22B03
Selective growth of single-walled carbon nanotubes by plasma enhanced
chemical vapor deposition
プラズマCVDによる単層カーボンナノチューブの選択合成
Tomohiro Nozaki, Yoshida Shinpei, Ken Okazaki
野崎智洋, 吉田新平，岡崎健
Department of Mechanicaland Control Engineering, Tokyo Institute of Technology
2-12-1 O-okayama, Meguro, Tokyo 152-8552, Japan
東京工業大学・機械制御システム専攻 〒152-8552 東京都目黒区大岡山2-12-1
Atmospheric pressure radio-frequency discharge (APRFD) has advantage over vacuum-oriented plasma
enhanced chemical vapor deposition (PECVD) in terms of single-walled carbon nanotube (SWCNT)
growth. Plasma-generated C2H2 is the main CNT growth precursor in PECVD. In the cathode sheath ofAPRFD, approximately 30% of the CH4 (initial feedstock) was converted into C2H6, C2H4 and C2H2. A
trace amount of C2H2 enabled the synthesis of SWCNTs in the thermal CVD regime. Hydrogen isnecessary to grow SWCNTs under non-thermal plasma environment because H2 suppresses formation ofexcess amount of C2H2; however, H2 does not eliminate amorphous carbon even at H2/C2H2 ratios of 300.
Atmospheric-pressure PECVD performed with a He/CH4/H2 system is equivalent to C2H2-assisted thermal
CVD without an etching gas. Depending on pressure, in other words C2H2 content, SW and MWCNTs canbe selectively synthesized.

1. Introduction
There were no reports on SWCNT synthesis inthe PECVD regime by 2003 [1]. Tremendous effort
has been made towards the control growth ofdesired CNTs using unique properties of reactive
plasma, enabling SWCNTs under the low-pressure
remote plasma CVD system [2]. However, due tolimited supply of plasma-generated reactive species,
SWCNT growth rate is unexpectedly low and dense
SWCNT film is hardly synthesized. Exceptionally,
microwave remote plasma CVD successfully grew
millimeter-tall SWCNTs [3]. Atmospheric pressureplasma enhanced CVD (AP-PECVD) was
originally highlighted because of its simplicity andflexibility over the process parameters. Morerecently, it is recognized as one of the promisingtechnique to grow vertically aligned SWCNTs [4,5].
In PECVD, conversion of initial feedstock into
acetylene (C2H2) is the key to determining CNT
morphology and to enhance overall growth rate.
Main role of H2, as supposed to be an etchant, isactually to suppress C2H2 formation during
plasma-induced decomposition of initial feedstock.
Note C2H2-sensitized thermal CVD does not fully
reproduce plasma-grown SWCNTs due to thepresence of ions [5]. In this paper, we focus on thegas analysis to identify the key species responsible
for SWCNT growth in the non-thermal plasma
environment. The role of plasma-generated C2H2 on
CNT growth is highlighted as a major CNT growth
precursor and selective growth of SW andMWCNTs are discussed.

