Synthesis of aligned carbon nanotubes using thermal chemical vapor deposition

Article abstract of DOI:10.1016/S0009-2614(99)01074-X

Aligned carbon nanotubes have been synthesized on transition metal-coated silicon substrates with C₂H₂ using thermal chemical vapor deposition. It was found that nanotubes can be mostly vertically aligned on a large area of plain Si substrates when the density of metal domains reaches a certain value. Pretreatment of Co-Ni alloy by HF dipping and etching with NH₃ gas prior to the synthesis is crucial for vertical alignment. Steric hindrance between nanotubes at an initial stage of growth forces nanotubes to align vertically. Nanotubes are grown by a catalyst-cap growth mechanism. Applications to field emission displays are demonstrated with emission patterns.

Full text of DOI:10.1016/S0009-2614(99)01074-X

2
9 October 1999
Chemical Physics Letters 312 Ž1999. 461–468
www.elsevier.nlrlocatercplett
Synthesis of aligned carbon nanotubes using thermal chemical
vapor deposition
a
a
a
b
Cheol Jin Lee , Dae Woon Kim , Tae Jae Lee , Young Chul Choi ,
b
b,c,)
d
d
Young Soo Park , Young Hee Lee
, Won Bong Choi , Nae Sung Lee ,
e
d
Gyeong-Su Park , Jong Min Kim
a
Department of Electrical Engineering, Kunsan National UniÕersity, Kunsan 573-701, South Korea
b
Department of Semiconductor Science and Technology, and Semiconductor Physics Research Center, Chonbuk National UniÕersity,
Chonju 561-756, South Korea
Department of Physics, Chonbuk National UniÕersity, Chonju 561-756, South Korea
c
d
Display Laboratory, Samsung AdÕanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea
e
Analytical Engineering Laboratory, Samsung AdÕanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea
Received 14 June 1999; in final form 2 September 1999
Abstract
Aligned carbon nanotubes have been synthesized on transition metal-coated silicon substrates with C H using thermal
2
2
chemical vapor deposition. It was found that nanotubes can be mostly vertically aligned on a large area of plain Si substrates
when the density of metal domains reaches a certain value. Pretreatment of Co–Ni alloy by HF dipping and etching with
NH3 gas prior to the synthesis is crucial for vertical alignment. Steric hindrance between nanotubes at an initial stage of
growth forces nanotubes to align vertically. Nanotubes are grown by a catalyst-cap growth mechanism. Applications to field
emission displays are demonstrated with emission patterns. q 1999 Elsevier Science B.V. All rights reserved.
Since the discovery of carbon nanotubes ŽCNT.
w1x, synthesis of carbon nanotubes for mass produc-
tion has been achieved by several methods such as
laser vaporization w2x, arc discharge w3x, and pyroly-
sis w4x. However, controllibility of diameters, lengths,
and chiralities has never been easily accessible with
such approaches. In spite of considerable efforts for
CNTs to apply for electron field emitters w5–8x,
practical approaches are still limited by the intricate
sample-preparation processes and large area synthe-
sis. In particular, synthesis of vertically aligned na-
notubes is of great importance for applications to
field emitters. Recently, vertically aligned CNTs have
been synthesized on glass by plasma-enhanced
chemical vapor deposition ŽPECVD. w9x. Aligned
CNTs can be also grown on mesoporous silica w10x
and Fe-patterned porous silicon w11x using the chemi-
cal vapor deposition ŽCVD. method. Despite such
breakthroughs in the synthesis, the growth mecha-
nism of the alignment are still far from being clearly
understood. Furthermore, flat-panel emission dis-
plays on a large area has not been clearly demon-
strated yet at a practical level. Here, we have synthe-
sized vertically aligned CNTs on a large area of
Co–Ni codeposited Si substrates by the thermal CVD
using C H gas. We demonstrate that CNTs can be
)
Corresponding author. Fax: q82-652-270-3585; e-mail:
leeyh@sprc2.chonbuk.ac.kr
2
2
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.01074-X

