Inhomogeneous elastic stretching of carbon nanosprings

Abstract

Atomistic modeling of carbon nanosprings under tension is performed. Two types of nanosprings are considered, the $l$-helicene (C${}_{l^2}$H${}_l$)${}_{\infty }$ in the form of a helicoid and the $l$-kekulene (C${}_{l^2-1}$H${}_{l+1}$)${}_{\infty }$ in the form of a spiral graphene nanoribbon. The molecules exhibit large elastic (reversible) deformations of 200%–500%. Surprisingly, $l$-helicene with $l>3$ and $l$-kekulene with $l>2$ are stretched inhomogeneously, so that the domains with small and large tensile deformation are observed within certain range of relative elongation. Moreover, within this range of elongation, the stretching occurs at constant tensile force. This behavior is explained by calculating the potential energy of homogeneously stretched nanosprings as a function of elongation. These curves have a non-convex shape within the range of relative elongation where the inhomogeneous deformation occurs. When the constraint of homogeneous tension is not applied, the system does not follow the non-convex dependence of energy on elongation, but rather the tangent line to this curve. Since energy is a linear function of elongation, the force is constant in the region of inhomogeneous deformation. The results presented demonstrate the possibility of creating graphene nanosprings that deform over a wide range of strain with a constant tensile force.