Conical Carbon Nanotube Arrays: Large Area Synthesis, Field Emission Characteristics

ABSTRACT

Conical carbon nanotube (CCNT) is a new morphological manifestation of carbon nanaotube with a unique structure, tapering from micron-sized base to nanometer scale tips with central hallow core consisting of a multiwalled carbon nanotube.1. The surfaces of these conical structures are composed of high density of graphene edge planes.2 Also, due to the conical geometry and high aspect ratio, the arrays of the conical carbon nanotubes contain their tips spaced from each other. So, these conical carbon tube arrays are expected to yield dramatically different behavior in terms of both electrochemical and field emission characteristics. However, for all the applications, large area (> 1 cm2 area) synthesis of these arrays on flat substrates has been challenging.

Here, we report our recent success with synthesizing these conical carbon tubular arrays over graphite and other metallic foils using microwave plasma enhanced chemical vapor deposition (MWCVD) reactor. Cylindrically rolled foils immersed vertically into the plasma, resulted in conical carbon tubes up to as long as 30 microns. The field emission characteristics for a CCNT array sample with a tip radius of 5 nm, density of 108/cm2 and having the highest aspect ratio exhibited a low turn-on electric field (< 0.7 V/µm) and a high field enhancement factor (β > 7,500). Other samples with lower emission characteristics were attributed either to the presence of field screening effect resulting from higher CCNT density or due to the corresponding tip and wall characteristics.

References:

1. R. C. Mani, X. Li, M. K. Sunkara, K. Rajan, Nano Lett. 2003, 3, 671

2. B. Chernomordik, S. Dumpala, Z. Chen and M.K. Sunkara, Chem. Vapor Depos. 2008, 14, 1-7.