APPLIED PHYSICS LETTERS 93, 261504 共2008兲

Plasma-controlled adatom delivery and „re…distribution: Enabling uninterrupted, low-temperature growth of ultralong vertically aligned single walled carbon nanotubes Eugene Tam and Kostya 共Ken兲 Ostrikova兲 Plasma Nanoscience, School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia and CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070, Australia

共Received 6 November 2008; accepted 5 December 2008; published online 31 December 2008兲 Large-scale 共⬃109 atoms兲 numerical simulations reveal that plasma-controlled dynamic delivery and redistribution of carbon atoms between the substrate and nanotube surfaces enable the growth of ultralong single walled carbon nanotubes 共SWCNTs兲 and explain the common experimental observation of slower growth at advanced stages. It is shown that the plasma-based processes feature up to two orders of magnitude higher growth rates than equivalent neutral-gas systems and are better suited for the SWCNT synthesis at low nanodevice friendly temperatures. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3058766兴 Single walled carbon nanotubes 共SWCNTs兲 are seen by many to be the future of nanoelectronics as well as numerous other uses including drug and gene delivery, hydrogen storage, and structural reinforcements.1–3 SWCNTs have unique properties such as exceptionally large surface area to volume and aspect ratios and, in the case of ultra thin nanotubes, can also be superconducting.4,5 However, there are several challenges to enable the commercial viability of SWCNTs. For SWCNTs fabricated for electronic devices, these challenges include vertical alignment, controlled chirality and diameter, maximum lengths, and placement of the SWCNTs onto the specified device locations.6 The high temperatures 共typically 700– 800 ° C and even higher兲, usually required to fabricate SWCNTs, lead to several significant problems ranging from excessive interlayer diffusion leading to the depletion of doping levels in functional layers to melting of interconnects, interlayers, and vias already present in electronic devices. This is why the most widely accepted method to fabricate nanotube-based devices for nanoelectronics is to grow the SWCNTs separately and then spin coat them onto the device.7 Spin coating, printing, and other means of placing the preformed SWCNTs onto the substrate suffer from major issues such as the inability to align SWCNTs properly and the necessity to implement several intermediate steps. Ultralong surface bound and vertically aligned SWCNTs 共VASWCNTs兲 are particularly promising for a number of applications. However, VASWCNTs quite often can only be grown to a certain and rather limited length before no further growth can be detected.8 The slow down of the growth of SWCNTs and the maximum heights achieved by various techniques are commonly attributed to catalyst poisoning as the predominant mechanism.9 The clue to resolve this long-standing problem is to maximize the delivery of carbon precursors to the catalyst particle without poisoning it, keeping precursor species delivery and incorporation rates high enough to sustain high a兲

Electronic mail: [email protected].

0003-6951/2008/93共26兲/261504/3/$23.00

SWCNT growth rates and enable this at nanoelectronic friendly temperatures. A viable possibility is to use low-temperature thermally nonequilibrium plasmas, which have been shown to provide numerous benefits in growing nanostructures such as vertical alignment, higher deposition rates, and precise control of the precursor flux.10–14 In particular, there have been reports on the SWCNT synthesis below 500 ° C and as low as 350 ° C; most remarkably, SWCNT growth at such low temperatures is not possible without the aid of plasmas.15,16 This technique will eventually allow the growth of SWCNTs on nanodevices without the interconnects melting. High-density plasmas can also be used for highly controlled SWCNT growth in the gas phase and then deposit them straight onto the substrate.17,18 However the issue of limited nanotube lengths still remains. The existing numerical simulation and modeling efforts also do not address this critical issue and generally examine only the early growth stages of SWCNTs, usually during nucleation, and mostly focus on the energetic aspects or separate modeling of processes in the gas phase.19–21 These simulations generally consider early nucleation stages and do not take into account plasma related effects. In this letter, we propose a way to maximize the VASWCNT growth rates/lengths through the plasma control of precursor deposition onto nanotube and substrate surfaces. Our simulations take into account controlled precursor placement, surface diffusion, and adatom evaporation processes and involve up to 109 carbon atoms. A Monte Carlo method was used to simulate the motion of carbon adatoms on both the substrate and the lateral surfaces of the 关armchair 共5,5兲兴 SWCNTs to quantify the influence of the surface temperature Ts and some other plasma-related effects. The growth rates of the SWCNTs are also examined and it is shown that in plasma the precursors can be delivered closer to the base of the SWCNTs, where they incorporate into the tubes and contribute to the growth,22 thus substantially increasing the nanotube lengths. Consider a single square cell in a rarefied nanotube array, each with a catalyst particle of the same size shown in Fig. 1. Our simulation determines the placements of adatoms on the substrate and the lateral surface of the SWCNTs based

