A review: controlled synthesis of vertically aligned carbon nanotubes
A review: controlled synthesis of vertically aligned carbon nanotubes
Carbon letters. 2011. Dec, 12(4): 185-193
Copyright ©2011, Korean Carbon Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : October 10, 2011
  • Accepted : November 11, 2011
  • Published : December 30, 2011
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About the Authors
Myung Gwan Hahm
Daniel P. Hashim
Robert Vajtai
Pulickel M. Ajayan
Carbon nanotubes (CNTs) have developed into one of the most competitively researched nano-materials of this decade because of their structural uniqueness and excellent physical properties such as nanoscale one dimensionality, high aspect ratio, high mechanical strength, thermal conductivity and excellent electrical conductivity. Mass production and structure control of CNTs are key factors for a feasible CNT industry. Water and ethanol vapor en-hance the catalytic activity for massive growth of vertically aligned CNTs. A shower system for gas flow improves the growth of vertically aligned single walled CNTs (SWCNTs) by controlling the gas flow direction. Delivery of gases from the top of the nanotubes enables direct and precise supply of carbon source and water vapor to the catalysts. High quality vertically aligned SWCNTs synthesized using plasma enhance the chemical vapor deposi-tion technique on substrate with suitable metal catalyst particles. This review provides an in-troduction to the concept of the growth of vertically aligned SWCNTs and covers advanced topics on the controlled synthesis of vertically aligned SWCNTs.
1. Introduction
Carbon nanotubes (CNTs) consist of rolled graphene sheets built from sp2 hybridized carbon atoms; they have extraordinary mechanical [1 - 15] , thermal [10 , 16 - 28] and electronic [29 - 43] properties and hold great promise for future applications. Their remarkable properties have attracted intense scientific interest as numerous theoretical and experimental studies of their properties [1 - 15 , 17 - 28] have put forward possible electronic [33 , 44 - 50] , field emission [51 - 55] ,electrochemical device, battery [56 - 59] , molecular sensor [60 - 64] , and gas storage [65 - 70] applications.The one dimensionality of the nanotube structure has given rise to exceptional physical and chemical properties and potential applications. For different applications, desired character-istics and different properties are needed. A single walled CNT (SWCNT), especially, can behave as a well-defined metallic, semiconducting or semi-metallic tube depending on the chirality and diameter, although there is no difference in the chemical bonding or any doping or additional functionalization between them. To accomplish the precisely controlled synthesis of an SWCNT structure in terms of the diameter, chirality, orientation, position and density, chemical vapor de-position(CVD) techniques [71 - 77] are becoming a strong manufacturing route compared to oth-er synthesis methods such as electric arc-discharge, laser vaporization and electrolysis [78 , 79] . Arc discharge and laser vaporization processes can produce only randomly tangled CNTs mixed with various impurities. Research on CNT growth using CVD techniques has been initiated by Ren and Huang [80] and was first used to demonstrate the control of the synthesis of aligned and ordered large diameter multi walled CNT (MWCNT) structures on the top surface of glass substrates by plasma enhanced CVD (PECVD). CNTs can self-align vertically during the CVD process, and can be made into patterns perpendicular to the substrate surface. The mechanism of
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Schematic drawing of ethanol based chemical vapor deposition system.
nanotube self-alignment typically involves van der Waals interac-tions between CNTs. CNTs interact with their neighboring tubes via van der Waals forces to gain rigidity, and that allows the nano-tubes to self-align and grow perpendicular to the substrate surface.To synthesize vertically aligned MWCNTs and SWCNTs, various CVD techniques have been developed. In this review, we review ethanol based CVD, water-assisted CVD and PECVD for the syn-thesis of vertically aligned SWCNTs.
