Selected Papers

Growth and Architecture of Single-Walled Carbon
Nanotubes on Patterned Silicon Substrates
Yoshikazu Homma†, Yoshihiro Kobayashi,
and Toshio Ogino
Abstract
Single-walled carbon nanotubes (SWNTs) were grown on Si and SiO2 patterns to form self-assembled
interconnecting networks. The growth was performed by chemical vapor deposition using methane andFe catalyst. Arrays of Si or SiO2 nano-pillars were prepared using synchrotron-radiation lithography.
The SWNT yield was controlled by choosing the catalyst particle size and growth temperature. When
the SWNT yield was low, a suspended network of individual SWNTs formed. The bridging SWNTs
were fairly straight, but the network density was low. At higher yields, bundles of SWNTs formed, andalmost all the pillars were connected in the SWNT bundle network. Suspended SWNTs will be usefulfor wiring or for the site-selective formation of SWNT-based transistors. They also show enhanced
Raman and photoluminescence signals. Thus, suspended SWNTs are of interest for both electronic andoptical applications.

1. Introduction
Carbon nanotubes, nano-scale cylinders of carbon,
have remarkable mechanical, electrical, and chemical
properties and are expected to be used for various
applications in nanoelectronics [1]. They are selforganizing and can be semiconducting or metallic.
Semiconducting carbon nanotubes act as field effecttransistors [2] or single electron transistors [3], andthey have shown single electron transistor characteristics even at room temperature [4]. Metallic carbon
nanotubes, on the other hand, exhibit ballistic conductivity at room temperature [5], and this along with
their mechanical strength, shape, and size makes
them ideal for the wiring of nano-scale devices. However, before we can build nanotube-based integrated
circuits, we must be able to control the chirality of
nanotubes*1 as well as their growth site, growth direction, and length. To control the growth site and direction, Cassell et al. used arrays of 10-µm-high silicon
towers and grew a network of nanotubes connecting† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: homma@will.brl.ntt.co.jp

28

the towers by chemical vapor deposition (CVD) [6],[7]. This self-directed growth is a remarkable featureof carbon nanotubes. However, the growth mechanism is not understood yet. We are attempting to use
the self-directed growth of nanotubes on patterned
substrates to fabricate nanotube interconnections
between nanostructures. In this paper, we report thesynthesis of single-walled carbon nanotube (SWNT)
networks on an array of sub-micrometer-scale Si or
SiO2 pillars. Carbon nanotubes were grown using
CVD with Fe catalysts. We succeeded in fabricating
various nanotube architectures. We found that
SWNTs suspended on the pillars showed enhanced
Raman and photoluminescence signals, which is useful in applications to nanotube devices as well as nanotube characterization. The self-directed growthmechanism is also discussed based on our observations using scanning electron microscopy.

*1 The chirality of nanotubes lets us characterize the arrangement of
atoms in the nanotube, and determine how a graphite layer isrolled into a cylindrical shape.

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2. Selective growth of SWNTs on silicon
substrates
Carbon nanotubes were grown by CVD using
methane as the carbon source and Fe as the catalyst.
For the catalyst preparation, a thin film of Fe wasdeposited on a Si or SiO2 substrate using a conventional vacuum evaporator. The thickness of thedeposited film varied from 0.2 to 5 nm. The deposited film turned into particles during thermal treatmentin CVD growth, and the particle size depended on thefilm thickness. The Fe particle sizes were 5 and 50
nm for 0.5- and 5-nm films, respectively. The furnace
for CVD experiments is described elsewhere [8]. An
argon gas flowed as the furnace was heated to reach
growth temperature (700–950°C). The argon gas wasthen replaced by pure methane at a flow rate of 300
cm3/min with the pressure of 67 kPa. The methane
flowed for 1 min and then was replaced by argon tocool the furnace to room temperature. After growth,(a)the specimens were observed with a high-resolution
scanning electron microscope (SEM) and a transmission electron microscope (TEM).
In methane CVD with Fe catalyst, the nanotube
species grown depends on the growth temperatureand the Fe catalyst particle size. When the particle
size is 10-30 nm, SWNTs grow at 950–900°C, andmulti-walled nanotubes (MWNTs) grow at around
800°C [8]. The SWNT growth temperature is lower
for smaller Fe particles. SWNTs can be obtained at
750°C or higher for 5-nm particles. The density of
SWNTs is higher for smaller Fe particles. Figure 1
shows SEM images of SWNTs grown on Si and SiO2
substrates. When the Fe film thickness was around 1
nm, most of the nanotubes were individual SWNTs(Figs. 1(a) and 1(c)), though there was a slight difference in the temperature range between Si and SiO2
substrates. For smaller Fe particles, SWNTs formed
bundles (Figs. 1(b) and 1(d)). Since the SWNT yield
was higher for the SiO2 substrates, the bundles were(b)

