Quasi-continuous growth of ultralong carbon nanotube arrays

Article abstract of DOI:10.1021/ja054454d

We report the growth of ultralong (>10 cm) multi-walled and single-walled carbon nanotubes such that the length is limited by the size of the furnace rather than by the termination of growth. The disturbance of microscale laminar flows results in disordered or shorter growth of carbon nanotubes. By downsizing reaction pipes, reaction gas flows are stabilized with low Reynolds numbers. In this way, the catalyst nanoparticles at the end of growing carbon nanotubes can travel a longer distance to grow ultralong nanotubes. Copyright

Full text of DOI:10.1021/ja054454d

Published on Web 10/12/2005
Quasi-Continuous Growth of Ultralong Carbon Nanotube Arrays
Byung Hee Hong,† Ju Young Lee,‡ Tobias Beetz,§ Yimei Zhu,§ Philip Kim,*,† and Kwang S. Kim*,‡
Department of Physics, Columbia UniVersity, New York, New York 10027, Center for Superfunctional Materials,
Department of Chemistry, DiVision of Molecular and Life Sciences, Pohang UniVersity of Science and Technology,
Pohang 790-784, Korea, and Center for Functional Nanomaterials, BrookhaVen National Laboratory,
Upton, New York 11973
Received July 6, 2005; E-mail: kim@postech.ac.kr; pkim@columbia.edu
Owing to outstanding mechanical and electrical properties of
carbon nanotubes (CNTs), intense research efforts have been made
to synthesize aligned long CNTs.1-3 Chemical vapor deposition
(CVD) methods using transition metal nanoparticle catalysts have
been widely used to produce long single-walled nanotubes
(SWNTs).4,5 The growth of ultralong CVD SWNTs, whose lengths
are up to ∼4 cm, has been reported in the literature.5 On the other
Figure 1. Schematic representation for the diameter-dependent stabilization
of reaction gas flows. By inserting the smaller tube inside the outer chamber,
hand, the multi-walled nanotubes (MWNTs) have great potential
for nanomechanical and nanoelectrical applications, employing their
microscopic turbulent flows (a) can be stabilized into laminar flows with
excellent mechanical properties and hierarchal structures.6-10
lower Reynolds numbers (b).
However, the scale and diversity of the MWNT structures have
been limited because of short lengths of available isolated MWNTs.
In this paper, we present a simple and reliable method to synthesize
several centimeters long MWNTs and SWNTs and discuss their
structural and electrical characteristics.
Solutions of 0.001-0.1 M FeCl3 in water and ethanol were used
as catalytic precursors that are decomposed at high temperatures
to produce Fe nanoparticles with different sizes and densities that
determine the diameters of CNTs.5 This simple procedure greatly
saves efforts to synthesize nanometer-sized catalyst particles from
solution chemistry. The catalyst patterns were created on the
substrates using drop-drying or stamping. After calcining the
catalysts at 950 °C for 30 min with the H2 and Ar gas mixture,
flowing at the rates of 60 and 200 cm3/min, respectively, the CH4
and H2 gases were flowed at 950 °C for 3 h at the rates of 100 and
60 cm3/min, respectively.
To grow extremely long CNTs with aligned geometries, a stable
laminar gas flow is required to stabilize the catalysts at the tip end
of growing CNTs and to travel longer distances. This can be
achieved by placing another small quartz tube inside the outer tube
Figure 2. SEM images showing the growth of CNTs depending on
Reynolds number (a, b) and the concentration of FeCl3 solutions (b-d).
(Figure 1). The stability of the gas flow depends on the tube
diameter (d) of the small inner growth tube and can be estimated
by the Reynolds number Re ) FVd/γ, where F is the density, V is
the speed, and γ is the coefficient of viscosity.11 Since catalysts
used for the growth of large-diameter MWNTs are less mobile than
those used for SWNTs growth, it is essential to have more stable
laminar flows with smaller Reynolds numbers. In our case, the
Reynolds number is estimated to be 50, which is near the lower
limit of conventional laminar flow ranges (below 2000). In the case
of the larger-diameter growth tube, which corresponds to a higher
Reynolds number, relatively short and disordered MWNTs were
produced (Figure 2a), presumably due to nanoscale turbulent flows
that disturb the growth of long MWNTs.
The concentrations of catalysts for b, c, and d are 0.1, 0.01, and 0.001 M,
respectively. Note that there are more curvy tubes in d, which indicates the
SWNT-dominant growth (because MWNTs are usually straight). Scale bars
) 50 µm.
of FeCl3 solutions, which determines the size distribution of catalyst
