Introduction

Carbon nanotubes have been studied extensively since their discovery [1] in 1991, because of the extraordinary physical properties they exhibit in electronic, mechanical, and thermal processes. A single-walled nanotube may be considered as a specific, one-dimensional giant molecule composed purely of carbon, whereas properties of multiwalled nanotubes are closest to those of graphite's in-plane properties, having sp2 hybridization of carbon bonds. To prepare closed-shell structures, one needs to insert topological defects into the hexagonal structure of graphene sheets. The extraordinary physical and chemical properties [2] and possible applications derived from these properties are attributed to the one-dimensionality and helicity of the nanotube structure.

For different applications, different properties and structures are important. Perfect carbon nanotubes have a high Young's modulus, or others are full of defects, thus providing the possibility for covalent or noncovalent functionalization; individual ones make use of quantum effects; and organized structures have millions of nanotubes to harness their synergy. Above all, the helicity in nanotubes is the most revealing feature to have emerged out of the first experimental [3] and theoretical papers [4-6]. This structural feature has great importance, because electrical properties of nanotubes pronouncedly change as a function of helicity and tube diameter.

Carbon nanotube growth methods can be classified based on the number of walls in a given tube. First, both multiwalled nanotubes [7] and single-walled nanotubes [8, 9] have been grown via arc-discharge carried out in an inert gas atmosphere between carbon or catalyst-containing carbon electrodes. Nowadays, carbon nanotubes and related materials are produced via a wide variety of processes [10], such as several types of the high-temperature arc-discharge method and laser vaporization of graphite targets, as well as numerous different techniques using chemical vapor deposition. The electric arc and laser methods are inherently impossible to scale up; however, these techniques are used routinely to make gram quantities of nanotubes. Nanotube samples produced by these methods are now commercially available in smaller quantities in "as-grown" or purified form.

Chemical vapor deposition (CVD) is a versatile and powerful tool in modern chemistry, chemical engineering, materials science, and nanotechnology. It is presently the most common method for carbon nanotube production, and it is also a well-known method [11] of carbon fiber production. In contrast to other methods, CVD can be scaled up, and there already exist industrial-scale production methods using this technique for nanofibers [12] which are dimensionally similar to the nanotubes discussed above. Furthermore, CVD can be tailored; that is, it can also be used to create oriented nanotube arrays on flat and 3-D substrates. Recent results point to the flexibility and power of the CVD technique, and ultimately, by tailoring the catalyst particles on substrates and controlling the CVD conditions, one hopes to achieve precise control of the nanotube architectures. By providing further control, the ultimate goal of the nanotechnology community is to provide carbon nanotube growth with a predefined number of tubes, with given diameter, helicity, and length along predefined locations.

In the first part of this chapter, we summarize the short history and achievements of the last several years of carbon nanotube growth. We also demonstrate our state-of-the-art methods of tailored nanotube growth and efforts to prepare nanotube structures capable of fulfilling the high expectations for these new and highly advanced materials. We then address applications of carbon nanotubes. Devices based on electron-field emission, low-voltage gas breakdown, filtering on the micro-, nano-, and even molecular scale, and equipment based on the enhanced properties of different composite materials consisting of nanotubes, are explored.

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