Plasma-enhanced chemical vapor deposition is the only technique for growing individual vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) at desired locations. Inferior graphitic order has been a long-standing issue that has prevented realistic applications of these VA-MWCNTs. Previously, these VA-MWCNTs were grown by a one-plasma approach. Here, we demonstrate the capability of controlling graphitic order and diameters of VA-MWCNTs by decoupling the functions of the conventional single plasma into a dual-plasma configuration. Our results indicate that the ionic flux and kinetic energy of the growth species are important for improving graphitic order of VA-MWCMTs

Geohegan, David

Ivanov, Ilia

Kayastha, Vijaya

Menda, Jitendra

Pan, Zhengwei

Puretzky, Alex

Ulmen, Benjamin

Vanga, Lakshman Kumar

Yap, Yoke Khin

English

Crossref

Michigan Technological University

Michigan Technological University Digital Commons @ Michigan Tech Department of Physics Publications Department of Physics 10-18-2005 Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas Jitendra Menda Michigan Technological University Benjamin Ulmen Michigan Technological University Lakshman Kumar Vanga Michigan Technological University Vijaya Kayastha Michigan Technological University Yoke Khin Yap Michigan Technological University 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 Menda, J., Ulmen, B., Vanga, L. K., Kayastha, V., Yap, Y. K., Pan, Z., Ivanov, I., Puretzky, A., & Geohegan, D. (2005). Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas. Applied Physics Letters, 87(17), 173106-1-173106-3. http://dx.doi.org/10.1063/1.2115068 Retrieved from: https://digitalcommons.mtu.edu/physics-fp/305 Follow this and additional works at: https://digitalcommons.mtu.edu/physics-fp  Part of the Physics Commons Authors Jitendra Menda, Benjamin Ulmen, Lakshman Kumar Vanga, Vijaya Kayastha, Yoke Khin Yap, Zhengwei Pan, Ilia Ivanov, Alex Puretzky, and David Geohegan This article is available at Digital Commons @ Michigan Tech: https://digitalcommons.mtu.edu/physics-fp/305 Appl. Phys. Lett. 87, 173106 (2005); https://doi.org/10.1063/1.2115068 87, 173106© 2005 American Institute of Physics.Structural control of vertically alignedmultiwalled carbon nanotubes by radio-frequency plasmasCite as: Appl. Phys. Lett. 87, 173106 (2005); https://doi.org/10.1063/1.2115068Submitted: 18 May 2005 . Accepted: 27 August 2005 . Published Online: 18 October 2005Jitendra Menda, Benjamin Ulmen, Lakshman K. Vanga, Vijaya K. Kayastha, Yoke Khin Yap, Zhengwei Pan,Ilia N. Ivanov, Alex A. Puretzky, and David B. GeoheganARTICLES YOU MAY BE INTERESTED INHigh-density vertically aligned multiwalled carbon nanotubes with tubular structuresApplied Physics Letters 86, 253105 (2005); https://doi.org/10.1063/1.1952575Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor depositionApplied Physics Letters 77, 2767 (2000); https://doi.org/10.1063/1.1319529Atomic layer coating of hafnium oxide on carbon nanotubes for high-performance fieldemittersApplied Physics Letters 99, 153115 (2011); https://doi.org/10.1063/1.3650471Structural control of vertically aligned multiwalled carbon nanotubesby radio-frequency plasmasJitendra Menda, Benjamin Ulmen, Lakshman K. Vanga,Vijaya K. Kayastha, and Yoke Khin YapaDepartment 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 37831-6056and Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee37996Received 18 May 2005; accepted 27 August 2005; published online 18 October 2005Plasma-enhanced chemical vapor deposition is the only technique for growing individual verticallyaligned multiwalled carbon nanotubes VA-MWCNTs at desired locations. Inferior graphitic orderhas been a long-standing issue that has prevented realistic applications of these VA-MWCNTs.Previously, these VA-MWCNTs were grown by a one-plasma approach. Here, we demonstrate thecapability of controlling graphitic order and diameters of VA-MWCNTs by decoupling the functionsof the conventional single plasma into a dual-plasma configuration. Our