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Aligning single-wall carbon
nanotubes (by W. Z.)
A single-wall carbon nanotube (SWNT) is a molecularly perfect fullerene and
might be the strongest, most electrically conductive, and thermally robust
material known. SWNTs are usually synthesized by laser ablation, arc
discharge, HiPco process (high pressure catalytic decomposition of CO) and
chemical vapor deposition (CVD) methods. These techniques (except CVD) grow
mainly SWNT ropes, which consist of several to hundreds of close-packed
nanotubes with similar diameters. Individual SWNTs often have diameters of
1-2 nm and lengths ranging from ~100 nm to several microns. Randomly
oriented SWNT ropes and individual tubes coexist forming entangled networks
(Fig.1) in the as-grown material. These entanglements are highly insoluble
and difficult to process. In order to maintain the excellent axial
properties expected from perfect single-wall carbon nanotubes, aligned SWNT
materials are highly desired. In the past several years, several methods
have been developed to obtain aligned SWNTs. Figure 2 shows schematically
various macroscopically oriented SWNT materials[1].
Fig.1. A SEM image (left) and a TEM image (right) of as-grown entangled
SWNTs.
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Fig.2. Schematic of various macroscopically oriented SWNT materials.
Tubes in a fiber are preferentially aligned along the fiber axis. For films,
the tube axes may either orient preferentially normal to the film plane
~e.g., aligned nanotube arrays grown on a substrate by chemical vapor
deposition method or lie preferentially in the film plane ~e.g., films
deposited from suspension onto filter membranes, or by drop casting or spin
coating onto flat substrates. |
CVD method was first used to grow aligned multi-wall carbon nanotube (MWNT)
arrays in 1998[2]. It was later found that CVD method can also produce
aligned SWNT arrays. Very recently (March 2004), Windle’s group at the Univ.
of Cambridge reported a fantastic work[3] on direct spinning nanotube fibers
from CVD synthesis, where nanotubes are highly aligned along the fiber axis.
It also demonstrated the versatility of CVD method.
CVD method usually does not produce large quantity of SWNTs, which makes it
worthwhile to align nanotubes (produced by other methods) after growth. To
achieve this purpose, the first step is usually to disperse SWNTs in water
with surfactant or in other chemical solvents (toluene etc.) and partially
break the nanotube entanglements. Then nanotubes can be further aligned by
mechanical shear, anisotropic flow, gel extrusion, or magnetic/electric
field. Aligned SWNT films, fibers, composites have been successfully made
using these methods. In this paper, we will compare various processes used
to align nanotubes.
Growth of aligned nanotube arrays on substrate
In a pioneer work[2] reported in 1998, carbon nanotubes aligned over areas up
to several square centimeters were grown on nickel-coated glass below 666°C
by plasma-enhanced hot filament chemical vapor deposition (PE-HF-CVD). These
lower temperature growth conditions are suitable for electron emission
applications, such as cold-cathode flat panel displays, which require carbon
nanotube emitters grown perpendicular to the glass surface. The carbon
nanotube arrays are fabricated by first depositing a thin nickel layer onto
display glass by radio frequency magnetron sputtering. The carbon nanotubes
are then grown on the nickel-coated display glass by PE-HF-CVD. Acetylene
gas was used as the carbon source and ammonia gas was used as a catalyst and
dilution gas. Nanotubes with controllable diameters from 20 to 400
nanometers and lengths from 0.1 to 50 micrometers were obtained, as shown in
Fig.3. Using this method, large panels of aligned carbon nanotubes can be
made under conditions that are suitable for device fabrication.
 
Fig.3. SEM micrograph of carbon nanotubes aligned perpendicular to the
substrate over large areas
The alignment of the carbon nanotubes is due to a nanotube nucleation
process catalyzed by ammonia and nickel. In the presence of ammonia, each
nickel cap efficiently catalyzes the continuous synthesis of carbon
nanotubes. As the nanotubes grow, the nickel cap remains on the tip of each.
The alignment and thickness of the carbon nanotubes may be determined by the
orientation and size, respectively, of the initial catalytic centers.
After the report of this work, several groups started to grow oriented
nanotube arrays on substrates. Various growth conditions/parameters were
used, and the morphologies of the final products were different. Later it
was demonstrated that SWNTs arrays can also be obtained. But the whole idea
is the same as the original one.
Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition
Synthesis
In a recently reported work3, Windle’s group at the Univ. of Cambridge spun
fibers and ribbons of carbon nanotubes directly from the CVD synthesis zone
of a furnace using a liquid source of carbon and iron nanocatalyst. This
process was realized through the appropriate choice of the reactants,
control of the reaction conditions and by the continuous withdrawal of
product with a rotating spindle used in various geometries.
