The structure of amorphous materials-continuous random networks (CRN) vs. CRN containing randomly dispersed crystallites-has been debated for decades. In two-dimensional (2D) materials, this question can be addressed more directly. Recently, controlled experimental conditions and atomic-resolution imaging found that monolayer amorphous carbon (MAC) is a CRN containing random graphene nanocrystallites. Here we report Monte Carlo simulations of the structure evolution of monolayer amorphous boron nitride (ma-BN) and demonstrate that it also features distorted sp2-bonding, but it has a purely CRN structure. The key difference is that, at low temperatures, C atoms easily form hexagons, whereas the probability to form canonical B-N-B-N-B-N hexagons is very low. On the other hand, hexagons have lower energy than non-hexagons, which results in hexagonal CRN regions that grow much like nanocrystallites in MAC. The net conclusion is that two distinct forms of amorphous structure are possible in 2D materials. The as-generated ma-BN is stable at room-temperature and insulating.
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