2. CVD for Growth of Vertically Aligned SWCNTs
- 2.1. Ethanol based CVD
The CVD method basically involves the decomposition of hydrocarbon molecules (e. g., methane, benzene, acetylene, naphthalene, ethylene, etc.) catalyzed by the transition metal. The synthesis process involves heating a thin film metal cata-lyst (e.g., Co, Ni, Fe, Pt or Pd deposited on substrates such as silicon, graphite or silica) to high temperature and flowing a hy-drocarbon gas through the tube reactor. To synthesize vertically aligned SWCNTs, not only is it necessary to have highly dense and well-dispersed catalyst nanoparticles but it is also necessary to have high activity and long lifetime for the catalyst [81 , 82] . In the CVD method, growth of CNTs is seriously limited by the short lifetime and the low activity of the catalyst nanoparticles. The resulting short lifetime and low activity of the CNTs syn-thesis has not only reduced the availability of CNTs, but also the amorphous carbon covered catalysts remain as impurities in the as-grown materials. Maruyama and coworkers first report-ed the synthesis of high-purity SWCNTs using alcohols such as methanol and ethanol as carbon sources [83] . In the ethanol based CVD process, controlled amounts of ethanol molecules (C 2 H 5 OH) not only work as a carbon source to grow CNTs but also serve as a weak oxidizer that can selectively remove the amorphous carbon layer on the catalyst particles [82] . It is hy-pothesized that the OH radical formed at high temperature from ethanol can remove the amorphous carbon efficiently during CNT growth, leaving only pure CNTs as product [84] . Fig.1
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Schematic drawing of chemical vapor deposition (CVD) proce-dure. (a) Si/SiO2 substrate. (b) Al2Ox film (20 nm thickness) was deposited by using a sputter coater or e-beam evaporator. (c) An ultra thin Co film(0.6-1 nm) was deposited as a catalyst on Al2Ox film for the growth of carbon nanotubes (CNTs) using e-beam evaporator. (d) Vertically aligned single walled CNT array grown by using ethanol based CVD process.
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(a) Optical microscope image of highly dense vertically aligned single walled carbon nanotubes (SWCNTs) grown with ethanol based chemical vapor deposition technique. (b) Low magnification scanning electron microscope (SEM) image of vertically aligned SWCNTs. (c) High magnification SEM image shows self-aligned SWCNTs. (d) Low magnifica-tion SEM image shows line pattern grown vertically aligned SWCNTs.
shows a schematic drawing of a typical ethanol based CVD sys-tem. In a typical ethanol based CVD procedure, a 20 nm thick Al film is deposited onto an Si/SiO 2 (1000 Å thick oxide) wafer by using a sputter coater or e-beam evaporator ( Figs. 2 a and b) and exposed to the air to allow the formation of an aluminum-oxide buffer layer that can support the growth of highly dense and ver-tically aligned CNTs. Then, a thin film of cobalt (Co) catalyst is deposited onto the Al 2 O x /SiO 2 multi-layer by using an e-beam evaporator ( Fig.2 c). The prepared substrate (Co/ Al 2 O x /SiO 2 ) is placed inside of a quartz tube and evacuated to 15 mTorr. Then,the quartz tube is heated to 850℃ meanwhile being exposed to an Ar-H2 mixture carrier gas (5% H2 balanced Ar gas) to provide a catalyst reducing environment with 100 sccm (standard cubic centimeter per minute) flow rate. When the temperature reaches 850℃, a controlled high purity anhydrous ethanol (99.95%) is supplied as a carbon source for the high density nucleation of
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single walled carbon nanotube (SWCNT) forest grown with water-assisted chemical vapor deposition. (a) Picture of a 2.5-mm-tall SWCNT forest on a 7-mm by 7-mm silicon wafer. The matchstick on the left and ruler with millimeter markings on the right are for size reference.(b) Scanning electron microscopy (SEM) image of the same SWCNT forest. Scale bar, 1 mm. (c) SEM image of the SWCNT forest ledge. Scale bar, 1㎛. (d) Low-resolution transmission electron microscopy (TEM) image of the nanotubes. Scale bar, 100 nm. (e) High-resolution TEM image of the SWCNTs. Scale bar, 5 nm. From Hata et al. [81]. Reprinted with permission from AAAS.
CNTs, resulting in vertically aligned CNT arrays. After a 30 min reaction time, the quartz tube is cooled to room temperature.Hahm and coworkers recently reported the millimeter scale syn-thesis of highly dense vertically aligned SWCNTs using ethanol based the CVD process depicted in Fig. 3 . Fig. 3 a shows a cross-sectional optical image of vertically aligned SWCNTs synthe-sized with the ethanol based CVD process. Figs. 3 b-d show representative scanning electron microscope (SEM) images of a highly dense and vertically aligned SWCNT forest ( Figs. 3 b and c are a low magnifica-tion SEM image and a high magnification SEM image, respectively) and a micro patterned growth of vertically aligned SWCNT wall structures ( Fig. 3 d). It can be clearly seen that the ethanol based CVD system is very effective in growing CNTs.
