Time period for the growth of single-wall carbon nanotubes inthe laser ablation process: evidence from gas dynamic studies

and time resolved imaging

Rahul Sen a, Y. Ohtsuka a, T. Ishigaki a, D. Kasuya a, S. Suzuki a, H. Kataura b,Y. Achiba a,*

a Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami±Osawa, Hachioji, Tokyo 192±0397, Japanb Department of Physics, Tokyo Metropolitan University, 1-1 Minami±Osawa, Hachioji, Tokyo 192±0397, Japan

Received 4 September 2000; in ®nal form 6 November 2000

Abstract

Single-wall carbon nanotubes (SWNTs) were synthesized by laser ablation of Ni±Co-graphite composite targets at

1200°C under argon gas. The e�ects of the temperature gradient near the target and the gas ¯ow rate on the diameter

distribution of SWNTs were studied in order to understand their growth dynamics. Raman spectroscopy was used to

analyze the diameter distribution of SWNTs. The ¯ow rate was found to a�ect the relative yields of SWNTs having

di�erent diameters when the temperature gradient around the target was large. Scattering images from the ablated

species at di�erent ¯ow rates, recorded by a high-speed video camera, indicated that 10 ms after the ablation the ve-

locities of backward moving species increased with increasing ¯ow rate. These ®ndings are used to estimate the time

required for determining the diameter distribution and the growth of SWNTs. Ó 2000 Elsevier Science B.V. All rights

reserved.

1. Introduction

Electronic properties of single-walled carbonnanotubes (SWNTs) depend crucially on theirstructural parameters. SWNTs behave like a metalor a semiconductor depending on their diameterand chirality [1]. Thus it is important to control thediameter distribution of SWNTs during thegrowth to produce nanotubes of desired propertyand to understand their growth mechanism. Laserablation of metal±graphite composite targets in

argon gas is a useful technique to grow SWNTs.However, the diameters and chiralities of the na-notubes vary considerably for a given set ofgrowth conditions [2±4]. Bandow et al. [5] reportedthat the mean diameter of the nanotubes increasedwith increasing temperature of the growth envi-ronment in laser ablation. Kataura et al. [6] stud-ied the e�ect of various metal catalysts on thediameter distribution of SWNTs grown by thelaser ablation technique. These authors reportedthat Ni±Co catalyst yielded SWNTs with diame-ters of 1.1±1.5 nm whereas Rh±Pd catalyst yieldedSWNTs having diameters as thin as 0.7±1.0 nm.Jost et al. [7] reported recently that for NixCo1ÿx

catalyst systems the yield of large-diameter

29 December 2000

Chemical Physics Letters 332 (2000) 467±473

www.elsevier.nl/locate/cplett

* Corresponding author. Fax: +81-426-77-2525.

E-mail address: achiba-yohji@c.metro-u.ac.jp (Y. Achiba).

0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 2 0 - 8

SWNTs increased with increasing Co ratio. De-spite these reports the process and mechanism ofSWNT growth is not well understood. In thisLetter, we report the e�ect of the temperaturegradient around the target and the gas ¯ow rate onthe diameter distribution of SWNTs obtained bythe laser ablation process. We also report thescattering images of the laser-ablated species re-corded by time-resolved photography. We esti-mate the velocities of the species inside the furnaceand correlate them with the ¯ow rates. Finally, thee�ect of the ¯ow rate on the diameter distributionof SWNTs and the actual ¯ow rate of the speciesare used to estimate the nucleation and growthtimes of SWNTs.

