Perfectly straight nanowires of fullerenes bearing long alkyl chains on graphite

Article abstract of DOI:10.1021/ja061450f

Fullerene derivatives bearing long alkyl chains epitaxially adsorb on the basal plane of graphite forming well-ordered one-dimensional lamellae. Within the lamellae, the C₆₀ moieties are organized in a zigzag-type fashion. The ordering is mainl

Full text of DOI:10.1021/ja061450f

Published on Web 04/26/2006
Perfectly Straight Nanowires of Fullerenes Bearing Long Alkyl Chains on
Graphite
Takashi Nakanishi,*,† Naoko Miyashita, Tsuyoshi Michinobu, Yutaka Wakayama, Tohru Tsuruoka,
‡
†
†
†
†
,†,‡
Katsuhiko Ariga, and Dirk G. Kurth*
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan, and Max Planck Institute of
Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany
Received March 2, 2006; E-mail: nakanishi.takashi@nims.go.jp
To handle, operate, and manipulate nanoscopic devices, self-
organization of molecular modules on solid surfaces and interfaces
has led to diverse research activities in nanotechnology and
1
nanoscience. Fine-tuning of interfacial and intermolecular interac-
tions permits building well-defined, ordered structures over large
areas, which have been visualized, characterized, and manipulated
2
by scanning probe techniques. Many molecules, such as derivatives
of long alkyl chains, self-assemble at suitable interfaces, such as
3
highly oriented pyrolytic graphite (HOPG), giving rise to close to
perfectly ordered structures. These ordered molecular structures can
4
serve as a template layer for biological molecules and for
5
visualization of aromatic molecules. Other examples include the
6
topological reaction of diacetylenes at the nanoscale or the
expression of molecular chirality.7
One-dimensional (1-D) nanostructures of organic semiconductors
are attractive candidates as active components in electronic nan-
odevices. A class of materials for which this is particularly relevant
is that of nanocarbon materials, including fullerenes. While the 2-D
8
arrangement of fullerenes on surfaces is well established, alignment
9
of fullerenes in 1-D architectures is far less common. We, therefore,
asked if it is possible to self-assemble nanowires in a predictable
way by using tailored fullerenes. As a structure directing component,
we attached to the fullerene long alkyl chains, which should induce
epitaxial assembly on HOPG in a facile way. In addition, the alkyl
chains act as insulating barriers, which spatially separate the
fullerene nanowires. In this communication, we report the formation
of 1-D lamellae, epitaxially oriented along the HOPG lattice, formed
by fullerene derivatives bearing long alkyl chains (Figure 1a). The
distance between adjacent nanowires can be regulated at nanometer
level by the molecular design.
Figure 1. (a) Structure of fullerene derivatives (1-5). AFM image (b) of
2
1
on HOPG spin-coated from CHCl3 solution (scan range 100 × 100 nm ,
Z range 2 nm). (c) High-resolution STM image of 1 on HOPG (scan range
2
3
0 × 30 nm ; Iset ) 40 pA; Vbias ) +3.0 V). STM (c) reveals the
organization of the C60 moieties within the lamellae. (d) Schematic
illustration showing the molecular organization of 1 in the lamellae.
Tapping-mode AFM images reveal that fullerene 1 forms a 1-D
lamellar structure on HOPG (Figure 1b). The length of the lamellae
exceeds 100 nm. The lamellae are characteristic for alkyl chain
3
The fullerene derivative with three hexadecyl chains (1) was
recently synthesized. In the volume phase, this molecule exhibits
a variety of solvent-dependent hierarchical architectures, including
fibers, spheres, disks, and cones.10 On the basis of these findings,
we decided to synthesize fullerene derivatives (2-5) and study their
assembly on HOPG. The newly synthesized fullerenes were
prepared following the same strategy as for 1 and were unambigu-
nanostructures on graphite. Presumably, the alkyl chains assemble
along the underlying lattice axis of the basal plane of graphite, thus
arranging the fullerene moieties on the surface. This hypothesis is
supported by the observation that stripes in different domains are
rotated by multiples of 60° with respect to each other, reflecting
the symmetry of the underlying lattice. The periodicity of the
nanostripes is determined to be 6.3 nm. On the basis of molecular
modeling, the length of 1 in an all-trans conformation is estimated
to be 3.1 nm,10 that is half the lamellar width. Therefore, we propose
that the lamellae are composed of fully extended alkyl chains, and
that the molecules are arranged in a head-to-head bilayer architec-
ture. The AFM image reveals a height difference in the dark and
bright regions (Figure 1b) of 0.4-0.5 nm. Alkanes are expected to
have a height of 2.5 Å and C60 of 7 Å.11 We associate the bright
regions with the C60 moieties and the darker ones with the alkyl
tails. The data suggest that all three alkyl chains adsorb to the
surface. In Figure 1c, the high-resolution STM image, measured
under the optimized conditions, shows lamellae composed of C60
arranged in a zigzag-type fashion. The periodicity of the lamellae
1
13
ously characterized by H and C NMR, FT-IR, and UV-visible
spectroscopy as well as MALDI-TOF mass spectrometry (Sup-
porting Information).
