Ni and Growth of Single-Wall Carbon

Since the discovery of single-wall carbon nanotubes (SWNTs),1,2 laser ablation ..... The similarity of the peak positions of C, Co, and Ar emissions a...
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J. Phys. Chem. B 2000, 104, 6777-6784

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Laser Ablation of Graphite-Co/Ni and Growth of Single-Wall Carbon Nanotubes in Vortexes Formed in an Ar Atmosphere F. Kokai,*,† K. Takahashi,† M. Yudasaka,§ and S. Iijima§,|,⊥ Institute of Research and InnoVation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan, Nanotubulites Project, JST-ICORP, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, and Meijo UniVersity, Department of Physics, Tenpaku-ku, Nagoya 468-8502, Japan ReceiVed: January 28, 2000; In Final Form: April 13, 2000

To study the Nd:YAG laser ablation of graphite-Co/Ni (1.2 at. %) and the growth of single-wall carbon nanotubes (SWNTs) in an Ar atmosphere at 1200 °C, we applied time-resolved imaging and spectroscopic techniques. These techniques are based on measuring the laser plume emission and light scattering of growing materials. Laser ablation was performed at a fluence of 1.7 J/cm2 at 1200 °C and at room temperature for comparison. At 1200 °C, we observed temporally and spatially dependent emissions of C, C2, Co, Ni, and Ar species. In addition, emission, probably due to hot clusters and/or particles grown through the interaction with Ar atoms, could be seen at up to 1.5 ms. This emission showed the images separating into two regions at >100 µs, resulting from the formation of vortexes. In the light scattering images at 2-1000 ms after 1200 °C ablation, we observed the vortexes propagating forward (about 7 cm from the target) during 200 ms and then flowing downward to the target. We suggest that the time available for the SWNT growth in the vortexes is from a few milliseconds to the order of 1 s.

Introduction Since the discovery of single-wall carbon nanotubes (SWNTs),1,2 laser ablation of graphite (containing a small amount of transition metal) has been extensively applied to synthesize them. Usually, nanosecond pulses from a Nd:YAG laser were used to ablate a target of graphite-metal composite in an inert gas atmosphere maintained at 1200 °C.3-12 The presence of transition metal or metal alloy together with carbon species is essential to form SWNTs, and catalytic growth is considered to be the mechanism of SWNT formation. More recently, continuous-wave13 or 20 ms pulse12,14,15 CO2 laser ablation was applied to synthesize SWNTs. In this case, laser ablation even at room temperature (RT) could produce SWNTs. Transmission electron microscope examination of weblike carbonaceous deposits, produced by laser ablation, revealed the presence of SWNTs with their ends embedded in carbon particles containing metal particles (20-100 nm in diameter). We believe that hot carbon-metal particles were formed in the gas phase and that the SWNTs grew from them by supersaturation and segregation of carbon. For the laser ablation process using 20 ms CO2 laser pulses, we have performed in situ diagnostic studies by high-speed video imaging, emission spectroscopy, shadowgraphy, etc.14,15 After the ejection of carbon and metal species with an initial expansion velocity of ∼1 × 103 cm/s, we found the formation of a mushroom or turbulent cloud, containing a vortex motion of clusters, particles, etc. From the propagation velocities of the clouds and the increase in their areas, we suggested that the effective time for * Corresponding author. Telephone: (81) 471-44-9079. Fax: (81) 47144-8939. E-mail: [email protected]. † Institute of Research and Innovation. § Nanotubulites Project, JST-ICORP. | NEC Corp. ⊥ Meijo University.

the growth of SWNTs with a length of at least 0.5 µm was several milliseconds in RT CO2 laser ablation. At 1100-1200 °C, quite a high yield (>60%) of SWNTs was produced, and the time available for the growth of SWNTs may be as much as 1 s from the presence of prolonged blackbody radiation. In nanosecond-pulse laser ablation of graphite, energetic carbon species with supersonic expansion velocities, 105-106 cm/s, are ejected from the graphite surface.16,17 Although the dependence of Ar gas flow rate on the yield of SWNTs suggested that the time contributing to growing them was also the order of 1 s,18 few in situ diagnoses have been reported to investigate the SWNT growth process.19,20 To control and improve the production process of SWNTs by Nd:YAG laser ablation, it is essential to understand the behavior of ablated carbon and metal species during the time span of about 1 s from the start of laser irradiation. In addition, it is necessary to know the detailed role of the temperature (1200 °C) of the Ar atmosphere. In this article, we apply time-resolved imaging and spectroscopic techniques, based on the measurement of emission from the laser plume and light scattering (LS) of the growing materials, to study Nd:YAG laser ablation of a graphite-Co/ Ni target and growth of SWNTs in an Ar atmosphere at 1200 °C. For comparison, results of laser ablation at RT, in which SWNTs are not formed, are also presented. The ejection of carbon and metal species and their interaction with Ar atoms to form clusters and particles are characterized. We also discuss the SWNT growth in emerging vortexes in the Ar atmosphere. Experimental Section Figure 1 shows our experimental setup for the image measurement of plume emission and LS of the growing materials. Laser ablation of a graphite-Co/Ni (1.2 at. %)

