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.