results indicate that theionic flux and kinetic energy of the growth species are important for improving graphitic order ofVA-MWCMTs. © 2005 American Institute of Physics. DOI: 10.1063/1.2115068Carbon nanotubes CNTs are among the important ma-terials for the advancement of future nanoscience and nano-technology. Recently, vertically aligned multiwalled carbonnanotubes VA-MWCNTs have gained additional attentionfor innovative applications, such as nanotubes antennas,1vertical transistors,2,3 and vertical biosensors.4 Ideally, theseapplications require VA-MWCNTs to be grown at desiredpatterns. Plasma-enhanced chemical vapor depositionPECVD is the only technique for growing individual VA-MWCNTs at desired locations. However, the graphitic orderof these VA-MWCNTs is inferior to the multiwall CNTsgrown by arc discharge5 and thermal chemical vapordeposition.6–8 This has been a long-standing issue for realis-tic uses of VA-MWCNTs in applications, such as electronfield emission devices.9–13VA-MWCNTs grown by PECVD are usually called car-bon nanofibers CNFs and have highly distorted structures.Previously, these CNFs were grown by a one-plasmaapproach.14–17 Here, we found that the graphitic order anddiameters of VA-MWCNTs are controllable by decouplingthe functions of single plasma into a dual-plasma configura-tion. Our results indicate that the ionic flux and kinetic en-ergy of the growth species are important for improving thegraphitic order of VA-MWCMTs.By a dual-radio-frequency rf PECVD technique dual-rf-PECVD, we shown that VA-MWCNTs can be grown toan area as of 25 cm2 at substrate temperatures as low as540 °C.18 The dual-rf-PECVD system has a pair of parallelelectrodes. The plasma produced on the top and the bottomelectrodes is referred as the top plasma and the bottomplasma, respectively. The spacing between the two electrodesis 5 cm with the substrate located 3.5 cm from the top elec-trode. This substrate is placed on a heater and electricallycontacted with the bottom electrode. The heater controls thegrowth temperatures by using a proportional-integration-differentiation system.The top plasma ionizes the hydrocarbon gas and deter-mines the ionic density of the growth species. This ionicdensity is controlled by the forward power of a rf generator.Another rf power generator produces the bottom plasma andinduces bias voltages on the substrates and the bottom elec-trode. This bias voltage will accelerate the ionic species tobombard the growth surface and control their kinetic energy.Ni films were used as the catalyst for growing VA-MWCNTs. These films are deposited by pulsed laser depo-sition on Si substrates that have a layer of SiO2 film500 nm. Before the growth of MWCNTs, these Ni filmswere heat treated at 600 °C for 10 min. Methane gas CH4,99.9% flowed at a rate of 350 sccm and resulted in a con-stant pressure of 0.2 mTorr. The two rf plasmas were thengenerated to initiate the growth of MWCNTs at 600 °C. Wechose to use thick Ni films 25 nm for growing thickMWCNTs to enable easier detection of diameter changes ofthe MWCNTs.We have investigated the effect of the top plasma whilekeeping the substrate biasing at −150 V. As shown in Fig. 1,the diameters of the MWCNTs increased accordingly withthe increase in the forward power 50 W to 200 W to thetop plasma. The increase in this power enhances the ionicdensity inside the top plasma. Plasma heating will increasethe surface-free energy ES, in energy per unit area of theNi nanoparticles on the adjacent substrates. The total freeenergy of a cluster can be expressed as E=ESA+EVV,where EV is the volume-free energy in energy per unitvolume, and A and V are the surface area and the volume ofaAuthor to whom correspondence should be addressed; electronic mail:ykyap@mtu.eduFIG. 