In situ optical emission study on the role of C2 in the synthesis of single-walled carbon nanotubes

In situ optical emission spectroscopy was used to study the temporal and spatial behavior of laser induced plasmas in the laser-furnace synthesis of single-walled carbon nanotubes (SWCNTs). A graphite composite target located within a sealed quartz tube with a chemical stoichiometric composition of 95:4:1 at. wt % of carbon, yttrium, and nickel, respectively, was ablated by a Q-switched Nd:YAG laser delivering colinear, focused laser pulses of 1064 and 532 nm temporarily separated by 20 ns. The ablation process was done at a furnace temperature of 1273 K in a flow of argon gas at either 150 or 200 SCCM (SCCM denotes cubic centimeter per minute at STP). The pressure was varied (100, 400, and 600 Torr) for each gas flow setting. The temporal and spatial behavior of the emission intensity associated with C2 Swan bands (d 3Πg-a 3Πu) was investigated and found to be influenced by the pressure and flow rate of the argon gas. At conditions optimal to SWCNT production, a sharp drop in C2 intensity followed by a rise in C2 intensity was observed. The temporal and spatial behavior of the electron density was determined by the Stark broadening profile of the CII emission peak at 283.7 nm and was found to decrease with the adiabatic expansion of the plume. We propose that the sharp drop in C2 intensity and the rise in electron density and electron temperature observed in this study are due to the accompanying rapid nucleation and growth of SWCNTs.