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A Mechano-Chemical Model of Growth Termination In Vertical Carbon Nanotube Forests

Jae-Hee Han1, Charles Robert Welch2, Charles P. Marsh3, Thomas A. Carlson3, and Michael S. Strano4. (1) Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, NE47-577, Cambridge, MA 02139, (2) Information Technology Laboratory, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180, (3) Construction Engineering Research Laboratory (CERL), U.S. Army Engineer Research and Development Center, 2902 Newmark Drive, Champaign, IL 61822, (4) 66-566 Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

Understanding the mechanisms by which vertical array of carbon nanotubes (CNTs) terminate their growth may lead to the production of aligned materials of infinite length. Both single- and doubled-walled carbon nanotubes (SWNTs and DWNTs) films demonstrate characteristic yet unexplained deflections of the top surface near the edges and corners of the film. We show that this upturn in the surface can be explained by assuming a mechanical coupling between neighboring nanotubes.1 A Monte Carlo simulation of film growth is able to qualitatively reproduce the shape by assuming that the coupling is limited by the enthalpy of the carbon forming reaction. The shape of the surface is approximately conic with hyperbolic cross sections that allow for the calculation of a threshold force (Fmax = 34 to 51 nN for SWNTs and 25 to 27 nN for DWNTs) and elastic constant (k = 384 to 547 N/m for SWNTs and 157 to 167 N/m for DWNTs) from the images of experimentally synthesized films. Despite differences in nanotube type and precursor chemistry, the values appear consistent. The origin of the mechanical coupling is discussed.

Reference

[1] Han, J. H.; Graff, R. A.; Welch, B.; Marsh, C. P.; Franks, R.; Strano, M. S. ACS Nano 2008, 2, 53-60.