The nucleation and growth of aligned multiwall carbon nanotubes by microwave plasma-enhanced chemical vapor deposition have been studied. The nanotubes nucleate and grow from catalytic cobalt islands on a silicon substrate surface, with both their diameter and length dependent on the size of the cobalt islands. Electron microscopy reveals that the nanotubes grow via a “base growth” mechanism. The nanotubes grow initially at a very rapid and constant rate (∼100 nm/s) that decreases sharply after the catalyst Co particles become fully encapsulated by the nanotubes. We propose a detailed model to explain these experimental observations on nucleation and growth of nanotubes

Bower, Chris

Jin, Sungho

Werder, D. J.

Zhou, Otto

Zhu, Wei

Carolina Digital Repository

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 17 23 OCTOBER 2000Nucleation and growth of carbon nanotubes by microwave plasmachemical vapor depositionChris Bowera) and Otto Zhoub)Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599Wei Zhu, D. J. Werder, and Sungho JinBell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974~Received 23 May 2000; accepted for publication 29 August 2000!The nucleation and growth of aligned multiwall carbon nanotubes by microwave plasma-enhancedchemical vapor deposition have been studied. The nanotubes nucleate and grow from catalyticcobalt islands on a silicon substrate surface, with both their diameter and length dependent on thesize of the cobalt islands. Electron microscopy reveals that the nanotubes grow via a ‘‘base growth’’mechanism. The nanotubes grow initially at a very rapid and constant rate~;100 nm/s! thatdecreases sharply after the catalyst Co particles become fully encapsulated by the nanotubes. Wepropose a detailed model to explain these experimental observations on nucleation and growth ofnanotubes. ©2000 American Institute of Physics.@S0003-6951~00!00443-5#oertcandvtalannnsnf-gaerVsatumsuaathnedtivevreick-pro-ybecle-VDerr toltlestoe,ers.dno-i-isfi-li-Carbon nanotubes with different structure and morphogy can now be fabricated with several techniques. Expments performed, mostly oni dividual nanotubes, show thathey have extraordinary electrical and mechaniproperties.1 To investigate their macroscopic properties apotential technological applications it is essential to hananotube materials with controlled diameter, length, oriention, location, and microstructure. Although the laser abtion method2 can produce high quality single-walled carbonanotubes with some control in diameter and electroproperties,3 the process is not compatible with conditiorequired for device fabrication.Considerable progress has been made in fabricatioaligned multiwalled nanotubes~MWNTs! by chemical vapordeposition~CVD! methods.4–7 Although these processes difer in growth conditions such as substrate, catalyst, feedand temperature, the growth model proposed by sevgroups essentially adopts the established concepts of Ccarbon fiber growth developed in the 1970s.8,9 It is believedthat nanotubes grow as carbon precipitates from a superrated metal catalyst that resides at either the base~i.e., ‘‘basegrowth’’! or the tip of a growing nanotube~i.e., ‘‘tipgrowth’’!. Catalyst–substrate interactions and temperagradients across the catalyst particle are considered toimportant factors that determine the growth mechanisHowever, most of these models were proposed withoutficient and systematic supporting experimental evidence,they often lacked details about the physical mechanismsthe effects of various process parameters. In addition,kinetics of nanotube nucleation and growth is largely uknown.We have previously reported the growth of alignMWNTs using microwave plasma-enhanced CVD.10 Herewe present a mechanistic study of the nanotube nucleaand growth and the kinetics involved in the microwaplasma CVD process. We found that the nanotubes growa!Electronic mail: cbower@bell-labs.comb!Curriculum in Applied and Materials Sciences.2760003-6951/2000/77(17)/2767/3/$17.00Downloaded 24 Feb 2003 to 132.239.1.230. Redistribution subject to Al-i-le--icofs,alDtu-rebe.f-ndnde-oniaa ‘‘base growth’’ mechanism in our CVD system, and theis a strong correlation