200 nm(c)

200 nm(d)

200 nm

200 nmFig. 1. SEM images of SWNTs on Si and SiO2 substrates grown at 900°C using Fe catalyst films with different thickness.
(a) 1-nm Fe film on Si, (b) 0.5-nm Fe film on Si, (c) 0.5-nm Fe film on SiO2, and (d) 0.2-nm Fe film on SiO2.

Vol. 2 No. 2 Feb. 200429

Selected Papers

much denser on them. The reason for the differences
between Si and SiO2 substrates lies in how the Fe catalyst reacts with each substrate.
On Si substrate, SiO2 surrounds the Fe particles
after annealing of the substrate in argon. Figure 2(a)shows an SEM image of a Si substrate surface after
CVD growth. The particles are surrounded by a substance of weaker contrast, which is SiO2 [8]. Similar
images can be seen in Figs. 1(a) and (b). The freeenergy difference between SiO2 and Fe2O3 might
contribute to the favorable reduction of oxide aroundthe Fe particles, so SiO2 is formed selectively aroundthem. The SiO2 base is thought to prevent the reaction
between Fe particles and Si substrate. When the substrate temperature is high, a reaction occurs between
the catalyst and the Si substrate. At silicide formation
temperatures, nanotubes are rarely formed [9]. However, Fe particles are not always fully isolated fromthe Si substrate by SiO2. When they partially touch
the substrate, silicidation can occur (Figs. 2(b) and2(c)). This might be the reason for the lower SWNT
yield on Si compared to that on SiO2. On the SiO2
surface, Fe particles do not form silicide below
1000°C, but they are often embedded under the surface (Fig. 2(d)). Even so, the SWNT yield is rather
high on the SiO2 surface. This is a curious phenomenon and should be investigated further.

(a)

200 nm(b)
Catalyst particles

Precipitates

5 nm

Si
(c)

3. Suspended SWNT network growth on
patterned substrates
By using the selectivity in growing individual
SWNTs or bundles of SWNTs, we could obtain various SWNT architectures on patterned substrates.
Arrays of Si or SiO2 pillars were prepared using synchrotron-radiation lithography. The pillar patterns
consisted of a two-dimensional square lattice of
cylindrical pillars (diameter: 150 nm, height: 360 nm
for Si and 300 nm for SiO2, pitch: 400–1000 nm).
SWNTs grown on pillar structures frequently showedsuspended growth between pillars. Figure 3 shows
SEM images of individual SWNTs and bundles of
SWNTs suspended from the SiO2 pillars. When the
SWNT yield was low, the suspended nanotubes consisted of individual SWNTs. The SWNTs were fairly
straight, but the density was low. Some pillars had nosuspended nanotubes (Fig. 3(a)). At higher yields,
SWNT bundles were formed (Fig. 3(b)). Interestingly, nanotubes from each pillar bundled together tomake a single bridge between neighboring pillars.
Figure 4(a) shows individual SWNTs grown on
100-nm-diameter Si pillars [10]. Most of the SWNTs
30

Fe

FeSi2

SiO2

Si

(d)

Catalyst particles

5 nm

SiO2

Fig. 2. SEM and TEM images of Fe particles on Si and
SiO2 substrates after CVD. (a) SEM image of Fe
particles on Si, (b) cross-sectional TEM image of
Fe particles on Si, (c) schematic illustration of Fe
particles on Si, and (d) cross-sectional TEM
image of Fe particles on SiO2.

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bridges suspended between neighboring pillars wasroughly one. The bundles were often curved, butalmost all the pillars were connected in the SWNT
bundle network. Bundles also formed bridges
between the SiO2 pillars, but many individual bundles
extended from sidewall to sidewall (Fig. 4(c)). Thatis, the pillars were connected by many bundles, whichis in sharp contrast to the Si pillar case. The SWNT
yield difference between Si and SiO2 is the reason forthe marked change of the suspended structures.

