**5. Perspective approaches for the control of array morphology**

Dense arrays of highly ordered surface-bound vertically aligned nanotubes on silicon and metal oxides have a great potential for the fabrication of various advanced nano- and microdevices such as fuel cells, sensors, and field emission element [39,40,41,42] One possible way to integrate the carbon nanotube array in the silicon platform is the use of anodized aluminum oxide (AAO) membranes to grow the pre-structured CNT patterns, bonded to the template surface. Indeed, the use of AAO membranes as growth templates was successful for the fabrication of, i.e., electron emitters [43]. Synthesis of carbon nanotubes on AAO templates allows precise and reproducible control of the dimensions of nanotubes [44, 45]. In this section we will review in short the AAO template characteristics important for growing the carbon nanotube arrays, and discuss the most important control parameters.

ordered structure of very thin AAO templates can be fabricated. For example, AAO templates fabricated on silicon wafers have already been used to fabricate highly ordered carbon nanotubes [48]. The AAO templates fabricated on non-aluminum substrates can be compatible with much higher processing temperatures, well above the 700 °C. Silicon substrates may be

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33

However, quartz is more advantageous substrate for AAO membranes to be used as templates for the CNT synthesis. Quartz has a very high melting point, allowing for much higher temperatures, and can protect the AAO templates from cracking during the thermal treatment. Another advantage of quartz is transparency enabling the use of AAO templates as photonic crystals, and thus significantly broadening the application of fabricated AAO templates in

**Figure 15.** Schematic of the fabrication of AAO on quartz. High purity aluminum is deposited onto cleaned quartz,

**Figure 16.** SEM images of the quartz-based AAO template suitable for the fabrication of carbon nanotube arrays. (a)

also useful for protecting AAO from distortion during the CNT growth.

other optics related applications.

and the template is fabricated by anodization.

Side view and (b) top view.

An AAO template can be prepared by the anodic oxidation of aluminum in various acid solutions. The thickness, pore size and interpore distance can be easily controlled by varying conditions of anodization such as composition of electrolyte, process temperature, applied voltage, process time and pore widening time [46,47]. Figure 14 shows SEM images of the free standing AAO templates fabricated by the two-step anodization.

**Figure 14.** A Free-standing AAO fabricated using a two-step anodization. (a) Top view and (b) side view.

However, the free-standing AAO templates and membranes fabricated on aluminum foil are not be suitable for growing carbon nanotube arrays due to the thermal instability. Under thermal treatment, which is inevitable in the nanotube fabrication process, the AAO templates fabricated on aluminum foil easily crack due to the difference in thermal expansion coefficient of the alumina oxide and underlying aluminum. Moreover, the growth temperature cannot exceed the aluminum melting point e. In addition, a free-standing AAO template easily cracks due to its ceramic nature. Therefore, the conventional approach based on the use of the aluminum foil is not suitable for the CNT growth and fabrication of the carbon nanotube-based electronic devices.

To avoid this problem, it is necessary to fabricate AAO templates on other functional sub‐ strates. The alternative materials include silicon, quartz and ITO glass, on which the highly ordered structure of very thin AAO templates can be fabricated. For example, AAO templates fabricated on silicon wafers have already been used to fabricate highly ordered carbon nanotubes [48]. The AAO templates fabricated on non-aluminum substrates can be compatible with much higher processing temperatures, well above the 700 °C. Silicon substrates may be also useful for protecting AAO from distortion during the CNT growth.

**5. Perspective approaches for the control of array morphology**

32 Syntheses and Applications of Carbon Nanotubes and Their Composites

nanotube arrays, and discuss the most important control parameters.

standing AAO templates fabricated by the two-step anodization.

Dense arrays of highly ordered surface-bound vertically aligned nanotubes on silicon and metal oxides have a great potential for the fabrication of various advanced nano- and microdevices such as fuel cells, sensors, and field emission element [39,40,41,42] One possible way to integrate the carbon nanotube array in the silicon platform is the use of anodized aluminum oxide (AAO) membranes to grow the pre-structured CNT patterns, bonded to the template surface. Indeed, the use of AAO membranes as growth templates was successful for the fabrication of, i.e., electron emitters [43]. Synthesis of carbon nanotubes on AAO templates allows precise and reproducible control of the dimensions of nanotubes [44, 45]. In this section we will review in short the AAO template characteristics important for growing the carbon

An AAO template can be prepared by the anodic oxidation of aluminum in various acid solutions. The thickness, pore size and interpore distance can be easily controlled by varying conditions of anodization such as composition of electrolyte, process temperature, applied voltage, process time and pore widening time [46,47]. Figure 14 shows SEM images of the free

**Figure 14.** A Free-standing AAO fabricated using a two-step anodization. (a) Top view and (b) side view.

electronic devices.