between the catalyst metal layer thness and the nanotube diameter. We observed that thecess kinetics involved a very rapid initial growth followed ba saturation period in which nanotube growth appears tostunted. We propose a four-step model of nanotube nuation and growth to explain the experimental data.The nanotubes were grown in a microwave plasma Csystem connsisting of a 2.45 GHz 5 kW microwave powsupply and an inductively heated substrate stage. Priogrowth, a thin film~2–60 nm! of metal catalyst~cobalt! wassputter deposited onto a silicon~100! substrate. Scanningelectron microscopy~SEM! showed that the sputtered cobafilms were continuous with only a few pinholes. The sampwere transferred~in air! to the growth chamber and heated825 °C in flowing hydrogen~200 sccm, 20 Torr!. A 1 kWmicrowave plasma was subsequently ignited. At this timammonia (NH3) was introduced to completely replace thhydrogen, and acetylene (C2H2) was added shortly afte~;0–5 min! to start the nucleation and growth of nanotubeTypically, a deposited film could be visually observe5–10 s after introduction of the acetylene. The carbon natube films were dull black in color and could be grown unformly over 4 in. silicon wafers. Figure 1~a! shows a cross-sectional SEM image of a highly oriented MWNT filmdeposited for 3 min with an initial cobalt layer of 2 nm. Thnanotube film was grown at 825 °C and 20 Torr with a 3 to 1~150:50 sccm! ammonia to acetylene ratio. Higher magniFIG. 1. ~a! Aligned 30-nm-diameter MWNTs grown on cobalt coated sicon. The initial cobalt layer thickness was 2 nm.~b! MWNTs grown whenthe initial cobalt layer thickness was 20 nm.7 © 2000 American Institute of PhysicsIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspeo-gtrenmnt-untigshisobthamireveiluiohllpethdeseef ttuaDrt-aln,n–ac-lystlystbyngliodinrewanhatttomasimertheaftertrateetubeoc-sedonno-medelyfedofein.f them2768 Appl. Phys. Lett., Vol. 77, No. 17, 23 October 2000 Bower et al.cation SEM and transmission electron microscopy~TEM!studies showed that the nanotubes had a narrow diamdistribution around 30 nm and were about 12mm in length.For comparison, Fig. 1~b! shows a MWNT film grown for 10min with an initial cobalt film thickness of 20 nm. The nantubes deposited here had larger diameters, shorter lenand were substantially less uniform in orientation, compato the nanotubes deposited on the 2 nm cobalt film showFig. 1~a!.The initially continuous sputter-deposited cobalt filwas seen, prior to the nanotube growth, to transform ithree-dimensional islands during the temperature rampheating process. The size of the Co islands was dependethe initial thickness of the sputtered Co, as shown in F2~a! and 2~b!. Thicker cobalt films~.10 nm! yielded largerCo islands on average with a broader size distribution, wthinner cobalt films~,10 nm! resulted in smaller Co islandon average with a much narrower size distribution. Theserved sizes of Co islands and their distributions prior tonanotube growth were consistent with the nanotube dieters and their distributions after the growth, as shownFig. 2~c!. The nanotubes produced using 2 nm of sputteCo had smaller average diameters of nanotubes and anarrow diameter distribution centered around 30 nm, whthe nanotubes grown on the 20 nm sputtered Co had a mlarger average diameter and a wide diameter distributThis result suggests that smaller diameter nanotubes, pereven single-walled nanotubes, could be produced if smacatalytic metal islands can be formed.TEM, performed on nanotubes that had been scrafrom the substrate and then briefly ultrasonicated in menol, showed that the nanotubes were typically compose20–40 concentric graphitic shells. Many of the nanotub~>50%! had ‘‘bamboo’’ type defects11,12along a part of theirlength. Cone-shaped Co particles were found enclowithin the ends of the nanotubes, as shown in Fig. 3~a!. SEMperformed on a sample grown for only 10 s@inset in Fig.3~a!