In 2001, Zhu et al. reported the formation of a 20 cm long nanotube thread
following the pyrolysis of hexane, ferrocene, and thiophene[4]. While this
work shows the possibility of fiber formation directly in a furnace, the
product was isolated strands. By mechanically drawing the carbon nanotubes
directly from the gaseous reaction zone, Windle’s group found it possible to
wind up continuous fiber without apparent limit to length. The key
requirements for continuous spinning are the rapid production of high purity
nanotubes to form an aerogel in the furnace hot zone and the forcible
removal of the product from reaction by continuous wind-up. They selected
ethanol as the carbon source in which 0.23 to 2.3 wt% ferrocene and 1.0 to
4.0 wt% thiophene is dissolved, the solution being injected at 0.08 to 0.25
ml/min from the top of the furnace into a hydrogen carrier gas flowing at
400 to 800 ml/min, with the furnace hot zone in the range of 1050 to 1200°C
(Fig. 4).
Fig.4. (A) Schematic of the direct spinning process. The liquid feedstock,
in which small quantities of ferrocene and thiophene are dissolved, is mixed
with hydrogen and injected into the hot zone, where an aerogel of nanotubes
forms. This aerogel is captured and wound out of the hot zone continuously
as a fiber or film. Here, the wind-up is by an offset rotating spindle. (B)
Schematic of the wind-up assembly that operates at a lower temperature,
outside the furnace hot zone.
The fibers obtained from the ethanol-based reactions have been characterized
by electron microscopy, as shown in Fig.5. The nanotubes are actually quite
well aligned along the fiber axis. Using the fibers, they can further make
twisted ropes (Fig.5D). These fibers and ropes are expected to have
exceptional mechanical and electrical properties.
 
Fig. 5. (A) Photograph of nanotubes being wound from the spindle (left) onto
a second spindle (right). (B and C) SEM micrographs of a fiber that consists
of well-aligned MWNTs. (D) A permanent twist introduced into a nanotube
fiber after its removal from the furnace.
Post-growth methods to align nanotubes
In the early days, thick films of nanotubes were made by filter deposition
for the measurement of macroscopic properties of nanotubes. It was later
realized that filter deposition actually introduced partial alignment of
nanotube in the film plane. The reason is very simple: these nanotubes (long
1D objects) will lie preferential on the flat surface. It was also found
that the thick films made by other methods (spin coating, direct deposition
on flat surface) also show similar alignment. Of course, within the plane,
nanotubes are randomly oriented, which makes these materials not very
interesting.
In 2001, thick nanotube films with in-plane alignment were obtained by
deposition from suspension on to a nylon filter membrane using a strong
magnetic field [5]. The aligned force comes from the magnetic anisotropy of nanotubes. Ropes lie preferentially in the plane of the film, while addition
of the magnetic field during filter deposition introduces a preferred
direction in the plane. This alignment was confirmed by electron microscope
imaging (Fig.6).
Fig.6. TEM image of a magnetically aligned SWNT film, showing an area near
the torn sample edge. The linear texture, running from the lower left to
upper right of the image, is attributed to aligned ropes lying on average
parallel to the applied magnetic field (scale bar 0.5 µm)
With the help of extensional flow, nanotubes can get aligned. Mechanical
shear, anisotropic flow, gel extrusion etc. can all provide extensional
force.
One such example is nanotube composite fibers produced by Winey’s group at
Univ. of Penn.[6]. Nanotubes were dispersed in polyethylene (PE) using a
twin screw compounder for an efficient fabrication method. Composite fiber
was melt spun to achieve highly aligned nanotubes. Optical micrographs show
that the fibers have smooth surface (Fig.7). X-ray measurement and polarized
Raman measurement show that the nanotubes within the fiber are very well
aligned along the fiber axis. It was believed that the extensional flow of
polymer surrounded the nanotubes helped the alignment of nanotubes.
Figure 7: Optical micrograph of SWNT/PE composite fibers.
Various methods used to make aligned nanotubes are reviewed in this article.
Macroscopically aligned nanotube film and fibers do show anisotropic
properties, as expected[7]. It is also found that the better the alignment,
the better the property along the aligned direction. Researchers expect that
perfectly aligned nanotube fibers composed of extremely long nanotubes will
have exceptional mechanical, electrical and thermal properties. A lot of
work is still needed to be done to achieve this goal.
Reference:
[1] Zhou et al., Appl. Phys. Lett. (2004)
[2] Z F Ren et al., Science, 282, 1105 (1998)
[3] Ya-Li Li et al., Science (2004) , just published online.
[4] Zhu et al., Science 296, 884 (2002)
[5] B.W.Smith et al., Appl. Phys. Lett., 77(5),2000
[6] R. Haggenmueller et al., J. Nanoscience and Nanotechnology, 3(1), (2003)
[7] J. E. Fischer et al., Journal of Applied Physics, 93, 2157 (2003) |
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