Ammonia (NH3) gas was thought to be essential for the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) and led to the formation of bamboo-like structures. Here, we show that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixed gases with or without NH3 gas. The growth of these VA-MWCNTs is guided by a growth model that combined the dissociative adsorption of acetylene molecules (C2H2) and the successive vapor-liquid-solid growth mechanism. Results indicate that the key factor for growing these VA-MWCNTs is a balance between the decomposition rate of the C2H2 molecules on the iron catalyst and the subsequent diffusion and segregation rates of carbon

Geohegan, David

Ivonov, Ilia

Kayastha, Vijaya

Pan, Zhengwei

Puretzky, Alex

Yap, Yoke Khin

English

Michigan Technological University

Michigan Technological University Digital Commons @ Michigan Tech Department of Physics Publications Department of Physics 6-15-2005 High-density vertically aligned multiwalled carbon nanotubes with tubular structures Vijaya Kayastha Michigan Technological University Yoke Khin Yap Michigan Technological University Zhengwei Pan Oak Ridge National Laboratory, Ilia Ivonov Oak Ridge National Laboratory, Alex Puretzky Oak Ridge National Laboratory, See next page for additional authors Follow this and additional works at: https://digitalcommons.mtu.edu/physics-fp  Part of the Physics Commons Recommended Citation Kayastha, V., Yap, Y. K., Pan, Z., Ivonov, I., Puretzky, A., & Geohegan, D. (2005). High-density vertically aligned multiwalled carbon nanotubes with tubular structures. Applied Physics Letters, 86(25), 2583105-1-253105-3. http://dx.doi.org/10.1063/1.1952575 Retrieved from: https://digitalcommons.mtu.edu/physics-fp/306 Follow this and additional works at: https://digitalcommons.mtu.edu/physics-fp  Part of the Physics Commons Authors Vijaya Kayastha, Yoke Khin Yap, Zhengwei Pan, Ilia Ivonov, Alex Puretzky, and David Geohegan This article is available at Digital Commons @ Michigan Tech: https://digitalcommons.mtu.edu/physics-fp/306 Appl. Phys. Lett. 86, 253105 (2005); https://doi.org/10.1063/1.1952575 86, 253105© 2005 American Institute of Physics.High-density vertically aligned multiwalledcarbon nanotubes with tubular structuresCite as: Appl. Phys. Lett. 86, 253105 (2005); https://doi.org/10.1063/1.1952575Submitted: 15 March 2005 . Accepted: 13 May 2005 . Published Online: 15 June 2005Vijaya Kumar Kayastha, Yoke Khin Yap, Zhengwei Pan, Ilia N. Ivanov, Alex A. Puretzky, and David B.GeoheganARTICLES YOU MAY BE INTERESTED INControlling dissociative adsorption for effective growth of carbon nanotubesApplied Physics Letters 85, 3265 (2004); https://doi.org/10.1063/1.1806558Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequencyplasmasApplied Physics Letters 87, 173106 (2005); https://doi.org/10.1063/1.2115068The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspendedmembranesApplied Physics Letters 92, 133107 (2008); https://doi.org/10.1063/1.2903702High-density vertically aligned multiwalled carbon nanotubeswith tubular structuresVijaya Kumar Kayastha and Yoke Khin Yapa!Department of Physics, Michigan Technological University, Houghton, Michigan 49931Zhengwei Pan, Ilia N. Ivanov, Alex A. Puretzky, and David B. GeoheganCondensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge,Tennessee 37831-6056 and Department of Materials Science and Engineering,University of Tennessee, Knoxville, Tennessee 37996sReceived 15 March 2005; accepted 13 May 2005; published online 15 June 2005dAmmonia sNH3d gas was thought to be essential for the growth of vertically aligned multiwalledcarbon nanotubes sVA-MWCNTsd and led to the formation of bamboo-like structures. Here, weshow that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixedgases with or without NH3 gas. The growth of these VA-MWCNTs is guided by a growth model thatcombined the dissociative adsorption of acetylene molecules sC2H2d and the successivevapor-liquid-solid growth mechanism. Results indicate that the key factor for growing theseVA-MWCNTs is a balance between the decomposition