2. Experimental

SWNTs were grown by the laser ablation ofmetal±graphite composite targets using the metalcatalysts, Ni(0.6 at.%)±Co(0.6 at.%). The targetwas supported by a graphite holder and ®xed to arotating molybdenum rod inside a quartz tube.The tube was ®rst evacuated by a rotary pump,and then ¯owing argon gas was introduced into it.The pressure of argon gas inside the tube wasmaintained at 500 Torr and the ¯ow rate was set toa desired value. The tube was heated by an electricfurnace, and the temperature was maintained at1200°C. Laser ablation was carried out by usingthe second harmonics of a Nd:YAG laser (532 nm,10 Hz), focused to a 5 mm diameter spot on thetarget. The laser power was 300 mJ/pulse, and theablation was carried out for 30 min. Duringthe laser ablation the ¯owing argon gas carried thecarbon products downstream, where a mat-likematerial was found to deposit on the molybdenumrod near the outlet region of the furnace. Thismaterial was collected by scraping and analyzed byRaman spectroscopy and transmission electronmicroscopy (TEM). Raman scattering was mea-sured using a 1 m double monochromator, JobinYvon U-1000, and a photo-multiplier, Hamama-tsu Photonics R943-02. The laser excitation linefor Raman spectroscopy was 488 nm, and thespectral resolution was 4 cmÿ1. The radialbreathing mode (RBM) of the Raman spectrum

was used for estimation of the diameter distribu-tion of SWNTs. Scattering images from the laserablated species were recorded as follows. The ab-lation pulse was the fundamental of Nd-YAGlaser (1064 nm, �10 ns pulse width), and thescattering images were observed due to the secondharmonic (532 nm, long pulse mode �200 ls ispulse width) operating at a ®xed time delay. Theablation and scattering lasers were co-axial, andthe images were collected at a direction perpen-dicular to them. The images were recorded by ahigh-speed video camera (Kodak Ektapro HSmotion analyzer 4540), and the recording rate was9000 frames/s (�100 ls time window for eachframe).

3. Results and discussion

3.1. Analysis of the Raman spectra

Diameter-selective Raman scattering forSWNTs is particularly important for the Ramanband at about 180 cmÿ1, which is associated withthe RBM of the carbon nanotube [8]. According totheory the RBM frequency is inversely propor-tional to the diameter of SWNT [5,9]. The ob-served RBM bands of SWNTs, shown in Fig. 1,could be ®tted by a sum of multiple Lorentzianpeaks. Two constraints were used for the ®ttingprocedure. A minimum number of frequencies thatcould ®t all the RBM bands were ®rst selected andall the Lorentzian peaks were then given the samewidth, since the damping constant for the RBMwas not expected to depend much on the diameterof 1±1.6 nm [4]. Peaks were ®rst selected by in-spection; ®ve Lorentzian peaks with widths rang-ing from 5 to 12 cmÿ1 were found to ®t theexperimental data reasonably well. Then all thepeaks were given an equal width, and the ®ttingwas repeated. A width of 8:4 cmÿ1 gave a good ®tto the observed data. The observed spectra and thecomponent peaks indicated with arrows are shownin Fig. 1. Each of these peaks corresponds to theRBM frequency of SWNT having a particulardiameter, and the tube diameter is inversely pro-portional to the RBM frequency. The relative yieldof each SWNT was obtained by dividing the area

468 R. Sen et al. / Chemical Physics Letters 332 (2000) 467±473

of the corresponding RBM peak to the total areaof all the RBM peaks. The RBM frequenciesshown in Fig. 1 indicate that the SWNTs havediameters ranging from 1.1 to 1.5 nm. This wasalso con®rmed by TEM observation. For con®r-mation of the appropriateness of correlating theintensities of the RBM peaks to the relative yieldsof SWNTs, it is important to consider resonancee�ects. The intensity of the RBM peaks is en-hanced when the photon energy of the excitationlaser is in resonance with an allowed opticaltransition between van Hove singularities in theelectronic structure of SWNTs [8]. ResonanceRaman scattering for an excitation wavelength of488 nm occurs for semiconducting SWNTs withdiameters of 1±1.6 nm due to the third and fourthvan Hove singularities and for metallic tubes ofabout 0.8 nm diameter [10]. The diameters ofSWNTs evaluated from the peak positions of theRBM frequencies for our samples fall in the rangeof 1.1±1.6 nm. Therefore, the detected RBMmodes are of the semiconducting SWNTs of this

diameter range. This enables a direct comparisonof the RBM intensities for semiconductingSWNTs with diameters of 1.1±1.5 nm and corre-lation of these intensities to the relative yields ofthe SWNTs.

