Lattice Defects and the Mechanical Anisotropy of Borophene

Using density functional theory combined with a semiempirical 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 Σ5 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 Σ5 grain boundaries could be rather high given that their formation energies are as low as 0.10 and 0.06 eV/Å, respectively. In addition, our results also suggest that borophene can react easily with H2, O2, and N2 when exposed to these molecules. We find that the mechanical strength of borophene is remarkably reduced 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 negative Poisson’s ratio in borophene, and the substitutional C or N can significantly increase the Poisson’s ratio by constrast.