Auxeticity of monolayers induced by superimposed effect of hinged hexagons and tetragons with various bonding characteristics

Two-dimensional auxetic materials exhibit an unusual mechanical behavior of expanding in response to tensile strain, which is primarily attributed to specific re-entrant geometric structures while regardless of the chemical composition and electronic structure. In this work, we systematically analyze the mechanical behaviors of phosphorene, group-IV monochalcogenide (GeS) and graphane monolayers composed of hexagons (H-), tetragons (T-) and two combined structures (HT1- and HT2-) to explore their auxetic behaviors based on first-principles calculations. Most of them exhibit an intrinsic in-plane negative Poisson’s ratio (NPR), which is dominated both by their special geometric structures and their electronic structures. Our findings unveil that HT2-P exhibits a remarkably low NPR of −0.174 at small strain, which is the smallest among these monolayers without negative frequencies in phonon spectrum. Additionally, HT1-P demonstrates all-angle negative Poisson's properties and showcases the most pronounced NPR (−0.262) at large strain, surpassing that of H-P and T-P. The superposition of these ring structures can further enhance the auxetic properties. Furthermore, the electronic effect exerts a significant impact on the auxetic property, and generally speaking, systems possessing lone-pair orbitals exhibit superior auxetic behavior. The unusual auxetic behavior in combination with other remarkable features of monolayer two-dimensional materials may lead to novel multifunctionality.

A series of monolayers with auxeticity induced by superimposed effect of hinged hexagons and tetragons with various bonding characteristics.