STEM - Internal Home
 
 

Nucleation Mechanism for Single Wall Carbon Nanotubes

X. Fan1,2*, R. Buczko1,3,4, A. A. Puretzky5,1, D. B.  Geohegan1, J. Y. Howe6, S. T. Pantelides1,4 and S. J. Pennycook1,4

Phys. Rev. Lett. 90, 145501

Full Article (PDF 428 KB)

Carbon nanotubes have unique properties with a host of potential applications in nanoelectronics, displays, structural materials and hydrogen storage. The key challenge is to induce nanotubes to grow in the right place, in the right direction, and at the right time. An atomic-level understanding of the nucleation process is highly desirable to guide new approaches to synthesis. While average conditions during growth can be determined, no experimental technique can watch individual carbon atoms forming into clusters on the surface of the metal catalyst clusters that seed the nanotubes. Theory, however, is able to "see" a nanotube form through calculations. In laser ablation growth, both the carbon and catalyst atoms condense from the gas phase into metal/carbon liquid drops. As they cool through the eutectic point, carbon will be rejected from the metal and nucleates on the surface. With first-principles theory we have examined various possible forms of carbon nuclei, with increasing size, so as to elucidate the general form of nucleation pathway. We do not need to follow each atom's motion, as the process is statistical in nature, but just determine the lowest energy form of nucleus as a function of the number of carbon atoms. The results are given in the Fig. 1, and show that it is energetically favorable to introduce pentagons at the earliest stages of nucleation. Although pentagons introduce curvature and have an associated energy cost, they allow the dangling bonds around the periphery of the flake to saturate in the metal surface. The lowest energy structure on a metal surface is a hemispherical cap or capped tube. Subsequently the structure can grow by a root growth mechanism.

   
 
Figure 1

 

  Fig. 1. Determination of the nucleation pathway for single wall carbon nanotubes during laser ablation growth. First-principles calculations show the excess energy per carbon atom relative to a graphite for various clusters on a Ni surface. Structures containing hexagons are shown as solid points, structures containing pentagons as squares. Points are labeled according to the number of (pentagons, hexagons) in each structure. It is clear that pentagons are incorporated at the earliest stages in order to facilitate curvature, and reduce the energy associated with dangling bonds at the perimeter.
   
 
  1. Solid State Division, ORNL
  2. Department of Chemical and Materials Engineering, University of Kentucky, Lexington,
  3. Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland
  4. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235
  5. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN
  6. Metals and Ceramics Division, ORNL
    * Present address: Center for Advanced Microscopy, Michigan State University, East Lansing, MI, 48824

 Oak Ridge National Laboratory