rate of the C2H2 molecules on the ironcatalyst and the subsequent diffusion and segregation rates of carbon. © 2005 American Institute ofPhysics. fDOI: 10.1063/1.1952575gSince their discovery in 1991,1 multiwalled carbon nano-tubes sMWCNTsd have gained significant research interestdue to their unique structures, properties, and potentialapplications.2 Recently, high-density vertically aligned sVAdMWCNTs have gained attention for innovative applicationssuch as nanotubes membranes,3 nanotubes filter,4 nanotubeyarns,5 three-dimensional microbatteries,6 etc. However, thegrowth mechanism of these VA-MWCNTs on substrates bycatalytic thermal chemical vapor deposition sCVDd is stillnot understood. Recently, catalytic thermal CVD was alsoused for the growth of vertically aligned single wall carbonnanotubes.7The vapor-liquid-solid sVLSd growth mechanism is wellaccepted for the growth of carbon nanotubes.8,9 However, itdoes not provide sufficient details on the decomposition pro-cess of the hydrocarbon molecules on the catalytic nanopar-ticles. The decomposition process is important for under-standing how to maintain the activity of the catalyst. This isimportant for the growth of VA-MWCNTs by the catalyticthermal CVD technique, in which high-density growth sitesare necessary so that van der Waals forces between adjacentMWCNTs can restrict their growth direction toward the freespace, thus achieving vertical alignment. Ammonia sNH3dgas was thought to be essential for the growth of these VA-MWCNTs. However, this approach leads to the formation ofbamboo-like structures,10,11 which are not suitable for appli-cations such as nanotube membranes, filters, and microbat-teries, where tubular structures are required.Recently, we found that carrier gases sAr,N2,H2d canchange the growth mode of MWCNTs from a saturatedgrowth to a continuous growth.12 We have explained thisphenomenon by a growth model that combined the dissocia-tive adsorption of acetylene sC2H2d molecules on iron sFedcatalysts and the successive vapor-liquid-solid sVLSd growthmechanism. This model emphasizes the adsorption of C2H2molecules on the Fe nanoparticles and a charge transfer fromthe molecule to the catalyst that will reduce the energy re-quired for molecular decomposition. The decomposition ratemust be controlled for achieving equilibrium with the ratesof subsequent carbon diffusion into the catalysts and carbonsegregation from the catalysts. Excessive decomposition ratewill cause the formation of amorphous carbon sa-Cd films onthe catalyst surfaces and will prevent further adsorption anddecomposition of C2H2 molecules. Guided by this model, weshow here that high-density VA-MWCNTs with ideal tubularstructures can be grown with and without the use of NH3gas. This result also indicates that the use of NH3 gas is notthe sole factor for the formation of bamboo-like structures.In our experiments, catalytic Fe thin films were coatedon SiO2/Si substrates in a pulsed-laser deposition system.13We used the fourth-harmonic generation of Nd:YAG laserswavelength, l,266 nmd at an energy density of ,1 J cm−2on the Fe target. Depositions were carried out at room tem-perature in a vacuum s,10−5 mbard. The film thickness wasdetermined by using an in situ thickness monitor. Pretreat-ment of these Fe/SiO2/Si substrates was carried out for 15adAuthor to whom correspondence should be addressed; electronic mail:ykyap@mtu.edu.FIG. 1. SEM images of MWCNTs grown on 2.5 nm thick Fe film at sad800 °C, sbd 750 °C, scd 700 °C and scd 650 °C. Scale bar: 3.0 mm. Scalebar for the inset: 7.5 mm.APPLIED PHYSICS LETTERS 86, 253105 s2005d0003-6951/2005/86~25!