Invited Paper PS+SS-ThA1
Plasma Prize Lecture - The Role of Atomic Hydrogen on Plasma Synthesis of Carbon Nanotubes

Thursday, November 3, 2011, 2:00 pm, Room 202

Hydrogen containing discharges are used widely in plasma synthesis of a variety of nanustructures including nanoparticles and carbon nanotubes. We developed a method for measuring the H-atom flux at the plane of the substrate surface during H₂ plasma exposure. Our method is based on infrared measurements of the change in free-electron absorption in a polycrystalline ZnO film when this film exposed to H atoms. Hydrogen acts as an electron donor in ZnO, and thus the concentration of hydrogen-generated free carriers can be extracted from their absorption in the infrared. The change in the concentration of free carriers can in turn be related to the flux of H atoms impinging on a ZnO film placed on the substrate platen. Using this and a suite of additional plasma and material characterization techniques, including optical emission, infrared, and Raman spectroscopy and electron microscopy we systematically investigated the interrelation among plasma gas phase composition, catalysts morphology, catalyst structure, and carbon nanotube structure in plasma enhanced chemical vapor deposition of carbon nanotubes. The structures of carbon nanotubes grown from catalytic nanoparticles via PECVD in CH₄ /H₂ mixtures show a strong dependence on the H₂-to-CH₄ ratio in the feed gas. Hydrogen plays a critical role in determining the final carbon nanotube structure through its effect on the catalyst crystal structure and morphology. At low H₂-to-CH₄ ratios, iron catalyst nanoparticles are converted to Fe3C and well-graphitized nanotubes grow from elongated Fe₃C nanoparticles. High H₂-to-CH₄ ratios in the feed gas result in high atomic hydrogen concentrations in the plasma and strongly reducing conditions, which prevents conversion of Fe to Fe₃C. In the latter case, poorly-graphitized nanofibers grow from ductile bcc iron nanocrystals that are easily deformed into tapered nanocrystals that yield nanotubes with thick walls. In the limit of pure hydrogen the cylindrical graphene walls of a nanotube are etched and amorphized by the H atoms. Etching is not uniform across the length of the CNT but rather, small etch pits form at defective sites on the CNT walls along the entire nanotube length. Once an etch pit is formed, etching proceeds rapidly, and the remainder of the CNT is quickly etched away.