Lattice Defects and the Mechanical Anisotropy of Borophene

TL;DR

This study uses density functional theory to examine how various lattice defects affect the mechanical anisotropy of borophene, revealing defect-induced changes in its elastic properties and potential for tunable Poisson's ratio.

Contribution

It provides a detailed analysis of defect formation energies and their impact on borophene's mechanical anisotropy, including the possibility of inducing negative Poisson's ratio.

Findings

Defects significantly degrade mechanical properties.

Certain defects can tune Poisson's ratio.

Borophene reacts easily with H₂, O₂, N₂.

Abstract

Using density functional theory combined with a semi-empirical van der Waals dispersion correction, we have investigated the stability of lattice defects including boron vacancy, substitutional and interstitial X (X=H, C, B, N, O) and $\Sigma$5 tilt grain boundaries in borophene and their influence on the anisotropic mechanical properties of this two-dimensional system. The pristine borophene has significant in-plane Young's moduli and Poisson's ratio anisotropy due to its strong and highly coordinated B-B bonds. The concentration of B vacancy and $\Sigma$5 grain boundary could be rather high given that their formation energies are as low as 0.10 eV and 0.06 eV/$\AA$ respectively. In addition, our results also suggest that borophene can react easily with H$_2$, O$_2$ and N$_2$ when exposed to these molecules. We find that the mechanical properties of borophene are remarkably degraded by these defects. The anisotropy in Poisson's ratio, however, can be tuned by some of them. Furthermore, the adsorbed H or substitutional C may induce remarkably negative Poisson's ratio in borophene, and the substitutional C or N can significantly increase the Poisson's ratio by contrast.