Understanding and Controlling Chirality

Controlling the radii and chirality of as-grown nanotubes is critical to their wide-scale incorporation as components of nano-electro-mechanical devices.  In catalyst-assisted chemical vapor deposition growth of nanotubes, small catalyst nanoparticles serve as sites initiating nanotube growth. In one model of nanotube formation, small graphitic caps nucleate and grow on the surface of catalyst nanoparticles.  At some critical point, the is

land lifts off the surface of the catalyst, and nanotube growth begins.  The size and defect topology of the cap at the point of lift-off is an important parameter for determining nanotube chirality and radius.  

We are using statistical methods to study ensembles of caps that form under a prescribed growth environments.  This provides us a systematic manner in which to control growth parameters such as temperature, precursor pressure, carbon - catalyst interaction strength, and catalyst size.  Our approach is based on grand canonical Monte Carlo simulations which utilize parameters extracted from ab initio simulations to describe the atomic interactions that take place.  Using this statistical approach, we predict how chirality distributions vary under different growth environments, with the ultimate aim of learning how to tailor chirality-specific growth processes.  

The Role of Facets in Nanotube Growth

During growth of carbon nanotubes via catalyst-assisted chemical vapor deposition, the role of the crystalline facets of the catalysts, if any, is not well understood.  In this collaboration, we work with Professor Alex Zettl's experimental group to explain their obse

rvations of the changing facet structure of iron catalysts during nanotube growth.  Experimental observations demonstrate that the facet structure of the catalyst varies significantly and reversibly with precursor carbon concentration and nanotube formation.

To explain these observations, we have explored via density functional theory the surface energies of BCC iron under different surface terminations (for instance, surfaces terminated with isolated carbon atoms and surfaces terminated with graphene layers, as illustrated in the figure below).  Our predictions of the Wulff shape of the iron catalyst correspond nicely with experimental observations.

Nanotube Growth

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