Controlled growth of carbon nanotube films for high-current field emission

Carbon nanotubes (CNTs) offer great potential for numerous cold-cathode field emission applications. A less studied need is for high-current cathodes. While work to date has focused on the use of tangled webs of single-wall CNTs, much understanding about field emission has occurred from studies using multi-wall CNTs with controlled geometries. However, the crystalline nature of these multi-wall CNTs typically is far inferior to that of single-wall CNTs. We use high-resolution transmission electron microscopy to demonstrate that growth at temperatures ≤ 630 °C via thermal chemical vapor deposition can produce highly crystalline multi-wall CNTs, with structures consisting entirely of concentric graphene cylinders. Conversely, growth at temperatures ≥ 650 °C results in crystalline CNTs embedded in a nanocrystalline graphite, or glassy carbon, sheath. This sheath material is likely a poor electrical conductor, due to phonon scattering, and will have deleterious effects on field emission. Field emission measurements taken from such films are consistent with the best field-emitting multi-wall CNT films in the literature, in terms of total current for a given applied field, but without the benefit of the preferred perpendicular orientation. These results are promising toward the development of reliable high-current field emission cathodes.