93, 261504-1

© 2008 American Institute of Physics

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

261504-2

E. Tam and K. 共. Ostrikov

Appl. Phys. Lett. 93, 261504 共2008兲

FIG. 2. 共Color online兲 Height of SWCNTs vs time for 共a兲 neutral and 共b兲 plasma systems.

FIG. 1. 共Color online兲 Schematic of the 共5,5兲 SWCNT-plasma system and main processes involved.

on the results of previous simulations, which allow one to determine the precursor trajectories in the gas phase.22 In addition to changes in precursor trajectories, plasmas introduce effects such as heating due to ion bombardment and surface charging and some other polarization-related effects that lead to changes in the diffusion activation and desorption energies.23 The plasma parameters were chosen such that the potential drop across the plasma sheath is optimum to maximize the number of precursor species landing close to the base of the SWCNTs, which grow via the root growth mechanism. The simulation code then traced the motion of every adatom along the substrate and nanotube surface using a random walk approximation. It was assumed that the nucleating SWCNT is always the largest that can fit on the catalyst particle, also that Ts increases due to ion bombardment and associated heat transfer, and also that the adatom diffusion activation and desorption energies are reduced in plasma.23,24 The change in the surface temperature depends on the precursor influx and the potential of the sheath relative to the plasma bulk. The substrate and the lateral surface of the SWCNTs have been treated separately, each of them having different precursor distributions, boundary conditions, and characteristic energies. A Monte Carlo method was employed 共for every adatom兲 to determine whether adatoms desorb/ evaporate from the surface or not. The diffusion 共D兲 and evaporation 共R兲 coefficients that quantify those effects can be approximated as

From Fig. 2 one can see that the growth rate of the SWCNTs continuously decreases with time, which is consistent with experimental results.26 As time passes, the height of the SWCNTs increases leading to a reduction in the proportion of precursors landing on the substrate surface. Since fewer adatoms are able to land on the substrate surface, a smaller number of them eventually reach the catalyst and the growth rates decrease with time. A common way to boost the nanotube growth is to further increase the substrate temperature to enhance adatom mobility. However, higher substrate temperatures lead to several adverse effects such as the reduction in the average distance, which adatoms can travel before desorption/evaporation in addition to the already mentioned melting of temperature sensitive on-chip circuitry elements. From Fig. 3共a兲, one can see that as Ts increases, the average rate of the SWCNT growth initially increases exponentially at low temperatures for both cases, however, levels off for the neutral-gas case. Changes in the surface temperature directly affect the mobility of adatoms as well as the deposition and desorption/evaporation rates. This also significantly affects the average distance an adatom is able to travel and hence the size of the area of the depleted adatom density around the nanotube. It can be seen in Fig. 3 and Eq. 共1兲 that, as the temperature decreases, adatoms become less mobile leading to extremely low growth rates at low temperatures. It is also seen that, in the plasma, the growth rate can be up to two orders of magnitude higher than in neutralgas-based systems. Our numerical experiments suggest that due to lower diffusion activation energy and additional ion heating of the substrate in a plasma, SWCNT can be grown at temperatures as low as 700 K, with the average VASWCNT height gained over a period of 10 s, being approximately 500 nm, which is consistent with experimental results.4–6,15 Note that from Fig. 3共b兲, the dominant route through which the majority of adatoms reach the catalyst particle changes from the substrate to the lateral surface as the latter

