Dynamics of single-wall carbon nanotube synthesis by laser vaporization

Abstract

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C2 and C3 molecules, and clusters. The ablation plume expands in stages during the first 200 μs after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a ∼1-cm3 volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 μs) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (∼100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 μm/s) was obtained.