7292
J. Phys. Chem. C 2007, 111, 7292-7297
Growth Mechanism of Long and Horizontally Aligned Carbon Nanotubes by Chemical
Vapor Deposition
Alfonso Reina,† Mario Hofmann,‡ David Zhu,‡ and Jing Kong*,‡
Department of Material Science and Engineering and Department of Electrical Engineering and Computer
Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed: February 10, 2007; In Final Form: March 29, 2007
The selective production of long, horizontally aligned carbon nanotubes (>1 mm) or short, randomly oriented
carbon nanotubes (<50 µm) was achieved in a chemical vapor deposition process by influencing the catalyst
pretreatment and reaction conditions. A detailed investigation was undertaken to elucidate the mechanism
yielding the two different morphologies. It was found that the duration of the catalytic growth of a nanotube
plays a vital role; that is, the actual growth period of long nanotubes is significantly higher (up to 15 min or
more) compared to short nanotubes (10 s or less). Alignment with the gas flow occurs only when a nanotube
reaches a critical length, which suggests that short growth durations limit not only the length of CNTs but
also their alignment with the gas flow. Furthermore, it is concluded that differences in the nanoparticle’s
catalytic lifetime is the most probable factor determining the extension of growth duration and lengths to
obtain long, horizontally aligned CNTs. This work represents a step forward toward the integration of CNTs
in electronic applications.
1. Introduction
The growth of long and aligned carbon nanotubes (CNTs) is
highly desirable for a variety of applications including nanoelectronics1,2 and high strength composite materials.3 However,
most of the CNTs directly produced on substrates by chemical
vapor deposition (CVD) are short (<50 µm) and lie in random
orientations (will be termed SR (short, randomly oriented)
nanotubes further on). During the past few years it was
demonstrated that CNTs with lengths up to centimeters could
be grown and they exhibit alignment with the gas flow direction
(these will be termed LA (long, aligned) nanotubes further
on).4-14 It was argued that, for the synthesis of these LA
nanotubes, interactions with the substrate5-7 have to be prevented. Consequently, the focus of efforts has been on controlling the physical aspects in the CVD process. Various methods
were applied to keep the CNTs floating, such as putting the
catalyst on high terraces5 or using “fast heating” to generate
local turbulence so that the catalyst particles can be lifted above
the substrate while growing CNTs.6,7 With increasing experience
in this field, it appeared that neither the high terraces nor fast
heating is decisive in obtaining long and horizontally aligned
nanotubes. Instead, LA nanotubes can also be grown with
normal CVD syntheses which usually generate SR nanotubes.8,9
The mechanism that distinguishes these two different types of
nanotubes thus remains unclear.
In this paper, a detailed investigation is undertaken to clarify
the differences in the growth of LA and SR nanotubes. It is
found that the two CNT morphologies are determined by catalyst
pretreatment. The study of CNT length distributions for different
synthesis times suggests that the duration of the actual growth
of LA nanotubes is significantly higher (15 min or more) than
* To whom correspondence should be addressed. E-mail:
jingkong@mit.edu.
† Department of Material Science and Engineering.
‡ Department of Electrical Engineering and Computer Science.
SR nanotubes (10 s or less). This indicates that for the latter
case the synthesis time does not equal the growth duration.
Furthermore, statistical data on the correlation between CNT
length and orientation is presented which implies that alignment
with the gas flow occurs only when a nanotube exceeds a critical
length. These two observations suggest that short growth
duration limits not only the length of CNTs but also their
alignment with the gas flow. Finally, a series of studies was
carried out to investigate possible growth termination mechanisms yielding SR nanotubes. It is concluded that differences
in the nanoparticles catalytic lifetime is the most probable factor
limiting CNT growth duration. This result relates the morphology of CVD grown carbon nanotubes directly to the chemical
effects of the catalyst and emphasizes their influence as equally
important as physical and fluid dynamical conditions.
2. Experimental Details
A. CNT Synthesis. Two approaches were used to prepare
the catalyst for nanotube growth in our experiments: Iron
nanoparticles derived from ferritin and FeCl3 based catalyst.