/253105/3/$22.50 © 2005 American Institute of Physics86, 253105-1minutes in a thermal CVD system at 800 °C. This was donein the flow of hydrogen s270 sccmd and nitrogen s150 sccmdgases, although other research groups prefer to use NH3 gas.The growth gases then replaced the pretreatment gases afterthe system was tuned to the growth temperature. After thegrowth, all samples were examined by scanning electron mi-croscopy sSEMd, transmission electron microscopy sTEMd,and micro-Raman spectroscopy.The VLS mechanism indicates that the growth tempera-ture sTgd will change the diffusion and the segregation ratesof carbon. However, excessive Tg can enhance both thermaldecomposition and dissociative adsorption of hydrocarbonmolecules, which have not been considered in the VLSmechanism. Thus, we scrutinize the effect of Tg by growingMWCNTs for 15 min with 2.5 nm thick Fe films and Ars50 sccmd /C2H2 s70 sccmd growth gases. At Tg=850 and900 °C, random and sparse nanotubes were grown snotshownd. At Tg=800 °C, denser but random MWCNTs weregrown, as shown in Fig. 1sad. A similar growth mode wasdetected for the case of 750 °C, as shown in Fig. 1sbd. At700 °C, the length of the tubes and the degree of their align-ment increased, as shown in Fig. 1scd. This tendency wasenhanced at Tg=650 °C and lead to the growth of VA-MWCNTs, as shown Fig. 1sdd and its inset. We think that atTg.700 °C, thermal decomposition of C2H2 molecules be-comes significant and causes the formation of a-C films onsome catalysts, which prevent continuous dissociative ad-sorption, reduces the growth density, and thus results in ran-dom MWCNTs.Once the vertically aligned MWCNTs were achieved atTg=650 °C, we examined the effect of the catalyst filmthickness. The SEM images of the MWCNTs grown at Tg=650 °C by different Fe film thicknesses are shown in Fig.2. As shown in Figs. 2sad–2scd, the growth rate of the VA-MWCNTs increases with the decrease of Fe film thickness of3.5, 2.5, and 1.5 nm, respectively. According to Fick’s law ofdiffusion, the net flux of carbon atoms scrossing unit area inunit timed is J=−DsDN /dd.14 The minus sign means thatFIG. 2. SEM images of VA-MWCNTs grown at different Fe film thick-nesses: sad 3.5 nm, sbd 2.5 nm, scd 1.5 nm, and sdd 0.5 nm. All samples weregrown at 650 °C for 15 min and their growth rates are summarized in sed.Scale bar: 30 mm.FIG. 3. SEM images of the VA-MWCNTs grown on 1.5 nm thick Fe film atsad 550 °C, sbd 600 °C, scd 650 °C, sdd 700 °C, and sed 750 °C. Allsamples were grown for 15 min and their growth rates are summarizedin sfd. Scale bar: 30 mm.FIG. 4. Patterned growth of VA-MWCNTs with 1.5 nm thick Fe film at650 °C for 30 min. Scale bar for the inset: 20 mm.FIG. 5. Raman spectra for VA-MWCNTs grown with 1.5 nm thick Fe filmat 650 °C by various mixed gases.253105-2 Kayastha et al. Appl. Phys. Lett. 86, 253105 ~2005!diffusion occurs away from regions of high concentration. Dis the diffusivity and DN is the change of carbon concentra-tion at the top and the bottom surfaces of the Fe particles.Both D and DN are constant for identical growth tempera-tures. Thus, faster growth rates are observed in cases withthinner Fe films ssmaller particle diameter dd. Besides, thin-ner catalyst film leads to the growth of thinner nanotubes,which require fewer carbon atoms for the growth of a unittubular length. Thus, faster growth rates are observed incases with thinner Fe films. However, the growth rate is re-duced for the case with 0.5 nm thick Fe film fFig. 2sddg. Thisis an energy-limited phenomenon wherein the growth ratesof thin nanotubes are reduced at low growth temperaturess650 °C in this cased due to the need of higher energy forgrowing smaller cylinders with higher curvature of graphenesheets.15 As summarized in Fig. 2sed, the optimum growthrate at 650 °C occurred for the case of 1.5 nm thick Fe film.We further refine the optimum growth temperature sTgdfor samples grown by using 1.5 nm thick Fe films. Images inFigs. 