10.1021/jp000359+ CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000

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Figure 1. Experimental configuration for time-resolved imaging.

composite target (10 mm in both diameter and length) was performed using a second-harmonic beam (532 nm, 3 ns pulse width) from a Q-switched Nd:YAG laser in a quartz glass tube.5-9,12,14,15 The tube had an inner diameter of 36 mm and a length of 900 mm. Ar gas was flowed at 0.5 L/min, and its pressure was kept at 600 Torr. The laser spot diameter at the target surface was 4 mm, and the laser fluence was set to be 1.7 J/cm2. The target was maintained at either room temperature or 1200 °C by using an electric furnace equipped with a viewport (65 mm × 250 mm)15 through which the laser plume and growing materials could be observed. The length of the heated zone of the tube was 250 mm in the furnace, and target position was 125 mm relative to the border of the heated zone. The region of 1200 °C along the tube axis was located 200 mm around the center of the heated zone. The temperature gradually reduced to 1100 °C at the edge of the heated zone.

Kokai et al. We used a lens-coupled gated intensified-CCD (ICCD) camera system (Princeton Instruments ICCD-MAX with a 105 mm Nikon UV lens) for the emission images of the plumes, as in previous reports.16,21 Emission images in the wavelength range of 200-820 nm were taken at 90° to the ablation laser beam at delay times of 0.01-1500 µs after the start of laser irradiation. The gate widths of the camera were between 0.003 and 5 µs. Band-pass filters (center wavelengths ) 248, 345, 516, and 764 nm; full widths at half-maximum bandwidths (fwhms) ) 5-10 nm) were placed before the lens to measure the wavelengthselected image. In addition, time-resolved emission spectra of the plume rising from the target surface to a height of up to 2 mm were taken at delay times of 0.02-100 µs by using the ICCD camera (gate width ) 3 ns) combined with a monochromator (Acton SpectraPro 750) with a spectral resolution of 0.1 nm. For the LS images of growing materials, the second-harmonic beam (532 nm, 5 ns pulse width, 100 mJ output) from another pulsed Nd:YAG laser was used as the probe light at delay times between 250 µs and 1 s following the ablation laser irradiation. The 532 nm beam, converted to a thin sheet by a lens system, was inserted into the quartz cell from the same direction as the ablation laser. The sheet thickness was approximately 2 mm and the height was 5 cm near the target. Light scattered at 90° to the sheet was then imaged onto the ICCD camera through a band-pass filter (Sigma Koki, fwhm ) 1 nm) at delay times of 2-1000 ms after ablation. The gate width of the ICCD camera was 10 ns. Results and Discussion Total Visible Emission Image. Figure 2 shows typical gated ICCD images of the total visible emission from laser plumes. These images were taken at various delay times following laser

Figure 2. Typical gated ICCD images of the total visible emission for laser ablation at (a) 1200 °C and (b) RT. These images are two sets of those taken for different laser shots at various delay times. The images at 0.02-2 µs were taken with a gate width of 0.003 µs, those at 5-50 µs with gate widths of 0.01-0.1 µs, and those at 100, 500, and 15 000 µs with gate widths of 0.15, 0.3, and 5 µs, respectively. All the images are normalized relative to each maximum signal level. Also shown are the scale for the length from the target and the color look-up table for emission intensity, where the red background color is zero counts and the maximum signal is purple.

