**1. Introduction**

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18 Syntheses and Applications of Carbon Nanotubes and Their Composites

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Large arrays and networks of carbon nanotubes, both single- and multi-walled, feature many superior properties which offer excellent opportunities for various modern applications rang‐ ing from nanoelectronics, supercapacitors, photovoltaic cells, energy storage and conversation devices, to gas- and biosensors, nanomechanical and biomedical devices etc. At present, arrays and networks of carbon nanotubes are mainly fabricated from the pre-fabricated separated nanotubes by solution-based techniques. However, the intrinsic structure of the nanotubes (mainly, the level of the structural defects) which are required for the best performance in the nanotube-based applications, are often damaged during the array/network fabrication by sur‐ factants, chemicals, and sonication involved in the process. As a result, the performance of the functional devices may be significantly degraded. In contrast, directly synthesized nanotube arrays/networks can preclude the adverse effects of the solution-based process and largely pre‐ serve the excellent properties of the pristine nanotubes. Owing to its advantages of scale-up production and precise positioning of the grown nanotubes, catalytic and catalyst-free chemi‐ cal vapor depositions (CVD), as well as plasma-enhanced chemical vapor deposition (PECVD) are the methods most promising for the direct synthesis of the nanotubes.

On the other hand, these methods demonstrate poor controllability, which results in the unpre‐ dictable properties, structure and morphology of the resultant arrays. In our paper we will dis‐ cuss our recent results obtained by the application of CVD and PECVD methods. Specifically, we will discuss carbon nanotube arrays and networks of very different morphology. The fabri‐ cation of the arrays of vertically aligned and entangled nanotubes, as well as arrays of arbitrary shapes grown directly on the pre-patterned substrates will be considered with a special atten‐ tion paid to the fabrication methods and the influence of the process parameters on the array

© 2013 Levchenko et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Levchenko et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

growth morphology (see Figure 1). Besides, the possibility of creating the 3D structures of car‐ bon nanotubes through post-processing of the arrays by liquids will be discussed.

CVD. This array was grown by using Ar/H2/CH4 gas mixture on Fe/Al2O3 catalyst. By tuning the growth condition (e.g., temperature and pressure), it was demonstrated that the SWCNTs could be of a high quality (a high IG/ID in the Raman spectra) and could contain a significantly higher content of metallic nanotubes as compared to the 'standard' metallic nanotube content of 33%

Large Arrays and Networks of Carbon Nanotubes: Morphology Control by Process Parameters

http://dx.doi.org/10.5772/52674

21

On the other hand, PECVD refers to the CVD process that uses plasma environment as an extra dimension to control the growth of CNTs. Plasma by definition contains ionized species and is generally considered as the fourth state of matter along with solid, liquid and gas. Recent advan‐ ces in the plasma-based nanofabrication offer unprecedented control over the structure and sur‐ face functionalities of a range of nanomaterials [4]. One of the major advantages, as compared to the conventional CVD processes, is that nanostructures can grow vertically-aligned due to the electrical field in the vicinity of surface [5]. Another benefit of using plasma is that the tempera‐ ture required to dissociate carbon feedstock could be greatly reduced [6]. Figure 3 illustrates the isolated CNTs grown in a PECVD system using Ni/SiO2 as the catalyst, C2H2/NH3 as the gas pre‐ cursors, and a DC glow discharge. It can be seen clearly that these nanotubes are aligned vertical‐ ly to the substrate surface, due to the plasma sheath-directed growth. These freestanding

nanotubes could give many opportunities to custom-design novel functional devices.

**Figure 2.** Typical randomly-oriented SWCNT networks with a unique "bridging" morphology grown in catalytic CVD [3].

**Figure 3.** Low- and high-resolution SEM images of the typical arrays of vertically-aligned CNTs grown in PECVD proc‐ ess with a glow discharge. The growth followed the 'tip-growth' mode as the catalyst nanoparticles are noticeable on

the top of each nanotube [4].

(1/3 metallic and 2/3 semiconducting) produced in many CVD processes [3].

The fabrication methods involved are the conventional CVD utilizing various gases such as methane, ethane, acetylene, argon, and hydrogen; plasma-enhanced CVD based on induc‐ tively-coupled low-temperature plasma reactor; microwave PECVD. The advantages of the plasma-based CVD process will be shown and discussed with a special attention. We will also discuss the influence of the process parameters such as process temperature, pressure, gas composition, discharge power etc. on the morphology of the nanotube arrays and networks, and demonstrate that the proper selection of the parameters ensures very high level of the process controllability and as a result, sophisticated control and tailoring of the growth structure and morphology of the carbon nanotube arrays.

Characterization technologies used are scanning and transmission electron microscopy (SEM and TEM), as well as atomic force microscopy (AFM), Raman and X-ray photoelectron spectroscopy techniques. The results of the numerical simulations will also be used to support the growth models and proposed growth mechanisms.

**Figure 1.** Morphologies of the representative CNT arrays grown by CVD (a) and PECVD (b).
