2.1 Historical Overview

Very small diameter (less than $ \mathrm{10~nm}$) carbon filaments were prepared in the 1970's and 1980's through the synthesis of vapor grown carbon fibers by the decomposition of hydrocarbons at high temperatures in the presence of transition metal catalyst particles with diameters of less than $ \mathrm{10~nm}$ [13,14,15]. However, no detailed systematic studies of such very thin filaments were reported in these early years, and it was not until the observation of CNTs in 1991 by IIJIMA of the NEC laboratory using high resolution transmission electron microscopy [2].

It was IIJIMA's observation of multi-wall CNTs (MW-CNTs) (see Fig. 2.1) in 1991 [2] that heralded the entry of many scientists into the field of CNTs, stimulated at first by the remarkable one-dimensional quantum effects predicted for their electronic properties, and subsequently by the promise that the remarkable structure and properties of CNTs might give rise to some unique applications. Although the initial experimental observations were for MW-CNTs, SW-CNTs had been the basis for a large body of theoretical studies and predictions that preceded their experimental observation. The most striking of these theoretical developments was the prediction that CNTs could be either semiconductors or metals depending on their characteristics, namely their diameters and the orientation of their hexagons with respect to the CNT axis (chiral angle) [16,17,18]. Though predicted in 1992, it was not until 1998 that these predicted remarkable electronic properties were corroborated experimentally [19,20].

Figure 2.1: The observation of coaxial MW-CNTs by IIJIMA in 1991 [2].
M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors