# Direct Deposition of Bamboo-Like Carbon Nanotubes on Copper Substrates by Sulfur-Assisted

Authors: Sri Lakshmi Katar; Adolfo González-Berríos; Joel De Jesus; Brad Weiner; Gerardo Morell
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Films of bamboo-like carbon nanotubes (BCNTs) were grown directly on copper substrates by sulfur-assisted hot filament chemical vapor deposition (HFCVD). The effects of substrate temperature and growth time over the BCNT structure were investigated. The films were characterized by scanning electron microscopy (SEM), Raman spectroscopy (RS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron field emission (EFE) studies. SEM and Raman characterization indicate a transition from the growth of microcrystalline diamond to the growth of a dense entangled network of carbon nanotubes or fibers as the substrate temperature is increased from 400 to900∘C that is accounted for by the base growth model. TEM images show that the nanotubes have regular arrays of nanocavities. These BCNTs show good electron field emission properties as other carbon films.


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## Body

## 1. Introduction

Diamond deposition on copper substrates was studied back in 1992, morphology of both the
as-plated  copper/diamond matrix and the film that resulted after further CVD
diamond deposition were reported [1].
The influence of bias-enhanced nucleation (BEN) on polycrystalline copper substrates was reported [2]. Copper exhibits a good lattice
match with diamond; therefore, this material has been studied previously [3, 4].
Copper does not form carbide but instead maintains a stable graphitic surface
carbon [2]. When the diamond-scratched Cu substrates were molten,
owing to insolubility of C in Cu, a carbon-rich surface layer was formed [5].
Freestanding diamond films can be prepared using copper substrate by CVD; thin films
(~10 μm) are obtained using a two-step growth method
for stress relief. Thicker films (>20μm) can be prepared through direct deposition [6].
The growth of graphitic layers becomes more and more distorted when going away
from the copper surface and finally, when a critical size corresponding to 5-6 layers is
reached, diamond nucleation and growth takes place [7].A strong interaction of the graphite layer
with the copper surface is reported in a diamond growth characterization study
using Auger and X-ray photoelectron spectroscopy. High-resolution electron microscopy results indicate
that there exists a graphite/amorphous carbon intermediate layer between CVD
diamond and its Cu substrate [8]. Ab initio calculations show that diamond
crystallites can be grown on polycrystalline copper up to 20 micrometer in
thickness with the preferable orientation of 〈111〉 [9].
In our laboratory, we are engaged in an effort to grow diamond films on copper
substrates for lithium rechargeable battery applications [10]. While studying the growth properties of
these diamond films, we found a dense cluster of bamboo-like carbon nanotubes.Carbon
nanotubes (CNTs) have been of great interest to scientists and industrial
communities due to their unique properties and diverse applications such as in
nanoelectronics [11–13],
scanning probes [14],
supercapacitors [15–19],
and field emitters [20–22]. Recently, carbon nanotubes with special
structures such as bamboo-shaped [23, 24], octopus [24], fish-bone [25], and coils [26], which are different from the
conventional straight tubes, have attracted immense interest. As the most
common member of this family, bamboo-shaped CNTs, consisting of many separated
hollow compartments, [24–26], have been investigated to explore their unique
structure-associated properties. Several studies on the synthesis of these
periodic structures were reported using hot filament chemical vapor deposition
[27],
direct current plasma-enhanced hot filament chemical vapor deposition method [28],
and microwave plasma chemical vapor deposition system [29, 30].
The sulfur- and phosphorus-doped diamond films were grown epitaxially by a
quartz tube-type microwave-assisted plasma chemical vapor deposition method [31].
Another group reported the growth of BCNTs using nickel electroplated thin
copper foils [22].
It has been found that
these structures are stable. Bamboo-like carbon nanostructures resemble zigzag
carbon nanotubes capped with a plain graphene sheet [32].Recently,
we have synthesized multiwalled  carbon nanotubes and nanofibres on molybdenum
substrates
[33]. Carbon-based films have been
considered as potential materials for field emission (FE) devices, such as FE
displays and micro X-ray sources because of their unique properties and
characteristics: relatively low-threshold field, robustness, chemical
inertness, radiation hardness, and ease of production. Although available field
emission data from structures similar to the ones reported in this paper are
not abundant, the ones reported indicate that these bamboo-like structures show
good field emission properties [34, 35].
While investigating the diamond film growth on copper substrates in greater
detail, we have found a mass of bamboo-like carbon nanotubes, which can be
prepared at a temperature of 900C° without depositing a catalyst
layer on the substrate prior to carbon film deposition. In this paper, we
report the growth of these periodic structures at different deposition
temperatures and times, the copper substrate and sulfur doping effect the
structure of the films prepared by hot filament chemical vapor deposition.