(a)

4. Properties of suspended SWNTs
200 nm(b)

Fig. 3. SEM images of SWNT bridges on SiO2 pillars. (a)Individual SWNT bridges and (b) bridges
consisting of SWNT bundles.

started growing from the pillar tops and reached adjacent pillars, forming a network structure. Some nanotubes extended from one pillar to another, and others just reached a neighboring pillar and stopped.
Most reached the nearest neighbor, but some connected to the second-nearest neighbors.
Figures 4(b) and 4(c) show the bundle growth on Si
and SiO2 pillars, respectively. The catalyst film wasdeposited after tilting the substrate +45° and then
–45° [11]. Thus, the catalyst film formed on the sidewalls as well as on the tops of pillars. Nevertheless,the bundles were suspended only between the tops of
the Si pillars (Fig. 4(b)). This means that the SWNTs
that grew on the sidewalls of the pillars might havebundled together at the pillar top. The number ofVol. 2 No. 2 Feb. 2004

The difference between the suspended SWNTs andthe SWNTs on the substrate is that the suspended
ones do not interact with the substrate. This is a simple and obvious difference, but it has a significant
effect on SWNT properties. In this respect, individual
suspended SWNTs are the most interesting because
they are also free from interaction with other nanotubes.
We have found some remarkable properties specific to suspended SWNTs. One is the observation of
intense Raman signals from suspended SWNTs [12].
SWNTs show various Raman modes, by which thestructures of individual nanotubes can be characterized. The Raman signals from suspended SWNTs aremuch more intense than those from SWNTs on thesubstrate. Figure 5 shows Raman signals obtained byscanning the probe laser beam from the flat region to
the pillar region. In the measured wave number range,
there is a G-band (≈1590 cm–1) originating from thein-plane stretching mode of the graphite sheet and a
D-band (≈1530 cm–1) originating from defects. The
number of SWNTs on the substrate surface was morethan 100 times that of suspended SWNTs. Nevertheless, the intensity of the G- and D- bands was stronger
in the pillar region. This indicates that even one
SWNT can be detected by Raman spectroscopy when
it is suspended.
Another interesting and very important result is thatthe suspended SWNTs are photoluminescent. Semiconducting SWNTs have a direct-band-gap structure,and thus should be luminescent for the band-gap transition. However, so far, SWNT bundles or SWNTs on
the substrate have not shown luminescence; only
solutions of purified SWNTs individually isolated
inside surfactant micelles were reported to be luminescent [13]. We recently found that the suspended
SWNTs show intense band-gap luminescence in thenear-infrared region [14], [15]. The precise mechanism has not been clarified yet, but isolating the31Selected Papers(a)200 nm

400 nm(b)

400 nm
200 nm(c)

200 nm

400 nmFig. 4. SEM images of various SWNT architectures at glancing and top views. (a) Individual SWNT bridges (Pt-coated)on Si pillars, (b) bundle SWNT bridges on Si pillars, and (c) multiple bridges of SWNT bundles on SiO2 pillars.

SWNTs by minimizing their interaction with theenvironment is the key to obtaining luminescence.
This is a very important finding because it means thatsuspended SWNTs may be able to emit light at optical communication wavelengths of 1.3 or 1.55 µm.
The excitation and emission wavelengths depend on
the chirality of SWNTs. Therefore, controlling thechirality is a crucial issue for future applications.
32

5. Mechanism of suspended growth
The growth of nanotubes suspended between thepillars was investigated statistically through SEM
observations of about 2000 pillars [10]. The results
for a mostly individual SWNT case are summarized
in Fig. 6. About 76% of the pillars examined showednanotube growth in some style. Among them, 45%
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1591

were bridging nanotubes, 40% fallen ones, and 15%
halfway terminated ones. Among the bridging nanotubes, 85% achieved nearest-neighbor connection,

Pillarregion

1316

1566
1544
1524

Excitation wavelength:514.5 nm2 × 103

Flat
region
1800

1700

1600

1500

1400

1300

1200

1100

Raman shift (cm–1)

Fig. 5. Comparison of Raman spectra between the pillar
region and flat region.