Laser Ablation of Graphite-Co/Ni and Nanotube Growth

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Figure 3. Distance from the target to the front edge of the total visible emission image as a function of the delay time observed for the 1200 °C ablation. The inset is a plot at earlier delay times of up to 1.0 µs.

ablation at 1200 °C and RT. All the images are normalized relative to each maximum signal level. The color look-up scale is defined so that the red background color is zero counts and the maximum signal is purple. At both temperatures, plume emissions were detected as soon as the laser pulse irradiation (pulse width: 3 ns) started and decomposition of the graphite surface began to occur. The plume emissions continued for longer times (up to 1.5 ms at 1200 °C) than the laser pulse width. In addition, the areas of the emission images became larger and their shapes changed with increasing delay time.22 These plume emission behaviors are thought to be governed by the following two processes. One is the ejection of electrons, carbon and metal atoms, etc. from the target. This process may continue for more than 1 µs, similar to that during laser ablation of graphite in a vacuum,16 although confinement of the ablated species by the surrounding Ar atmosphere makes it difficult to determine when the ejection from the target actually stops. The other is the collisional excitation processes taking place consecutively among the ejected carbon and metal species, growing clusters and particles, etc. The plume emissions continued for longer times than intrinsic emission lifetimes of the plume-forming species. For example, the lifetime of the C2 Swan band emission is 101.8 ns.23 In particular for the 1200 °C ablation, the area and shape of the plume emission changed greatly with increasing delay time. Over 20 µs, with increasing emission area, its front edge gradually separated from the target, while there was a region still remaining near the target surface. Subsequently, the entire plume region began to leave the target, as seen in the image at 50 µs. As time proceeded, the plume separated into two regions, nearly symmetrical to the incident laser beam, as seen in the images at 100 µs, 500 µs, and 1.5 ms. The distance from the target to the front edge of the emission image observed for the 1200 °C ablation is plotted as a function of the delay time in Figure 3. We think that this plot provides the propagation velocity of an ensemble consisting of ablated species, growing clusters and particles, etc.24 From the distances at the earlier times of 20 and 50 ns, a high velocity of 1.9 × 106 cm/s was calculated, which is almost the same as the velocities observed for ablation in a vacuum.16 However, the velocity of the ensemble appeared to be affected by ambient Ar gas and decreased progressively during its propagation, similar to the behavior of carbonaceous materials in CO2 laser ablation.15 Around 100 µs, when plume separation began, the calculated velocity of 2.2 × 103 cm/s was lower. The lowest calculated velocity of 5.7 × 102 cm/s was at 1.0 and 1.5 ms. At 1200 °C, after the deceleration of ablated species, clusters,

Figure 4. Emission spectra at (a) 241.5-254.0, (b) 338.5-351.5, (c) 510-520, and (d) 758.5-767.5 nm measured at 0.2 µs after laser irradiation. The peaks assigned to C (248 nm), Co (340.5 and 345.5 nm), Ni (341.5 and 346.2 nm), C2 (516.5 nm), and Ar (764 nm) emissions are marked.

particles, etc., we think that the formation of vortexes, as suggested by the plume separation at >100 µs, is induced by the flow of the background Ar gas around the plume. The behavior of the vortexes will be further mentioned below (see also Figure 10). Emission Spectroscopy and Wavelength-Selected Emission Image. To identify carbon and metal species in the plume and to investigate their temporally dependent behaviors resulting in the vortex formation, we measured the time-resolved emission spectra of the plume following 1200 °C ablation. Examples of the emission spectra measured at 0.2 µs after the initiation of laser irradiation are shown in Figure 4. We collected the emission in the region within 2 mm of the target surface. In Figure 4a, a strong peak observed at 248 nm was assigned to the emission from C atoms.25 Many fairy strong peaks in Figure 4a,b were assigned to the emissions from Co and Ni atoms.25 For example, the peaks at 340.5 and 345.4 nm are emission from Co atoms, and the peaks at 341.5 and 346.2 nm are emission from Ni atoms, although some peaks of Co and Ni atoms are overlapped. Although signal-to-noise ratios are not high in longer wavelength regions (Figure 4c,d), the C2 Swan

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Figure 5. Emission spectra showing the 248 nm C peak taken at delay times of (a) 0.1, (b) 0.3, and (c) 1 µs.

