The structure of amorphous two-dimensional materials: Elemental monolayer amorphous carbon versus binary monolayer amorphous boron nitride

TL;DR

This paper investigates the atomic structure of amorphous two-dimensional materials, revealing that elemental amorphous graphene contains crystallites, while binary amorphous boron nitride has a disordered pseudo-CRN structure with pseudocrystallites.

Contribution

It provides new insights into the atomic arrangements of amorphous 2D materials, especially distinguishing between elemental and binary monolayer amorphous structures.

Findings

Amorphous graphene favors a crystallite-containing Z-CRN structure.

Binary amorphous BN forms a disordered pseudo-CRN with pseudocrystallites.

Implications extend to other non-elemental 2D and bulk amorphous materials.

Abstract

The structure of amorphous materials has been debated since the 1930's as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely honeycomb regions made of noncanonical hexagons. Implications for other non-elemental 2D and bulk amorphous materials are discussed.