# showed that the Co particles were located at the basthe nanotubes. Cross-sectional TEM, shown in Fig. 3~b!, alsoconfirmed the presence of the Co particles at the base onanotubes, and no Co was ever observed at the nanotips. This is strong evidence for a ‘‘base growth’’ mechnism, similar to that reported by Baker and Harris for CVgrowth of carbon fibers.8 This ‘‘base growth’’ mechanismwas further supported by the experimental evidence, pFIG. 2. SEM images of the Co islands which formed from~a! 2-nm-thickand~b! 20-nm-thick Co layers.~c! A histogram showing the distribution onanotube diameters found with different cobalt layer thickness.Downloaded 24 Feb 2003 to 132.239.1.230. Redistribution subject to Aterhs,dinopon.le-e-ndryechn.apserda-ofsdofhebe-e-sented in our earlier report,10 that a growth process consising of an initial plasma deposition followed by a thermdeposition yielded a straight-to-curly nanotube transitiowhich occurred at the base. Cobalt silicide (CoSix) was ob-served by TEM and cross-sectional SEM at the silicocobalt interface@see the inset of Fig. 3~b!#, indicating that thecobalt reacts with the silicon during the process. This retion might increase the adhesion between the metal cataand the substrate, which could explain why the metal catastays at the base of the nanotube during growth.A growth rate curve, shown in Fig. 4, was obtainedmeasuring the height of nanotubes grown for differiamounts of time~10 s–30 min!, under otherwise identicaconditions. The curve shows a very short incubation per~,10 s! followed by a period~2–3 min! of rapid and con-stant growth, which is then followed by a sharp decreasethe growth rate. During the rapid growth, the nanotubes gat a rate of 100 nm/s. This is roughly 30 times higher tht e growth rate observed with a thermal CVD process.10The decrease in growth rate is attributed to the fact tthe catalytic Co particles became encapsulated at the boof the nanotubes after an extended period of growth. It wnoticed that nanotubes deposited for a short period of t~e.g., less than 3 min! do not contain Co particles within theishells at the bottom ends when observed by TEM; rather,cone-shaped cobalt remained on the substrate surfacethe nanotubes were removed, as shown in Fig. 3~c!. In con-trast, crater shaped marks were left behind on the subssurface after longer-growth nanotubes~i.e., after the decreasin the growth rate occurred! were removed@see Fig. 3~d!#,indicating that the Co particles were enclosed at the nanoends. We postulate that the decrease in the growth ratecurs as a result of the cobalt particle becoming fully enclowithin the carbon layers@such as shown in Fig. 3~a!#. Con-s quently, it became more difficult for the reactive carbspecies to reach the catalyst particle at the root of the natube and continue the growth process through a presudiffusion and precipitation process.The nanotube growth rate was also found to be inversFIG. 3. ~a! TEM micrograph of a cone-shaped cobalt particle fully encloswithin one end of the MWNT. The inset shows a nanotube after 10 sgrowth. ~b! A cross-sectional TEM image of an aligned MWNT film. Thinset shows a cross-sectional SEM image of an aligned MWNT film.~c! ASEM image of the ‘‘scratched’’ surface of a nanotube film grown for 1 mThe inset is a close-up of one of the cone-shaped cobalt particles left osurface.~d! A SEM image of the ‘‘scratched’’ surface of a nanotube filgrown for 10 min.IP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspthehe.isteraeteaanclac-calbasein-ndate.i-n tondsandbes.on-horsareialthbalt-pewthar-s il-ny ofro-hellyandswthryth,byrezup-mH.ek,O..andll,.n,aheo2769Appl. Phys. Lett., Vol. 77, No. 17, 23 October 2000 Bower et al.proportional to the nanotube diameter. Figure 4 showsaverage nanotube height measured for four films that wgrown simultaneously under identical conditions, with tonly variable being the initial thickness of the cobalt filmNote that the cobalt film thickness determines the cobaltland size, which in turn determine the nanotube diameClearly, the smaller diameter nanotubes grew at a fasterin their height, compared to the larger diameter tubes. Thresults seem to suggest a constant mass deposition racarbon for each nanotube, and this constant rate of mdeposition determines the corresponding height that a ntube can reach associated with its diameter.We have developed a step model for nanotube nuation and growth in our microwave plasma CVD system,FIG. 4. ~a! A