Elastic Properties of BCN Alloys, Graphene, and h‑BN Monolayers Containing Point Defects

Point defects can strongly affect the mechanical properties of two-dimensional (2D) materials, causing an overall detrimental effect on the strength, stiffness, and elasticity. However, the opposite has also been reported in the literature, which indicates that our understanding of the role of defects at the atomic level remains incomplete. This computational study provides a systematic assessment, based on first-principles calculations, of the mechanical properties of the archetypal 2D materials (h-BN and graphene monolayers) containing substitutional impurities and vacancies, which is further extended to 2D BCN alloys representing the case of high concentration of substitutional impurities in h-BN and graphene. In general, the stiffness of these materials, as described by Young's modulus, decreases in the presence of point defects. The Young's modulus of h-BN decreases rapidly with increasing concentration of C atoms in the N positions, while the drop is smaller for C impurities in the B positions. Notably, a defect configuration, in which carbon atoms replace the neighboring N and B atoms as a pair, results in the values of the Young's modulus in the range between that of pristine graphene and h-BN. In h-BN, B vacancies give rise to a greater decrease in stiffness than N vacancies, as explained by the analysis of the local defect-mediated strain fields formed near the point defects. The effects of graphene weakening through the introduction of substitutional defects and vacancies are similar to those observed in h-BN. This mechanical behavior persists in materials with few atomic percent of point defect concentration and agrees with most experimental results found in the literature. As the mechanical properties of 2D BCN alloys can be manipulated by a preferential substitution of B and N atoms with C atoms, our predictions may guide future efforts in defect-mediated engineering of the mechanical properties of 2D materials.