3sad–3sed correspond to samples grown at 550, 600,650, 700 and 750 °C. VA-MWCNTs were obtained for allcases and their growth rates are summarized in Fig. 3sfd. Asshown, the optimum growth temperature was between 650and 700 °C. The drop of growth rate at 750 °C is due to theadditional thermal decomposition of C2H2 molecules. Thereduced growth rates at 600 and 550 °C are believed to bedue to energy-limited formation at low temperatures, as dis-cussed earlier. At the optimum growth condition sTg=650 °C, Fe films of 1.5 nm thickd, VA-MWCNTs can begrown in desired patterns by using prepatterned catalystfilms. As shown in Fig. 4, VA-MWCNTs are organized intomicrotowers of 60 mm360 mm with uniform heights. Theheight of these microtowers was ,150 mm after growing for30 min.Under this optimum growth condition, we have repeatedthe growth by replacing C2H2/Ar mixed gas with C2H2/H2or C2H2/NH3 mixed gases. High-density VA-MWCNTswere grown in all these cases. We have examined the struc-tures of these VA-MWCNTs by Raman spectroscopy sRen-ishaw 100, He–Ne laser excitationd and TEM sHitachiHF2000d. As shown in Fig. 5, the G band s,1580 cm−1,zone center phonons of E2g symmetryd and the disorder “D”band s,1320 cm−1, K-point phonons of A1g symmetryd havesimilar intensity ratios sIG / IDd, which indicates a similar gra-phitic order.13 This is confirmed by TEM; that is, all theseVA-MWCNTs have an ideal tubular structure. For example,the sample grown by using C2H2/NH3 mixed gas fFig. 6sadgis similar to that grown by C2H2/Ar mixed gas fFig. 6sbdg.This means the use of NH3 gas in the growth is not the solefactor for the formation of the bamboo-like structures. Inaddition, we have examined the growth of MWCNTs bychanging the pretreatment gas from N2/H2 mixed gas to pureNH3 gas, a reported pretreatment procedure that producedbamboo-like nanotubes.10,11 High-density VA-MWCNTswere obtained at our optimum growth condition with variousgrowth gases, such as C2H2/Ar, C2H2/H2 or C2H2/NH3.VA-MWCNTs with tubular structures are revealed in allthese cases.In summary, guided by a growth model that combineddissociative adsorption and the VLS mechanism, high-density VA-MWCNTs can be grown by various types ofgrowth gases and pretreatment gases. Under our optimumcondition, nanotubes with tubular structures are formed withsimilar graphitic orders. The use of NH3 gas either in thegrowth or in the pretreatment process is not the only factorfor the formation of bamboo-like structures.One of the authors sY.K.Y.d acknowledges support fromthe Michigan Tech Research Excellence Fund, the Depart-ment of the Army sW911NF-04-1-0029, through the CityCollege of New Yorkd, and the Center for Nanophase Mate-rials Sciences sCNMSd at Oak Ridge National Laboratory.1S. Iijima, Nature sLondond 354, 56 s1991d.2Carbon Nanotubes: Synthesis, Structure, Properties and Applications, ed-ited by M. S. Dresselhaus and G. Dresselhaus sSpringer, Berlin, 2001d.3B. J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Cavlas, and L. G.Bachas, Science 303, 62 s2004d.4A. Srivastava, O. N. Srivastava, S. Talapatra, R. Vajtai, and P. M. Ajayan,Nat. Mater. 3, 610 s2004d.5M. Zhang, K. R. Atkinson, and R. H. Baughman, Science 306, 1358s2004d.6C. Wang, R. Zaouk, L. Taherabadi, M. Madou, V. Kayastha, and Y. K.Yap, in 206th Meeting of the Electrochemical Society s2004 Joint Inter-national Meetingd, Honolulu, Hawaii, 3–8 October 2004, Symposium Q1,abstract 1492.7K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, and S. Iijima,Science 306, 1362 s2004d.8G. W. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 s1964d.9E. I. Givargizov, in Current Topics In Materials Science, edited by E.Kaldis sNorth-Holland, Amsterdam, 1978d, Vol. I, Chap. 3.10C. J. Lee and J. Park, Appl. Phys. Lett. 77, 3397 s2000d.11Y. T. Lee, N. S. Kim, S. Y. Bae, J. Park, S.-C. Yu, H. Ryu, and H. J. Lee,J. Phys. Chem. B 107, 12958 s2003d.12V. Kayastha, Y. K. Yap, S. Dimovski, and Y. Gogotsi, Appl. Phys. Lett.85, 3265 s2004d.13Y. K. Yap, M. Yoshimura, Y. Mori, and T. Sasaki, Appl. Phys. Lett. 80,2559 s2002d.14C. Kittel, Introduction to Solid State Physics, 6th ed. sWiley, New York,1986d, p. 518.15G. B. Adams, O. F. Sankey, J. B. Page, M. O’Keeffee, and D. A. Drabold,Science 256, 1792 s1992d.FIG. 6. TEM images of MWCNTs grown by sad C2H2/NH3, and sbdC2H2/Ar mixed gases. Scale bar: 5 nm.253105-3 Kayastha et al. Appl. Phys. Lett. 86, 253105 ~2005!

High-density vertically aligned multiwalled carbon nanotubes with tubular structures

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High-density vertically aligned multiwalled carbon nanotubes with tubular structures

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