3.2. E�ects of temperature gradient and gas ¯owrate

The temperature gradient near the target waschanged by changing the target position with re-spect to the center of the furnace. Fig. 2a shows aschematic representation of the changes in thetarget position in the furnace. The temperaturepro®le inside the furnace, shown in Fig. 2b, wasmeasured experimentally by placing a thermo-couple at di�erent positions inside the quartz tubelocated in the furnace. As shown in Fig. 2b, asigni®cant temperature gradient was indicated at

Fig. 2. (a) Schematic representation of the experimental setup

for changing the target position inside the furnace and, (b)

temperature pro®les inside the furnace at di�erent gas ¯ow

rates. When the target is placed at the center it is termed as 0;

when placed upstream it is negative, and is positive when placed

downstream.

Fig. 1. Raman spectra of the RBM of SWNTs grown using

Ni(0.6 at.%)±Co(0.6 at.%) catalyzed graphite rods at 1200°C.

Ar gas pressure was 500 Torr and the ¯ow rate was 0:55 cm3=s.

Arrows denote the RBM frequencies in cmÿ1.

R. Sen et al. / Chemical Physics Letters 332 (2000) 467±473 469

positions far away from the center of the furnace.We ®rst examined the e�ect of the target positionon the diameter distribution of SWNTs. RBMspectra were measured for SWNTs grown at dif-ferent target positions with respect to the center ofthe furnace, where the temperature was kept at1200°C. Fig. 3 indicates that the relative yields ofthe small-diameter SWNTs increases whereasthose of large-diameter SWNTs decreases as thetarget is placed away from the center of the fur-nace. Since the ambient temperature away fromthe furnace center is lower than that at the centerposition, this ®nding suggests that lower ambienttemperature favors the growth of SWNTs havingsmaller diameters. This result agrees with those ofthe e�ect of ambient temperature on the diameter

distribution of SWNTs [5]. In this connection, theyields of fullerenes, obtained by laser ablation ofgraphite targets, are also sensitive to the ambienttemperature [11].

Having established that the target position in-side the furnace, and hence the ambient tempera-ture around the target, is important for controllingthe relative yield of SWNTs having di�erent di-ameters, we next studied the e�ect of ¯ow rate ofthe gas on the diameter distribution of SWNTs.Fig. 4 presents the Raman spectra associated withRBM of the SWNTs grown at di�erent targetpositions under di�erent ¯ow rates. When thetarget was placed at the furnace center, the ¯owrate had only a small e�ect on the diameter dis-tribution. When the target was placed at ÿ40 and)80 mm, however, the ¯ow rate strongly a�ectedthe relative yields of SWNTs having di�erent di-ameters. In both cases, the intensity of the163 cmÿ1 peak increased and that of the 203 cmÿ1

peak decreased with increasing ¯ow rate. In otherwords, an increase in the ¯ow rate increased therelative yields of large-diameter SWNTs and de-creased the yields of small-diameter SWNTs. Inthe 0 mm case, the temperature gradient aroundthe target was uniform (see Fig. 2b), and the di-ameter distribution was essentially una�ected bythe ¯ow rate. In the )40 and )80 mm cases,however, the temperature gradient around thetarget was larger and the relative yields of SWNTswere more sensitive to the ¯ow rate.

