Hydrostatic pressure effects on the structural and electronic properties of carbon nanotubes

We study the structural and electronic properties of isolated single-wall carbon nanotubes (SWNTs) under hydrostatic pressure using a combination of theoretical techniques: Continuum elasticity models, classical molecular dynamics simulations, tight-binding electronic structure methods, and first-principles total energy calculations within the density-functional and pseudopotential frameworks. For pressures below a certain critical pressure P-c, the SWNTs' structure remains cylindrical and the Kohn-Sham energy gaps of semiconducting SWNTs have either positive or negative pressure coefficients depending on the value of (n, m), with a distinct "family" (of the same n - m) behavior. The diameter and chirality dependence of the pressure coefficients can be described by a simple analytical expression. At P-c, molecular-dynamics simulations predict that isolated SWNTs undergo a pressure-induced symmetry-breaking transformation from a cylindrical shape to a collapsed geometry. This transition is described by a simple elastic model as arising from the competition between the bond-bending and PV terms in the enthalpy. The good agreement between calculated and experimental values of P-c provides a strong support to the "collapse" interpretation of the experimental transitions in bundles. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.