However, the free-standing AAO templates and membranes fabricated on aluminum foil are not be suitable for growing carbon nanotube arrays due to the thermal instability. Under thermal treatment, which is inevitable in the nanotube fabrication process, the AAO templates fabricated on aluminum foil easily crack due to the difference in thermal expansion coefficient of the alumina oxide and underlying aluminum. Moreover, the growth temperature cannot exceed the aluminum melting point e. In addition, a free-standing AAO template easily cracks due to its ceramic nature. Therefore, the conventional approach based on the use of the aluminum foil is not suitable for the CNT growth and fabrication of the carbon nanotube-based

To avoid this problem, it is necessary to fabricate AAO templates on other functional sub‐ strates. The alternative materials include silicon, quartz and ITO glass, on which the highly However, quartz is more advantageous substrate for AAO membranes to be used as templates for the CNT synthesis. Quartz has a very high melting point, allowing for much higher temperatures, and can protect the AAO templates from cracking during the thermal treatment. Another advantage of quartz is transparency enabling the use of AAO templates as photonic crystals, and thus significantly broadening the application of fabricated AAO templates in other optics related applications.

**Figure 15.** Schematic of the fabrication of AAO on quartz. High purity aluminum is deposited onto cleaned quartz, and the template is fabricated by anodization.

**Figure 16.** SEM images of the quartz-based AAO template suitable for the fabrication of carbon nanotube arrays. (a) Side view and (b) top view.

The high quality AAO templates were fabricated directly on quartz, using a two-step anodi‐ zation without using any inter-layers between the deposited aluminum and quartz substrate. Figure 15 illustrates the schematic of this process. Prior to the fabrication of AAO template, quartz samples are cleaned in boiling solution of 30% w.t. of H2SO4 and 70% W.t. of H2O2. After that, the samples were etched in HF solution (0.1 w.t.% for 30 seconds), placed into an e-beam evaporator and coated with high purity (99.999%) aluminum to a thickness of 1.0 µm at a deposition rate of ≈1.5 nm×s-1. The deposited Al film was then anodized to produce porous alumina templates in an electrolytic cell using a two-step anodization process. As a result, high quality AAO templates on quartz were fabricated. Figure 16 shows the SEM image of the AAO templates.

led carbon nanotube and semiconducting carbon nanowire networks. Nanoscale

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35

[4] Z. J. Han, B. K. Tay, M. Shakerzadeh, K. Ostrikov. Superhydrophobic amorphous car‐

[5] I. Levchenko and K. Ostrikov, M. Keidar, S. Xu. Deterministic nanoassembly: Neutral

[6] M Meyyappan. A review of plasma enhanced chemical vapour deposition of carbon

[7] W. Zhou, L. Ding, S. Yang, J. Liu. Synthesis of high-density, large-diameter, and aligned single-walled carbon nanotubes by multiple-cycle growth methods. ACS

[8] K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima. Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes. Science

[9] G. Zhang, D. Mann, L. Zhang, A. Javey, Y. Li, E. Yenilmez, Q. Wang, J. P. McVittie, Y. Nishi, J. Gibbons, H. Dai. Ultra-high-yield growth of vertical single-walled carbon nanotubes: Hidden roles of hydrogen and oxygen. PNAS 2005;102, 16141-16145. [10] D. Sun, M. Y. Timmermans, Y. Tian, A. G. Nasibulin, E. I. Kauppinen, S. Kishimoto, T. Mizutani, Y. Ohno. Flexible high-performance carbon nanotube integrated circuits.

[11] J. Wu, K. S. Paudel, C. Strasinger, D. Hammell, A. L. Stinchcomb, and B. J. Hinds, Programmable transdermal drug delivery of nicotine using carbon nanotube mem‐

[12] Z. J. Han, K. Ostrikov, C. M. Tan, B. K. Tay, S. A. F. Peel. Effect of hydrophilicity of carbon nanotube arrays on the release rate and activity of recombinant human bone

[13] S. Kumar, I. Levchenko, Q. J. Cheng, J. Shieh, K. Ostrikov. Plasma enables edge-tocenter-oriented graphene nanoarrays on Si nanograss. Appl. Phys. Lett. 2012;100,

[14] Z. J. Han, K. Ostrikov. Uniform, dense arrays of vertically aligned, large-diameter

[15] M. P. Garrett, I. N. Ivanov, R. A. Gerhardt2,3, Alex A. Puretzky2, and David B. Geo‐ hegan. Separation of junction and bundle resistance in single wall carbon nanotube percolation networks by impedance spectroscopy Appl. Phys. Lett. 97, 163105 (2010);

single-walled carbon nanotubes. J. Am. Chem. Soc. 2012;134, 6018–6024.

bon/carbon nanotube nanocomposites. Appl. Phys. Lett. 2009;94, 223106.

or plasma route? Appl. Phys. Lett. 2006;89, 033109.

nanotubes. J. Phys. D: Appl. Phys. 2009;42, 213001.

2011;3, 3214-3220.

Nano 2011;5, 3849–3857.

12004;306, 1362-1364.

053115.

Nature Nanotech. 2011;6, 156–161

branes. Proc. Natl. Acad. Sci. USA 2010;107, 11698.

http://dx.doi.org/10.1063/1.3490650 (3 pages)

morphogenetic protein-2. Nanotechnology 2011;22, 295712.

Above results demonstrate that AAO template technology not only can be used in a piece of aluminum foil, but also can be combined with silicon and other functional substrate technol‐ ogy. AAO template on functional substrate were used in the fabrication of CNT arrays can be realize the field emitters or possible become optical devices when CNT in quartz-AAO. Moreover, since the crystallinity of CNTs increase with the synthesis temperature, the emission current density increases with the synthesis temperature of CNTs. This again demonstrated that only AAO on functional substrate can realize high quality of CNTs array fabrication.