act /kBTs兲, D = ␯␭sub,lat/4 exp共− ␧共sub,lat兲 ads R = ␯ exp共− ␧共sub,lat兲 /kBTs兲,

共1兲

where ␭sub = 0.54 nm and ␭lat = 0.14 nm are the lattice parameters, ␯ ⬃ 1013 Hz is the surface vibrational frequency, Ts is the temperature of the substrate and SWNCT surfaces, and act act ads ads = 0.9 eV, ␧lat = 0.8 eV, ␧sub = 1.9 eV, and ␧lat = 1.8 eV ␧sub are surface energies, with subscripts that either denote it to be attributed to the lateral surface of the SWCNTs 共lat兲 or the substrate surface 共sub兲; the superscripts act and ads represent the activation and adsorption energy, respectively.20,25

FIG. 3. 共Color online兲 共a兲 Average SWCNT growth rate of SWCNTs vs Ts in 共i兲 plasma and 共ii兲 neutral systems. 共iii兲 Ratio of the SWCNT growth rates in plasma and neutral systems. 共b兲 Proportion of adatoms that directly deposit onto the lateral SWCNT surface among all precursor species that become adatoms in neutral 关共i兲 and 共iv兲兴 and plasma 关共ii兲 and 共iii兲兴 processes at Ts = 700 K 关共i兲 and 共ii兲兴 and 1000 K 关共iii兲 and 共iv兲兴.

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

261504-3

E. Tam and K. 共. Ostrikov

FIG. 4. 共Color online兲 The product of the probability of the adatoms reaching the base of SWCNTs and the number of adatoms landing on that region of the SWCNTs in a 共a兲 neutral and 共b兲 plasma systems visualized by a graded color scheme. Drawn not to scale.

begins to form, adatoms begin to populate the SWCNT surface, and fewer precursors land on the substrate surface. Thus, as the nanotube grows in length, precursors deposit further away from the catalyst nanoparticle, effectively slowing down the nanotube growth. To understand why this is the case, other details, such as the distance traveled by adatoms before desorption and the precursor distribution during the growth of the SWCNT, have been examined. Figure 4 quantifies and illustrates the effectiveness of incorporation 共through the substrate bound catalyst nanoparticle兲 of adatoms deposited on different sections of the nanotube lateral surface. It shows the distribution of the adatom density in different surface areas multiplied by the probability of the adatoms to reach the SWCNT base from those areas. The upper section of the nanotube is more exposed to the precursor influx, hence, the adatom density deposited onto those areas is quite high. However, due to intense desorption/evaporation processes at larger nanotube lengths, only very few of the adatoms landing near the exposed tips of the SWCNTs actually make it all the way to the base were they can then be incorporated into the developing nanostructure. Ideally, to maximize the growth rates of the SWCNTs, precursors should be deposited as close to the catalyst nanoparticle as possible. If this is not possible, then the temperature should be selected such that the diffusion rates and hence the characteristic distance traveled by adatoms are optimized. In a neutral-gas-based system, control of the precursor flux in the gas phase is limited, thus temperature control is the only possibility. However, in a plasma-based process, precursors can be controlled in the gas phase by varying the substrate bias, discharge power, and the operating gas pressure. Therefore, much higher rates of SWCNT growth can be achieved at all temperatures, in particular, at low temperatures where the adatom mobility is low. Our simulations do not include any effects related to catalyst poisoning, yet the rate of growth decreases with time at rates consistent with experimental observations. In addition, the reduction in the growth rates with time of the SWCNTs in neutral-gas-based processes is much greater than it is in plasma systems. In the latter case this slow down does not cause the SWCNT growth to stop over the time

Appl. Phys. Lett. 93, 261504 共2008兲

period in which our numerical experiments were run. Moreover, controlled ion deposition closer to the SWCNT base can help and prevent catalyst particles from unwanted poisoning through increasing the catalyst temperature 共and hence, its extrusion ability兲 and removing amorphous deposits around the nanotube. Finally, we stress that under the same conditions, much longer VASWCNTs can be grown in a plasma environment, with growth rates up to two orders of magnitude higher and surface temperatures significantly lower compared to equivalent neutral-gas-based processes. The results of this work are generic and may be applied to a broader range of materials and other one-dimensional nanostructures such as nanorods and nanowires. This work was partially supported by the ARC and CSIRO. 1

