Structure and mechanical properties of monolayer amorphous carbon and boron nitride

TL;DR

This study investigates how the degree of disorder in monolayer amorphous carbon and boron nitride influences their mechanical properties, revealing structure-property relationships and toughening mechanisms through simulations.

Contribution

It introduces a new order parameter Fx to quantify disorder and links it to mechanical responses, providing insights into amorphous 2D material toughness.

Findings

Fx effectively describes the degree of disorder in amorphous monolayers.

Mechanical responses depend mainly on Fx, regardless of crystallite size and distribution.

Crack propagation is influenced by crystallites, which can enhance toughness.

Abstract

Amorphous materials exhibit various characteristics that are not featured by crystals and can sometimes be tuned by their degree of disorder (DOD). Here, we report results on the mechanical properties of monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (maBN) with different DOD. The pertinent structures are obtained by kinetic-Monte-Carlo (kMC) simulations using machine-learning potentials (MLP) with density-functional-theory (DFT)-level accuracy. An intuitive order parameter, namely the areal fraction Fx occupied by crystallites within the continuous random network, is proposed to describe the DOD. We find that Fx captures the essence of the DOD: Samples with the same Fx but different sizes and distributions of crystallites have virtually identical radial distributions functions as well as bond-length and bond-angle distributions. Furthermore, by simulating the fracture process with molecular dynamics, we found that the mechanical responses of MAC and maBN before fracture are solely determined by Fx and are insensitive to the sizes and specific arrangements of the crystallites. The behavior of cracks in the two materials is analyzed and found to mainly propagate in meandering paths in the CRN region and to be influenced by crystallites in distinct ways that toughen the material. The present results reveal the relation between structure and mechanical properties in amorphous monolayers and may provide a universal toughening strategy for 2D materials.