1. Scanning electron microscope SEM images of VA-MWCNTsgrown at different top plasma forward powers: a 50 W, b 100 W, c150 W, and d 200 W. The substrate biasing is maintained at −150 V forall cases. Scale bar=2 m.APPLIED PHYSICS LETTERS 87, 173106 20050003-6951/2005/8717/173106/3/$22.50 © 2005 American Institute of Physics87, 173106-1the cluster, respectively.19 At stable mode, ES is positivewhile EV is negative in value. As plasma heating increasesthe surface-free energy ES, smaller catalyst particles willdiffuse and merge as larger clusters. This will decrease A andincrease V. Thus, the volume-free energy EVV term willdominate and minimize the total-free energy E becomesmore negative. The formation of large particles will result inthe growth of VA-MWCNTs with larger diameters.We use Raman spectroscopy to examine the graphiticorder of these VA-MWCNTs. This is done by comparing theintensity of the graphitic G and disordered D Ramanbands. The G and D bands represent the zone center phononsof E2g symmetry and the K-point phonons of A1g symmetry,respectively.20 The intensity ratio of these two bands IG / IDis commonly used as a measure of graphitic order of carbon-based materials. All measurements are carried out by a con-focal micro-Raman system, using a HeNe excitation laser=632.8 nm. The Raman spectra for MWCNTs samplesgrown by various top plasma forward power are shown inFig. 2a. The relation of IG / ID to the forward power isshown in Fig. 2b. As shown, the IG / ID ratio increases at aforward power of 200 W, which indicates the enhancementof graphitic order of the VA-MWCNTs.We have then investigated the effect of the bottomplasma by varying the negative dc substrate bias voltagewhile keeping the top plasma forward power at 200 W. Asshown in Fig. 3, an increase in the diameter of the tube isobserved when the biasing varied from Fig. 3a −50 V,Fig. 3b −100 V, to Fig. 3c −150 V. The increase insubstrate biasing enhanced the kinetic energy of the imping-ing ions. The ionic flux near the substrate region will also beincreased. Thus, the energy transfer to the substrate surfacewill be enhanced through ion bombardment. This will in-crease the surface energy of the Ni nanoparticles and inducelarger Ni clusters, as discussed earlier. Thus, the diameters ofthe MWCNTs increased. However, when the bias voltagereaches a level between −150 and −200 V Figs. 3c and3d, a decrease in the diameters of the MWCNTs is de-tected. This can be explained by the sputtering of Ni nano-particles by the energetic ions. The diameters of MWCNTscontinue to decrease at a biasing of −250 V Fig. 3e. NoMWCNTs were formed when the biasing is higher that−350 V, which is the total re-sputtering region.Figure 4a shows the Raman spectra for these VA-MWCNTs. The corresponding relation of IG / ID to the sub-strate biasing is shown in Fig. 4b. As shown, the IG / ID ratiois low 0.48 at low substrate biasing −50 V, but is in-creased to IG / ID of 0.80 at higher substrate biasing. Thisresult indicates that a biasing threshold occurred between−50 to −100 V for growing MWCNTs with high graphiticorder. These are consistent with the obtained transmissionelectron microscope TEM images shown in Fig. 5. At abiasing of Fig. 5a −50 V, the structure of MWCNTs arehighly distorted. Defects/debris at the sidewalls are clearlyFIG. 2. a Raman spectra for VA-MWCNTs grown at various top plasmaforward power and b the corresponding IG / ID relation.FIG. 3. SEM images of VA-MWCNTs grown at different substrate biasing: a −50 V, b −100 V, c −150 V, d −200 V, and e −250 V. The top plasmaforward power is maintained at 200 W for all cases. Scale bar=1 m.FIG. 4. a Raman spectra for VA-MWCNTs grown at various substratebiasing and b the corresponding IG / ID relation.173106-2 Menda et al. Appl. Phys. Lett. 87, 173106 2005shown in the inset. At higher biasing −150 V, MWCNTswith higher graphitic order are grown as shown in Fig. 5b.These MWCNTs have straighter and smoother sidewalls asshown in the inset. This result indicates that the growth spe-cies with a high kinetic energy is important for the formationof MWCNTs with improved graphitic order. We believe thathigh substrate biasing will generate directional ion flux andpromote covalent bonds of carbon along the tubular axis, aswe proposed previously.18 Our results are