while only 9% showed second-nearest neighbor connection. Therefore, 29% of the nanotubes originatingfrom the pillar top achieved interconnection between
nearest-neighbor pillars.
Such a relatively large fraction of the nearest-neighbor interconnection can be obtained when the pillar
aspect ratio is large and the pillar spacing is less than
twice the pillar height. When the pattern spacing wasmuch larger than the pillar height, nanotubes often
failed to connect to pillars directly and fell to the substrate. However, the difference in the spacing
between the nearest neighbor and the second-nearest
neighbor cannot explain the large difference in yield,
because when the pillar spacing was less than twice
the pillar height, the percentages of nearest- and second-nearest-neighbor bridging were almost constant.
The methane flow, which is supposed to keep the nanotubes floating [7], is unlikely to be the cause of thesuspended growth because of the lower pressure andsmaller methane flow rate in the present case. In addition, we did not observe any flow direction effects.
Nanotubes originating from pillar tops can extend
in any direction. However, if a nanotube starts togrow on the plane of the pillar top, the tube is likely
to grow along the pillar top surface and thus extend

Halfway terminatedNo growth

Total: 2004

Second-nearest neighborconnection

76%

Growth55%
45%

Bridging

85%
FallenNearest-neighborconnection

Fig. 6. Percentages of nanotube types grown on Si pillars using Fe catalyst.

Vol. 2 No. 2 Feb. 200433

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parallel to the substrate surface. As a nanotube
extends, it forms a cantilever with a large aspect ratio,
so the open end of the nanotube vibrates duringextension. If the vibration amplitude is large, the nanotube contacts a neighboring pillar. The thermal
vibration amplitude of a SWNT at 900°C was estimated to be as large as 6 µm when the tube length L
was 20 µm [16]. However, the amplitude is proportional to L3/2 [17], and it is estimated to be only 18 nm
for a 400-nm-long SWNT. This value is quite modest
and cannot explain the high percentage of nearestneighbor bridging. Furthermore, the vibration frequency of a SWNT calculated using the equation in
Ref. 17 is of the order of 108 MHz, which could hardly include any enhancement from the mechanicalvibration of the CVD apparatus. One explanation forthis discrepancy is imperfections in the CVD-grown
nanotubes. The above values assume a Young’s modulus of 1 TPa. However, because of the lower growthtemperature in CVD, the nanotubes are defective
compared with those grown by laser ablation or arc
discharge. In addition, when bundles are formed, theeffective Young’s modulus decreases with increasing
bundle diameter [18]. If the effective Young’s modulus decreases by a factor of 100, the thermal vibration
amplitude becomes 180 nm for a 400-nm cantilever.
This explanation can be applied to the bundle case,where the bridges curve substantially, showing a
reduced effective Young’s modulus. However, forindividual SWNTs, a decrease in Young’s modulus
by a factor of 100 cannot be attributed only to defects.
We have shown that the catalyst particles melt in
the growth ambient [9]. Therefore, when a nanotube
contacts a pillar only at a catalyst particle, the nanotube may swing or even revolve around the particle
due to the mechanical vibration of the CVD apparatus. Thus, the probability of making contact with aneighboring pillar should be high. The nearest-neighbor bridging of at least 80% can only be explained bya swing amplitude as large as the pillar spacing. Oncea nanotube contacts a neighboring pillar, the growthorientation is fixed and the tube extends from one pillar to another.
Nanotubes can grow upward or downward from thepillar top. Downward-growing nanotubes just fall tothe substrate. Upward-growing ones often form anarch between pillars. If the arch is long enough, its
central part can touch the substrate surface due tovibration or rotation. This causes a nanotube to fallfrom both ends at the pillar tops, and explains theabsence of tall arches. If two adjoining arches areformed, they can touch each other and become entan34

gled (see Fig. 3(d) in Ref. 10).
6. Conclusion
We have succeeded in fabricating various types ofsuspended SWNT architectures on nano-scale pillar
structures. These suspended SWNTs are either individuals or bundles, depending on the catalyst particle
size and growth temperature. When the particle size
is rather large, the SWNT yield is low and individual
SWNT bridges are formed. For smaller catalyst particles, 5 nm or less, the SWNT yield increases, resulting in bundle formation. A higher yield is obtainedfor SiO2 than for Si, as a result of the degree of silicidation of the catalyst particles. Thus, densely suspended bridges are formed between SiO 2 pillars.
These suspended SWNTs show enhanced Raman andphotoluminescence signals. Suspended SWNTs areformed in a self-assembling manner, probably due tovibration of the tips as the SWNTs grow. This growth
characteristic is useful for site selective and directional synthesis of SWNTs. For future device applications, control of chirality is a crucial issue because
the electrical and optical properties depend on thechirality of SWNTs.
7. Acknowledgments
We thank Takayuki Yamashita (then at Tokai University, presently at ULVAC Corp.) and Daisuke Takagi (Meiji University) for performing the CVD experiments. This work was supported by the NEDO
International Joint Research Grant Program.
References
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[4]

