NANO
LETTERS
Structural Characterizations of Long
Single-Walled Carbon Nanotube Strands
2002
Vol. 2, No. 10
1105-1107
Bingqing Wei,* Robert Vajtai, Yoon Young Choi, and Pulickel M. Ajayan
Department of Materials Science and Engineering, Rensselaer Polytechnic Institute,
Troy, New York 12180
Hongwei Zhu, Cailu Xu, and Dehai Wu
Department of Mechanical Engineering, Tsinghua UniVersity, Beijing 100084, China
Received July 23, 2002
ABSTRACT
Centimeter long single-walled carbon nanotube strands synthesized using chemical vapor deposition method were characterized using
microscopy, micro-Raman spectroscopy, and small-angle X-ray diffraction techniques. The strands are large collections of well-aligned nanotube
bundles, which consist of well-arranged single-walled nanotubes in a two-dimensional triangular lattice. The diameter of the single-walled
nanotubes varies from 1.1 to 1.7 nm, with domination of the 1.1 nm tubes and the corresponding lattice constant of 1.42 nm.
The ability to control the structure and arrangement of carbon
nanotubes (CNTs) is a prerequisite to realizing many practical
applications. Single-walled carbon nanotubes (SWNTs), for
all their remarkable and promising properties, will never
totally fulfill their potential until an efficient way is found
to manipulate and organize them into ordered arrays. Recent
advances in SWNT growth by chemical vapor deposition
(CVD) approaches have been achieved in producing high
quality SWNT materials at large scale and in bridging SWNT
networks on patterned substrates in a controllable manner.1-5
Our new approach for producing macroscopic SWNT strands
enables us to test the bulk properties of nanotube structures.6
The materials synthesis and in part the characterizations have
been published.6 Here, detailed results on structural characterizations of the SWNT strands are presented to reveal the
inner symmetry and length scale of the alignment in the
strands.
Synthesis of long strands of ordered single-walled carbon
nanotubes have been accomplished by an optimized catalytic
CVD technique using a floating catalyst method in a vertical
furnace where n-hexane is catalytically pyrolyzed when a
sulfur-containing compound and hydrogen are added to the
process.6 The n-hexane (carbon source) solution with a given
composition of ferrocene (catalyst, 0.018 g/mL) and thiophene
(sulfur additive, 0.4 wt %) is introduced into a vertical CVD
furnace at a flow rate of 0.5 mL/min after heating the furnace
to the pyrolysis temperature (1423 K) with hydrogen as the
carrier gas flowing at a rate of 250 mL/min. Macroscopically
* Corresponding author: email [email protected]
10.1021/nl025719e CCC: $22.00
Published on Web 09/12/2002
© 2002 American Chemical Society
Figure 1. Micrographs of a typical single-walled nanotube strand.
(a) SEM image of the strand consisting of thousands of nanotube
bundles; approximately 15 µm in diameter. (b) HRTEM image of
a cross sectional view of single-walled nanotube bundles showing
their two-dimensional triangular lattice.
long single-walled nanotube strands as long as 20 centimeters
are formed during this continuous process.6
Because the nanotube strands are of macroscopic lengths
and can be manipulated quite easily, direct structural
characterizations can be performed on individual strands. For
Raman and X-ray measurements, the samples were used asgrown, several cm long, and a few hundred micrometers in
diameter; while for microscopic observations, the strands
were first separated mechanically with a tweezers into several
smaller strands having diameters from about 5 to 20 µm and
then observed using field emission scanning electronic
microscopy (FESEM) and high-resolution transmission electronic microscopy (HRTEM).
The high-resolution SEM view along single strands
indicates that the strands consist of thousands of well-aligned
bundles of SWNTs (Figure 1a). Each of the bundles is
Figure 2. Raman spectrum of long aligned single-walled nanotube
strands. Nanotubes having 1.1 nm in diameters in the strands
(corresponding to the peak at 215.3 cm-1) dominate as derived from
the radial breathing mode peak calculations. Splittings of the E2g
mode (peaks turn to shoulders) observed for the sample are
characteristic of SWNTs. Excitation energy: 1.96 eV (632.8 nm).
composed of SWNTs forming a two-dimensional triangular
lattice in their cross-sectional view as shown in the HRTEM
image (Figure 1b).