band emission with a band head at 516.5 nm26 is seen. In addition to the emissions of these carbon and metal species, a broad peak due to the excitation of ambient Ar atoms is seen around 762-766 nm centered at about 764 nm.26 Several characteristic emission peaks of Ar atoms were also observed in the region of 700-800 nm. The emission peaks of these atomic and molecular species are superimposed on a continuum emission, probably due to Bremsstrahlung radiation (free-free transitions) from a hot plasma.27 We note that emissions from ionic species such as C+ and Ar+ were also observed in laser ablation of graphite at a higher fluence.21 We observed that the widths of the emission peaks in Figure 4 changed dramatically depending on the delay time after ablation. For example, emission spectra showing the 248 nm C peak taken at delay times of 0.1, 0.3, and 1 µs are shown in Figure 5. At 0.1 µs, a broad emission peak, superimposed on a continuum emission, was observed with an fwhm of 1.10 nm. At 0.3 and 1 µs, narrower peaks (0.28 and 0.12 nm) were seen together with a decrease in intensity of the continuum emission. Similar variations of the peak width were also observed for emissions of other species, fwhms were 4.8-0.14 nm for the Ar peak and 0.36-0.14 nm for the Co and Ni peaks. In addition to these changes in peak widths, a shift in the peak position was also observed. In Figure 5, compared to the peak position in the spectrum at 0.1 µs, the peaks at 0.3 and 1 µs are shifted by 0.14 nm toward a short-wavelength side. For the broadening of the emission peaks from ablation plumes, there are three likely mechanisms: Stark, resonance, and Doppler mechanisms.27 Among these, the Stark mechanism, due to collisions with electrons or ions, can provide a larger width together with a peak shift.28 Therefore, we think that the Stark mechanism is dominant in our peak broadening. The broadening suggests that carbon and metal species (and electrons) exist with a high density caused by the confinement by ambient Ar atoms, leading to an enhanced growth of clusters and particles. Emission intensities of the C (248 nm), Co (345.4 nm), C2 (516.5 nm), and Ar (764 nm) peaks were found to vary

Figure 6. Temporal distributions of emission intensities of the C (248 nm), Co (345.4 nm), C2 (516.5 nm), and Ar (764 nm) peaks in Figure 4. The emission intensity at 246 nm (Bremsstrahlung radiation) is also shown.

significantly with the delay time following ablation. Emission intensities of these species are plotted as a function of delay time in Figure 6.28 The emission intensity at 246 nm (Bremsstrahlung radiation) is also shown for comparison. Rise times of the emission intensities at 0.02-0.08 µs indicate that first the Bremsstrahlung radiation becomes observable following laser ablation. Then the emissions of C, Co, and C2 appear, and finally

Laser Ablation of Graphite-Co/Ni and Nanotube Growth the emission of Ar appears. The intensity distributions of C, Co, and Ar show the presence of two emitting components revealing peaks at 0.1-0.2 and 0.5-0.6 µs, respectively. The intensity distribution of the emission peak of Ni atoms at 341.5 nm was similar to that of Co atoms. The similarity of the peak positions of C, Co, and Ar emissions at 0.1-0.2 µs to that of Bremsstrahlung radiation suggests that the origins of these earlier emerging emissions are electron-impact excitation of the corresponding species at each ground state and plasma recombination, although the contribution of the direct ejection of excited C and Co atoms may not be negligible. The origin of the delayed component peaking at 0.5-0.6 µs is not clear at present. If the arrival of C and Co atoms ejected from the target is dominant, the propagation velocity of the delayed component should be one-fifth that of the earlier component. We do not think that such a process occurs. Rather, backward motion of the ejected C and Co atoms induced by collisions with Ar atoms, excitation of Ar atoms by backward moving C and Co atoms, etc. may contribute to the appearance of the components peaking at 0.50.6 µs. Compared to the intensity distributions of C, Co, and Ar emissions, it is surprising that C2 emission continues for 70 µs. We think that this continuation results from the formation and excitation of C2: recombination of C atoms decelerated in the confined plume, dissociation of growing hot clusters, etc. Wavelength-selected emission images of the laser plume following 1200 °C ablation were found to provide speciesspecific information on the temporally and spatially dependent behaviors of plume-forming species. Figure 7 shows wavelengthselected emission images at 248, 345, 516, and 764 nm measured at delay times of 0.1 and 1 µs following laser ablation. At 0.1 µs, three emission images at 248, 345, and 764 nm (Figure 7a,b,d), due to the presence of C, Co and Ni, and Ar, respectively, have similar shapes, although their areas and locations are slightly different. The emission image at 516 nm (Figure 7c), having a different shape, indicates the presence of C2 close to the target, as well as the C2 propagating forward. We note that the Bremsstrahlung radiation, whose intensity distribution is shown in Figure 6a, is not neglected in the frontedge region of each image. At 1 µs, the four images show clearly different shapes with increasing emission areas. The 248 nm image (C emission) exists far from the target (1.5-2.8 mm). The 345 nm image (Co and Ni emissions) has its strong emission region around 2 mm from the target and its weak emission region close to the target. The presence of this weak emission region is consistent with the emission distribution up to 10 µs in Figure 6c. The 516 nm image (C2 emission) exists close to the target with its strong emission around 1 mm. This strong emission is also consistent with C2 emission distribution continuing for 70 µs (Figure 6d). The 764 nm image shows two emitting regions: strong emission at 1.5-3.2 mm and weak emission at 0.2-1.5 mm. For the 764 nm emission image, images taken at delay times of 0.2-10 µs are also shown in Figure 8. The emission area at 0.5-1.5 mm in the 0.2 µs image became larger with forward propagation up to 2.0 µs, whereas this emission disappeared with a further increase in the delay time, as seen in the images at 5 and 10 µs. On the other hand, a new emission appeared in the region close to target at 0.5 µs, and its area increased with increasing delay time. We attribute this new emission appearing after 0.5 µs to blackbody radiation from growing clusters and/or condensing particles, probably composed of carbon-metal composites. In connection with its appearance, it is important to note that emission images at 248, 345, 516, and 764 nm at >50 µs were similar to those of the

