Metastable twin boundary mediating superelasticity and ferroelasticity in monolayer group IV monochalcogenides

Abstract Although covalent and ionic inorganic semiconductors have traditionally been excluded from exhibiting superelastic behavior, recent atomic-scale high-resolution transmission electron microscopy has revealed superelasticity in bulk group-IV monochalcogenides (e.g., GeSe). In this study, we extend the concept of superelasticity to the two-dimensional (2D) atomic limit using ab initio simulations, with GeSe serving as a paradigmatic example. Under uniaxial tensile strain under zigzag direction, resonant-bonding-induced transverse phonon softening is activated, triggering a reversible structural transition mediated by metastable twin boundary—an essential characteristic that distinguishes superelasticity from ferroelasticity. Through systematic analysis, we identify GeSe, SnS, and GeS as promising superelastic candidates, while SnSe, Bi, and Sb exhibit ferroelastic behavior under similar zigzag tensile strain. Additionally, we demonstrate that uniaxial compressive strain along the armchair direction universally induces ferroelasticity in Bi, Sb, GeSe, SnS, and SnSe. These findings not only confirm the existence of superelasticity in 2D materials but also establish a clear criterion for differentiating superelasticity from ferroleasticity. This work provides a transformative framework for understanding and designing flexible electronic devices based on superelastic monolayer group-IV monochalcogenides.