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62
C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
8
y2
mostly vertically aligned on catalyzed Si substrate
when the domain density reaches a certain value.
Controlling the domain density of Co–Ni alloy by
HF dipping and dry etching with NH3 gas prior to
the deposition is of crucial importance. Steric hin-
drance between nanotubes at an initial stage of the
growth forces nanotubes to align vertically. Na-
notubes are then further grown by the catalyst-cap
growth mechanism. We further demonstrate the field
emission patterns and I–V characteristics obtained
from vertically aligned carbon nanotubes.
density of nanotubes is about 3=10 cm , about
100 times larger than the typical density of microtips
in conventional Spindt-type field emission arrays.
Some nanotubes are kinky at the initial stage of
growth, as shown in Fig. 1d. We evaluated the wall
structures by the transmission electron microscopy
ŽTEM. ŽHitachi H-9000NA, 300 kV., as shown in
Fig. 1e,f. Peeled nanotubes were dispersed on a
copper carbon-microgrid. TEM images show long
and straight multi-wall nanotube with an hollow
inside ŽFig. 1e.. The lattice images are shown in Fig.
We used p-type silicon substrates with a resistiv-
ity of 15 V-cm, since it is of importance to use
silicon substrates for future Si-based optoelectronic
devices. Co–Ni ŽCorNi, 1:1.5. metal alloys with
1f. However, long-range lattice images are not
formed due to relatively low growth temperature.
This was also confirmed in our Raman scattering
measurement that a relatively large defective peak at
y1
1
00 nm in thickness were thermally evaporated at
1295 cm
was observed with the main graphitic
y6
y1
room temperature in a vacuum of 10
torr on
G-peak at 1595 cm . Fig. 2 shows the Raman
spectrum of alinged carbon nanotubes using FT-Ra-
man spectrometer ŽBRUKER RFS 100rS. with the
excitation laser of Nd:YAG Žwavelength: 1064 nm..
oxidized SiŽ100. substrates. These samples were fur-
ther annealed at 4008C in Ar ambient for 20 min,
which enhanced the adhesion of metal layers to the
oxidized Si substrates. The annealed samples were
dipped for 100–200 s in HF solution ŽHFrDI water,
Besides two main peaks, several peaks at low
wavenumbers were also observed in our multiwall
nanotubes, although these were usually observed in
single-wall nanotubes w12–14x.
1
:12. and were then loaded on the quartz boat inside
the CVD quartz reactor of 60 mm in diameter. In
order to prevent the oxidation of Co–Ni alloys, Ar
gas was flowed into the quartz reactor during in-
creasing the temperature to a desirable value. Sam-
Nanotubes were reproducibly grown with the
above conditions. We have also tried various growth
conditions w14x. CNTs were grown well independent
ples were pretreated using NH gas with a flow rate
of growth temperatures within the temperature range
3
of 80 sccm for 5–20 min at 800–9008C. Then, CNTs
of 800–9008C, since C H gases are highly decom-
2
2
were synthesized using C H gas with a flow rate of
posed in this range by the catalytic assistance of
2
2
1
5–40 sccm for 10–20 min at the same temperature
transition metals w10x. Tube lengths increased with
as the NH3 pretreatment. The reactor was cooled
down slowly to room temperature in Ar ambient
after the synthesis.
Fig. 1a–d shows scanning electron microscope
ŽSEM. ŽJEOL JSM-6400, 20 kV. images of CNTs
increasing growth time. However, some carbona-
ceous particles appeared with longer growth time.
The growth rate was 30 mmrh, similar to that from
the CNT growth on porous Si w10x. We note that the
HF dipping and NH3 pretreatment are crucial steps
to obtain high density of nucleation sites. Without
with various magnifications. CNTs are well aligned
over the large area Ž20=30 mm . of the substrate,
2
NH pretreatment, CNTs were grown uniformly but
3
as shown in Fig. 1a. The surface morphologies of
CNTs are clean and uniform with the length of about
not vertically aligned. Therefore, the NH3 pretreat-
ment is a very critical step in vertically aligning
carbon nanotubes on Si substrates.
5
mm and the diameter of about 200 nm. The white
bundles of CNTs shown in Fig. 1a are peeled off due
to scratches. Note that top of the aligned nanotubes
is terminated by a transition-metal cap, the white
spots, as shown in Fig. 1b, which is similar to the
previous reports w9,10x. Fig. 1c reveals high density
Fig. 3 shows the SEM images of surface morphol-
ogy for Co–Ni alloy. The HF dipping etches metal
surface and increases the surface roughness with
increasing the HF dipping time, as shown in Fig.
3a,b. The subsequent NH pretreatment further etches
3
of nanotubes, where nanotubes are curly at the bot-
tom but rather straight near the top of nanotubes. The
the surface and forms small domains inside the metal
cluster, as shown in Fig. 3c, acting as nucleation

C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
463
Fig. 1. SEM images of carbon nanotubes grown with gas flow rate of 40 sccm at 8508C for 10 min. Edges are vividly visualized by peel-off
2
with a razor. Ža. A uniformly distributed morphology of vertically aligned carbon nanotubes in large area of 20=30 mm . Žb. Magnified
top view. Žc. Magnified tilted view of carbon nanotubes at the peeled edge. Žd. Magnified view of Žc.. Že. TEM image of a single carbon
nanotube in higher resolution. Žf. The lattice image of a nanotube.

