PERGAMON Carbon 38 (2000) 1445–1451

Gas and pressure effects on the production of single-walledcarbon nanotubes by laser ablation

a a , a a b*˜ ´E. Munoz , W.K. Maser , A.M. Benito , M.T. Martınez , G.F. de la Fuente ,c d d dY. Maniette , A. Righi , E. Anglaret , J.L. Sauvajol

a ´ ´Instituto de Carboquımica, CSIC, Marıa de Luna 12, 50015 Zaragoza, Spainb ´ICMA, CSIC-Universidad de Zaragoza, Marıa de Luna 3, 50015 Zaragoza, Spain

c ´ `Serveis Cientıfico-Tecnics, Universitat de Barcelona, 08028 Barcelona, Spaind ´GDPC, Universite de Montpellier II, 34095 Montpellier Cedex, France

Received 2 November 1999; accepted 13 December 1999

Abstract

We report the production of single-walled carbon nanotubes (SWNTs) under argon, nitrogen and helium atmospheres inthe pressure range of 50 to 500 Torr using a continuous wave 10.6-mm CO -laser. Both argon and nitrogen can be equally2

used as buffer gas to synthesize SWNTs in high yield between 200 and 400 Torr. Below 200 Torr the amount of SWNTs inthe soot is drastically decreased and the material is dominated by amorphous carbon. Negligible amounts of SWNTs wereformed using helium. These gas and pressure effects are assigned to changes on the local temperature environment. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Carbon nanotubes; B. Laser irradiation; C. Electron microscopy, Raman spectroscopy

1. Introduction nitrogen atmospheres in the pressure range between 200and 400 Torr. Below 200 Torr the soot is dominated by

One efficient method to produce single-walled amorphous carbon. On the other hand, helium does notnanotubes (SWNTs) is the ablation of heated graphite induce favorable conditions for the growth of SWNTs.targets by the pulsed Nd:YAG laser vaporization technique These gas and pressure effects are discussed in terms of[1,2]. Recently, we have shown that high yields of SWNTs changes on the local conditions near the evaporation zone,can also be synthesized by using a very simple CO -laser i.e. specifically to changes on the local temperature en-2

system operating in continuous wave (cw) mode [3,4]. vironment.Here, the absence of an external furnace around thegraphite target clearly shows that the local conditions nearthe evaporation zone induced by this laser radiation are 2. Experimentalplaying an important role in the formation process ofSWNTs. Therefore this system is ideally suited for study- A CO -laser operating in continuous wave mode at2ing the parameters important for the growth of SWNTs and 10.6 mm was employed and focused onto a graphite–their still not well understood growth mechanism. Ni(4.2 at.%) /Y(1 at.%) composite target rod (spot size:

2 2This letter describes the influence of different buffer |0.8 mm ; power density: 12 kW/cm ), as reported ingases (argon, nitrogen and helium) in the pressure range previous studies [3,4]. Experiments were carried out underfrom 50 to 500 Torr on the formation of SWNTs using the static atmospheres of argon and nitrogen (50, 100, 200,above mentioned cw CO -laser system. High yields of 300, 400 and 500 Torr), and of helium (400 Torr). The2

SWNTs material can be produced under both argon and produced carbonaceous material was collected on anentangled copper wire that filled the evaporation chamber,as well as on the reactor’s walls. Characterization measure-*Corresponding author. Tel.: 134-976-733-977; fax: 134-ments of the as-produced material were performed using976-733-318.

E-mail address: wmaser@carbon.icb.csic.es (W.K. Maser). scanning electron microscopy (SEM) (JEOL JSM-6400),

0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 99 )00277-8

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transmission electron microscopy (TEM) (Philips CM30, For both buffer-gases, argon and nitrogen, we observe the300 kV), and micro-Raman spectroscopy (argon ion laser same pressure behavior. Furthermore, no difference con-

2with l 5514.5 nm, power density: ,50 mW/mm , cerning quantity and quality has been observed performingexc

collection: Jobin-Yvon T6400 spectrometer, resolution: 2 experiments under either dynamic or static conditions.21cm ). In order to get a representative picture of the However, using helium as buffer gas, a powdery soot

sample, the Raman spectra were collected on different material containing no significant amount of filaments andareas on each sample, and then summed and averaged. SWNTs was produced as seen by electron microscopy

(SEM and TEM, respectively). Therefore, our electronmicroscopy results demonstrate again that the existence of

3. Results and discussion high amounts of SWNTs homogeneously distributed in thesoot material seems to be connected to its web-like

Laser-produced plumes of about 0.5–1 cm long were appearance.generated when the CO -laser ablation processes were More insights into the gas and pressure effects on the2

carried out under argon and nitrogen atmospheres ranging formation of SWNTs can be obtained by Raman spec-from 200 to 500 Torr. The materials synthesized under troscopy (Fig. 3). Fig. 3a shows the Raman spectra of thethese experimental conditions were collected mainly on the samples produced under argon atmospheres. For allcopper wire system in the form of a filamentous soot, pressures, one can easily find a broad band consisting of