- 2.2. Water assisted CVD
The water assisted CVD process is one of the well-known techniques for synthesis of vertically aligned SWCNTs. Hata et al. [81] reported millimeter-scale vertically aligned SWCNTs using the water assisted ethylene CVD process. 2.5 mm long vertically aligned SWCNTs were synthesized using Fe/Al or aluminum oxide multilayers on Si wafers, as shown Fig. 4 . They found that the water vapor acts as a promoting and preserving agent for the catalytic activity of the metal catalyst nanoparti-
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(a) Optical images of carbon nanotube (CNT) forest with a height of 1 cm synthesized by top-flow growth. (b) Single walled CNTs grown on A4 substrate. Reprinted with permission from Yasuda et al. [85]. Copyright (2009) American Chemical Society.
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Schematics of gas flow direction to catalysts at initial stage of the growth and with the presence of a forest for (a and b) conventional lateral-flow and (c and d) top-flow growth, respectively. Reprinted with permission from Yasuda et al. [85]. Copyright (2009) American Chemical Society.
cles. Balancing the relative levels of ethylene and water is one of the critical factors to maximize catalytic lifetime for milli-meter-scale growth of SWCNTs [81] . Hata et al. [81] also demonstrated ~1 cm long vertically aligned SWCNT growth on A4 paper size substrate using gas supply direction control, as shown Fig. 5 [85] . Under lateral gas flow growth, the gases can diffuse by typical molecular diffusion on the surface of the substrate. This typical diffusion allows a uniform and optimum supply of gases to the catalysts, as shown in Fig. 6 a [85] . However, for top gas flow growth, the shower head forces the gases to flow into the gap of the SWCNTs with minimum flow around the vertically aligned SWCNTs, and this facilitates a uniform and optimum supply of the carbon and of water vapor to the cata-lysts for growth of SWCNTs. The gases can also diffuse paral-lel to the SWCNT alignment direction and encounter minimal interruption, as depicted in Fig. 6 [85] . These results confirm the observed sensitivity, stability, uniformity, high carbon efficiency and high yield of top-flow growth. Noda et al. [86] reported mil-limeter thick SWCNT forest growth using an aluminum oxide supported Fe catalyst system and a water assisted CVD process. They demonstrated the hidden role of Fe catalyst support. Their CVD was carried out by using C2H4/H2/H2O/Ar. Fig. 7 shows the effect of the catalyst supports on vertically aligned SWCNT
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Effect of support materials for Fe catalyst on carbon nanotube(CNT) growth. CNTs were grown for 10 min under standard conditions. (a) Cross-sectional optical images of CNTs grown by using combinatorial catalyst libraries, which had a nominal Fe thickness profile ranging from 0.2 nm (at left on each sample) to 3 nm (right) formed on either SiO2, Al2Ox, or Al2O3. (b) Relationship between the thickness of CNTs and the nominal Fe thickness of the catalyst. (c) Raman spectra of the same samples. Copy-right 2007 The Japan Society of Applied Physics.
growth using the water assisted CVD process. Differences of height of vertically aligned SWCNTs are evident between the Fe catalysts with aluminum oxide (Al 2 O x and Al 2 O 3 ) supports. SWCNT forests grow thick by using either of these two catalysts when Fe catalyst layer is wider than 0.6 nm. However, vertically aligned SWCNTs grow longer only with the Fe/Al 2 O x catalyst when the Fe catalyst layer is thinner than 0.6 nm.
- 2.3. Plasma enhanced CVD
High quality SWCNTs and MWCNTs can be grown using high pressure arc-discharge, high pressure CVD and high pres-
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Molecular oxygen assisted plasma enhanced chemical vapor deposition (PECVD). (a) Atomic force microscopy image of Fe catalyst nanoparticles. (b) Optical image of vertically aligned single walled carbon nanotubes (SWCNTs) synthesized on 4 in wafer. (c) Low magnification scanning electron microscopy (SEM) image of vertically aligned SWCNTs. (d) Cross-sectional SEM images of vertically aligned SWCNTs. (e) Raman spectrum of vertically aligned SWCNTs grown with PECVD. (f ) High resolu-tion transmission electron microscopy image of SWCNTs. Reprinted with permission from Zhang et al. [96]. Copyright (2005) National Academy of Science, USA.
