11兲 SWNT
共circles兲.
sulation of fullerene molecules indicate that nanotubes of
diameter greater than 13 Å are capable of hosting C60 or C70
fullerenes.
In order to elucidate the mechanism for molecular alignment, we have modeled a C70 molecule encapsulated in
nanotubes of varying diameter. The calculations were obtained using the density-functional based tight-binding
共DFTB兲 method20 which has been implemented within an
efficient DFT package for linear combination of atomic type
orbitals.21,22 This method combines the accuracy of density
functional theory with the speed of the nonorthogonal tight
binding method with minimal basis set. This allows performing computations with systems containing 102 – 103 atoms
with a single CPU. In the DFTB approach, the total energy
of the system can be approximated as a sum over the band
structure energy and a short range repulsive two-body potential as
E tot⫽
兺i n i ␧ i ⫹ 兺k 兺l V Rep共 兩 R l ⫺R k兩 兲 ,
where n i and ␧ i are the occupation number and the eigenvalues, respectively, obtained by solving the Kohn–Sham equations. The repulsive part is fitted with self-consistent local
density approximation data. It is known that DFT does not
reproduce van der Waals interactions.23 However, a combination of long-ranged cutoff of hopping and overlap integrals
with the local density approximation can give the correct
interlayer distance for graphite and other s p 2 carbon structures. The calculations were performed with an energy resolution of 10⫺6 eV and k points mesh of ten points to achieve
full convergence. The basis sets used in these calculations
have been extensively tested and found to predict correctly
properties of carbon structures such as fullerenes and nanotubes.
The total energy for different orientations of C70 inside
共10,10兲 and 共11,11兲 armchair nanotubes is shown in Fig. 2, as
a function of the polar angle ␪ between the long axis of the
fullerene and the axis orthogonal to the nanotube. The polar
angle ␪ is the most important geometrical parameter in
C70@SWNTs systems, as only ␪ can be directly measured by
Khlobystov et al.
793
FIG. 3. HRTEM micrographs of C70@SWNT with 共a兲 transverse and 共c兲
longitudinal orientation of C70 .
high-resolution transmission electron microscopy 共HRTEM兲
and the energy of C70@SWNTs depends significantly more
on ␪ than on other angular parameters. The total energy is
lower for the longitudinal C70 orientation in the narrower
共10,10兲 nanotubes 关Fig. 3共d兲兴 whereas the transverse orientation is more stable for the wider 共11,11兲 nanotubes 关Fig.
3共b兲兴. This tendency is consistent with a general interaction
for graphitic systems,17,18 in which the optimum graphene
sheet van der Waals separation is 3.34 Å. For the ellipsoidal
fullerene C70 , with a short axis of 6.91 Å and a long axis of
7.92 Å, this long-range interaction is maximized by adjusting
the orientation of the ellipsoidal molecule with respect of the
nanotube.
The DFTB energy difference between transverse and
longitudinal orientations of isolated chains of C70 fullerenes
with van der Waals intermolecular separations at the chain
repeat distances of Fig. 2 was found to be less than 2 meV,
making orientational dependence of fullerene–fullerene interactions
negligible.
Therefore,
calculations
for
C70@SWNTs structures have been performed considering
one C70 molecule per five unit cells of the nanotube with a
periodic boundary condition. The supercells used in calculations contain 270 and 290 atoms for the C70@(10,10) and
C70@(11,11), respectively. The fullerene–fullerene separation was therefore over 4.5 Å for both diameters so that the
fullerene–fullerene interactions are negligible.
We filled the nanotubes with the C70 fullerene 共99%
pure, Aldrich兲 in the gas phase at 405 °C (5⫻10⫺6 Torr, 24
h兲, resulting in C70@SWNT peapods in a high yield 共⬎90%兲
as evidenced by HRTEM 共JEOL JEM-4000EX, LaB6 , accelerating voltage 100 kV, information limit ⬍0.12 nm兲. The
nanotubes were of two different diameters, corresponding to
those indicated by the Raman spectroscopy, and were aggregated in small bundles. Within the nanotubes there were two
orientations of the C70 molecules ( ␪ ⫽0° and ␪ ⫽90°), corresponding to the theoretical modeling. No nanotubes containing C70 in both orientations were found in the sample,
thus indicating that only one type of orientation can exist
within a given nanotube, confirming that the C70 alignment is
associated with the geometrical parameters of the nanotube
共Fig. 3兲. Extensive HRTEM analysis also revealed that
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794
C70@SWNTs with transverse orientation are more abundant
than those with longitudinal orientation. Raman spectroscopy
of the filled nanotubes gave all vibrational modes of C70 at
the same position as for crystalline C70 . However, nanotubes
RBM peaks were downshifted by 2–3 cm⫺1 for C70@SWNT
关Fig. 1共c兲兴 compared to the empty nanotubes thus indicating
the presence of C70 inside nanotubes. This shift is related to
the interaction between ␲-electrons of the fullerene with the
interior of nanotube and comparable to the RBM shifts reported earlier for C60@SWNT structures.24
In summary, by a careful choice of nanotubes with certain diameters we have observed directly two alignment orientations for ellipsoidal C70 molecules encapsulated in the
SWNT. HRTEM examination demonstrated that all fullerenes within a given nanotube have the same type of orientation: transverse or longitudinal. No intermediate orientations were observed. Our theoretical modeling of the
C70@SWNT systems indicates that the orientation of the
molecules is precisely controlled by the nanotube diameter,
thus revealing the mechanism for spontaneous molecular
alignment.
The work described here is supported through the Foresight LINK Award Nanoelectronics at the Quantum Edge,
funded by DTI/EPSRC 共GR/R66029/01兲 and Hitachi Europe
Ltd., and by a NEDO grant. The authors also thank Dr.
James Owen, Dr. Andrew Horsfield, Dr. Steven Kenny, and
Dr. Seung Mi Lee for useful discussions.
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