Hierarchical and size dependent mechanical properties of silica and silicon nanostructures inspired by diatom algae

Abstract

Biology implements fundamental principles that allow for attractive mechanical properties, as observed in biomineralized structures. For example, diatom algae contain nanoporous hierarchical silicified shells that provide mechanical defense from predators and virus penetration. These shells generally have a morphology resembling honeycombs within honeycombs, meshes, or corrugated folds, and are surprisingly tough when compared to bulk silica, which is one of the most brittle materials known. However, the reason for the enhanced mechanical properties has remained elusive. Here, it is proposed that one reason for the superior mechanical properties lies in the geometric arrangement, size, and shape of the structures. By carrying out a series of molecular dynamics simulations with the first principles based reactive force field ReaxFF, it is shown that when concurrent mechanisms occur, such as shearing and crack arrest, toughness is optimally enhanced. This occurs, for example, when structures encompass two nanoscale levels of hierarchy: an array of thin walled foil silica structures, and a hierarchical arrangement of foil elements into a porous silica mesh structure. For wavy silica, unfolding mechanisms are achieved for increasing amplitude, and allow for greater ductility. Furthermore, these deformation mechanisms are governed by the size and shape of the structure. The ability to transform multiple mechanical properties, such as toughness, strength, and ductility, is extremely important when looking into future applications of nanoscale materials. Altering the mechanical properties of one of the most brittle and abundant minerals on earth, silica, allows a new window of opportunity for humanity to create applications and reinvent materials once thought to be impossible. The transferability of the concept allowing for massive transformation of mechanical responses, such as brittle to ductile or weak to tough, through geometric alterations at the nanoscale, is a profound discovery that may unleash a new paradigm in the way materials are designed.