## 2. Experimental

The
carbon films deposited on copper substrates used in this study were grown in a
custom-made HFCVD chamber which has been previously described in detail
elsewhere [36].Tungsten wire (99.95% pure, 0.5 mm diameter)
was wound as a helical spring filament in the HFCVD.Before the thin films deposition, the
tungsten filament was carburized. For this, we pumped down the CVD chamber to 8×10−6 mbar (6.0×10−6 torr) and then filled it with a
mixture of 2.0%CH4, 98.0%H2 with 500 ppm of H2S.
Maintaining the combined flow of gases at 100 sccm, and a pressure of 20 torr,
we kept a constant current of 20 amps through the tungsten wire, for 8 hours.
Starting the carburization process, the voltage across the filament was 7.7 V,
indicating a low resistance for pure tungsten, but as the time progressed, the
voltage increased, reaching 17.0 V after 8 hours.Polycrystalline
copper substrates (99.9% pure, 0.5 mm thick, 14 mm disk diameter) were hand
polished with 600 grit sandpaper on both sides to make them flat. One side was
then further polished with 1000, 1500, and 2000 grit sandpaper to smooth the
surface. The last step of the polishing was made with diamond powder (<1
micron particle size) to achieve a mirror-like surface and for diamond scratching
and seeding to enhance the nucleation. Before the diamond powder polishing, the
substrates had a natural copper color, and after the powder polishing, the
substrates looked darker. The substrates were then cleaned in an ultrasonic
bath with 2-propanol for 15 minutes, dried with helium, and inserted in the
HFCVD chamber on top of a molybdenum substrate holder.Prior
to each deposition, we pumped down the CVD chamber to8×10−6 mbar (6.0×10−6 torr) and then introduced the
gases. For all the samples discussed in this paper, we kept constant the gas
mixture of 2.0%CH4, 98%H2 with 500 ppm of H2S,
the combined flow of the gases to 100 sccm, the deposition pressure at 20 torr,
the filament temperature at ~2500C°, and the filament-substrate
distance to 8 mm. The only parameters changed were the substrate temperature
and the deposition time. We grew samples at 500C° for 2 hours, at
700C° and 800C° for 5 minutes, and at 900C°
for 15 minutes. The deposition time was selected in order to achieve a film
that covered the whole substrate that did not experience peeling or
delamination from the copper substrate for the selected substrate temperature.Scanning
electron microscopy using a JEOL model 35 CF microscope revealed the surface
morphology. Raman spectroscopy is used to analyze the structure. The Raman
spectra were recorded using a triple monochromator (ISA J-Y Model T64000) using
the 514.5 nm line of Ar laser. The spectra were recorded using an 80x
objective, and the probed area was of 1-2μm2. The power on the sample was kept
below 10 mW to avoid damage. Silicon was used to calibrate the Raman peak
position. A Carl Zeiss TEM LEO 922 transmission electron microscope operating
at 200 kV was used to observe the morphology and microstructure of the BCNT
samples.The
sample was scraped from the copper disk and mixed in water to make a
suspension. A drop of the suspension was
spin coated on the copper grids covered with a formvar carbon film. A JEOL 3010 high-resolution transmission
electron microscope operating at 300 kV was used to observe the morphology and
microstructure of the BCNTs samples.Field emission I-V
characteristics were measured in a custom-made system, described in detail
elsewhere [37]. Briefly, a diode configuration is
used, in which a molybdenum rod of 3 mm diameter (area: 0.071 cm2)
serves as the anode. Voltage is applied using a Stanford Research Systems PS350
power supply. The emitted current is measured with a Keithley 6517A
electrometer. For the configuration employed, the macroscopic surface electric
field (ES) on the sample (i.e., cathode) can be estimated accurately
by ES=V/dCA, where V is the voltage applied to the anode
and dCA is the distance between the anode
and the cathode [38]. All the measurements were taken at
dCA=100±2μm, and at a pressure of 1–2×10−7 Torr (1.3–2.7×10−7 mbar). Currents lower than 1×10−12 A were considered at the background noise level. The
current density was calculated by dividing the current over the area of the
sample subjected to ES=V/dCA, which, in our specific
configuration, is well approximated by using the area of the anode (0.071 cm2)
[38].
The turn-on field (Et) was defined as the electric field necessary
to emit a current density of 10 μA/cm2. Each data point in the
I-V curves is the  average of 8 measurements taken 250 milliseconds apart. For
data acquisition, a custom Lab View (National Instruments) program was developed. As a measure of
precaution to avoid the influence of displacement or charging current in the
field-emitted current, specifically in currents smaller than 1×10−11 A, a delay of 4 seconds
between the change in voltage and the data acquisition was employed.