Fullerenes 1-5 were spin-coated from chloroform solutions (c
-
5
)
2 × 10 M) onto freshly cleaved HOPG under conditions to
achieve submonolayer coverage in order to reveal structural details
of the interfacial layers. AFM measurements were carried out in
tapping-mode under ambient atmosphere, and STM measurements
were performed at 100 K under ultrahigh vacuum.
†
National Institute for Materials Science.
Max Planck Institute of Colloids and Interfaces.
‡
6328
9
J. AM. CHEM. SOC. 2006, 128, 6328-6329
10.1021/ja061450f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
The electrochemical activity of the C60 groups is fully maintained
in the surface-confined assemblies.
In summary, we presented an approach to direct the assembly
of C60 groups on graphite in a predictable way. We used alkanes
as structure directing components, covalently bonded to C60. The
epitaxial assembly of the alkyl chains on graphite results in
formation of lamellar structures and a 1-D alignment of the
fullerenes. Derivatives with three alkyl chains adsorb in a head-
to-head bilayer structure. The C60 moieties are organized in a zigzag-
type fashion within the lamellae. If only one alkyl chain is present,
an interdigitated lamella is formed. The spacing between lamellae
is determined by the alkyl chain length. Our work indicates that
the substitution pattern and the chain length of the alkyl chains
determine the order and packing motive. We show that the C60
moieties are electroactive, opening the way to address individual
Figure 2. AFM image (a) of 2 on HOPG spin-coated from CHCl3 solution
2
(
scan range 100 × 100 nm , Z range 2 nm). Cyclic voltammogram of 2 (b)
on HOPG (0.1 M n-Bu4NClO4, CH3CN, Ar atmosphere, scan rate 0.1 V
-
1
s
, 20 °C).
is again 6.3 nm. The C60 moieties appear larger (approximately
.8 nm) than their actual size (approximately 1 nm)9,12 perhaps
C
60 and to investigate the conductivity of individual (polymerized)
1
nanowires.
because of the tip-object convolution. Apparently, the C60 moieties
are not close packed as one would expect from the spatial mismatch
of three alkyl chains and C60. The organization of 1 within the
lamellae is schematically illustrated in Figure 1d. Epitaxial ordering
of the alkyl chains on the basal planes of graphite forces the
molecules into a lamellar architecture, while intermolecular π-π
interaction gives rise to the zigzag ordering of the C60 moieties.
To further verify the hypothesis of the ordering process, we
synthesized fullerene derivatives with different alkyl chain length.
We expected that longer alkyl chains should result in a larger
periodicity of the lamellae. Derivative 2 has three eicosanyl chains
in the 3,4,5-positions of the phenyl group and an estimated length
of 3.6 nm in a fully extended conformation. Similar to 1, AFM
imaging of 2 shows close to perfect lamellae (Figure 2a). In contrast
to 1, the length of the lamellae is several hundred nanometers, and
the domain sizes of lamellae are larger than that of 1 (Supporting
Information). The periodicity of the nanostripes is determined to
be 7.2 nm, which corresponds nicely to twice the molecular length
of 2, indicating the same structural motive as derivative 1. This
result supports the structural model as shown in Figure 1d.
In contrast, molecules having three dodecyl chains (3) in the
Acknowledgment. This work was supported, in part, by a
Grant-in-Aid for Young Scientists (B) (No. 17750140) from the
Ministry of Education, Science, Sports and Culture, Japan, and
Iketani Science and Technology Foundation. We are grateful to
Dr. T. Hasegawa and Dr. Y. Okawa, NIMS, for assistance during
the STM measurements.
Supporting Information Available: Synthetic procedures for
compounds 2-5, AFM, and CV in solutions. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(
1) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (b)
Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton,
P. H. Nature 2003, 424, 1029.
(
2) (a) Kunitake, M.; Uemura, S.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu,
K. Angew. Chem., Int. Ed. 2002, 41, 969. (b) Samori, P. Chem. Soc. ReV.
2005, 34, 551. (c) Gyarfas, B.; Wiggins, B.; Zosel, M.; Hipps, K. W.
Langmuir 2005, 21, 919.
(
(
(
3) (a) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (b) Xu, S. L.; Zeng,
Q. D.; Wu, P.; Qiao, Y. H.; Wang, C.; Bai, C. L. Appl. Phys. A 2003, 76,
209.