particles (Figure 2b-d). For three different concentrations of FeCl3
solutions (0.001, 0.01, and 0.1 M), the average heights measured
by atomic force microscopy were 1.8(0.4), 2.4(0.7), and 3.3(1.0)
nm, respectively, where the numbers in parentheses are the standard
deviation. This implies that the number of MWNTs increases as
the concentration and the size of catalyst particles increase. The
growth condition was optimized to synthesize the horizontally
aligned ultralong MWNT/SWNTs on the SiO2 substrates. Typically,
well aligned extremely long (up to ∼10 cm) individual CNTs,
whose length is limited by the size of silicon wafers or the length
of the hot zone of the furnace, were observed (Figure 3a). The
growth is likely to continue as long as the reaction gases keep
flowing within the laminar range. The CNTs can also grow across
100-500 µm trenches, showing that even large-diameter MWNTs
The size of catalysts is known to determine the diameters as
well as the wall thicknesses of CNTs.12 In our experiment, usually
SWNTs and MWNTs grow together under the same growth
conditions, but their ratio changes depending on the concentration
† Columbia University.
‡ Pohang University of Science and Technology.
§ Brookhaven National Laboratory.
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10.1021/ja054454d CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (a) SEM images of centimeter-long CNTs. The inset in (a) shows the optical image of a 10 cm long SiO2 substrate with ultralong CNT arrays
grown from end to end. (b) HRTEM images showing single-, double-, triple-, and multi-walled CNTs. Scale bars ) 5 nm. (c) Optical and SEM images of
a 1.5 cm long CNT device. The inset image shows the counter electrodes of the CNT marked with the white arrow. Scale bars ) 100 µm. (d, e) The I-V
curves of a 1.5 cm long CNT. The upper inset in (d) shows the linear I-V curve for the 10 µm long channel. The lower inset shows the gate dependence
at Vsd ) 20 V.
float over the surface due to laminar flows. These suspended
ultralong MWNT/SWNTs are useful for high-resolution transmis-
sion electron microscopy (HRTEM) investigations. The HRTEM
images show that various MWNTs, including double- or triple-
walled nanotubes, were synthesized along with SWNTs (Figure 3b).
Since CNTs grown on the substrate are parallel without overlap
to each other, we were able to characterize the electron transport
properties of extremely long CNTs by contacting individual tubes.
Arrays of electrodes were fabricated using TEM grids as stencil
masks for thermal evaporation of Au. Within the array, 100 µm2
Au pads were separated by 20 µm contact individual CNTs (Figure
3c lower panel). By contacting the last and the first electrode pads
of the two arrays separated by macroscopic distance (>1 mm), we
probed the current (I)-bias voltage (V) characteristics between two
terminals as we changed the gate voltage (Vg) at the degenerately
doped Si substrate below the 300 nm SiO2 dielectric layer (Figure
3c). While the I-V characteristic shows a linear ohmic behavior
(Figure 3d upper inset), the CNT devices whose channel lengths
are longer than ∼10 mm often indicate strongly nonlinear charac-
teristics that the current turns on only above certain bias voltages
(Figure 3d). Figure 3e shows the I-V characteristics of a 15 mm
long individual nanotube. The current increases appreciably for V
> Von ) 3 V. For a large positive bias (V > 10 V), the device
behaves like a field-effect transistor, where the current can be
modulated by the gate voltage. The rectifying I-V behavior has
been observed in SWNTs with intermolecular junctions, where tubes
with different chirality join together.13,14 The turn-on voltages and
the nonlinearity of I-V curves can be modulated by gate voltages,
implying that the potential barriers of conduction induced by local
defects or intramolecular junctions can be controlled by electric
field (Figure 3e).
linear I-V characteristics of centimeter-long CNTs would bring
about a potential application of these CNTs as multifunctional
electronic devices.16
Acknowledgment. This research was supported by the NSF
Nanoscale Science and Engineering Initiative (CHE-0117752), New
York State Office of Science, Technology, and Academic Research,
the Korea Science and Engineering Foundation (KOSEF) Creative
Research Initiative and postdoctoral fellowship, Brain Korea 21,
and the U.S. Department of Energy under Contracts DE-AC02-
98CH10886 and FG02 96ER45610.
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In conclusion, we have developed the method of growing
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