Several methods are known for synthesizing CNTs. In the arc-discharge method used in the initial experiments in the early 1990's, a high DC current is passed between carbon electrodes, leading to carbon evaporation and the formation of MWCNTs. The first demonstration of controlled SWCNT synthesis also used arc discharge, but with the inclusion of a transition metal catalyst. Later studies showed that ablating a carbon target using a high-powered laser could also be used to form CNTs. However, as shown in Figure , the most common method for synthesis of CNTs over the last two decades is based on chemical vapor deposition (CVD), wherein passage of a carboncontaining precursor gas over a substrate in a reactor allows for reaction between the precursor and the surface leading to gradual buildup of CNTs. The substrate can incorporate a catalyst, commonly nanoparticles made of Fe, Ni, Co, or combinations of these metals, which can nucleate CNT growth and allow it to proceed at lower temperature. A catalyst is generally not needed for MWCNT synthesis, but is needed to make SWCNTs, since the size and composition of the catalyst determines the initial stages of CNT growth, which determines the diameter and chirality of the final nanotube.