Control of growth orientation for carbon nanotubes

Article abstract of DOI:10.1063/1.1535269

Control of growth orientation was studied for laterally aligned carbon nanotubes synthesized on substrates over iron nanoparticles using chemical vapor deposition. A magnetic field that was applied in the process of adhering catalyst particles on silicon oxide substrates from dispersion was used to control the growth direction of the carbon nanotubes. It was suggested that carbon nanotubes preferentially grow from certain facets of the catalyst particles, indicating a crucial clue in investigating the growth mechanism of carbon nanotubes.

Full text of DOI:10.1063/1.1535269

Control of growth orientation for carbon nanotubes
Ki-Hong Lee, Jeong-Min Cho, and Wolfgang Sigmund
Citation: Applied Physics Letters 82, 448 (2003); doi: 10.1063/1.1535269
View online: http://dx.doi.org/10.1063/1.1535269
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/82/3?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 3
20 JANUARY 2003
Control of growth orientation for carbon nanotubes
Ki-Hong Lee, Jeong-Min Cho, and Wolfgang Sigmunda)
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
͑Received 25 September 2002; accepted 14 November 2002͒
Laterally aligned carbon nanotubes were synthesized on substrates over iron nanoparticles using
chemical vapor deposition. In addition, aligned carbon nanotubes grown vertically and with tilt
angle to the substrates were produced, which means that it is possible to grow aligned carbon
nanotubes at any angle relative to the substrate. The growth direction of the carbon nanotubes was
controlled by a magnetic field that is applied in the process of adhering catalyst particles on silicon
oxide substrates from dispersion. The ferromagnetic property of the iron nanoparticles fixes them in
a defined orientation under magnetic field, which results in aligned growth of the carbon nanotubes.
These results indicate that carbon nanotubes preferentially grow from certain facets of the catalyst
particles, suggesting a crucial clue in investigating the growth mechanism of carbon nanotubes. The
laterally aligned carbon nanotubes could make it possible to integrate them in nanoelectronic
devices, such as a channel for field-effect transistors. © 2003 American Institute of Physics.
͓DOI: 10.1063/1.1535269͔
Carbon nanotube ͑CNT͒ growth on silicon substrates in
controllable ways is a challenge for use in nanoelectronic
devices. Vertically aligned CNTs have been produced by us-
ing chemical vapor deposition ͑CVD͒ over catalyst particles
patterned on a porous substrate,1 and on a glass substrate by
plasma-enhanced CVD.2 There is a general tendency for
CNTs to form bundles as they grow, which makes it difficult
to interpret their growth mechanism.
tween the substrate and the magnetic field direction. The best
alignment of carbon nanotubes is usually produced closer to
the edge of the substrates. In our work, the growth direction
of CNTs could be correlated with the applied magnetic field,
that is, the growth direction is approximately perpendicular
to the magnetic field. This result suggests the possibility to
control the growth orientation of the CNTs. Furthermore,
evidence is given for the growth mechanism of the CNTs
from the metal catalyst particles by CVD.
Several research groups showed that field-effect transis-
tors ͑FETs͒, using semiconducting nanotubes as a channel,
operate successfully at room temperature.3,4 Collins et al. re-
ported a method that tailors the electrical properties of car-
bon nanotubes by using electrical breakdown.5 However,
they produced FETs circuit one at a time, and needed much
effort to attach a nanotube to metal electrodes. Thus, this
approach is not suitable for the production of microchips. In
order to integrate nanotube-based FETs, it is essential to find
a way to control the growth orientation of the CNTs, and to
synthesize many aligned nanotubes laterally on the sub-
strates. In contrast to vertical growth, lateral growth of CNTs
has received little attention because of its difficulty. Dai and
coworkers showed the possibility that CNTs could be pro-
duced parallel to the substrate by using suspended catalysts
made by a lift-off process.6,7 They attributed parallel growth
of nanotubes to reaction gas flow. One other research group
has reported CNTs grown parallel to substrates by using an
oxide layer to suppress vertical growth of the CNTs.8 Until
now, however, no one has reported the natural growth of
CNTs laterally on the substrate surface.
N-type ͑resistivity: 3 ⍀ cm, thickness: 625 ␮m͒ silicon
͑100͒ wafers with thermal oxide film ͑250 nm͒ were used as
substrates. Iron catalyst nanoparticles were prepared by ther-
mal decomposition of an organometallic precursor
Fe(CO) in melted trioctylphosphine oxide ͑TOPO͒.9,10
Iron nanoparticles were dispersed in hexane, and adhered on
the substrate before growing carbon nanotubes. A magnetic
bar ͑140 mT͒ was used for adhering iron particles on the
substrate from solution. Substrates were attached to the mag-
netic bar in two different positions ͓cases 1 and 2 shown in
Fig. 1͔, followed by dipping into the dispersed iron nanopar-
ticle slurry for 1 to 2 s directly after sonicating the
dispersion.11 The magnetic bar was removed from the sub-
strate after complete evaporation of the solvent. Carbon
nanotubes were then grown at 1050 °C using argon, methane,
and hydrogen gas with flowing speeds of 1000, 50, and 50
sccm, respectively.12
Scanning electron microscopy ͑SEM͒ was used to exam-
ine carbon nanotubes grown on silicon oxide substrates. Fig-
ure 2 shows low magnification SEM images of nanotubes
grown from case 1 in Fig. 1. Figure 2͑e͒ shows the four