3.3. Scattering images of the laser ablated species

In order to understand this ¯ow rate e�ect, wemeasured the scattering images of the laser ablatedspecies at di�erent ¯ow rates. Fig. 5 shows a seriesof photographs taken by a high-speed video cam-era at di�erent time intervals after the laser abla-tion. Fig. 5 shows that 10 ms after the ablation theablated species has moved about 40 mm from thetarget, and this distance is independent of the ¯owrate. This initial movement, caused by the ablationprocess, is in opposite direction to the gas ¯ow andis termed forward movement. Images taken after30 and 50 ms show that the particles have startedmoving in the direction of the gas ¯ow, termedbackward motion. From these time-resolved scat-

Fig. 3. Raman spectra of the radial breathing mode of SWNTs

grown at di�erent target positions with respect to the center of

the furnace. The ¯ow rate of Ar gas was 0:55 cm3=s, and the

pressure was 500 Torr. The temperature at the center of the

furnace (position� 0 mm) was 1200°C.

470 R. Sen et al. / Chemical Physics Letters 332 (2000) 467±473

tering images, the velocities of the backwardmoving species are estimated to be about 38, 123and 255 mm/s for the ¯ow rates of 0.55, 1.66 and2:77 cm3=s, respectively. Since the velocities of thebackward moving species increase with the ¯owrate, their positions inside the furnace after a cer-tain time interval are di�erent. This is clear fromthe bottom frames in Fig. 5, 90 ms, for di�erent¯ow rates. When the target is placed at the center,this di�erence in position of the backward movingspecies at di�erent gas ¯ow rates does not corre-spond to a large change in the temperature insidethe furnace. When the target is placed at )40 or)80 mm, however, this di�erence means that afterabout 100 ms the backward moving species are ata much higher temperature when the ¯ow rate ishigher. Since a higher ¯ow rate gives higher yieldsof SWNTs having larger diameters, it indicatesthat the temperature of the growth species ishigher at a higher ¯ow rate. This also suggests thatthe SWNT growth process continues to occur even100 ms after the laser ablation. Thus the growthperiod is long enough for the ¯ow rate to signi®-cantly in¯uence the relative yields of SWNTshaving di�erent diameters, especially when thegrowth species ¯ow through a large temperaturegradient.

Fig. 4. RBM of SWNTs grown at di�erent target positions and di�erent gas ¯ow rates. The dotted curves represent individual

Lorentzians, and the solid curve represents ®t to the experimental data. Target positions are: (a) 0 mm (furnace center), (b) )40 mm

and (c) )80 mm.

Fig. 5. Scattering images of the laser ablated species recorded

at di�erent time intervals after the ablation pulse. The dashed

line is given as a visual guide, showing the movement of the

species moving backward with time.

R. Sen et al. / Chemical Physics Letters 332 (2000) 467±473 471

Gorbunov et al. [12] studied the e�ect of the¯ow rate of argon gas on the overall yield ofSWNTs; their results suggest that the growth timesare of the order of 1 s. Recent studies of time-re-solved imaging and spectroscopy on the dynamicsof SWNTs growth process [13,14] conclude thatSWNTs' growth occurs in vortexes in long timeperiods (a few ms to a few s). Fig. 4b and c showthat the relative yields of large-diameter SWNTsincrease with increasing ¯ow rate whereas theoverall range of the diameter distribution remainsuna�ected. In other words, the same ®ve types ofSWNTs grow but their relative yields depend onthe temperature of the growth species. Therefore,we conclude here that the initial condensation andnucleation processes, which may determine therange of diameter distribution, occur much faster(<100 ms) but the growth continues for more that100 ms after the ablation. There is a signi®cantchange in the position of the backward movingspecies for di�erent ¯ow rates only after 90±100 msand change in the ¯ow rate changes the relativeyield of SWNTs when a temperature gradient ex-ists in the furnace. This means that the relativeyields of SWNTs can change by a change in thetemperature of the backward moving species even100 ms after the ablation. We cannot estimate theupper limit of the growth period of SWNTs at thismoment, but our preliminary results indicate thatSWNTs can grow even 500 ms after the laser ab-lation.

