A Comparison of Single-Wall Carbon Nanotube Production Using Continuous Wave and Pulsed Laser Vaporization

Abstract

We have produced single-wall carbon nanotubes (SWNTs) by Nd:Yag laser vaporization of porous cobalt-nickel/graphite targets for the first time without applying external heat to the target. Both continuous wave (c.w.) and pulsed laser-techniques were explored. In the pulsed experiments, the energy density per pulse was varied between 0.12 and 0.62 J/cm² and pulse rates ranged from 24 kHz down to 3 kHz corresponding to average powers of 25.5 to 16 W. Continuous wave experiments were conducted at average powers which corresponded to those measured for each of the pulsed runs. An additional run at 30 W was also performed. A rigorous method developed for the analysis of transmission electron microscopy (TEM) images was used to estimate SWNT content in the laser-generated carbon samples. The pulsed and continuous wave processes both produced a linear increase in SWNT content with increasing average power. These results are not intuitive for the pulsed laser-production since a higher average power corresponds to a lower peak pulse power. In fact, a pulsed run with the maximum pulse energy of 0.62 J/cm² (3 kHz, 16 W), produced no detectable SWNTs. A maximum SWNT content of 78% was observed for c.w. experiments at a power of 30 W. Additional laser experiments performed on dense cobalt-nickel / graphite targets at room temperature enabled a better understanding of the carbon removal mechanisms for the pulsed versus c.w. processes. Cumulatively, these investigations indicate that high energy laser pulses produce particles from highly porous targets by ablation which are too large to readily be incorporated into growing nanotubes. Successful high-yield production of SWNTs relies upon remaining in a vaporization regime during synthesis.