distinct locations of CNT growth as observed by SEM. In
regions ͑a͒ and ͑b͒, carbon nanotubes appear to grow later-
ally and in alignment with the substrate ͓Figs. 2͑a͒, 2͑b͔͒. On
the side wall of substrate ͓Fig. 2͑c͔͒, where the magnetic
field is parallel to the surface, the CNTs grew vertically on
the surface. Figure 2͑d͒ also shows vertically grown CNTs in
the center region, with parallel magnetic field lines to the
substrate surface. In the work presented here, the reaction
͓
͔
5
Here, we report the assembly of laterally aligned CNTs
on silicon oxide substrates grown over iron catalyst particles
by applying magnetic fields to the iron nanoparticles in the
process of adhering them onto the substrates from dispersion.
Aligned carbon nanotubes were also synthesized vertically or
with tilt angle to the substrates by changing the angle be-
a͒
Author to whom correspondence should be addressed; electronic mail:
wsigm@mse.ufl.edu
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0003-6951/2003/82(3)/448/3/$20.00 448 © 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 82, No. 3, 20 January 2003
Lee, Cho, and Sigmund
449
FIG. 1. Schematic diagram of a magnetic bar used for adhering iron nano-
particles to the silicon substrates. ͑Circled arrow shows the orientation of the
easy axis of the iron nanoparticles͒. ͑a͒ The substrate is attached parallel to
the side of the magnetic bar in case 1. ͑b͒ The simplified magnetic field
around the substrate in case 1. The field is perpendicular at the edge, and
parallel at the center. ͑c͒ The substrate is attached perpendicular to the side
of the magnetic bar in case 2. ͑d͒ The simplified magnetic field around the
substrate in case 2. The magnetic field tilts to the substrate.
FIG. 2. SEM photographs of carbon nanotubes grown on the substrate in
case 1. The carbon nanotubes were synthesized at 1050 °C for 10 min. ͑a͒
The CNTs grow laterally with alignment at the edge. ͑b͒ CNTs overlap with
each other during the growth when a large number of CNTs are grown in a
small area. ͑c͒ CNTs grow vertically at the side-wall region of the substrate.
͑d͒ Vertical growth of CNTs at the inside region of the substrate ͑d͒ region
of Fig. 2͑e͒, showing top growth mode ͑SEM tilt angle is 45°͒. ͑e͒ The
schematic diagram showing the position observed in ͑a͒–͑d͒ respectively.
The opposite side of the substrate shows the same trend in the growth
characteristics.
gas flow was perpendicular to the silicon wafer, that is, per-
pendicular to the growth direction of CNTs in regions ͑a͒–
͑d͒, which means that the growth direction of the CNTs is not
derived by gas flow. Compared to vertical growth, which
allows unlimited space for growing, lateral-growing carbon
nanotubes will overlap with each other if their length ex-
ceeds the space between the iron nanoparticles. As a result,
they may form bundles, or some CNTs’ growth direction will
be deflected away from the original growth orientation, as
observed in Fig. 2͑b͒.
For case 2 ͑see Fig. 1͒, the substrate was attached verti-
cally onto the magnetic bar for dip coating. Here, growth of
aligned CNTs tilting from the substrates is found, as shown
in Figs. 3͑a͒–3͑c͒. They appear to arch over the substrate,
which is assumed to be caused by the weight of the catalyst
particle at the ‘‘top’’ of the growing CNT. Figure 3͑d͒ shows
a schematic diagram for the suggested growth mode. As in
case 1, the tilted CNTs grow in the same orientation in rela-
tion to the magnetic field, and the region with the best CNT
alignments are found closest to the edge of the substrates.
Several exciting phenomena were observed in relation to
the growth orientation of CNTs and the applied magnetic
field. First, CNTs have a tendency to grow approximately
perpendicular to the applied magnetic field lines, as is shown
in Fig. 2 for regions ͑a͒–͑d͒. Second, the alignment of CNTs
is usually observed close to the edge of the silicon wafer.
Third, the growth direction of CNTs is dependent on the
boundary shape of the edge.
the growth characteristics could be excluded. We suppose
that the most probable reason for these phenomena should be
related to the crystallographic orientation of iron nanopar-
ticles.
Magnetic anisotropy tends to force the magnetization
into a well-defined direction. Uniaxial anisotropy favors
alignment of the magnetization along an easy axis. Ferro-
magnetic materials, such as iron, nickel, and cobalt, have one
crystallographic easy axis. For example, iron ͑bcc͒ favors
magnetization parallel to the ͗100͘ direction. The easy axis
tends to be parallel to the magnetic field. As a result, the
applied magnetic force fixes the adhering particles to a de-
fined orientation on the substrate ͓Figs. 1͑c͒ and 1͑d͔͒. Re-
gardless of the phase transition of the iron nanoparticles,
they seem to maintain a discrete crystallographic orientation
at the CNT growth temperature. From the results, it is as-
sumed that the CNTs grow along a certain crystallographic
orientation or plane that is about perpendicular to the easy
axis. However, at least two orientations should be fixed in
order to arrange three-dimensional nanoparticles in a defined
direction. One orientation, the easy axis, should be fixed par-
allel to the magnetic field direction. An inhomogeneous mag-
netic field could possibly fix the other orientation. Magnetic
In our work, the CNTs were produced at 1050 °C, which
is much above the Curie temperature of iron ͑770 °C͒. The
iron nanoparticles are demagnetized at the growth tempera-
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ture; as a result, the magnetic effect of the iron particles on
field lines are deflected at the boundary of the silicon sub-
99.233.106.76 On: Fri, 25 Apr 2014 01:54:37