This estimate is based on our recent experi-ments conducted by placing the target at )150mm. At this position, the ablation at a ¯ow rateof 0:55 cm3=s did not yield any SWNT, butSWNTs were obtained at 1:66 cm3=s and above.Within 10 ms of the laser ablation the particlesmoved to )190 mm position in these ¯ow ratesand then started moving backwards with di�erentvelocities for di�erent ¯ow rates. From a velocityof 123 mm/s corresponding to the ¯ow rate of1:66 cm3=s we estimate that the backward movingspecies are located around )130 mm after 500 ms.The temperature at this position, �850°C, isconsidered to be the threshold temperature abovewhich SWNTs grow. We therefore conclude thatSWNT can grow even 500 ms after the laserablation.

Let us again take a look at Fig. 3, which showsthe diameter distribution of SWNTs grown atdi�erent target positions under the same ¯ow rate�0:55 cm3=s�. The relative yields of SWNTs havingdi�erent diameters grown at target positions of�80 and )20 mm are qualitatively similar, butFig. 2(b) shows that the temperatures at thesepositions are signi®cantly di�erent. The similarityin the relative yields can be explained as follows:Within 10 ms after the laser ablation the speciesmove 40 mm in the forward direction but afterthat they start moving backwards (see panel for0:55 cm3=s in Fig. 5). From the velocity of thebackward moving species, estimated to be 38 mm/s,they are expected to move 4±11 mm downstreamafter a time interval of 100±300 ms. When thetarget is placed at )20 mm, the species are at )56to )49 mm position, and when the target is placedat �80 mm the species are at �44 to �51 mm aftera time interval of 100±300 ms. From Fig. 2b, thetemperatures at these positions are similar andthus the relative yields of SWNTs having di�erentdiameters are also similar. More quantitatively,the )20 mm case shows slightly higher yields oflarge-diameter SWNTs (23%) as compared tothose in the �80 mm case (12%). This di�erencemay arise from the fact that in the )20 mm casethe backward moving species are still moving intothe higher temperature zone even after 300 mswhereas in the �80 mm case they simply move outto the lower temperature zone. It should also benoted that the relative yields of SWNTs havingdi�erent diameters in these target positions are notsimilar when the ¯ow rate is increased. This isbecause at a higher ¯ow rate the species movefurther downstream and the corresponding tem-perature is varied. This result further indicates thatthe growth times for SWNTs are of the order of afew hundred ms.

4. Conclusions

The results of the e�ect of the target positionreveal that the ambient temperature around thetarget is crucial for controlling the relative yieldsof SWNTs having di�erent diameters. Therefore,it is important to have a uniform temperature

472 R. Sen et al. / Chemical Physics Letters 332 (2000) 467±473

distribution around the target for a better controlof the relative yields of SWNTs. The ¯ow rate ofargon gas also a�ects the relative yield of SWNTshaving di�erent diameters, especially if there is alarge temperature gradient around the target.This indicates that the ¯ow rate a�ects the dy-namics of the ablated carbon species involved inthe SWNT growth. Results of scattering experi-ments show that an increase in the gas ¯ow rateincreases the velocity of the backward movingspecies. Therefore, if the target is placed at theupstream positions ()40 or )80 mm), an increasein the gas ¯ow rate will enhance the velocity withwhich the growth species move to the highertemperature zone. Thus the e�ective temperatureof the growth species will increase, and this willresult in higher yields of large-diameter SWNTs.Since the overall range of the diameter distribu-tion stays essentially invariant while the relativeyields of SWNTs change, we conclude that initialnucleation process occurs much faster whereasthe growth process of the SWNTs takes muchlonger time.

Acknowledgements

This work was supported by the Japan Societyfor the Promotion of Science, Research for theFuture Program. Partial support was obtainedfrom the Grant-in-Aid for Scienti®c Research onthe Priority Area `Fullerenes and nanotubes' bythe Ministry of Education, Science, Sports andCulture of Japan.

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