Thin Solid Films 457(2004) 7–11

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2003.12.033

Optical measurement in carbon nanotubes formation by pulsed laserablation

Tomoaki Ikegami, Futoshi Nakanishi*, Makoto Uchiyama, Kenji Ebihara

Graduate School of Science and Technology, and Department of Electrical and Computer Engineering, Kumamoto University,2-39-1 Kurokami, Kumamoto, 860-8555, Japan

Abstract

Carbon nanotubes(CNTs) were produced by laser ablation of a graphite composite target in argon and nitrogen ambient gas.To investigate the effect of nitrogen gas on CNTs formation, the plasma plume was examined using optical emission spectroscopy.The vibrational temperature of C molecules was estimated by fitting of a Swan band spectrum. The temperature in N ambient2 2

gas is lower than that in Ar ambient gas. In a nitrogen atmosphere, the spectrum intensity of C Swan band was enhanced and2

CN violet system was also observed. Soot collected in the reaction tube was observed using FE-SEM and TEM. The sootdeposited in the nitrogen gas contained more bundled CNTs than those in Ar ambience.� 2003 Elsevier B.V. All rights reserved.

Keywords: Pulsed laser ablation; Carbon nanotubes; Nitrogen; C Swan band2

1. Introduction

Since their discovery by Iijima from the soot of anarc process used for making fullerenesw1x, carbonnanotubes(CNTs) have been the focus of considerableattention, because of many possible applications innanostructurew2x, super-strong materialsw3x and hydro-gen storagew4x. Several process methods such as arcdischarge, chemical vapor deposition(CVD) and laserablation have been used to produce CNTs. In particular,pulsed laser ablation(PLA) is well known to producehigh yield single wall carbon nanotubes(SWNTs) w5x.In production of SWNTs, a graphite composite target

containing Ni and Co as catalyst have been ablated bya pulsed Nd:YAG laser in argon ambient gas. So far,there has been much research on this process. Scott etal. reported the growth mechanism for SWNTs usingspectral emission and laser-induced fluorescence meas-urement of plasma plume from a composite carbontarget w6,7,10x. Yudasaka et al. studied Raman spectraof SWNTs grown by laser ablation at different ambientgas pressuresw8x. Recently Nishide et al.w9x reported

*Corresponding author. Tel:q81-96-342-3620; fax:q81-96-342-3630.

E-mail address: f-naka@st.eecs.kumamoto-u.ac.jp(F. Nakanishi).

that SWNTs can be produced in nitrogen ambient gasat higher yield than in an argon atmosphere. Moreover,it was found that SWNTs produced in nitrogen gas werethinner than those formed in other gases. However, adetailed growth mechanism for SWNTs under nitrogenambient gas has not been investigated. In the presentwork, optical emission spectroscopy was performed ona plasma plume produced by the laser ablation toinvestigate the effect of nitrogen gas on formation ofSWNTs. Collected soot in the reactor was examinedusing scanning electron microscopy(SEM) and trans-mission electron microscopy(TEM). The results wereanalyzed and compared with those measured in argonambient gas.

2. Experiment

Fig. 1 shows an experimental set-up for CNT forma-tion by PLA and an optical measurement of the plasmaplume. A graphite composite target rod containing 1at.% nickel(Ni) and cobalt(Co) was used. The targetset in a quartz glass tube(i.d. 36 mm) is ablated by apulsed Nd:YAG laser. The repetition rate, wavelengthand energy of the laser are 10 Hz, 532 nm(secondharmonic light), and 240 mJypulse, respectively. Laser

8 T. Ikegami et al. / Thin Solid Films 457 (2004) 7–11

Fig. 1. Experimental set-up.

fluence at the target surface is approximately 2.6 Jycm . The quartz tube is heated to 1273 K by an openable2

and closable electric furnace and filled with pure argonor nitrogen gas. A water-cooled copper block was set inthe quartz tube at the exit of the furnace behind thetarget to collect CNTs. During ablation, gas pressure iskept at 80 kPa(600 Torr) and the gas flow rate is 100sccm. The gas flow velocity at a heated zone in thequartz tube is approximately 0.7 cmys.Spontaneous emission from the plasma plume near

the target surface was collected by a lens and transferredto a spectroscope(Acton Research Corp., SpectraPro-308i) via an optical fiber. Spectrum images of thespectrometer were observed using an ICCD detector(Princeton Instruments, ITEyCCD-576G). A delayedpulse generator was use to trigger the laser and to gatethe ICCD detector. The ICCD was exposed for 100 nsat 100 ns to 20ms after laser ablation. The plasmaplume was observed at 1 mm from the target surface byopening one side of the furnace by 2–3 cm. Usingmeasured spontaneous emission spectra, vibrational tem-peratures of C molecules were estimated by fitting2

Swan band spectrum calculated theoretically. The sootin the reactor tube were observed without purificationby FE-SEM and TEM to compare the SWNTs producedin ambient argon gas with those in nitrogen gas.

3. Measurements and discussion

3.1. SEM and TEM images of soot

Fig. 2 shows the images of FE-SEM(a,b,c,d) andFE-TEM (e,f) of the soot deposited at the collector afterlaser ablation in argon gas(a,b,e) and in nitrogen gas(c,d,f). Since an amount of the amorphous carbon foundin Fig. 2c,d is less than that in Fig. 2a,b, the production

yield of SWNTs in nitrogen ambient gas seems to belarger than that in argon ambient gas.From TEM images, the CNT bundles consisted of

some SWNTs of approximately 1 nm in diameter. It isdifficult to discuss the influence of ambient gas on theCNT diameter using the low-resolution TEM images.Raman spectra measurement may be necessary for fur-ther discussion.

