(247a) Understanding the Mechanism of Single-Walled Carbon Nanotube Growth towards Designing a Chiral-Selective Catalytic System

Due to their outstanding properties, single-walled carbon nanotubes (SWCNTs) possess the potential to revolutionize several fields through their use in the fabrication of power-efficient electronic devices, ultrasensitive and selective bio- and chemo-sensors, as well as in the design of new diagnosis and therapeutic strategies.  A prerequisite for the exploitation of these technologies is the large-scale production of nanotubes with specific properties (e.g. metallic vs. semiconductor).  Since such properties depend on the nanotube chirality, it is necessary to design a process where commercial quantities of predetermined nanotube chirality can be selectively synthesized.           

A rationale design of such catalytic system must be guided by a thorough understanding of the nanotube growth mechanism, which is still subject to debate.  Using molecular simulation techniques, we investigate the potential use of catalyst nanoparticles as templates to guide the nanotube growth toward a specific chirality during chemical vapor deposition (CVD) synthesis.  Density functional theory (DFT) calculations show a correlation between nanotube and nanoparticle structures during the nucleation stage.  It is found that, given the relative strengths of carbon-carbon, metal-carbon, and metal-metal bonds, an unsupported nanoparticle adapts to the nascent nanotube (inverse template effect), rather than imposing its structure to said nascent nanotube.  To solve this issue we propose exploiting the interactions of supported nanoparticles with the support material to help control the nanoparticle structure during nanotube growth, thus achieving a direct template effect.

Reactive molecular dynamics simulations demonstrate that depending on the strength of the metal/support interaction and on the nanoparticle size, the nanoparticle structure and dynamics can be altered in such a way that may favor the growth of nanotubes of specific quality and chirality.   As the nanotube nucleates and grows, it tries to maximize the occupation of hollow sites (by carbon atoms) on the nanoparticle surface.  Thus the ever-evolving nanoparticle structure (at 1000 K) can act as template that influences the structure of the growing nanotube.  The effectiveness of this template effect is positively affected by: 1) a good nanotube/nanoparticle contact, and 2) a high rate of defect annealing, whereas is negatively affected by: 3) a heterogeneous nanoparticle surface, and 4) excessive nanoparticle mobility.   Since there are some opposing trends on how these factors are affected by nanoparticle size and interaction with the support, a compromise must be made in optimizing the conditions that favor templated nanotube growth.  Based on this we suggest a range of conditions that can potentially lead to templated growth in experimental nanotube synthesis.

In order to take advantage of templated growth it is necessary to find conditions that promote nanoparticle structure stability, but also growth of high quality SWCNTs.  But to correctly predict these conditions, it is first necessary to elucidate key aspects of the nanotube growth mechanism.  We use reactive molecular dynamics to investigate the carbon transport mechanism and carbide formation. We aim to demonstrate that some of the nanoscale synthesis conditions established by the widely known vapor-liquid-solid  (VLS) mechanism are not mandatory for successful nanotube growth, thus in principle widening the range of experimental conditions within which nanotube synthesis can be optimized.