Dec 5, 2008
Growing chiral carbon nanotubes by design
Single-wall carbon nanotubes (SWCNTs) have arisen as promising building blocks in the synthesis of novel composite materials for micro and nanoelectronics, micro and nanofluidics, micro and nanopower devices, and biomedical applications. All of this seems feasible due to the unusual SWCNT’s physico-chemical properties. However, these properties are strongly dependent on the helicity degree or so-called SWCNT chirality. Thus, designing a selective synthesis process capable of producing SWCNTs of specific chiralities is crucial to take full advantage of these properties.
Typical SWCNTs syntheses use a carbon-containing source and a suitable catalyst that facilitates the formation of stable nanotube structures. It is generally accepted that a nascent nanotube emerges upon nucleation of carbon atoms on a metal nanocatalyst surface, and it grows by subsequent addition of carbon species to the nanotube rim. A bottleneck remains in understanding how the growth of certain nanotube chiralities might be favored over others during synthesis. Therefore, uncovering the reasons that bias the formation of near-armchair instead of near-zig-zag chiralities is a first step in the ultimate goal of obtaining specific chiral SWCNTs by design.
A natural way to improve the understanding of the synthesis and properties of nanostructures is the use of ab initio computational tools. In a systematic theoretical study recently published in Nanotechnology, these tools are applied to investigate possible mechanisms for chirality selectivity, using the hypothesis that such behavior might be determined by the chiral nanotube geometric and electronic structure.
It is found that nascent nanotubes formed during nucleation are more stable in conformations leading to near-armchair SWCNTs resulting in energetic differences of up 0.21 eV per carbon atom between some chiralities. A postulated growth mechanism suggests that the growth kinetics of near-armchair SWCNTs is more favorable than that of near-zig-zag tubes, since successive additions of C2 radicals to the geometric/electronic conformation of nascent near-armchair structures lead to an increase of the number of active sites in the nanotube rim as the reaction progresses. Favorable near-armchair growth is also suggested by frontier orbitals reactivity and tendency of tube realignment controlling mechanical growth stoppage. Current work focuses on the metal nanocatalyst role on the selective reaction.
About the author
Perla B Balbuena is professor of chemical engineering, and materials science and engineering at Texas A&M University. Diego A Gómez Gualdrón received his BS in chemical engineering from Universidad Industrial de Santander, Bucaramanga, Colombia, and is currently a graduate student at the Materials Science and Engineering Graduate Program of Texas A&M University.