For the first type, a drop of ferritin solution (Sigma Aldrich,
type I from 0.66 vol % in H2O) was deposited along a strip
near the edges of Si/SiO2 substrates (substrate size 6.4 mm ×
11 mm). The drop did not wet the surface which enabled the
selective deposition of ferritin particles only on the region where
the drop was cast. Deposited ferritin was then annealed in air
at 700 °C for 5 min to burn the organic shell leaving behind
the Fe compound nanoparticles as catalyst. An AFM height
image of the deposited nanoparticles after the removal of the
organic coating is available in the Supporting Information. For
catalyst derived from FeCl3, poly(dimethyl siloxane) (PDMS)
was used to stamp the catalyst from a 0.01 mM FeCl3 (iron(III)
chloride hexahydrate, Sigma-Aldrich, 98% minimum) catalyst
solution in ethanol.
The nanotube CVD synthesis was carried out in a tube furnace
with a 1 in. diameter fused silica tube and a 15 in. long heating
10.1021/jp0711500 CCC: $37.00 © 2007 American Chemical Society
Published on Web 05/03/2007
Growth Mechanism of Carbon Nanotubes
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7293
Figure 1. SEM images of samples containing short, randomly aligned CNTs (a-c) and samples containing long, gas flow aligned CNTs (d-f) at
different magnifications and locations; (a and d) overall view of catalyst region, (b and e) edge of catalyst region, and (c and f) inside catalyst
region. Gas flows from right to left. All CNTs shown were grown using ferritin catalyst and ethanol CVD with Ar + H2 (a-c) and Ar + O2 (d-f)
gas mixtures during catalyst pretreatment.
TABLE 1: Summary of Different Results Regarding CNT Characteristics Obtained with Different Process Parameters
(Catalyst, Carbon Source, and Gases during Pregrowth Heating)
methane CVD
ethanol CVD
ferritin
Ar + O2
Ar + H2
short, random: variable density
long, aligned: variable density (up to 5 mm)
FeCl3
Ar + O2
Ar + H2
short, random: high density
long, aligned: low density (100 µm - 6 mm)
long, aligned: variable density (up to 1 cm)
short, random: high density (90% of the growths) +
long, aligned: 10% of the growths; variable density (up to 8 mm)
long, aligned: low density (<1 mm)
short, random: high density (90% of the growths) +
long, aligned: 10% of the growths; variable density
zone. The process consisted of two steps. First, the substrates
were heated from room temperature to growth temperature (900
°C) in a fixed amount of time, a step that will be subsequently
referred to as catalyst pretreatment. The effect of pretreatment
under different mixtures of gases was investigated using 600
sccm Ar + 10 sccm O2 or 600 sccm Ar + 440 H2. Once the
furnace temperature reached 900 °C, for ethanol CVD, a gas
flow of 600 sccm Ar (bubbled through ethanol) + 440 sccm
H2 was used. For methane CVD, a gas flow of 1000 sccm CH4
+ 440 sccm H2 was used. After the synthesis, the furnace was
cooled down under the flow of 600 sccm Ar + 440 sccm H2 to
room temperature.
B. CNT Characterization. Samples were characterized using
a JEOL JSM-6060 scanning electron microscope (SEM) and a
Dimension 3100 atomic force microscope (AFM). Because of
an electron emission contrast of a differently charged SiO2/Si
substrate in the vicinity of carbon nanotubes, they could be
distinguished in images with magnifications as low as 10X using
an acceleration voltage of 1 kV.15
3. Results and Discussions
Two types of results-SR nanotubes vs LA nanotubes are
obtained respectively by tuning different CVD parameters.
Figure 1 shows SEM images representative of the two aforementioned scenarios. These CNTs were synthesized using
ferritin-derived nanoparticles under ethanol CVD. Figure 1a-c
shows different magnification SEM images of SR nanotubes.