plot of the nanotube length vs growth time~s! and a plot ofthe nanotube length vs initial cobalt film thickness~l!. The growth ratedata~s! were obtained from samples grown on 2-nm-thick Co films andT5825 °C and P520 Torr, and with a 3:1 ammonia to acetylene ratio. TCo film thickness data~l! were obtained from four samples of varying Cthickness grown at the same time under identical conditions.FIG. 5. Schematic of the proposed four-step growth model.Downloaded 24 Feb 2003 to 132.239.1.230. Redistribution subject to Aere-r.teseofsso-e-sschematically illustrated in Fig. 5. The model takes into acount the formation of catalytic cobalt islands, the chemireactions between cobalt and Si substrate, and thegrowth mechanism involved. It also identifies the determing factors for controlling both the nanotube diameter alength. In step 1, a layer of Co is deposited on a Si substrIn step 2, the Co film on the surface starts to form semspherical islands during heating, driven by surface tensiolower the total energies. These three-dimensional islaprovide the nucleation and growth sites for nanotubes,the size of these islands dictate the diameter of nanotuCobalt silicides are formed at the Co–Si interface that csume a portion of the metallic Co and also serve as ancor adhesion promoters for the Co islands. These silicidesseen by cross-sectional SEM and TEM to be in epitaxrelationship with the Si substrate. During the initial growstage in step 3, nanotubes nucleate and grow from the coislands with plasma-induced alignment,10 likely through car-bon reactions with cobalt~dissolution, saturation, and precipitation!. The cobalt islands transform into a conical shaand are confined at the bottom ends of the tubes. The groof nanotubes continues in length until the conical cobalt pticles are completely enclosed by the nanotube shells, alustrated in step 4. At this time, the growth slows dowdramatically or possibly completely stops.In summary, we have presented a mechanistic studthe nucleation and growth of aligned nanotubes by micwave plasma CVD. We found that the initial thickness of tcatalytic Co layer or the size of the Co islands essentiadictates the nanotube diameters, and that the catalyst islremain at the base of the nanotube throughout the groprocess. We found that nanotubes grow initially at a verapid rate followed by a dramatic decrease in its growapparently after the catalytic particles are fully enclosedthe nanotube shells.The authors acknowledge R. Tung and A. G. Ramifor useful discussions. C.B. acknowledges fellowship sport from NASA. O.Z. acknowledges funding support froONR MURI Grant No. N00014-98-1-0597.1M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund,Science ofFullerenes and Carbon Nanotubes~Academic, New York, 1996!.2A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. TomanJ. E. Fischer, and R. E. Smalley, Science273, 483 ~1996!.3X.-P. Tang, K. A. H. Shimoda, L. Fleming, K. Y. Bennoune, C. Bower,Zhou, and Y. Wu, Science288, 492 ~2000!.4W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, RA. Zhao, and G. Wang, Science274, 1701~1996!.5Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegal,P. N. Provencio, Science282, 1105~1998!.6S. Fan, M. G. Chapline, N. R. Franklin, T. W. Tombler, A. M. Casseand H. Dai, Science283, 512 ~1999!.7J. Li, C. Papadopoulos, J. M. Xu, and M. Moskovits, Appl. Phys. Lett.75,367 ~1999!.8R. T. K. Baker and P. S. Harris, inChemistry and Physics of Carbon,edited by P. L. Walker and P. A. Thrower~Marcel Dekker, New York,1978!, Vol. 14.9G. G. Tibbetts, J. Cryst. Growth66, 632 ~1984!.10C. Bower, W. Zhu, S. Jin, and O. Zhou, Appl. Phys. Lett.77, 830~2000!.11Y. Saito, in Carbon Nanotubes, edited by M. Endo, S. Ijima, and M. SDresselhaus~Elsevier, Oxford, 1996!, p. 159.12H. Cui, D. Palmer, O. Zhou, and B. R. Stoner, inAmorphous and Nano-structured Carbon, edited by J. Sullivan, J. Robertson, O. Zhou, T. Alleand B. Coll@Mater. Res. Soc. Symp. Proc.593, 39 ~1999!#.tIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition

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Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition

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