consistent with thetheory that both the kinetic energy Ek and the potentialenergy U of the growth species are contributing to thestructural properties of solids.21 Structural orders ofMWCNTs will be enhanced when the total energy E=Ek+U of the growth species well exceeds the cohesive energyEc of the sp2-bonded carbon networks 7.4 eV/atom.The exceeding energy E-Ec will be quenched to produceatomic scale heating21 that promotes atomic diffusion andenhances the structural order of VA-MWCNTs. As guided bythese findings, we have then optimized the growth of VA-MWCNTs at thinner catalyst films. For example, at 200 Wof top plasma and substrate biasing of −100 V, straight VA-MWCNTs with uniform diameter 60 nm are grown at450 °C as shown in Fig. 6.In summary, the diameters and graphitic order of VA-MWCNTs can be effectively controlled by the dual-rf-PECVD technique. This is accomplished by adjusting theionic flux and the negative dc bias voltages on the substratesindependently by a dual-rf-plasma approach.One of the authors Y.K.Y. acknowledges support fromthe Michigan Tech Research Excellence Fund, the Depart-ment of the Army Grant No. W911NF-04-1-0029, throughthe City College of New York, and the Center forNanophase Materials Sciences sponsored by the Division ofMaterials Sciences and Engineering, U.S. Department of En-ergy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.1Y. Wang, K. Kempa, B. Kimball, J. B. Carlson, G. Benham, W. Z. Li,T. Kempa, J. Rybczynski, A. Herczynski, and Z. F. Ren, Appl. Phys. Lett.85, 2067 2004.2J. Li, R. Stevens, L. Delzeit, H. T. Ng, A. Cassell, J. Han, and M. Meyyap-pan, Appl. Phys. Lett. 81, 910 2002.3A. M. Cassell, J. Li, R. M. D. Stevens, J. E. Koehne, L. Delzeit, H. T. Ng,Q. Ye, J. Han, and M. Meyyappan, Appl. Phys. Lett. 85, 2364 2004.4C. V. Nguyen, L. Delzeit, A. M. Cassell, J. Li, J. Han, and M. Meyyappan,Nano Lett. 2, 1079 2002.5S. Iijima, Nature London 354, 56 1991.6W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, R. A.Zhao, and G. Wang, Science 274, 1701 1996.7R. Kamalakaran, M. Terrones, T. Seeger, P. Kohler-Redlich, and M. Rühle,Y. A. Kim, T. Hayashi, and M. Endo, Appl. Phys. Lett. 77, 3385 2000.8V. K. Kayastha, Y. K. Yap, Z. Pan, I. N. Ivanov, A. A. Puretzky, and D. B.Geohegan, Appl. Phys. Lett. 86, 253105 2005.9A. G. Rinzler, J. H. Hafnet, P. Nikolaev, L. Lou, S. G. Kim, D. Tománek,P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, 15501995.10W. A. de Heer, D. T. Châtelain, and D. Ugarte, Science 270, 1179 1995.11N. de Jonge, Y. Lamy, K. Schoots, and T. H. Oosterkamp, NatureLondon 420, 393 2002.12Y. Wei, C. Xie, K. A. Dean, and B. F. Coll, Appl. Phys. Lett. 79, 45272001.13J.-M. Bonard, C. Klinke, K. A. Dean, and B. F. Coll, Phys. Rev. B 67,115406 2003.14Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegal, andP. N. Provencio, Science 282, 1105 1998.15C. Bower, W. Zhu, S. Jin, and O. Zhou, Appl. Phys. Lett. 76, 1776 2000.16M. Chhowalla, K. Teo, C. Ducati, N. Rupesinghe, G. Amaratunga,A. Ferrari, D. Roy, J. Robertson, and W. Milne, J. Appl. Phys. 90, 53082001.17A. V. Melechko, V. I. Merkulov, T. E. McKnight, M. A. Guillorn, K. L.Klein, D. H. Lowndes, and M. L. Simpsona, J. Appl. Phys. 97, 413012005.18T. Hirao, K. Ito, H. Furuta, Y. K. Yap, T. Ikuno, S. Honda, Y. Mori,T. Sasaki, and K. Oura, Jpn. J. Appl. Phys., Part 2 40, L631 2001.19L. E. Brus, J. A. W. Harkless, and F. H. Stillinger, J. Am. Chem. Soc. 118,4834 1991.20Y. K. Yap, M. Yoshimura, Y. Mori, and T. Sasaki, Appl. Phys. Lett. 80,2559 2002.21A. Anders, Appl. Phys. Lett. 80, 1100 2002.FIG. 5. TEM images of MWCNTs grown a substrate biasing of a −50 Vand b −150 V. The top plasma forward power is maintained at 200 W forboth cases.FIG. 6. SEM images of straight VA-MWCNTs at optimum growthconditions.173106-3 Menda et al. Appl. Phys. Lett. 87, 173106 2005

Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

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Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

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