These CNTs form a “mat” due to the high density and random
orientation. In contrast, Figure 1d-f illustrates samples with
LA nanotubes at different magnifications. A sharp contrast in
morphology can be noticed from these images. Each result was
achieved only by the manipulation of chemical process param-
eters, namely the catalyst pretreatment step. A summary of the
parameters tested and their results are presented in Table 1. For
ethanol CVD, it was found that LA nanotubes grow from both
FeCl3 and ferritin derived catalyst if O2 is used during the
catalyst pretreatment stage of the process. For methane CVD,
the presence of H2 in the pretreatment stage resulted in LA
nanotubes from both kinds of catalysts, whereas O2 did not have
this effect.16 The significance of these chemical process
parameters in the control of nanotube morphology are addressed
in the subsequent sections. The results aforementioned were
obtained with fused silica tubes that have been used previously
for a period of time (generally, 30-50 CVD growths). Processes
performed with new fused silica tubes give rise to SR CNTs
exclusively. This suggests that a possible conditioning of the
tubes is influencing the stability of our experiments. Previous
reports have observed a similar influence of the CVD chamber.17
Further efforts are being devoted to improve the understanding
of this observation.
3.1. Growth Differences between SR and LA CNTs.
Variation of synthesis time was performed for CVD processes
resulting in SR and LA CNTs (Figures 2 and 3, respectively).
The accurate termination of short time synthesis was achieved
by a special flow reversal arrangement that allowed quick
displacement of hydrocarbons with an inert gas (see the
Supporting Information). Figure 2a indicates that some nanotubes have already grown after 10 s. CNT length distributions
do not change significantly over time, and they show a
maximum around the same length value for all synthesis times.
This observation has two implications. First, it indicates that
nanotube growth starts and ends within 10 s or less, and
therefore beyond these 10 s, the length of the CNTs is not
proportional to growth time as assumed previously.4 Second,
7294 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Reina et al.
Figure 2. SEM images of SR nanotubes grown using ferritin catalyst and ethanol CVD with different synthesis times (a-d) and their respective
length distributions (e-h); 10 s (a and e), 30 s (b and f), 1 min (c and g), and 15 min (d and h). Gas flows from right to left. Scale bar 20 µm.
Figure 3. SEM images of LA nanotubes grown using FeCl3 catalyst and methane CVD with different synthesis times (a-d) and their respective
length distributions (e-h); 2 min (a and e), 5 min (b and f), 10 min (c and g), and 15 min (d and h). Gas flows from right to left. Scale bar 1 mm.
Figure 4. Statistical data showing the correlation between CNT orientation and length. Orientation is taken as the angle between flow direction
and nanotube orientation (a). Map of orientation vs length (b), standard deviation of orientation vs length (c). Data was obtained from CNTs grown
with both ethanol and methane CVD. Scale bar 20 µm.
as the synthesis time increases, the only feature that is changing
is the density of nanotubes, suggesting that particles do not
initiate growth of nanotubes at the same time. Figure 3 shows
results from experiments with different synthesis times under
CVD conditions favoring LA nanotubes and their respective
length distributions. From the low magnification images, it can
already be inferred qualitatively how the longest nanotubes
stemming from the catalyst region tend to get longer as synthesis
time increases. Although the peak in the length distribution does
not shift significantly with synthesis time, the distribution
broadens its longer-length side tail, demonstrating the proportionality between the actual growth duration and the synthesis
time for the longest nanotubes.
Inside the catalyst region of the LA samples, there are both
LA nanotubes and short random orientated nanotubes. This
allows us to perform a length vs orientation angle mapping,
which is illustrated in Figure 4a. The distribution demonstrates
a strong correlation between the degree of alignment and the
length of the carbon nanotubes. At short lengths, the standard
deviation approaches 52 degrees (Figure 4c), which is in
agreement with the value expected for a uniform probability
function of the orientation angle (see the Supporting Information). The alignment sets in at a length of 40-50 µm which is
illustrated by the narrow angular distribution after this length
and confirmed by the standard deviation. This demonstrates that
the gas flow effect becomes significant after the growing
nanotubes have achieved a critical length. This characteristic is
independent of the type of CVD (ethanol or methane) and
catalyst (ferritin or FeCl3) used which suggests that it is mainly
influenced by fluid dynamics.
Growth Mechanism of Carbon Nanotubes
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7295
Figure 5. CNTs grown using reduced concentration of ferritin particles,
ethanol CVD and catalyst pretreatments favoring short, randomly
aligned CNTs (a) and gas-flown aligned CNTs (b). Gas flows from
right to left. Scale bar 20 µm.
From these last two experimental observations, we find that,
in order to obtain LA nanotubes, it is necessary to have extended
growth durations and the achievement of a critical length. This
implies that the early growth termination of SR CNTs causes
both short length and random orientation. Growth termination
in the case of SR CNTs can be caused by different reasons which
are analyzed in the following sections.
