NANO LETTERS
Formation of Conical Carbon Structures on Vapor-Grown Carbon Filaments
2002 Vol. 2, No. 6 673-676
Nazim Muradov*,† and Alex Schwitter‡ Florida Solar Energy Center, Cocoa, Florida 32922, and Materials Characterization Facility, UniVersity of Central Florida, Orlando, Florida 32826 Received April 22, 2002
ABSTRACT We report two novel conical carbon structures of nano-to-micrometer dimensions that spontaneously grow on the surface of carbon filaments produced by thermal decomposition of certain hydrocarbons. The first carbon structure occurs in the form of an inverted truncated cone attached with its base to the surface of a carbon filament. Another carbon structure grows in the form of a conical spike with an apex angle of 6°. Interestingly, these two structurally dissimilar carbon species can grow adjacent to each other.
Carbon nanostructures have been attracting great interest in the research community because of their remarkable electronic and mechanical properties that could potentially impact many areas of science and technology. Following the discovery of fullerenes1 and nanotubes,2 new carbon structures have recently been reported. For example, carbon “onions”,3 helical nanotubes,4 nanotube-based tori,5 nanocones,6 nanohorns,7 microtrees,8 nanotube caps,9 nanocapsules,10 nanorods,11 etc. have been observed as free-standing structures produced by different techniques. The formation of most of these nanotube-based structures can be attributed to topological defects in the lattice. Since the defects introduce a nonzero Gaussian curvature into hexagonal lattice, they result in the formation of smooth, seamless interfaces between different nanotubes.12 Conical carbon structures have recently become an active area of research due to their interesting morphologies and potential applications.6,7,13-17 It was determined that conical carbon nanostructures occur when there are fewer than six pentagons in the end cap of a nanotube (as opposed to six pentagons required for the formation of a hemispherical end cap). For instance, the body of a carbon nanocone with the angle of 19.2° is constructed of a hexagon network, while its apex contains five pentagons.6 Conical carbon nanostructures with the cone angles of ∼20° have recently been produced by CO2 laser ablation7 and arc discharge.6,10 These methods involve the condensation of carbon atoms produced by evaporation of solid carbon (e.g., graphite) at temperatures in excess of 3000 °C. Recent studies indicated that a variety of conical carbon nanostructures could be produced by pyrolysis of organic * Corresponding author. E-mail
[email protected]. Tel. 321-6381448. Fax 321-638-1010. † Florida Solar Energy Center, University of Central Florida. ‡ University of Central Florida. 10.1021/nl0255909 CCC: $22.00 Published on Web 05/02/2002
© 2002 American Chemical Society
compounds. For example, conical carbon structures were produced by chemical vapor deposition of carbon (1300 °C) from CH4-H2 mixtures onto carbon filaments made by a catalytic method.13 Graphitic conical nanofibers (GCNF) with average diameters of 5-70 nm and lengths of
