Atomic-Scale Investigations of Multiwall Carbon Nanotube Growth

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. Precise structural control of this arrangement remains the key challenge of CNT growth to realizing their technological potential. Plasma-enhanced chemical vapor deposition (PECVD) from methane-hydrogen gas mixtures using catalytic nanoparticles enables large-scale growth of CNT films and controlled spatial placement of CNTs on a substrate, however, much is still unknown about what happens to the catalyst particle during growth, the atomistic mechanisms involved, and how these dictate the final nanotube structure. To investigate the fundamental processes of CNT growth by PECVD, a suite of characterization techniques were implemented, including attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical emission spectroscopy (OES), Raman spectroscopy, convergent-beam electron diffraction (CBED), high-resolution transmission and scanning-transmission electron microscopy (TEM, STEM), 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. At low hydrogen concentrations in the plasma iron catalysts are converted to Fe3C, from which high-quality CNTs grow; however, catalyst particles remain as pure iron when hydrogen is in abundance, and produce highly defective CNTs with large diameters. The initially faceted and equiaxed catalyst nanocrystals become deformed and are elongated into a teardrop morphology once a tubular CNT structure is formed around the catalyst particles. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (∼1°-3°) 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. Fe 3C catalyst nanoparticles that are located inside the base of well-graphitized CNTs of similar structure and diameter do not exhibit a preferred orientation relative to the nanotube axis. Thus, it does not appear that the graphene nanotube walls of a CNT are necessarily produced in an epitaxial process directly from Fe3C faces. Chemical processes occurring at the catalyst-CNT interface during growth were inferred by measuring, ex situ, changes in atomic bonding at an atomic scale with a 0.15 nanometer electron probe. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe 3C, near where the CNT walls terminate at the catalyst base. An amorphous carbon-rich layer at the catalyst base provides the source for CNT growth. EELS measurements and Z-contrast STEM imaging provide evidence that carbon diffuses on the Fe3C catalyst surface, along its interface with both the iron oxide shell and CNT walls. Atomic-scale EELS measurements at the catalyst surface in locations of CNT wall formation revealed no change in the iron L23 edge compared to the bulk of the catalyst, indicating that Fe3C did not decompose to BCC iron and graphite during CNT wall formation.;Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls can be etched and amorphized. 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. By examining the H-etching behavior of planar sheets of graphite, it is determined that H etching occurs preferentially at the graphite edges, or equivalently, at the exposed graphene edges present at nanotube etch pits.