3.2. Causes of CNT Growth Termination. The growth
duration is limited by factors that can be divided into mechanical
and chemical aspects of CNT growth. For the mechanical aspect,
interaction between nanotubes or between nanotubes and the
substrate are generally expected to be unfavorable for extended
growth duration. These issues are addressed in the following
discussion.
CNT-CNT Interaction. The high density of SR nanotubes
in Figure 1c suggested that possible entanglement may prevent
CNTs from continuously growing and thus aligning with gas
flow. However, further experimentation demonstrates that the
density of nanotubes cannot account for the difference between
SR and LA nanotubes. Figure 5 shows SEM pictures of CNTs
synthesized from ferritin nanoparticles with 50 times more
dilution in concentration than Figure 1. Even with a greatly
reduced density of nucleated CNTs, SR and LA nanotubes
(Figure 5, panels a and b, respectively) can still be obtained
selectively as in the case of Figure 1. In fact, the density of SR
nanotubes in Figure 5a is lower than the density of LA nanotubes
in Figure 1d-f. This demonstrates that CNT-CNT interaction
during growth is not a limitation for length and gas flow
alignment in these experiments.
CNT-Substrate Interaction. CNT-substrate interactions are
usually expected to obstruct CNT growth. For example it is
assumed that flow effects could force the nanotubes into contact
with the substrate and inhibit growth.8 Our experiments show
that synthesis takes place in a macroscopically laminar fluid
dynamical condition which is very stable since different sample
loading geometries (see the Supporting Information) that are
usually expected to change the flow characteristics12 yield the
same results. More importantly, selective growth of SR and LA
Figure 6. Base growth of SR CNTs grown by methane CVD. AFM
images of the same region with Fe nanoparticles before (a) and after
CNT growth (b). Scale bar 200 nm.
nanotubes can be achieved using the same total gas flow, sample
loading geometries and hydrocarbon gas concentration by only
changing catalyst pretreatment and conditioning of growth
chamber (Table 1). Therefore, gas flow mechanical effects,
unlike other reports,5,12 are not the determining parameters
leading to LA CNTs in this work.
The growth rate of a CNT could also determine if it is
susceptible to substrate interaction. For instance, an increased
growth rate can have a beneficial influence on a growing CNT
since it minimizes the time that the CNT can interact with the
substrate under deflection caused, i.e., by thermal vibration or
micro turbulences in the gas flow. However, in our experiments,
the growth rates of SR CNTs is likely to be of the same order
of magnitude or even higher than LA CNTs: A constant growth
rate for LA CNTs of approximately 2 µm/s can be assumed
from the extension of the length distribution’s tail over time
(Figure 3e-h). Similarly, the growth rate of the longest SR
nanotubes is at least 1 µm/s, making a growth termination due
to significantly lower growth rates unlikely in these experiments.
Another aspect of CNT growth relevant in CNT-substrate
interactions is the growth mechanism (tip vs base growth).
Figure 6 shows the AFM images of the substrate before and
after the CVD synthesis of SR nanotubes using ferritin nanoparticles. It can be clearly seen that the SR nanotubes are grown
via the “base growth” mechanism, meaning that the catalyst
nanoparticle stays attached to the substrate while the nanotube
is being extruded. This is consistent with previous findings.18
If a growing CNT is expected to interact with the substrate, a
tip growth mechanism could favor the extension of growth
duration. In this case, the catalyst particle would move away
7296 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Figure 7. AFM images of two different LA CNT ends suggesting
base growth (a and b) and tip growth (c and d); Imaging taken after
growth (a and c) and after annealing in air (b and d). CNTs were grown
using both ferritin and FeCl3 under ethanol and methane CVD
respectively. Scale bar 200 nm.
