Myung Gwan Hahm
Department of Mechanical and Industrial
Engineering,
NSF Nanoscale Science and Engineering Center
for High-Rate Nanomanufacturing,
Northeastern University,
Boston, MA 02115
e-mail: mghahm@coe.neu.edu
Young-Kyun Kwon
e-mail: ykkwon@khu.ac.kr
Department of Physics and Research Institute for
Basic Sciences,
Kyung Hee University,
Seoul 130-701, Korea
Ahmed Busnaina
Yung Joon Jung
1
e-mail: jungy@coe.neu.edu
Department of Mechanical and Industrial
Engineering,
NSF Nanoscale Science and Engineering Center
for High-Rate Nanomanufacturing,
Northeastern University,
Boston, MA 02115
Structure Controlled Synthesis of
Vertically Aligned Carbon
Nanotubes Using Thermal
Chemical Vapor Deposition
Process
Due to their unique one-dimensional nanostructure along with excellent mechanical,
electrical, and optical properties, carbon nanotubes (CNTs) become a promising material
for diverse nanotechnology applications. However, large-scale and structure controlled
synthesis of CNTs still have many difficulties due to the lack of understanding of the
fundamental growth mechanism of CNTs, as well as the difficulty of controlling atomic-
scale physical and chemical reactions during the nanotube growth process. Especially,
controlling the number of graphene wall, diameter, and chirality of CNTs are the most
important issues that need to be solved to harness the full potential of CNTs. Here we
report the large-scale selective synthesis of vertically aligned single walled carbon nano-
tubes (SWNTs) and double walled carbon nanotubes (DWNTs) by controlling the size of
catalyst nanoparticles in the highly effective oxygen assisted thermal chemical vapor
deposition (CVD) process. We also demonstrate a simple but powerful strategy for syn-
thesizing ultrahigh density and diameter selected vertically aligned SWNTs through the
precise control of carbon flow during a thermal CVD process. DOI: 10.1115/1.4002443
1 Introduction
Nanotechnologies based on carbon nanotubes CNTs are de-
veloped very rapidly from the discovery in 1993 1 because of
their exceptional mechanical, electrical, and optical properties 2.
The applications of CNTs are their use in nanoscale electronics
such as nanosensors, interconnects, and field effect transistors by
their specific electronic structures, superior transport properties,
and unique one-dimensional nanostructures 3–5. In the synthesis
of CNTs, the catalytic chemical vapor deposition CVD method
has been developed actively for the large-scale synthesis of CNTs
6–12. However, still challenging difficulties are the control of
morphology and structure of CNTs with the ability of synthesizing
them in a large quantity. The key parameters in CNT growth using
thermal CVD processes are chemical and physical characteristics
of catalyst nanoparticles, hydrocarbons, and reaction environment
during growth of CNTs. In the CVD process, the dissociation of
hydrocarbon molecules catalyzed by the transition metal and the
precipitation of sp
2
carbon bonds from supersaturated metal cata-
lyst nanoparticles lead to the formation of CNTs. Therefore, the
diameter of CNTs is closely related to the size of metal catalyst
nanoparticles. However, it is very difficult to control the size of
catalyst nanoparticles precisely with the uniform distribution. In
this paper, we report the large-scale selective growth of vertically
aligned VA SWNTs and DWNTs using an ethanol based thermal
CVD process. We also demonstrate the large-scale synthesis of
diameter controlled vertically aligned SWNTs by controlling the
flow rate of ethanol vapor. To understand the diameter selected
growth of SWNTs, we also carried out a computational investiga-
tion of the fundamental SWNT growth mechanism and kinetics
under different ethanol flow rates in the CVD process using vari-
ous computational techniques, including the first-principles for-
malism.
2 Experimental Method
Vertically aligned CNTs were synthesized by employing a ther-
mal ethanol CVD technique 13. Figure 1 is a schematic showing
our ethanol CVD system and experimental procedure for the
growth of high density and vertically aligned CNTs. First, a 20 nm
thick Al film was deposited onto a SiO
2
layer using a sputter
coater and exposed to the air for the formation of aluminum-oxide
buffer layer to grow highly dense and vertically aligned CNTs
Fig. 1b. Then, an ultrathin Co catalyst film with 0.5–1 nm
thickness was deposited on an Al
2
O
x
/ SiO
2
multilayer using an
e-beam evaporator Fig. 1c. The prepared substrate
Co / Al
2
O
x
/ SiO
2
was placed inside of a quartz tube and the CVD
chamber was evacuated to 15 mTorr. Then the temperature was
increased to 850° C while being exposed to an argon-hydrogen
mixture gas 5% hydrogen balanced Ar with 100 SCCM SCCM
denotes cubic centimeter per minute at STP flow rate. In a de-
sired reaction temperature 850°C, controlled high purity anhy-
drous ethanol 99.95% was supplied as a carbon source for the
high density nucleation and growth of CNTs resulting in vertically
aligned CNT arrays Fig. 1d. For the characterization of the
CNT structure and morphology, transmission electron microscope
TEM, Raman spectroscopy, and scanning electron microscopy
SEM were used. Especially, to investigate the large-scale diam-
eter distribution of synthesized vertically aligned SWNTs, a Ra-
man radial breathing mode RBM mapping process was em-
ployed with an excitation wavelength at 785 nm. Raman RBM
maps were recorded using a Raman microscope LabRAM HR
800, HORIBA Jobin Yvon and a mechanical-optical mapping
stage.
3 Results and Discussion
Figure 2a shows a cross-sectional optical image of CNT film
grown using an ethanol CVD process. During the CVD reaction,
controlled amounts of oxygen in ethanol molecules C
2
H
5
OH
work as a weak oxidizer that would selectively remove amor-
1
Corresponding author.
Manuscript received June 15, 2009; final manuscript received January 13, 2010;
published online November 15, 2010. Assoc. Editor Wilson K. S. Chiu.
Journal of Heat Transfer MARCH 2011, Vol. 133 / 031001-1 Copyright © 2011 by ASME
Downloaded 15 Nov 2010 to 168.7.242.64. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm