Atomic-scale investigations of multiwall carbon nanotube growth.

Abstract

The fundamental processes of carbon nanotube (CNT) growth by plasma-enhanced chemical vapor deposition (PECVD) were investigated using a suite of characterization techniques, 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 are deformed by the surrounding CNT structure during growth. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (~1-3 degree) rotations, twists, and bends along their axial length between adjacent locations. Fe3C 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, indicating that the graphene nanotube walls are not 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 EELS. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe3C, 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. These results suggest that what is required for CNT growth is a graphene seed and a source of decomposed carbon. Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls are etched nonuniformly. Etch pits form at defective sites along the CNT, from which etching proceeds rapidly. It is determined that H etching occurs preferentially at graphene edges.