| AVS 57th International Symposium & Exhibition | |
| In Situ Microscopy and Spectroscopy Topical Conference | Tuesday Sessions |
| Session IS+SS-TuA |
| Session: | In Situ Microscopy/Spectroscopy – Interfacial Chemistry/Catalysis |
| Presenter: | A. Mkhoyan, University of Minnesota |
| Authors: | A. Mkhoyan, University of Minnesota M.J. Behr, University of Minnesota E.S. Aydil, University of Minnesota |
| Correspondent: | Click to Email |
The combination of unique mechanical, thermal, optical, and electronic properties of carbon nanotubes (CNTs) make them a desirable material for use in a wide range of applications. Many of these unique properties are highly sensitive to how carbon atoms are arranged within the graphene nanotube wall. Plasma-enhanced chemical vapor deposition (PECVD) from methane-hydrogen gas mixtures using Fe catalytic nanoparticles enables large-scale growth of CNT films, however, much is still unknown about what happens to the catalyst particle during growth and how it dictates the final nanotube structure. To investigate the fundamental processes of CNT growth by PECVD (S)TEM based characterization techniques were used including convergent-beam electron diffraction (CBED), high-resolution (S)TEM imaging, energy dispersive x-ray spectroscopy and electron energy-loss spectroscopy (EELS).
It is found that hydrogen plays a critical role in determining the final CNT structure through controlling catalyst crystal phase and morphology. A variety of tube structures grow, via a base-growth mode, from single crystalline BCC iron and cementite catalyst particles. At low hydrogen concentrations in the plasma, well-graphitized nanotubes grow from elongated Fe3C crystals, while at high hydrogen concentrations, poorly-graphitized nanofibers grow from BCC iron crystals. Although catalyst particles are single crystals, they exhibit combinations of small-angle rotations, twists, and bends along their axial length between adjacent locations. Distortions are most severe away from the base up into the nanotube where the number of walls is large. This suggests that the stresses generated by the surrounding nanotube distort the catalyst particle during growth. The much larger thermal expansion coefficient of Fe3C compared to that of the nanotube may also play a role in shaping the crystal into the observed tear-drop morphology. No preferential catalyst orientation relative to the nanotube axis was observed, suggesting that what is required for nanotube growth is not an epitaxial relationship with the catalyst, but rather, only formation of an initial graphitic carbon seed. Z-contrast STEM images combined with atomic-scale EELS measurements also revealed an iron-oxide shell at the very base of each BCC and Fe3C catalyst crystal.