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Figure 7. Wavelength-selected emission images at (a) 248, (b) 345, (c) 516, and (d) 764 nm measured at delay times of 0.1 and 1 µs following laser irradiation. The 0.1 and 1 µs images were taken with gate widths of 0.003-0.01 and 0.005-0.02 µs, respectively. The length scale and color look-up table are shown as in Figure 2.

total emission (Figure 2a), also suggesting that the new emission is due to the clusters and/or particles. LS Images of Carbonaceous Materials. More information on growing materials at a later time was obtained by comparing LS images following RT and 1200 °C ablation. Figure 9 shows LS images at 0.25-5 ms following RT ablation. The images indicate condensational growth of carbon particles in an Ar atmosphere, probably some of them contain Co/Ni particles. The grown particles gradually disappeared as they reached about 3 cm from the target due to deposition of the grown particles on the wall of the quartz tube. In the RT ablation, most of the deposited carbon particles were found to be amorphous from their Raman spectra.7 We point out here that the growth of the amorphous particles was almost completed within 0.5 ms after ablation. The rate of this condensational growth is extremely high (at least 10 times) relative to that after 1200 °C ablation, as mentioned below. Figure 10 shows LS images observed at 2-1000 ms following 1200 °C ablation. These images are completely different from

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Figure 8. 764 nm emission images measured at delay times of 0.210 µs. The images were taken with gate widths of 0.005-0.5 µs. The length scale and color look-up table are shown as in Figure 2.

those after RT ablation. Growing materials became visible around 2 ms after ablation, and then two vortexes of growing materials appeared. Although the vortexes propagated forward up to about 7 cm from the target during 200 ms, they flowed down toward the target due to the flow of Ar gas. Their flow velocity was estimated to be about 3.7 cm/s in the quartz tube from their positions at 0.5 and 1 s. The emerging vortex here is consistent with the observation by a laser-induced luminescence technique.20 In RT ablation, the emission area at 100 µs was located at up to about 3 mm from the target, as seen in the images in Figure 2b. However, the LS image at 250 µs in Figure 9 shows the presence of clusters and particles at up to about 8 mm. On the other hand, in 1200 °C ablation the emission area at 1.5 ms was located at up to about 24 mm from the target (Figure 2a), and the LS image at 2 ms was located at about 17 mm (Figure 10). These detected positions in RT and 1200 °C ablation may be attributed to the rate of condensational growth, depending on the temperature of ambient Ar gas, and the formation of vortexes in 1200 °C ablation. In 1200 °C ablation, the clusters and particles were still hot at 1.5 ms. We could trace a series of events after ablation to 1 s from the combination of emission and LS imaging techniques. We believe that two small spots observed in the LS image at 2 ms in Figure 10 correspond to the central parts of two vortexes. Therefore, the LS image showed the spots at 17 mm located closer to the target compared to the positions of clusters and particles in the emission image at 1.5 ms, although accurate distribution of clusters and particles

Figure 9. LS images at delay times of 0.25-5 ms following RT ablation. The region from the target to about 7 cm is shown. The color look-up scale is defined as in Figure 2.

cannot be compared because of the difference in sensitivity between the two detection methods.