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64
C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
Fig. 2. Raman spectrum of vertically aligned carbon nanotubes.
seeds for vertical alignment of nanotubes. The size
of small domains within the metal cluster in Fig. 3c
is about 200 nm, the same as the diameter of CNTs.
This strongly suggests that carbon diffusion within
the small domain is limited by the domain wall. With
the absence of NH3 pretreatment, nanotubes were
laid down with a low density. Both HF dipping and
NH3 pretreatment were necessary to control the
surface morphology to align nanotubes vertically in
case of using the Co–Ni alloy.
the density of nucleation sites reaches a certain
value, nanotubes grown in directions other than ver-
tical direction are prohibited from growing due to the
steric hindrance from the adjacent nanotubes and
then the growth direction is changed to further grow
vertically. This can be confirmed from some kinky
nanotubes at the early growth stage, as can be clearly
seen in Fig. 1d.
Fig. 4 illustrates our proposed growth model.
C H molecules are adsorbed on the metal domains.
2
2
Nanotubes grown on Ni-coated substrates had a
diameter of about 100 nm, whereas diameters of
nanotubes grown on Co-coated substrates were about
As carbon atoms are further supplied, a carbon–metal
eutectic alloy can be formed, decreasing the melting
temperature of the alloy. Formation of carbon–metal
eutectic alloy promotes the diffusion of carbon in the
metal alloy, initiating carbon aggregations followed
by the nanotube formation, as shown in Fig. 4a. The
carbon diffusion is limited by the domain size within
the metal particles and thus the diameter of na-
notubes should not be larger than the domain size.
As the nanotube grows further, part of the metal
domain is pushed upward, forming a metal cap, as
shown in Fig. 4b. The metal cap saturates dangling
bonds of the nanotube at the edge, stabilizing nan-
otube edges. Presuming sufficient supply of carbon
atoms and high density of domains within the metal
2
00 nm. However, nanotubes were not vertically
aligned in both cases w14x. In case of the Co–Ni
alloy, Ni particles have faster etching rate than Co
particles during dry NH etching. We found that Ni
3
particles were etched away during dry etching of
NH , which resulted in the increase of the density of
3
nucleation sites within Co particles. This was con-
firmed by the energy-dispersive X-ray ŽEDX. spec-
trum ŽFig. 3d.. The amount of Co left on the surface
after NH treatment was about six times larger than
3
that of Ni. Note that the amount of Ni was 1.5 times
larger than that of Co in as-deposited samples. When

C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
465
Fig. 3. SEM images of surface morphology for Co–Ni alloy. Ža. No HF dipping, Žb. HF dipping for 140 s, and Žc. HF dipping for 140 s
followed by NH3 pretreatment for 10 min.
cluster, nanotubes are forced to align vertically, as
shown in Fig. 4c. Some nanotubes at the edge of the
cluster may not be aligned vertically at the initial
stage but will be eventually aligned vertically by the
steric hindrance from other nanotubes for further
growth, as shown in Fig. 4c. As CNTs become
longer, C H gases may not easily reach the bottom
of the substrate due to the compact CNTs highly
populated on the surface. Instead, they will have a
better chance to reach the top metal cap and then,
adsorbed carbon atoms can diffuse into the edge of
nanotubes. Formation of complete hexagons are cata-
lytically promoted by the assistance of the metal cap
w15x, giving rise to the continuous cap growth. This
model is able to provide uniform height of nanotubes
in large area and control the height with growth time.
Uniform growth in height may not be achieved on
catalyst-patterned substrates with the model of base
growth suggested in another report w11x. Our model
can also explain the existence of the metal caps at
the top of nanotubes after the growth, as observed
from our SEM and TEM images and other reports
w9,10x.
2
2
The field emission patterns were observed from
vertically aligned carbon nanotubes. Fig. 5 illustrates
2
the emission patterns from 1.4=1.6 cm in size
with bias conditionings for Ža. 30, Žb. 60 and Žc. 90
min. Fig. 5d shows typical I–V curve from Fig. 5c

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C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
Fig. 4. Schematic diagram of our growth model. Ža. Formation of nucleation process, Žb. cap growth, and Žc. mechanism for vertical
alignment at the initial stage.

C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
467
Fig. 5. Emission patterns from 1.4=1.6 cm in size with bias conditionings for Ža. 30 min, Žb. 60 min, and Žc. 90 min, and Žd. typical I–V
curve from Žc.. Open squares in Žd. show emission currents from patterned graphite powder-epoxy composite. The inset shows
Fowler–Nordheim plot from vertically aligned carbon nanotubes.
along with the corresponding Fowler–Nordheim plot
Žinset.. The anode, an ITO Žindium tin oxide.-phos-
phor-coated glass, was separated from the bottom
layer of nanotubes by a spacer in distance of 570
mm. The chamber was maintained at 1=10y6 Torr
during the I–V measurements. Although samples
were clean, as seen in Fig. 1a, they were annealed
initially by repeatedly applying 100 and 1000 V in

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C.J. Lee et al.rChemical Physics Letters 312 (1999) 461–468
order to burn unintentionally protruded nanotubes.
This improves the emission patterns as shown in Fig.
the Semiconductor Physics Research Center at Chon-
buk National University.
5
a–c. The turn-on voltage was about 1.2 Vrmm
with a current of 10 nA, which is much smaller than
the other reports obtained from patterned nanotube–
epoxy composite w8x and from single-wall nanotubes
produced by arc-discharge w16x. Turn-on voltage also
References
varies with the resistance of the transition metals.
For comparison, we have also fabricated patterned
graphite powder–epoxy composite, which resulted in
higher turn-on voltage, as shown in Fig. 5d. In spite
of smaller effective emission area from CVD-grown
samples, the emission current was much larger than
that of graphite. The current increases sharply after
turn-on voltage and saturates near 3 Vrmm. The
inset clearly shows that this field emission follows
the Fowler–Nordheim equation. The presence of
metal particles at the top of nanotubes prohibited
efficient emission and some protruded nanotubes
gave rise to non-uniform emission patterns. A keen
control with bias annealings is required to have large
emission currents by removing metal caps and uni-
form emission patterns over a large area. We empha-
size that the present approaches are easily integrable
within the current semiconductor process.
In summary, we have synthesized aligned carbon
nanotubes on plain Si substrates using thermal CVD
method. Vertically aligned nanotubes are uniformly
synthesized on a large area of Si substrates. A new
metal-cap growth mechanism is proposed based on
the experimental observations. This approach should
be easily applicable to a large area synthesis without
loss of generality. We have also demonstrated the
field emission patterns from the aligned nanotubes.
w1x S. Iijima, Nature 354 Ž1991. 56.
2
w x
A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C.
Xu, Y.H. Lee, S.G. Kim, D.T. Colbert, G. Scuseria, D.
Tom a´ nek, J.E. Fisher, R.E. Smalley, Science 273 Ž1996. 483.
w3x C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de
la Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer,
Nature 388 Ž1997. 756.
4
w x
M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H. Ter-
rones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D. Townsend, K.
Prassides, A.K. Cheetham, H.W. Kroto, D.R.M. Walton,
Nature 388 Ž1997. 52.
w5x W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 Ž1995.
179.
1
w x
6
P.G. Collins, A. Zettl, Phys. Rev. B 55 1997 9391.
Ž .
w7x Y. Saito, K. Hamaguchi, K. Hata, K. Uchida, Y. Tasaka, F.
Ikazaki, M. Yumura, A. Kasuya, Y. Nishina, Nature 389
Ž1997. 554.
w8x Q.H. Wang, A.A. Setlur, J.M. Lauerhaasi, J.Y. Dai, E.W.
Seelig, R.P. Chang, Appl. Phys. Lett. 72 Ž1998. 2912.
Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P.
Siegal, P.N. Provencio, Science 282 Ž1998. 1105.
9
w x
w10x W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y.
Zhou, R.A. Zhao, G. Wang, Science 274 Ž1996. 1701.
w11x S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M.
Casell, H. Dai, Science 283 Ž1999. 512.
w12x J.M. Holden, P. Zhou, X. Bi, P.C. Eklund, S. Bandow, R.A.
Jishi, K.D. Chowdhury, G. Dressehaus, M.S. Dresselhaus,
Chem. Phys. Lett. 220 Ž1994. 186.
w13x A.M. Rao, E. Richer, S. Bandow, B. Chase, P.C. Eklund,
K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A.
Thess, R.E. Smally, G. Dresselhaus, M.S. Dresselhaus, Sci-
ence 275 1997 187.
Ž
.
w14x C.J. Lee, D.W. Kim, T.J. Lee, Y.C. Choi, Y.S. Park, W.S.
Kim, W.B. Choi, N.S. Lee, J.M. Kim, Y.G. Choi, S.C. Yu,
Y.H. Lee, Appl. Phys. Lett. 75 Ž1999. 1721.
w15x Y.H. Lee, S.G. Kim, D. Toman e´ k, Phys. Rev. Lett. 78
Ž1997. 2393.
Acknowledgements
We acknowledge the financial support by the
Korea Science and Engineering Foundation through
w16x
J.M. Bonard, J.P. Salvetat, T. Stockli, W.A. de Heer, L.
Forro, A. Chatelain, Appl. Phys. Lett. 73 Ž1998. 2113.