Superior Mechanical and Electronic Properties of Novel 2D Allotropes of As and Sb Monolayers

Novel monolayer allotropes of As and Sb monolayers are predicted to be energetically and dynamically stable by calculations based on density functional theory. Remarkably, these monolayers possess superior mechanical flexibility and can withstand tensile strain as large as 58% in the armchair direction and 24% in the zigzag direction, which are higher than the strain limits of 2D materials such as graphene, MoS2, and phosphorene. The predicted mechanical flexibility is mainly due to the highly puckered nature of these monolayer structures. Tensile strain along the armchair direction expands the puckering of the structure by increasing the dihedral angle without a significant increase in the bond lengths. Moreover, the mechanical properties are found to be highly anisotropic: Young's modulus in the armchair direction is 3 times less than that in the zigzag direction. Furthermore, we show that these monolayer allotropes undergo semiconductor-to-metal transition on application of uniaxial strains and a transverse electric field. The calculated results show the possibility of wide-range tuning of the band gap of these monolayers while keeping their direct gap behavior intact, which can be useful in optoelectronic applications including light-emitting diodes and solar cells.