High density of multiwalled carbon nanotubes observed on nickel electroplated copper substrates by microwave plasma chemical vapor deposition

Article abstract of DOI:10.1016/S0009-2614(02)00133-1

High density and long multiwalled carbon nanotubes are deposited using nickel clusters formed from an electro-deposited nickel coating treated by ammonia. Tubes of length 40-50 μm having bamboo shaped structure are aligned nearly parallel to the copper substrate. The inner diameter of the tubes is in the range 20-50 nm. The selected area and nano area diffraction spots clearly show the crystalline nature of nickel clusters. At some locations of the samples in the vicinity of graphite layers, Ni₃C phase is also detected. The formation of the Ni₃C phase that is unstable at high temperature supports the idea of a vapor-liquid-solid (VLS) mechanism for the growth of the tubes.

Full text of DOI:10.1016/S0009-2614(02)00133-1

12 March 2002
Chemical Physics Letters 354 (2002) 331–336
www.elsevier.com/locate/cplett
High density of multiwalled carbon nanotubes observed
on nickel electroplated copper substrates by microwave
plasma chemical vapor deposition
a
a
a,
a
b
Manoj K. Singh , P.P. Singh , E. Titus *, D.S. Misra , F. LeNormand
a
Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India
b
GSI, IPCMS, UMR 7504 CNRS, Strasbourg 67037 Cedex, France
Abstract
High density and long multiwalled carbon nanotubes are deposited using nickel clusters formed from an electro-
deposited nickel coating treated by ammonia. Tubes of length 40–50 lm having bamboo shaped structure are aligned
nearly parallel to the copper substrate. The inner diameter of the tubes is in the range 20–50 nm. The selected area and
nano area diffraction spots clearly show the crystalline nature of nickel clusters. At some locations of the samples in the
vicinity of graphite layers, Ni3C phase is also detected. The formation of the Ni3C phase that is unstable at high
temperature supports the idea of a vapor–liquid–solid (VLS) mechanism for the growth of the tubes. Ó 2002 Published
by Elsevier Science B.V.
1. Introduction
sized clusters on the substrates. It is observed
[13–15] that the diameter of the tubes is related to
the size of the clusters present on the substrates
and therefore the generation of the nanoclusters
on the growing surface has become very important
for the tube synthesis. A combination of parame-
ters such as the size of the clusters, the smoothness
of the substrate, and the temperature of deposition
play an important role in the synthesis of the sin-
gle-wall nanotubes [16]. Several groups have also
carried out a number of very careful measurements
to understand the nature and the properties of the
tubes. Scanning electron microscopy (SEM),
transmission electron microscopy (TEM), high
resolution transmission electron microscopy
(HRTEM), scanning tunneling microscopy
(STM), Raman spectroscopy have proved very
successful in the characterization of the tubes.
Since the first report of the observation of the
carbon nanotubes [1], huge interest has been gen-
erated in the research of this novel material,
promising vast potential for applications [2,3],
pyrolitic [4], chemical vapor deposition (CVD)
[5,6], laser ablation [7], and arc discharge [8,9]
techniques have been used successfully to synthe-
size the carbon nanotubes. It is established that the
presence of suitable clusters [10] of metals such as
Ni, Co, Fe, etc. is essential for the deposition of
the tubes. Several techniques [11,12] have been
used and proposed for the formation of nanometer
* Corresponding author.
E-mail address: elby@phy.iitb.ac.in (E. Titus).
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0009-2614(02)00133-1

