Growing carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

Article abstract of DOI:10.1063/1.121658

A processing route has been developed to grow bundles of carbon nanotubes on substrates from methane and hydrogen mixtures by microwave plasma-enhanced chemical vapor deposition, catalyzed by iron particles reduced from ferric nitrate. Growth takes place at about 900°C leading to nanotubes with lengths of more than 20 μm and diameters on the nanometer scale.

Full text of DOI:10.1063/1.121658

Growing carbon nanotubes by microwave plasma-enhanced chemical
vapor deposition
L. C. Qin, D. Zhou, A. R. Krauss, and D. M. Gruen
Citation: Appl. Phys. Lett. 72, 3437 (1998); doi: 10.1063/1.121658
View online: http://dx.doi.org/10.1063/1.121658
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APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 26
29 JUNE 1998
Growing carbon nanotubes by microwave plasma-enhanced chemical
vapor deposition
L. C. Qin,a) D. Zhou,b) A. R. Krauss, and D. M. Gruen
Materials Science and Chemistry Divisions, Argonne National Laboratory, MSD/CHM 200, Argonne,
Illinois 60439
͑
Received 4 November 1997; accepted for publication 27 April 1998͒
A processing route has been developed to grow bundles of carbon nanotubes on substrates from
methane and hydrogen mixtures by microwave plasma-enhanced chemical vapor deposition,
catalyzed by iron particles reduced from ferric nitrate. Growth takes place at about 900 °C leading
to nanotubes with lengths of more than 20 ␮m and diameters on the nanometer scale. © 1998
American Institute of Physics. ͓S0003-6951͑98͒01826-9͔
1
The extraordinary properties of carbon nanotubes have
made this allotrope of nanocarbon the most studied nanoma-
terial for the past few years. Mechanical measurement of the
axial Young’s modulus of multiwalled carbon nanotubes2
obtained extremely high values of about 1800 GPa, which
suggest that carbon nanotubes are the stiffest material known
to date. Electronically, a carbon nanotube can behave either
as a semiconductor or as a metal, depending on its diameter
method ͑estimated to be 5000–20 000 °C͒ or the laser evapo-
ration method ͑4000–5000 °C͒. It also seems that for grow-
ing well-aligned nanotube arrays1
6,19
on solid substrates for
potential nanodevice applications, chemical vapor deposition
͑CVD͒ methods appear to be most promising.
In this letter we report a method that enables growth of
carbon nanotubes of large aspect ratios on supporting solid
matrices by microwave plasma-enhanced chemical vapor
deposition. The apparatus is of the same type that has been
widely employed to grow nanocrystalline diamond thin
films.20 However, different conditions have been identified
that allow the deposited carbon to grow into graphitic tu-
bules. For example, the growth of carbon nanotubes requires
higher methane-to-hydrogen ratios compared with the condi-
tions under which micro- or nanodiamond thin films are
grown.
3
and helicity. This unique electronic property makes carbon
nanotubes a novel material that allows tuning of its elec-
tronic properties by manipulating its atomic structure in or-
der to fabricate electronic devices. Carbon nanotubes have
also been investigated for applications as electric-field-
induced electron emitters4 and a very low onset field of 0.8
V/␮m has been observed from multiwalled carbon nanotubes
–6
6
produced by arc discharge. Recent experimental studies
Alumina substrates (Al O ) used for growing the carbon
have demonstrated that carbon nanotubes possess quantum
2
3
7
nanotubes in our system were first coated with ferric nitrate
wire behavior and electronic property enhancement can be
achieved by chemical doping.8
3 3
2
͓
Fe͑NO ͒ •6H O͔ solution. The coating of such an iron-
containing compound has been found to be necessary as the
iron catalyst particles are essential for both the initial nucle-
ation and the subsequent growth of carbon nanotubes.
Growth takes place in a vacuum chamber in which the sub-
strate can be heated to about 850–900 °C. The substrate tem-
perature was kept constant throughout the deposition pro-
cess. A mixture of methane (CH ) and hydrogen (H ) gases
One of the challenging issues has been the synthesis of
this form of nanocarbon on large scales ever since it was first
observed at the cathode in an electric arc evaporation experi-
1
ment where the anode had been consumed. In addition to
the refinement of the arc discharge technique,9 subsequent
,10
developments have led to the realization of many alternative
1
1,12
3
processing techniques, such as laser evaporation;
pyroly-
4
2
1
were admitted to the deposition chamber through mass flow
controllers. The total pressure in the chamber was kept con-
stant at 15 Torr while the flow rates were 15 and 10 sccm for
sis of hydrocarbon gases using benzene (C H ), acetylene
6
6
1
4
15,16
17,18
(
C H ), methane (CH );
etc.
However, given the
2
2
4
sensitive relationship between the atomic structure and the
properties of the tubules, in order to make carbon nanotubes
of practical importance, the criteria for assessing any synthe-
sis technique must include the feasibility and potentiality for
scaled-up production at low cost as well as the ability to
control structure. In this regard, gas phase deposition meth-
ods have received a great deal of attention since they can be
conveniently scaled up in production levels, while growth
takes place at moderate temperatures ͑around 1000 °C͒, well
below the operating temperature for the arc-discharge
CH4 and H , respectively. The microwave plasma input
2
power was 600 W. A schematic of the experimental setup is
given in Fig. 1. The lower edge of the plasma was in contact
with the substrate surface during the deposition process. The
