Structure and mechanical properties of monolayer amorphous carbon and boron nitride

Amorphous materials exhibit characteristics that are not featured by crystals and their degree of disorder (DOD) may be tunable. Here, we report results on the mechanical properties of monolayer amorphous carbon (MAC) and monolayer amorphous boron nitride (maBN) with different DOD [1]. Pertinent structures are obtained by kinetic-Monte-Carlo simulations using DFT-based machine-learning potentials. An intuitive order parameter, namely the areal fraction F_x occupied by crystallites within the continuous random network (CRN), is proposed to quantify the DOD. We find that F_x captures the essence of the DOD: Samples with the same F_x but different sizes and distributions of crystallites have virtually identical radial distributions functions as well as bond-length and bond-angle distributions. Furthermore, the mechanical responses of MAC and maBN before fracture are solely determined by F_x 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. These results may provide a universal toughening strategy for 2D materials.