Carbon Nanotube Synthesis and Growth Mechanism

A carbon nanotube (CNT) is a tubular structure made of carbon atoms, having diameter of nanometer order but length in micrometers. Right from its discovery, we have been listening exciting quotations about CNT, viz. “CNT is 100 times stronger than stainless steel and six times lighter...” “CNT is as hard as diamond and its thermal capacity is twice that of pure diamond...” “CNT’s current-carrying capacity is 1000 times higher than that of copper...” “CNT is thermally stable up to 4000K...” “CNT can be metallic or semiconducting, depending on their diameter and chirality...” However, it is important to note that all those superlative properties were predicted for an atomically-perfect ideal CNT which is far from the CNTs we are practically producing today. Despite a huge progress in CNT research over the years, we are still unable to produce CNTs of well-defined properties in large quantities by a cost-effective technique. The root of this problem is the lack of proper understanding of the CNT growth mechanism. There are several questions at the growth level awaiting concrete answer. Till date no CNT growth model could be robustly established. Hence this chapter is devoted to review the present state of CNT synthesis and growth mechanism. There are three commonly-used methods of CNT synthesis. Arc-discharge method, in which the first CNT was discovered, employs evaporation of graphite electrodes in electric arcs that involve very high (~4000°C) temperatures (Iijima, 1991). Although arc-grown CNTs are well crystallized, they are highly impure; about 60–70% of the arc-grown product contains metal particles and amorphous carbon. Laser-vaporization technique employs evaporation of high-purity graphite target by high-power lasers in conjunction with high-temperature furnaces (Thess et al., 1996). Although laser-grown CNTs are of high purity, their production yield is very low (in milli gram order). Thus, it is obvious that these two methods score too low on account of efficient use of energy and resources. Chemical vapor deposition (CVD), incorporating catalyst-assisted thermal decomposition of hydrocarbons, is the most popular method of producing CNTs; and it is truly a low-cost and scalable technique for mass production of CNTs (Cassell et al., 1999). That is why CVD is the most popular method of producing CNTs nowadays. Here we will review the materials aspects of CNT synthesis by CVD and discuss the CNT growth mechanism in the light of latest progresses in the field.