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M.K. Singh et al. / Chemical Physics Letters 354 (2002) 331–336
However, despite huge research a large number of
the technological and basic issues still remain to be
resolved. These are mainly the origin and mecha-
nism of the tubular growth, the deposition of
oriented tubes on the substrates, the yield of var-
ious processes of carbon deposition.
In the present Letter we report the use of an
innovative electroplating technique to deposit the
nickel coatings and to subsequently produce clus-
ters which catalyze the synthesis of multiwalled
carbon nanotubes. The structure of the tubes is
cross-sectional (bamboo shaped) because the
cluster size is large. The density of the tubes is high
and the diameter to length ratio (aspect ratio) is
very small. We have carried out careful HRTEM
measurements to study the structure of the tubes.
The selected area and nano area electron diffrac-
tion patterns (SAD and NAD, respectively) have
shown the presence of Ni and Ni3C phases in the
vicinity of the graphite layers.
during the nanotubes growth is 840 Æ 20 °C. The
nanotubes are examined using SEM and HRTEM
techniques. The samples for HRTEM are prepared
using replica technique on a copper grid and no
special treatment is required for the observation in
TEM. The selected area diffraction and nanodif-
fraction are performed using HRTEM.
3. Results and discussion
A typical SEM micrograph of the nanotubes
grown in our system is shown in Fig. 1. The tubes
are long and show typical multiwalled structure.
Also it is interesting to note that the density of the
tubes is quite high and they appear to be aligned
parallel to the substrate plane. Typically the di-
ameters of the tubes grown using our technique
(Fig. 2a) range from 60 to 110 nm. The thickness
of the core region is approx. 20–30 nm while the
walls are 20 nm thick. The structure of the tubes
matches with that of the ‘bamboo’ shaped tubes
[19–21] indicating that a further optimization of
the cluster size may be necessary to obtain the
‘smooth walled’ tubes with relatively smaller di-
ameter. An expanded high resolution picture of
the graphite walls of the multiwalled tubes is
shown in Fig. 2b. The spacing between the
graphite planes is estimated to be 0.32 nm (Fig. 2b)
2. Experimental
Thin copper foils are used in our case as the
substrate for the nanotube deposition. The foils
have polycrystalline nature and have commercial
grade purity (98%) without any special pre-treat-
ment to improve their smoothness. The electro-
plating of the Ni coatings on copper foils is
performed in an electroplating bath whose details
are described elsewhere [17]. The copper foils are
used as cathode and a thin platinum wire is used as
anode. Commercial grade nickel sulfate
ðNiSO4; 7H2OÞ; nickel chloride ðNiCl2; 6H2OÞ and
boric acid ðH3BO3Þ are used as electrolytes in a dc
electroplating process. The bath temperature is
maintained at 60 °C and its pH is 3.5. The Ni
layers having a thickness 15 lm are treated in
plasma of ammonia ðNH3Þ gas for 2 min prior to
the deposition of carbon nanotubes in MPCVD
apparatus. The temperature of the NH3 treatment
is 800 Æ 20 °C and its flow rate is 180 sccm at a
pressure of 20 mbar. Methane ðCH4Þ and hydro-
gen ðH2Þ are used as the precursor gases and their
flow rates are 6 and 20 sccm, respectively. The
details of the MPCVD apparatus are described
elsewhere [18]. The substrate temperature ðTsÞ
Fig. 1. SEM micrograph showing the morphology of the tubes
on copper substrate. The exceptionally long carbon nano tubes
nearly parallel to the substrate are noteworthy.

M.K. Singh et al. / Chemical Physics Letters 354 (2002) 331–336
333
(a)
(a)
(b)
Fig. 2. (a) A magnified image of a typical bamboo shaped tube.
The diameter of the core area and the walls are 18 and 22 nm,
respectively. The Ni filling at the tube end can be noted.
(b) HRTEM image of the graphite planes of the tube.
(b)
Fig. 3. (a) The location of nickel clusters (darker particles) in
the HRTEM image. The dark large particle is carbon. (b) A
large faceted nickel cluster showing the graphite layers around
it.
instead of 0.34 nm as expected from turbostratic
graphite [22].
We also observe several clusters of nickel par-
ticles ranging from 8–10 nm to 100 nm) embedded
in our samples. The clusters are mostly coated with
graphite layers that can be observed in HRTEM.
Some of the clusters are so tiny that they move
under the influence of the high voltage applied in
TEM. Fig. 3a,b show the positioning and distri-
bution of the clusters observed in TEM. Graphite
layers around the clusters may also be seen. At
many locations amorphous or diamond like car-
bon (DLC) impurities are seen in or on the surface
of the tubes. The energy dispersive X-ray analyses
(EDS) reveal the composition of the clusters to be
only Ni. No other metallic impurities can be ob-
served within the limitations of the EDS. Several
areas of the metallic clusters were selected to per-
form SAD and NAD. The area selected for SAD
was a few microns while for NAD it was in the
nanometer range. Fig. 4a–c show the SAD and
NAD patterns obtained with different zone axes