21which exhibited a web-like texture. These web-like col- eight to nine peaks between 120 and 200 cm . Thislected materials were particularly long and abundant when clearly indicates the presence of SWNTs with a diameteroperating between 300 and 400 Torr. SEM and TEM distribution ranging from 1.1 to 1.7 nm [7], where thestudies on samples produced between 200 and 500 Torr amount of large diameter SWNTs is significant. Peaks in(Fig. 1a–d, and Fig. 2a) reveal that very high yield the intermediate frequency region (not shown here) located

21productions of SWNT material were achieved under both between |750 and 970 cm suggest the presence of aargon and nitrogen atmospheres. It seems that within this mixture of SWNTs with different chiralities [8,9]. Finally,pressure interval the most suitable conditions for the in the high frequency region, the strongest band with a

21formation of SWNTs can be found. From several SEM peak at ¯1588 cm can be seen. The intensity of all theseimages the yield of SWNTs was estimated to be around 60 described features, which is more or less directly related toto 80 vol.% with respect to the total collected carbonace- the amount of SWNTs in the samples, is found to beous material. TEM micrographs (Fig. 2a and b) show that slightly pressure dependent with a maximum between 200the SWNTs appear mainly self-organized in bundles with and 400 Torr. The low frequency SWNT peaks of thediameters ranging from 10 to 20 nm. Many thinner bundles samples produced at 500 and 100 Torr are around 50% andemerge from thicker ones, that could in turn be composed 30% lower in intensity, respectively, while the signal forof several bundles stuck together next to each other. the 50 Torr sample (Fig. 3a, inset) is almost negligible andAccording to these SEM and TEM studies, it is difficult to indicates significant amounts of disordered carbon.estimate the length of these bundles, however, they should The Raman results of samples produced under nitrogenbe of the order of one micron, according to atomic force atmospheres are presented in Fig. 3b. These spectra, whosemicroscopy studies performed on SWNT samples pro- intensities are in general somewhat lower than the onesduced under argon at 400 Torr by this laser ablation synthesized under argon as buffer-gas, show more or lesstechnique [5]. the same broad diameter distribution and behavior, i.e.

On the other hand, working below 200 Torr, more pressure dependence. Only the sample produced at 100diffuse plumes, and occasionally very long ones, were Torr of nitrogen seems to be an exception. Here, thegenerated. When the laser ablation processes took place intensities of the Raman features assigned to SWNTs areunder 50 and 100 Torr argon and nitrogen atmospheres, the significantly lower than those that correspond to nanotubesmaterial produced was a powdery soot containing very low contained in materials synthesized under argon at the samedensities of nanotubes, according to SEM observations pressure. The mean peak in the low frequency region of(Fig. 1e and f). A similar pressure threshold has been the Raman spectrum of the sample produced under nitro-

21reported by Yudasaka et al. [6]. These SEM micrographs gen at 100 Torr appears at |180 cm , which correspondsshow that the synthesized material has a different appear- to SWNTs having diameters of |1.2 nm [7]. In comparisonance. The observed filamentous structures seem to be to the materials produced at higher pressures, the diametercovered almost completely by agglomerates of other distribution in this sample is narrower, and the contributionspecies, probably less organized carbon structures. TEM of tubes having big diameters smaller. Unfortunately, duemicrographs reveal that in the sample materials produced to the weak intensity and the disappearance of the lowat 100 Torr, areas containing high densities of SWNTs can frequency peak structure in the 50 Torr sample, it remainsbe found only occasionally (Fig. 2b) while in the case of unclear if this change in the diameter distribution belowthe 50 Torr samples areas of amorphous carbon are 200 Torr represents a general trend in the case of nitrogen.dominating and almost no SWNTs have been observed. Altogether, the Raman results reflect well the former

˜E. Munoz et al. / Carbon 38 (2000) 1445 –1451 1447

Fig. 1. Typical SEM micrographs of the as-produced nanotube material synthesized by cw CO -laser ablation under static argon (a, c, e),2

and nitrogen (b, d, f) atmospheres, respectively. While high densities of entangled filaments were observed under pressures between 400 (a,b) and 200 (c, d) Torr, only low densities of these filaments were observed in the samples generated under 100 Torr (e, f).

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Fig. 2. TEM micrographs of areas containing high density nanotube material generated by cw CO -laser ablation of a Ni /Y (4.2 /1 at.%)2

target under static nitrogen 400 Torr (a) and argon 100 Torr (b) atmospheres.