Raman spectra were excited with the 632.8 nm line (∼1.96
eV) of a He/Ne laser in the backscattering configuration with
a micro-Raman spectrometer (Renishaw 2000) fitted with a
50× objective at room temperature. Total power to the
sample was filtered down to 10 mW over the approximately
3-5 µm diameter laser spot. The SWNT strands were
adhered to a polished silicon wafer surface by a drop of water
to prevent their vibration. The Raman spectra taken from
different areas along the strands are very similar to each
other, indicating a good homogeneity along the strands. Two
peaks (1311.1, 1585.3 cm-1) can be observed in the range
of 1200-1700 cm-1 in a typical Raman spectrum, as shown
in Figure 2. The peak centered at 1585.3 cm-1, which has
been assigned to the E2g (stretching) mode of graphite, is
the strongest peak in the spectrum, indicating a good
arrangement of the hexagonal lattice of graphite, i.e., only a
few structural defects along the nanotube wall. The diameter
of SWNTs can be determined by the frequency of the radial
breathing modes from 100 to 300 cm-1.7 There are three
clear peaks of the radial breathing mode observed in Figure
2. According to the expression ω ) 224/d (nm) + 14,8
nanotubes with diameter of 1.1, 1.2, and 1.7 nm are present
in the strands. However, the sharp and intense Raman peak
at 215.3 cm-1 suggests that there is a dominant diameter of
1.1 nm for the nanotubes in the strands. This is also
confirmed by HRTEM observations. From the diameter
dependence of the allowed optical distribution,9,10 we can
also conclude that semiconducting (around 146.7 cm-1) and
metallic (around 197 and 215.3 cm-1) SWNTs coexist in
the samples.
To carry out characterizations of symmetry and alignment,
further optical and X-ray diffraction methods were published;11,12 we used X-ray diffractometry with Cu KR (λ )
1.542) line in an X-ray diffractometer (Scintag, model XDS
2000) operated at 45 kV and 40 mA. The sample holder
used has a rectangular hole with a size of 1.5 cm × 2.5 cm.
1106
Figure 3. (a) Schematic of X-ray measurements of the long
nanotube strands. Small angle scans (2θ from 4 to 10°) and angular
scans (at Q ) 0.51) were performed. A typical result is shown in
(b) and in its inset. Identical peak (1,0) of the triangular lattice is
at about Q ) 0.51. Sharp peaks at 90 and 270 in the angular scans
(inset) indicate a good alignment of nanotube bundles in the strands.
The SWNT strands were vertically mounted across the center
of the hole with both ends of the strands attached to the
sample holder as shown schematically in Figure 3a. The
X-ray diffraction studies on the strands were focused on the
low-frequency regions (the scanning angle of 2θ was 4-10°),
and we obtain a well-defined peak at Q ) 0.51 Å-1 (2θ )
7.3°, scattering vector Q ) 4π sin θ/λ) corresponding to d
(1,0) of the nanotube triangular lattice as shown in Figure
3b.12,13 By fixing Q ) 0.51 Å-1, an angular scan was
performed to get information on the alignment of nanotube
bundles in the strands. The sample was rotated clockwise
from 0° to 360° (note: the angle along the scan is defined
as 0° along the strand axis) and the intensity was collected
at every 5° for 100 seconds.
The peak at Q ) 0.51 Å-1 in Figure 3b is one of the
feature peaks of the triangular structural distribution of
SWNT bundles. The corresponding lattice parameter calculated from this peak position is 1.42 nm, which is very near
to the lattice parameter12,14 of an ideal lattice assembled from
1.1 nm diameter nanotubes. The small difference from the
calculated value is fully explainable by taking into account
the signal superposition from finite crystallites made of
nanotubes with different diameters.15 These results also
Nano Lett., Vol. 2, No. 10, 2002
In conclusion, the long nanotube strands, consisting of
bundle-like single-walled nanotubes, are self-assembled
during the CVD growth process and are well ordered into
two-dimensional triangular structures. These very long
crystalline strands of single-walled carbon nanotubes could
be handled and manipulated easily and macroscopically,
suggesting that these are not as brittle as most other nanotube
aggregates prepared by other techniques.
Acknowledgment. We gratefully acknowledge financial
support from the NSF-NSEC, at RPI, for the directed
assembly of nanostructures.
References
Figure 4. Photographs of two samples of similar mass (one
commercially available HiPCO sample (left) and the other nanotube
strand prepared by our technique (right)) ultrasonicated for 10 min
each in ethanol solutions. Arrow points to the tangled SWNT strand
in the solution. The strand does not break up during sonication,
compared to the normal SWNT samples (left) which break up easily
into smaller fragments.
confirm the results from Raman measurements that 1.1 nm
tubes dominate the strands. The fwhm of the peaks at 90°
and 270° in the angular scans (inset of Figure 3b) is about
44°, almost half of that in the nanotube fibers (the fwhm is
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nanotube bundles in the strands. Note that the peak signals
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strand. The strong peaks obtained demonstrate once again
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dominant.
Another interesting observation is worth mentioning here.
We attempted to break up and disperse the nanotube strands
by ultrasonication in ethanol solution for up to several tens
of minutes. We did not observe any breakup or dissolution
of the strands, unlike SWNT samples prepared by the arcmethod or obtained commercially (HiPCO). The nanotube
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structures of SWNTs in our experiment, in contrast, demonstrates that the nanotubes in the strands are continuous
and can not be broken up unless individual nanotubes break
or exfoliate from the strands.
Nano Lett., Vol. 2, No. 10, 2002
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