## 3. Results and Discussions

Scanning electron microscopy images (Figure1) show
significant qualitative differences between films grown below and at 900C°.
The films deposited on copper at 400, 500, 700, and 800C° are
composed of submicron particles. The films grew faster as the substrate
temperature was increased, to the point that films deposited at 700C°
are delaminated due to thickness. The films deposited at 900C° for
15 minutes show entangled clusters of nanotubes or nanofiber structures that
were analyzed in more detail by TEM (Figure 2). As shown in Figures 2(a) and 2(b)
with different magnifications, the films grown at 900C° are composed
of bamboo-like carbon nanotubes (BCNTs) with diameters ranging from 50–100 nm and
variable lengths. They contain nanocavities that
are stacked one over the other with closed walls consisting of a number of
graphene layers (Figure 3(c)).SEM micrograph of the BCNT directly deposited on Cu at different temperatures
showing (a) 400C°,
2 hours; (b) 500C°, 5 minutes; (c) 700C°, 5 minutes; (d) 900C°,
15 minutes.
(a)(b)(c)(d)TEM images of the BCNT directly deposited on Cu showing (a) bamboo-like carbon
nanotubes, (b) high-resolution image of the nanocavities, and (c) graphitic sidewalls of a nanocavity.
(a)(b)(c)Figure 3
Raman spectra of carbon films grown at different temperatures
showing the transition from microcrystalline diamond to BCNTs.The
energy dispersive X-ray analyses (EDS) reveal the presence of sulfur and carbon
in the bamboo-like carbon nanotubes clusters. These results are confirmed by
the results from X-ray photoelectron spectroscopy, which indicate the carbon
and sulfur peaks. The XPS results revealed a sharp C 1s peak at 284.4 eV and an
S 2p peak at 165.9 eV. The atomic concentrations from XPS data showed that the
sample contains 98.4% carbon, 1.44% oxygen, and 0.15% sulfur.The addition of
trace amounts of sulfur, in the form of hydrogen sulfide, to the diamond
chemical vapor deposition reaction produces profound changes in the gas phase
chemistry and the surface chemistry. We have reported a four-fold increase in
the deposition rate, from 0.1 to 0.4μm with 500 ppm of hydrogen sulfide [39],
and the formation of BCNTs instead of nanocrystalline diamond films as we
change the substrate from Mo to Cu [10]. In the gas
phase, H2S leads to the formation of CS in the heat-activated volume
around the filament, which turns back into H2S in the cooler region
above the substrate, while leaving C atoms deposited on the substrate surface
to form diamond [40].
This process results in a new C-transport mechanism to the substrate. In
addition, at the growing surface, sulfur acts as a cross-linker that promotes
the formation of graphitic bonds at the growing surface before it is released
back to the vapor phase [41].
Moreover, sulfur has the effect of lowering the melting point of the Cu
substrate [42]
rendering it in a quasiliquid state at the surface, thus enabling its catalytic
activity for the growth of BCNTs at temperatures falling short from the melting
point of bulk Cu. Hence, the formation of BCNTs by HFCVD at 900C° is
directly related to the presence of sulfur and the use of Cu substrates.Raman spectra of the all the films
were recorded at four different places on the substrate to confirm the
homogeneity and reproducibility of the results. The Raman spectra (Figure3) show the evolution
of carbon bonding nature for the films deposited as a function of substrate
temperature at 400, 500, 700, 900C.° The results agree with those
found by SEM and described above. All of the films grown below 900C°
have the signature of diamond and nanocomposite carbon in different
proportions. The Raman peak at around 1332 cm−1 corresponds to the
longitudinal optical (LO) mode of diamond and is characteristic of
microcrystalline sp3-bonded C. The peak around 1580 cm−1 corresponds to the Raman active E2g mode in graphite and is called
the G-band [43].
The peak at around 1360 cm−1 is ascribed to the Raman inactive A1g mode frequency and is called the D-band. A breakdown in selection rules due to
disorder in the graphite structure (defects, curved graphite sheets, dislocations, and lattice
distortions) renders it to be active. These two peaks are characteristic
of graphitic structures [44].
For the films under study, the D-band is larger than the G-band, indicating
that they contain trigonally bonded carbon with a large degree of disorder in
relation to crystalline graphite [5, 45–48].
The analysis of the Raman spectra indicates that disorder in the graphitic
structures increases as the substrate temperature increases, while the diamond
component diminishes and finally disappears for films grown at 900C°,
which corresponds to BCNTs. The breathing mode peak at 190.5 cm−1 was not
observed, as expected for BCNTs.The
above-described characterization indicates a transition from a growth of microcrystalline diamond
to the growth of a dense entangled network of carbon nanotubes or fibers as the
substrate temperature is increased from 400 to 900C°, approaching
the melting point of copper (1085C°)
within 14% in absolute scale due to the active role of surface copper atoms.
Nanocrystalline diamond is instead obtained at 900C° when molybdenum
is employed as substrate, for which the temperature stays 63% below its melting
point in absolute scale. Due to the
capillary action of the nanotubes and the variation of the surface tension of
the copper particles, which act as a catalyst during growth, the nanotubes grow
with a regular bamboo-like shape. These results indicate that the underlying
growth mechanism can be described by the base growth model [49].
Assuming that this model is operating in this case, the highly movable Cu
particles on the surface act as catalysts as the Cu substrate temperature
approaches its melting point (1084C°). The base growth model is also
consistent with the lack of metal particle filling the nanocavities.