2. Experimental
A capacitively coupled RF discharge (13.56MHz) was generated between two parallel-plate
metallic electrodes separated by 5 mm. The RF
power was transmitted through the upper electrode(40 mm diameter) equipped with water-cooled
sintered metallic mesh plate [6]. The sintered metal
plate was manufactured from stainless steel
powders with an average grain size of 100 m.
A catalyst-coated substrate was located on theheated bottom electrode. A detailed catalyst
preparation procedure is described in Refs. [4].
Briefly, an aluminum oxide thin film with estimatedthickness 20 nm was initially coated on a silicon
substrate (15 × 15 mm). The iron catalyst wasdip-coated using an iron-acetate-containing ethanolsolution. The dip-coated substrate was annealed in
air at 400 C for five minutes. The iron oxide wasreduced by atmospheric pressure RF discharge inthe He and H2 mixtures (1500, 10 cm3min-1). After afive-minute plasma treatment at 500 C, thesubstrate temperature was elevated to 700 C, andthe flow rates of He, H2 and CH4 were adjusted for
CNT growth.
3. Online gas analysis in the plasma sheath
The gas component in the cathode plasma sheath
was analyzed qualitatively using a quadrupole mass
spectrometer (QMS; Prisma, Pfeiffer Vacuum
Technology). A thin metallic capillary tube (outerdiameter, 450 m; inner diameter, 250 m) with anouter diameter one-half of the sheath thickness wasinserted into the sheath. The reacting gas wasextracted directly for online gas analysis, andfragment peaks from m/e = 1 to 100 were recorded.
A clear difference is observed between m/e = 25and 30. Except for methane, the main hydrocarboncomponent is C2H6, although a trace amount of
C2H2 and C2H4 are possible. Other fragment peaks
related to higher hydrocarbons, such as propane(m/e < 44) and butane (m/e < 58), were not detected.
For quantitatively gas analysis, the reacting gas wasaccumulated in a gas bag and then analyzed by gas
chromatography (Shimadzu, GC-8A).
Figure 1 shows the CH4 conversion andselectivity for the C2 hydrocarbons at different
conditions. At 100 kPa, the main product is C2H6
that corresponds well with mass spectrometry
analysis. In addition, 20% of the C2H4 and a trace
amount of C2H2 were detected and the productselectivity is influenced by H2 content. C2H2
content was approximately 70 ppm with H2 dilution
and 220 ppm without H2. C2H6 selectivity isremarkably decreased at 20 kPa. C2H6 is produced
by a three-body recombination of CH3 (CH3 + CH3+ M = C2H6 + M). The overall reaction rate ispressure dependent, thus C2H6 formation issuppressed as the total pressure decreases.
Correspondingly, the selectivity for C2H4 and C2H2was increased at reduced pressures.

pressure decreases, implying formation of defective
graphite structure is enhanced in the reducedpressure. In other word, G/D ratio deteriorates as
C2H2 increases. TEM analysis clearly showed that
MWCNTs were preferentially synthesized atreduced pressure. In addition to total pressure, theG/D ratio depends greatly on initial amount of
catalyst metal [8]. There is an optimum value forthe initial amount of catalyst precursor for SWCNT
growth.

Figure 2 Pressure-dependent change in the G/D ratio ofCNTs: He/CH4/H2, 3000/48/90 cm3min-1; 60 W; 700C;
3 min synthesis; Fe catalyst, ● 0.15 and ■ 0.05 wt%

Concluding remarks
Plasma-produced C2H2 is the key component togrow CNTs. Atmospheric pressure RF discharge
enabled SWCNTs because it suppressed C2H2formation. SWCNTs turned MWCNTs as totalpressure decreased where C2H2 content increased.
Note that ions are unique in plasma process thus
C2H2-assisited thermal CVD does not fully
reproduce plasma grown CNTs.
Acknowledgements
Figure 1 CH4 conversion and selectivity for the C2hydrocarbon.  20 kPa, H2 = 0; █ 100 kPa, H2 = 0; ░100 kPa, H2 = 90 cm3min-1. He/ CH4 = 3000/48 cm3min-1
and 60 W.

3. Pressure-dependent selective growth of CNTs
CNTs produced at different total pressure andfixed input power (60 W) were characterized by
micro-Raman spectroscopy equipped with 514 nm
excitation laser (Seki Technotron STR750 Laser
Raman Spectrometer). Figure 2 shows pressuredependent change in the peak intensity ratio ofG-band to D-band of Raman spectrum (hereafter
the G/D ratio). The G/D ratio decreases as total

The project was supported by KAKENHI. T.N.
expresses great thank to Prof. Shigeo Maruyama(Department of Mechanical Engineering, The Universityof Tokyo) for the catalyst preparation.

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