Growth of highly oriented carbon nanotubes by plasma-enhanced hot filament chemical vapor deposition

Article abstract of DOI:10.1063/1.122912

Highly oriented, multiwalled carbon nanotubes were grown on polished polycrystalline and single crystal nickel substrates by plasma enhanced hot filament chemical vapor deposition at temperatures below 666°C. The carbon nanotubes range from 10 to 500 nm i

Full text of DOI:10.1063/1.122912

Growth of highly oriented carbon nanotubes by plasma-enhanced hot
filament chemical vapor deposition
Z. P. Huang, J. W. Xu, Z. F. Ren, J. H. Wang, M. P. Siegal et al.
Citation: Appl. Phys. Lett. 73, 3845 (1998); doi: 10.1063/1.122912
View online: http://dx.doi.org/10.1063/1.122912
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APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 26
28 DECEMBER 1998
Growth of highly oriented carbon nanotubes by plasma-enhanced hot
filament chemical vapor deposition
Z. P. Huang, J. W. Xu, Z. F. Ren,a) and J. H. Wang
Materials Synthesis Laboratory, Departments of Physics and Chemistry, and Center for Advanced Photonic
and Electronic Materials (CAPEM), State University of New York at Buffalo, Buffalo, New York 14260
M. P. Siegal and P. N. Provencio
Sandia National Laboratories, Albuquerque, New Mexico 87185-1421
͑Received 13 July 1998; accepted for publication 27 October 1998͒
Highly oriented, multiwalled carbon nanotubes were grown on polished polycrystalline and single
crystal nickel substrates by plasma enhanced hot filament chemical vapor deposition at temperatures
below 666 °C. The carbon nanotubes range from 10 to 500 nm in diameter and 0.1 to 50 ␮m in
length depending on growth conditions. Acetylene is used as the carbon source for the growth of the
carbon nanotubes and ammonia is used for dilution gas and catalysis. The plasma intensity,
acetylene to ammonia gas ratio, and their flow rates, etc. affect the diameters and uniformity of the
carbon nanotubes. © 1998 American Institute of Physics. ͓S0003-6951͑98͒01952-4͔
Following the first reported observation of carbon
nanotubes,1 numerous letters have reported studies on the
yield of carbon nanotubes, their diameter and wall
thickness,2–6 growth mechanisms,2,7,8 alignment,6,9 electron
glass provided by Corning Inc.͒ since the display glass sit
side by side with the Ni did not show any noticeable defor-
mation after the experiments16 and also Ni is not red hot by
visual observation. Growth durations were from 10 min to 5
h depending on the desired carbon nanotube lengths.
Samples were examined by scanning electron microscopy
͓͑SEM͒, Hitachi S-4000͔ to measure tube lengths, diameters,
site distributions, alignment, density, and uniformity. High-
resolution transmission electron microscopy ͑TEM͒ was
used to determine the microstructure of individual tubes.
Samples were also examined by x-ray diffraction, Raman
spectroscopy, and x-ray photoemission spectroscopy to study
the structure, crystallinity, composition, and central core and
tube wall structures.
Figure 1͑a͒ is an SEM micrograph showing the align-
ment of carbon nanotubes grown on polycrystalline Ni.
Growth conditions are described in Table I͑a͒. Clearly, the
carbon nanotubes are oriented perpendicular to the substrate
surface and are quite uniform in height. Note that the carbon
nanotubes do not grow well along the Ni grain boundaries,
shown by the two empty tracks running from upper left and
from upper right down to bottom, which is due to the fact
that grain boundaries do not have enough Ni as catalysis.
Figure 1͑b͒ is a higher magnification image of an area within
a single Ni grain and clearly shows that the distribution uni-
formity within these grains is reasonably good. However,
there exists a wide distribution of carbon nanotube diameters
ranging from 60 to 500 nm. Fortunately, the uniformity in
both diameter and site distributions can be controlled via the
growth conditions.
emission
properties,10–14
nanodevices,15
theoretical
predictions,2 and potential applications.2 Nanotube alignment
is particularly important to enable both fundamental studies
and applications, such as flat panel displays, vacuum micro-
electronics, chargeable batteries, etc. However, only one re-
port exists on the growth of aligned carbon nanotubes by
thermal decomposition of acetylene in nitrogen gas at tem-
perature above 700 °C on mesoporous silica containing iron
nanoparticles6 before our report on growth of large arrays of
well-aligned carbon nanotubes on glass.16 Here we report the
growth of highly oriented, multiwalled carbon nanotubes on
nickel substrates at low temperatures by the same method
͑plasma enhanced hot tungsten-filament chemical vapor
deposition͒ described in our previous letter.16 The motivation
to grow carbon nanotubes on Ni substrates is for the appli-
cations of using carbon nanotubes as battery electrodes and
energy storage. We use acetylene (C2H2) to provide carbon
for the growth of the carbon nanotubes and ammonia (NH3)
gas for both dilution gas and catalysis. The catalytic role of
ammonia is discussed in our previous letter.16
The base pressure of the deposition chamber is Ͻ6
ϫ10Ϫ6 Torr. We grew carbon nanotube films in a pressure
of 1–20 Torr maintained by flowing acetylene and ammonia
gases with a total flow rate of 120–200 sccm. We varied the
