C2F,BN, and C nanoshell elasticity fromab initiocomputations

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 ${\mathrm{C}}_x\mathrm{F}$ $(x<~2).$ 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 $71°$ at each fluorine row attachment. Our analysis yields a simple physical model coupling the mechanical strain with chemical transformation energies.