(20f) A Mechanochemical Model of Growth Termination In Vertical Carbon Nanotube Forests and Experimental Approaches toward Infinite Film Synthesis

Authors: 
Han, J., Massachusetts Institute of Technology
Welch, C. R., U.S. Army Engineer Research and Development Center
Marsh, C. P., U.S. Army Engineer Research and Development Center
Strano, M. S., Massachusetts Institute of Technology
Carlson, T. A., Construction Engineering Research Laboratory (CERL)


Understanding the mechanisms by which vertical carbon nanotubes (CNTs) array 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, 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 will be discussed. With this theoretical hypothesis, up to now, we have experimentally synthesized the millimeter-thick vertical CNTs array via water injection route during synthesis to circumvent the growth termination of the film in chemical vapor deposition system. Notably we observed such characteristic deflections of the top surface of the films grown and were able to control the degree of both deflections and straightness, possibly originated from the mechanical coupling between arrays of CNTs in the forests. For the further detailed quantitative analysis, small angle X-ray scattering (SAXS) studies on different forests grown under various water injection modes will be presented.

Reference

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