Elemental Amorphous Carbon versus Binary Amorphous Boron Nitride Monolayers

The structure of amorphous materials has been debated since the 1930s as a binary question: Zachariasen continuous random networks (Z-CRNs) vs. 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 by kinetic Monte Carlo simulations that emulate chemical-vapor deposition (CVD) growth on a substrate. We find that crystallite-containing Z-CRN is by far the energetically preferred structure of elemental monolayer amorphous carbon (MAC), as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN (ma-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 [1]. The ma-BN is thermally stable and insulating, and the thermal conductivity is two orders of magnitudes smaller than h-BN due to vibrational-mode localization [2].

[1] Y.-T. Zhang et al., https://doi.org/10.1021/acs.nanolett.2c02542 (2022).

[2] Y.-T. Zhang et al., Appl. Phys. Lett. 120, 222201 (2022).