4) (a) Severin, N.; Barner, J.; Kalachev, A. A.; Rabe, J. P. Nano Lett. 2004,
4
, 577. (b) Mao, G.; Chen, D.; Handa, H.; Dong, W.; Kurth, D. G.;
M o¨ hwald, H. Langmuir 2005, 21, 578.
3,4,5-positions or two hexadecyl chains in the 3,5-positions of the
5) (a) Xu, B.; Yin, S.; Wang, C.; Qui, X.; Zeng, Q.; Bai, C. J. Phys. Chem.
B 2000, 104, 10502. (b) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew.
Chem., Int. Ed. 2002, 41, 3681.
phenyl group (4) do not form ordered lamellae but random
aggregates. The findings correlate with the thermal and structural
properties determined by DSC13 and FT-IR.
(6) Okawa, Y.; Aono, M. Nature 2001, 409, 683.
14
(
7) (a) Gesqui e` re, A.; Jonkheijm, P.; Hoeben, F. J. M.; Schenning, A. P. H.
J.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. Nano. Lett. 2004, 4,
In the case of derivative 5, which has just one hexadecyl chain,
the length and the domain size of the lamellae are smaller than
those found in 1 and 2 (see Figure S3). Interestingly, the periodicity
of the nanostripes is only 4.3 nm, which is much shorter than twice
the molecular length as in the previous examples. Apparently,
derivative 5, with an estimated molecular length of approximately
1
175. (b) Stepanow, S.; Lin, N.; Vidal, F.; Landa, A.; Ruben, M.; Barth,
J. V.; Kern, K. Nano Lett. 2005, 5, 901.
(8) (a) Uemura, S.; Sakata, M.; Hirayama, C.; Kunitake, M. Langmuir 2004,
2
0, 9198. (b) Yoshimoto, S.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu,
K.; Ito, O.; Itaya, K. J. Phys. Chem. B 2005, 109, 8547.
(9) Bonifazi, D.; Spillmann, H.; Kiebele, A.; de Wild, M.; Seiler, P.; Cheng,
F.; G u¨ ntherodt, H.-J.; Jung, T.; Diederich, F. Angew. Chem., Int. Ed. 2004,
43, 4759.
3.1 nm, forms a different arrangement. We propose that 5 forms
(10) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga, K. Chem.
Commun. 2005, 5982.
interdigitated lamellae with a head-to-tail configuration. Such an
arrangement maximizes the van der Waals interactions of adjacent
alkyl chains as well as C60 moieties. The alternative structure of a
tilted bilayer is discarded because of the significant spatial mismatch
of the C60 moiety and the alkyl chain.
These results demonstrate that the distance between adjacent
fullerene nanowires can be tailored at the nanoscopic level through
the molecular design (1, 6.3 nm; 2, 7.2 nm; and 5, 4.3 nm).
Cyclic voltammetry (CV) of 1 and 2 (Figure 2b) on HOPG shows
a single redox event corresponding to the generation of the fullerene
monoanion15 at a potential of E1/2 ) -1.29 V for 1 and -1.30 V
for 2. The current passed during the reduction process, and the
electroactive quantity is calculated to be 25% for 1 and 63% for 2
of full monolayer coverage. These results are comparable with the
surface coverage determined by AFM (Supporting Information).
(
11) The nearest-neighbor distance of C60 is 1.0 nm, and the carbon cage
diameter is 0.7 nm: (a) Kr a¨ tschmer, W.; Lamb, L. D.; Fostirovios, K.;
Huffman, D. R. Nature 1990, 347, 354. (b) Klyachko, D.; Chen, D. M.
Surf. Sci. 2000, 446, 98.
(
12) (a) Kobayashi, K.; Yamada, H.; Horiuchi, T.; Matsushige, K. Appl. Surf.
Sci. 1999, 140, 281. (b) Spillmann, H.; Kiebele, A.; St o¨ hr, M.; Jung, T.
A.; Bonifazi, D.; Cheng, F.; Diederich, F. AdV. Mater. 2006, 18, 275.
13) Differential scanning calorimetry (DSC) patterns showed a phase transition
behavior with the endothermic peak at 30.6 °C for 1, at 58.9 °C for 2,
and at -37.1 °C for 3; in contrast, there were no phase transition peaks
for 4.
(
2
(14) FT-IR at room temperature displays CH stretching vibrations for 1 and
2
at 2849 (νsym) and 2918 cm- (νasym), indicating an all-trans conformation
1
-1
of alkyl chains, while 3 and 4 show peaks at 2852 and 2922 cm
characteristic of disordered alkyl chains.
,
(
15) (a) Fullerenes: Chemistry, Physics, and Technology, Kadish, K. M., Rouff,
R. S., Eds.; Wily-Interscience: New York, 2000. (b) Nakanishi, T.; Morita,
M.; Murakami, H.; Sagara, T.; Nakashima, N. Chem.sEur. J. 2002, 8,
1641.
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