## 4. Field Emission Properties

These BCNTs show
field emission properties that indicate the existence of a good interface
between the substrate and the BCNTs, which formed by direct deposition (i.e.,
no catalyst or buffer layer). For reproducibility of the results, we have
chosen two BCNT samples (a, b) deposited at 900C° for this study.
The turn-on field was estimated to be around 3.43 V/μm for one sample (see Figure 4(a)) and
around 3.35 V/μm for the second sample (see Figure 4(b)).
These turn-on fields are slightly above those reported by Ding et al. [34]
and Srivastava et
al. [50]
on samples containing bamboo-like carbon nanotubes (1.4 V/μm and about 1.5 V/μm, resp.) [51]
and they are similar to turn-on fields reported from nonaligned carbon
multiwalled nanotubes by Sveningsson et al. [52].
The Fowler-Nordheim (F-N) plots (see Figures 4(c) and 4(d)) show two
distinctive linear regions: one on the high-electric field region (located at
the left side of the plot) and the other on the low-electric field region
(located at the right side of the plot). These two regions featured in F-N
plots appear in field emission data from various materials, including
diamond-like carbon films and carbon nanotubes [33, 37]. It was shown by Xu et al. [53]
that this feature is not caused by adsorbates and that it could be caused by
space charge effects [54].
However, Liao et al. [55]
while studying field emission from diamond films proposed that this feature is
caused by the quantum tunneling of the electrons through multiple barriers
caused by the presence of different materials or structures. The F-N plots in
this paper indicate that such tunneling through multiple barriers does occur in
the BCNTs films, enhancing in this way the electron field emission properties
of the films by providing more than one channel for electron tunneling [56].(a)
and (b) Field emission plots of two samples of BCNTs
showing the behavior of the current density as a function of electric field, (c) and (d) the corresponding Fowler-Nordheim plots revealing two distinct
slopes.
(a)(b)(c)(d)


## 5. Conclusions

HFCVD was
employed to synthesize films of BCNTs directly on copper substrates without any
intentional catalysts. The structural and compositional characterizations
indicate that carbon films deposited on copper undergo a transition from
microcrystalline diamond to a dense entangled network of carbon nanotubes or
fibers as the substrate temperature is increased from 400 to 900C°.
This result was ascribed to the active role of surface copper atoms as the
substrate temperature approaches the melting point of copper (1085C°)
within 14% in absolute temperature scale. The BCNTs have regular arrays of
nanocavities and their growth is consistent with the base growth model. These
BCNTs appear to have good electrical contact to the substrate from the fact
that they show excellent electron field emission properties similar to other
carbon films grown on carbide-forming substrates.



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