Pressure-induced hard-to-soft transition of a single carbon nanotube

Abstract

Carbon nanotubes (NTs) exhibit a wealth of fascinating structural, mechanical, and electronic properties. As an ideal one-dimensional structure, their properties are highly anisotropic. For example, mechanically the NTs are extraordinarily hard in the axial direction but soft in the radial direction; the compressibility anisotropy in the two directions exceeds orders of magnitude. As the NTs are virtually incompressible in the axial direction, much attention has been paid to their structural and mechanical behavior in the radial direction. 1‐8 The radial deformation (i.e., the change of cross-section shape) of single-walled carbon nanotubes (SWNTs) in turn influences their mechanical 1‐5 and electronic properties. 7‐9 The hardness, as one of the most important parameters characterizing the mechanical properties of SWNTS, has been intensively studied. 10‐12 However, so far, most studies have focused only on the ground-state hardness of SWNTs at ambient conditions. In this Paper, we investigate the mechanical properties, especially the hardness, of a single SWNT under hydrostatic pressure, using constant-pressure molecular-dynamics (MD) simulations and linear elastic analysis. We discover a pressure-induced hard-to-soft transition at which the radial modulus of the SWNT decreases by as much as two orders of magnitude. We show that this mechanical (hardness) transition is caused by a pressureinduced structural (shape) transition of SWNT characterized by a transformation of its cross section from a circular to elliptical shape. The critical transition pressure decreases with increasing tube radius. We use a constant-pressure MD method especially suited