How Do Carbon Nanotubes Grow?
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| According
to an ORNL computer model, possible configurations of 60-atom carbon
nuclei during laser ablation include (a) a flat graphite flake perpen-dicular
to a metal surface; (b) an open-ended nanotube on metal; and (c) a
capped nanotube on metal. (Illustration enhanced by Jamie Payne.) |
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| High-resolution transmission electron microscope image showing a carbon nanotube emerging from a nickel-cobalt catalyst particle. (Image by Jane Howe) |
How and why do carbon nanotubes
grow during laser ablation and chemical vapor deposition? What role do
metal catalyst particles play in inducing carbon atoms to form structures
that grow into nanotubes?
To address these questions,
Steve Pennycook of ORNL’s Solid State Division and Xudong Fan (now at
the University of Michigan) used a computer to model the laser ablation
process, in collaboration with Richard Buczko of the Institute of Physics,
Polish Academy of Sciences, and Socrates Pantelides, a distinguished guest
scientist at ORNL from Vanderbilt University.
According to Pennycook, here’s what the modeling suggests: When the liquid droplets are formed, the cobalt and nickel atoms cluster in the center and the carbon atoms migrate to the outside of the drop. The carbon atoms form a flat graphite flake in hexagonal configurations.
“The problem is that the carbon
atoms at the edges of the sheet-like graphite flake have dangling bonds,”
Pennycook says. “The graphite flake wants to be at its lowest energy level,
so it tries to put down roots and bond with the surface of another material.
“To get rid of the dangling bonds, which require lots of energy, the graphite flake curls into a dome-shaped structure, and its atoms at the edges bond with nickel atoms in the metal catalyst particles. From there the curved graphite sheet grows as a hollow tube. If the metal particles were not there, the graphite sheet would eventually get rid of its dangling bonds by curving into a sphere of 60 carbon atoms, which is the fullerene called a buckyball.”
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