acetylene-to-ammonia volume ratio from 1:2 to 1:10 for dif-
ferent experimental runs. Both polished polycrystalline and
single-crystal Ni substrates were used. After stabilizing the
working pressure, the tungsten filament coil powered by a
direct current ͑dc͒ source and the plasma generator were
turned on to generate heat and plasma. Under the present
experimental setup, the temperature of samples is estimated
to be below 666 °C ͑which is the strain point of the display
Figure 2 is an SEM micrograph showing carbon nano-
tubes grown on polycrystalline Ni under a different condition
as described in Table 1͑b͒. The tube diameter is smaller and
its distribution is narrower, ranging from 200 to 300 nm.
This difference results mainly from increasing the plasma
intensity. The increase of plasma intensity apparently re-
duced the catalytic Ni particle size. Smaller Ni particles re-
sult thinner carbon nanotubes. Based on this observation, we
further increased the plasma intensity in an ensuing experi-
a͒
Electronic mail: zren@acsu.buffalo.edu
0003-6951/98/73(26)/3845/3/$15.00
3845
© 1998 American Institute of Physics
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3846
Appl. Phys. Lett., Vol. 73, No. 26, 28 December 1998
Huang et al.
FIG. 2. SEM micrograph of carbon nanotubes grown at a higher plasma
intensity under the conditions listed in Table I͑b͒. The diameter uniformity
is substantially narrowed and the average diameter size is reduced. Most
of the tube diameters are ϳ250 nm. The density increased to
4ϫ106 nanotube/mm2, about four times higher than that shown in Fig. 1.
we note that when the diameters are Ͻ20 nm, the tubes are
less straight than for those with diameters Ͼ50 nm.16 The
good alignment of carbon nanotubes on glass16 and silicon17
will be very important to the development of cold-cathode
electron emitters for applications such as flat panel displays,
microelectronics, etc.
Finally, we performed TEM analysis on the carbon
nanotubes to determine whether or not these film structures
are truly carbon nanotubes, as opposed to carbon fiber-like
structures. Samples with carbon nanotubes grown to several
microns in length were easily scraped off a nickel substrate
directly onto a copper TEM grid for analysis. Figure 4 shows
a typical image obtained for these carbon nanotubes. The
disorder is due entirely to the random process of nanotube
collection on the TEM grid. The dark spot at the end of each
structure is a small ball of Ni that appears to cap each tube.
This image is typical to those reported elsewhere demon-
strating carbon nanotube structure.2,3 Nevertheless, the insert
to Fig. 4, a high-resolution image of a portion of a typical
carbon nanotube structure, is more convincing. The width of
FIG. 1. ͑a͒ SEM micrograph shows alignment of carbon nanotubes grown
on a large area of polycrystalline Ni substrates. Note that the carbon nano-
tubes grow well inside each grain, but not along the grain boundaries. ͑b͒ A
magnified view of ͑a͒ to show the diameter size and distribution, height
uniformity, density, etc. The tube diameters range from ϳ60 to 500 nm. The
density is about 106 nanotubes/mm2.
ment. The growth conditions are listed in Table I͑c͒. In order
to keep the substrates at low temperature, the filament cur-
rent was reduced from the usual 9 to 6 amperes. Figure 3 is
an SEM micrograph showing the carbon nanotubes grown
under these conditions. This further increase in plasma inten-
sity results in two structural changes. First, there is a sub-
stantial decrease in the average tube diameters from ϳ250
nm as shown in Fig. 2 to ϳ100 nm as shown in Fig. 3.
Second, the tube lengths increase dramatically. This high
growth rate is very attractive for the potential large-scale
production of carbon nanotubes with long lengths. Finally,
TABLE I. Growth conditions for the nanotube films shown in Figs. 1͑a͒ and
1͑b͒, 2, and 3.
C2H2 /NH3
Filament current
Plasma intensity
Growth time
͑sccm/sccm͒
͑amperage, A͒
͑amperage/V/W͒
͑min͒
͑a͒: For Figs. 1͑a͒
and 1͑b͒
20/100
͑b͒: For Fig. 2
80/160
͑c͒: For Fig. 3
40/80
9
9
6
0.09/460/50
0.2/640/140
0.3/700/220
90
25
20
FIG. 3. SEM micrograph shows that carbon nanotubes with higher aspect
ratios are synthesized using a higher plasma intensity than that used for the
samples in Fig. 2. The length of the carbon nanotubes is at least 10 ␮m and
the diameters are Ͻ100 nm.
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Appl. Phys. Lett., Vol. 73, No. 26, 28 December 1998
Huang et al.
3847
ameter and length͒, and range of both site and height
distributions within a given film.
This material is based upon work supported in part by
the U.S. Army Research Office under Grant No. DAAG55-
97-1-0139. The management of Dr. Robert R. Reeber to this
program is greatly appreciated. The authors would also like
to thank G. Sagerman for his technical support and P. Bush
for his insightful discussions and valuable help in SEM stud-
ies. High-resolution transmission electron microscopy
͑TEM͒ was performed on a JEOL 2010 at The University of
New Mexico, using phase contrast digital imaging with a
CCD slow scan camera. Sandia is a multiprogram laboratory
operated by Sandia Corporation, a Lockheed Martin Com-
pany, for the U.S. DOE under Contract No. DE-AC04-
94AL85000.
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