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Fig. 3. Original Raman spectra of samples generated under, respectively, argon (a) and nitrogen (b) atmospheres within the pressure range21from 50 to 500 Torr. For clarity, the intermediate frequency region from 300 to 1400 cm has been omitted. The spectra obtained on the 50

Torr samples are additionally presented as inset figures.

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electron microscopy observations: the maximum SWNT and subsequently reduces the efficiency in SWNT forma-tion. This process seems to start at a pressure of 500 Torr.yield can be found in the web-like soot material producedOn the other hand, under very low pressures, the vaporbetween 200 and 400 Torr. There are no obvious differ-plume will expand too rapidly leading to an increasedences between samples produced in this pressure rangecooling rate for the evaporated species and possibly as wellunder argon or nitrogen atmospheres. Below 200 Torr theto a decrease of the concentration of the reacting speciesformation of powdery-like soot material can be related to aresulting in a decreased SWNT yield. This process appar-strong decrease of the amount of SWNTs reaching theently starts at pressure below 200 Torr. Here, long vaporpoint where the samples are dominated by amorphousplumes have been observed and the amount of SWNTs incarbon at 50 Torr. Additionally, no SWNT signal wasthe samples was found to be drastically reduced. Belowobtained from material produced using helium as buffer-200 Torr, the cooling process seems to proceed moregas.rapidly using nitrogen as buffer gas, as can be seen byThe cw CO -laser leads to the evaporation of target2comparing the Raman intensities of the spectra for argonspecies and heats at the same time the target rod. Tempera-and nitrogen at 100 and 50 Torr. Furthermore, the fact thattures of 12008C have been measured in a zone 1 cmthe 100 Torr nitrogen sample contains larger amounts ofaround the laser spot (when operating under a 400 Torrtubes with smaller diameters could be related to theargon atmosphere). The heat radiation from this zoneobservation of other authors [2,7,13] that with lowerreduces the temperature gradient experienced by thetemperature of the growth environment the diameterevaporated species. This helps to keep the environment atdistribution shifts to smaller values. Our results thereforetemperatures favorable for the growth of SWNTs. There-show that the employed gas and its pressure affect both thefore, the local conditions i.e., especially the temperaturetemperature of the tube growth environment and theand the temperature gradient near the evaporation zone,confinement of the evaporated species in the plasma

play an important role in the formation process of SWNTsplume, playing a crucial role in the SWNT formation.

[4]. The observed gas and pressure effects can be directlyrelated to the change of these local conditions. Sincelighter gases are more efficient for rapid cooling than

4. Conclusionheavier ones (proportional to 1 /m where m correspondsa a

to the molecular weight) [10,11] the temperature gradient, In this letter we have shown that both argon andi.e. the cooling rate, becomes too large to favor the SWNT nitrogen can be equally used as the buffer gas in the cwformation. This is the case when helium with a molecular CO -laser ablation technique to produce high yields of2weight of m 54 a.m.u. is used as buffer-gas. On the othera SWNTs. Here, the optimum pressure range lies betweenhand, with a molecular weight of m 540 a.m.u., argon isa 200 and 400 Torr. Below this pressure interval the amountable to reduce the temperature gradient and creates favor- of SWNTs gets drastically reduced. Negligible amounts ofable conditions for the formation of SWNTs. Nitrogen SWNTs have been formed using helium as ambient gas.with a molecular weight of m 528 a.m.u., also leads toa All these observations, i.e. the influence of the nature oftemperature gradients still small enough to produce SWNT the gas and its pressure on the formation of SWNTs, canyields comparable to the ones obtained for argon, as has be consistently explained by correlating these two parame-been observed in our measurements. This as well might be ters directly with the change of the temperature anda hint that not too many nitrogen molecules dissociate, concentration of the species contained in the vapor plumebecause this would result in an increased cooling rate [11]. and the temperature gradient they experience. These resultsEELS experiments are currently being carried out in order further corroborate the importance of the local conditionsto see if nitrogen atoms have been formed during the near the evaporation zone for the formation of SWNTs.process and incorporated into the carbonaceous network ofthe samples, as has been recently reported by other authors[12]. Acknowledgements

The gas pressure is also acting on the temperature andtemperature gradient. A pressure between 200 and 400 This work was supported by the DGICYT (ProjectTorr restricts the expansion of the vapor plume and by PB94-0224). E.M. acknowledges funding from the De-avoiding excessive cooling of the evaporated species high ´partamento de Educacion y Cultura de la Comunidadyields of SWNTs can be produced. The results shown here ´ ´Autonoma de Aragon.suggest that in this range of pressures an optimum confine-ment in the vapor plume is achieved, which provides themost suitable temperature and concentration conditions of Referencesthe evaporated species leading to high SWNT yields.Increasing the pressure will result in a too high collision [1] Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J et al.probability of the evaporated species with the ambient gas Crystalline ropes of metallic carbon nanotubes. Sciencemolecules resulting therefore in an increased cooling rate, 1996;273:483–7.

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