deposited film was studied by scanning electron microscopy
͑SEM͒ to examine the structure of the as-grown material
prior to any postdeposition treatment.
A typical as-grown morphology of the carbon deposits
taken with a SEM is shown in Fig. 2, where large bundles of
tangled carbon nanotubes can be seen. A striking feature is
that the as-grown nanotubes have quite a uniform diameter
of about 10–50 nm, while the typical length is well beyond
20 ␮m, resulting in a high aspect ratio of about 1000 or
larger. Two major components are identified: carbon nano-
a͒Present address: IBM Corp., Department MXQ/014, 5600 Cottle Road,
San Jose, CA 95193. Electronic mail: qin@ibm.net
b͒
Present address: Advanced Materials Processing and Analysis Center, Uni-
versity of Central Florida, Orlando, FL 32816.
0003-6951/98/72(26)/3437/3/$15.00
3437
© 1998 American Institute of Physics
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3438
Appl. Phys. Lett., Vol. 72, No. 26, 29 June 1998
Qin et al.
FIG. 1. Schematic of the microwave plasma-enhanced CVD apparatus.
Methane and hydrogen gases enter the chamber ͑with 1 mTorr base pres-
sure͒ where a coated substrate can be heated to elevated temperature ͑370–
FIG. 3. SEM image of tangled carbon nanotubes produced by the electric
arc-discharge technique. Note the striking resemblance in morphology of the
arc-produced nanotubes with those produced by microwave plasma-
enhanced CVD shown in Fig. 2.
9
50 °C͒. The plasma is excited above the substrate on which carbon nano-
tubes are grown.
catalyst particles while the continued heating of the substrate
would quickly bring the iron particles back to thermal equi-
librium. Since the temperature of the substrate is kept con-
stant, the bottom of the catalytic particle should be hotter
than the top. This should result in the growth of carbon tu-
bules from the iron particle while the catalyst particles stay
on the surface of the supporting substrate. The presence of
atomic hydrogen in the plasma should also enhance the de-
composition of ferric nitrate associated with the initial coat-
ing into iron particles on the substrate surface, in addition to
enhancing the diffusion of carbon into the iron particles.
The growth rate of carbon nanotubes in the microwave
plasma-enhanced CVD described here has been found to be
tubes and soot. It is interesting to note that the carbon nano-
tubes appear superficially very similar to those produced by
electric arc-discharge evaporation, despite the fact that the
two processes operate in vastly different temperature re-
gimes. The similarity is displayed in Fig. 3, which gives a
SEM image of carbon nanotubes deposited at the cathode in
an arc-discharge experiment.21 It should also be emphasized
that in our experiment the nanotubes are quite evenly distrib-
uted over the total surface of the substrate, which measures
2
2
5ϫ50 mm ; while in the arc-discharge method, they oc-
cupy only a small portion of the central region of the cath-
ode. The large-area uniformity of the deposition in the
present study has important practical implications for array
fabrication and other applications.
We propose that the growth mechanism is catalyst as-
sisted. Figure 4 depicts a two-step model for the nucleation
and growth of carbon nanotubes by catalytic reaction in
plasma-enhanced CVD. Nucleation is assumed to occur at
the surfaces of the iron catalyst particles, as indicated in the
Fe–C phase diagram. The decomposition of hydrocarbons at
a particle surface would result in carbon deposition at its
surface. In the meantime, the plasma formed above the sub-
strate would enhance diffusion of carbon species into the
much slower than that in the pyrolytic growth reported
earlier.1
5,16
We believe that the difference in growth rate is
caused by the temperature difference between the two cases,
noting that the pyrolytic deposition was performed at about
1
200 °C. The growth of the nanotubes has been observed to
occur in both axial and radial directions. Although the nano-
tubes appear to be closely associated with the catalytic par-
ticles, it is not yet clear what effect the size of the iron
catalyst particles has on the diameter of the carbon nano-
tubes.
The significance of the present results is twofold: ͑1͒
FIG. 4. Growth model for the formation of carbon nanotubes in microwave
plasma-enhanced CVD. The catalyst particle ͑coated onto the substrate sur-
face as indicated by a solid arrow͒ can contain larger amount of carbon in
equilibrium while temperature is raised; the heat needed to decompose
methane molecules ͑moving towards the catalytic particle as indicated by
the open arrow͒ at the catalytic particle surface lowers the temperature,
resulting in the particle being supersaturated, and therefore, precipitation of
excess carbon occurs—forming graphitic tubules at the surface.
FIG. 2. SEM image of tangled carbon nanotubes of uniform diameter grown
on an alumina substrate by microwave plasma-enhanced CVD. Typical di-
ameters of the shown carbon nanotubes are 10–50 nm, while their lengths
go well beyond 20 ␮m.
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Appl. Phys. Lett., Vol. 72, No. 26, 29 June 1998
Qin et al.
3439
1
2
appropriate physical conditions have been identified for the
growth of carbon nanotubes by microwave plasma-enhanced
CVD, and ͑2͒ the successful growth on solid substrates
opens potentially new processing routes for growing well-
aligned carbon nanotubes. The latter should be of great tech-
nological interest as carbon nanotubes have shown excellent
electric-field-induced electron emission properties. In par-
ticular, this method should allow more precise control over
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