C2F, BN, and C nanoshell elasticity from ab initio computations -: art. no. 235406

Two-dimensional lattices of carbon, boron-nitride, and fluorine-carbon compositions are treated with ab initio methods in order to evaluate and compare their mechanical properties in a uniform fashion. The demonstrated robustness of continuum elasticity up to very small length-scale allows one to define and compute the in-plane stiffness and flexural rigidity moduli of the representative nanoshells of C, BN, and CxF (x less than or equal to2). While only small deviations from linear elasticity are observed for C and BN, fluorination causes significant spontaneous shell folding. We discover that spontaneous curvature in fluorinated nanotubes shifts the energy minimum from a plane sheet towards the very small diameter tubes of (4,4) and even (3,3) indexes. Moreover, their equilibrium cross sections are distinctly polygonal, due to curvature self-localization, with an equilibrium angle of 71 degrees at each fluorine row attachment. Our analysis yields a simple physical model coupling the mechanical strain with chemical transformation energies.