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EFFECT OF TEMPERATURE GRADIENT NEAR THE TARGET AND GAS FLOWRATE ON THE DIAMETER DISTRIBUTION OF SINGLE-WALLED CARBON
NANOTUBES GROWN BY THE LASER ABLATION TECHNIQUE
Rahul Sen1, Hiromichi Kataura2, Yohsuke Ohtsuka1, Toshinobu Ishigaki1,
Shinzo Suzuki1 and Yohji Achiba1
1
Department of Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, JAPAN
2
Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, JAPANABSTRACTGas dynamic and time resolved imaging studies have been performed on the growth of
single-walled carbon nanotubes (SWNTs) in the laser ablation process. SWNTs weresynthesized by laser ablation of Ni-Co catalyzed graphite targets at 1200 oC under argon gas.
The effects of the temperature gradient near the target and the gas flow rate were studied inorder to understand the effect of gas dynamics over the diameter distribution of SWNTs. The
gas flow rate affects the diameter distribution of SWNTs especially when the growth species
flow through a large temperature gradient. Scattering images from the growth species atdifferent flow rates was recorded by high-speed video imaging. The results indicate that thevelocities of these species are dependent on the gas flow rate but this dependence is evident
30 ms after the laser ablation. These findings are used to estimate the time period for the
nucleation and the growth of SWNTs.
INTRODUCTION
Electronic properties of single-walled carbon nanotubes (SWNTs) depend crucially on
their structural parameters such as diameter and chirality [1]. It is important to understand thegrowth mechanism of SWNTs to achieve a control over their diameter distribution and to
produce nanotubes of desired property. Laser ablation of metal-graphite composite targets inargon gas can produce high quality SWNTs, however the diameters and chiralities of thenanotubes vary considerably for a given set of growth conditions [2-4]. Bandow et al. [5]reported that the mean diameter of the nanotubes increased with increasing temperature of thegrowth environment in laser ablation. Diameters of SWNTs can also be changed by changing
the catalyst system [6] or by changing the catalyst ratio [7]. Despite these reports the process
and mechanism of SWNT growth is not well understood. We consider that by changing
various control parameters in the laser ablation process and by studying the dynamics of theablated species, we can obtain an insight into the growth process of the SWNTs. We havestudied the effect of the temperature gradient around the target and the gas flow rate on the
diameter distribution of SWNTs. We also observed the scattering images of the laser-ablated
species recorded by time-resolved photography. We estimated the velocities of the species
inside the furnace and correlated them with the flow rates. Finally, the effect of the flow rateon the diameter distribution of SWNTs and the actual flow rate of the species are used toestimate the nucleation and growth times of SWNTs.
EXPERIMENTAL
SWNTs were grown by the laser ablation of metal-graphite composite targets using themetal catalysts, Ni(0.6 at.%)-Co(0.6 at.%). The target was supported by a graphite holder andfixed to a rotating molybdenum rod inside a quartz tube. A rotary pump first evacuated the1