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M.K. Singh et al. / Chemical Physics Letters 354 (2002) 331–336
Fig. 4. (a) and (b) show the selected area electron diffraction spots on a nickel cluster with electron beam along the ð100Þ and ð110Þ
zone axes and (c) shows nano area electron diffraction spots for nickel cluster with electron beam along the ð110Þ zone axis.
on various locations of Ni clusters. Clearly all the
spots correspond to the different planes of the
cubic Ni structure. While at most places we ob-
served the Laue spots corresponding to Ni clusters
at some locations amongst the clusters we also
observe the formation of Ni3C. The layers corre-
sponding to 100 planes of Ni3C observed in the
samples at some locations are seen in Fig. 5. The
separation between the layers is 0.226 nm and they
appear to be relatively defects free. The NAD and
SAD patterns corresponding to the Ni3C phase are
shown in Fig. 5a,b, respectively. Rings of the
ð012Þ, ð113Þ and ð119Þ index planes are evident
which means that the Ni3C domains are mainly
polycrystalline.
The formation of the carbon nanotubes in our
case appears to be mostly bamboo shaped. It is,
however, interesting to note that the tubes are
mostly aligned parallel to substrates and are very
long. The bamboo shaped tubes have mostly cross-
sectional structure with the tube comprising of
several sections. The large inner diameter of 20 nm
(Fig. 2a) of the tubes in our case may be related to
the size of the clusters. In some cases Ni cluster
encapsulating one termination of the tube is ob-
served. The nanosized clusters of Ni mostly appear
to be well faceted in HRTEM pictures (Fig. 3).
The faceting of the Ni particles is observed by
many workers [23] and plays an important role in
the growth of the tubes. The crystal facets are most
likely produced during the NH3 treatment and
might be related to the differential etching rates for
different crystal directions. The large clusters (of
the size 100 nm), as is apparent from our study,
normally do not support tubular growth and only
a few graphite layers are formed around these (Fig.
3). This may be due to the fact that the weight of
the cluster is too high to be supported by the tubes
in the catalytic growth mode, within the assump-
tion of a top growth model [23]. The walls of the
tubes (Fig. 2b) appear defective in the present case
with a high density of planar dislocations.
The SAD as well as NAD spots clearly indicate
the structure of the clusters to be cubic nickel. Fig.
5a,b correspond to SAD whereas Fig. 4c corre-
spond to NAD patterns. The Laue spots are ob-

M.K. Singh et al. / Chemical Physics Letters 354 (2002) 331–336
335
Ni3C phases as the faint reflections of Ni could
also be obtained in the pattern. Also the ð111Þ
reflections of the Ni fall at the same position as
ð113Þ reflections of Ni3C. The formation of a
Ni3C phase appears to be interesting as the depo-
sition conditions at the time of CVD are not
conducive. It must therefore be formed during the
cooling phase at 200–450 °C, as the nickel carbide
phase is not thermodynamically stable at high
temperature. This will however support the vapor–
liquid–solid (VLS) growth model mechanism for
the growth of nanotubes implying that the carbon
saturation of the nickel particles occurs during the
CNTgrowth [24].
4. Conclusions
The ‘bamboo shaped’ exceptionally long mul-
tiwalled carbon nanotubes are deposited using
electroplated nickel coatings as catalyst. The cap-
ping of tubes with nickel is observed in few cases.
The nano area and selected area electron diffrac-
tion spots indicate the cubic structure of nickel
particles. At some locations the evidence for the
synthesis of Ni3C phase is also found having in-
teresting consequences for the growth models.
Acknowledgements
Fig. 5. (a) The HRTEM image of the planes of Ni3C. The
spacing of 0.226 nm match closely with ð100Þ planes of Ni3C.
The faint image of graphite planes can also be seen in the vi-
cinity. (b) Shows the nano area diffraction spots of the Ni3C
phase.
We acknowledge gratefully the kind assistance
of Gabi Ehret (IPCMS-GSI) and Dr. I. Samajdar
for HRTEM observations. The financial assistance
from Indo-French Center for Promotion of Ad-
vanced Research to visit Strasbourg by one of the
authors (DSM) is gratefully acknowledged.
tained with different zone axes. The formation of
Ni3C phase is however observed near some of the
clusters. This is interesting in view of the fact that
the Ni3C is well known to be difficult to form. The
spacing between the layers of Ni3C is measured to
be 0.226 nm and matches well with the 1 0 0 inter-
planer spacing of cubic nickel carbide. The NAD
and SAD patterns corresponding to the carbide
phase are shown in Fig. 5a,b, respectively. The
Laue spots are obtained with zone axis h110i and
h100i, respectively. Strong reflections from ð113Þ,
(3 0 6), ð110Þ, and ð116Þ planes of Ni3C are seen.
We should mention here that this phase never
appears to be pure Ni3C but a mixture of Ni and
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