450
Appl. Phys. Lett., Vol. 82, No. 3, 20 January 2003
Lee, Cho, and Sigmund
growth direction of the CNTs and the crystal orientation of
the catalyst particles. Until now, there has been much con-
troversy about the growth mechanism of CNTs. These find-
ings may suggest a solution to a fundamental question in
growth of CNTs and the parameters that determine CNT
growth direction over, and orientation relative to, catalyst
materials. The finding that the orientation of CNTs can be
varied with a magnetic field indicates the following: ͑i͒ nano-
tubes must grow from defined crystallographic facets of the
iron nanocrystals, ͑ii͒ the catalyst particle stays aligned with
the magnetic field beyond its Curie temperature of 770 °C,
and ͑iii͒ the iron nanoparticles are not a liquid catalyst, but a
solid material that dissolves and reprecipitates carbon. This
study focused on iron particles, but similar principles may
apply to other catalysts such as cobalt and nickel that have
different crystal structures and an easy axis for magnetiza-
tion. Finally, this work shows that it is possible to produce
laterally aligned CNTs on silicon wafers naturally, which in-
creases the ability of CNT integration in nanoelectronic de-
vices.
FIG. 3. SEM photographs of CNTs grown on the substrates in case 2 ͑SEM
tilt angle is 45°͒. The CNTs were synthesized at 1050 °C for 10 min. All of
them have an arch-like shape and grow in the same direction. ͑a͒, ͑b͒ SEM
shows that CNTs grow in the same direction over a large area ͓N pole side
in Figure 1͑c͔͒. ͑c͒ SEM photograph of CNTs grown at S pole side in Fig.
1͑c͒. ͑d͒ Schematic diagram of the shapes of CNTs produced in ͑a͒–͑c͒. The
CNTs have several shapes according to the growth length ͓each case is
indicated in ͑b͔͒.
This work was supported by DARPA/Army Research
Office under Grant No. DAAD19-00-1-0002 through the
Center for Materials in Sensors and Actuators ͑MINSA͒.
strate, which is diamagnetic, resulting in an inhomogeneous
field in this region. Iron nanoparticles arranged themselves
so as to minimize the magnetic anisotropy energy in an in-
homogeneous field. This could explain why alignment of car-
bon nanotubes is achieved in the edge region, and why the
growth direction of the CNTs was affected by boundary
shape in this region. In our work, it was observed that the
orientation of CNT growth is strongly dependent on the in-
homogeneous character of the local magnetic field providing
an inhomogeneous magnetic field.
As a matter of fact, there exists a strong magnetostatic
interaction between magnetized nanoparticles due to their
external magnetic field. In our work, the magnetic bar was
removed from the substrates after drying the sample. While
dipping the substrates into the dispersion, most of the par-
ticles were concentrated in the region of high magnetic field.
In this region, the magnetostatic interaction between the iron
nanoparticles may have forced them deflect each other after
the magnetic bar was removed from the substrate. Nanopar-
ticles that are in contact with each other aggregate during
heating to form larger particles, which delay or prevent the
growth of CNTs. In addition to this, the tendency of the
CNTs to form bundles makes it more difficult to maintain the
original growth orientation. As a result, the relationship be-
tween the growth direction of the CNTs and the applied mag-
netic field is less obvious in this region.
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10 The nanoparticle synthesis process was slightly changed from the previ-
ously reported method. A 0.2 mL of Fe(CO)5 was added to 6.0 g of TOPO
at 330 °C under argon atmosphere, and then aged for 30 min at the same
temperature. The reaction mixture was added to excess ethanol and cen-
trifuged several times. Transmission electron microscopy showed iron
nanoparticles with diameters from 10 to 20 nm.
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magnetic characteristics and high specific surface energy. Therefore, the
slurry is ultrasonicated before dipping the substrates.
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the CVD furnace. The substrate temperature was increased by 6 °C/min up
to the growth temperature in argon atmosphere with a flowing speed of
600 sccm. On reaching growth temperature, the furnace temperature was
held constant for 10 min, then annealed in hydrogen ͑50 sccm͒ and argon
͑1000 sccm͒ before growth of the CNTs. After growing the CNTs, the
furnace was cooled to room temperature in argon ͑600 sccm͒.
Our results thus indicate a correlation between the
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