Growth of Nanocarbons in a Low Pressure Inductively Coupled Plasma
Katsuyuki Okada
National Institute for Materials Science
Abstract: A 13.56 MHz low pressure inductively coupled CH4/CO/H2 plasma
has been applied to prepare nanocarbons, which include nanocrystalline diamond
(NCD) particles, carbon nanotubes (CNTs), and carbon nanocapsules (CNCs).
The characterizations were performed with transmission electron microscopy
(TEM) and electron energy loss spectroscopy (EELS). The TEM observations
have revealed that the NCD particles are 200-700 nm in diameter and that each
particle is composed of small particles of about several ten nm in diameter. It
also was found that CNTs are multi-walled of 3-5 nm in diameter, whereas the
CNCs are multi-layered hollow capsules of 10-20 nm in diameter. The EEL
spectrum of multi-walled CNTs exhibits two peaks at 284 eV and at 292 eV
corresponding to π* states and σ* states of sp2-bonded carbons, respectively.
The energy loss near-edge structure (ELNES) is similar to that of graphite rather
than that of sp2-rich amorphous carbon.
Keywords: Nanocarbon, Nanocrystalline diamond, Carbon nanotube,
Carbon nanocapsule, Low pressure inductively coupled plasma,
1. Introduction
Covalently bonded disordered thin-film materials
have been of considerable interest from both
fundamental and applied perspectives in the last 25
years since the chemical vapor deposition of
diamond was developed, followed by that of
fullerenes and carbon nanotubes [1,2]. Among them,
amorphous and nanostructured carbon films are
currently being extensively studied for use as
electron emitters, cold-cathode sources, and hard
low-friction coatings. From the fundamental
perspective, on the other hand, the structure of these
materials contains both threefold-coordinated (sp2bonded) and fourfold-coordinated (sp3-bonded)
carbon atoms. Nanocrystalline diamond (NCD)
films have also attracted considerable attention
because they have a low coefficient of friction and a
low electron emission threshold voltage [3]. The
small grain size (approximately 5-100 nm) gives
films valuable tribological and field-emission
properties comparable to those of conventional
polycrystalline diamond films. Furthermore,
applications for micro-electro-mechanical systems
(MEMS) devices, metal-semiconductor field effect
transistors (MESFETs), electrochemical electrodes,
and biochemical devices have been proposed that
take advantage of these excellent properties [4-6].
A 13.56 MHz low pressure inductively coupled
CH4/CO/H2 plasma has been applied to prepare
NCD particles of 200-700 nm in diameter. Twodimensional platelet-like graphite, carbon nanotubes
(CNTs), and carbon nanocapsules (CNCs) also were
deposited with different growth conditions. The
characterizations were performed with transmission
electron microscopy (TEM) and electron energy loss
spectroscopy (EELS).
2. Experiment
Figure 1 shows the schematic view of the low
pressure inductively coupled plasma chemical vapor
deposition (ICP-CVD) system. The detailed
description of the ICP-CVD system and deposition
procedures were reported previously [7]. To be brief,
a low pressure ICP was generated in a growth
chamber by applying 13.56 MHz rf powers of 1 kW
to a three-turn helical antenna. The flow rates of CH4
and H2 were kept at 4.5 and 75 sccm, respectively,
whereas the flow rate of CO ([CO]) was varied
between 0, 1.0, and 10 sccm, respectively. The total
gas pressure was accordingly varied from 45 to 50
mTorr. Silicon (100) wafers (10 mm in diameter)
were used as a substrate. The substrate temperature
was kept at 900 ℃. The deposition duration was 2
hours.
EELS measurements [8] were carried out by using
a post-column energy filter (GATAN, GIF2002)
equipped with a transmission electron microscope
(TEM; Hitachi HF-3000) at 297 keV. The vacuum in
the microscope had a pressure of less than 1.2 x 10-6
Pa, to ensure that the samples were not contaminated
with carbon during TEM observations. Twodimensional arrays of a charge-coupled device
(CCD) were used for digitally recording the TEM
images, EEL spectra, and chemical maps. The
typical CCD readout times were 5 sec for acquiring
EEL spectra and 50 sec for chemical mappings. The
energy resolution of the instrument was
approximately 0.5 eV, which was defined as the
zero-loss full width at half maximum (FWHM).
Figure 1. Schematic view of low pressure inductively
coupled plasma chemical vapor deposition system.