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quartz tube, and then flowing argon gas was introduced into it. The pressure of argon gas
inside the tube was maintained at 500 Torr and the flow rate was set to a desired value. An
electric furnace heated the quartz tube, and the temperature was maintained at 1200 oC. Laser
ablation was carried out by using the second harmonics of a Nd:YAG laser (532 nm, 10 Hz),focused to a 5 mm diameter spot on the target. The laser power was 300 mJ/pulse, and theablation was carried out for 30 min. During the laser ablation the flowing argon gas carried
the carbon products downstream, where a mat-like material was found to deposit on the
molybdenum rod near the outlet region of the furnace. This material was collected byscraping and analyzed by Raman spectroscopy and transmission electron microscopy (TEM).
Raman scattering was measured using a 1 m double monochromator, Jobin Yvon U-1000, anda photo-multiplier, Hamamatsu Photonics R943-02. The laser excitation line for Raman
spectroscopy was 488 nm, and the spectral resolution was 4 cm-1. The diameter distributionof the SWNTs was analyzed from the Radial Breathing Mode (RBM) of the Raman spectrum.
Diameter-selective Raman scattering for SWNTs is important for the Raman band at
about 180 cm-1, which is associated with the RBM of the carbon nanotube [8]. According totheory the RBM frequency is inversely proportional to the diameter of SWNT [5]. The
observed RBM bands of SWNTs could be fitted by a sum of multiple Lorentzian peaks. Twoconstraints were used for the fitting procedure as follows. Peaks were first selected byinspection; five Lorentzian peaks with widths ranging from 5 to 12 cm-1 were found to fit theexperimental data reasonably well. Then all the peaks were given an equal width, and thefitting was repeated. A width of 8.4 cm-1 gave a good fit to the observed data. Each of the fivepeaks corresponds to the RBM frequency of SWNT having a particular diameter, and the tube
diameter is inversely proportional to the RBM frequency. The relative yield of each SWNT
was obtained by dividing the area of the corresponding RBM peak to the total area of all theRBM peaks. The RBM frequencies shown in Fig. 1 indicate that the SWNTs have diameters
ranging from 1.1 to 1.5 nm. This was also confirmed by TEM observation. For confirmationof the appropriateness of correlating the intensities of the RBM peaks to the relative yields of
SWNTs, it is important to consider resonance effects. The intensity of the RBM peaks isenhanced when the photon energy of the excitation laser is in resonance with an allowed
optical transition between van Hove singularities in the electronic structure of SWNTs [8].
Resonance Raman scattering for an excitation wavelength of 488 nm occurs forsemiconducting SWNTs with diameters of 1–1.6 nm due to the third and fourth van Hove
singularities and for metallic tubes of about 0.8 nm diameter [9]. The diameters of SWNTs
evaluated from the peak positions of the RBM frequencies for our samples fall in the range of
1.1–1.5 nm. Therefore, the detected RBM modes are of the semiconducting SWNTs of thisdiameter range. This enables a direct comparison of the RBM intensities and correlation ofthese intensities to the relative yields of the SWNTs.
Scattering images from the laser ablated species were recorded as follows. The ablation
pulse was the fundamental of Nd-YAG laser (1064 nm, ~10 ns pulse width), and thescattering images were observed due to the second harmonic (532 nm, long pulse mode ~200µs pulse width) operating at a fixed time delay. The ablation and scattering lasers wereco-axial, and the images were collected at a direction perpendicular to them. The images wererecorded by a high-speed video camera (Kodak Ektapro HS motion analyzer 4540), and therecording rate was 9000 frames/s (~100 µs time window for each frame).
RESULTS AND DISCUSSION
Changing the target position with respect to the center of the furnace changed thetemperature gradient near the target and this is schematically represented in Figure 1. The

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temperature profile inside the furnace, shown in Figure 1(b), indicates that a significant
temperature gradient exists at positions far away from the center of the furnace. We haveexamined the effect of the target position on the diameter distribution of SWNTs and thedetails are discussed elsewhere in the literature [10]. In brief, the relative yields of thesmall-diameter SWNTs increases whereas that of large-diameter SWNTs decreases as thetarget is placed away from the center of the furnace. Since the ambient temperature awayfrom the furnace center is lower than that at the center position, this finding suggests thatlower ambient temperature favors the growth of SWNTs having smaller diameters. This
result is consistent with the earlier report on the effect of ambient temperature on the diameter
distribution of SWNTs [5].
Mo RodDirectionoflaser beam andAr gas flow(b)

Graphite holder

TargetFurnace(a)

Quartz tube

Furnace center

1300
1200

o

Temperature ( C)

1100
1000
900
800

3

0.55 cm /s

700

3

600

1.66 cm /s

500

32.77 cm /s

400
300-200 -160 -120 -80

-40

0

40

80

120 160 200

Target position (mm)Figure 1 (a) Schematic representation of the experimental set-up for changing target position
inside the furnace. When target is placed at the center it is termed as 0; when placed upstream
it is negative and when placed downstream it is positive. (b) Temperature profiles inside thefurnace at different gas flow rates.

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After establishing the fact that the ambient temperature around the target is important forcontrolling the relative yield of SWNTs having different diameters, we studied the effect ofgas flow rate on the diameter distribution of SWNTs. Figure 2 presents the Raman spectra
associated with RBM of the SWNTs grown at different target positions under different flow
rates. When the target was placed at the furnace center, the flow rate had only a small affecton the diameter distribution (Figure 2(a)). When the target was placed at –40 and –80 mm,
however, the flow rate strongly affected the relative yields of SWNTs having different
diameters (Figure 2(b) and (c)). In both cases, the intensity of the 163 cm-1 peak increased
and that of the 203 cm-1 peak decreased with increasing flow rate. In other words, an increasein the flow rate increased the relative yields of large-diameter SWNTs and decreased theyields of small-diameter SWNTs. In the 0 mm case, the temperature gradient around thetarget was uniform (see Figure 1(b)), and the diameter distribution was essentially unaffectedby the flow rate. In the –40 and –80 mm cases, however, the temperature gradient around thetarget was larger and the relative yields of SWNTs were more sensitive to the flow rate.

(c)

(b)

(a)

3

3

100

150

200

250
-1

Raman shift (cm )300 100

3

1.66 cm /s

30.55 cm /s

150

200

250

2.77 cm /s

Intensity (arb. units)3

0.55 cm /s

Intensity (arb. units)

Intensity (arb. units)3

1.66 cm /s

32.77 cm /s

2.77 cm /s

300 100-1

Raman shift (cm )31.66 cm /s

30.55 cm /s

150

200

250

300
-1

Raman shift (cm )Figure 2. Raman spectra (RBM) of SWNTs grown at different target positions under
different gas flow rates. The dotted curves represent the individual lorentzians, and the solid
curve represents fit to the experimental data. Target positions are (a) 0 mm (furnace center),(b) –40 mm and (c) –80 mm.

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To understand this flow rate effect, we recorded the scattering images of the laser ablated
species at different flow rates. A series of photographs taken by a high-speed video camera atdifferent time intervals after the laser ablation is shown in Figure 3. From the top frames in
Figure 3, it is clear that 10 ms after the ablation the ablated species has moved about 40 mm
from the target, and this distance is independent of the flow rate. This initial movement,
caused by the ablation process, is in opposite direction to the gas flow and is termed forward
movement. Images taken after 30 and 50 ms show that the particles have started moving in
the direction of the gas flow, termed backward motion. From these time-resolved scattering
images, the velocities of the backward moving species are estimated to be about 38, 123 and255 mm/s for the flow rates of 0.55, 1.66 and 2.77 cm3/s respectively. Since the velocities ofthe backward moving species increases with the flow rate, their positions inside the furnace
after a certain time interval are different. This is clear from the bottom frames in Figure 3, 90
ms, for different flow rates. When the target is placed at the center, this difference in positionof the backward moving species at different gas flow rates does not correspond to a largechange in the temperature inside the furnace. When the target is placed at –40 or –80 mm,
however, this difference means that after about 100 ms the backward moving species are at amuch higher temperature when the flow rate is higher. Since a higher flow rate gives higher
yields of SWNTs having larger diameters, it indicates that the temperature of the growth
species is higher at a higher flow rate. This suggests that the SWNT growth process continues
to occur even 100 ms after the laser ablation. Thus the growth period is long enough for theflow rate to significantly influence the relative yields of SWNTs having different diameters,
especially when the growth species flow through a large temperature gradient.
Gas flow rate in cm3/s
0.55

Delay time in ms
1.66

2.77

10
30
50
70
90

TargetDirection of gas flow
and laser beam

40 mm

Figure 3. Scattering images from the laser ablated species recorded at different time intervals
after the ablation pulse. The dashed line is given as a visual guide, showing the movement of
the backward moving species with time.

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CONCLUSIONS
Gorbunov et al. [11] studied the effect of the flow rate of argon gas on the overall yieldof SWNTs; their results suggest that the growth times are of the order of 1 s. Recent studiesof time-resolved imaging and spectroscopy on the dynamics of SWNTs growth process [12,13] conclude that SWNTs’ growth occurs in vortexes in long time periods (a few ms to a fews). Figure 2(b) and (c) show that the relative yields of large-diameter SWNTs increase withincreasing flow rate whereas the overall range of the diameter distribution remains unaffected.
Therefore, we conclude here that the initial condensation and nucleation processes, which
may determine the range of diameter distribution, occur much faster (<100 ms) but thegrowth continues for about 100 ms and more.
ACKNOWLEDGEMENTS
This work was supported by the Japan Society for the Promotion of the Science (JSPS)(Research for the Future Program) and by the Ministry of Education, Science, Sports andCulture of Japan.
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