Laser Ablation of Graphite-Co/Ni and Nanotube Growth

Figure 10. LS images at delay times of 2-1000 ms following 1200 °C ablation. The region from the target to about 7 cm is shown at 2-50 ms, and the region at 2-8 cm from the target is shown at 0.2-1 s. The color look-up scale is defined as in Figure 2.

The sensitivity in an LS experiment is generally proportional to d6N, where d and N are respectively the diameter and concentration of the particle to be detected.29 It is most likely that the LS image of vortexes became observable with increasing particle sizes due to the promotion of condensation. Recently, Gorbunov at al.18 reported the synthesis of SWNTs under the

J. Phys. Chem. B, Vol. 104, No. 29, 2000 6783 condition (1150 °C, 1.5 J/cm2, and 490 Torr) similar to our experiment, in which an Ar flow rate was varied from 0.1 to 40 cm/s and the highest yield of SWNTs was found at 1-2 cm/s. Considering the length of the heated zone (250 mm) in our study, we think that SWNTs are grown in the vortexes existing on the order of 1 s. This is consistent with our occasional finding of weblike deposits retaining a vortex-like shape at the edge of the electric furnace. Growth Dynamics of SWNTs. Laser ablation of graphiteCo/Ni composite, using a nanosecond laser pulse, results in the ejection of energetic carbon and metal species. Their initial velocities were estimated here to be of 1.6 × 106 cm/s. Their temperatures are considered to be more than several thousand Kelvin from temperature evaluations of Cn (n ) 1-3) detected by mass spectrometry17 and of the blackbody radiation of carbon particles30 during laser ablation of graphite. We believe the ejected carbon and metal species were mainly composed of atomic and small molecular species; indeed, the emissions of C, Co, and Ni atoms were observed here. In the case of the carbon species, the major ejection of small species such as C, C2, and C3 was seen in laser ablation of graphite.17 Ejected carbon and metal species begin to collide with surrounding Ar atoms. This results in the dissipation of kinetic and thermal energies of the carbon and metal species, together with electronic excitation of Ar atoms. A shockwave is probably generated and propagates in the Ar atmosphere.21 With pushing Ar atoms, the formation of carbon (main component) and metal clusters begins through collisions among the carbon and metal species in the volume confined by the Ar atmosphere. After 0.5 µs from ablation, the clusters continue to grow, resulting in the formation of larger clusters such as linear chain and ring carbon clusters, and condensational growth of carbon and metal particles (and carbon-metal composite particles) seems to occur, together with the diffusion of Ar atoms (acting as third bodies to cool clusters and particles) into the confined volumes. Since the growth rates of metal clusters are lower than those of carbon clusters,20 the formation of carbon clusters and particles probably proceeds early. The further decrease in kinetic energies and the backward motion of clusters and particles, induced by collisions with Ar atoms, lead to vortex formation at >100 µs, as suggested by the images between 50 µs and 1.5 ms in Figure 2. From the LS images revealing different condensation behaviors after ablation at RT and 1200 °C, the formation rate of clusters and particles and their gas dynamic phenomena are considered to be significantly affected by the temperature of the Ar atmosphere. The temperatures of the clusters and particles in the vortex continue to fall and approach the temperature at which nucleation and segregation of carbon to grow SWNTs is thought to occur in carbon-metal particles. The temperature is about 1200 °C for the catalysts of Co/Ni. Above or below this temperature, a decrease in the yield of SWNTs was observed.18,31 We think that a key issue is when the temperature of carbonmetal composite particles in the vortex becomes about 1200 °C to start the SWNT growth. In laser ablation of graphite at 1150 °C in an Ar atmosphere (200 Torr), the temperature of carbon particles was estimated to be about 1500 °C at 2.5 ms after laser ablation from the analysis of their blackbody emission.30 Although our higher Ar gas pressure (600 Torr) may lead to a somewhat higher rate of temperature reduction, we deduce that the temperature of the carbon-metal particles in the vortex becomes about 1200 °C after a few milliseconds. Our LS images indicate that the vortexes, which flowed down toward the edge of the electric furnace, are maintained on the

6784 J. Phys. Chem. B, Vol. 104, No. 29, 2000 order of 1 s in the Ar atmosphere. It is most likely that vortex motion accumulates clusters and particles into the central part of the vortex ring existing in the confined volume (