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M. S. Dresselhaus, G. Dresselhaus, and Ph. Avouris (Eds.), “Carbon
Nanotubes Synthesis, Structures, and Applications,” Springer, Berlin,
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sive Nanotube Networks with Directionality,” Adv. Mater. Vol. 12,
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[9] Y. Homma, Y. Kobayashi, T. Ogino, D. Takagi, R. Ito. Y. J. Jung, andP. M. Ajayan, “Role of Transition Metal Catalysts in Single-walled
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[10] Y. Homma, T. Yamashita, Y. Kobayashi, and T. Ogino, “Growth ofSuspended Carbon Nanotube Networks on 100-nm-scale Silicon Pillars,” Appl. Phys. Lett., Vol. 81, No. 12, pp. 2261-2268, 2002.
[11] Y. Jung, Y. Homma, T. Ogino, Y. Kobayashi, D. Takagi, B. Wei, R.
Vajtai, and P. M. Ajayan, “High-Density, Large-Area Single-Walled
Carbon Nanotube Networks on Nanoscale Patterned Substrates,” J.
Phys. Chem. B, Vol. 107, No. 28, pp. 6859-6864, 2003.
[12] Y. Kobayashi, Y. Homma, T. Ogino, Y. Ueno, O. Niwa, and T.
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[15] J. Lefebvre, J. M. Fraser, Y. Homma, and P. Finnie, “Photoluminescence from Single Walled Carbon Nanotubes: a Comparison between
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[8]

Vol. 2 No. 2 Feb. 2004

Yoshikazu HommaGroup Leader, Device Physics Laboratory,
NTT Basic Research Laboratories.
He received the B.S. and M.S. degrees inphysics from Tohoku University, Sendai, Miyagi
in 1976 and 1978, respectively. He received the
Ph.D. degree in applied physics from the University of Tokyo, Tokyo in 1987. He joined theMusashino Electrical Communication Laboratories, Nippon Telegraph and Telephone Public
Corporation (now NTT) in 1978. Since then hehas engaged in research of ultra-trace analysis ofsemiconductors, in situ imaging of dynamic surface processes, and ordered nanostructure fabrication on semiconductor surfaces. His current
interest is the growth control of carbon nanotubes
and its in situ observation. He is a member of the
Physical Society of Japan, the Japan Society ofApplied Physics (JSAP), the Surface Science
Society of Japan (SSSJ), the Japanese Society ofMicroscopy, and three other societies. He hasreceived the Japanese Society of Microscopy
award and several other awards.

Yoshihiro Kobayashi
Senior Research Scientist, Device Physics
Laboratory, NTT Basic Research Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in chemistry and applied chemistry from Waseda
University, Tokyo in 1983, 1985, and 1994
respectively. He joined the Musashino ElectricalCommunication Laboratories, NTT in 1985.
Since then he has been researching reaction control on semiconductor surfaces and the fabrication of ordered nanomaterials. His current interest is the growth control of carbon nanotubes andits in situ observation. He is a member of the
Physical Society of Japan, JSAP, and SSSJ.

Toshio Ogino
Professor, Dept. of Electrical and Computer
Engineering, Yokohama National University,
Hodogaya, Yokohama.
He received the B.E, M.E., and Ph.D. degrees
in engineering from the University of Tokyo,
Tokyo in 1974, 1976, and 1979, respectively. He
joined the Musashino Electrical Communication
Laboratories, Nippon Telegraph and Telephone
Public Corporation (now NTT) in 1979. Since
then he has researched semiconductor devices,surface processes, and ordered nanostructure
fabrication on semiconductor surfaces. He
moved to Yokohama National University in
2002. His current interest is the growth control ofcarbon nanotubes and its application to smart
nano-interconnections. He is a member of JSAP,
SSSJ, the Institute of Electronics, Information
and Communication Engineers, and the Materials Research Society.

35