A. Sidorenko, T. Krupenkin, A. Taylor, P. Fratzl, and J. Aizenberg, Science 315, 487 共2007兲. 2 T. E. McKnight, A. V. Melechko, D. K. Hensley, D. G. J. Mann, G. D. Griffin, and M. L. Simpson, Nano Lett. 4, 1213 共2004兲. 3 K. Ostrikov, Rev. Mod. Phys. 77, 489 共2005兲. 4 Y. F. Li, T. Kaneko, and R. Hatakeyama, Appl. Phys. Lett. 92, 183115 共2008兲. 5 T. Kato and R. Hatakeyama, Appl. Phys. Lett. 92, 031502 共2008兲. 6 T. Nozaki and K. Okazaki, Plasma Process. Polym. 5, 301 共2008兲. 7 A. Schindler, J. Brill, N. Fruehauf, J. P. Novak, and Z. Yaniv, Physica E 37, 119 共2007兲. 8 W. Kim, H. C. Choi, M. Shim, Y. Li, D. Wang, and H. Dai, Nano Lett. 2, 703 共2002兲. 9 Y. Wang, Y. Liu, X. Li, L. Cao, D. Wei, H. Zhang, D. Shi, G. Yu, H. Kajiura, and Y. Li, Small 3, 1486 共2007兲. 10 F. J. Gordillo-Vazquez, V. J. Herrero, and I. Tanarro, Chem. Vap. Deposition 13, 267 共2007兲. 11 Z. Chen, U. Cvelbar, M. Mozetič, J. He, and M. K. Sunkara, Chem. Mater. 20, 3224 共2008兲. 12 D. Mariotti, V. Švrček, and D.-G. Kim, Appl. Phys. Lett. 91, 183111 共2007兲. 13 X. P. Lu, Z. H. Jiang, Q. Xiong, Z. Y. Tang, X. W. Hu, and Y. Pan, Appl. Phys. Lett. 92, 081502 共2008兲. 14 K. Ostrikov and A. B. Murphy, J. Phys. D 40, 2223 共2007兲. 15 M. Cantoro, S. Hofmann, S. Pisana, V. Scardaci, A. Parvez, C. Ducati, A. C. Ferrari, A. M. Blackburn, K.-Y. Wang, and J. Robertson, Nano Lett. 6, 1107 共2006兲. 16 J. Robertson, G. Zhong, H. Telg, C. Thomsen, J. H. Warmer, G. A. D. Briggs, U. Detlaf-Weglikowska, and S. Roth, Appl. Phys. Lett. 93, 163111 共2008兲. 17 M. Keidar, Y. Raitses, A. Knapp, and A. Waas, Carbon 44, 1022 共2006兲. 18 M. Keidar, I. Levchenko, T. Arbel, M. Alexander, A. M. Waas, and K. Ostrikov, Appl. Phys. Lett. 92, 043129 共2008兲. 19 F. Ding, P. Larsson, J. A. Larsson, R. Ahuja, H. Duan, A. Rosen, and K. Bolton, Nano Lett. 8, 463 共2008兲. 20 O. A. Louchev, Y. Sato, and H. Kanda, Appl. Phys. Lett. 80, 2752 共2002兲. 21 K. N. Ostrikov, M. Y. Yu, and H. Sugai, J. Appl. Phys. 86, 2425 共1999兲. 22 I. Levchenko, K. Ostrikov, and E. Tam, Appl. Phys. Lett. 89, 223108 共2006兲; E. Tam, I. Levchenko, and K. Ostrikov, J. Appl. Phys. 100, 036104 共2006兲. 23 K. Ostrikov, I. Levchenko, and S. Xu, Pure Appl. Chem. 80, 1909 共2008兲. 24 K. Ostrikov, Vacuum 83, 4 共2008兲. 25 A. V. Krasheninnikov, K. Nordlund, P. O. Lehtinen, A. S. Foster, A. Ayuela, and R. M. Nieminen, Phys. Rev. B 69, 073402 共2004兲. 26 S. Huang, X. Cai, and J. Liu, J. Am. Chem. Soc. 125, 5636 共2003兲.

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp