An investigation into the cap deformation of carbon nanotube tips using tight-binding molecular dynamics simulation

Abstract

This article employs a tight-binding molecular dynamics simulation approach to investigate the cap deformation of a carbon nanotube tip when used as a probe in a tapping mode measurement process. The simulation results indicate that the deformation behavior of the cap during its interaction with a rigid graphite surface obeys Hooke’s law until a critical repulsive force is attained. At this point, a radial deformation of the cap occurs as the bond orientations and lengths change. This deformation causes a reduction in the magnitude of the repulsive force, and is accompanied by an increase in the axial compression. Consequently, the cap deformation no longer conforms to Hooke’s law. It is observed that when the capped carbon nanotube is withdrawn from the graphite surface, the cap recovers its original shape. By considering the deformation within the Hookean region, this study derives the spring constants of caps of different radii. It is shown that the spring constant decreases as the radius of the carbon nanotube increases toward that of an (8,8) carbon nanotube. Thereafter, there is no significant change in the spring constant as the radius of the nanotube continues to increase. Specifically, the results indicate that the spring constant of a (4,4) armchair carbon nanotube is approximately 1.4 nN/Å, and that this value reduces to 0.4 nN/Å for carbon nanotubes of radii greater than that of an (8,8) carbon nanotube.