Radial deformation and stability of single-wall carbon nanotubes under hydrostatic pressure

In this paper we have developed a theory of energetics for isolated single-wall carbon nanotubes (SWNTs) deformed in the radial direction, and applied this theory to investigate their deformation characteristics and stability under hydrostatic pressure. The starting point of the theory is the strain energy of SWNTs predicted by ab initio calculations based on the density functional theory (DFT), which shows the same behavior as that obtained for the continuum elastic shell model. We extend this result for inflated SWNTs with circular cross section to calculate the deformation energy of a deformed SWNT without performing further DFT calculations. This extension is then complemented by a van der Waals interaction, which is not fully taken into account in the DFT approximations currently in use but becomes important in highly deformed tubes. We find that the minimum pressure, P-1, for the radial deformation to occur is proportional to the inverse cube of tube diameter, D, in agreement with the recent theoretical predictions as well as the classical theory of buckling. The radial deformation of SWNTs with D < 2.5 nm is found to be elastic up to very high pressure and they hardly collapse. On the other hand, SWNTs with D > 2.5 nm show a plastic deformation and collapse if the applied hydrostatic pressure exceeds a critical value, which is about 30-40% higher than P-1 and also varies as D-3 though approximately. These SWNTs with large D collapse when the cross-sectional area is about 60% reduced with respect to the circular one. It is also found that for SWNTs with D > 7.0 nm, the plastically deformed (collapsed) state is more stable than the inflated one. This critical value of D is somewhat larger than previously predicted.