Jae-Hee Han1, Rachel A. Graff1, Bob Welch2, Charles P. Marsh3, Ryan Franks3, and Michael Strano1. (1) Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 66-366, Cambridge, MA 02139, (2) Information Technology Laboratory, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, (3) Construction Engineering Research Laboratory (CERL), 2902 Newmark Drive, Champaign, IL 61822
Understanding the mechanisms by which vertical arrays of carbon nanotubes (CNTs) terminate their growth may lead to the production of aligned materials of infinite length. Both single- and doubled-walled carbon nanotube (SWNT and DWNT) 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. 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, 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.