**4. Types and characteristics of carbon nanotubes (CNTs)**

Elemental carbon in sp<sup>2</sup> hybridization can form a variety of amazing structures, such as graphite, graphene, CNTs, and fullerene [5]. Carbon nanotubes are allotropes of carbon with a concentric cylindrical shape of diameter in the order of nanometer and length of micrometer [2]. In particular, CNTs with very high lengthto-diameter ratios (132,000,000:1) have been constructed [6]. The carbon network of the shells is closely related to the honeycomb arrangement of the carbon atoms in the graphite sheets [5]. The structural shape of SWNTs is a tube several nanometers in diameter and several microns in length. They are also made of perforated graphene sheets. They are placed in one direction and next to each other to form the pipes. A MWNT is an arrangement of several to tens of hundreds of concentric tubes of graphite plates with adjacent 0.34 nm shells separated (**Figure 1**) [5].

They are tubular cylinders of carbon atoms with extraordinary mechanical, electrical, thermal, optical, and chemical properties. CNTs were first discovered by Iijima in 1991; however, the first microscopic production of CNT was made by two researchers at Nippon Electric company limited fundamental research laboratory, and since then, there have been several developments of CNT [2]. Carbon nanotubes can be classified into chair-shaped and zigzag (**Figure 2**). Among them,

**Figure 1.** *Scheme of MWNT [5].*

**Figure 2.** *Scheme of different SWNTs: armchair (a); zigzag (b); and chiral (c) [5].*

only nanotubes have metal sex seats, chiral semiconductor tubes and zigzag tubes have 1–3 narrow slits and 2–3 large slits, respectively [5].

Methods of producing CNTs include arc evacuation, chemical vapor deposition (CVD), laser ablation, electrolysis, pyrolysis, flame synthesis, electron or ion beam