3. Results & Discussion
Figure 2 shows SEM photographs of the resultant
deposits on a Si(100) substrate. Figs. 2(a), 2(b), and
2(c) correspond to [CO]=0, 1.0, and 10 sccm, which
are referred to as samples A, B, and C, respectively.
The morphology of sample A was platelet-like, as
shown in Fig. 2(a), and no crystal facets were clearly
seen. When CO was added to the CH4/H2 plasma,
particles of 200-300 nm in diameter as well as
platelet-like deposits appeared, as shown in Fig. 2(b).
With the increase in [CO], only particles were
deposited on the Si substrate, as shown in Fig. 2(c).
The diameters of the particles were 200-700 nm.
Detailed observation reveals that the particles consist
of small particles of about 20-50 nm in diameter,
and that the particle size remains are almost the
same regardless of increasing [CO]. It is therefore
speculated that increasing [CO] results in a large
supersaturation degree of carbon; thus, the number
of encounters between particles is increased.
The previous TEM observations have revealed
[7] that the two-dimensional platelet-like deposits
consist of disordered microcrystalline graphite,
whereas the particles are composed of only diamond
nanocrystallites. The high-resolution TEM (HRTEM) images clearly show that each particle is
composed of small particles of about several ten nm
in diameter. The X-ray diffraction pattern for the
sample C exhibits the diffraction peaks of diamond
(111) and (220) planes [9]. The crystallite size was
estimated to be approximately 20 nm from the
FWHM of the diamond peaks by using the
Scherrer’s equation. It is consistent with the TEM
observations.
The addition of CO to CH4/H2 plasmas is
considered to produce oxygen-containing radicals,
e.g., atomic oxygen, OH radicals, and CO radicals
themselves in the plasmas. As mentioned above, a
morphological change from a plate-like deposit to
the deposition of particles took place upon adding
CO. Also, the number of encounters between
particles increased with an increase in [CO].
According to the TEM and XRD patterns [7,9],
nondiamond carbon was effectively removed with an
increase in [CO]. We therefore presume that oxygen
containing radicals produced by the addition of CO
play an effective role in the removal of nondiamond
carbon under diamond growth conditions and that
the CO additive results in a large supersaturation
degree of carbon. This is consistent with the
previously reported hypotheses [10,11] that OH
radicals and atomic oxygen gasify sp2 carbon and
that they suppress the formation of amorphous
carbon and graphitic carbon.
Figure 3 shows a HR-TEM image of the resultant
deposits with different growth conditions. The
morphology clearly exhibits multi-walled CNTs.
The diameters range from 3 to 5 nm.
The outer part of the deposit, as shown in Fig. 4,
reveals that multi-layered hollow CNCs also are
partially deposited. The diameters range from 10 to
20 nm.
The EEL spectrum of multi-walled CNTs is shown
in Fig. 5. It exhibits two peaks at 284 eV and at 292
eV corresponding to π* states and σ* states of sp2bonded carbons, respectively. The energy loss nearedge structure (ELNES) is similar to that of graphite
rather than that of sp2-rich amorphous carbon [12].
Figure 3. HR-TEM image of multi-walled carbon nanotubes.
Figure 2. SEM micrographs of obtained deposits:
(a) [CH4/CO]=4.5/0 sccm, (b) [CH4/CO]=4.5/1.0 sccm,
and (c) [CH4/CO]=4.5/10 sccm.
Figure 4. HR-TEM image of multi-layered hollow carbon
nanocapsules.
Figure 5. EEL spectrum of multi-walled carbon nanotubes.
4. Summary
Nanocarbons including NCD particles of 200-700
nm in diameter, two-dimensional platelet-like
graphite, CNTs, and CNCs were prepared in a 13.56
MHz low pressure inductively coupled CH4/CO/H2
plasma. The HR-TEM images clearly show that
NCD particles are composed of small particles of
about several ten nm in diameter.
The morphology of CNTs clearly exhibits multiwalled ones. The diameters range from 3 to 5 nm.
The EEL spectrum of CNTs exhibits two peaks at
284 eV and at 292 eV corresponding to π* states
and σ* states of sp2-bonded carbons, respectively.
The ELNES is similar to that of graphite rather than
that of sp2-rich amorphous carbon. The CNCs were
found to be multi-layered hollow capsules of 10-20
nm in diameter.
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