from the substrate while growing a CNT and therefore interactions of any CNT segment with the substrate will not hinder
the growth as in the case of base growth. Indeed, previous
reports have suggested a tip growth mechanism in the synthesis
of LA CNTs.6 We investigated the downstream end part of LA
nanotubes after growth as an attempt to clarify if tip growth
occurs exclusively in the case of LA nanotubes. Substrates with
markers were used to identify the position of the inspected
nanotubes. After AFM imaging of the downstream LA CNT
ends, the substrate was annealed in air at 450 °C for 30 min to
remove a small part of these end sections. Subsequent AFM
imaging of the same location clarifies if a catalyst nanoparticle
was present at the end of the nanotube. Figure 7 shows two
different scenarios observed within the set of nanotubes
inspected. Figure 7a-d shows AFM pictures of the end sections
of two LA nanotubes. After annealing, the nanotubes start to
disappear leaving behind some residues but a catalyst particle
is only present in the second case (Figure 7c,d). These results
indicate that tip growth is not a necessary condition for growing
the LA nanotubes since base growth can also occur.
Based on the above discussions, the aforementioned scenarios
do not seem to limit CNT growth duration. Therefore growth
termination by chemical aspects is favored as presented
consequently.
Catalyst DeactiVation. The growth duration of CNTs can also
be affected by the catalytic lifetime or time between activation
and deactivation of the catalyst nanoparticles. It is known that
this characteristic time can be affected by CVD process
parameters19 which is in agreement with our experimental
results. Catalytic deactivation of nanoparticles in CNT growth
is usually attributed to their excess carbon intake during
hydrocarbon decomposition or strong chemisorption of carbon
atoms on the catalyst surface.19 Previous reports have successfully extended catalyst lifetimes by introducing H2O or O2
during CNT syntheses.20,21 It was proposed that the presence
of oxidizing agents is able to extend CNT growth duration by
helping avoid excessive carbon exposure of the catalyst20 or by
providing control over the carbon/hydrogen ratio to optimize
Reina et al.
sp2 carbon production.21 In this report, the use of H2O or O2
during the CNT growth step was not beneficial to obtain LA
CNTs. However, due to observations in Table 1, we believe
that the catalyst pretreatment under different environments such
as Ar + O2 or Ar + H2 has a similar influence in extending the
growth duration. For instance, in ethanol CVD, the presence of
oxygen within the catalyst particle after its pretreatment could
also react with carbon species and therefore minimize the effects
of excessive carbon exposure of the catalyst. On the other hand,
in methane CVD, it may not be necessary to have fine control
of excessive carbon since methane has a higher decomposition
temperature over ethanol due to the higher bond strength of
C-H bonds with respect to C-O and C-C bonds.22
From the discussions above, we conclude that the last factor,
catalytic lifetime of the nanoparticles, is the most likely reason
determining the production of LA carbon nanotubes in our
experiments. If this is the case, the requirements satisfied by
LA CNTs, extended growth time and achievement of a critical
length, would be a consequence of the catalyst chemical
characteristics only. Furthermore, laminar and smooth gas flow
would be a necessary yet not a sufficient condition in order to
obtain LA nanotubes, just as it seems to be the case in the
experiments summarized in Table 1.
4. Conclusions
In summary, the growth of long, gas flow-aligned and short,
randomly oriented nanotubes from iron-based catalyst was
compared using atmospheric methane and ethanol CVD. It was
found that the two types of nanotubes result from differences
in growth duration. It is significantly higher (up to 15 min or
more) for long nanotubes than short ones (10 s or less).
Alignment with the gas flow sets in once the nanotubes become
longer than a critical length. Further experiments suggest that
the difference in catalytic life time of the nanoparticles is the
most probable reason leading to longer growth duration and
lengths in the case of aligned nanotubes. This extends the list
of influential parameters to carbon nanotube growth and helps
the understanding of growth mechanisms of nanotubes and their
integration on substrates for electronic device applications.
Acknowledgment. The authors acknowledge the support of
the Interconnect Focus Center, one of five research centers
funded under the Focus Center Research Program, a Semiconductor Research Corporation program. The authors also
acknowledge Intel Higher Education Program.
Supporting Information Available: (1) AFM image of
ferritin derived nanoparticles, (2) diagram arrangement used in
order to enable accurate CVD process termination, (3) description of “fast heating” process used, (4) calculation of standard
deviation for a uniform probability distribution of orientation
angles, and (5) schematic description of different sample holding
geometries. This material is available free of charge via the
Internet at http://pubs.acs.org.
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J. Phys. Chem. C, Vol. 111, No. 20, 2007 7297
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