3.2. Spontaneous emission from the carbon plume

Figs. 3 and 4 show the spontaneous emission spectrumobserved in argon and nitrogen ambient gas at varioustime delays, respectively. Each spectrum is measured ata delay time of(a) 100 ns,(b) 500 ns,(c) 1 ms, (d) 5ms, (e) 10 ms, and(f) 20 ms, is obtained by integrationof 20 spectra. In Fig. 3, C a Swan band spectrum2

(a P –d P ) consists of various vibrational transitions,3 3u g

450–470 nm(Dnsy1), 480–520 nm(Dns0) and520–570 nm(Dns1), clearly recognized. The intensitydecreases rapidly after 1ms. In Fig. 4 for nitrogenambient gas, the intensity of the C Swan band spectrum2

is larger than those in argon gas, and the CN violetsystem is also observed at approximately 360–390 nm.It indicates that C molecules are excited by nitrogen2

molecules more effectively than argon gas, and CNmolecules are formed by the reaction between the Cand N atoms produced by dissociation of the nitrogenmolecule.In Fig. 5 C Swan band spectrum(Dns0) from the2

plasma plume in argon ambient gas observed at 100 nsafter laser ablation is shown(thick line). In this figure,to estimate the vibrational temperature, C Swan band2

(Dns0) spectrum calculated for 6700 K(thin line) isfitted to measured data. Fig. 6 shows the C vibrational2

temperatures estimated by the method mentioned aboveas a function of time from laser ablation. Within 1msafter ablation, estimated C temperatures for both argon2

and nitrogen ambient gases are over 8000 K, which ishigher than the furnace temperature. In argon ambientgas, C temperature increases until 300 ns and then2

decreases. In nitrogen gas, an initial temperature ishigher and decreases faster than that in argon atmos-phere. This seems to be explained that C molecules2

excited by laser loose less energy by collision withnitrogen molecules of lighter mass than argon at earlystage. At a later phase C molecules seem to lose much2

energy due to inelastic collisions with reactive nitrogengas.

4. Conclusions

Spontaneous emission spectra of the plasma plumeare measured by ablating a graphite composite target in

9T. Ikegami et al. / Thin Solid Films 457 (2004) 7–11

Fig. 2. SEM images of soot deposited at 600 Torr of argon ambient gas(a,b), nitrogen ambient gas(c,d) and TEM images of soot of argonambient gas(e) and nitrogen ambient gas(f).

argon and nitrogen ambient gas. In both gases, C Swan2

band spectra were dominant and the CN violet systemwas observed in nitrogen atmosphere. It was foundnitrogen gas enhanced the C emission and decreased2

vibrational temperature of C molecules after ablation.2

From the results of SEM images of soot, amorphousphase contained in the soot in the nitrogen ambient gaswas less than those in argon gas. PLA in nitrogen

10 T. Ikegami et al. / Thin Solid Films 457 (2004) 7–11

Fig. 3. Spontaneous emission spectra measured at various delay times after laser ablation in argon ambient gas:(a) 100 ns,(b) 500 ns,(c) 1 ms,(d) 5 ms, (e) 10 ms and(f) 1 ms.

Fig. 4. Spontaneous emission spectra measured at various delay times after laser ablation in nitrogen ambient gas:(a) 100 ns,(b) 500 ns,(c) 1ms, (d) 5 ms, (e) 10 ms and(f) 1 ms.

11T. Ikegami et al. / Thin Solid Films 457 (2004) 7–11

Fig. 5. Spontaneous emission spectrum in argon ambient gas, 100 nsafter laser ablation(thick line) and C Swan band spectrum(Dns0)2

calculated for vibrational temperature of 6700 K(thin line).

Fig. 6. Estimated vibrational temperatures in argon and nitrogen ambi-ent gas at various delay times after laser ablation.

ambient gas seems to be more effective than in argongas atmosphere from the viewpoint of suppressing amor-phous carbon formation.

Acknowledgments

The authors would like to thank Professor M. Nishidaand Mr M. Matsuda of the Department of MaterialScience for TEM measurement, and Mr K. Sakamotoand Kumamoto Industrial Research Institute for SEMmeasurements.

Referencesw1x S. Iijima, Nature 354(1991) 56–59.w2x P.M. Ajayan, O. Stephan, Ph. Redlich, C. Coltrex, Nature 375

(1995) 564.w3x R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties

of Carbon Nanotubes, Imperial College Press, London, 1998.w4x C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S.

Dresselhaus, Science 286(1999) 915–926.w5x T. Guo, P. nikolaev, A. Thess, D.T. Colbert, R.E. Smalley,

Chem. Phys. Lett. 243(1995) 49–54.w6x C.D. Scott, S. Arepalli, P. Nikolaev, R.E. Smalley, Appl. Phys.

A 72 (2001) 573–580.w7x S. Arepalli, C.D. Scott, Chem. Phys. Lett. 302(1999) 139–145.w8x M. Yudasaka, T. Komatsu, T. Ichihashi, Y. Achiba, S. Iijima,

J. Phys. Chem. B 102(1998) 4892.w9x D. Nishide, H. Kataura, S. Suzuki, K. Tsukagoshi, Y. Aoyagi,

Y. Achiba, Chem. Phys. Lett. 372(2003) 45–50.w10x S. Arepalli, P. Nikolaev, W. Holmes, C.D. Scott, Appl. Phys.

A 70 (2000) 125–133.