sure laser ablation [78 , 79 , 87 - 94] . Vertically aligned SWCNTs can be synthesized using the PECVD technique on substrates with suitable metal catalyst particles [95 - 99] . Amaratunga and coworkers reported the growth of vertically aligned CNTs us-ing a direct current PECVD technique with an Ni and Co cata-lyst system. Their results show that the alignment of vertically aligned CNTs depends on the electric field and that the growth rate can be changed depending on the CNT diameter, which reaches a maximum as a function of growth temperature [95] . Dai and coworkers developed large scale ultra-high-yield syn-thesis of vertically aligned SWCNTs using the PECVD method.They reported the growth of vertically aligned SWCNTs us-ing a molecular oxygen-assisted PECVD technique on a full 4 inch wafer scale, as shown Fig. 8 [96] . Their results show that PECVD also requires the formation of a dense and relatively uniform layer of catalyst nanoparticles. This layer is essential for vertically aligned SWCNT growth. For 10 min growth, the length of the vertically aligned SWCNTs is ~10 ㎛, as shown Fig. 8 d. Figs. 8 e and f show that the vertically aligned CNTs are single walled. Kawarada and coworkers also reported a long,millimeter scale growth of vertically aligned SWCNTs using
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Optical images of 0.5 cm long vertically aligned single walledcarbon nanotubes (SWCNTs) grown on different substrates. (a) A honey-combmat (pitch size: 900 ㎛, hole diameter: 700 ㎛). (b) A cross-sec-tional-image. (c) Vertically aligned SWCNT rod array. (d) A crosssectionalimage. Reprinted with permission from Zhong et al. [97]. Copyright (2007) American Chemical Society.
the microwave plasma CVD method. They synthesized highly dense vertically aligned SWCNTs on Si wafer using a sandwich like catalyst system of Al 2 O 3 (0.5 nm)/Fe (0.3-0.5 nm)/Al 2 O 3 (0.5 nm) [99] . The first Al 2 O 3 layer works as a buffer layer to prevent the reaction between Fe and Si. On the other hand, the second Al 2 O 3 layer increases the surface diffusion barrier of catalytic atoms to prevent the aggregation of Fe nanoparticles [99] . Recently, Kawarada and coworkers reported on a devel-oped PECVD system to produce 0.5 cm long vertically aligned SWCNTs, as shown in Fig. 9 . The sandwich like catalyst system was heat-treated at 600℃ for 5 min with H 2 and CH 4 . The heat-treatment restructures the thin Fe film and redistributes the Fe catalyst nanoparticles [97] . They also suggested a diffusion limit for the carbon source for CNT growth. In the case of a fully coat-ed Si wafer with Fe catalyst nanoparticles, the SWCNTs stand-ing on the edges attempt to grow faster than their neighbors, causing a slight buckling along their length, as shown in Fig. 9 b. On the other hand, in the case of an isolated pillar substrate, the vertically aligned SWCNT pillars bend over to allow vertically aligned SWCNTs to grow faster on one side with better carbon source access [97] . Dai and coworkers demonstrated fast heating PECVD for preferential synthesis of semiconducting vertically aligned SWCNTs. To construct the desired CNT based semicon-ductor devices with high performance, it is critical to synthesize metallic SWCNTs and semiconducting SWCNTs, selectively. Dai and coworkers used C 2 H 2 gas without additional carrier gas under low pressure (30 mTorr) to grow semiconducting verti-cally aligned SWCNTs, selectively [98] . They measured Raman spectra using different excitation laser wavelengths to confirm the semiconducting properties of the as-grown SWCNTs, as de-picted in Fig. 10 . The HIPco SWCNTs have the typical radial
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Raman spectra of the as-synthesized and HIPco single walled carbon nanotubes (SWCNTs) using excitation laser with wavelengths of 488, 514, 633 and 785 nm. Reprinted with permission from Qu et al. [98]. Copyright (2008) American Chemical Society.
breathing mode (RBM) peaks for semiconducting and metallic nanotubes. On the other hand, as-synthesized vertically aligned SWCNTs have the dominant Raman spectra of semiconducting SWCNTs in both RBM and G band regions [98] . In three different Raman RBM spectra with differ-ent laser wavelengths (488, 633 and 785 nm), RBM modes also show semiconducting nano-tubes, as depicted in Fig. 10 b.
3. Summary
CVD techniques are becoming a strong manufacturing route for the controlled synthesis of SWCNTs, especially with a view to controlling their structure (diameter and chirality) and build-ing organized SWCNT networks with desired growth orienta-tion, position and density. There have been great developments in nanotube growth techniques, and it is now possible to syn-thesize high quality vertically aligned CNTs. In this review, we reviewed the various growth techniques for controlled synthesis of vertically aligned SWCNTs, including ethanol based CVD,water assisted CVD and PECVD processes. We also introduced the hidden role of the OH radical and aluminum oxide buffer layers in the growth of highly dense vertically aligned SWCNTs.
MGH and RV acknowledge financial support from ARL/ARO (No.W911NF). DPH acknowledges financial support from National Science Foundation Graduate Research Fellow-ship (No. 0940902).
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