Mechanical Response of a 2D SiO2 Bilayer: Vitreous Behavior Under Stretching and Anomalous Behavior Under Bending

Over the last decade, a 2D form of silica (2D-SiO2) has been synthesized in laboratory [1]. It consists of two mirror-image external sublayers of SiO2 tetrahedra, where the mirror plane is composed of a middle-sublayer of oxygen atoms in shared tetrahedra vertices. This SiO2 bilayer is a fully-chemically-saturated, mechanically-stable, and van der Waals-interacting 2D insulator, with an experimental band gap of 6.7 eV, a strong candidate to function as an insulating layer in van der Waals stackings of 2D materials. We have addressed the mechanical response of a 2D-SiO2 bilayer to uniaxial tensile strains and bending deformations, employing SIESTA-code ab initio calculations. We find that two fundamental structural modes dominate the response of the 2D-SiO2 bilayer to mechanical deformations: nearly unconstrained scissor and rotation displacements of oxygen atoms in the middle of the Si-O-Si bonding chains in the two external sublayers, and to some extent also in the middle sublayer. These structural modes lead to a nonlinear elastic response up to 30% strains. Moreover, they enable the 2D-SiO2 bilayer to essentially "never break", with atomic rebonding observed at tensile uniaxial strains larger than 30%. The bending response was analyzed by means of nanotubes of various radii, formed by rolling up a 2D-SiO2 bilayer. The O-atom structural modes lead to the formation of kinks in the nanotube surface, such that no discenible pattern can be identified for the evolution of the morphology of the nanotubes as the tube diameter increases. The elastic energy involved in forming the nanotubes also shows no identifiable scaling with the nanotube radius.

[1] K. Büchner and M. Heyde, Prog. Surf. Sci. 92, 341 (2017).